Stock and water systems of the paper machine

Transcription

1 Ulrich Weise, Jukka Terho and Hannu Paulapuro Chapter 5 Stock and water systems of the paper machine 5.1 Definitions The following terms are commonly used to specify certain areas and systems as part of the entire paper mill water system: Short circulation: The system in which paper machine wire water is separated from the stock in web forming and used for dilution of the thick stock to be delivered to the headbox. Long circulation: The system in which excess white water from the short circulation and other waters are collected at the paper machine (PM) and used for stock dilution and other purposes in stock preparation. Within the long circulation loop, usually fiber recovery and water cleaning equipment is installed. Approach flow system: The system extends from the machine chest to the headbox lip. The main purpose is to meter and dilute the stock including blending with other components like fillers, chemicals, and additives unless not already added in stock preparation. Then, the low-consistency stock is pumped and screened before feeding to the headbox. Stock cleaning by hydrocyclones and deaeration can be included. Stock preparation: Stock preparation or "stock prep" includes mechanical treatment of the stock before the machine chest, proportioning, and blending of the main stock components. Stock preparation begins with repulping or the dilution of pulp from integrated mill operations at the pulp storage towers and ends at the machine chest. 5.2 Design principles Elements and operations The purpose of the stock and water systems is to supply the paper machine (PM) with stock and water in such way that - The quantity of supplied stock is sufficient for the production capacity of the PM - The supply is even and of such quality in order to reach a high PM productivity - The product at the reel meets the given quality parameters. Additionally, process design and operations are carried out in such a way that the above-mentioned aims are met at minimum cost. Hence, the process design has to be optimized according to energy consumption and stock quality refinement in particular. Other constraints have to be considered like process water quality, especially at low fresh water consumption, or pulp quality characteristics. Finally, these aims have to be met at all possible operation conditions. The process and the PM are usually designed so that their highest efficiencies are reached with the most produced grade, which is preferably in the middle of the range of grades aimed for. A small increase in running time efficiency on the main grade is often more beneficial than aiming for highest efficiency with all possible grades. The target is to achieve the maximum total production efficiency. This requires a good knowledge of the product mix to be realized before building or rebuilding the mill. Thus, process and installed equipment can be tailored for the specific grade, which gives advantage in specific consumption of energy and other resources. Papermaking Part 1, Stock Preparation and Wet End - Page 1

2 On the other hand, producing grades other than those initially considered is less economical or even impossible. Generally speaking, the larger the PM production is, the more narrow is the window of grades which can be produced on this machine. Hence, large and fast running machines are usually "single-grade machines." In order to avoid unfavorable interferences and in order to gain steady operation of certain equipment or sub-processes, these units are, at least to some degree, de-coupled from each other. This is usually achieved by buffers with sufficient capacity or by the possibility to switch or to connect process streams differently. The higher the amount of storage or buffer capacity is, the larger the variation may be, which can be leveled out. However, the smoothening effect of large buffers on very low frequency variation can be limited. On a multigrade machine, the time needed for changing grades should be short and the amount of broke produced during the grade change should be small. Hence, stock chests should be relatively small and recirculations short. The total production efficiency on a multigrade machine is usually lower than for especially designed single-grade processes. Table 1 shows typical total efficiencies for modern paper mills; the efficiency figure considers scheduled and unscheduled downtime and broke. Table 1. Typical total efficiencies of modern paper machines. Grade produced Total efficiency Newsprint 90% Supercalendered paper 90% Lightweight coated paper 82% Woodfree paper 92% Tissue paper 85% Linerboard 85% Folding boxboard 82% The sub-processes of the stock and water systems of the paper machine are: - Stock preparation - Stock approach flow system - White water and fiber recovery system - Broke system. Figures 1 and 2 show stock preparation and short circulation as blackbox models. For process design as well as for troubleshooting in mill practice, it is useful to be aware of the borders and interfaces between the sub-processes and operations, especially in complex and sophisticated systems. Addressing the criteria, needs, and concerns for each process step makes it evident as to whether all pieces are suitable to meet their particular goal. Designing the processes accordingly will maximize profitability. Categorizing the activities and defining the process steps is also known as system analysis. Possible bottlenecks in processes can be identified. In an interconnected system, the entire system is only as good as its weakest part. In this respect, malfunction or disturbances originating from auxiliary equipment, fittings, piping, controls, etc., can spoil process performance significantly, even if the main equipment is well chosen and, as such, operating perfectly. Separation of processes into functional units makes complex and sophisticated systems more comprehensible and easier to understand, which means a potential improvement in controllability, system stability, operation safety, etc. Clearly structured processes also ease troubleshooting because reason and cause of problems can be more quickly and easily separated and identified. Figure 1. Stock preparation, system principle. Papermaking Part 1, Stock Preparation and Wet End - Page 2

3 The number of used pulp components depends on their availability and on the product properties desired. Accordingly, in stock preparation, the fiber furnish is determined by: - Selection and proportion of the stock components - Improvement and development of the fibers, i.e., beating. Chapters 3 and 4 describe in detail the main operations like slushing, defibration, and refining of pulp. The consumption of fresh water in stock preparation is very low if not zero. Fresh water is used for the dilution of chemicals and eventually as supplement water in startup situations. In a simple case, stock preparation consists of dilution of a single pulp component and mixing it with recovered fiber and broke. Figure 2 shows a typical short circulation as a blackbox model; the streams of air from deaeration and filler are optional according to the paper produced. The "rejects" stream here also includes the end-stage screen accept, if this is fed into the couch pit. The heating energy added to the wire pit controls the process temperature; its application can be restricted to the startup situation. All fluid streams vary purposely and arbitrarily in flow, concentration, the composition of suspended and dissolved materials, and temperature to some extent. In this respect, the component with the most impact on this system is the thick stock flow. Figure 2. Short circulation, system principle. The main operations in the approach flow system are: - Dilution to headbox consistency - Removal of product and production disturbing contaminants (solids and air) - Conditioning with chemicals and additives - Feeding the headbox - Supply of additional water for PM cross-profile control in case of a headbox dilution system. Figure 3 shows the approach flow system of a printing paper mill. The thick stock is pumped from the machine chest (1) for dilution at the wire pit (2). After the thick stock pump a shut-off valve, a magnetic flowmeter, and a connector for the consistency measurement are shown. Thus, the basis weight is here controlled by the speed of the thick stock pump. At the stock mixing zone of the wire pit (2), right before the stock fan pump (3), the additive dosage points are shown. The stock fan pump (3) feeds the first stage cleaner banks (4), from where the accept flows to the deaeration tank (5). The air from the stock deaeration tank (5) and the headbox dilution water deaeration tank (not shown) passes through an on/off valve and the condenser to the first-stage vacuum pump (6) followed by a water separator. On the small branch pointing to the right side, the vacuum breaker valve is shown. The second stage vacuum pump followed by a water separator is partly hidden. The stock flows from the deaeration tank (5) to the headbox fan pump (7). There are connections shown for filler and additive dosage right before the fan pump (7). There are two horizontally mounted pressure screens (8), which are equipped each with an automatic shut-off valve on the feed side and a manual shut-off valve at the accept side. Reject collection and treatment are not shown here. From the screens (8), the machine stock flows to the headbox of the PM. Retention aid is dosed via a ring pipe header and several radial dosage points (9). Figure 3. Stock approach flow system of a fine paper machine (refer to text for itemization of position numbers) System stability Papermaking Part 1, Stock Preparation and Wet End - Page 3

4 Production conditions and parameters can be quite different according to the wide range of different paper and board grades. Papermaking succeeds even with highly loaded process water in closed cycle, i.e., effluent-free, production. However, process conditions have to be stable to reach the required product quality and a high production efficiency. Steadiness and uniformity of flow are vital and are determined by the design as well as by the operation of the process. Especially in modern operations with high-speed machines and with low fresh water consumption, the physical system stability is as important as the chemical stability of the water systems and the wet end. Attention is paid to the variation of several parameters, separately acting or interacting with each other, when designing or improving the operation of the flow approach system and the wet end. Problems can be caused by variation of: - Flow speed - Pressure (pulsation) - Temperature - Flow conditions and the degree of turbulence - Consistency - Furnish composition and its homogeneity including inorganic solids - Specific surface of fibers, content of fines, freeness - Charge content, cationic demand, ph - Content and distribution of chemicals and additives - Content and distribution of undesired substances and contaminants including air. In the stock and water system of the PM, the importance of steadiness and uniformity of flow increases when approaching the sheet forming. Unevenness in any flow can occur for each of the above-mentioned parameters: - In the cross-direction of the flow - In the flow direction. The latter means a time variation of the flow, which appears in: - Short term: in frequencies > 1 Hz, or in wavelengths < 1 s - Long term: in frequencies < 1 Hz, or in wavelengths > 1 s. Additionally to these periodical (deterministic) variations, there are random (stochastic) variations in the short term as well as in long term Grade changes Contrary to the principle of steady operation are grade changes on the PM. Carrying out grade changes has to be considered carefully when designing the approach flow system and the long circulation of a paper or board machine to ensure stable operation over the entire product range. This applies for so-called "single-grade" as well as for "multigrade" machines; on the single-grade machine, the difference between the produced grades is small. The following points are important on multigrade machines: - Quick response upon changes to gain a short transition period when changing to another grade. This means an improvement in production time efficiency and less broke. Large storage capacity dampens the response. - Adaptation without production disturbance. Machine runnability or production should not be lower right after the grade change compared to steady-state operation. - Fast leveling-out of variations, e.g., caused by instability in the wet end chemistry. Besides the PM runnability, product quality also should not be on a lower level after the grade change Papermaking Part 1, Stock Preparation and Wet End - Page 4

5 compared to steady-state operation. - Stable operation of the system and all its components at all operation levels. Flows, concentrations, and chemistry can differ significantly between different grades, and this has to be paid attention to in design and operation. Stability is to be achieved hydrodynamically, mechanically, and electrically. Control loops have to be tuned accordingly. The short circulation shows, according to Norman's simplified short circulation model1, a t dynamic response for the normalized time, where V is the wire pit volume and Q V=Q 2 2 is the dilution water flow for the thick stock (Q 0, c 0 ) in the wire pit. Figure 4 b shows the dynamic response for different retention values R, when starting the system in Fig. 4 a with clear water. Consider that each stock fraction has a different retention value. The curves in Fig. 4 b could be, for example, the retention curves for different Bauer McNett fractions of a certain stock composition. Their different response causes variations in the basis weight as well as in the web composition as a reaction to quality or quantity changes in the thick stock flow. An increase in thick stock feed leads to a quicker change in the long fiber fraction of the web and to a slower change in the fines and filler content. Vice versa, when decreasing the thick stock flow, fines and fillers are over-represented in the web, due to the less retained material circulating1. This can cause oscillation in the product properties, especially after grade changes, which can reduce the total efficiency of the PM. If a change between particular grades is not possible without major material losses or process complications, emptying and washing of the system is necessary. In this case, the production must be scheduled accordingly. Such washing periods are carried out, for example, in colored paper production, where the dyestuff is changed grade by grade from light to dark shades. After producing the darkest shade, the system is cleaned and production can be started again with the lightest shade. Figure 4. (a.) Short circulation model. (b.) Dynamic response for different retention values R as a function of normalized time Pressure variations and pulsation Of the parameters possibly varying in short-term pressure variations or pulsation are the most critical in the short circulation. High-speed, single-layer machines for printing paper production are affected in particular. Changes in pressure alter the headbox pressure causing undesired variations in the headbox lip flow and in the fiber orientation, etc. In the PM wet end, flow variations are experienced as "barring." Pressure pulses can cause segregation of fibers, fines, and fillers in the suspension, which cause small-scale stock concentration variations and, thus, affect the formation of the sheet. Also fiber spinning, plugging of screens, and deposits can be caused. These variations not only reduce the flow uniformity in the machine-direction, but also make the control of the cross-direction profile more difficult. As a result, the runnability, thus the production efficiency of the PM is reduced. Besides runnability, also the product quality can be reduced and, in many cases, printability in particular. Couching several plies like in board machines dampens significantly small-scale variation, typically basis weight variations, which might be critical for a single-layered sheet. Pulsation analysis The stock approach system is a collection of elements that can - create, - transmit, Papermaking Part 1, Stock Preparation and Wet End - Page 5

6 - dampen or amplify pressure variations. The effect of these pulses can be studied from the basis weight variations, either on-line or off-line. A more direct measurement of pulsation is possible by installing pressure transducers at pipes or by taking vibration measurements with accelerometers outside of pipes or machines. The occurring frequencies and amplitudes characterize the pulsation. Usually, the higher the amplitude is, the greater is the disturbance. Signal analysis including the Fast Fourier Transformation (FFT) is used to extract frequency components2. Figure 5 shows the typical frequency bands of sources in which the disturbances occur at the short circulation. Figure 5. Frequency bands of hydraulic pulsation sources in the short circulation. The detrimental effect of mechanical vibrations cannot be underestimated. Sufficient support of pipes and solid foundations of machinery are important3. Separate foundations and substructure of the headbox and wire section reduce vibrations in the wet end. In the approach flow system, a well-designed framework and proper mounting of the piping gives enough support to avoid vibrations but allowing for thermal expansion. Origin of pressure variations All rotating equipment in contact with stock can contribute to pulses, which affect the uniformity of the headbox lip flow, e.g.: - Stock and headbox fan pump - Pressure screen(s) - Headbox rectifier rolls. If pulses occur at the rotational frequency, usually a mechanical problem is indicated like out-of-roundness caused by damage or improper installation or misalignment due to worn bearings. Pulses can also occur at the multiples of the rotational frequency. For example, for the pressure screen, the possibly occurring frequencies are according to the number, the geometry, and the interaction of the rotating elements with the screen. The amplitude of pulses depends on the design, the manufacturing precision, and the degree of wear. Besides mechanical problems, also hydraulic overloading can cause or amplify pulsation. Air in stock has a similar effect. Stochastic (random) pressure variations can have different origins, including: - Air in stock and air pockets in the system, e.g., at pipe bends - Mixing and dilution sites, e.g., by the headbox re-circulation loop - Controllers and faulty measurements Stock consistency variations The constant thick stock feed flow to the PM has been a major concern since the development of continuous papermaking. Long-term thick stock concentration variations can be caused by: - Large and sudden variations in the component flows to the blend chest - Problems with consistency measurement and control by: - Faulty measurements- Insufficiently tuned controllers- Strong pressure variations in the dilution water header- Other physical problems such as hysteresis of valves and pressure variations in control air, etc. - Wrong design of the machine chest or insufficient agitation. Figure 6. Stock Sankey diagram of supercalendered (SC) paper machine at design production. Papermaking Part 1, Stock Preparation and Wet End - Page 6

7 Figure 7. Water Sankey diagram of supercalendered (SC) paper machine at design production. Changes in the amount of stock circulating in the short circulation, i.e., by changes in the wire retention, have an effect on the stock concentration in the headbox. This includes slow variations in retention, which might be initiated by changes in the amount, size distribution, or charge of the fillers and fines fraction, changes in the amount of dissolved and detrimental substances, or in cationic charge demand, temperature, ph, etc Headbox approach flow stability Pipe bends, joints, and installations cause pipe flow disturbances, which appear as turbulence and vortices causing a head loss4,5. Especially immediately before the headbox, secondary flows or vortices have to be avoided as far as possible to ensure the most even flow conditions. The number of bends in the pipe between the machine screen(s) and headbox should be as small as possible, also for lessening the costs. Figure 3 shows one possible solution. The last bend before the headbox has a radius of at least three times the pipe diameter. If that is not possible for some reason, the pipe should be carried out as a so-called "hydraulic knee," where the pipe diameter is reduced over the bend. This solution is usually more expensive; however, in any case, cavitation and flow separation must be avoided. The bend is followed by a straight section of about five times the pipe diameter right before the tapered inlet header of the headbox (if an attenuation unit or other special headbox approach is not used)6. No control valves or other measuring devices are installed in the headbox feed pipe. Properties of the headbox feed stock are measured after the headbox at the recirculation line. See also the section about stock transport Stock and water balance Determining the stock and water balance of the paper mill is the starting point for any new process design as well as for an existing system analysis. The balances can vary substantially for different situations, e.g., production of the lowest and the highest basis weight or in the case of web breaks. An illustrative manner of representing balances is in the form of a Sankey-diagram as shown in Figs. 6 and 7 for a supercalendered paper line in steady operation. Figure 6 shows the flow of solids relative to the amount of paper at the reel, and Fig. 7 shows the specific water flow for the same situation. Note that, for this grade, there is a large amount of solids in the short circulation in this case, at a retention of 54%. For clarity reasons, the figure shows the cleaner and screening system as one block each and the PM shower water system is very simplified. Collected waters are of different quality, and they are not fed all together into the white water (WW) tank, as shown in Figs. 6 and 7. Note also that the PM comprises a dilution water headbox. Dilution at the second and third cleaner stages is done with wire water, which is shown by the smaller of the two parallel streams between the blocks "wire pit and fan pump" and "cleaner system" in Figs. 6 and 7. The paper mill surplus water is usually fed upstream, like in this case, into the mechanical pulp production unit Multi-ply and multilayer systems The number of separate stock approach flow systems increases according to the number of different stock components used either in multi-ply or multilayer paper and board production. Figures 8 through 10 show the block diagrams for a printing paper-producing machine, a multilayer-producing PM like tissue or linerboard, and a multi-ply board or linerboard machine. The multilayered sheet is produced from one multichannel headbox and one former; thus, there is only one wire pit (see Fig. 9). The multichannel headbox combines two or three stock flows into one jet leaving the headbox lip. This flow consists typically of two different furnishes, which means in the case of a triple layered flow that one furnish is used for the middle layer and the other one for the two outer layers. Both approach flow systems have to be dimensioned Papermaking Part 1, Stock Preparation and Wet End - Page 7

8 according to the desired sheet structures and grades. The requirements on availability and trouble-free operation of the stock approach flow systems are as high, if not even higher, as in single systems because the failure of one approach flow system means that the entire paper production fails. Figure 8. Printing paper machine, block diagram. On a multi-ply board machine, either on a multifourdrinier machine or on a multiple former machine, the wire waters are collected separately and the water circuits are usually separated (see Fig. 10). This is important, if the furnishes of the plies are of very different kinds, e.g., a white top on a brown or gray mid or base layer. Investment costs are higher, due to the higher degree of complexity of the entire stock approach flow system including separate blend and machine chests for each ply or component. The larger amount of equipment also requires more floor space, electric power, and instrumentation and control loops. The benefits of multi-ply forming are7: - Production can be possibly increased, - Raw material costs can be optimized by using a cheaper furnish and still getting the same strength or optical properties, - Better quality white-lined grades can be produced with lower basis weight of white pulp layer. Figure 9. Multilayer headbox machine with two stock components, block diagram. Figure 10. Two-ply machine, block diagram System cleanliness Cleanliness refers to freedom from dirt and contaminants in the process and in the product. Cleanliness refers also to absence or to a low level in dissolved and colloidal material contamination, which can cause scale or dirt formed by precipitation, coagulation or biological activity appearing as slime. Slime occurs as lumps or films causing product defects such as holes, specs, and smell, as well as production problems by plugging and scaling. The latter leads to micro-biologically induced corrosion and possibly to decreased heat transfer. Micro-organic activity is unavoidable due to the large content of nutrients in paper mill waters and the usually favorable temperature. Hence, control of biological activity is needed in order to protect the production and the product from disturbing slime. System cleanliness should be a concern everywhere in paper production. Cleanliness is supported by correct process design. On the one hand, standing water or low flow speed, dead ends, edges, corners, rough surfaces, and low-quality materials have to be avoided in piping and machinery where material can accumulate and slime or scaling can build up. Note, for example, that it is the first and not the last cleaner bank seen in feed flow direction in Fig. 3, which can be disconnected, thereby ensuring full flow without dead ends in the distribution feed pipe. Figure 11 shows that a fine finish of the steel surface hampers the growth of microbes8. This is important in the stock approach system and nearer the headbox6, especially at locations where the flow velocity is not high, like in the deaeration tank. On the other hand, cleaning of equipment and pipes has to be possible. This includes well-positioned wash fluid connectors and drainage valves to empty pipes during shutdowns. The latter is essential for all thick stock pipes, which should be inclined and equipped with a drain at the lowest and a vent at the highest point. Air pockets are a prominent place for slime to build up, which then releases as lumps from time to Papermaking Part 1, Stock Preparation and Wet End - Page 8

9 time. Avoiding the buildup of air pockets is important when positioning valves and selecting the appropriate type of valve. At open surfaces, splashing should be avoided because air is entrained and material builds up on surfaces above the waterline. This then accumulates, dries, and eventually re-enters the process as detrimental chunks. System washings have to be performed during planned maintenance shutdowns, which can include pressure rinse, high-temperature treatment, and washing with chemicals according to needs9. Timer-controlled washing systems and showers can be installed on equipment and tanks, which are prone to build up dirt or chunks of dried stock. Finally, keeping floors and machinery clean improves the mill operators' safety in general. Figure 11. Microbe population on metal surfaces with different degree of finishing Stock flow operations Stock blending The functional paper properties are determined in great part by the properties of the stock components used. Type, quality, and quantity of the different components are determined by the specific recipe for each grade. Therefore, the stock is a blend of several components in order to reach the desired paper properties under the most economic circumstances. Generally speaking, stock blending can take place continuously or in a batch system. In modern papermaking, batch blending is used only for specialty papers produced on machines with small production rates or even in discontinuous operation, applying very special furnish components, dyes, or chemicals. Figure 12 shows a typical example for a continuous system. All fiber components are diluted to the same pre-set concentration for blending. Each pulp typically has a separate pulp chest, the proportioning chest, to ensure a constant supply at the dosage point. In an integrated mill, pulp is usually picked up at a medium-consistency storage tower by dilution with water from the main PM dilution header. The concentration in the pulp chest is usually adjusted to 0.2% 0.3% points higher than in the blend chest. The stock is then diluted to the blending concentration and pumped to blending via refiners or directly. The components are proportioned to the blend chest by flow metering and flow ratio controllers. The setpoints for the controllers are given to the process control system according to the current recipe. The level controller regulates the total amount of stock entering the blend chest. Occasionally, the blend chest is also called a "mixing chest." Despite the name, the function of this chest is not only to create complete motion of the stock, which is referred to as "mixing," but also to gain complete stock uniformity, referred to as "blending"10. There are three or more components mixed in the blend chest: - Primary stock component(s), flow ratio controlled and consistency corrected - Broke, flow ratio controlled and consistency corrected - Recovered fiber from the saveall. Possible consistency differences between the stock component flows can be corrected by calculating the mass flow in the process control system. The components are typically fed via a common header pipe to the blend chest. The header pipe at the side of the blend chest is also a possible dosage point for functional chemicals, e.g., dyes. The sweetener stock pump on the other side of this header pumps sweetener stock to the saveall disc filter. The amount of required sweetener can be large, cf. Fig. 6. The arrangement of the pipes determines which furnish component is used predominantly as sweetener (see Fig. 12). The concentration in the blend chest is similar to the pre-set consistency of the blending streams. Under all possible production conditions, the residence time of stock in the blend chest has to be longer than the blending time, Papermaking Part 1, Stock Preparation and Wet End - Page 9

10 i.e., the time until a reasonably homogenous mixture is reached. Therefore, agitation has to be sufficient. The blended pulp is pumped at a constant rate to the machine chest. The stock is diluted by a small concentration decrement, typically about 0.2% 0.3%. All connections to the blend and machine chest are designed in such a way that the best possible mixing occurs and entrainment of air is low, e.g., by avoiding splashing and large vortices in the vicinity of the open surface. In some installations, additional equipment like refiner or thick-stock screens can be found at the position between the blend chest and the machine chest. Constant consistency and steady hydraulic load at this position ease the operation of such equipment. On the other hand, it has to be considered that any extra equipment at this location requires extra maintenance or might otherwise cause extra downtime, which decreases the total PM efficiency. Uneven operation, e.g., by wear of refiner fittings or varying reject flow from the screen, can cause variations with adverse effects on the performance of the stock approach flow system. Figure 12 shows also how a sampling site can be integrated into the blending system. Constant flow and the possibility to flush and to clean are important in order to collect representative samples of the pulps for laboratory analysis. In addition, or instead of a station for manual sampling, automated or robotized pulp analysis equipment can be also installed. Figure 12. Stock blending and machine chest including sampling station, an example Stock dosage The basis weight of the sheet is controlled by the amount of thick stock from the machine chest to the PM. It is affected by the amount of filler added to the short circulation. For information on basis weight control, refer to Volume 14: Process Control of this book series. Stock dosage or metering is commonly carried out in either of the following two alternative ways: - By a thick stock valve right before dilution at the wire pit - By speed control of the thick stock feed pump. In either case, the thick stock concentration is kept constant. Advantages of the speed-controlled pump are a lower energy consumption, higher feed flow accuracy, and simpler piping layout. The benefits are worth the effort, especially for machines producing a range of grades with large variation in the basis weight. A stuff box is not recommended in modern paper machines in either case because it is a source of slime problems. If stock is metered by valve control, the basis weight valve is located directly before stock dilution at the wire pit. The pipe at the location of the valve should point upward in order to avoid the accumulation of air on the downstream side of the valve. Valve control can be also carried out with two valves in parallel, i.e., with one valve for coarse metering and the other one for fine control Stock dilution Principle In mill practice, reducing the stock consistency means the mixing of a high-consistency stream with a low-consistency stream. Hence, consistency variations in the blended stream can be caused by variations in the flow and in the consistency of either of the streams to be blended. Consistency variations in the lean stream are usually not significant to the diluted stream, except if the rate of dilution is high. The flows and the size of the mixing volume or the mixing zone determine how much variation can be leveled out according to the amplitude and wavelength of variation. For example, the machine chest is dimensioned according to this principle Mixing Papermaking Part 1, Stock Preparation and Wet End - Page 10

11 Mixing is achieved by turbulence created by moving parts, surge on static elements, or turbulence created when feeding streams together. In coaxial pipe arrangements, like in wire pit stock dilution, the thick stock is fed into the dilution water and not vice versa, in order to gain the best mixing and a stable flow. The streams are mixed by secondary flows, which are created by the speed difference of the streams to be mixed. In wire pit stock dilution, the turbulence created in the fan pump provides good mixing. Consistency variations can also be filtered by dividing the flow into branches, in which the stock is retained for varying lengths of time before the separate flows are recombined. Figure 13 shows schematically an arrangement of the multiple flow-lag principle through a divided manifold11. The stock flowing through the left-hand cleaner of the parallel cleaners has the longest retention time. The principle applies in the same manner for pressure screens, deaeration vessel feed pipes, other piping, and the entire PM water system. Figure 13. Multiple flow-lag principle Machine stock dilution In a typical stock approach system, the thick stock from the machine chest is diluted at the bottom of the wire pit (see Fig. 14). The diluted stock consistency depends on the retention, i.e., the wire water consistency, and the amount of thick stock dosed, which again is adjusted to meet the desired basis weight of the product. The actual consistency, which is the primary cleaner feed consistency, can differ according to the produced grade, its basis weight, the current retention, filler content, etc. The thick stock consistency is kept constant. No consistency control occurs after the machine chest. The geometry of the bottom as well as of the outlet of the wire pit is very important to ensure stable hydraulic conditions and a good mixing of the different components. A fan pump, which is feeding the primary cleaner stage in most systems, is directly connected to the asymmetrically tapered mixing zone. Additives like dyes and possibly filler are dosed at the topside of the fan-pump suction piece. The coaxial flow of the main components is steady, if the flow velocity difference between the components is large enough under all possible operation conditions: V 1 > V 2 > V 3 > V 4 where V 1 V 4 are the flow speeds as shown in Table 2 and in Fig. 14. The outer concentric feed pipe is the end of the standpipe collecting circulation flows other than the wire water, possibly: - Second- and third-stage cleaner accept - Second- and third-stage machine screen accept - Headbox recirculation, especially if there is no deaeration tank - Deaeration overflow - Overflow and circulation from the headbox dilution system. Figure 14. PM wire pit with single stock dilution. Feeding the thick stock and other circulated stock top-down at the fan-pump suction side, thus, at the position which is today exclusively used for additives (see Fig. 14), is an outdated solution. The disadvantages of this arrangement are, on the one hand, poor mixing due to the type of arrangement itself and to low velocity differences. On the other hand, feed pipes that are arranged side-by-side can cause hydraulic interaction between the flows12. Table 2. Common wire pit flow velocities as shown in Fig. 14. Papermaking Part 1, Stock Preparation and Wet End - Page 11

12 Flow velocity [m/s] Thick stock V ± 0.5 Circulation V ± 0.5 Wire water V ± 0.5 Wire water V If a low headbox consistency is required, a two-stage dilution system might be needed. In the first stage, the stock is diluted to the cleaner feed consistency and, in the second stage, the cleaner accept is diluted further to reach the desired headbox consistency after screening. Hence, cleaner feed accept consistency and screen feed consistency are de-coupled. As an advantage, the cleaner plant can be operated constantly at the optimal consistency, while the headbox consistency may change according to the grade produced. Two-stage systems are usually considered for multiple grade producing machines with stock cleaning, if the headbox consistency is about 0.7% or lower. On the contrary, tissue paper machines without cleaners and little product variations often have one-stage dilution, even though the consistency can be as low as 0.15%. Similar to a multichannel headbox system with two approach flows, the wire pit also has for a traditional two-stage dilution two separate dilution zones, which are usually located opposite to each other. Another design consists of connected parallel wire water tanks each with a dilution zone. Such a system of connected wire water tanks has been applied in, e.g., multiple layer production with two-stage dilution. However, the two-stage dilution system operates only then stable, if enough wire water is consumed at the second dilution stage Headbox dilution system The aim of dilution at the headbox is to control the basis weight cross-profile at the PM (see Chapter 6). The amount of dilution water ranges from a few percent of the headbox lip flow up to over 20%. Basis weight control is more efficient the lower the solids content is of the dilution water13. To ensure efficient control, a certain dilution flow is needed at all dilution positions all the time. According to the design and recommendations by the headbox manufacturer, the amount of required dilution water can be quite high, even if there is a high one-pass retention like on board machines. In order to avoid an increase in hydraulic load on the saveall disc filter, the headbox dilution water is usually taken directly from the wire pit. For products with characteristically low retention, the required water for dilution has to be increased accordingly. The approach flow system for the headbox dilution water consists of similar elements as the system for the diluted stock main flow: - A speed-controlled fan pump - A fine screen - Deaeration equipment similar to the stock system - Overflow from the headbox dilution header, usually similar to the stock flow. The dilution water is deaerated in a separate unit or in a separate compartment, which is integrated into the stock deaeration tank. The same design criteria are applied as for the stock approach flow system and its equipment concerning low pulsation and the proper design of surfaces and materials Medium- and high-consistency stock dilution In integrated mills, the prepared pulp is often collected from medium-consistency (MC) storage towers, with a storage consistency of typically 8% 12%. Dilution to a pumpable suspension takes place in the bottom part of the storage tower by injecting dilution water, which is filtrate or white water taken from the main header of the PM water system. Figure 15 shows a medium-consistency storage tower. The dilution water is fed to the suction zone of the agitator or Papermaking Part 1, Stock Preparation and Wet End - Page 12

13 injected along the propeller shaft directly into the zone of highest turbulence at the agitator14. Its amount is usually controlled by the diluted stock flow. The fine adjustment of the diluted stock consistency is by injection of dilution water at the suction side of the pump according to the consistency measurement, as shown in Fig. 15. For long distance transport of pulp in high consistency of 20% 50%, either lorries or belt/tube conveyors are used. High-consistency pulp is transported over short distances by screw conveyors. Stock from lorries is usually slushed in pulpers, while the use of a dilution screw is possible for diluting the pulp, which is continuously fed by the conveyor. The pulp is diluted in the screw to storage consistency and drops into a storage tower from where pulp proceeds as described above. The higher the transfer consistency is from integrated pulping to the paper machine system, the better is the separation of the water circuits of the systems. Thus, the less detrimental substances are carried over into PM water, when the white water exchange is made at highest stock consistency. Figure 15. Medium-consistency storage tower and stock dilution Cleaning and screening Chapter 9 of Volume 5: Mechanical Pulping of this book series describes the principles of cleaning and screening and related equipment. Depending on the stock components, pulp has already been subjected to various classification processes like cleaning and screening upstream. Hence, the amount of foreign material to be removed is small; thus, the reject stream should be small. Therefore end-stage cleaners are of special design, and screens are often not discharging reject continuously in an effort to reduce the loss of fibers. The main purpose of cleaning and screening is to ensure clean stock by removing bundles, flakes, and occasional debris and strings, which are partly created within the system; refer to the above section about system cleanliness Hydrocyclone cleaning Centrifugal cleaners are used to remove dense debris of fiber size or smaller from the diluted stock within the short circulation15,16. This debris can be sand, grit, shives, pitch, or other dense particles. Practically all low basis weight and printing paper machines have a multistage cleaner cascade system, which can be attached to a deaeration tank. In the hydrocyclone, the suspension path involves a double vortex with the suspension spiraling downward at the outside and upward at the inside. At the beginning of the conical part of the cyclone, the stream velocity undergoes a redistribution so that the tangential component of velocity increases with decreasing radius. The spiral velocity in the cyclone might reach a value several times the inlet velocity. The separation of accepted and rejected particles depends on the velocity profile and the location of the layer of zero-vertical velocity. The smaller the main diameter of the cyclone is, the more efficient is the separation of debris but the smaller is the hydraulic capacity. At small diameter, the risk of plugging is higher. In the PM cleaner cascade, however, plugging is a minor problem due to dilution after every stage and due to oversized particle removal already performed during stock preparation by screening or high-density cleaners. No cutoff size or critical particle diameter exists for cyclone separation. Centrifugal and shear forces determine the separation or fractionation The flow pattern in the cyclone is very complex, and the separation efficiency curve is unique for a given cleaner geometry. Figure 16 shows the flow pattern schematically. Hence, the debris removal efficiency must be determined experimentally. The conical part of the cleaner can contain baffles and helical guides to modify or Papermaking Part 1, Stock Preparation and Wet End - Page 13

14 direct the flow. These tend to increase the hydraulic capacity and to reduce the fiber reject rate and pressure drop, but they also reduce the cleaning efficiency to some degree15. The removal of vapor and air from the cleaner can also stabilize cleaner flow. Air accumulates along the longitudinal axis of the cleaner, thus in the core of the vortex. Besides the cleaner geometry and design, particle separation is determined by the specific gravity difference between fluid and particle, by the specific surface of the particle, by the stock viscosity, which varies with the stock temperature and the air content, and by the velocity field in general. The operational cleaner parameters are: - Stock flow rate and feed pressure - Ratio of underflow and overflow, i.e., the reject ratio - Feed consistency - Back pressure on the reject side. Depending on these parameters and on the cleaner design, a certain pressure drop and reject thickening occurs. Figure 16. Flow pattern in the hydrocyclone (forward cleaner). Hydrocyclones can remove heavy debris, if designed as forward cleaners, or light debris, as reverse or through-flow cleaners. In the stock approach flow system of the PM, only forward cleaners are used, of which up to seven stages are commonly connected in cascade fashion. The overall debris removal efficiency is best in the cascade system. The higher the number of stages, the higher is the debris concentration in the reject and the smaller the reject stream. Mounting of PM cleaners is either linearly in banks or racks, or radially, e.g., in canisters, often with the reject pipe at the center. Cleaners can operate in a horizontal as well as in a vertical position. The advantage of a linear arrangement is good accessibility of individual cleaners for service, while canisters usually require less floor space. Accept and reject lines are equipped with pressure transmitters for monitoring the cleaner operation. The filler and abrasive pigment content of the stock increases toward the higher stages of the cascade, which can cause excessive wear and reduce the cleaner efficiency and ultimately cause the cleaner to burst. The highest wear is at the narrow side of the cone, where flow velocity as well as consistency are highest. Filtrate is often used instead of white water for dilution at later cleaner cascade stages to improve cleaning efficiency and to reduce fiber loss. End-stage cleaner rejects are usually rich in pigments or filler. Due to the separation principle of cleaners, especially coarse filler particles and agglomerated pigments from coated broke are rejected. It may prove to be feasible to recover these minerals by dispersion and to feed them back to the PM. Thereby, the disposed amount of reject is reduced. In such a filler recovery process, fractionation of the filler containing rejects takes place at first, possibly including coating kitchen wash water. The fine fraction contains most of the filler, which is again classified according to particle size. The coarse mineral particles are concentrated to 30% 50% solids content, treated by a disperser and looped back to previous classification. Undispersable, coarse particles are classified along with the coarse fraction from fractionation and the reject from initial classification. The accepts of both of these classification stages are recovered Screening In nearly all papermaking operations, the installation of at least one pressure screen is mandatory right before the headbox. Exceptions could be, for example, for the lowest quality level of bogus board, or if using extremely long specialty fibers, which would become wrapped up in a screen. The main functions particular to the PM screen are: - Protecting the wet end from occasional coarse foreign material which could damage forming Papermaking Part 1, Stock Preparation and Wet End - Page 14

15 fabrics, i.e., the function of a police filter - Removing debris and dirt - Deflocculation of the stock and improvement of formation. Even if all stock components including broke were previously screened with a finer screening medium, the flakes, bundles, and lumps can be created, e.g., by deposit on the walls of chests and tanks. Secondary stickies and pitch particles can also form in the PM system. The PM screen is located directly before the headbox without any control valves or other installations on the accept side other than retention aid dosage nozzles. Due to its particular position in the process, this screen has to fulfill the following special characteristics: - Very low pulsation generating operation - Polished surfaces - Metal-to-metal flanged connections, especially on the accept side21 - Highest possible availability, i.e., virtually trouble-free operation - Dimensioning according to simplicity in layout, often use of a single screen is preferred - Optimized design to prevent air pockets. Because of these special features, the PM screen typically has a lower screening efficiency in comparison to pressure screens in stock preparation22. On the other hand, the higher the screening efficiency is, the better the runnability of the PM usually is. In order to improve screening efficiency, the use of slotted screen plates has become common. The slot size of the machine screen is a compromise between high screening efficiency and a large open area. The finer the slotting is, the higher is the flow velocity at the slot or the larger is the screen or the number of screens. High slot velocity increases the risk of plugging and pressure pulses, which are in contrary to the above-mentioned requirements. Generally speaking, there is also a higher risk of stringing or fiber spinning for low-consistency screening with slots in comparison to holes. An alternative system for high screening efficiency is a combination of thick stock screening with narrow slot width and machine screening with wider slots or holes. In thick stock screening outside the stock approach system, no special requirements exist concerning the screen surfaces, the generation of pulsation, or other system design features compared to a location right before the headbox. In this respect, it is possible to install a thick stock screening system between blend chest and machine chest23,24. If several stock components are not well screened, a decrease in PM breaks and a product quality increase by fewer spots and holes in the product can be achieved with a smaller number of screens, if installed at this position. This alternative is particularly to be considered for rebuilds, where available floorspace is restricted. However, the load of debris reaching the short circulation can often be reduced in existing mills by installation of a new broke cleaning and screening system or by improving the existing one. The end-stage machine screen reject flow is often discontinuous with a timer-controlled flushing system. Thereby, the retention time within the screen is increased, and more time is given to separate valuable fiber from discharged debris Deaeration Air in stock and water Gases in stock, in the following called "air," refer to dispersed air, i.e., air bubbles, and dissolved gas in water. Additionally, gases can be created by chemical or biological reactions within the water system. Deaeration, or more precisely degasification, refers usually to mechanical vacuum treatment of the stock suspension in order to reduce its air content significantly. Free air consists of air bubbles, which rise to the surface if not interfered by flow. The remaining dispersed air is often called "bound" or "residual" air. Papermaking Part 1, Stock Preparation and Wet End - Page 15

16 Water temperature, surface tension, and pressure affect the adsorption and desorption of air, the bubble stability, the ability of bubbles to coalesce, water viscosity, and thus the bubble retention in the fluid phase25. Average bubble diameters usually range from 50 to 120 μm. With increased temperature, the amount of dispersed air and the air solubility in water decreases and the weighted average of the bubble size decreases. On the other hand, air solubility in water increases linearly with pressure according to Henry's law. With a pressure increase of about, e.g., kpa after a fan pump or even higher after the headbox dilution water pump, most of the dispersed air is dissolved. With the pressure decrease to ambient pressure at the headbox lip, many small-sized bubbles are created by desorption. This redispersed air remains partly with the drained water because small bubbles do not coalesce and, hence, are able to accumulate in the short circulation25. Thus, the mean size of the bubbles is in the circulation water smaller than at the point where air is mechanically entrained by splashing Effect of air on papermaking Air in papermaking stock has adverse implications on the production process as well as on product quality. The critical amount of air leading to noticeable problems depends on the type of PM, the paper grade, and the kind of furnish being used26. Consider as an illustrative example a headbox lip flow concentration of 1% by volume dispersed air for a PM without mechanical deaeration and a headbox solids concentration of 0.7%27. In this case, the volume of dispersed air is about as large as the volume of the solids. It is therefore well understandable that air in stock reduces the production performance, e.g., by: - Pumping efficiency decrease - Screening efficiency decrease - Reduction of drainage - Foam problems at open surfaces, which can also cause accumulation of hydrophobic and sticky material similar to froth flotation - Increase in microbiological activity leading to slime problems - Pressure and flow velocity variations due to air pockets within the system - Instability and noise by cavitation. Air in stock and water adversely affects the product quality28 30 in the form of: - Pinholes and holes - Spots, specks, and lumps in the paper, due to foam or accumulation at the surface of larger air bubbles - Decreased wet-web strength - Decrease in smoothness and tensile strength due to deteriorated formation. On the other hand, finely dispersed air bubbles can also improve formation in some cases31. A higher level of bulk due to the presence of small air bubbles can be a beneficial side effect. In addition to paper quality improvement, the investment into a mechanical deaeration system is often justified due to increases in: - PM runnability and more stable production - Possible speed increase of the PM - Savings on defoaming agents - Savings due to decreased energy losses in pumps Sources of air From the hydrodynamics standpoint, dispersions of gas bubbles in liquids are basically unstable. Papermaking Part 1, Stock Preparation and Wet End - Page 16

17 Forming bubbles means an expansion of the normal surface area of the liquid and, thus, requires a work input. Dissolved substances can stabilize bubbles and retard their collapse. Bubbles can also attach to hydrophobic sites in the stock or get trapped between fibers. Air is either brought with the feed flows into the system, or it is brought in, either due to inherent process operation or by action, which could be avoidable. For example, the splashing occurring in PM wire dewatering is unavoidable. Possibly avoidable sources of air are: - Air via incoming stock and water flows - Splashing in open cleaner reject discharge - Faulty operation of the deaeration unit, e.g., due to insufficient vacuum generation - Liberation of gas by chemical reactions; e.g., acidic process water or acidic additives in combination with calcium carbonate pigments cause a release of CO 2 - Leakage - Biological activity. As already mentioned above, pressure, temperature, and flow speed differences can desorb dissolved air into dispersed air. The release of carbon dioxide from carbonate pigments in acidic conditions causes distinct problems, due to the higher solubility of CO 2 compared to air, and due to resistant scaling by calcium oxalate Chemical deaeration The principle of defoaming chemicals is to increase the rate of bubble coalescence. This means that two colliding bubbles have a higher probability of joining and forming a bigger bubble in the presence of a surface-active defoaming agent, commonly a product on mineral oil basis. Defoaming agents usually affect to little extent, or not at all, the amount of dissolved gases in the paper stock, which can be up to 2% 3% by volume, even if the amount of dissolved air is low. Hence, the ratio of dissolved gas to total gas is increased with the addition of a defoaming agent. If a low level of dispersed air in stock is required, the exclusive use of defoaming chemicals becomes expensive and thus the installation of a mecha-nical deaeration unit is feasible. The addition of defoaming agents in large amounts often causes a decrease in system cleanliness, deposition of dirt on fabrics and felts, a decrease in retention, or an increased need of retention aids respectively. Over-dosage of some defoaming agents can even cause the reverse effect and stabilize bubbles. Defoaming chemicals are also used in combination with mechanical deaeration units for a maximum in air removal. The mechanically deaerated stock is "hungry" and has a high capacity to dissolve more gas into water27. Some types of defoaming agents can prevent the pulp suspension from reabsorbing significant amounts of air when exposed to it after deaeration treatment Deaeration tanks Deaeration tanks remove dissolved and dispersed air very efficiently. Air is desorbed above the boiling point and effectively driven out, when creating a large fluid surface. The typical deaeration tank is designed to treat the headbox stock or headbox dilution water by: - Spraying - Impingement - Boiling. With spraying and by impingement against the interior surface of the tank, a large fluid surface is created and trapped or bound bubbles are released and removed. The air-containing suspension enters upward into the deaeration tank from coaxially connected centrifugal cleaners Papermaking Part 1, Stock Preparation and Wet End - Page 17

18 or feed nozzles through pipes in helical flow pattern. The helical flow causes the desired spraying. The headbox circulation flow is usually fed back to the deaeration tank, but without spraying. Boiling takes place at a vacuum, which is high enough so that no additional heating of the suspension is required. The minimum required vacuum depends therefore on the feed temperature. The system temperature is controlled by heat or steam injection into the wire pit. In practice, the absolute pressure should be about 1 kpa lower than the boiling point at the given temperature. To support the required amount of vacuum, the deaeration tank is placed in an elevated location, and all flows from the unit are barometric drop-leg lines; this includes also the reject lines, in case cleaners are directly connected to the deaeration. Likewise, the primary cleaner feeding fan pump is provided by sufficient back-pressure. Besides deaeration, positive side effects by tank deaeration are as follows: - Constant weir overflow within the tank is maintained by controlling the fan pump speed. - Excellent ability to level out short-term stock consistency variations, especially if the flows are connected obeying the multi-lag principle as shown in Fig Good mixing is obtained by the stock spraying. - Upstream pressure pulses are well dampened. The vacuum is generated by a one- or two-stage water ring pump system. Figure 3 shows a two-stage system. The removed gases are cooled by cooling water in a pre-condenser, either by direct contact or in a noncontact condenser. The more expensive and more cooling water-consuming indirect condenser has the advantage that the cooling liquor does not get in contact with the exhaust; thus, it cannot become contaminated. The use of a steam ejector to discharge the deaeration tank receiver33 causes heat loss and loads the condenser thermally. The use of a sufficiently large vacuum pump or two connected in series usually proves to be more economical. Each vacuum pump is followed by a water separator. Condensed and separated water is collected via drop-legs at the deaeration seal tank in the cellar and typically added to the PM vacuum pump system. The removed air is exhausted via a silencer. Two different deaeration and process design alternatives are commonly used: - All cleaner stages are on the PM floor level and, thus, separated from the deaeration tank. - One to three cleaner stages are connected directly to the deaeration tank. For mid-sized or large PMs, the number of required cleaners is large; thus, the deaeration tank is extended by wings to collect the cleaner accepts. This arrangement is called a "flying wing" system. Table 3 shows the advantages of both alternatives. In several cases, the deaeration tank is supplied with only the first stage cleaner accept. If also the second- and third-stage accepts are fed into the deaeration receiver, the discharge is into the weir overflow, which means that the third-stage accept is fed forward. The last stages are located on the PM floor level and connected in cascade fashion. High requirements for air-free headbox feed flow and operational constraints often make deaeration necessary for the headbox dilution water also, if such a PM cross-direction control system is applied. The deaeration of this dilution water can be either integrated into the stock deaeration tank, or it can occur in a separate vessel. The combined stock and headbox dilution water deaeration tank consists of two compartments located opposite to each other with facing weirs discharging either into the same or into different overflow discharging pipes. The separate headbox dilution water deaeration tank is connected to the vacuum pump system of the stock deaeration. Principle and operation are similar to the stock deaeration system. The headbox dilution header circulation can also be fed back to the dilution deaerator similarly to the headbox stock circulation. Papermaking Part 1, Stock Preparation and Wet End - Page 18

19 Table 3. Advantages of the two most common deaeration tank design solutions. Deaeration tank with cleaners connected Deaeration tank and cleaners separately High cleaner efficiency Standard cleaners can be used Low pressure drop cleaners with low reject Lower investment costs for the deaeration thickening receiver tank Larger reject orifice diameter due to air core in Smaller volume to exhaust, which may save a cleaner causing less plugging and less wear second stage vacuum pump More space on the machine floor level All cleaner stages operators on PM level, regular inspection by operators is easier Lower energy consumption More suitable to add in rebuild design Deaeration by other equipment Dispersed air can leave the system by bubbling out at any open surface, i.e., the wire pit in the short circulation. To allow bubbles to rise, the downward flow velocity in the wire pit must not exceed 0.15 m/s. In order to lower this velocity, the diameter of the wire pit and, thus, the amount of water in the short circulation has to be increased. However, only the larger bubbles of about 1 mm diameter are removed from the wire pit27, and its deaerating capacity remains poor, even with the addition of defoaming agents. Air is removed from stock suspension to some extent when exposing to gravitational field like, e.g., in centrifugal cleaners. Cleaners with light reject removal therefore have a slight deaerating effect. The principle of deaeration by centrifugation is applied in a special type of pump, which is connected to a vortex chamber (see Fig. 17). By means of this deaeration pump, it is possible to redesign the conventional PM short circulation, as mentioned below in the section about novel approaches. Finally, deaeration outlets from pressure screens or other equipment keep the device free from air pockets formed by accumulation of single air bubbles. Instead of active deaeration, the redispersion of air is avoided and the buildup of dirt and slime is prevented. Figure 17. Stock pump with deaeration by centrifugation, courtesy of POM Technology Chemical conditioning Wet end chemistry The wet end chemistry of each PM is unique. A plethora of different and partly unknown substances governs the chemistry of all papermaking process waters. These substances are in interaction, which is influenced by a large number of parameters. In practice, process behavior depends on a complex balance, and predicting a reaction upon process changes or upon addition of a chemical can be difficult, if not sometimes impossible. While principles of wet end chemistry are explained in Volume 3: Forest Products Chemistry and Volume 4: Papermaking Chemistry of this book series, some implications by the process design are stated in the following. Chemicals are added either to the stock or to the process water for the following reasons: - To improve the product properties, e.g., fillers, strength enforcing additives, sizing agents, dyes, etc. - To support and to maintain efficiency of the product properties improving additives, e.g., ph-control, retention aids, fixatives, etc. - To maintain system stability and cleanliness, e.g., defoaming agents, biocides, fixatives, pitch dispersants, washing detergents, etc. Another way of classification is according to functional and process additives34. However, Papermaking Part 1, Stock Preparation and Wet End - Page 19

20 some of the process additives are added in order to support the functional paper chemicals because the performance of the functional aid is measured only from that part, which remains with the paper product35. The dosed chemical reacts with the dispersed fibers and pigments as well as with the dissolved material and potentially with previously added chemicals circulating with the process water. The less process water is discharged and, therefore, the higher the rate of recirculation, the higher is the threat of unfavorable effects in the process caused by accumulating substances. Hence, process chemicals should only be used when absolutely necessary. Overdosing is to be avoided for cost reasons and because of unnecessary loading of the process water. In mill practice, the addition of each wet end chemical should be checked on a regular basis. Practice has shown that changes in the process or in its chemistry can make the use of some chemicals obsolete, which should be consequently turned off. This tends to be "forgotten" because their exact effect on the system is often unknown Dosage points The right dosage point for a particular chemical can differ from system to system. At the dosage point, there is often a trade-off between the best possible mixing with the targeted reaction components and undesired reaction with other components or process disturbance. The flow conditions and the flow speed difference at the dosage point have to be known to ensure proper mixing with the stock or water stream. The order of addition of some additives matters, for example, in a two-component retention system. Possible dosage sites are: - In stock preparation, e.g., before or after refining - To single stock components before blending - At the blend chest, the machine chest, or in between - Before the thick stock feed pump - Before thick stock dilution - At stock dilution, before the primary fan pump - Before the headbox fan pump - Before the machine screen - Before the headbox - To the wire pit Dosing In the paper mill, dosage of chemicals means blending a stream of usually small volumetric flow but high concentration with a large flow of stock water. In order to ensure good mixing with the main stream, the additive is often diluted with water, which has to be in most cases of purified fresh water quality. The chemical is dosed directly into the mixing zone or into the middle of the stream. A multiple radial injector feed system is used typically for high-molecular retention polymer or bentonite proportioning before the headbox (see Fig. 18). In this particular case, mixing is supported by the turbulence created at the joint of the accepts of the two parallel machine screens. The location between the PM screens and headbox can also be seen in Fig. 3. The multiple injection ensures better distribution of the chemical component compared to a single nozzle; this is needed, if consecutive mixing with high shear forces is to be avoided. Figure 18. Bentonite dosage at the headbox feed pipe after pressure screens Measurements Various sensors and measuring devices are installed for control and to supply the operators with Papermaking Part 1, Stock Preparation and Wet End - Page 20

21 necessary information. Besides the basic process parameters like flow, consistency, temperature, and pressure, other data more specific to the papermaking process can be obtained, as shown in Table 4. Reliable metering and measuring is most important for good control of stock operations and high process performance. Table 4. Parameters for wet end chemistry monitoring and control36. Important parameters: Often useful parameters: Temperature Dissolved organic compounds, COD ph Cationic demand Conductivity Inorganics (Ca+, Ca2+, Al3+, SiO 2 ) Consistency Charge Freeness, fines, and filler content Turbidity Flow Hole and specks count Air content According to the type of measuring device, the installation is either at the main pipe or at a measurement line or at a bypass pipe. Advanced, timer-controlled operating stock or wet end chemistry measuring devices can also be grouped together in an analysis center. Sampling pipes and bypass lines are needed if the sensor has to be disconnected occasionally or regularly for cleaning or calibration. Sampling and bypass pipes have to be designed and placed at the main pipe so that no process disturbances like dirt accumulation occur, which might adversely affect the process or distort the measured values or even harm the device. Flow conditions in particular in bypass lines must be steady, and the buildup of air pockets must be avoided. All sensors and advanced measurement devices have to be installed in such a way that the measured values are reliable and consistent. Therefore the following points have to be considered: - The sensor is processwise positioned at a suitable location. The sensor is usually installed prior to the control valve, if connected in series. In some instances in chemical conditioning, the sensor is situated downstream, for example, in retention control. - The sensor has to be installed at a suitable location in order to gain reliable measurement results and according to the requirements of connected controls, e.g., to avoid unnecessary dead time. - At all times, sensors have to be in full contact with the medium to be measured. - Good accessibility to operators and maintenance people, especially if tuning, regular calibration, or other maintenance is required. - Maintenance and cleaning is carried out regularly according to a schedule. - Flushing or scavenging media have to be suitable and sufficient. - External sources of error and disturbance should be avoided, e.g., magnetic field or vibration Stock transport Piping According to the flow rate calculated during the process design phase, a stream velocity is chosen, which determines the diameter of the pipe. In practice, the stream velocity is determined among other things by the following factors: - Investment cost - Operating costs Papermaking Part 1, Stock Preparation and Wet End - Page 21

22 - Properties of the fluid - Pipe erosion - Vibrations - Pressure shocks - Noise (at valves) - Cleanliness - Potential for production increase - Functional location of the pipe - Pressure or position on a suction side - Function as main- or sideline. The price of the pipe and costs resulting from the needed parts, valves, insulation, and support, the capital cost, and the pay-back period affect the total investment cost. The operating costs arise from the energy consumed for fluid transport and from maintenance costs. The piping head loss is proportional to the square of the flow velocity of water and of pulp slurry within certain limits37. The total pipe friction loss curve is non-linear and consists of several areas of different flow regimes. These can be simplified into a plug flow, a transition flow, and a turbulent flow region38. The actual stock flow conditions depend mainly on: - Velocity - Consistency - Pipe diameter - Temperature - Pipe roughness - Type of pulp - Pigment or filler content - Pretreatment of the pulp, like drying and beating. In practice, the stream velocity is chosen according to generally agreed upon empirical values that have been proven sound in practice. When pumping stock over long distances or wherever else possible, it might be useful to calculate the optimum consistency for pumping pulp and adjusting accordingly Pumping The following types of pumps are usually used in paper mills: - Centrifugal pumps - Process pumps for water and stock up to 5% consistency- Fan pumps: cleaner feed pump and headbox pump- Medium-consistency (MC) pumps - Displacement pumps - Screw pumps for sludge, pigment slurry, coating color, etc.- Plunger pumps and membrane pumps for chemicals and additive dosage- Water ring pumps for vacuum generation. The main parameters for selecting a centrifugal pump are: - Capacity, Q - Head, H - Solids concentration - Temperature. Papermaking Part 1, Stock Preparation and Wet End - Page 22

23 At a given speed, each pump has a characteristic curve H P (Q). Also the system response is described by a characteristic curve H S (Q), which consists of a static part and an operational part according to factors such as the downstream load characteristics, pipe friction, and valve performance. Typically, the head changes as a square of the flow rate. The operating point of the pump is determined by the point, where the characteristic curve of the pump H P (Q) meets the characteristic curve of the system H S (Q). In process design, the desired pump capacity is usually given and the required pumping head is calculated. The flow through the system can be altered, by the following means: - Speed control of the pump, which changes the characteristic curve H P (Q) - Control by a throttling valve, which creates additional system friction and thus changes the characteristic curve H S (Q) - Control by bypass circulation - Irreversible, mechanical reduction of the impeller diameter by rotary cut, or replacement by an impeller of a different size. Speed control is often feasible for larger pumps operating at variant conditions because of the energy savings compared to throttling valve control, which has the lowest investment cost. The more throttling there is, the higher are the losses and the less economic is the type of flow control compared to pump speed control. Throttling losses are however less if the characteristic curve of the pump H P (Q) is flat, which means a moderate decrease in head, H, with increased flow, Q. Furthermore, the type of solids and the ions contained in the fluid determine the material requirements for the impeller and the pump casting in order to avoid erosion and corrosion during long-term operation. Stock pumps are especially designed to avoid plugging and fiber spinning as well as excessive wear. The air content of the fluid also has to be considered because air accumulates around the rotation axis on the suction side of the impeller and lowers the characteristic curve of the pump and reduces pumping efficiency. The largest pumps in the paper mill are the fan pumps feeding the first cleaner stage and the headbox. Especially the headbox pump has to operate at lowest possible pulsation, which is achieved by design and manufacturing precision, for example, by a staggered position of vanes between the two suction sides. The characteristic curve of the headbox pump H P (Q) should be steep in order to keep the transformation of pressure variations into flow variations as low as possible Broke system Introduction Broke is paper that is discarded at any point of the manufacturing and finishing processes inside the paper mill. Broke occurs on a continuous basis as trims from the wire and from winders, and broke occurs occasionally as, e.g., reel slab-offs, in the finishing room, or during breaks. Usually, all broke is repulped, cleaned, and stored in the broke system. The processed broke is blended with other components at the blend chest and thus fed back into the production process. The amount of broke dosed to the furnish depends on web breaks and the broke line capacity. Pulps are irreversibly altered during the first drying, which impairs some paper properties as known from paper made of recycled fibers. Hence, the properties of dry-broke pulp is different when compared to the fresh pulp used. There is no specific quality difference between dry broke and clean, unprocessed paper, e.g., unprinted printing shop waste, which is called "wastepaper" or "recovered paper" by definition. In specialty paper or dyed paper production, the reuse of broke might be somehow limited by Papermaking Part 1, Stock Preparation and Wet End - Page 23

24 the required product quality or due to other reason. Depending on the paper grade and the degree of processing, the broke might be pulped and used at another time or elsewhere. However, when this broke leaves the mill, it becomes per definition "recovered paper" or secondary fibers. One way to classify broke handling together with other processes connected to the PM is to consider it as a system of its own. Another way is to consider the broke system as part of stock preparation because the same operations of slushing and defibration are performed. Finally, broke handling means that fibers are recovered for use on the PM. However, the broke system should not be integrated into the fiber recovery or saveall operation because it can lead to system instability. During steady operation of the PM, a steady flow of white water with constant solids content is handled in the saveall, while broke occurs at an inconstant rate. The broke system should therefore be decoupled from the white water treatment. Also the use of broke as sweetener of the disc filter is not favorable. The broke thickener filtrate, however, is normally fed into the fiber recovery, while the dilution water to the broke pulpers is taken from the white water or clear water tower, hence, from water after fiber recovery Broke system requirements Uniform and good stock quality is the main requirement to a properly operating broke system. If breaks are long-lasting or occur repeatedly, the amount of broke in the stock has to be increased. Poor performance of the broke system due to malfunctions or faulty design can cause further breaks, which in turn raises pressure to increase the proportion of broke in the stock. Hence, proper design and sufficient storage capacity of the broke system are essential. The total broke storage capacity is commonly equivalent to 2 4 hours of net production, depending on the paper grade; complex machines with coating stations require large broke storages like an on-line lightweight-coated (LWC) paper machine. Broke handling is determined by the following functional steps: - Broke transport - Pulping - Storage - Cleaning and homogenizing - Dosage. The capacity of all machine pulpers and equipment of the broke system has to be sufficient to handle the amount of paper produced at maximum gross production. On the other end, the broke system also has to function properly from the broke storage tower to broke dosage, if no breaks occur over a long period of production. In this case, the broke consists only of trims and slab-offs and eventually occurring broke from rejected rolls, which accounts together for only a few percent of the maximum capacity. Therefore, broke can be circulated to assure proper functioning of the broke screens and deflakers. On multigrade machines, grade changes can create a special problem if furnishes of two grades are not compatible. A running grade change in such a situation is not possible, but a time-consuming cleanup of the system is necessary. Therefore, it is not usually feasible to use a large single broke storage tower on a multigrade machine. At the next grade change, the stored broke might not be suitable for the new grade and would have to be discarded. For this reason, wet broke is used immediately. At least on small machines, dry broke can be stored and added together with other raw material components at the beginning of the stock system. The flexibility required by a multigrade machine is achieved by sacrificing the stability of the system, especially during web breaks. During a web break, the proportion of broke in the stock pumped to the PM increases, which can easily cause new breaks. This is one reason among several others causing Papermaking Part 1, Stock Preparation and Wet End - Page 24

25 usually a lower operation speed of a multigrade machine than the speed of a single-grade machine operating under similar conditions Broke handling Transport and auxiliary equipment The location and the number of broke pulpers is chosen for each machine as a compromise between the minimum transport effort of broke and the cost for installation and operation. Right after a break begins, the web is cut at the next pulper before the break location to avoid accumulation of broke in the cellar or on a conveyor. Showers spray some of the pulper dilution water in order to ensure proper transport of the web into the pulper. Broke pulpers are equipped with an exhaust fan to avoid moisture entering the machine hall and to ensure the paper web feed into the pulper. Loose bits of dry broke are transported under the PM drying section on demand by conveyor belts of full machine-width to the pulpers. Older and slower running machines may lack such conveyor systems; thus, broke is collected in the cellar and fed into pulpers manually. Winder trims are often carried through a pneumatic system over longer distances to a dry-broke pulper or to a separate trim-pulper. The conveying air is separated in a cyclone separator, which can be integrated into the trim-pulper. Spray showers are installed, which might also be used to de-dust exhaust from certain dust removal sites, like the slitting stations. Especially for wide machines, a continuously operating trim pulper with a single bottom rotor is usually more practical than running a pulper of machine-width, e.g., the winder pulper, continuously. Rejected rolls are either fed into the PM dry-end pulper, into some other finishing room pulper, or into a separate broke roll pulper with a single bottom rotor. The rejected rolls are opened by a guillotine cutter, which can be equipped with two conveyor belts, one on each side of the cutter. Large rolls are moved back and forth for cutting into small segments, which are repulped bit by bit to avoid upsetting the broke system Pulpers Old and very slowly running PMs might have no broke pulper or just one located beside the machine. Also for a modern tissue machine with a Yankee dryer and especially with one wire/one felt design, one broke pulper at the dry end is sufficient. In contrast, for large board machines and modern high-speed machines for graphic paper production, the number of broke pulpers can be large. A couch pit, press, and dry end pulper are needed at least. The total number of pulpers depends on the paper grade and particular process. For example, a modern uncoated wood-free paper machine could have the following pulpers: - Couch pit - Press pulper(s) - Size-press pulper - Calender pulper - Reel pulper - Winder pulper - Winder trim pulper - Finishing room pulper - Finishing room trim pulper - Broke roll pulper. The broke pulpers under the PM are dimensioned according to the PM width. The construction is either concrete or steel. On wide machines, usually two rotors are located next to each other at the long side of the pulper. The direction of rotation is opposite, so that the web is Papermaking Part 1, Stock Preparation and Wet End - Page 25

26 pulled into the vat. The pulped stock leaves the vat through a screen plate behind the rotor. Also cross-shaft agitated pulpers are used with several impellers on the shaft. There, the discharge is from the short side of the pulper. Figure 19 shows the principle of a discontinuously operating one-pump broke pulping system. In the case of a break, the pulper is started and dilution water is added immediately. This is commonly initiated by break automatics. The pulping consistency ranges usually between 3% and 6%. The pulped broke is pumped into the couch pit, which operates continuously. Two pumps with different capacity discharge the couch pit and possibly other pulpers with continuous broke feed like, e.g., trims. During normal running condition, the smaller pump runs and the level is controlled similarly to that shown in Fig. 19. In a break situation, the level rises quickly and the larger pump starts. In case trims at the PM dry end are not pulped in a separate trim pulper, the receiving broke pulper is also equipped with a two-pump system. Finally, finishing room broke pulping can operate in some cases at high consistency in a separate system. Figure 19. Broke pulping system. Coated broke is not fed to the couch pit, but collected separately. The pigments of the coating color appear in coated broke like filler. In order to control the amount of coated broke, thus the amount of pigment added to the paper stock, collection, thickening, and storage are separated from the uncoated broke system. The separate system has its own broke storage, while cleaning and other broke treatment can be together. Wet-strength broke requires sometimes chemicals and elevated temperature by the addition of steam in order to reduce pulping time. Steam or chemicals are added to pulpers, in particular, to finishing room or broke roll pulpers in order to slush broke, which has been dry for some time Slushed broke-handling system The degree of sophistication of a broke system can vary from plain recirculation of pulped broke to the blend chest without any treatment up to complex systems with multiple cleaning stages. The following equipment can be part of a broke-handling system: - Broke storage tower - Broke thickener - Pressure screen, possibly in multiple stages - High-density cleaner - Open screen, like vibrating screen or scalping screen - Deflaker. Broke storage towers are usually operated in combination with a thickener, e.g., a gravity decker or sometimes inclined screens. The thickener increases the consistency of the stored pulp, and water can be moved back to the clear water system. Consistency fluctuations are thereby reduced. The thickened pulp is collected in a chest, from where part of the broke is circulated to the tower. In a few special cases of dyed paper, broke bleaching chemicals are added to the broke storage. If the saveall is employed also as a broke thickener, which is sometimes done for wire pit trims, the saveall is loaded with fines. Especially at PM web breaks, variations in the saveall operation affect the white water system and possibly thereby cause system instability. A clear separation between broke handling and the white water system is preferred. For coated paper production, double broke lines are used for thickening and storage, one for uncoated broke and one for coated broke. The degree of sophistication of broke cleaning systems is determined by the demands in quality and in quality constancy of the stock. For example, a high-speed PM for graphical paper Papermaking Part 1, Stock Preparation and Wet End - Page 26

27 production can have multiple-stage pressure screening. To reduce fiber losses, the end-stage screen is often operated with sequential flushing. Also open screening devices, of which the vibrating screen is most common, have been used at the last stage. However, modern broke cleaning systems are operated at the same consistency as the entire broke system, i.e., 2.5% 3.5%. High-density cleaners can be installed to remove, e.g., sand and other heavy dirt. Coated and uncoated broke are often treated together in a single system, in order to operate the pressure screens under desirably constant load. Thus, the proportioning of coated and uncoated broke is done at the broke screening chest. More efficient than the removal of dirt from the stock is to avoid contamination by dirt entering the broke pulpers. The way broke is collected and transported (see above) affects its dirt content. The cleanliness of areas where broke falls onto the ground determines the dirt entry into the stock system. The deflaker (see Chapter 3) creates high hydraulic forces when passing the stock through the gaps between static and rotating plates, which are either perforated or equipped with bars. Flocks, flakes, and fiber bundles are sheared due to high acceleration and deceleration of the stock and by impact of surfaces. The content of flakes in broke is usually high due to the short dwelling and repulping time. Deflakers are needed in particular for coated broke and wet-strength paper broke. Possible arrangements for deflakers are: - Deflaking of whole broke before dosage - Deflaking of first-stage screen reject only - Deflaking of coated or dry broke only - No deflaking. If needed, deflakers are connected in parallel. In special cases or for certain specialty papers, broke handling arrangements can be different Broke dosage Broke is usually added to the stock at the blend chest. It should be considered that the composition and the quality of broke might be different from the fresh furnish components. Broke contains fillers and other dispersed and dissolved material according to the additives and materials applied to the stock or to the paper surface in the PM. Fluctuations in the broke dosage, especially those of coated broke, can disturb or even upset the wet end chemistry. Consider, for example, a sudden increase of broke added to the furnish, which causes an increase in fine and filler material resulting in an increased cationic demand. In consequence, wire retention drops which affects production and the demand for retention aids. Quality variations can also originate from changes in the proportion of wet and dry broke. In some cases, separate storage systems for wet and dry broke are applied. This might be considered in particular for an integrated mill using never-dried chemical pulp or for specialty papers. The fiber properties and the needed amount of drying energy change at most when chemical pulp fibers are dried for the first time. In multi-ply board production or at multilayer headboxes, broke can be exclusively dosed to certain layers or plies of the sheet, e.g., in order to hide dirt or specks in the middle layer Coated broke systems In particular applications, like special coating colors with high binder content and for high-end demands, a disperser or a kneader can be installed to crush coat particles. Platelets or grainy, insufficiently broken-up coat particles can cause problems in forming and streaks especially in low-weight coat application. The coarse particles are also entering the cleaner system of the stock approach system and cause an increased loss of pigments, if they are not recovered from the cleaner rejects. A dewatering press is usually required because the disperser or kneader runs Papermaking Part 1, Stock Preparation and Wet End - Page 27

28 at a consistency of 30% or higher. Recycling the press filtrate back to coated broke pulping ends up in a separate water circulation, which reduces the amount of detrimental substances passed to the PM. Treatment of this circulating water by, e.g., dissolved air flotation (DAF) can also reduce the amount of hydrophobic substances ("white pitch") significantly. 5.5 Fiber recovery and water clarification Equipment installed in the white water system to separate suspended solids and water has a truly twofold function. On the one hand, stock components are recovered and, on the other hand, process water is clarified for further use in the mill. If separation machinery is installed in a series, the fiber recovery unit, or synonymously the saveall, is located in the first place, followed by possible steps of more advanced water purification for special uses, like for high pressure showers. The equipment at each stage is optimized for its particular purpose. Generally, the separation of suspended solids and water is achieved by40: - Filtration - Flotation - Fractionation - Sedimentation. All of these techniques can also be applied in combination. Hydraulic capacity and purification performance are the key parameters. Other parameters that have to be considered include: concentration of suspended solids in the feed, presence of colloidal and dissolved substances, chemicals needed, available space, power requirement, all-over energy consumption, need for auxiliary equipment, etc. As is applicable for every type of machine or system, the most important factor is cost, which includes the cost for the equipment and its installation, operational cost, and maintenance. Cost effectiveness is gained by a reduced amount of raw material losses, increased process performance, and the benefits due to stable and efficient paper production. Fiber recovery feed water consists of water from: - The wire pit overflow - Water separators in the PM vacuum system, i.e., the water removed from the web by vacuum at the wet end and by wet pressing - PM wet end tray - Broke thickener. The process connected to the saveall has to be designed so that a constant feed flow is maintained. Despite that, variations in the saveall load, either in solids content or in flow, originate from: - Variations in wire retention - Variations in sweetener quality and quantity - PM grade changes - Changes in fresh water supply. If the fiber recovery system is designed correctly, no variations originate from web breaks on the PM. Besides a constant filtrate quality, the amount of recovered fines and filler should be as constant as possible Filtration Disc filter Papermaking Part 1, Stock Preparation and Wet End - Page 28

29 The disc filter is a unit where multiple discs rotate in a vat. Each of the 30 or more discs consists of several segments covered with fine wire and rotates over the stationary filtration zones, as shown in Fig. 20. During filtration, a fiber mat builds up with the aid of pre-coating pulp, the so-called "sweetener," which is added to the white water feed flow. The thicker and denser this filter cake becomes during vacuum filtration, the less solids are passed through. Hence, the filtrate consistency is getting lower, until the filtration process is interrupted, when the filter mat emerges from the filled vat. The filtrate of the different phases is therefore collected separately as richer cloudy filtrate, leaner clear filtrate, and the optionally leanest super-clear filtrate. Table 5 shows the typical filtrate properties41. The disc segments of the same radial shaft position discharge into the same discharge channel in the shaft, i.e., the channel to which these segments are firmly connected. The different filtrate qualities are collected at one end of the shaft, where the pipes or drop legs to the filtrate tanks are attached. The split ratio of the 2 3 filtrates is either adjustable by altering the access to the shaft channels of the disc filter, or it is determined by a fixed design of the rotary filtrate valve at the shaft. Cloudy filtrate is discharged under atmospheric conditions via a free-fall pipe. Clear filtrate and super-clear filtrate are collected in two separate drop legs, each discharging the filtrates into a seal chamber in the filtrate tanks (see Fig. 21). The height difference between disc filter center-shaft and the level in the filtrate tank is 7 8 m for creating the required vacuum generated by the virtue of a sufficient velocity in the drop legs. Vacuum can be also generated by a vacuum pump, especially if a high dryness of the filter cake is desired. The filter cake maintains the vacuum in the super-clear filtrate line, even at the end of filtration when it emerges above the waterline. After that, vacuum is released from the mat, which is then removed from the wire by the knock-off shower or by an air jet. Another shower keeps the trough clean from bits of discharged pulp. An oscillating shower cleans the filter cloth. The water from this shower enters into the cloudy filtrate tank, or it is collected separately in a discharge channel according to the design of the disc filter (see Fig. 20). All filter shower waters are usually taken directly from the clear filtrate tank. The cloudy filtrate is often circulated as shown in Fig. 21. Table 5. Typical filtrate properties41. Property White water Cloudy filtrate Clear filtrate Super-clear filtrate Consistency, mg/l Consistency variation, mg/l << Particle size index ª Pigment to fiber ratio ª 1: coarse... 5: fine Figure 20. Filtration principle of the disc filter. Figure 21. Disc filter saveall system. The initial fiber mat is fluffy and porous, which is formed when passing cloudy filtrate under atmospheric condition. Thus, the filter cake stays permeable enough that a sufficient quantity of Papermaking Part 1, Stock Preparation and Wet End - Page 29

30 clear and super-clear filtrate is gained. The rotational speed of the disc filter determines the time given to the fibers to build up a mat. The typical speed in the range of 0.2 to 1.5 rpm can be controlled by the level signal of the filter vat. Another mode of operation is to run with a selected speed and to maintain the vat level with cloudy filtrate circulation, as shown in Fig. 21. Variations in the following feed flow parameters cause changes in the filter mat properties and, hence, fluctuation in the hydraulic capacity and the quality of the filtrates: - Fines and filler content - Feed flow - Freeness - Type and properties of the sweetener. If there is for example a jump in freeness, the level in the disc filter vat can drop even at low disc speed. In such a case, the hydraulic load can be increased by adding white water from the header to the cloudy filtrate circulation loop, as shown in Fig. 21. The sweetener is dosed at a certain ratio to the white water flow to the saveall. The dilution of the recovered solids at the discharge screw of the disc filter is proportional to the amount of sweetener supplied. Further on, an exhaust can be connected to the filter hood, especially if the white water temperature is high. Instead of vacuum-driven filtration, pressure can be applied to increase the filtration rate by an increased pressure difference. Air is blown from the filtration tank into the disc filter to create a certain pressure under the hood. If no drop legs are needed, it is not necessary to install the pressure disc filter at an elevated location. Pressure disc filters are used, e.g., in mechanical pulping, where the water temperature might be high. In the PM water system, low-pressure disc filters are sometimes used to post-treat the saveall clear filtrate to produce a super-clear filtrate for PM showers and similar purposes. The paper mill discharge point is at the clear filtrate tank where water leaves the system; this is the point where the paper mill effluent occurs regularly. This effluent stream is flow-controlled, for example, according to the level in the filtrate tower. This surplus water is fed upstream into the water system of the pulping or the recycled fiber processing plant. If no such integrated operation exists, the surplus filtrate is let into the sewer Sweetener The fiber content of the white water entering the filter vat is increased by adding pulp, the so-called "sweetener," or precoating pulp. The vat concentration is in a similar range as known for PM headboxes. Sweetener is needed to form a sufficient fiber mat. The function of the filter fabric is to support this filter cake, which acts as the filter medium. In cake filtration, the fineness of the fabric has rather little significance on the filtrate quality, if chosen within a range of reasonable mesh size. The sweetener acts as filter medium; hence, its quality determines: - Filtrate quality - Constant and high filtration rates to reach the desired filtrate quality - Reliable filter cake discharge. Sweetener fibers are not circulating in the saveall system; thus, sweetener fibers are always used once and then blended with the PM stock. The fines content of the sweetener pulp is preferably low and constant. Thus, long-fiber chemical softwood pulp is the best possible sweetener. The following are less preferable sweeteners in descending order: TMP, GW, mixed recovered paper stock, and broke42. The sweetener stock is taken from the stock feed header to the blend chest (see also Fig. 12). The recovered fibers are fed back to the blend chest. Due to the increased fines content, the recovered stock should not be forwarded into the broke system in order to avoid fluctuation of fines content entering stock blending. Papermaking Part 1, Stock Preparation and Wet End - Page 30

31 In addition to the sweetener, the filtration result can be influenced by the dosage of charged polymers as flocculating agent. If no optimal sweetener is available for example, in paper production with 100% recycled fiber furnish, addition of a chemical aid can benefit the filtrate quality. Polymer addition reduces the amount of required filter area; hence, it might be a suitable approach in de-bottlenecking a disc filter Drum filter A drum filter is a filter fabric-covered drum rotating in a vat with filtration toward the inside of the drum, i.e., similar to the disc filter. Filtration can be supported by vacuum created in drop legs, similar to the disc filter. Drum filters with a low-pressure differential are either gravity deckers or valveless drum filters. Gravity deckers are the most simple drum filters, where filtration is driven by the head difference between the vat level and the filtrate level inside the drum. The low-pressure difference causes a gentle flow through the filter medium, which results in a fairly good filtrate quality. A valveless drum filter contains tubes inside the drum, which are arranged in a helical pattern. One end of the tubes is connected to a longitudinal channel under the drum surface, and the other opening is inside the drum. As the longitudinal chamber emerges from the stock, the open end of the connected tube is still immersed in the filtrate inside the drum, which creates a siphon for vacuum40. Drum filters can be used for high freeness long fiber pulp, which quickly builds up thick pulp mats and which does not discharge cleanly from disc filter elements43. However, for most applications, the hydraulic capacity of drum filters is too low and only one filtrate quality is produced. This usually makes them unsuitable for a saveall application. Drum filters are often used as simple but reliable thickeners, e.g., in the broke-handling system Other filters - Special pressure filters or some bag filters are used for post-treatment of clear or super-clear filtrate for some shower applications. - Sludge and wet rejects can be dewatered by gravity tables and screw or wire filter presses. For low-quality board grades, fiber-containing sludge or even effluent treatment sludge can be returned into the papermaking process. The same applies for the pressate, which may be returned into the process water system. In various instances, some rejects sludge from flotation and from other locations in the integrated mill are dewatered and treated together. In such a case, water should not be returned to the process without advanced treatment. - Sand filters are often used in fresh water preparation. Sand filtration has been applied in the special case of total effluent-free paper production to post-treat process water44. Also if effluent from the effluent treatment plant is recycled, it can be sand filtered before reintroduction into the paper mill process. In some devices, flotation and sand filtration are combined into one unit. Sand filters are periodically cleaned by backwashing. - Classifying filters are dicussed below Flotation Dissolved air flotation (DAF) units are used in the paper mill to clarify fresh water, process water, or effluent. Due to the size of the created air bubbles, DAF is also called microflotation. Unlike the other water clarification techniques, the physical treatment is usually preceded by addition of chemicals, which can also reduce the amount of dissolved organic substances in process water to some degree. The removal of colloidal material by DAF is typically in a range of 10% 40% if chemicals for precipitation and fixing are added. Inorganic salts, similar to the other techniques discussed in this section, are not reduced. Usually, DAF does well in removing hydrophobic particles and colloids, which are prone to form stickies and pitch45. In a process with little effluent Papermaking Part 1, Stock Preparation and Wet End - Page 31

32 discharge, colloidal and potentially sticky material accumulates, especially when recycled fiber furnish is used or coated broke occurs. In these cases, DAF improves process water and thus the PM runnability efficiently. The brightness of graphical paper made from recycled fiber furnish is improved when applying DAF to grayed process water filtrates. In these cases, the flotation sludge must be discarded and not reintroduced into the process. The removal of dispersed microscopic and fine material is also very good. According to the solids content of the feed flow and the type and amounts of chemical aids dosed, the removal efficiency is usually between 85% and 98%. Typically, a coagulation aid, fixative, or alternatively bentonite is added at first. The dosage point is at the suction side of the feed pump or at another location with high turbulence to ensure good mixing. Then, a long-chained polymer is added to the feed flow via a chemical injector to flocculate suspended solids. Right after this, the microscopic air bubbles are introduced, which make the created flocs buoyant (see Fig. 22). Figure 22. Dissolved air flotation unit for process water treatment. The air bubbles are created according to Henry's law when releasing pressure from water saturated by dissolved air. The over-saturation with dissolved air by the sudden pressure decrease is the so-called "soda-bottle effect." The generation of large amounts of microscopic bubbles is crucial for the operation of DAF. Air is dissolved into water at the air saturation reactor, where water is mixed at high turbulence with compressed air at an absolute pressure usually of kpa. The reactor can contain turbulence-creating and flow-guiding elements; also two-stage systems are available. Air is supplied by a separate compressor system or from the mill's compressed air network, if the supplied pressure is sufficient. Either cleared water or feed water is delivered to the air saturation reactor by the booster pump. There are three system modes for flow through the reactor: - Partial flow mode - Full flow mode - Recycle flow mode. In recycle flow mode, a part of the clear water is used for aeration and mixed with the feed flow. Figure 22 displays this most commonly used mode in operation while the valve for partial flow operation is closed. In recycle flow mode, the hydraulic load is increased by about the amount of circulating water and the flotation unit has to be dimensioned accordingly. In partial flow mode, part of the feed passes the aeration system. In full flow mode, all feed water has to pass aeration and the reactor as well as the compressed air system has to be increased in size accordingly. The advantages of recycling vs. partial flow mode are: - Slightly lower chemicals consumption because the effect of the fixative may be reduced for the partial flow passing the air saturation system - No plugging and less dirt accumulation in the air saturation system The pressure of the aerated water is released by a decompression valve or by a turbine right before mixing with the feed stream (see Fig. 22). Pressure can also be released at decompression nozzles feeding the air-saturated stream into the feed pipe. The microbubbles are created immediately at the pressure drop and attach to the fibers and flocs to create buoyant sludge forming a blanket at the surface of the flotation tank. The bubble size is typically μm. Heavy particles are removed from the bottom of the flotation tank, typically via a discontinuously operating trash trap. The flow conditions inside the flotation tank are important for the separation performance. In round tanks, the surface is large and the radial flow velocity toward the clear water side is low. In rectangular tanks, parallel guiding plates installed inside the Papermaking Part 1, Stock Preparation and Wet End - Page 32

33 tank control the flow profile and the bubble rise. Particularly in older devices, the feed and the aerated stream are fed separately to the flotation tank. The floatage is removed by one or by a combination of the following devices: - Scoop or skimmer - Scraper or sometimes a blower - Flow over a weir. The solids content of the sludge depends on various parameters, which determine, e.g., the floc size and density. When treating PM white water and filtrates, the solids content of the flotation sludge is typically in a range of 3% 5%. Similarly to the saveall principle, the recovered solids can be returned to the process; otherwise the sludge is disposed. In the latter case, the sludge is either dewatered and post-treated, e.g., together with other mill sludges and wet rejects, or it is pumped into an effluent treatment plant. In some instances, the solids content of the flotation sludge is controlled by the speed of the scoop or scraper. Finally, by applying special measurements to determine, for example, charge or turbidity, the chemicals dosage and the operation and efficiency of the DAF unit can be controlled and improved Classification Screening or fractionation of white water in order to recover fibers and fines is not feasible due to the large flow to be treated and due to relatively high demands in fines recovery. Inclined screens, i.e., curved or bow screens, which are sometimes called scalping screens, are used for various applications including fiber recovery, thickening, or protecting downstream operations from fibers or debris. Inclined screens have no moving parts, except an oscillating cleaning shower, which removes plugging material and which prevents buildup and blinding by slime or scaling. Washing is often realized by backspraying, flushing nozzles, or by a combination of both. Screen plate design and slot size depend on the application. Inclined screens require a constant feed flow, which can also be pressurized, and have a limited hydraulic capacity. Microscreens and shower water screens are used to post-treat clear and super-clear disc filtrate for mid- and high-pressure shower applications. Microscreens are sometimes called "polishing filters." In contrast to drum or disc filters, the type of medium determines the separation of the solid and the filtrate phases, although the design of the screen can be rather similar to filtration. If contaminated white water is fed to a microscreen, inclined screens are often installed upsteam to retain coarse particles from entering in order to avoid damage of the filter medium. Even at a low solids concentration, as low as 0.001%, long fibers can be still present in the filtrate44, which can plug needle jets or high-pressure spray showers. Showers particularly sensitive to coarse particles are therefore protected by a screen, which in this respect is called a "police filter." Membrane filtration, in particular ultra- and nano-filtration, is discussed in the section about advanced water treatment Sedimentation Settling chests and hopper-bottomed tanks were commonly used before saveall equipment like disc filters was introduced. The clarification of PM white water by sedimentation has several disadvantages, which make its application as process water purification unit unpractical. Some of these disadvantages are: - Long retention time - Low concentration of recovered fibers Papermaking Part 1, Stock Preparation and Wet End - Page 33

34 - Fouling and microbiological activity causing slime and deposits - Heat loss - Sensitivity to changes in hydraulic load - Requiring a larger amount of water circulating in the process. Sedimentation units are used in effluent treatment. The use of flocculating agents in DAF can form aggregates which are too dense to float and thus settle in the flotation tank. Therefore, sediment removal is needed from DAF units. Otherwise, sedimentation is not used for PM process water treatment in modern mills. 5.6 Mill water systems Paper mill water household The function of water in papermaking is to transport and to distribute the fibers and to consolidate the sheet when water is removed from the web. In mill operation, water is needed to also fulfill various other functions, as listed in Fig. 24 under the topic "purposes." In the following, only the process waters are considered, i.e., which is the water in contact with the stock. Figure 23 shows the paper mill water household in principle. Water enters the system as fresh water and via stock from integrated pulping, which is usually stored at a concentration of 4% 12%. There are also other minor water sources. A small amount of water enters in the form of steam, as indicated in Fig. 23, which comprises steam added to the wire pit for temperature control and possibly used for the preparation of additives. In the modern integrated mill, the effluent from the PM to the sewer is small if not zero. This means that most, if not all, surplus water is possibly used upstream in integrated mechanical pulping or in deinking. Fresh water is entering the integrated mill mainly at the PM; see also Fig. 25. Figure 23. Paper mill water household, system principle. Matching supply and demand of water reduces purging and process water supplement by fresh water; hence, it reduces water consumption as well as the amount of effluent. Figure 24 shows the principle to improve water usage. In a water conservation program, the first step is to determine the minimum water quality required for each particular purpose. Other water than process water is kept in separate water systems, e.g., cooling water in the cooling water system, which can also circulate in there, via a cooling tower if needed. Uncontaminated water can be discharged from the mill without further treatment, hence, without loading the effluent treatment plant hydraulically. Instead of discharging the uncontaminated nonprocess water, it can be added to the process as fresh water, which decreases the total water consumption of the mill. Figure 24. Working cycle for segregated and optimized internal mill water use. Process water is stored as clear water or as white water. In the normal operation of the PM, all white water passes the saveall and only clear filtrate is stored. If two process water storage towers are available, one is used for filtrate exclusively, while the other one might also contain white water. In this way, the best separation of waters of different quality is achieved. However, the coupling of the towers and the filtrate tanks should allow flexibility. Process water storage towers are always equipped with agitators, and they are often insulated. A rule of thumb says that the process water storage should have the same volume as the total pulp storage. Another rule of thumb determines the broke storage as 1.5 times the longest sustained trouble period that is acceptable without taking in fresh water46. A lack of sufficient process water storage means that a large amount of fresh water has to be Papermaking Part 1, Stock Preparation and Wet End - Page 34

35 added during startups and web breaks compared to normal PM operation. This can cause process instability. Fresh water can be adjusted in temperature and ph, but the content of suspended and dissolved solids is different. This can imbalance wet end chemistry; variations imposed on the effluent treatment plant are often increased. The fresh water to supplement lacking process water is heated to the process water temperature and added to the process at one location only, i.e., the white water tank. This location is processwise distant from the headbox; thus, a possible impact on the wet end chemistry is attenuated. Similarly, consistency variations in the dilution water header are avoided. A supply tower for mechanically and/or chemically pretreated fresh water is useful, for example, if varying amounts of fresh water are needed in the cold season of the year. In this case, fresh water circulates from the tower through a heating loop utilizing recovered heat from the PM hood exhaust or from the mechanical pulping heat recovery system, if available. Otherwise, steam is injected. For many grades, the optimum stock temperature is found within a range of 46 C to 54 C (115 F to 130 F)47. Higher temperatures of up to over 60 C (140 F) limit microbiological growth significantly, while fungal growth stops already when exceeding 50 C (122 F) Fresh water use As shown in Table 6, the specific fresh water consumption of modern mills is in a range of 2 20 L/kg paper49,50 with the exception of technical or specialty paper mills which might consume up to 100 L/kg or more. In older mills, the fresh water consumption can be higher due to suboptimal process design. Table 6 shows average consumption figures in to indicate the progress in the reduction of fresh water consumption. The large reduction in the last decades was possible due to the introduction of saveall equipment, the installation of sufficient process water storage, the use of more efficient machinery, the separation of streams of different water quality, and an all-over improved process design. Table 6. Typical specific fresh water consumption of modern paper mills in comparison to the situation in Paper grade Today Today 1971a 1971a L/kg gal/lb L/kg gal/lb Newsprint Wood-free fine paper b 14b Supercalendered (SC) paper Lightweight coated (LWC) paper Tissue Liner and fluting Multiply board a Mills in Sweden b "Magazine paper" 51 The driving forces to reduce the fresh water consumption are: - Legislation and permissions in respect to either the fresh water consumption or the effluent discharge Papermaking Part 1, Stock Preparation and Wet End - Page 35

36 - Cost: - Fresh water and its treatment- Effluent treatment and possibly effluent discharge cost- Material savings: fibers, fines, and filler- Energy savings - Fresh water availability - Higher process stability, if fresh water reduction means a reduction in process water supplement. A mill is called closed if no process water is discharged. Due to process inherent evaporation, mainly in the drying section of the PM, and due to the discharge of water contained in cleaning and screening rejects, a certain input of fresh water is always needed. This amount is smaller the more water enters with moist raw materials, pigment slurry, chemicals, etc., cf. Fig. 23. If a rather dry raw material like recycled fiber is used, then the specific fresh water consumption is about 2 L/kg in a closed mill operation. In this case, the amount of water evaporated at the PM drying section is about 50% of the fresh water intake. Effluent-free papermaking is possible, but the system stability and PM runnability are likely reduced and quality problems can occur if process design and internal water purification are inadequate. Fresh water is used for selected shower applications at the PM, and no fresh water is consumed to supplement process water. Table 7 shows the shower water need of a wide PM52. Fresh water can be replaced by cleaned process water, if the content of suspended solids is sufficiently low, and if detrimental substances do not deposit or precipitate, which can plug the orifices of showers, cf. Table 853. High-pressure showers are operated with fresh water for that reason. Fresh water is also used at other locations, where dissolved organic and inorganic compounds can cause problems or where scaling is harmful. As already mentioned, up to 10% 15% of fresh water is needed for preparation and dilution of chemicals. Table 7. Shower water demand of a modern 9-m-wide paper machine52. Wire section Warm fresh water 300 kpa kpa 55 L/s (14.5 gal/min) 30 L/s (7.9 gal/min) 25 L/s (6.6 gal/min) In total, of which: Washing and lubrication High-pressure showers Post-treated clear filtrate 300 kpa kpa 80 L/s (21 gal/min) 70 L/s (18.5 gal/min) 10 L/s (2.5 gal/min) In total, of which: Former and roll showers Trim knock-off and edge showers Additionally during breaks 60 L/s (15.9 gal/min) Knock-off Condensate 1 L/s (0.3 gal/min) Trim squirt, roll edge moistening Press section Warm fresh water 300 kpa kpa 35 L/s (9.2 gal/min) 25 L/s (6.6 gal/min) 10 L/s (2.5 gal/min) In total, of which: Washing and lubrication High-pressure showers Post-treated clear filtrate 300 kpa 55 L/s (14.5 gal/min) 55 L/s (14.5 gal/min) In total, of which: Internal roll washing Table 8. Shower water quality demands53. Solids load < 50 ppm ppm ppm Possible application of water Equivalent to filtered fresh water Usable in 1 mm orifice Usable in 1.5 mm orifice Papermaking Part 1, Stock Preparation and Wet End - Page 36

37 ppm Usable in 3 mm orifice ppm Brush type shower recommended > 500 ppm Purgable shower recommended Dissolved and detrimental substances Detrimental substances are non-ionic and anionic dissolved and colloidal substances54,55. Anionic trash, which can be determined by the cationic demand, is therefore a subgroup of detrimental substances; it has been shown that non-ionic substances can also be detrimental. Anionic trash especially consumes retention aids and thus decreases the PM wire retention. Detrimental substances can adsorb or precipitate onto the surfaces of fibers, fillers, and fines, which adversely affects fiber-to-fiber bonding, brightness, and the accessibility of process chemicals55. Temperature has important influence on these sorption kinetics. Dissolved and chemically active substances: - Enter with all raw materials, including fresh water - Are created by operations in integrated pulping, e.g., by bleaching or deinking - Are created by paper surface treatment entering via the broke system. Table 9 lists the composition of detrimental substances according to their origin55. Table 9. Composition and origin of detrimental substances55. Chemical compound(s) Origin Sodium silicate Peroxide bleaching, deinking, recovered paper Polyphosphate Filler dispersing agent Polyacrylate Filler dispersing agent Organic acids Pitch dispersing agent Carboxymethylcellulose Coated broke Starch Recovered paper, broke, strengthening agents Humic acids Fresh water Lignin derivatives Kraft pulp, mechanical pulp Lignosulfonates Sulfite and NSSC pulp, CTMP Hemicelluloses Mechanical pulp Fatty acids Mechanical pulp Organic dissolved and colloidal substances (DCS) are an excellent nutrient for microbe populations to propagate fast at usually favorable process water temperature. According to the level of dissolved oxygen in process water, either aerobe or anaerobe microbiological activity causes a decrease in the system cleanliness by slime and smell. Slime deposits by anaerobic micro-organisms propagate corrosion in particular. This can also occur with the presence of oxygen because anaerobic conditions are reached on the metal surface underneath deposits of aerobic micro-organisms56. Inorganic dissolved substances, namely salts, are also deteriorating the process performance and potentially the product properties. A high content of electrolytes, in particular chloride, increases the potential for corrosion. According to the osmotic equilibrium, the swelling of fibers decreases with an increased content of electrolytes in the process water; hence, beatability and the behavior of fibers in web consolidation deteriorate to some extent. In mill practice, a rough estimate of the amount of detrimental substances is obtained by the chemical oxygen demand (COD). Other values, like the amount of dissolved organic compounds (DOC) can also be determined on-line. Particle charge on-line titration can be applied to samples from the thick stock flow as well as to low concentration stock. Sensors measuring the zeta-potential, turbidity, and the amount of total organic compounds (TOC) can give additional Papermaking Part 1, Stock Preparation and Wet End - Page 37

38 information. The effect of salts in the process water can be monitored on-line by measuring the conductivity of the fluid. Specific metal cations and some anions are usually determined in the laboratory; only a few are measured on-line, for example, calcium, aluminum, and silicate. See also Table Accumulation mechanisms Continuous feed or generation of a substance within a process with internal circulation causes this substance to accumulate. This means that the concentration in the circulation loop increases reaching levels the higher the less effluent is discharged, which is sometimes described by the (misleading) term "degree of closure"57. Each organic and inorganic dissolved compound eventually reaches equilibrium. The equilibrium appears earlier and a lower concentration level is reached if part of the substance is consumed in chemical reactions by a phase change like precipitation, by flocculation, by microbiological degradation, or in some other way. The particular equilibrium of each substance is determined by the process conditions, which can be monitored by measuring, for example temperature, ph, and charge density. If a substance is not consumed, a higher level of concentration is reached, which is governed by the streams discharged from the process. In order to show trends and possible levels of accumulation, so-called "enrichment" or "accumulation" curves are plotted, which are often based on short circulation balance calculations. For example, the term "enrichment factor" has been defined as the ratio of the headbox concentration of a given substance at a certain degree of white water recycling to that at zero-recycling58. To study the accumulation of detrimental or other substances, the simple short circulation model has to be extended to the entire mill water system, as shown below in Fig. 26. In order to gain a better understanding of mill water systems, it can be useful to incorporate adsorption parameters into model calculations to study, e.g., wire retention or the concentration of chemical aids59, or pursuit accumulation mechanisms by dynamic simulation60. Table 10 summarizes the disadvantages of decreased fresh water use. Table 10. Disadvantages of decreased fresh water consumption61. Increase in suspended Increase in dissolved Increase in temperature solids material Blocking of showers Scale deposits Sizing problems Increase of fines and change Alteration of wet end Reduction in vacuum pump in retention chemistry capacity Spots and dirt in the product Increase of biological activity Increase of biological activity Deposit formation Deposit formation (at low temperature) Abrasion Corrosion Reduction of wire life Color, smell The PM headbox should always be the reference point for evaluating changes in water quality and in the load by detrimental substances62. The following points have relevance in respect to accumulation of detrimental substances and system stability: - Correct choice of source for water to discharge from the mill as effluent - Correct selection of water to be treated by advanced purification - Suitable application of the purified water in the process. While savealls improve process water reuse, advanced water purification techniques are needed just as well in order to remove dissolved material, if the fresh water consumption and the discharge of effluent should be reduced further Vertically integrated mills Papermaking Part 1, Stock Preparation and Wet End - Page 38

39 Many paper mills are connected to a pulp plant, producing chemical or mecha--nical pulp or processing recovered paper. The interface to the paper mill is usually a medium- or sometimes a high-consistency storage, from where the pulp is picked up by dilution water from the paper mill. The reduction in fresh water consumption in an existing process has to be accompanied by two means in order to keep the load of detrimental substances low in the headbox. On one hand, the increase of the transfer consistency at the interface between pulping and paper mill decreases the carryover of detrimental substances that are created in pulping and bleaching. On the other hand, fresh water is used in high-pressure showers and other selected locations at the PM and if needed for preparation and dilution of chemicals. Further on, effluent is not discharged from the PM system, but surplus PM filtrate is added to the white water system in the pulping plant. Figure 25 shows this principle. The separation of the pulping and paper mill water systems leads to a higher dissolved matter concentration in the upstream water loops, from where all effluent of the integrated mill is discharged. Figure 25. Vertically integrated mill water household, system principle. Designing the integrated mill water system according to this counter-current flow principle, or as a cascade of mill water loops, results in: - Decreased load of detrimental substances in the PM water - Reduced effluent flow and lower fresh water demand - Possibly improved efficiency in pulping, e.g., in deinking63. The separation of the water systems is enhanced further, when introducing a "washing" loop by installing a second dewatering press at the interface between pulping and the PM. This is particularly useful to reduce detrimental substances caused by the high alkalinity in peroxide bleaching of deinked or mechanical pulp, which is often carried out as a final step before transferring the pulp to the PM. In addition to organic dissolved and colloidal material, supporting chemicals like sodium silicate used in deinking and in peroxide bleaching also can be detrimental. After the bleaching tower, pulp is diluted to a lower consistency and pumped to a second dewatering press prior to the pulp storage. The pressate is fed upstream. Figure 26 shows the lower level in detrimental substances at the headbox of the two-press system against the effluent discharge from the integrated mill. For comparison, a situation without press dewatering is also shown, assuming a 12% solids content after thickening compared to 35% after press dewatering. The specific load of detrimental substances is plotted relative to the concentration at the kink point of the two-press curve. At this point, the amount of surplus PM filtrate meets exactly the amount of dilution water required to reach the desired consistencies at bleaching and pulp storage towers. Reducing further the amount of the fresh water added to the paper mill would create a lack of PM filtrate in this particular example; hence, it would require supplement by highly contaminated pulping filtrate. Figure 26 shows clearly that this must not be done in mill practice without applying advanced water purification treatment to this recycle water. On the other hand, applying advanced treatment to this stream is most feasible, when closing the water system of the integrated mill62. Figure 26. Accumulation of detrimental substances in the PM headbox water at reduced effluent discharge for an integrated paper mill with different degrees of process water separation Horizontally integrated mills Horizontal integration here means parallel paper production lines. Even if parallel paper machines were identical, or if the same product is produced on both lines, the amount of Papermaking Part 1, Stock Preparation and Wet End - Page 39

40 dissolved organic and inorganic substances can vary significantly. In most cases, it is feasible to separate the water systems of neighboring machines, especially if the fresh water consumption is low or different for both lines. This requires separated stock preparation, wet broke systems, savealls, and process water storage for each line. Hence, the independent and trouble-free operation of either line is guaranteed regardless to whether changes occur on the other line, either in wet end chemistry, in production state, or due to grade change. A case in which collected mill effluent is re-introduced as process water requires sufficient treatment by advanced water purification Advanced water purification techniques Advanced water purification techniques are applied to paper mill process water in order to reduce the amount of dissolved and colloidal substances circulating in the process. Advanced techniques might become feasible or necessary when reducing the fresh water consumption below a certain level as described above. Investment and operational costs of advanced equipment are reduced by the savings from decreased flows through fresh and wastewater treatment. From the many known techniques for advanced water purification, the following have been successfully applied to paper mill process water in industrial scale: - Biological treatment - Membrane filtration - Evaporation. The selection of the suitable technique is determined by parameters similar to those for choosing equipment to remove suspended solids, namely: - Desired quality of purified water - Cost: - Investment cost- Operational cost- Spare parts and replacement media; maintenance - Energy demand, power, and steam consumption - Reliability, availability, and responsiveness - Properties and reachable concentration of concentrate - Required feed flow properties and pre-treatment - Uniformity requirements on the flow rate and properties of the feed - Floor space. The reject stream from advanced treatment consists either of a concentrate with increased viscosity or of sludge. Required post-treatment of this reject stream and costs resulting from it are often crucial for the feasibility of a certain technique. Possible further uses or treatments of the concentrate or sludge are: - Drying and/or combustion - Trading or use elsewhere - Disposal, if allowed - Treatment in the wastewater treatment plant - Re-use within the process. The most frequently applied method for fluid concentrate handling is combustion Membrane filtration Membrane filtration is used in the paper industry to purify process water or to recover valuable Papermaking Part 1, Stock Preparation and Wet End - Page 40

41 material like pigments or latex from coating kitchen effluents. The membrane works as a molecular screen; thus, the properties of the membrane determine the quality of the cleaned stream. Table 11 shows classification of membrane filtration according to the particle cut-off size in separation. The separation sizes and operating pressures in Table 11 are guideline figures. Table 11. Membrane filtration techniques. Method Separation size Operating pressure Membrane type Reverse osmosis < 1.5 nm 3 6 MPa Non-porous (RO) 0.5 nm 7 nm 1 4 MPa Micro-porous Nano-filtration (NF) 3 nm 0.1 μm MPa Micro-porous Ultra-filtration (UF) 50 nm 5 μm MPa Porous Microfiltration (MF) Membranes can be shaped in various module types such as tubes, plates or frames, hollow fibers, monoliths, or spiral wound. Membrane material can also be charged when applied to UF or RO. The cross-flow principle is generally applied to paper mill process water to avoid excessive fouling. Membrane fouling means a decrease in flux due to adsorption, blockage of pores, or deposition on the membrane surface by gel or layer formation. Periodical or continuous cleaning is necessary. Good pretreatment of the feed, like microfiltration, is required. NF and especially RO usually post-treat ultra-filtrate. Despite pretreatment and cleaning, membranes have to be replaced every 1 5 years. The specific energy consumption is kwh/m3(65). The filtrate quality as well as the flux depend on the type of membrane used and on the feed. The reduction in COD depends on the contributing amount of large molecules and colloids. The COD reduction from PM filtrates by UF is usually in the range of 20% 40%66,67, but by NF possibly up to 90%68. NF or RO reduces the content of ions to some degree Evaporation Evaporation is the most effective technique to concentrate all nonvolatile substances for removal from the process. The quality of the condensate is equivalent to fresh water quality or better69. For paper mill process water, multi-effect (ME) and mechanical vapor recompression (MVR) evaporation are used at mill scale, both employing falling film technology and vacuum evaporation The feed water is usually pretreated by filtration and/or dissolved air flotation. In the ME cascade, the evaporated vapor is used as heating steam in the next lower pressure effect. The vapor from the final effect is condensed by water cooling. Waste heat in form of secondary or low-pressure steam is used for heating. According to the number of effects and the amount of heating, a final contaminant concentration of 50% or higher is achieved. In MVR, a fan or compressor to be used as the heating medium recompresses the vapor. No steam or cooling water is required. By using polymeric film as heat-exchanging material, a large heating surface is provided at low investment cost and relatively low power requirements. For the latter, the specific energy consumption is 8 10 kwh/m3(70), and the concentrate flow is about 10% of the feed flow. Post-treatment of the MVR condensate by ME evaporation or direct steam concentration is needed to reach a higher concentration level. The COD removal efficiency by evaporation is about 95%, and the removal of electrolytes is even better. The removal of organic material can be improved by stripping volatile organic compounds at the evaporator and by treating the foul condensate, e.g., in an anaerobic reactor71. Evaporation of volatile low-molecular organic acids can be reduced by increasing the ph Biological in-process treatment Paper process water is loaded with non-toxic substances, the majority of which are Papermaking Part 1, Stock Preparation and Wet End - Page 41

42 carbohydrates. It is therefore well suited for biological degradation. The type of treatment required does not differ appreciably from that used for ordinary paper mill effluent treatment. Contrary to wastewater treatment, the biological treatment becomes part of the manufacturing process, which requires the process design optimized for reliability and responsiveness. Especially for mills without effluent discharge, anaerobic treatment is feasible to degrade the highly concentrated dissolved organic substances in the process water. To avoid dissipation of anaerobic conditions into the paper production process, an aeration tank follows the anaerobic reactor. The water cleaned in a clarification tank after aeration can be purified further by sand filtration or directly by membrane filtration. Biological water treatment is not efficient to remove colored substances from water, especially from mechanical pulping filtrates. An integrated post-treatment by membrane filtration73 can be useful to remove color and to detain microorganisms from entering into the paper process if required74. Sludge can be recycled back into the process as furnish used in brown or gray liner and board production75,76. Compared to the other techniques, biological treatment does not require prior solids removal. Anaerobic micro-organisms are sensitive against unsuitable temperature and ph, as well as against major variations in nutrition feed. The advantages compared to aerobic cultures are a higher degradation efficiency of organic material if fed at high concentration, about ten times lower generation of sludge, and possible energy recovery from produced biogas. The reduction in BOD, especially in a combined aerobic and anaerobic system, is naturally high, typically up to 95% 99%. Depending on the amount of biodegradable material contributing to the COD, a reduction of 90% in COD is also reachable. Precipitation occurring in biological treatment reduces the content of dissolved inorganic material like calcium and sulfate to some extent. 5.7 Novel approaches and possible future The major challenges for improving the PM stock and water system are good control of the wet end chemistry and prediction and management of circulating fines and filler. Variations in the form of responses upon changes are overlaying due to different propagation times through the system. Response times are in the range of minutes in short circulation and of several tens of minutes, up to hours, in long circulation. An increase in tank sizes cannot well attenuate such low frequency variations. Contrary to that, reducing the amount of circulating stock and water in the approach flow system will improve attenuation by shorter circulation times and, thus, reduce the system reaction time. This allows faster grade changes and reduces the amount of broke, which is the more important the faster the machine runs. The volume of stock and water should be reduced in the short circulation to improve system reaction upon changes because sufficient process water storage capacity is needed in the long circulation to achieve a low fresh water consumption. The novel approaches77 80 require air-free dilution water. Accurate on-line measurements of consistency are also necessary and, additionally, on-line measurements are required of fines and filler content and of fiber properties in order to allow reliable feed-forward control81. By in-pipe blending and dilution of well-controlled and air-free streams, chests might become obsolete in the stock approach flow system, namely blend chest, machine chest, and wire pit. Air-free dilution water is provided by using either a centrifugal deaeration pump (see Fig. 17) or a deaeration tank for the dilution water. The use of deaerated dilution water instead of deaerating the entire machine stock has the following additional advantages: - Increased and stabilized cleaner consistency, due to possible air-free post-dilution - Improved attenuation of variations due to applied flow-lag principle, cf. Fig. 13, by post-dilution - Improved system cleanliness and pumping efficiency in an air-free dilution water system. Papermaking Part 1, Stock Preparation and Wet End - Page 42

43 The removal of the blend and machine chests from the mill layout requires a constant level in all stock proportioning chests and consistence constancy. This can be achieved by stock circulation between the proportioning chest and the dilution zone of the pulp storage tower79. Operation without wire pit and short circulation with a centrifugal deaeration pump showed on a pilot machine an about 10 times improved stabilization time and 2 4 times faster basis weight adjustment on a grade change82. Examples of stock and water systems of modern machines producing different paper and board grades are given in Chapter 1. References 1. Norman, B., "The water and fiber flow system in the paper and board mill," EUCEPA 1990 Print, Paper and Board Products for Printing in the Nineties Conference Proceedings, EUCEPA, Paris, p Seifert, P., Tappi 63(10):135 (1980). 3. Perrault, J., Tappi J. 67(7):62 (1984). 4. Kershaw, T.N., "Basic wet end hydraulics," TAPPI 1985 Wet End Operations Seminar Notes, TAPPI PRESS, Atlanta, p Boyce, M.P., in Perry's Chemical Engineers' Handbook (Perry, R. H. and Green, D. W. Eds.) 7th edn., McGraw-Hill, New York, 1997, Chap Reijonen, Y., "Optimising the short circulation," EUCEPA 1988 XXIII Conference Proceedings, vol. 1, EUCEPA, Paris, p Seppälä, S.I., "Driving forces to multiply forming," 1987, New available techniques and current trends, part I, SPCI, Stockholm, p Kießling, H., Das Papier 41(7):370 (1987). 9. Guillory, K., Tappi J. 81(9):81 (1998). 10. Reed, C.S., Tappi J. 78(7):241 (1995). 11. Cutshall, K.A., Ilott, G.E., Brooks, B.W., Pulp Paper Can. 80(6):T193 (1979). 12. Stadelmann, J., Wochenbl. Papierfabr. 116(21):902 (1988). 13. Nyberg, P. and Malashenko, A., Pulp Paper Can. 99(1):T5 (1998). 14. T oukonummi, O., Paperi Puu 80(6):430 (1998). 15. Bliss, T., "Centrifugal cleaning in the stock preparation system," TAPPI 1996 Stock Preparation Short Course Notes, TAPPI PRESS, Atlanta, p Moore, S.J., "The practical aspects of cleaner system diagnosis for optimum performance," TAPPI 1986 Papermakers Conference Proceedings, TAPPI PRESS, Atlanta, p Wood, J.R. and Karnis, A., Pulp Paper Can. 80(4):T116 (1979). 18. Rehmat, T. and Branion, R., "Fibre fractionation in hydrocyclones," CPPA st Annual Meeting Notes, Technical Section, CPPA, Vol. B, Montréal, p. B Sevilla, E.M. and Branion, R.M.R., J. Pulp Paper Sci. 23(2):J85 (1997). 20. Vikio, P.A., "Recovering fiber and filler from mineral loaded paper mill rejects," TAPPI 1997 Engineering and Papermakers Conference Proceedings, Book II, TAPPI PRESS, Atlanta, p Johnson, M., "The headbox approach system," TAPPI 1991 Papermakers Conference Proceedings, TAPPI PRESS, Atlanta, p Papermaking Part 1, Stock Preparation and Wet End - Page 43

44 22. Bliss, T., "Screening in the stock preparation system," TAPPI 1996 TAPPI Stock Preparation Short Course Notes, TAPPI PRESS, Atlanta, p Fredriksson, B., Hämäläinen, T., Koikkalainen, J., Paperi Puu 79(1):30 (1997). 24. Kervin, D. and Coffey, C.A., "Thick stock screening for fine debris removal," TAPPI 1997 Engineering and Papermakers Conference Proceedings, Book II, TAPPI PRESS, Atlanta, p Isler, W. and Widmer, F., Das Papier 32(11):473 (1978). 26. Pietikäinen, T., Tappi J. 75(11):185 (1992). 27. Matula, J.P., "Deaeration and the Approach System," TAPPI 1996 Stock Preparation Short Course Notes, TAPPI PRESS, Atlanta, p May, O.W. and Buckman, S.J., Tappi 58(2):90 (1975). 29. Lorz, R.H., Pulp Paper Can. 88(10):T361 (1987). 30. Ajersch, M., Pelton, R., Towers, M., Loewen, S., J.Pulp Paper Sci. 18(4):J121 (1992). 31. Karras, M. and Springer, A., Tappi J. 72(2):155 (1989). 32. Rauch, R., Hofer, H.-H., Sangl, R., Weigl, J., Wochenbl. Papierfabr. 125(17):794 (1997). 33. Steward, J., Pulp and Paper 55(3):170 (1981). 34. May, O.W., Tappi J. 74(7):67 (1991). 35. Roberts, J.C., Paper Chemistry, 2nd edn., Blackie Academic and Professional, London, Korpi, T., Paperivalmistusprosessin kemian peruskoulutus, KCL, Espoo, Moller, K., Duffy, G.G., Titchener, A.L., Appita 26(4):278 (1973). 38. Duffy, G.G., Appita 42(5):358 (1989). 39. TAPPI TIS "Optimum consistency for pumping pulp," TAPPI PRESS (1993). 40. Doucette, J.A., "White water and savealls," TAPPI 1996 Stock Preparation Short Course Notes, TAPPI PRESS, Atlanta, p Pelkiö, A., "Kuiduntalteenotto paperikoneen 0-vedestä," 1995 Paperitehdaspäivät, Ahlstrom Machinery, Savonlinna, Paper No Pelkiö, A., "Fiber recovery from paper machine white water," 1997 Paperitehdaspäivät, Ahlstrom Machinery, Savonlinna. 43. Perrault, R.R., Tappi J. 76(12):189 (1993). 44. Diedrich, K., Hamm, U., Knelissen, J.H., Das Papier 51(6A):V153 (1997). 45. Strauß, J. and Großmann, H., Wochenbl. Papierfabr. 125(9):468 (1997). 46. Mardon, J., O'Blenes, G., Robertson, A.G.,Tkacz, A., Pulp Paper Can. 99(5):35 (1998). 47. Panchapakesan, B., "Closed white water system designs," TAPPI 1993 Papermakers Conference Proceedings, TAPPI PRESS, Atlanta, p Hoekstra, P.M., in Chemical Processing Aids in Papermaking: A Practical Guide, (K.J. Hipolit, Ed.), TAPPI PRESS, Atlanta, 1992, pp Terho, J., Pulp Paper Eur. 3(3):25 (1998). 50. Myréen, B., Water Sci. Tech. 29(5-6):1 (1994). 51. Franzén, T., Heinegard, C., Martin-Löf, S., Söremark, C., Wahren, D., "Establishment of a closed system for the paper making process," EUCEPA 1973 XV Conference Proceedings, Papermaking Part 1, Stock Preparation and Wet End - Page 44

45 EUCEPA, Paris, p Komppa, A., "Vedenkäytön optimointi," 1995 AEL-INSKO-koulutus P907101/95 IV, AEL, Helsinki. 53. Dexter, R., Paper Asia 13(11):22 (1997). 54. Auhorn, W., Wochenbl. Papierfabr. 112(2):37 (1984). 55. Linhart, F., Auhorn, W.J., Degen, H.J., Lorz, R., Tappi 70(10):79 (1987). 56. Geller, A. and Göttsching, L., Wochenbl. Papierfabr. 109(15):525 (1981). 57. Wahren, D., Pulp Paper Can. 86(3):T88 (1985). 58. Alexander, S.D. and Dobbins, R.J., Tappi 60(12):117 (1977). 59. Melzer, J., Das Papier 38(10A):V55 (1984). 60. Orccatoma, J.A., Stiee, D., Paris, J., Perrier, M., Pulp Paper Can. 98(9):T336 (1997). 61. Negro, C. and Tijero, J., in COST E1, Paper recycling (Blanco, M.A., Negro, C., and Tijero, J., Eds.), European Commission, Luxembourg, 1997, Chap Weise, U., Terho, J., Myrén, B., "The effluent-free SC paper mill a simulation study," nd EcoPaperTech Conference Proceedings, KCL & PI, Helsinki, p Kappen, J., Demel, I., Öller, H.-J., Wochenbl. Papierfabr. 124(20):910 (1996). 64. Lilja, K. and Ullman, P., "Evaporation in pulp and paper mill water pollution control comparison with membrane filtration and other methods," SPCI th International Conference Proceedings on New Available Techniques, Vol. 2, SPCI, Stockholm, p Perry, R.H. and Green, D.W., Perry's Chemical Engineers' Handbook, McGraw-Hill, New York, 1997, pp , Nuortila-Jokinen, J. and Nystöm, M., J. Membrane Sci. 119:99 (1996). 67. Sierka, R.A., Folster, H.G., Avenell, J.J., "The treatment of whitewaters by adsorption and membrane techniques," TAPPI 1994 International Environmental Conference Proceedings, TAPPI PRESS, Atlanta, p Nuortila-Jokinen, J., "The closed paper mill white water system and the internal paper mill white water treatment," Monograph No. 59, Lappeenranta University of Technology, Lappeenranta, Legnerfält, B., Hallgren, O., Nygren, A., "Evaporation as a CTMP mill kidney," 1997 International Mechanical Pulping Conference Proceedings, SPCI, Stockholm, p Koistinen, P.R., "Treatment of pulp and paper industry effluent using new low cost evaporation technology with polymeric heat transfer surfaces," TAPPI 1996 Minimum Effluent Mills Symposium Notes, TAPPI PRESS, Atlanta, p Gartz, R., Pulp Paper Eur. 3(1):20 (1998). 72. Pekkanen, M. and Kiiskilä, E., "Options to close the water cycle of pulp and paper mills by using evaporation and condensate reuse," TAPPI 1996 Minimum Effluent Mills Symposium Notes, TAPPI PRESS, Atlanta, p Tardif, O. and Hall, E.R., "Membrane biological reactor treatment of recirculated newsprint whitewater," TAPPI 1996 Minimum Effluent Mills Symposium Notes, TAPPI PRESS, Atlanta, p Mönnigmann, R. and Schwarz, M., Das Papier 50(6):357 (1996). Papermaking Part 1, Stock Preparation and Wet End - Page 45

46 Chapter 75. Göttsching, L., "Completely closed water system a German case study," 1997 Johan Gullichsen Colloquium, PI, Helsinki, p Pichon, M., Nivelon, S., Charlet, P., "Paper mill whitewater deconcentration to move towards complete closure of circuits," TAPPI 1996 Minimum Effluent Mill Symposium Notes, TAPPI PRESS, Atlanta, p Meinander, P.O., Paper Tech. 36(4):26 (1993). 78. Meinander, P.O., Pulp Paper Eur. 3(1):13 (1998). 79. Pekkarinen, T. and Kaunonen, A., "New Approach to Wet End Management," XI Valmet Paper Technology Days 1998 Proceedings, Valmet Inc., Jyväskylä, p Matula, J. P., Paperi Puu 82(3):174(2000). 81. Kaunonen, A., "Runnability and quality improvement through consistency measurement and control," 1996 Valmet Paper Machine Days Proceedings, Valmet Inc., Jyväskylä, 7 pp. 82. Nykänen, R., "Technological and Economic Potential of the POM Concept," Master's Thesis, Helsinki Univ. of Technology, Espoo, Papermaking Part 1, Stock Preparation and Wet End - Page 46

47 Figure 1. Stock preparation, system principle. Figure 2. Short circulation, system principle.

48 Figure 3. Stock approach flow system of a fine paper machine (refer to text for itemization of position numbers). Figure 4. (a.) Short circulation model. (b.) Dynamic response for different retention values R as a function of normalized time 1.

49 Figure 5. Frequency bands of hydraulic pulsation sources in the short circulation.

50 Figure 6. Stock Sankey diagram of supercalendered (SC) paper machine at design production.

51 Figure 7. Water Sankey diagram of supercalendered (SC) paper machine at design production.

52 Figure 8. Printing paper machine, block diagram.

53 Figure 9. Multilayer headbox machine with two stock components, block diagram.

54 Figure 10. Two-ply machine, block diagram.

55 Figure 11. Microbe population on metal surfaces with different degree of finishing 8. Figure 12. Stock blending and machine chest including sampling station, an example.

56 Figure 13. Multiple flow-lag principle. Figure 14. PM wire pit with single stock dilution.

57 Figure 15. Medium-consistency storage tower and stock dilution.

58 Figure 16. Flow pattern in the hydrocyclone (forward cleaner).

59 Figure 17. Stock pump with deaeration by centrifugation, courtesy of POM Technology. Figure 18. Bentonite dosage at the headbox feed pipe after pressure screens.

60 Figure 19. Broke pulping system.

61 Figure 20. Filtration principle of the disc filter. Figure 21. Disc filter saveall system.

62 Figure 22. Dissolved air flotation unit for process water treatment. Figure 23. Paper mill water household, system principle.

63 Figure 24. Working cycle for segregated and optimized internal mill water use. Figure 25. Vertically integrated mill water household, system principle.

64 Figure 26. Accumulation of detrimental substances in the PM headbox water at reduced effluent discharge for an integrated paper mill with different degrees of process water separation.