HOWARD RAPSON. Bleach plant and screen room modernization

Transcription

1 bleaching T178 Bleach plant and screen room modernization HOWARD RAPSON Memorial Award By J. Bennett and L. Garant Abstract: Prince George Pulp produces 800 t/d of softwood Kraft paper and market Kraft pulp, as unbleached or semi-bleached product. Customer requirements and effluent treatment restrictions led to a new screen room and bleaching sequence. The screen room employs a primary pressure screen in a four-stage process. The bleach plant operates as two independent processes, which can provide stock to either the pulp machine or paper machine. The project was completed on schedule and on budget. Operator acceptance has been very good. Performance has exceeded expectations. T HIS PROJECT had two main components; the first focused on changes within the bleach plant and the second within the brown stock screen room. The Bleach Split portion of the project consisted primarily of re-routing stock and filtrate flows to permit the operation of the bleach plant in a 3 stage / 2 stage split configuration to optimize the production cost of bleached grades, while maintaining the ability to produce high-quality unbleached stock. The Screen Room portion of the project involved replacing obsolete pulp and paper brownstock screening systems with a single line pressure screening system to improve pulp cleanliness and reduce fibre and BOD losses to the effluent system. The project was fully operational at the end of the September 2004 shutdown. BLEACH PLANT HISTORY Before 1996, the canfor Prince George mill produced full bleached, semi bleached, and unbleached pulps, along with unbleached sack Kraft paper. In , a bleached grade of paper was developed, which could be produced using the final three stages of the bleach plant. The bleach plant used the following configurations for the production of the various pulp and paper grades (refer to Figs. 1, 2): 1. Unbleached pulp (UBK) and Bleached paper (Polar): UBK on stages 1 and 2 to the 45 tonne high density storage (HD) to the pulp machine Polar on stages 3,4,5 to the 300t HD to the paper machine with a bleaching sequence of DEpD 2. Semi Bleached (SB) or Full Bleached (FB) and Unbleached paper (Kodiak) SB or FB on stages 1,2,3,4,5 to the 45t or 300t HD to the pulp machine. The bleaching sequence is based on DEopDEpD. When on SB, bleaching chemicals were applied only to the first two or three stages with the remaining stages being wash only. Kodiak paper bypassed the bleach plant, en route to the paper machine. This set of bleaching sequences was used from 1996 until SCREEN ROOM HISTORY Prior to the 2004 modernization, most of the process equipment in the PG mill screen room was mill original (1965-vintage), based on Cowan screens. A number of changes were made to the process layout over the years, with the overall goal of improving pulp and paper cleanliness. The screen room in the Prince George mill was a two-line design with a primary screen line dedicated to each of the pulp and paper machines, and a common rejects screening system. The prime purpose of the screen room is to remove shives (not fully delignified fibre bundles) from the brownstock pulp before the bleaching and papermaking processes. A secondary purpose is to wash the stock to remove residual chemicals and dissolved organics (lignin) remaining after the pulping process. The screen room had the following configuration (Fig. 3): 1. Paper Line: Primary Stage: Five Centrifugal Cowan Screens. Primary accepts go to the vacuum decker, to the 90t screened-stock HD and then to the paper machine directly when making unbleached paper, or to the third stage in the bleach plant when making bleached paper grades. Primary rejects go to the pulp secondary stage feed for further processing. 2. Pulp Line Primary Stage: Single Hooper Pressure Screen Primary accepts go to the gravity decker and then to the first stage of the bleach plant. Primary rejects go to the secondary stage feed. 3. Rejects Screening Secondary Stage: Two Centrifugal Cowan screens Secondary accepts go to the Cowan primary screen feed. Secondary rejects go to the tertiary stage feed. Tertiary Stage: One Centrifugal Cowan screen (There is a spare swing screen that can act either as a secondary or tertiary screen). Tertiary accepts go to the fibre reclaim system feed. Tertiary rejects are pumped to the Intercon clarifier for further processing and are burned in the power boiler at Intercon (sister mill located adjacent to PG pulp). J. BENNETT Senior Process Engineer Prince George Pulp Prince George, BC L. GARANT Project Engineer Allnorth Consultants Ltd (formerly Duncan Industrial Engineering Inc.) Prince George, BC PULP & PAPER CANADA 107:9 (2006) 27

2 T179 bleaching FIG. 1. OLD - FBK/SBK Pulp & Brown Paper. FIG. 2. OLD - UBK Pulp & Bleached Paper. Fibre Reclaim System: One (1) primary radiclone canister (20 cones) and one (1) secondary radiclone canister (8 cones). Radiclone accepts go to the secondary stage feed. Radiclone rejects are pumped to the Intercon clarifier for further processing and are burned in the power boiler at the adjacent Intercon mill. One significant change in the screen room came in 2003 as a result of pulp customer concerns about shive levels in the market unbleached pulp. The Hooper screen basket was changed from slots to slots. The rotor speed was increased to maintain the production rate with the smaller opening. These changes satisfied the customer that PG Pulp was looking for a solution to the shive problem, but it was recognized as an interim solution, not a final one. PROJECT JUSTIFICATION A combination of factors came together in 2003 that justified the bleach plant and screen room modernization. The key issues the project addressed were: High bleaching costs for bleached paper High shive count in unbleached pulp Increased variability of the BOD5 loading in the effluent treatment system Unsteady bleach plant operation due to frequent shutdowns and startups when making grade changes Production rate limitation through the screen room DESIGN CONSIDERATIONS It was realized going into this project that major changes were required in the bleach plant and the screen room layout if all the objectives were to be achieved. The design for the new bleach plant and screen room had to address the cost and quality concerns as well as fit into the plans for the future direction of the Prince George pulp and paper products. Bleach Plant The increased demand for bleached paper made it apparent early in the design that the full-bleached (FB) grade of pulp could not be made in the new PG bleach plant. The production requirements for bleached paper and semibleached pulp used all the available days in the year, with no time left over for producing full bleached pulp. Canfor s Intercon and Northwood Pulp Mills, also in Prince George, took over supplying pulp to the FB pulp customers from the PG mill. Also, it would be very expensive, if not impossible, to satisfy the brightness and quality specifications of the full bleach pulp using a 3-stage bleach plant on non-oxygen delignified pulp. A significant amount of the unbleached pulp from the Prince George Mill is used in electrical applications, due to the low conductivity and ash content of the pulp. In the existing process, this was achieved by washing the stock through two stages of the bleach plant. This ability was retained in the new bleach plant. The bleaching cost for bleach paper was significantly higher than for the semi bleach or full bleach pulp grades. This was attributed mainly to the lack of an Eop stage in the bleach paper sequence. A further consideration was that a significant energy reduction project was commissioned in early The changes with the new bleach plant had to retain the benefits gained from the energy projects. Once the initial design was determined, with brownstock washing in the last two stages, the issue of filtrate contamination was addressed by separating the bleach and unbleached filtrate overflows. A detailed Operability Study (the Op in HazOp) identified a number of scope changes early enough in the project to be included into the design at little or no extra cost. The new bleach plant configuration addressed the major design considerations by becoming two bleach plants. There is a three-stage process for producing the semi-bleach pulp and bleach paper grades and a two-stage process to provide additional washing for the unbleached grades of pulp and paper. The three-stage bleach plant uses the sequence DEopD to produce stock with a final brightness between 68% ISO and 84% ISO, depending on the customer requirements. The two-stage sequence provides two washers to remove as much of the residual lignin and soda as possible from the unbleached stock. The chemistry of the new bleach plant is the same as the old one, albeit with fewer stages. However, the water (filtrate and white water) management is significantly more complex. The brownstock stages (old 4-5) are downstream of the bleached stages (old 1-2-3). The filtrate contamination issue has been addressed with both stages 4 and 5. Filtrate from sealbox #4 is sent to the screen room for white water chest make-up, with any excess being pumped directly to the caustic effluent line. Stage 5 acts as an open washer, with all excess filtrate overflowing directly to the effluent treatment system. The downstream process water is also managed according to the destination of the bleaching stages. Paper machine pressate (from the twin-roll press) is used as shower water and high-density dilution for the stages providing stock to the paper machine. Pulp machine white water is used for the same purposes on the stages feeding the pulp machine. The switching is done with automatic valves at the bleach plant warm water system pumps. Refer to Fig. 4 for the new bleaching sequence, and Fig. 5 for the shower and filtrate management system. Screen Room The first design consideration was choosing to retrofit the existing screen room rather than installing a completely new screening system. The preliminary cost estimates showed that a new installation would cost at least 25% more than the retrofit option. Once the retrofit decision had been made, the existing screening equipment, tanks, and pumps were examined to determine the optimum layout. The decision to use a single large primary screen rather than two or three smaller units in parallel was closely tied to this assessment. As BOD5 reduction was a cornerstone of the justification, the screen room water system had to be closed as much as possible. This aspect tied the bleach plant changes (unbleached stages) into the brown white water system and then to the brownstock washer showers. The balance between final product quality and production capacity determined the slot size for the pressure screens. The decision was made to use slots, assess the quality and production, and change to :9 (2006) PULP & PAPER CANADA

3 bleaching T180 FIG. 3. Old Screen Room. FIG. 4. NEW - All Grades Pulp & Paper. FIG. 5. Shower & Filtrate Management. FIG. 6. Metso Primary Screen slots if necessary. An operability study was also completed for the new screen room, which identified areas that would benefit from re-engineering to allow easier, safer, or more efficient operation of the new screen room. The new screen room uses a singleline, four-stage screening process. The Metso primary screen is capable of processing up to 1400 ADt/d of accept stock and is guaranteed to remove at least 95% of the incoming shives (see Fig. 6). In the Canfor configuration, the screen capacity is rated at 1150 ADt/d of accepts. The existing Hooper pressure screen now serves as the secondary screen. A rebuilt Ingersoll-Rand pressure screen from Canfor s Northwood mill has been placed as the tertiary screen. All the pressure screens have slotted baskets, with a different rotor design in each screen. Two Cowan screens were retained for use in the quaternary position (one on-line, one spare), for fibre recovery purposes. The fibre recovery system was retained as-is. Refer to Fig. 7 for a layout of the new screen room. PROJECT JUSTIFICATION The justification for this project was quite complex, involving elements of cost reduction, revenue enhancement, and strategic benefits that were difficult to quantify in terms of production or cost increases. The cost reduction elements were: Bleaching cost - bleached paper 19% reduction Bleaching cost - coloured paper 55% reduction Bleaching cost - semi-bleach pulp 4% reduction Screen room defoamer use 50% reduction Fibre loss from screens 1 t/d reduction Electrical energy $1.50/t increase The revenue enhancement elements were: Increased UBK pulp sales 5% increase Production increase due to reduced BOD loading 2 t/d increase The strategic elements were: Improved pulp quality, due to reduced shives Improved environmental performance Increased operational consistency PROJECT MANAGEMENT AND IMPLEMENTATION First, a project team was created and specific roles and responsibilities were assigned to each team member. A project implementation schedule was prepared and project milestones were identified. Team member deliverables were identified based on the project schedule. Two major target dates were identified early in the project; one was a major mill outage in the fall of 2004 and the other was a smaller outage in the spring of The success of the project was dependant on being able to meet those dates and completing all of the work required in those outages to allow mill operations to continue after the outages. The detailed project scope was developed from the process and control diagram drawing through team meetings including personnel from operations, maintenance, and engineering. A final project scope of work meeting and operability review meeting was arranged in the two weeks following the project approval by Canfor s board of directors in March Any changes requested to the project after those two meetings were treated as scope change, requiring a process and cost justification. Project steering committee meetings were held on a regular basis to monitor key project milestones, to approve scope changes and to make critical project decisions. Project team meetings were also held regularly to discuss design, process PULP & PAPER CANADA 107:9 (2006) 29

4 T181 bleaching and operations problems, construction progress and to update the project schedule. Interaction between the various engineering disciplines was also coordinated during the team meetings. During the team meetings, each team member was asked to provide a verbal progress report of the respective portion of the project and delegation of tasks were issued based on progress. A scope change evaluation document was created. Each scope change was evaluated based on specific criteria such as total cost of scope change, impact on project budget, necessity of the scope change for project success, and cost justification. The scope change requestor had to provide a process operability justification for the scope change before it was heard by the steering committee. There were five scope changes approved for the Bleach Split portion, and another four scope changes approved for the Screen Room portion. Some of those scope changes were necessary to the success of the project and others with a smaller impact on the overall project budget were approved to improve the mill operator s work conditions, such as large knife gate valves. The project went extremely well considering the fast track schedule. Good communication channels were established from the project kick-off meeting and all deadlines were met. The project team understood that good communication and prioritizing of tasks was key to ensuring good construction progression and collaboration between the various trades involved. Good planning and overcommunication proved to be the key to the success of this project. DETAILED PROJECT SCHEDULE The project was divided into two distinct projects: The Bleach Split Project The Screen Room Upgrade Project Early on in the detail engineering development, it was identified that it would be advantageous to start-up the Bleach Plant Split portion of the project before the September shutdown to avoid having to commission two large projects following the September 2004 shutdown. There was an opportunity in May 2004, during a scheduled recovery boiler water wash, to install most of the tie-ins for the Bleach Plant Split Project. By thinking outside the box, the area Process Engineer and Operators identified all of the tie-ins that could be installed without interrupting operations, before and after the May outage. Each process connection on the project was evaluated and scheduled based on mill production operations. The project implementation schedule was updated and incorporated in the mill production schedule. This allowed a minimum of tie-ins and final connections to be scheduled during outages. Installation of some final tie-ins on the effluent system required a full mill outage and had to be delayed until the September shutdown, but again, an ingenious operation strategy allowed the mill to fully commission and start-up the Bleach Split portion of the project in July 2004, without a major mill outage. The Bleach Plant Split portion project was started-up in July 2004 during a scheduled one-day outage. The construction logistics for the Screen Room Upgrade project were quite different from the Bleach Plant Split project, hence the necessity of splitting the project in two distinct projects. Most of the piping and equipment had to be installed in the screen room without interrupting operations, leaving the final tie-ins and commissioning to be done during the fiveday mill wide September 2004 outage. To reduce project costs and to simplify piping arrangements, a large number of pipes from the old screen room were reused in the new screen room. The engineering team was again asked to think outside the box to design this project. Because of a limited amount of space in the old screen room, the design team was asked to minimize new piping runs and reuse as much as possible existing piping runs. This also limited the amount of redundant pipelines to be left after the project completion. This made the shutdown schedule and pipeline tie-in identification process more complicated, but a significant amount of the budget was saved in new pipeline costs and installation. A detailed shutdown construction schedule was prepared in conjunction with the mill shutdown coordinators to avoid conflicts between the screen room project construction and regular maintenance work during the major shutdown. The final schedule, incorporating maintenance activities, screen room construction, final commissioning and start-up, was updated daily before and during the shutdown. The project shutdown construction schedule was the critical path during the mill outage and the schedule was evaluated and optimized to perform the work in the shortest time span possible with a limited budget and resources. A schedule review meeting was held with each trade foreman, so they were fully aware of the impact of their schedule on the other trades, which made the coordination between trades a much easier task. There were 71 piping ties-ins, five chest inserts, and three centrifugal pumps installed in a very small area during the five-day September outage. A large construction crew was required and a detailed completion schedule was prepared, signed off and updated daily to monitor progression. Coordination between the day and night shifts was critical to assure good progression of the project. Construction trades were scheduled to avoid overhead work as much as possible and safety was most important. A large portion of the instrumentation was pre-commissioned before the shutdown. As well, the control system was modeled on a simulator to identify and fix logic and operation bugs before start-up, resulting in very short final commissioning and start-up. BUDGET AND COST CONTROL The project was completed within 1% of the budget. A significant amount of pipe and existing control valves were re-used to reduce the equipment purchase costs. Swapping the existing titanium ClO 2 mixer from the D2 stage with an SMO grade ClO 2 mixer from the Intercon Pulp Mill saved the cost of purchase of a new chlorine dioxide mixer. Equipment was also refurbished and reused for the screen room portion of the project, such as the HiQ 210 pressure screen obtained from Northwood for use as a tertiary screen. The quaternary feed pump and the secondary screen feed pump were reused. This resulted in a cost savings of about 3% for the complete project. SCOPE CHANGES There were five approved scope changes on the Bleach Plant Split portion of the project. Relocate an existing Kajaani brightness probe to the discharge of washer #3 Install a new Kajaani probe in the brownstock line to tower #4 Replace 100 of 12 SS pipe for bleach bypass with brownstock (line was totally rotten) Replace 45t Hi D stock pump Install On/Off actuators on five knife gate valves on the bleach hot water tank There were four approved scope changes on the Screen Room Upgrade portion of the project, plus one for shutdown help. Install a flowmeter and control valve in the primary screen accepts recirculation line Provide independent flow control on both of the primary screen rejects lines (requirement by Metso) Install an On/Off actuator on the primary screen feed pump suction valve A contractor had to be brought in last minute to complete 90% of the tie-ins during the September shutdown. The extra costs are due to contract requirements attached to the short notice of the call. Two full working days during the shutdown were lost due to under-manning. Install an 8 line from the turpentine condenser to tank farm hot water tank (to take this water out of the brown white water chest). The scope changes for the bleach plant screen portions of the project amounted to about 6% of the total project cost. There were significant cost savings in the mechanical equipment supply for the :9 (2006) PULP & PAPER CANADA

5 bleaching T182 FIG. 7. NEW Screen Room. TABLE I. Bleaching cost reduction. % Reduction Bleach Paper 14.4% SB-77 Pulp 14.5% SB-67 Pulp 3.1% FIG. 8. Shive Count (Before & After). Screen Room Project, including: Savings on the Primary Screen and Tertiary Screen Feed Pumps, due to aggressive pricing from the supplier Savings in Metso spares for the Primary Screen, after it was decided that a spare rotor would not be required, as it was a low risk item Re-using the Secondary Screen Feed Pump rather than replacing it as had been anticipated Savings on the Tertiary Screen, by obtaining a surplus screen from Northwood, rebuilding the housing, re-using a demonstration rotor and salvaging spare parts from the Northwood mill s extra stock. The total cost savings in mechanical equipment was about 3% of the overall capital cost. The tight control of the original scope of work allowed the extras to be purchased and installed without over-running the approved budget for the complete project. SAFETY PERFORMANCE Safety was a top priority on this project. The contractors were briefed every morning in toolbox meetings about various safety issues. Housekeeping was also a top priority as many different trades were working in a relatively small space. The safety performance from all workers on this project was exceptional. Approximately a total of 3,000 contractor manhours and 4,500 mill crew manhours were worked on the Bleach Plant Split project. Only one first aid (a chlorine dioxide gas exposure) was reported. The worker returned to work the same day. A total of 8,000 contractor manhours and 3,000 mill crew manhours were worked on the Screen Room Upgrade project. No first aids were reported. PRELIMINARY PERFORMANCE RESULTS The main Key Performance Indicator (KPI) for the Bleach Split project was reduced bleaching costs on bleached paper due to the change to a sequence using the more efficient first three (DEopD) rather than the last three (DEpD) bleaching stages. Since August, the bleach paper and SB-77 costs have been reduced by an average of 14%. The SB-67 pulp, which uses less overall chemical, has also seen reduced bleaching costs, but not as dramatically, as shown in Table I (as % of cost of bleached paper). The Screen Room Upgrade KPI was reduced shives, particularly when making UBK pulp. The first UBK production with the new screen showed total shive count cut in half compared to the previous runs, with the large shive count about 10% of the previous level. The performance guarantee tests completed in 2005 confirmed the 96% shive removal claimed by the manufacturer. (Figs.8, 9, 10). There have been two very significant operational improvements as a result of this project, one in the bleach plant and one in the screen room. The bleach plant operational stability has improved dramatically as a result of the new sequence. In the old bleaching sequence, a complete shutdown of the bleach plant was required every two weeks as the sequence was changed from DEopDEpD (full bleach or semi-bleach pulp) to WW & DEpD (unbleached pulp and bleached paper). At each switch, tower #3 changed from low consistency to medium consistency or back, causing stock channelling after most grade changes. In the new sequence, adjusting the chemical application rate on the Eop and D1 stages changes the brightness and the grade. The switching between pulp machine and paper machine is done by automatic switching valves on the stock lines from the two bleach plant high-density storage towers. The screen room operation has also become much more consistent. The old screen room had two primary screening lines and a common rejects screening line, with a total of nine atmospheric screens and one pressure screen, along with a two-stage radiclone cleaning system. It was operated in manual control mode most of the time, except for a few flow controllers around the pressure screen. The new screen room has three pressure screens and a single atmospheric screen, plus the radiclones. All the screens as well as the stock chest level controls run in auto mode, based on chest level or production rate. The screen room has become a relatively simple area to operate rather than an area that required constant operation attention. LEARNINGS There was a great deal of time spent on the AFE preparation phase of this project. As a result, there were relatively few scope changes during the project. Careful cost control of the major purchases and during construction allowed the extras to be accommodated within the project budget. The involvement of operations personnel helped with the design, commissioning and start-up phases. There were very few fixes required once the rebuilt processes were on-line, indicating that the majority of the operators concerns were addressed in the original design. The use of mill crews as opposed to PULP & PAPER CANADA 107:9 (2006) 31

6 T183 bleaching FIG. 9. Total Shive Count. FIG. 10. Large Shive Count. dedicated contractors for the fabrication and installation of capital projects can present a vulnerability to the cost control of a project. Mill crews are often subject to the shifting priority of the mill needs and are not always able to be dedicated to completing their capital project work on a timely basis. Unless the mill crew s priorities are firmly with the project, the project budget can be at risk. ACKNOWLEDGEMENTS The technical assistance of Mona Henderson, Process Engineer, of Metso was greatly appreciated during the training, commissioning and start-up phases of the project. The prime contractor IIC is also thanked for working safely and for the high quality work of their tradesmen during the entire project. The cooperation between Allnorth Consultants Ltd (formerly Duncan Industrial Engineering) - Mechanical, Universal Dynamics - Electrical and Instrumentation, and the mill personnel was critical to the project s success. The assistance of the mill tradesmen and operators during the design, construction, and commissioning was an essential part of the success of this project. Finally, the assistance and support of Peter Lovell, Team Leader and Jim Dunn, Area Supervisor and the other mill personnel was invaluable. Résumé: Prince George Pulp produit 400 t/j de pâte commerciale kraft et de papier kraft de résineux ainsi que du papier écru et mi-blanchi. Les exigences des clients et les restrictions en matière de traitement des effluents ont entraîné l installation d une nouvelle salle de classage et d une nouvelle séquence de blanchiment. La salle de classage utilise un classeur primaire sous pression et un procédé en quatre étapes. L atelier de blanchiment comprend deux procédés de blanchiment indépendants, qui peuvent fournir de la pâte à la machine à pâte ou à la machine à papier. Le projet a été achevé dans les délais prévus et selon le budget établi. Les opérateurs ont très bien accepté les nouvelles installations. La performance a dépassé les attentes. Reference: J. BENNETT, L. GARANT. Bleach Plant and Screen Room Modernization. Pulp & Paper Canada 107(9) T: (September, 2006) Paper presented at the 2005 PACWEST Conference in Harrison Hot Springs, BC, May 11-14, Not to be reproduced without permission of PAPTAC. Manuscript received February 27, Revised manuscript approved for publication by the Review Panel on February 6, Keywords: MODERNIZATION, BLEACH PLANTS, SCREEN ROOMS, KRAFT MILLS, BRITISH COLUMBIA, PERFORMANCE EVALUATION :9 (2006) PULP & PAPER CANADA