Guidelines for Produced Water Evaporators in SAGD

Transcription

1 Guidelines for Produced Water Evaporators in SAGD DAN PETERSON, HPD West, Bellevue Washington IWC KEYWORDS: Produced Water, Zero Liquid Discharge (ZLD), evaporator, evaporation of wastewater, crystallizer, crystallization of wastewater, Steam Assisted Gravity Drainage (SAGD), enhanced oil recovery (EOR), bitumen production. ABSTRACT: An evaporative process for treating Produced Water has been demonstrated in SAGD. This process is an improvement over conventional treatment methods in that it produces half the waste with three orders of magnitude improvement in recovered water quality. Design concepts are illustrated and explained in this paper. INTRODUCTION The ever increasing world demand for oil in the last decade has stretched existing oil supply infrastructure to production limits. Canada is in the mainstream of the production frenzy with huge reserves of heavy oil. Recovery of the heavy oil is well defined through several methods but the one gaining most prominence is Steam Assisted Gravity Drainage (SAGD). This method recovers heavy oil using steam to heat oil sands, which are sand formations containing the bitumen heavy oil. These oil sand formations are found throughout northern Alberta. The fluidized oil and condensed steam (Produced Water) are collected and brought to the surface. The oil is separated from the water and shipped to an upgrading process. The SAGD facility is left with the task of recycling the Produced Water to make more steam while minimizing makeup water resources and waste disposal quantity. The water treatment and steam production portion of heavy oil recovery requires substantial capital equipment with an ever increasing need to satisfy regulatory agencies who want maximum recovery potential with the least impact on the environment. This means maximum recycle with minimum waste. Due to the many SAGD projects in the planning and study phases, having information readily available to make technical and economic decisions on processes that will easily meet system design criteria becomes essential. The use of evaporators as a water recovery process is demonstrating reliable and robust operation on SAGD Produced Water, therefore, more information on this technology is needed to properly evaluate this option. Producers and engineering firms are interested in how this technology is applied and whether it is applicable to their systems. The evaporation process provides a single unit operation to reduce wastewater volumes to a minimum and maximize water recovery in the form of high quality distillate for boilers. This paper discusses the attributes of Produced Water Evaporators and how this system fits into the SAGD process. 1

2 CONVENTIONAL SAGD WATER TREATMENT USING LIME SOFTENING AND ION EXCHANGE Presently, the general guideline on water recovery is to recycle 90% of the Produced Water as boiler feedwater. With the discharged wastewater and other losses, a makeup water source is required to close the water balance. A conventional water balance incorporating Warm Lime Softening (WLS) and Weak Acid Cation (WAC) ion exchange is shown as Figure 1. FIGURE 1 SAGD SYSTEM WATER BALANCE WITH WLS AND WAC (ALL UNITS TONNES/DAY EXCEPT WHERE NOTED) This approach has been utilized for decades in Produced Water recovery and there has been much experience gained in the operation of these systems. This process removes hardness and silica by physicalchemical treatment and ion exchange. Some variation is possible to what is shown, but in general, this is the accepted process to recycle Produced Water to a conventional once through steam generator (OTSG) boiler. Overall recovery of water is on the order of 90% and the wastewater stream from the facility is about 10%. Makeup water and boiler blowdown comprise about 28% of the water treated with Produced Water being about 72% in the example of Figure 1. An OTSG must be used in this application as the treated water is poor quality relative to typical boiler feedwater standards (by ABMA/ASME guidelines, ref 1). The OTSG produces high pressure steam ( psig) and requires that the feedwater be less than 8,000 ppm TDS (Total Dissolved Solids) and near zero total hardness. Silica (SiO 2 ) can be tolerated up to about 50 ppm. The OTSG can produce up to 80% steam quality (80% steam and 20% boiler blowdown) concentrating the brine about 5X as boiler blowdown (BBD). A train of vapor-liquid 2

3 separators is used to produce 100% quality steam which is required for the SAGD process. A portion of the liquid from the vapor-liquid separators, or blowdown is recycled to minimize wastewater discharge. Recycling BBD to the front end of the process increases the overall TDS of the treatment train and approaches boiler inlet limits on the OTSG. To treat this combination of boiler feedwater, the conventional system - WLS/WAC treatment system requires the use of several chemical additions. The major bulk chemicals used are lime (Ca(OH) 2 ), magnesiun oxide (MgO), soda ash (Na 2 CO 3 ), caustic (NaOH), and hydrochloric acid (HCl). Several of these chemicals are in solid form requiring silos and mixing systems to inject the chemical into the process. Minor amounts of coagulant and polymer are used to aid in solid separation. Streams are recycled as shown in the schematic to achieve the highest recovery of Produced Water and minimize the waste volume at the same time. Typically, the waste generated from this process is deep well injected. The conventional water treatment process has been around a long time in enhanced oil recovery (EOR) operations and is now being applied to a relatively new EOR process, SAGD. Much has been learned about the process when applied to SAGD in Northern Alberta through numerous lessons learned experiences. Equipment suppliers have stepped up to the challenges and modified the systems to adapt to the SAGD process and be as resilient as possible under the unique operating conditions of SAGD in Northern Alberta. EVAPORATION AS A PRODUCED WATER TREATMENT TECHNOLOGY FOR SAGD Evaporation technology has been introduced into the SAGD heavy oil recovery process. This is not a new technology in the wastewater business but it is relatively new to SAGD when compared to the conventional process. The number of conventional systems in SAGD compared to evaporation is an order of magnitude more. However, the few operating Produced water Evaporators have exhibited admirable operating traits: A high percentage (>95%) of Produced Water is recovered as high quality boiler feedwater makeup. The product from the evaporators - high quality distillate, allows for the use of drum boilers. This is not an option in conventional treatment. A small volume of waste (less than the conventional system) is generated. High reliability has been demonstrated. The existing Produced Water Evaporators have continuously operated with very little operator attention over a wide variety of normal and adverse operating conditions. Having positive operating attributes of this kind has generated interest among heavy oil producers who want to know if the installation of Evaporators has tangible economic advantages when compared to the more conventional process of physical chemical treatment. Many studies have compared the basic OPEX and CAPEX numbers of the systems but have excluded some important economic factors. Several papers (such as ref 2) have been presented which have many additional economic factors that may be considered. Two very important factors are: (1) Plant reliability and availability (2) The use of drum boilers 3

4 These two factors add a few percentage points of availability for oil production (steam production means oil production) and options for boiler fuel and potential efficiency gains (savings on natural gas). Introducing these factors into the life cycle costs will favor the economics of Evaporators. UNDERSTANDING THE USE OF MECHANICAL VAPOR COMPRESSION (MVC) VERTICAL FALLING FILM EVAPORATORS IN SAGD The Produced Water Evaporator system fits into the SAGD water balance as shown in Figure 2. FIGURE 2 SAGD SYSTEM WATER BALANCE WITH EVAPORATION SYSTEM (ALL UNITS TONNES/DAY EXCEPT WHERE NOTED) The basic MVC process is represented in Figure 3. The feed to the Evaporator requires chemical additions that are introduced in the Feed Tank. After chemical addition, the feed is heated by outgoing distillate in the Feed Preheater. After being heated up, the feed is steam stripped in the Deaerator with a small flow of steam. The Deaerator removes non-condensible gases and oxygen from the feed stream. The deaerated feed is added to the bulk recirculaing slurry in the Brine Evaporator vapor body. Brine slurry is recirculated from the vapor body/retention chamber through an internal return circuit. External piping is used only to connect the recirculation pump. The major portion of the internal recirculation circuit is a preheat pipe centrally located in the tube bundle. The recirculation flow exits the return section in the upper liquor box and flows radially across the top liquor distributor 4

5 FIGURE 3 MVC EVAPORATOR SYSTEM plate. From there it falls onto the lower distributor plate, which evenly distributes the brine to the top tubesheet between all the tubes. The brine flows to the perimeter of the tubes (in the tubesheet) and down the tubes as a thin falling film. Heat is transferred to the brine by condensing steam on the outside of the tube. The heat produces steam vapor in the tubes and the vapor body. The steam vapor and brine separate in the Vapor Body space above the brine level. Some concentrated brine slurry is discharged from the system to control the concentration. Vapor generated within the vapor body is cleaned of entrained brine by the Vapor Washer. If any foaming should occur in the system, the vapor washer serves to knock down the foam. The cleaned vapor stream is passed through a vapor compressor where the vapor pressure is increased to the condensing temperature of the heater shell. The compressed vapor is desuperheated and then sent to the heater shell. Desuperheating is essential to reduce scaling of heat transfer surfaces and to maintain the overall heat transfer coefficient. The compressed vapors are introduced into the heater near the bottom of the tube bundle through a vapor distribution plenum. This design provides a sweep of vapors upward through the bundle, assuring that any residual non-condensible gases are carried out through the vent lines. Excess steam is vented from the heater shell and sweeps the shell of non-condensable gasses, ultimately entering the deaerator as stripping steam. The majority of the vapor is condensed in the falling film heating element and discharged through the feed preheater. The cooled process distillate is used for boiler feedwater. SAGD WATER BALANCE WITH EVAPORATION TECHNOLOGY APPLIED The Water balance of Figure 2 is based on the following parameters: Oil production of 25,000 bbls/day 5

6 Steam to Oil Ratio (SOR) of 2.5 Reservoir loss of 10% of the steam input Boiler blowdown (BBD) of 2% Vent losses of 1% Evaporator Concentration factor of 28.6 by weight (feed to waste ratio as determined by the chemistry. In this case, the concentration is limited by chemistry which affects material selection) This basis is achievable using Evaporation as the water treatment technology. Overall recovery of water is over 95% and the wastewater generated is less than 4%. These numbers show an improvement over conventional treatment where there is 2.5X the waste rate and only 90% water recovery. Evaporation achieves these values since the concentrated brine leaving the system is about 10% total solids (TS) compared to about 4% TS for conventional treatment. The boiler blowdown (BBD) shown is 2% of the overall boiler feedwater (BFW) rate. This is accomplished since the distillate produced from an Evaporator is of very high quality (near ABMA/ASME quality for steam use in a turbine, ref 1) when compared to the conventional process. A 2% BBD would have about 150 ppm of TDS content with a properly designed Evaporator system incorporating a Vapor Washer as shown in Figure 3. The boiler selection possibilities are expanded over that of conventional treatment due to the high quality BFW derived from an Evaporator system. OTSG can also be used in this system where a major portion of the normally 20 to 30% BBD can be recycled back to the boiler as BFW. An alternative to having steam separation and BBD recycle equipment within the boiler system is to have a no BBD approach as shown in Figure 4. FIGURE 4 SAGD SYSTEM WATER BALANCE WITH NO BOILER BLOWDOWN (ALL UNITS TONNES/DAY EXCEPT WHERE NOTED) 6

7 Since the water quality of boiler makeup water is so high as a result of using evaporator distillate, an option to boiler operation is to have no boiler blowdown. Figure 4 illustrates this concept as it fits into the overall SAGD water balance. The boiler produces 98% steam quality which is sent out to the individual well formations. The 98% quality steam is handled by the steam distribution system prior to well injection (which is normal operation) or by the well formation itself. SAGD WATER CHEMISTRY, PRODUCED WATER AND MAKEUP WATER SOURCES With the oil production defined at 25,000 bbls/day and a SOR (steam to oil ratio) of 2.5, the steam requirement is 10,952 Tons/Day (9,936 Tonnes/Day) and the Boiler feedwater flow rate is 1,891 GPM at 172 o F. The derived water balance (flows) are shown in Figure 1 for a c onventional water treatment system and Figures 2 and 4 for an evaporator treatment system. The amount of steam (100%) required for the formation is the main factor which sets the overall flow rate for water treatment. In this example, two (2) large Mechanical Vapor Compression (MVC) Evaporators are required. The system water balance, as with any water balance, is highly affected by the chemistry. All the numbers previously presented are based on a well defined chemistry. For discussion purposes a single typical Produced water chemistry is chosen with two (2) potential makeup sources. Figure 5 shows the input chemistries to a typical SAGD process located in Northern Alberta. Produced Water MU Chem MU Chem Typical Chemistry Normal TDS High TDS BASE BASE ALTERNATE Calcium (Ca) Magnesium (Mg) Sodium (Na) Sulfate (SO4) Chloride (Cl) Bicarbonate (HCO3) Carbonate (CO3) Silica (SiO2) Total Dissolved Solids (TDS) Suspended Solids (SS) <25 <2 <10 Total Organic Carbon TOC) 200 <1 35 Oil and Grease (O&G) 20 <1 <1 ph Temperature, deg F FIGURE 5 SAGD INPUT CHEMISTRIES Generally, Produced Water in Northern Alberta has the following characteristics: Has a TDS range of 1500 to 5000 Is saturated in silica (SiO 2 ) as high as 350 (reported value) Has high alkalinity and Cl (major inorganic anions) High Na (major inorganic cation) High dissolved organic content (as TOC measurement) 7

8 Generally low immiscible organic content (as O&G measurement) Low hardness, low SO 4, low misc other components Makeup water in the area can have quite a wide variance in total dissolved content (TDS). Generally, there is an ample supply of good, quality makeup. Concerns about over use of these sources with the number of upcoming projects causes alternatives to be evaluated. These alternate makeup sources are generally from underground formations which have higher TDS values. The alternate listed in Figure 5 is typical of what is available but higher salinities (higher TDS) are possible. At some point, the high salinity of the makeup water requires separate treatment. For conventional treatment systems this has more of an impact but the same effect occurs for an Evaporator system. Figure 6 shows the effect of the two (2) makeup waters used in the SAGD water balance using evaporation where the formation loss is varied. The evaporator system, two (2) 1000 GPM MVC Evaporators, is designed under specific chemistry conditions and the resulting design is applied to varying chemistries. In all cases the amount of distillate produced is the same to provide the steam necessary for oil production. The BASE case design chemistry combines the Produced Water and the makeup water in the ratios as listed in Figure 2 (or 4) for the 10% formation loss. This provides the basis for Evaporator design in the BASE case that is applied to different scenarios of makeup chemistry and formation loss. 220 Two Evaporator Waste Rate, GPM GPM with 17.1 % High TDS Makeup 92 GPM with 5.6% High TDS Makeup 65 GPM with 13.8% BASE system Makeup BASE Design operating with higher TDS makeup Alternate Makeup Water (About 18,000 TDS) BASE Makeup Water (About 3,000 TDS) Formation Loss, % of Steam Injection FIGURE 6 FORMATION LOSS AND MAKEUP WATER SOURCES The individual plot for each of the makeup water supplies shows what will happen when the formation recovery changes (as formation loss). For the BASE makeup water, not much changes with formation loss variation. The reason is that the TDS s of the Produced Water and the makeup water are about equal. Having more or less normal makeup water in the evaporator feed does not change how the evaporator operates. Changing the makeup source to higher TDS 8

9 water, more than 5X higher in TDS, has an effect on how the evaporator operates. Figure 6 shows that the waste rate goes up by a factor of two (2) at the same formation loss of 10%. As the formation loss is increased, the waste rate increases as well. Even at zero formation loss, the amount of waste using the higher TDS makeup water is 50% higher than the BASE makeup. These changes are a direct result of the water balance. While producing the required amount of distillate to be used as BFW, the makeup must account for all losses, which in this water balance, includes the Evaporator waste and vent. MECHANICAL VAPOR COMPRESSION (MVC) LIMITS OF OPERATION BY CHEMISTRY Once the MVC Evaporator system is designed to a specific flow and chemistry, several operating parameters cannot be altered. In the case presented here, the following evaporator parameters were set in the BASE design by the parameters of the water balance and input chemistries as described earlier: The concentration of the combined feedwater was limited to maintain reasonable materials of construction which reduces capital cost. In this case, 316L SS (or equivalent) limited the concentration of chloride (Cl ) ion in the final waste brine. This concentration occurred at a weight concentration factor of 28.6 with a feed Cl level of about The amount of heat transfer area for a single evaporator was defined to allow use of a Turbo-Fan (compressor). This allows for more reliable operation due to the lower head requirement and the capital cost is much less than a conventional compressor. The compression cycle limitation for a Turbo- Fan is about a 10 o F delta temperature difference between the saturated inlet and outlet conditions of the compressor. This temperature difference (the compressor temperature difference) must account for heat transfer delta temperature in the evaporator, brine boiling point elevation, and pressure losses of the evaporator system. Once these parameters are set, operation of the Evaporator cannot, or should not, exceed these design parameters. Figure 7 shows the effect of varying the evaporator concentration factor (CF) on the BASE chemistry. If higher concentrations are demanded, then alternate materials and a higher head compressor can be incorporated into the design to accomplish this. At the BASE design chemistry and a CF of 28.6, the concentrated brine TDS is about 10% (100,000 ppm) and the waste rate is 3.5% of the feed. This recovers 95+% of the Evaporator feedwater as high quality distillate. If the BASE case feed TDS were higher or lower, a different set of curves would be derived. There is a practical limit of concentration for a falling film evaporator shown on the graph of Figure 7 at about 27% TDS. At this point, further concentration would produce heavy precipitation of salt that would plug the falling film distribution system. Other alternate evaporator designs can handle heavy salt precipitation, but they have a much lower energy efficiency which results in excessive electrical or steam consumption. 9

10 300, Total Dissolved Solids (TDS) as ppm (w/w) 250, , , ,000 50,000 Definition: The Concentration Factor (CF) in an Evaporator is defined as the ratio of Evaporator Feed to Evaporator Waste. As the CF is increased, more distillate production is achieved (higher recovery) and less waste produced. In these examples, weight CF is used. BASE Design Concentration (316L SS limited by Chloride) BASE Design Concentration (Resulting Waste Rate at 3.5%) Practical TDS Limit For Falling Film Evaporators Minimum Waste Rate For System Evaporator Waste Rate, % of Evaporator Feed Concentration Factor (CF) by Weight 0 FIGURE 7 RELATION BETWEEN CF, TDS, AND WASTE RATE FOR BASE CHEMISTRY MECHANICAL VAPOR COMPRESSION (MVC) DESIGN CONFIGURATION Figure 8 shows a single falling film evaporator system very similar to that of Figure 3. The only real difference between Figure 3 and Figure 8 is the presentation of the falling film evaporator itself. This falling film evaporator has two (2) recirculation pumps that split the evaporator heat transfer area FIGURE 8 EFFICIENT FALLING FILM EVAPORATOR SYSTEM DESIGN FOR HIGH QUALITY DISTILLATE PRODUCTION 10

11 for evaporation from different brine concentrations. The brine cascades through the system so evaporation of the bulk of the water is achieved at a lower concentration that has a lower boiling point elevation. The vapor (steam) and condensate (distillate) side of the system are common within the unit. This design saves energy by requiring less compressor head to accomplish the same evaporation as that of a single area design. The split area design is the basis in the BASE design presented here. Figure 9 illustrates the advantage of the split area design over that of the single area design. The amount of energy (KWH per KGAL of distillate produced) is plotted as a function of evaporator concentration factor. Electrical Energy, KWH/KGAL of Distillate Produced Notes: Turbo-Fan Limit - As brine concentration (CF) is increased, more compressor head is required to overcome the boiling point of the brine. The Turbo-Fan limit is exceeded at some point. Make Up Steam - The MVC process is a net energy process. Properly designed, there is little excess (a vent). As the concentration factor decreases, the compression cycle does not have to work as hard producing less and less vent until Make Up steam is required. BASE Design Concentration Single Area Design Split Area Design Double Stage Design Turbo-Fan Limit in each design Make Up Steam Requirement Concentration Factor (CF) by Weight FIGURE 9 EFFECT OF CONCENTRATION FACTOR ON EVAPORATOR ENERGY There are three (3) curves in Figure 9. Two have been discussed: the Single Area Design and the Split Area Design. The third design configuration presented is a Double Stage MVC design and is shown in Figure 10. All designs presented here use identical Evaporator designs in that they all have the same amount of heat transfer area with the same recirculation flow per area. The graphs of Figure 9 show that there is substantial energy savings to be gained by going to a split area design or 2-Stage design. The energy savings between the split area design and the Double Stage design is minimal at the concentration (CF=28.6) of the BASE system being discussed here. As the concentration is increased beyond the BASE system, the effect of cascading the evaporation from lower concentrated sumps becomes evident; a higher percentage of energy is saved. The choice of the Split Area Design for the BASE case design presented here can be justified by noticing the diamond markings on each curve. This is the point where the Turbo-Fan is at the compression limit of design for that device. Concentrations exceeding this diamond marked point would require a more expensive centrifugal 11

12 FIGURE 10 DOUBLE STAGE MVC EVAPORATOR SYSTEM compressor design. The Single Area Design requires a much higher head through the range of the graph and can only use a Turbo- Fan design up to about a CF=15. Beyond CF=15, the Single Area Design would require a more expensive compressor (and use more energy). The Split Area and Double Stage Designs have about the same limit for the Turbo-Fan application (at CF~35) when compressing steam (from the final concentrate stage for the Double Stage design). Another aspect of MVC design is the net energy of the MVC cycle. The same steam that is evaporated must be compressed and used to evaporate itself on a pound-perpound basis and compensate for heat losses. This requires inefficiency in the system to compensate for the fact that the heat of vaporization per pound of water decreases at increased pressure and the waste brine leaves at boiling temperature. Since the compression cycle is not perfect and develops superheat in the compressed steam, enough extra energy is available to operate the system without any makeup steam. Figure 10 also notes where each system would require makeup steam by the triangle markings. As the CF is decreased to below 20, each system starts to require makeup steam as a result of the waste stream exiting the system with too much heat. This reduces the ratio of distillate to feed and there is simply not enough distillate to heat the feed up to the temperature needed to run makeup steam free. In the selection of the heat transfer area for the evaporator system, enough area was put into the design to allow operation of a Turbo- Fan. Figure 11 shows the effect on energy by changing the heat transfer area in each of the potential evaporator designs. All three designs are presented (Single Area, Split Area, Double Stage). As area is added to the evaporation system, less energy is used by the compressor cycle. If the project economics can determine what the cost of 12

13 110 Electrical Energy, KWH/KGAL of Distillate Produced BASE Design Area Selection Single Area Design Split Area Design Double Stage Design Evaporator Area, % of BASE Syxstem FIGURE 11 EFFECT OF AREA ON ENERGY CONSUMPTION energy is in terms of net present value (NPV) over the life of the plant, this may influence evaporator design. More heat transfer area may be added due to the savings in energy. Generally, these systems are energy efficient to start with and adding large amounts of area or more evaporators may not prove to be economically feasible. EVAPORATOR CHEMICAL CONSUMPTION Figure 12 lists the chemicals used to operate the BASE design Evaporator system on Produced Water for SAGD. 50% Caustic Chemical Dosage Pounds/Hour 500 (as 100%) 1394 Antifoam Scale Inhibitor / Dispersant Basis: 1968 GPM of feed to system producing 1890 GPM of Distillate FIGURE 12 CHEMICAL CONSUMPTION The major chemical used in the evaporator system is 50% caustic. Antifoam and Scale Inhibitor/Dispersant are added in minor amounts to the incoming feed of the evaporator. Antifoam dosing of the feed will generally control foaming throughout the evaporator system. The main issue with foaming is potential carryover of brine material to the compressor. The dosing technique works well but is not certain. To augment foam control, cameras are installed to view through the vapor body ports and allow operators to observe the recirculating brine and determine if changes in antifoam addition are needed. Antifoam control starts with dosing the feed but can also be added directly to a vapor body to stop foam if observed. A combination scale inhibitor/dispersant is used to control scale formation throughout the evaporator system. This material is added to the feed and helps to prevent the feed heat exchangers from fouling. This is severe service and historically, front end heat exchangers of Produced water evaporators foul on a routine basis and require a chemical 13

14 cleaning within several months. Prudent design allows for redundant heat exchangers to be installed. The main function of the dispersant part of the additive (Inhibitor/Dispersant) is to keep hardness deposits suspended in the system rather than become deposits on evaporator heat transfer surfaces. The amount of hardness and high operating ph of Produced Water evaporators precipitates what little hardness there is in the system. Without the dispersant, hardness deposits would certainly occur on heat transfer surfaces. A high ph is required to solubilize silica at the high brine concentrations achieved by the evaporator system. The bulk of the caustic will be added to the recirculating brine of the first section of an evaporator to increase the ph. It is also possible that a small portion of the caustic may be added to the incoming feed to improve clarity due to organic content. Caustic may also be added to subsequent sections as needed. The following reactions occur as the result of the ph increase: (1) SiO 2 + 2NaOH Na 2 SiO 3 (aq) + H 2 O (2) Mg++ + 2NaOH Mg(OH) 2 + 2Na+ (3) H 3 BO 3 + xnaoh H (3-x) BO 3 +x + xh2o + xna+ (4) HCO 3 - +NaOH CO Na+ +H 2 O Additionally some calcium and other metals present may precipitate as calcium hydroxide or similar to magnesium as in equation (2). These reactions must be taken into account when estimating caustic consumption. The conversion of silica to soluble silicate is essentially complete at the operating brine ph (12 to 13), and prevents silica fouling of the internal evaporator surfaces. The high temperature and ionic strength of the evaporator concentrate will drive reaction (2) to a much higher level of completion than what may be expected in a conventional precipitation softener. Reaction (3) is much more complex because the form of boron may not be as stated. This is the simplest form that can be listed. Reaction (4) is the simplest in this system and is explained in most water treatment literature. EVAPORATOR DISTILLATE QUALITY AND USE IN DRUM BOILERS Figure 13 represents the typical values of distillate quality when processing Produced Water in evaporators. The distillate quality during normal operation is expected to be much better. The distillate quality produced from an evaporator system processing Produced Water has been heavily discussed throughout the industry. In general, the quality can be stated to be near ABMA/ASME quality water required for high pressure boilers. This quality standard is based on producing steam with <0.1 ppm of TDS for turbine operation. In SAGD facilities there is no turbine and the steam quality does not have to meet these high standards. This begs the question as to what standard is acceptable for drum boilers used in oil field operations. Figure 13 presents a conservative distillate quality that would be produced (in this example) and becomes a basis for project decisions. Two distillate qualities are presented. One based on distillate derived from a well designed evaporator and the other based on using a Vapor Washer to polish the vapor leaving the evaporator. The Vapor Washer is a separate vessel and will improve distillate quality by at least a factor of 2. 14

15 FIGURE 13 DISTILLATE QUALITY The values presented in Figure 13 are based on non-volatile components except for the organic content, which is presented in three ways: (1) A total amount which would be measured by standard TOC methods. (2) A non-volatile amount which is derived empirically for distillate. (3) An O&G measurement which is an extractable portion of the total organics present, probably non-volatile in nature. The total amount is estimated based on experienced volatility of organics in Produced Water. Most of the organics in are not volatile. The non-volatile amount in distillate is based on atmospheric boiling of distillate where the majority of distillate (>95%) is boiled off at atmospheric pressure leaving non-volatile organics behind. Whether this remaining material is really non-volatile is open to discussion since it is present in the distillate by being volatile. The use of a vapor washer also improves the quality of the stream. TREATMENT ALTERNATIVES FOR EVAPORATOR BRINES The concentrated brine from a Produced Water Evaporator is disposed of by two (2) methods: (1) Deep well disposal (2) Zero Liquid Discharge (ZLD) treatment Both approaches have been installed in SAGD facilities. Deep well disposal is the most common approach for disposal of derived Produced Water brines. The difficulty in the disposal is with the reactions that may occur in the disposal formation. High ph brine containing silica needs to be neutralized and filtered prior to deep well disposal. To accomplish this requires a series of reaction tanks followed by a clarifier/thickener arrangement with filtration. This multi-step process has not been fully employed to treat evaporator concentrate and some additional work is required for this process to become as reliable as the evaporator process itself. The ZLD (zero liquid discharge) approach is a brute force method to dispose of solids with no aqueous waste. Most of the water is removed from the brine leaving a solids cake that is disposed off site. The ZLD process employs two (2) additional process steps to accomplish this. The evaporator waste is first concentrated to a highly concentrated slurry in a forced circulation crystallizer 15

16 (concentrator). An additional falling film preconcentrator if justified by the chemistry and size of the system may augment the concentration process. The heavy slurry produced by the concentrator process is dried by use of a rotary drum dryer to produce transportable waste solids material. The forced circulation crystallizer can be driven by an MVC process or direct steam while the rotary drum dryer requires natural gas. If a falling film concentrator were used, it would be an MVC design. SUMMARY Evaporation is a viable process to recover and reuse Produced Water in the SAGD heavy oil recovery process. A higher percentage of recovered water is achieved at three orders of magnitude better water quality than chemical treatment. The higher quality treated water allows more boiler configuration choices than just OTSG's. Different boiler configurations can utilize different fuels, be more efficient, require less installation resources, and improve overall economics of a SAGD facility. The higher recovery also means that the overall waste produced is more than 50% less, allowing consideration of ZLD instead of deep well disposal. optimum evaporator design to establish recovery percentages in overall SAGD designs to identify how the evaporation process fits into the system. Also presented, were evaporator arrangements which improve overall energy efficiency with the same heat transfer area. With all the important benefits gained from using the evaporation process, this process is destined to become a major contributor to the SAGD industry. REFERENCES 1. Feedwater Quality Task Group, Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers CRTD-Vol. 34, prepared for the industrial subcommittee of the ASME Research and Technology Committee on Water and Steam in Thermal Systems, HEINS, W., PETERSON, D., Use of Evaporation for Heavy Oil Produced Water Treatment, prepared for presentation at the Canadian International Petroleum Conference, June 10 12, The basic design parameters for Produced Water evaporators have been presented. Methodology has been shown and graphically presented showing chemistry effects on evaporator design. The chemistry is all important in establishing evaporator materials of construction and the compression system used. Evaporator heat transfer area and chemistry concentration factor are the main design parameters to optimize evaporator economics. Operating electrical power and capital cost can be minimized based on adjusting these factors. Project considerations should first look at 16