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ExxonMobil Proprietary WATER POLLUTION CONTROL

WATER REUSE DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section XIX-B

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December, 2002 Changes shown by ➧

CONTENTS Section

Page

Scope..............................................................................................................................................................3 References .....................................................................................................................................................3 Definitions ......................................................................................................................................................3 BACKGROUND...............................................................................................................................................4 GUIDELINES FOR WATER REUSE PROGRAM DEVELOPMENT ...............................................................4 SUMMARY ..............................................................................................................................................4 WATER REUSE JUSTIFICATION ..........................................................................................................4 Reduce Raw Water Costs or Overcome Water Supply Limitation .......................................................4 Reduce Wastewater Treatment Cost and Lower Tax for Treated Effluent Discharge..........................4 Reduce or Eliminate Wastewater Treatment Facilities Expansion.......................................................4 Better Meet Environmental Regulations...............................................................................................5 WATER REUSE STUDY.........................................................................................................................5 Conduct Water Balance .......................................................................................................................5 Identify Potential Reuse Opportunities .................................................................................................5 Prioritize Reuse Options ......................................................................................................................6 Further Evaluate the Most Attractive Options.......................................................................................6 Confirm Incentives for the Most Attractive Options ..............................................................................7 Conduct Lab, Pilot, or Field Test..........................................................................................................8 Hazard Identification & Risk Assessment ............................................................................................8 Implementation.....................................................................................................................................8 WATER REUSE BY PROCESS AREA...........................................................................................................9 WATER TREATMENT/STEAM GENERATION.......................................................................................9 Reduction .............................................................................................................................................9 Reuse...................................................................................................................................................9 COOLING TOWER ...............................................................................................................................10 Reduction ...........................................................................................................................................10 Reuse.................................................................................................................................................10 DESALTING..........................................................................................................................................11 STRIPPED SOUR WATER (SSW) .......................................................................................................12 Reuse.................................................................................................................................................12 POWERFORMER .................................................................................................................................12 MTBE UNIT ...........................................................................................................................................12 WET GAS SCRUBBER.........................................................................................................................13 WASTEWATER ....................................................................................................................................13 Treatment for Water Reuse.........................................................................................................................13 ExxonMobil Research and Engineering Company – Fairfax, VA

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WATER REUSE

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PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES

TYPICAL WATER QUALITY PARAMETERS CHANGED BY TREATMENT.........................................13 TREATMENT AND REUSE ISSUES.....................................................................................................14 TREATMENT FOR DISSOLVED SOLIDS REMOVAL ..........................................................................15 Municipal Wastewater Effluent Treatment and Reuse .......................................................................15 Cooling Tower Blowdown Treatment and Reuse ...............................................................................15 Refinery Wastewater Effluent Treatment and Reuse .........................................................................16 CATIONS AS IONS, PPM..............................................................................................................................25 ANIONS AS IONS, PPM................................................................................................................................25

FIGURES Figure 1 TABLES Table 1 Table 2 Table 3 Table 4

Water Reuse Application Decision Tree ...........................................................................17

Water Consumers and Wastewater Producers in Refineries............................................18 Water Chemistry Analysis Parameters .............................................................................19 Water Reuse Already Practiced at ExxonMobil.................................................................22 Water Reuse Matrix...........................................................................................................23

APPENDICES A B C D E F G H I

REUSE IN DEMIN PLANT.................................................................................................24 QUALITIES OF RECLAIMED MUNICIPAL WASTEWATER USED AT XOM FACILITIES ..............................................................................................................25 COOLING TOWER BASIN WATER QUALITY LIMITS .....................................................26 HYDROGEN SULFIDE DISTRIBUTION IN DESALTER VERSUS PH..............................27 MTBE WASTEWATER COMPOSITION ...........................................................................28 STRIPPED SOUR WATER ANALYSES ...........................................................................29 DISSOLVED SOLIDS REMOVAL PROCESS ...................................................................30 AUGUSTA SCREENING STUDY: CTBD TREATMENT AND REUSE TO DEMIN............31 AUGUSTA SCREENING STUDY: CTBD TREATMENT AND REUSE TO DEMIN............32 Revision Memo 12/02

Moved Tables 3 – 7 and all Figures to Appendices

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WATER REUSE DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section XIX-B

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SCOPE The objective of this section is to provide guidelines to: 1) justify water reuse programs, 2) conduct a water reuse study and 3) implement water reuse options. This document provides the framework for a water reuse study and introduces the issues involved in each stage of the process. The first section includes guidelines for developing a water reuse program. The subsequent sections address water reuse opportunities by process unit. Water reduction opportunities are discussed as well as wastewater streams that have been successfully reused or technically evaluated for reuse. Critical parameters for that reuse are discussed. As new opportunities for water reuse are evaluated, they will be added to subsequent updates of the DP. In general, calculation procedures are not provided. Blending of water streams to determine pH and scaling potential are time consuming calculations and are generally performed by computer. A third party water chemistry computer program has been licensed to ExxonMobil and is available for use in evaluation of water reuse options. ExxonMobil Research and Engineering (EMRE) should be contacted for information on obtaining and running the program. Alternatively, EMRE can be used as a resource to run the program. Since water reuse generally involves an operating change, experts in the affected process areas, water treating areas, materials, and safety should be consulted during evaluation of a reuse option. The final section gives a brief introduction to treatment for reuse.

REFERENCES 1. 2. 3. 4. 5. 6.

EE.102E.78, “Cooling Tower Water Treatment Guidelines - Second Edition,” E. I. Wolfe, 1978. Meller, Floyd H., “Electrodialysis (ED) & Electrodialysis Reversal (EDR) Technology,” Ionics Incorporated, March 1984. EE.2E.86, “New Guide to Boiler Water Treatment,” E. I. Wolfe, R. J. Franco, K. R. Walston, January 1986. Raycheba, John M. T., “Membranes Technology Reference Guide,” Ontario Hydro, 1990. EE.50E.94, “Guidelines for Sour Water Reuse for Desalting,” C. P. Feerick, C. M. Schinner, E. D. Carlson, 1994. 95 GCD 88, “Water Reuse Workshop Follow-up: Guidelines for Water Reuse Program Development,” E. Kang, June 29, 1995.

DEFINITIONS Anion - A negatively charged ion resulting from the dissociation of salts, minerals or acids in water. Blowdown - The amount of water purged from a recycling system in order to maintain contaminant levels within a specified range. Cation - A positively charged ion resulting from the dissociation of salts, minerals or acids in water. Conductivity - The ability of a solution to conduct electrical current, commonly expressed in microsiemens/cm or micro mhos/cm. Electrodialysis (ED) - A process in which ions are transferred through membranes from a less concentrated to a more concentrated solution as a result of the passage of direct electric current. Electrodialysis Reversal (EDR) - An electrodialysis process in which the polarity of the electrodes is reversed on a prescribed time cycle thus reversing the direction of ion movement in a membrane stack. Hardness - A quality of water defined by the amount of calcium and magnesium present. Recycle - The reuse of wastewater in the process from which it originated. Reduction - The modification ofa process that lowers the amount of wastewater produced. Reuse - Routing or cascading wastewater from one process unit to another. Reverse Osmosis (RO) - The process by which pressure is applied to the more concentrated solution so that the solvent flows through the semi-permeable membrane into the less concentrated solution. ➧

Silt Density Index (SDI) - A measure of how likely or rapidly a colloid-containing solution will foul or plug a filter or membrane. Total Dissolved Solids (TDS) - The material residue from the filtrate that is left in the vessel after evaporation and subsequent drying in an oven at 356°F (180°C). Total Suspended Solids (TSS) - The material residue left after evaporation and subsequent drying (at 217-221°F [103-105°C]) of the portion of the sample retained on the filter.

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WATER POLLUTION CONTROL

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WATER REUSE

December, 2002

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES

BACKGROUND Water reuse is not a novel concept. It has been around for more than 25 years. Water reuse can result in a significant reduction in the volume of water (raw and discharged wastewater) used/generated at a refinery or chemical plant. Interest in water reuse is increasing for two main reasons: the availability and rising cost of raw water and its treatment and the rising cost of wastewater treatment. Increasingly stringent effluent water quality requirements and other new regulations are resulting in the need for additional investment in control/treatment of wastewater. To minimize this investment, a long term water management plan including reduction, recycle, and reuse should be implemented.

GUIDELINES FOR WATER REUSE PROGRAM DEVELOPMENT SUMMARY The Water Reuse Management Process can be broken down into 8 major steps: •

Conduct a Water Balance

•

Identify Potential Reuse Opportunities

•

Prioritize Potential Reuse Options

•

Further Evaluate the Most Attractive Options

•

Confirm Incentives for the Most Attractive Options

•

Conduct Lab, Pilot, or Field Test, if necessary

•

Hazard Identification & Risk Assessment of Water Management Project

•

Implementation

These steps are illustrated in a decision tree (Figure 1) and are described in the subsequent sections.

WATER REUSE JUSTIFICATION In general, cost reduction policies will promote conservation/recycle projects and wastewater minimization. Limited freshwater supply, more stringent environmental regulations and increasing costs will drive conservation and reuse. The following describes four potential justifications for water reuse projects. Reduce Raw Water Costs or Overcome Water Supply Limitation Public demand for water continues to grow, while supplies dwindle. In some areas, aquifers are being depleted and communities are switching to surface water supplies. Municipalities that control water resources may seek to reduce water usage by increasing the cost of water, or limiting the supply. Refineries and chemical plants that purchase raw water, such as locations that use surface water supplies, may be affected by these measures. Generally, most of the raw water requires treatment which can be as simple as clarification or as complex as ion exchange. Water reuse provides a means to reduce the cost of both raw water procurement and treatment by reducing the amount used. Reduce Wastewater Treatment Cost and Lower Tax for Treated Effluent Discharge In addition to wastewater treatment costs, certain locations are taxed based on treated effluent rate. Reducing, recycling, or reusing water upstream of the wastewater treatment plant (WWTP) results in savings of wastewater treatment and discharge costs in addition to the savings realized for raw water consumption. Reduce or Eliminate Wastewater Treatment Facilities Expansion Environmental regulations are likely to result in modification or expansion of WWTP's in the near or long term. A decrease in the hydraulic, organic, and/or total dissolved solids (TDS) loadings to the WWTP may decrease, delay, or eliminate this investment. Furthermore, process water treating facilities such as sour water strippers and wastewater benzene strippers (used in the U.S. to satisfy regulatory requirements) may be operating at or close to their design capacities. It may be necessary to accommodate additional process water from future projects. Debottlenecking these units may be feasible via water reuse.

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WATER REUSE DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

Section XIX-B

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December, 2002

Better Meet Environmental Regulations Reduction in the hydraulic load to the WWTP may result in improved operation of the plant as well as reduction in mass loading of contaminants. However, the potential disadvantages of water reuse are increased TDS levels, such as metals, and possibly increased concentration of organics in the raw wastewater due to lower wastewater flow rates. This in turn may increase the toxicity in the wastewater discharged. The increased TDS level can be partially alleviated by segregating high dissolved solids streams such as demin or ballast water, or by removal of dissolved solids from selected streams upstream of the WWTP. Improved WWTP operation due to lower flow rates may result in no increase in organics in treated wastewater.

WATER REUSE STUDY Conduct Water Balance The first step in a water reuse study is to generate a water balance around the entire refinery/chemical plant. An overall water balance must identify water sources to the refinery or chemical plant, wastewater produced, and process water evaporation rates. The water balance is then developed further to identify water consumers and individual wastewater producers. Users and producers will be matched in the next step, identifying potential reuse options. 1.

Water Sources There are four primary sources of water: •

Raw Water

•

Water in Crude

•

Ballast Water

• Stormwater Raw water, such as municipal water, onsite well water, or surface water are common sources of water which are metered. Water in the crude can be estimated based on changes in crude tank levels during crude water draw-off, or may already be measured for auditing purposes. Ballast water can be estimated from changes in ballast tank levels. Stormwater flow rates are seasonal and vary significantly from location to location. They can be estimated from yearly rainfall amounts and site area and runoff coefficients. In addition, a minor source of water at chemical plants is that produced by chemical reactions. 2.

Wastewater and Evaporation Wastewater can be divided into major categories as follows: •

Utility Wastewater (boiler blowdown, cooling tower blowdown, water treating wastewater)

•

Process Wastewater (stripper waters, desalters, process wash waters, etc.)

• Stormwater (stormwater may already be included in wastewater rates) To complete the balance, one must estimate the evaporation rate, which is primarily from the cooling tower and wet gas scrubber. Steam losses and evaporation from wastewater treatment plant basins or lagoons can also be significant. Total wastewater flow rates are generally available as they are required for environmental compliance reporting. 3.

Identify Water Consumers and Wastewater Producers Once an overall balance is achieved, individual water consumers and wastewater producers should be identified. A list of potential consumers and producers for a refinery is provided in Table 1. While it is nearly impossible to identify every consumer and producer, major ones should be identified along with an estimate of their flow rates. Note that some wastewaters are cascaded from one unit to another unit. Therefore, be cautious not to repeat the same stream more than once. When conducting the facility water balance, portable ultrasonic flowmeters can be used to quantify flow through pipes that do not have permanent, dedicated flowmeters.

Identify Potential Reuse Opportunities 1.

Types of Reuse There are several categories of water reuse, listed below in approximate order of increasing cost of implementation. •

Reduction

•

Recycle within the unit

•

Reuse Direct or cascade reuse Segregation and reuse Treatment and reuse (including treated WWTP effluent)

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WATER REUSE DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

Reduction of the amount of water used in a process generally results in an increase in the contaminant level and the temperature of the effluent wastewater. If this is acceptable from a process, safety, and materials standpoint, it can be used to lower raw water usage. A typical large reduction in raw water usage is obtained by increasing the cooling tower cycles. Recycle is the reuse of wastewater in the process from which it originated. Examples of this are return of sample coolers to the cooling tower or a pump-around on a water scrubber tower. A purge stream off of the recycle loop is needed to control the level of contaminants. Other measures, such as heat removal, chemical treatment, or contaminant removal may also be required. Reuse is the routing or cascading of wastewater from one unit to another. For example, stripped sour water can be reused in the desalter. The advantage of reuse over recycle is that contaminant build-up is less likely. Each process has minimum water quality requirements. If the quality of the wastewater from a process meets the minimum water quality requirements of another process, it can be reused directly. If not, segregation of certain wastewaters from a combined stream or treatment of the wastewater before reuse may be necessary. 2.

Concentrate on Major Wastewater Producers and Water Consumers As with the water balance, when identifying potential water reuse opportunities, concentrate on major water users and wastewater producers. The effort needed to evaluate high or low flow streams is usually the same but the incentives for higher flow streams will generally be higher. Also, consider reduction and recycle before reuse as these are generally less expensive to implement.

3.

Consider General Water Quality Parameters When Available To the extent they are known, consider general water quality parameters when identifying potential reuse options. Parameters such as pH, pressure, temperature, oxygenated or reduced state, oil content, suspended solids content, and dissolved solids content may be known and can be used as a first pass in matching water sources to users. A list of water chemistry analysis parameters along with methods of analysis can be found in Table 2. If these qualities are not known, potential matches based on water flow rate should be identified for later investigation of water quality. Parameters, such as pressure or pH may be altered at relatively low cost. Wastewater can also be blended with the current water source to affect contaminant levels or meet flow rate requirements.

4.

Practiced Water Reduction Options A list of reuse options already being practiced at ExxonMobil is included in Table 3. These should be considered in an evaluation of potential reuse options. Site specific review of these options is still needed as wastewater composition can vary significantly from one location to another. For example, stripped sour water has been identified as a potential for reuse to the cooling tower at Antwerp. However, stripped sour water may contain phenols or ammonia at levels that make it undesirable for this use at other locations. In addition, the TDS level in stripped sour water can vary significantly, for example, from 18 ppm at Antwerp to over 1,400 ppm at Benicia.

Prioritize Reuse Options 1.

Develop Cost/Benefit Analysis The number of water reuse options evaluated in detail will depend on the water reuse goals and resources available. In order to prioritize the options identified, each option should be categorized qualitatively as resulting in a low, medium, or high water reduction and requiring low, medium, or high investment. Consider preparing a table as follows: WATER REUSE OPTION

WATER REDUCTION

INVESTMENT

Option 1

Low

High

Option 2

High

Low

Option 3

Medium

Low

Since the water reuse options have not been determined to be feasible yet, a more detailed evaluation of the cost and benefit of each option is not warranted. 2.

Prioritize Options and Select Those to be Worked Further Prioritize the options from high water reduction and low investment to low water reduction and high investment. Depending on the water reduction goal and the resources available, select the number of options to be worked in detail.

Further Evaluate the Most Attractive Options 1.

Identify Receiving Process Constraints Determine the contaminants of concern for the receiving process and set the maximum level for each one. The limiting contaminant level may be the current operating level.However higher levels may be tolerable and should be considered. Consulting with process, safety, and materials specialists is recommended in setting these limits. Consideration should be given to the following:

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Section XIX-B

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•

Potential Catalyst Poisons: If the receiving process is a catalytic one, water which contacts hydrocarbon feed in the process can transfer contaminants to the hydrocarbon stream, which may be poisonous to the catalyst.

•

Treatment Chemicals Compatibility: Boilers, cooling towers, desalters, etc., receive chemical treatment. The chemical treatment vendor should be consulted regarding compatibility of their treatment with the new makeup water composition or any treatment chemicals from the wastewater source.

•

Materials: Contaminants such as ammonia and chloride or oxygenated water may cause corrosion. engineer should be consulted regarding any increased potential for corrosion due to the reuse.

•

Safety: Reuse may cause increases in hydrogen sulfide or ammonia levels or changes in pH resulting in volatilization of these compounds. This and other safety concerns should be identified and reviewed with the safety engineer.

•

Product Contamination: When reuse water comes in direct contact with the product (e.g., scrubbers or direct contact coolers), contaminants from the water can be transferred to the product.

The materials

2.

Obtain Water Analysis To determine water reuse feasibility, accurate and complete water analyses are recommended. Past data, if representative of current operation, can be used to minimize analytical costs. Otherwise, new water analyses should be obtained. A list of parameters to be analyzed, if they are believed present, is listed in Table 2. Acceptable methods of analysis are also included. Even if certain parameters are not limiting in the receiving process, all parameters should be analyzed. A complete analysis of the water allows the reuse option to be accurately assessed by hand calculation or simulated through water chemistry modeling. Electrochemical balancing of the water ensures that no major contaminants in the wastewater have been overlooked.

3.

Computer Modeling of Reuse Unless contaminants are being removed by treatment, water reuse results in an increase in the concentration of contaminants in the affected water streams. This can result in formation of solids, free hydrocarbon, or evolution of gases, which generally has a negative effect on the process. Blending of water streams results in new water compositions and generally changes the pH or scaling potential of the water.This may have process or materials implications. Simulation of reuse options using computer modeling that predicts the water chemistry is recommended to ensure that the reuse option identified is feasible. As a result of the modeling, a limitation of the extent of reuse, pH adjustment, or other treatment may be identified. Modeling will help identify the need for further analyses or pilot testing. Modeling will also help define the operating limits of the reuse option, thereby reducing the risk in implementation. The Environmental Simulation Program (ESP) is a state-of-the-art program (licensed to ExxonMobil) that predicts both inorganic and organic water chemistry including pH, formation of solids (scaling), and evolution of gases. ESP also has the capability to model unit operations, such as: •

Stripping

•

Mixing

•

Splitting

•

Neutralization

•

Heat Exchange

•

Reactors

Confirm Incentives for the Most Attractive Options Once the technical feasibility of the reuse options has been established, screening quality cost estimates for each option should be generated. This will allow re-prioritization of the reuse options on a quantitative basis. The implementation cost may include: •

Capital Investment (screening quality)

• Operating Cost (chemicals, utilities, etc.) The savings may arise from: •

Operations Wastewater treatment cost + Chemicals + Utilities + Sludge disposal -

+ Discharge fees (hydraulic and/or mass) Raw water cost + Purchase

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+ •

PROPRIETARY INFORMATION - For Authorized Company Use Only

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Treatment

Capital Deferred investment due to debottlenecking Reduced scope of investment

Conduct Lab, Pilot, or Field Test The objective of testing is to confirm an operable, reliable, and safe reuse operation. Depending on the reuse option, lab or pilot testing may be appropriate. In some cases, however, if process conditions can not be reproduced in the lab or in the pilot test, a full scale test may be necessary. The field test plan should include the following activities: •

Schedule and responsibilities

•

Review of chemical treatment with applicable process specialists and the vendor

•

Design Specification preparation and equipment installation (if necessary)

•

Risk assessment and SOC/OIC (Safe Operating Committee/Operations Integrity Committee) Review

•

Baseline data collection, sampling, and analyses

•

Sampling and monitoring during the test

Monitoring should include: •

Critical Process Parameters

•

Corrosion Monitoring

•

Fouling (i.e., pressure drop or loss of heat duty)

•

Chemical Treatment Verification

Hazard Identification & Risk Assessment Consistent with the goals of Operations Integrity Management Systems (OIMS), a hazard identification and risk assessment should be conducted before implementing any process change. The purpose of this risk assessment is to review potential reliability, safety, and environmental issues, and to identify mitigating steps if necessary. The reliability of the reuse water source and potentially a backup source for periods during which the reuse water source unit is shutdown must be considered. If the water reuse option is field tested before implementation, the risk assessment should be performed before the field test. 1.

2.

Identify Hazards Consider the following areas when identifying the hazards: •

Environmental

•

Materials

•

Safety

Risk Assessment Methodology •

Identify Potential Impact or Consequence

•

Rate the Probability of Occurrence Before Mitigation

•

Identify Mitigation & Prevention Methods

•

Rate the Probability of Occurrence After Mitigation

Implementation The final step is permanent implementation of the reuse. A typical project will require: •

Process Basis or Design Basis Memorandum Cost Estimate (Class V and Class III or IV) Cost Reduction Task Force Review

•

Design Specification Cost Estimate (Class II) HAZOP and SOC/OIC Review

•

Appropriation of Capital/Expense

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Section XIX-B

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Detailed Engineering & Project Implementation Construction Start-up

WATER REUSE BY PROCESS AREA A matrix of water reuse options already practiced or evaluated is shown in Table 4. These options are discussed further in the sections below. As discussed in the above Guidance section, all water reuse options must be evaluated on a site specific basis. Success at one location does not guarantee success at another unless the water chemistry and other process issues are considered. A site specific evaluation includes analysis, modeling, consulting with water chemistry, process, materials, and safety experts, lab, pilot, or field testing, and a risk assessment.

WATER TREATMENT/STEAM GENERATION Water treatment is defined as the process used to pretreat water for steam generation. Water treatment processes may include clarification/lime softening, filtration, ion exchange, and possibly reverse osmosis. Wastewaters generated in these processes are filter backwashes, ion exchange regeneration wastewaters, and reverse osmosis reject waters. In steam generation, blowdown water is generated to control boiler water chemistry, e.g., dissolved solids concentrations. Reduction Methods for reducing water treatment wastewaters are: •

Implement operator training programs emphasizing wastewater minimization.

•

Increase ion exchange resin inspection programs. Perform annual filter and resin bed inspections to correct uneven flow application or insufficient backwash cleaning. Sample and analyze media or resin to determine state of degradation.

•

Review backwash or regeneration sequences to ensure all stages have been optimized for flux rate, duration, or chemical strength.

•

Add filter instrumentation (TSS/turbidity monitors, backwash cycle times, automatic backwash system).

•

Monitor and automate ion exchange regeneration cycles by installing continuous analyzers (conductivity) and flow instrumentation.

•

For new facilities, consider countercurrent (versus co-current) regeneration.

Reuse Methods of reuse within the water treating process are reuse of filter backwash to the clarifier, cation/anion/mixed bed backwash to the clarifier, strong base anion final rinse to demin feed tank (clearwell), and mix bed final rinse to demin feed tank. Appendix A shows these reuse options as part of a water treatment flow schematic. Water chemistry considerations for each of these reuse options are discussed below: Backwashes to the Clarifier: Filter and ion exchange backwashes can be recycled to the clarifier instead of routed to wastewater. Source water for the backwashes is generally filtered water. The backwash waters are contaminated with suspended solids, which are then flocculated and removed in the clarifier. Usually a sump is needed to equalize flows for a steady recycle to the clarifier. The resin bed backwashes may not be at neutral pH, so the buffering capacity of the raw water should be checked to ensure the blended water pH is within acceptable limits. Final Rinse to the Clearwell: Ion exchange regeneration is made up of several steps, including acid regeneration of the cation beds and caustic regeneration of the anion beds. The spent regenerants containing dilute acid or caustic are sent to a neutralization tank. These waters are too high in dissolved solids for reuse and are routed to the wastewater treatment plant. In some cases, this wastewater can be routed directly to the effluent since it does not contain any hydrocarbon. Following these washes the beds are rinsed with feed water. Due to the presence of residual acid or caustic, the rinses are also sent to the neutralization tank. The final rinse of the strong base anion or mixed bed removes residual dissolved solids from the process. When the final rinse conductivity drops below the raw water conductivity, the rinse water can be reused by recycling to the demin feed tank (clearwell). High Pressure Boiler Blowdown to Low Pressure Boilers: Reuse is also possible in steam generation. As mentioned above, blowdown is generated to maintain water chemistry (e.g., dissolved solids at levels that will not produce scaling). This amount of water, as well as steam or condensate lost from the system, must be replaced by makeup boiler feed water. Dissolved solids limits in a high pressure boiler are lower than in a low pressure boiler. As a result, high pressure boiler blowdown water can be reused as makeup water to a lower pressure boiler, assuming internal boiler chemical compatibility between the two systems. Low Pressure Boiler Blowdown to Cooling Towers: Low pressure boiler blowdown is relatively free of organics and oil, it is a potential stream for reuse as cooling tower makeup. This option is discussed in greater detail under the cooling tower section. Tertiary Treated Municipal Wastewater as Boiler Feedwater: In some cases, alternative water sources such as tertiary treated municipal wastewater or treated groundwater can be further treated to boiler feedwater quality. At the Torrance refinery, tertiary

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WATER POLLUTION CONTROL

WATER REUSE PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES

treated municipal wastewater is treated via ultrafiltration and reverse osmosis and fed to existing high pressure boiler pretreatment facilities. In any such case, boiler quality demands and source water qualities must be carefully evaluated to ensure compatibility. Appendix B indicates quality data for reclaimed municipal wastewater used at the Torrance, Jurong, and Singapore PACT refineries.

COOLING TOWER Total dissolved solids levels are maintained in the cooling water system by purging a certain amount of the cooling water. This is the cooling tower blowdown (CTB) and is usually the largest single wastewater stream. The CTB plus evaporation water must be made up in the cooling tower system with makeup water. Reduction The first consideration for cooling tower water management is CTB reduction through maximizing cooling tower cycles of concentration. Increasing cooling tower cycles may be limited by uncontrolled blowdown. In this case, in order to increase cycles, sources of uncontrolled blowdown must be located and recycled back to the cooling tower. If, however, there is a controlled blowdown stream, then this stream can be reduced to increase cooling tower cycles. The maximum cycles are based on makeup water quality and the current cooling water chemical treatment program. Appendix C lists constituents of concern for the cooling tower and guideline concentration limits for these constituents in the cooling tower basin. As a first pass, cycles of concentration can be increased until one of these limits is met. Computer modeling with site specific analyses should be used to confirm the level of concentration is acceptable. The site's cooling tower chemical vendor should also be consulted before increasing cycles or using reuse waters for cooling tower makeup. The cooling tower chemical vendor should assist in modifying chemical dosages, if applicable, and identifying critical control parameters. While some parameters are of concern due to fouling or scaling potential, chloride is of concern for materials reasons. Generally, chloride restrictions are placed on stainless steel equipment to prevent chloride stress corrosion cracking, and pitting. The chloride limit depends on a variety of factors including temperature, pH, stress level, and oxygen concentration. Chloride stress corrosion cracking rarely occurs below 126°F (52°C) and 250 wppm (mg/L) chloride and for operating velocities of greater than 7.9 ft/s (2.4 m/sec) (when chloride levels are between 50 to 500 wppm [mg/L]). Many refineries are able to successfully operate their cooling tower with higher chloride content depending on levels of other ions in the water. Local materials experts should be consulted regarding the materials and conditions in the refinery in order to determine if higher chloride levels can be tolerated. A limit of 500 wppm (mg/L) on chlorides may also be set by the typical stabilized phosphate treatment system because the chemicals cannot prevent pitting corrosion of carbon steel at higher levels. It may, however, be possible for the chemical treatment vendor to adjust the chemical levels to allow for a higher chloride limit. Routine maintenance inspection and continuous on-line monitoring should be used to ensure acceptable operation, especially when concentration limits are approached. Routine maintenance inspection includes corrosion coupons, test heat exchangers, and biofouling monitors and provides comprehensive information on corrosion and fouling. Low corrosion and fouling indicate that the amount of contaminants are acceptable for site operation. By using continuous on-line monitoring of cooling tower water conductivity, blowdown cycles can be controlled and blowdown minimized. Reuse High water requirements/wastewater production and less stringent makeup water criteria than boilers and some process stream water washes make the cooling tower a prime candidate for water reuse. However, ccases recycling water to the cooling tower must be carefully evaluated as fouling in the cooling tower circuit can adversely affect the entire refinery. As mentioned above, a list of parameters and their concentration limits for the cooling tower basin water are found in Appendix C. These levels are guidelines. Computer modeling using actual water analyses and reviewing present chemical treatment program limits will determine site specific limits. If modeling and site chemical program review show that the reuse is acceptable, a field test should be considered. Baseline and field test monitoring of heat exchanger performance and corrosion coupons should be performed. Stripped Sour Water (SSW) is an excellent candidate for cooling tower makeup. SSW generally has low total dissolved solids (TDS) when pH adjustment is not carried out in the stripping process. SSW is also typically low in metals content and suspended solids. However, in some refineries, high concentrations of hydrocarbons, hydrogen sulfide, ammonia, phenols, or dissolved organics may be present in the SSW and can cause problems such as heat exchanger fouling, odor, and corrosion when reuse to the cooling tower is implemented. The limits listed in Appendix C should be adhered to when considering reuse of SSW to the cooling tower. Computer modeling has shown that low pressure boiler blowdown is another source for makeup to the cooling tower. Low pressure boiler blowdown is known to be low in dissolved solids and relatively free of organics and oil. The sludge conditioner used in the boiler treatment may inhibit the function of dispersants used in the cooling water. This potential concern should be checked with the chemical treatment vendor. Reuse of wastewater treatment (WWT) effluent as cooling tower makeup has been of interest for many years. However, contaminant levels, especially BOD and ammonia, in the treated wastewater have prevented its use. More efficient/consistent WWT, generally required today to meet stringent effluent criteria, as well as better water chemistry predictive tools, have improved the potential for wastewater effluent reuse. Although the quality of wastewater has improved, there are some considerations that must be addressed before implementing this option. Continuous biocide addition (chlorination) of the feed water to the tower is necessary. If the WWT effluent is blended to the cooling tower water return (CWR) line, additional chlorination should be injected at the CWR line; if WWT ExxonMobil Research and Engineering Company – Fairfax, VA

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effluent is directly pumped to the cooling tower basin, chlorination should be injected directly to the effluent line just upstream of the cooling tower basin. This will ensure that bacteria from the treated wastewater are eliminated before the water enters the cooling system. Normal treatment of the cooling water with biocide should continue. With the addition of chlorine to the water for biological control, it is critical that the chlorides level be monitored. The effect of chlorides on the cooling system has been addressed earlier in this section.

DESALTING The purpose of the desalter is to remove salts or dissolved solids from the crude. Downstream of the desalter, the crude should have less than 2 lb salt per thousand barrels (< 2 ptb [6 gm/m3]). Some wastewaters can be reused to the desalter as wash water. The desalter is one of the few locations where the presence of some hydrocarbon in the reuse water is acceptable. In fact, some locations have reused high phenol-containing sour water to the desalter to get removal of phenol from the wastewater. When reusing water to the desalter, several parameters must be considered in order to ensure operation of the desalter will not be disrupted. EMRE reports EE.50E.94 and EE.105E.81 provide detailed descriptions of considerations for reuse to the desalter. Waters considered for reuse to the desalter should be low in dissolved solids (TDS), especially chlorides and calcium, and low in suspended solids (TSS). Dissolved solids reduce the driving force of transfer of solids from the crude to the water and suspended solids can increase the potential for emulsion formation at the oil/water interface. Dissolved solids can also precipitate out in the desalter, contributing to the suspended solids content. Desalter wash waters should also be low in ammonia and sulfides. Depending on the level of ammonia and the pH of the desalter brine, ammonia can be transferred to the crude and result in ammonium chloride deposition in the pipestill overhead circuit. Appendix A-1 shows an example of predicted partitioning of ammonia between the hydrocarbon and water phases in a desalter based on pH. Since desalter conditions and crude type will also affect partitioning, site specific simulations should be performed when considering reuse. Sulfide will also be distributed between the crude and wash water, depending on its level and the pH of the desalter brine. High sulfide levels in the desalter brine can be a concern from a safety standpoint if the brine is discharged to an open sewer and can be a wastewater treatment concern if the combined wastewater to biological treatment is above 30 - 50 wppm (mg/L) hydrogen sulfide. Sulfide can also combine with dissolved iron and precipitate out, contributing to suspended solids. Appendix D shows an example of predicted hydrogen sulfide level in the desalter brine based on the level in the wash water and the pH of the desalter brine. Again, site specific simulations should be performed. In addition, compounds which are or act like surfactants should be avoided as they can contribute to emulsion formation. If thiosulfates are present, they can break down to form elemental sulfur, another contributor to emulsion formation and corrosive acids. Thus, low thiosulfate streams are desirable. Water being considered for reuse should be sampled and analyzed for the above mentioned parameters of concern. A fairly complete cation and anion analysis should also be performed. Since several water sources are generally blended to produce the desalter wash water, the blend should be modeled to determine the blended pH and to ensure no precipitation of solids. A target brine pH of 5.5 to 7 is desired. Operation with brine pH in the 6 - 8 range is typical. Operation with pH above 8.5 has been associated with emulsion problems in the desalter when treating highly naphthenic crude blends. Operation at a brine pH of 7 or less minimizes the transfer of ammonia to the crude, hydrogen sulfide to the effluent brine, and minimizes emulsion formation potential. If brine pH is not within the desired range, the pH of the wash water can be adjusted. Reusing water can cause the desalter to be susceptible to corrosion. This can be prevented by operating above a brine pH of 5.5, avoiding oxygenated water sources, and minimizing chloride content in wash water. Low chloride content also maintains a good driving force for removal of chloride from the crude.

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Wastewaters which have been reused in the desalter as wash water include atmospheric pipestill (APS) and vacuum pipestill (VPS) overhead condensate, stripped sour water, and MTBE wastewater. All of these waters are low in dissolved solids, as their source is boiler feed water, softened water, or steam condensate. APS and VPS overhead condensate and stripped sour water contain ammonia and hydrogen sulfide. Reuse of stripped sour water is discussed in greater detail under the sour water stripper section of this document. MTBE wastewater may also contain ammonia and be at elevated pH due to caustic addition to the wash tower. MTBE wastewater analyses are listed in Appendix E. These show significant differences in ammonia levels which, as mentioned above, affects whether or not they can be reused in the desalter. As with all water reuse projects site specific water analyses should be obtained. Water chemistry, process, safety and materials issues should be considered, and modeling, lab or field testing may be recommended.

STRIPPED SOUR WATER (SSW) Sour water stripper bottoms have been identified as a good source of reuse water. Most refineries have sour water strippers, and the bottoms are generally low in contaminants such as total dissolved solids, ammonia, sulfides, and free organics. Stripped sour water also represents a significant water stream in terms of flow rate. The pH of a typical stripped sour water ranges from 8 to 10. The pH in the stripper may be increased or decreased to preferentially remove hydrogen sulfide or ammonia. This increases the dissolved solids content of the water, decreasing its reuse potential. Appendix F lists five stripped sour water streams from four refineries. These indicate the variability among stripped sour water sources and the importance in obtaining site specific water analyses. Stripped sour water from a particular tower can also vary significantly. This is especially the case if slop waters or spent caustics are routed to the sour water stripper. Sampling should be performed over time to determine the variability of the water, especially for critical parameters such as hydrogen sulfide, ammonia, and chlorides. Reuse Possible uses for SSW include reuse to the desalter, cooling tower, flare seal drums, wet gas scrubber, and as elutriator quench in the fluid coker. SSW can also be recycled to hydrotreating gas scrubber towers. Use of SSW to the desalter and the cooling tower are described in more detail in the Desalter and Cooling Tower sections. SSW to Flare Seal Drums: Water level is maintained in the flare seal drum to prevent flashback in the flare header. A constant makeup and purge are maintained to prevent contaminants from building up in the drum. Limits on the composition of this water are materials related, or there may be limits on hydrogen sulfide or ammonia if the purge water is routed to the sewer. Stripped sour water can be used as the makeup to the flare drum. The purge water can be recirculated to the stripper feed if sufficient capacity is available in the stripper. If sufficient capacity is not available, the purge water can be sent to the sewer. SSW as Elutriator Quench: Water is used in a Fluid Coker Elutriator to quench the coke. Both stripped sour water and sour water have been used for this service. The disposition of the contaminants in the water has not been determined conclusively. However, potential issues such as coke quality or overhead SOx and NOx levels can be screened by assuming that the contaminants of concern all lay down on the coke or all go overhead with the vapors. The material of the injection nozzle should be reviewed for compatibility with contaminant levels in the reuse water. SSW to a Wet Gas Scrubber: Covered under the Wet Gas Scrubber section. SSW to Hydrotreating Treat Gas Scrubbers: Treat gas scrubbers are used in some hydrotreating processes to remove hydrogen sulfide and ammonia from recirculating treat gas. Water may also be injected in-line to the reactor effluent circuit to prevent ammonium salt formation. The treat gas scrubber tower bottoms is sour and must be treated in a sour water stripper prior to being routed to the sewer. Alternatively, this water can be recycled to the hydrotreating scrubber after stripping. A former ExxonMobil refinery has a dedicated stripper for the treat gas scrubber water. Dissolved solids levels are maintained at low levels by purging a stream from the system. Issues to be addressed are adequate stripping of hydrogen sulfide and ammonia, adequate level of purge to prevent recycling of potential catalyst poisons, or scaling in the stripper.

POWERFORMER Cyclic POWERFORMING units may have dryer reactivation gas scrubbers for on-oil or regeneration gas dryers. The scrubbers cool and remove chloride and carbonate (if the dryers are on regeneration gas) from the dryer reactivation gas. Significant amounts of water are used on a once through basis. This water can be recycled if caustic is added to neutralize the chloride and if the water is cooled. A purge stream is also needed to maintain chloride levels below materials limits and to prevent precipitation of carbonate salts.

MTBE UNIT MTBE wastewater is a potential candidate for reuse in the desalter. Parameters required for reuse to the desalter are described in detail in the desalter section.

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WATER REUSE BY PROCESS AREA (Cont) WET GAS SCRUBBER To offset evaporation and purges, water must be added to the Wet Gas Scrubber (WGS) process. Materials compatibility and potential for solids formation should be evaluated when considering water reuse for this service. Reuse water should be free of insoluble hydrocarbon. Materials issues can be resolved by obtaining analyses and consulting with materials specialists. Water chemistry modeling at wet gas scrubber conditions can be used to determine whether or not solids will form. Wastewater sources that have been reused in the WGS include stripped sour water, boiler blowdown, and demin plant rinse water. Wastewater effluent has also been used for a portion of the makeup.

WASTEWATER Because wastewater effluent regulations are becoming more stringent, the quality of wastewater is improving. This characteristic coupled with the large volume of wastewater produced by a typical refinery makes wastewater effluent a potential candidate for reuse. However, since wastewater effluent is generally a combination of all wastewater streams, it may be too high in dissolved solids for reuse. Increased reuse of water in the refinery or chemical plant result in increased dissolved solids levels. The presence of residual bacteria also provides a challenge for reuse of this stream due to suspended solids limits or the potential for biofouling. One method of reusing wastewater is as firewater makeup. In order to implement this option, it is recommended that the wastewater be pre-chlorinated to control biological growth. Wastewater should be tested to ensure that a stable fire fighting foam can be generated. Once this option has been implemented, the firewater system remote dead legs should be flushed monthly to prevent anaerobic growth and corrosion. Wastewater can also be used as utility water in the plant for the washdown of process units. This can only be done if the utility water is a separate, segregated water system used for little else than unit washdowns. Again, pre-chlorination is recommended to control biological growth. Another option for reuse of wastewater is as cooling tower makeup. This option is discussed in greater detail under the cooling tower section.

TREATED GROUNDWATER Contaminated groundwater treated to remove trace organics or other contaminants may be potentially reused to boilers, cooling towers, firewater systems, or the delayed coker as elutriator quench. Thorough analysis should be performed to determine whether the treated groundwater quality is compatible with the receiving process or to determine the level of further treatment required. Depending on receiving process requirements, dissolved solids, metals, etc. concentrations may preclude the reuse of groundwater in some cases. Along with process approval, any potential groundwater reuse scenario should be evaluated by industrial hygiene and safety experts before implementation.

TREATMENT FOR WATER REUSE ➧

TYPICAL WATER QUALITY PARAMETERS CHANGED BY TREATMENT Treatment can be used to remove contaminants from wastewater and allow for its reuse. In addition, treatment may increasethe volume of wastewater that can be reused. Contaminants include: suspended solids, hardness (calcium and magnesium), free oil, soluble organics, dissolved gases (ammonia, hydrogen sulfide, carbon dioxide), dissolved solids, and microbes. The reasons for removing these contaminants are as follows: Suspended solids - Suspended solids cause plugging or fouling of equipment and would affect product quality if the reuse water comes in contact with product. Hardness - Calcium and magnesium may combine with hydroxide, carbonate or other anions in the water to form insoluble salts which cause scaling. In heat exchangers, scaling results in a reduction in heat transfer efficiency. Free oil and soluble organics - These contaminants cause fouling directly or indirectly by fostering biological activity. Underdeposit corrosion can also result from biofouling. Dissolved gases - Dissolved gases such as ammonia and hydrogen sulfide can be a safety or industrial hygiene concern if they evolve from the water. They can also cause biological activity, especially if a water containing ammonia is reused in the cooling tower. Waters containing carbon dioxide may cause scaling if the conditions are such that the carbon dioxide converts to carbonate and forms insoluble carbonate salts. Waters containing ammonia may cause corrosion of copper alloys. Dissolved solids - Dissolved solids can become insoluble if the pH or temperature of the water changes or if the water is concentrated. Insoluble salts are a potential scaling issue. Silica is also a scaling issue and becomes insoluble at about 150 wppm (mg/L). Chloride is a concern due to corrosion. Higher dissolved solids levels tend to increase corrosion potential.

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Microbes - Microbes may be a health issue, especially if the wastewater being reused is from a sanitary treatment facility. Otherwise, microbes are a fouling concern. In addition to contaminant removal, the temperature, pH or oxygen content of a water may be changed to allow for reuse. Changes to these are accomplished by various types of treatment. These treatment processes are common to water or wastewater treating and are therefore covered in other DP sections. The following table lists the type of treatment used for each parameter and the DP which covers it. PARAMETER

TYPE OF TREATMENT

DESIGN PRACTICE

Temperature

Heater or Cooler

pH

Caustic or acid addition, CO2 injection

XIX-A9

IX

Oxygen Content

Oxygen scavenger addition, aeration

XXVI-A

Suspended Solids

Flocculation, clarification, filtration

Hardness

Softening

Free Oil

Gravity separation, flotation (dissolved, induced), membranes

XIX-A1, A2

Soluble Organics

Biological, chemical oxidation, activated carbon, membranes

XIX-A5, A8, A11

Dissolved Gases

Degasifier, stripper, oxidation, absorption e.g. by amines

XIX-A10, XXVI-A

Dissolved Solids

Ion exchange, evaporation, precipitation, membranes (electrodialysis reversal, reverse osmosis)

Microbes

Biocides, membranes, filtration

XXVI-A, XIX-A3, A4 XXVI-A

XXVI-A

XXVII

These treatment steps range in complexity and cost from relatively simple and inexpensive (a heat exchanger) to relatively complex and costly (a reverse osmosis unit or an activated sludge plant). In some cases, the treatment process already exists in the refinery and effluent streams can be reused as described in previous sections. For example, sour water strippers remove ammonia and hydrogen sulfide from sour water and potentially allow for reuse in desalters and cooling towers. Refinery effluent wastewater treatment plants generally have several treatment steps to remove suspended solids, free oil, and soluble organics. Treated effluent wastewater is reused as fire water and cooling tower makeup. Other treatment processes, such as ion exchange, are generally used in raw water treatment but may also be used to pretreat wastewater for reuse. ➧

TREATMENT AND REUSE ISSUES Wastewater quality almost always varies more widely than that of raw water. This makes wastewater treatment more of a challenge. The treatment process must be flexible enough or have sufficient pretreatment to consistently meet the reuse quality demands. In addition, since the effects of some contaminants on the treatment or reuse process are difficult to predict, (for example, the effect of organics on membranes) pilot testing is more often needed to determine the viability of a treatment or reuse application. Justifying installation of a treatment process(es) to allow for reuse is generally difficult due to the relatively low cost of water at most locations. Multiple economic incentives, such as raw and wastewater treatment savings or avoiding installation of additional raw or wastewater treatment facilities, may provide sufficient economic justification. Alternatively, water supply limitations or government regulations could drive a reuse project. It is important to identify all water quality issues for reuse prior to selecting the treatment process(es). For example, if hardness is an issue, softeners may be used. However, if chloride is also limiting, ion exchange, electrodialysis reversal (EDR), or reverse osmosis (RO) can be used to remove both hardness and chlorides. It may also be possible to avoid some treatment by blending a reuse water with raw water or segregating wastewaters. In selection of the treatment process the disposition of the treated water and any purge streams must also be considered. Generally the treated water should be used in the highest quality service possible. For example, if hardness or dissolved solids are being removed as part of the treatment, the treated water quality may be better than the available raw water and offload existing raw water treatment facilities, thus providing additional incentives for reuse. Purge streams may include backwashes from media filters, sludges from clarifiers or softeners, or concentrate streams from membrane processes. These can be costly to dispose of and should be considered in the economics of the reuse and the treatment process selection. In the case of RO, discharging the concentrate with effluent wastewater is generally significantly less expensive than other disposal options so effluent wastewater discharge parameters should be considered in setting the RO recovery level (higher percent recovery means higher concentrate TDS level) and in selection of any chemicals (such as anti-scalants) added in the process.

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TREATMENT FOR DISSOLVED SOLIDS REMOVAL As more water is reused at a facility, and treatment is used to increase the amount of water that can be reused, dissolved solids levels in the wastewater will increase and eventually limit reuse. This is especially true in arid areas of the world where water supply is limited, or where salt water is intruding on fresh water supplies. In these locations, treatment for dissolved solids removal is being considered or is already implemented to improve the quality of water supplied. Treatment for dissolved solids may also be considered to allow for reuse of refinery cooling water or wastewater. There are several types of treatment for dissolved solids removal, including ion exchange, evaporation, precipitation, electrodialysis reversal and reverse osmosis. All of these processes, except for electrodialysis reversal, are covered in Design Practice XXVI Section A. A brief description of each process, its usage, target compounds removed, feed quality requirements and waste stream generated are provided in Appendix G. Some examples of applications of these processes for wastewater treatment and reuse are described below.

➧

Municipal Wastewater Effluent Treatment and Reuse Tertiary treated municipal wastewater effluent is being reused in both refinery cooling towers and boiler pretreatment plants. The Torrance refinery uses tertiary treated municipal wastewater effluent as cooling tower makeup and high-pressure boiler feedwater. The water undergoes a nitrification treatment step upstream of the cooling towers. Water fed to the high-pressure boilers is treated via ultrafiltration and reverse osmosis upstream of the onsite boiler water treatment plant. There is also an example in California of municipal wastewater undergoing tertiary treatment and being re-injected to groundwater. This water undergoes chemical clarification and disinfection using lime, multimedia filtration, cartridge filtration and RO. The pH of the water is lowered to 5.5 to minimize the potential for solids precipitation in the concentrate. The quality of the product water is better than that needed for cooling tower makeup and could likely be used to replace raw water in boiler feed water treatment. When considering reusing tertiary treated municipal wastewater, dissolved solids, suspended solids, organics, and microbes, are contaminants of concern. These contaminants are not only an issue for the reuse. They can also cause fouling in the dissolved solids removal process. Therefore, selection of pretreatment steps upstream of the dissolved solids removal step is critical to the success of the reuse. Reuse to Cooling Towers: The tertiary treatment steps used in each case are different, and of course depend on the quality of the wastewater and the desired quality of the product water. Additional treatment may allow for an increase in cooling tower cycles, thus reducing makeup water requirements, and adding treatment cost. The cost/benefit of increased treatment must be evaluated on an individual project basis. In one case a biological trickling filter is used to remove ammonia, followed by breakpoint chlorination to convert residual ammonia to chloramines and eliminate microbes. Hardness, chloride and silica levels are sufficiently low to allow for 5 cooling tower cycles without scaling concerns. Increased monitoring for corrosion, solids deposition, and microbiological activity have been implemented as well as a partially automated cooling tower chemical treatment program. In another example of municipal wastewater effluent reuse to cooling towers, lime softening, clarification and gravity filtration are used to remove hardness and suspended solids. The target for cooling tower cycles is 6, and again, increased monitoring for corrosion, deposition, and microbiological activity have been initiated. For implementation of municipal wastewater reuse to cooling towers, the following is recommended:

➧

•

Agreement on water quality specifications, which can be ranges, with the supplier and a quality assurance program for confirming compliance.

•

The potential for additional metals and other trace toxins contained in the municipal wastewater to impact the refinery's wastewater effluent permit should be considered. This may result in additional limits on the municipal wastewater supplied or obtaining allowances for use of the municipal wastewater from the permitting authority.

•

Tertiary treatment of the wastewater to allow achievement of the desired reuse water quality, e.g., based on cooling tower basin quality criteria provided in Appendix C of this DP and cooling tower cycles. Depending on economics, it may be acceptable to reduce the number of cycles from current operation. However, the effect of increased blowdown on wastewater treatment costs must be considered.

•

Breakpoint chlorination upstream of the cooling tower to ensure control of microbial activity. Review of health concerns with the refinery industrial hygienist.

•

Pilot testing the treatment scheme.

•

Risk assessment including evaluation of the need for increased corrosion, deposition, and microbiological monitoring.

Cooling Tower Blowdown Treatment and Reuse Reuse of cooling tower blowdown is generally only considered after maximum cycles have been achieved. This means that the water is limited by one or more dissolved solids, and some type of treatment for dissolved solids removal is needed. If hardness is limiting, it may be possible to soften the water and reuse it in the cooling tower. If however, higher cycles are limited by chloride or Total Dissolved Solids (TDS), EDR or RO may be a required treatment step. Before deciding on softening alone, the cost of ExxonMobil Research and Engineering Company – Fairfax, VA

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softening should be compared with the cost of EDR or RO treatment of the water. In addition, credit should be taken for the higher quality produced by EDR/RO as compared to softening alone. These credits may include debottlenecking or reducing the cost of boiler feed water pretreatment. A screening study of cooling tower blowdown reuse was performed for Augusta Refinery. A schematic process flow diagram and water qualities are provided in Appendices H and I, respectively. pH adjustment, dual media filters, and cartridge filters were specified upstream of EDR treatment. Issues including calcium phosphate precipitation potential and organic fouling potential necessitate pilot testing of this scheme prior to design and implementation. Consideration should be given to use of an activated carbon guard bed to reduce the potential for exchanger leaks to foul the membranes. Both RO and EDR membranes can be cleaned, but may be irreversibly fouled by some organics. In Augusta's case, EDR treatment was selected due to high silica levels in the cooling tower blowdown. This contaminant would precipitate out if concentrated by RO. EDR, on the other hand, does not concentrate silica. Demin plant feed was selected as the disposition for the treated water. This avoids further concentrating of silica, which would occur if the permeate was recycled to the cooling tower. This disposition also debottlenecks the demin unit because the treated water hardness is less than that of the raw water. Consequently, this allows for removal of silica in the demin unit. The EDR concentrate was reblended with the remaining wastewater, and the combined wastewater quality met effluent discharge limits. ➧

Refinery Wastewater Effluent Treatment and Reuse As mentioned in previous sections, treated effluent wastewater can be reused as fire water with only prechlorination as treatment if the TDS is sufficiently low to minimize corrosion In addition, the wastewater must form a stable fire fighting foam. If the wastewater treatment plant removes dissolved organics and ammonia, and dissolved solids are not limiting, reuse of effluent wastewater to the cooling tower can be considered. Generally minimal additional treatment is required: prechlorination and media filtration. If, however, dissolved solids are limiting, segregation of wastewater streams which are high in dissolved solids and low in oil (demin regenerant waste) should be evaluated to reduce the dissolved solids content of the wastewater. If there is still a need to reduce the dissolved solids level, treatment via evaporation, EDR, or RO for dissolved solids removal should be considered. Chlorination, media filtration, and cartridge filtration are generally required as pretreatment of biologically treated wastewater prior to reuse in any of these processes. If polyamide RO membranes are used, dechlorination is also required. Nanofiltration (NF) or ultrafiltration (UF) can be considered as pretreatment steps to remove submicron solids, microbes, or residual organics (these membranes may also be fouled by organics). As with dissolved solids removal treatment of cooling tower blowdown, the disposition of the concentrate stream should be considered. If it does not meet the effluent limits alone or via and reblending with the remaining effluent wastewater, , crystallization and disposal as a solid waste can greatly increase project cost. A potential low cost alternate disposition for a high dissolved solids concentrate stream is the Refinery Coker Unit coke quench drum (or elutriator vessel), where the impurities would be deposited on the product coke. Treatment of non-biologically treated wastewater has been considered. The main issue in this case is organics in the wastewater. EDR and RO can tolerate some soluble organics, but pilot testing is necessary to determine the fouling rates and cleaning efficiency. Pilot testing of RO has been performed by ExxonMobil on stripped sour water and by Texaco on produced water. The composition of the Exxon stripped sour water can be found in Appendix F, Benicia T-2831. Neither of these treatment systems have been commercialized. The ExxonMobil project did not proceed beyond the pilot phase due to high fouling rates which would likely have required removal of the organics prior to RO treatment. The Texaco case has not proceeded to full scale, but was felt by Texaco to be successful at the pilot scale without removal of the organics. In both cases, the pH of the water was adjusted to about 11 to minimize the fouling effect of the organics.

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FIGURE 1 WATER REUSE APPLICATION DECISION TREE

START

1. Is Raw Water Purchased? 2. Is Raw Water Supply Limited? 3. Would Like to Reduce Wastewater Treatment Cost? 4. Would Like to Lower Tax for Treated Effluent Discharge? 5. Would Like to Reduce or Eliminate Wastewater Treatment Plant Expansion?

Is answer to one or more questions YES? Yes

No

No Water reuse can be regulatory driven

Water Reuse Not Attractive

Yes

Water Reuse May Be Economically Justifiable

WATER REUSE In-coming:

Raw-Water Water in Crude Ballast Water Storm Water

Find Other Water Sources

No

Outgoing:

Utility Wastewater Process Water Segregated Stormwater Evaporation and Losses

Water Balance In = Out? Yes

Identify Consumers & Producers Identify Recycle/Reuse Opportunities Investment: Low, Medium, High Water Reuse Rate: High, Medium, Low

Recycle Direct or Cascade Reuse Segregation and Reuse Treatment and Reuse

Prioritize Reuse Options

Select Cases to be Worked Further Identify Constraints

Water Analyses

Receiving Process Constraints Treatment Chemicals Compatibility Materials Safety

ESP Modeling Capital Investment Operating Cost Savings Investment Offset

Confirm Incentives and Priority

Test Plan Monitoring

Lab, Pilot, or Field Test

Hazard Identification/Risk Assessment

Project Implementation DP19BF1

Design Specification Detailed Engineering

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TABLE 1 WATER CONSUMERS AND WASTEWATER PRODUCERS IN REFINERIES UNIT

WATER CONSUMERS

Atmospheric Pipestill (APS)

Desalter

WASTEWATER PRODUCERS Desalter Overhead Distillate

Vacuum Pipestill (VPS)

Overhead Distillate/Coalescer (including steam ejector condensates)

Hydrotreaters/ HYDROFINING Units

Treat Gas Scrubber

Treat Gas Scrubber Stripper Distillate Drum

Cat Feed HYDROFINING Unit

Recycle Gas Scrubber

Recycle Gas Scrubber/High Pressure Separator Low Pressure Cold Separator Wild Naphtha Stripper Overhead Distillate Drum

HYDROCRACKING Unit

Water Scrubber

Water Scrubber

Debutanizer Overhead Gas Water Wash Scrubber

Debutanizer Overhead Gas Water Wash Scrubber Low Pressure Cold Separator Debutanizer Overhead Drum (minor) Fractionator Overhead Drum

GOFINING Unit

Recycle Water Scrubber

Recycle Water Scrubber Low Pressure Cold Separator Product Stripper Overhead Distillate Drum

Cat Cracking Unit

Wet Gas Scrubber

Fractionator Overhead Distillate Drum DeC2 Absorber Feed Drum Wet Gas Scrubber Ammonia Scrubber

POWERFORMING Unit Fluid Coker

Dryer Reactivation Gas Scrubber

Dryer Reactivation Gas Scrubber

Regeneration Gas Scrubber

Regeneration Gas Scrubber

Elutriator Quench

Light Ends Water Scrubber Upstream of Amine Treating

Light Ends Water Scrubber Upstream of Amine Treating

Fractionator Overhead Distillate Drum

Coke Dewatering/Slurry Stripper

LBG Water Wash Tower

Venturi Scrubber

Coke Dewatering/Slurry Stripper

Caustic Wash Tower

Venturi Scrubber FLEXICOKING Unit

Elutriator Quench

Product Fractionator Overhead Drum

Light Ends Water Scrubber Upstream of Amine Treating

Coke Gas KO Drum Light Ends Water Scrubber Upstream of Amine Treating Coke Dewatering/Slurry Stripper

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ExxonMobil Proprietary WATER POLLUTION CONTROL

Section

WATER REUSE DESIGN PRACTICES

Page

XIX-B

PROPRIETARY INFORMATION - For Authorized Company Use Only

19 of 34

December, 2002

TABLE 1 (CON'T) WATER CONSUMERS AND WASTEWATER PRODUCERS IN REFINERIES UNIT

WATER CONSUMERS

Delayed Coker

WASTEWATER PRODUCERS

Quench Water

Coke Drum Gas Water Quench

Cutting Water

Fuel Gas Water Scrubber Upstream of Amine Treating

Coke Drum Gas Water Quench Fuel Gas Water Scrubber Upstream of Amine Treating MTBE Hydrogen Plant

Methanol Extraction Tower

Methanol Extraction Tower

Feed Wash Tower

Feed Wash Tower

Steam Drum Makeup Condensate

Condensate Blowdown Hydrofiner Stripper

EXOLFINING N Extraction

Vacuum Jet Ejector Condensers Caustic Treating

Wash Water

Wash Water

Boiler Feed Water Pretreatment and Boilers

Boiler Feed Water Makeup

Sand or Media Filter Backwash Ion Exchange Regeneration Waste Boiler Blowdown

Cooling Tower

Cooling Tower Makeup

Cooling Tower Blowdown

CLAUS Sulfur Plant

Waste Heat Boiler

Unit KO Drum

Sulfur Condensers Sour Water Stripper

Stripped Sour Water

CO Boiler

Waste Heat Boiler

Flare Gas Recovery System

Flare Drum Seal Water

Oil Movements

Flare Erum Seal Water Crude Tank Water Draws Product Tank Water Draws Ballast Water

Other

Segregated Storm Water

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WATER POLLUTION CONTROL

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WATER REUSE

December, 2002

PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES

TABLE 2 WATER CHEMISTRY ANALYSIS PARAMETERS PARAMETER (Level of Detection)

RAW WASTE WATER WATER

PRESERVATIVE

STORAGE TEMP, °F (°°C)

HOLDING TIME

SUGGESTED METHODS

Total Cations Aluminum (µg/L) Antimony (µg/L) Arsenic (µg/L) Barium (µg/L) Beryllium (µg/L) Boron (µg/L) Cadmium (µg/L) Calcium (µg/L)

X

X

6 months

ICPES(1)

HNO3 to pH < 2 for ICPES, No preservative for CVAF

6 months for ICPES, 28 days for CVAF

ICPES, CVAF(2)

HNO3 to pH < 2

6 months

ICPES

HNO3 to pH < 2

Chromium (µg/L) Cobalt (µg/L) Copper (µg/L)

X

Iron (µg/L)

X

X

X

X

Lead (µg/L) Magnesium (µg/L) Manganese (µg/L)

Mercury (µg/L, ng/L)

None X

X

X

X

Molybdenum (µg/L) Nickel (µg/L) Phosphorus (µg/L) Potassium (µg/L) Selenium (µg/L) Silicon (µg/L) Silver (µg/L) Sodium (µg/L)

X

X

Strontium (µg/L) Thallium (µg/L) Tin (µg/L) Vanadium (µg/L) Zinc (µg/L)

ExxonMobil Research and Engineering Company – Fairfax, VA

ExxonMobil Proprietary WATER POLLUTION CONTROL

Section

WATER REUSE DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

PARAMETER (Level of Detection)

RAW WASTE WATER WATER

PRESERVATIVE

STORAGE TEMP, °F (°°C)

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December, 2002

HOLDING TIME

SUGGESTED METHODS

Total Anions Bromide (µg/L) X

Chloride (µg/L)

X

Fluoride (µg/L) None

Cool, 39oF (4oC)

28 days

IC(3)

Nitrate (µg/L) Nitrite (µg/L) Orthophosphate (µg/L)

X

Sulfate (µg/L)

X

X

Sulfides (µg/L)

X

X

NaOH, ZnAc

Cool, 39°F (4°C)

7 days

Titrimetric, Iodometric

X

None

Cool, 39°F (4°C)

Immediatel y

Titrimetric, Iodometric, Ion Chromatography

X

X

H2SO4 to pH < 2

Cool, 39°F (4°C)

28 days

Potentiometric, Ion Selective Electrode, Distillation/ Titrimetric

X

X

None

Cool, 39°F (4°C)

28 days

Molybdosilicate

Sulfites (µg/L) Neutrals Ammonia (µg/L) Silica (mg/L)

(Plastic Container) m-Alkalinity, mg/L as CaCO3

X

p-Alkalinity, mg/L as CaCO3

X

None

Cool, 39°F (4°C)

14 days

Titration

X

None

Cool, 39°F (4°C)

14 days

Titration

TDS (mg/L)(4)

X

X

None

Cool, 39°F (4°C)

7 days

Gravimetric, dried at 356°F (180°C)

TSS (mg/L)

X

X

None

Cool, 39°F (4°C)

7 days

Gravimetric, dried at 217 221°F (103 - 105°C)

General Organics

X

X

H2SO4 to pH < 2

Cool, 39°F (4°C)

28 days

Combustion-Infrared, Oxidation

Cool, 39°F (4°C)

28 days

Spectrophotometric, Infrared, Gravimetric, Separation Funnel Extraction

TOC (mg/L)

(Glass Container) X

X

Oil & Grease, mg/L

Physical Parameters

HCl to pH < 2 (Glass Container)

X

X

None

Cool, 39°F (4°C)

Immediatel y

Electrometric

X

X

None

Cool, 39°F (4°C)

28 days

Conductivity Meter

pH Conductivity, mmho Notes: (1)

(2) (3) (4)

ICPES = Inductively Coupled Plasma Emission Spectroscopy. All cations listed under ICPES are typically covered in one ICPES scan. If selected analytical method does not provide all cations listed, parameters indicated with “X” are the necessary cations. Cold Vapor Atomic Fluorescence capable of Hg detection at ng/L levels. ICPES capable of Hg detection at µg/L levels. IC = Ion Chromatography. All anions listed under IC are typically covered in one IC scan. If selected analytical method does not provide all anions listed, parameters indicated with “X” are the necessary anions. TDS can be estimated from conductivity.

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ExxonMobil Proprietary Section XIX-B

WATER POLLUTION CONTROL

Page 22 of 34

December, 2002

WATER REUSE PROPRIETARY INFORMATION - For Authorized Company Use Only

DESIGN PRACTICES

TABLE 3 WATER REUSE ALREADY PRACTICED AT EXXONMOBIL

APPLICATION

AFFILIATES

Stripped Sour Water (SSW) reuse to treat gas wash tower (hydrocracker, hydrotreater, GOFINING unit)

Antwerp, Former XOM refinery

SSW reuse to flare drum as seal water

Antwerp, Augusta, Former XOM refinery, BTRF

SSW reuse to desalter

Antwerp, Augusta, Ingolstadt, Karlsruhe, Campana, BTRF, BRRF, Joliet, Chalmette

Sour water from TGCU unit as wash water in FLEXICOKING unit

BTRF

Sour water from FLEXICOKING unit as wash water in GOFINING unit

BTRF

Sour water as elutriator quench in a fluid coker

Former XOM refinery

Wastewater Treatment Plant (WWTP) effluent reuse as firewater

BTRF, BRRF, Former XOM refinery

WWTP effluent reuse as makeup to the cooling tower (low salt Biox unit)

Ingolstadt

MTBE wash water to desalter

BRRF

APS/VPS Overhead to desalter

Antwerp, Augusta, BTRF, BRRF, Former XOM refinery, Joliet

Recirculation of POWERFORMING Unit dryer reactivation gas scrubber water

Former XOM refinery

Boiler feed water pretreatment filter backwash to clarifier

Former XOM refinery, BTRF

Recycle final rinse water from strong base anion unit to the demin feed tank

Former XOM refinery, Baytown, Beaumont

Cooling Tower Cycle-up (greater than 3 cycles)

Former XOM refinery, Strathcona, BTRF, BTCP, BOP, Joliet, Beaumont

High pressure boiler blowdown as makeup to the low pressure boilers

BTRF, BTCP

Boiler blowdown as makeup to the cooling tower

Augusta, BOP

Cooling tower blowdown as industrial water

Augusta

WWTP Effluent as partial makeup to Wet Gas Scrubber

BTRF

Tertiary treated municipal wastewater to cooling towers

Torrance, Singapore Chem, Singapore PACT, Jurong

Stripped sour water as FCC wash water

Joliet

Reverse Osmosis (RO) reject water from low-pressure boiler treatment plant as makeup to firewater system

Torrance

Tertiary treated municipal wastewater to onsite high pressure boilers pretreatment facilities

Torrance, Singapore PACT

ExxonMobil Research and Engineering Company – Fairfax, VA

ExxonMobil Proprietary WATER POLLUTION CONTROL

Section XIX-B

WATER REUSE DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

Page 23 of 34

December, 2002

TABLE 4 WATER REUSE MATRIX REUSE SOURCE WATER

WATER CONSUMERS

NonReuse Source (1)

Boiler Feed Water Pretreatment

RW

Cooling Tower Make-up

RW

Desalter Wash Water

RW

Elutriator Quench in Fluid Coker

RW

APS and VPS Overhead Condensate

High Pressure Boiler Blowdown

Low Pressure Boiler Blowdown

MTBE Waste water

X X

Stripped Sour Water

BFW Pretreatment Filter and Ion Exchange Backwash

Strong Base Anion and Mixed Bed Final Rinse

X

X

X X

RW

Flare Drum Seal Water

RW

FCCU Emergency Vent Stack

RW

Lower Pressure Boiler Make-up

BFW

Hydrotreater Treat Gas Scrubber Water

RW

Wet Gas Scrubber

RW

Treated Groundwater

X

X

X

X X

X

Fire Water

X X

X

X X

X

X X X

X

X

Note: (1)

Wastewater Effluent

RW = Raw Water

BFW = Boiler Feed Water

ExxonMobil Research and Engineering Company – Fairfax, VA

X

ExxonMobil Proprietary WATER POLLUTION CONTROL

Page

Section XIX-B

24 of 34

WATER REUSE

December, 2002

DESIGN PRACTICES

PROPRIETARY INFORMATION - For Authorized Company Use Only

APPENDIX A REUSE IN DEMIN PLANT

Filter

Clearwell

Cation

Mix Bed

Anion

Lime Softener Raw Water Tank

Feed

Treated Water Tank

Degasifier

Flow Equalization Sump DP19BF03

Reuse - - -

AMMONIA DISTRIBUTION IN DESALTER VERSUS pH 8 pH = 9

Ammonia in Crude, wppm

7 6 5 4 3 2 pH = 8 1 pH = 7 0 0

200

400

600

800

Ammonia In Wash Water, wppm

DP19BF04

Notes:

(1) (2)

Based on Arabian Medium Crude and pH of brine at 77°F (25°C). Ammonia includes both NH3 and NH4+ forms.

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1200

ExxonMobil Proprietary WATER POLLUTION CONTROL

Section

WATER REUSE DESIGN PRACTICES

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PROPRIETARY INFORMATION - For Authorized Company Use Only

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December, 2002

APPENDIX B QUALITIES OF RECLAIMED MUNICIPAL WASTEWATER USED AT XOM FACILITIES

PARAMETER

TORRANCE

Denitrified Water1 CATIONS AS IONS, PPM

Water Pretreated Via Ultrafiltration / Reverse Osmosis2

JURONG

SINGAPORE PACT

Industrial Water3

High Grade Industrial Water4

Aluminum

< 0.01

Arsenic

< 0.02

Cadmium

< 0.01

Calcium

80

< 0.5

Chromium

< 0.01

Copper Iron

1.0

Manganese

< 0.003

< 0.01

< 0.1

0.01 – 0.04

< 0.001

< 0.01

Lead

< 0.01

Mercury

< 0.01

Sodium

700

< 6.8

9 – 32

Zinc

< 0.01

ANIONS AS IONS, PPM Chloride

450

Fluoride

< 3.2 < 0.1

Nitrate (as N)

36

Phosphate (as P)

4.9

Sulfate (as SO4)

600

< 0.05

< 1.6

pH Conductivity (microsiemens) 35

2 – 26 0.01 – 0.33

0.5 - 10

0.6 – 3.3

1-4

Total Dissolved Solids

Silica (as SiO2)

130 - 400

80 - 120

0.4 – 4

350 - 1050

10 – 124

7 – 7.4

6.5 – 7.5

850 - 1600

35 – 187

< 1.0

BOD5

0.1 – 0.9