By John Woodhull and Tabatha Pellerin
The initial efforts in planning for the expansion of a chemical or petrochemical facility always focus on the processes that will be used directly for production of the product. As the project continues, it often becomes clear that the utility system changes that will be needed to support the expansion will be more complex and costly than originally assumed.
Process engineers at ENSR Interna-tional have found that an integrated approach to addressing the expansion needs of all of the water-related utilities (raw water treatment, boiler feed water, steam, cooling water, and wastewater treatment) can yield huge dividends. Involving engineers who understand the limits and potential of existing utility systems early in the planning process, before decisions regarding process configuration are fixed, can lead to lower capital cost and greater operational efficiency for the total project.
For many chemical process industries, utility water systems generate a larger fraction of the hydraulic load seen at waste treatment than the production process itself. As a consequence, water system design and capacity decisions have a huge impact on wastewater treatment plant cost realized during plant expansion. As most water used throughout the plant is consumed by utility systems, these are the systems that must be evaluated to determine if significant volumes of water can be reused.
When the planning for all systems that use water is coordinated, and decisions are made based on overall water system economics, total project cost is reduced.
Water Balance
The reactors, distillation columns, heat exchangers, fired heaters and pumps that make up all chemical and petrochemical facilities cannot function without water-based utility systems. Steam, for example, is used throughout a chemical plant, including as a common source of heat in tubular exchangers. In reactor systems it can be used as a heat sink, as an inert diluent, or for mixing. It is also used for things as diverse as powering turbines, keeping instruments cool in an incinerator or as a barrier fluid in the seals of a compressor.
Steam condensate and boiler feed water are not only used to feed boilers —as a source of clean, low dissolved solids water they find use as make-up for chemical solutions, as a pump seal fluid or as wash water for removal of precipitated salts. Cooling water, either from a once-through or more commonly from a recirculating cooling water system, is supplied to users ranging from large tubular exchangers, to air conditioning units and sample coolers.
The aqueous utility streams pick up contaminants as they are used and piped throughout the system. As an example, steam condensate, if collected, can be recycled to produce more steam. However, a fraction is lost to the waste treatment as unrecovered condensate or is injected as live steam to the process. When steam contacts process fluids the resulting condensate becomes contaminated and this stream then is discharged to the wastewater treatment plant as process wastewater.
Similarly, boiler feed water or raw water added as part of a chemical processing step also picks up contamination as it is used and recycled throughout the processes. Cooling water can be recirculated, but dissolved solids, silica, and other contaminants are concentrated in the system as pure water evaporates in the cooling tower.
Blowdown from the recirculating water system to wastewater treatment is necessary in order to purge the solids that enter the system with the raw water and as products of corrosion. A raw water source is necessary since water circulated within each of the utility systems eventually is evaporated, becomes part of a product or byproduct, or makes its way to wastewater treatment.
In a refinery, process wastewater streams routed to the oily water sewer for treatment include large flows from crude desalting and sour water stripping. These process wastewater streams contribute the bulk of the contaminant flow to waste treatment. Total utility wastewater includes sand filter backwash, reverse osmosis reject, and regeneration from deionization, all of which result from raw water treatment. Blowdown streams from several cooling towers, a central boiler, and a number of process steam generators make up the balance of the utility water flow. Stormwater is an additional input to the overall water balance. Water leaves the system as vapor from the cooling towers and excess from the steam system. Wastewater is an additional significant outflow.
The raw water needed as make-up for utility systems must be pretreated before it can be used. Pretreatment requirements vary depending on where the water is to be used. Raw water that is to be used as makeup for a cooling water system must be free of suspended solids that could foul heat transfer surfaces. Raw water supplies for boiler feed water must be treated to remove both suspended and dissolved solids.
Treatment processes employed to condition raw water also generate wastewater streams, which are generally routed to a central wastewater treatment facility. These streams will often have high contents of suspended and dissolved solids. For example, a water stream used to regenerate a cationic ion exchange resin as part of the raw water treatment processes is acidic. Water used to regenerate an anionic resin is basic; thus pH adjustment is needed. The wastewater streams resulting from the production of steam and cooling water, in combination with the various blowdown streams, make utility wastewater the largest source of volumetric flow in a typical chemical or petrochemical facility.
Water collected throughout the facility from precipitation has the potential to pick up contaminants as it flows across equipment and pavement. For this reason runoff from areas where contamination is likely is often collected separately and routed to treatment. On an annual average basis stormwater runoff flows are not large, however runoff rates often set the instantaneous peak flowrate to waste treatment. In ENSR's experience, it is not unusual for peak runoff rates to exceed average wastewater flowrates by a factor of 100.
Expansion Planning
As illustrated in the previous section, planning for the wastewater from a plant expansion must include consideration of flow resulting from increased utility requirements if it is to accurately reflect the full impact of the change. Wastewater flow directly produced by the additional process equipment may well be only a small fraction of the total impact when increased utility flows are considered.
While in some cases the need for additional utility service can be met by expanding existing equipment, or adding new boilers/ cooling tower cells, in other cases this type of solution is difficult and expensive. Often it is necessary to find some other way to satisfy the water utility needs of the expansion project without the capital expense involved with expanding and upgrading utility systems. Wastewater treatment is an example. Many chemical and petrochemical facilities have continued to expand long after their wastewater systems were built, so that current loads exceed the original design capacity of the system.
Wastewater treatment plant (WWTP) capacity is tied to volumetric flowrate and contaminant mass loading, both of which are likely to be increased during a plant expansion. Typically a series of WWTP modifications have already been undertaken and any cost-effective changes that could be made to the treatment facilities have been made already. This leaves four approaches as the only viable options for matching treatment system capacity with wastewater load:
• Reduce need for water-based utilities,
• Reduce peak flow from stormwater,
• Segregation of clean streams for direct discharge, and
• Wastewater recycle/reuse.
A reduction in the need for water-based utilities (steam, condensate, cooling water) is almost certain to result in a corresponding reduction in wastewater volume. New plants can be designed to use less water than older ones by making use of technologies designed with reduced wastewater production in mind. Examples of these techniques include:
• Use of vacuum pumps as a replacement for steam jet ejectors,
• Meter/control wash-water use,
• Use countercurrent wash cascades,
• Add auto-shutoff provisions for hose stations and other water users,
• Provide freeze protection (tracing and insulation) to avoid the need to keep hose stations running during cold periods,
• Repair leaks as they arise.
Wastewater treatment system size is frequently set by the flowrate resulting from a storm of design intensity. As a consequence of the large plot area that plants cover, large stormwater volumes are realized during significant precipitation events. Therefore, with no flow equalization, the peak flowrate entering wastewater treatment for many chemical plants could be 100 times the average wastewater flow rate. The treatment system design that would result from this flow basis would be very uneconomical. The impact that stormwater has on treatment system design capacity can be reduced by measures such as the following:
• Routing non-contaminated stormwater to direct discharge,
• Holding water in tank dikes or other containment areas within the plant to lower stormwater surge volumes,
• Increasing equalization volume at the treatment plant.
Direct discharge of non-contaminated stormwater may provide significant relief for treatment plant design capacity limitations. In addition, other wastewater streams may also be sufficiently clean so that direct discharge without treatment is possible. Good candidates present at most facilities for direct discharge without treatment include:
• Boiler blowdown
• Cooling tower blowdown
It is important to recognize that there exists the potential for both of these streams to periodically be contaminated with hydrocarbons (from a leaking heat exchanger, for example). Therefore, using continuous on-line monitoring may be necessary so that if contamination is detected the stream can be diverted to the effluent treatment system.
A second issue relative to these streams can be metals content. Carefully managed corrosion control practices, such as making use of non-toxic chemicals that are free of heavy metals, can be used to produce a blowdown stream suitable for direct discharge. Additional cost for treatment chemicals or monitoring equipment can be well worthwhile if it allows the cost for expansion of a wastewater treatment system to be avoided.
There is also little question that in order to reduce total flow or contaminant load to treatment by recycle and reuse it is necessary to integrate treated effluent streams into utility systems, as these are the systems that have the greatest need for water. For this reason, it is very important to ensure that the standards for water being fed to utility systems are appropriate.
Standards that are needlessly tight have the effect of forcing the addition of expensive pretreatment steps, which may or may not be required. It is, of course, important to remember that if the objective of recycle-reuse is a reduction in treatment system load, then wastewater must be recycled prior to its entry to the treatment plant. Recycle of treated effluent reduces the need for make-up water, but does nothing to reduce requirements at the treatment plant.
Appropriate measures for recycle or reuse of wastewater differ depending on the specific situation. One feasible option often employed for recycling is to group streams based on TDS or contaminant content, e.g. recycle of low TDS sour water stripper bottoms (in preference to high TDS cooling tower blowdown). It is important that specifications for water to be reused are based on solid justification - being overly conservative drives up pretreatment cost, and limits possibilities for reuse.
In addition to recycle/reuse options that can be used to reduce treatment requirements, recycle of treated effluent may be advantageous in some cases as a means of reducing raw water consumption. Treated final effluent from central treatment facilities is finding use at hose stations as wash-water and for other uses that can tolerate a relatively high dissolved solids content.
Conclusions
Refineries and chemical plants are dependent on water-based utility systems, such as steam, boiler feed water, cooling water and wastewater treatment. The most significant impacts from plant expansion projects often will be felt by utility systems designed to treat raw water, generate steam, remove heat from re-circulated cooling water, and treat wastewater. Therefore, planning for cost-effective plant expansion must include a comprehensive evaluation of impacts to water-based utility systems.
The flow resulting from increased utility requirements can be a significant portion of the total wastewater. While expansion of utility services may be feasible, ENSR process engineers have determined that it is generally more cost-effective to employ the following measures (if possible):
• Reduce dependency on utility services (i.e., use counter-current wash cascades, use vacuum pumps instead of steam jet ejectors).
• Recycle/re-use various streams.
• Segregate clean streams for direct discharge.
• Reduce stromwater surge volume to wastewater treatment by providing additional equalization capacity or containment at the plant.
Another consideration is the raw water used for makeup to the water-based utility system, which generally requires pretreatment prior to use within the system. Treatment processes typically employed for this charge generate wastewater streams that can contribute quite significant loading to wastewater treatment. Standards for pre-treatment of water feed streams must be evaluated using cost-benefit analysis to determine treatment levels based on utility system requirements. It is often feasible to group streams based on TDS or contaminant level so these streams can be either pre-treated and then recycled/re-used or simply re-introduced to the system somewhere within the plant without prior treatment.
About the Authors: John Woodhull, P.E., is a senior program manager for ENSR International, an environmental and energy development services firm. As a chemical and process engineer, he has 22 years experience designing chemical plants, refineries, and waste treatment systems. Tabatha Pellerin, P.E., is a process engineer with ENSR. She is a registered professional engineer with over 11 years of experience in the environmental and process engineering fields. Most recently, she has evaluated and designed the wastewater treatment and collection systems for several refineries and other manufacturing facilities.
