Karen S. Eble, Jennifer Feathers
Betz Industrial
Trevose, Pa.
Before a refinery can optimize internal water reuse, all process water streams must be identified and their contaminants characterized.
They can then be matched to minimize total refinery water use, discharge, and treatment.
The identification of streams with similar contaminants (i.e., "matching") allows treatment to be tailored to the contaminants involved.
The cooling system is typically the largest consumer of water within a refinery and, consequently, it is the most likely place to reuse water. Therefore, the contaminant characteristics of process water streams should be compared with the level of contaminants that can be treated successfully in the cooling system.
This first of two articles outlines the development process for a water reuse plan, then identifies typical contaminants in the many effluents that make up waste treatment plant influent.
PROCESS WATER
Oil refineries use tremendous amounts of water. Refineries for which Betz Industrial has conducted water conservation/reuse studies have typically used 65-90 gal water/bbl of crude oil processed. That is 6.5-9 million gpd of water for a refinery processing 100,000 b/d of crude.
This range is slightly less than the 90-100 gal/bbl reported by NUS Corp. in 1986, and a significant decrease from the 2,000 gal/bbl reported in 1975.'
Clearly, gains in water conservation and reuse by refineries already have been tremendous. In many refineries, the easiest and least costly projects have been completed.
For this reason, this article assumes that water has been conserved wherever possible (e.g., cooling system and boiler cycles have been maximized). It also assumes that saving water throughout the utilities has been closely examined.
For example, a maximum of clean condensate (not process condensate) is now returned to the deaerator, and boiler blowdown is cascaded from high pressure to low pressure boilers, if applicable.
The focus is therefore on refinery process water streams, their reuse and, if used for makeup water, the effect that the streams' contaminants have on the cooling system. This information can be combined with knowledge of the utilities to provide the basis for an overall plan for refinery water reuse.
Many refineries have characterized their waste treatment plant effluent and considered possibilities for its reuse.
Certainly, waste treatment plant effluent is an excellent candidate for reuse. However, effluents from the process units that are the influent to the waste treatment plant must also be identified and thoroughly characterized; and the effects of their contaminants evaluated.
This information is important to a refiner who plans to optimize water use in the refinery while minimizing waste water treatment costs. It may be far less costly to repipe an existing stream for reuse within a unit than to pipe that stream to the waste treatment plant and back again-especially if no intermediate treatment is necessary.
Water reuse projects are driven by several variables:
- Purchased water cost
- Influent water treatment cost
- Effluent water treatment costs (on site or off site)
- Permit limitations on the amount or quality of discharge water
- Public relations.
In the development of a reuse plan, the refiner needs to identify water streams used at individual units throughout the refinery, as well as identify water streams coming from those units.
Contaminants allowed in the influent streams, and those found in the effluent streams, must be identified for their impact on the reuse area or treatment. Once this is known, the effluent contaminants and influent requirements can be matched to determine possible reuse schemes.
Effluent streams containing only contaminants at levels that can be tolerated by an influent stream can be matched with that stream for reuse as total or partial makeup.
Factors such as stream volume and physical proximity will also effect project viability.
Once effluent and influent streams have been matched, in terms of contaminants, the remaining effluent streams can be matched to determine if a specific treatment method might be more cost-effective than sending the streams to the waste treatment plant.
WATER REUSE PROGRAM
There are many small projects that will produce immediate water savings. But structuring the reuse investigation process before starting saves water, time, and money.
CONCEPTUAL PLANNING
The first step in a water reuse project-conceptual planning-includes defining the goals and scope of the project. The importance of water reuse must be clarified, with consideration given to all technical, environmental, and economic factors.
The analysis should provide answers to the following key questions:
- Should a reuse project be implemented (i.e., what is the payout in terms of the environmental and corporate economics)?
- Can the project be constructed (are the technology and capital available to complete the project goals)?
- Is water availability decreasing or requirements for water increasing?
- Does the refinery expect to save money on water costs?
- Will changing discharge permit requirements lead to future cost factors, such as capital equipment investment and/or fines?
- How much money will actually be saved (water not purchased, water not treated, equipment not built, fines not paid)? These savings should be estimated early to determine the amount of money that can be justified for the project.
DEFINING PARAMETERS
After initial planning, it is necessary to investigate and define project parameters. This requires that the refiner do the following:
- Define quantity and quality of available water supply, with consideration to future needs and seasonal variations.
- Identify federal and state permit limitations and anticipate future regulations to the extent possible.
- Identify water available for recycling (water streams coming from each process and utility unit).
- Characterize and measure contaminants in the recyclable water, including seasonal, daily, and crude related variations.
- Identify water users within the refinery and specify acceptable contaminant levels for each user.
IDENTIFYING REUSES
Once all streams are identified and characterized, they can be matched to achieve the proper quality water needed for various applications. A common example is reusing process overhead water from the crude unit in the desalter.
Condensed stripping steam and water injected into the crude and vacuum tower overhead streams for corrosion control are often of a quality suitable for use in the desalter as part of desalter makeup.
After all such direct reuse possibilities are addressed, streams containing low contaminant levels can be considered for reuse, with some prior cleanup. Use of sour water stripper bottoms is a good example, although sour water strippers were not built for the purpose of recycling.
The water to the sour water stripper is primarily process condensate, which is very low in salts, but contaminated with H2S, NH3, and phenols. Good operation of the sour water stripper will remove most of the H25 and NH3, and the effluent water will have a pH close to neutral. The resulting water makes excellent desalter makeup water.
WATER BALANCE
Water reuse projects can and do proceed without an extensive refinery-wide water balance. Without a good balance, however, the program may not utilize the most cost-effective or efficient combination of streams and treatment.
Before a water reuse project can progress, especially when the ultimate goal is "zero discharge," it is important to have a complete water balance around and within the refinery.
For the refinery as a whole, the water balance should be fairly straightforward: water in equals water out.
Usually "water in" is city water, well water, or surface water, and a relatively small amount of bs&w entering with the raw crude. "Water out" is waste treatment plant effluent, cooling tower evaporation and drift, and water lost to the process.
City, well, or surface water into the refinery is usually well-characterized because of its uses within the utilities and process units. Waste treatment plant effluent is usually well-characterized because of discharge permitting. Similarly, flow rates and water quality should be balanced around each unit in the refinery.
CONTAMINANTS
Information is available on waste treatment plant effluent characteristics and how the effluent can be recycled to water users in a refinery. 2 Contaminants such as hardness, solids, phosphates, and total organic carbon (TOC) are addressed in two ways: regarding both how much will be in the effluent, and how much will be acceptable as refinery influent.
But suppose a refiner wants to recycle a stream internally rather than process the stream at the waste treatment plant as part of the total water, then bring it back into the refinery. There is little information available on recycling sour water stripper bottoms to the desalter.
Contaminants in effluent streams from various process and utility units must be examined if internal recycle is to be considered. One must determine whether those contaminants are acceptable and to what level. If necessary, a treatment method must also be determined.
Contaminants from various units may include hydrocarbons, sulfides, cyanides, mercaptans, and ammonia, in addition to the contaminants already discussed.
PROCESS UNIT WATER
Each refinery process unit has a characteristic process water stream. Water-using units require water of specific quality, and water producers have typical major contaminant profiles.
The following is a discussion of various process units. The list is not intended to be comprehensive, but rather a starting point for discussions.
Pertinent information for each stream includes contaminants, possibilities for recycle or reuse, cleanup requirements, and effects on cooling systems and cooling system treatment if the stream is to be used for cooling system makeup.
CRUDE UNIT
Water is used at the desalter to wash the crude. The water should be low in salts (specifically chlorides) and suspended solids, and should have a low scaling potential. Occasionally, lowhardness clarified water is used for the entire makeup, but more frequently a lowerhardness water is used, such as boiler feedwater or a combination of boiler feedwater and clarified water.
Stripped sour water is used in many plants as desalter wash water. Occasionally, nonstripped sour water is used for a portion of the desalter water. The sulfur content of the crude affects the amount of nonstripped sour water that can be used.
In some refineries, it is possible to recycle some desalter effluent water back with desalter inlet wash water. Generally, desalter effluent brine has not reached saturation. Betz ProChem has treated refineries having up to 50% recycle of desalter water. However, certain types of crude may preclude this.
Desalter effluent water goes to the waste treatment plant. The major contaminants are hydrocarbon (free and emulsion), soluble salts, and suspended solids,
In many refineries, additional wash water is injected into the crude tower overhead upstream of the overhead condensers to minimize process-side corrosion of those bundles. Steam from stripping the crude in the distillation tower and side stream strippers is condensed in overhead condensers and collects with the wash water in the accumulator water boots.
The condensed steam and wash water are contaminated with hydrocarbons, H2S, NH3, and various acids and acid salts. This water may go to the sour water stripper or waste treatment plant, or it can be reused at the desalter.
In some cases, a portion of this water can be recycled to the upstream side of the overhead condensers as part of the makeup to the wash water system.
VACUUM UNIT
Overhead wash water and steam stripping operations are similar to those in the crude tower. The resulting water is similarly contaminated with hydrocarbons, H2S, NH3, and various acids and acid salts, and can be reused in the same way.
GAS PLANT
If wash water systems are used in the gas plant, the resulting water is contaminated with light hydrocarbons and H2S.
SULFURIC ACID ALKYLATION
Reacted product is treated with caustic and then washed with water to remove any residual caustic. The water used for caustic dilution and washing is usually condensate or boiler feedwater, although filtered water is also a candidate. There is normally a small bleed to the sewer from the caustic system to maintain the pH or caustic concentration.
Sometimes a small amount of esters and caustic are present in the effluent water from the water wash. Although small in volume, this effluent may be a good candidate for water reuse in a neighboring unit, or as makeup to the cooling system.
FCC UNIT
Steam may be injected with the feed to the fluid catalytic cracking unit (FCCU) reactor, and/or directly to the riser, to ensure good atomization. Condensed steam is collected in the accumulator water boots downstream of the main fractionator.
The water usually goes to the sour water stripper. It is contaminated with hydrocarbons, sulfur compounds, and NH3.
Wash water is injected upstream of the main fractionator overhead condensers and compressor intercoolers to help protect them from process-side corrosion. That water also accumulates in the accumulator water boots. Boiler feedwater and condensate are used.
Assuming a low ammonia content, sour water stripper bottoms can be considered as a makeup source for this system. It may be possible to eliminate water entirely with the use of process-side additives.
Contaminants found in the produced water include cyanides, sulfides, ammonium chlorides, and phenolic compounds (phenols, cresols, xylenols, etc.). Because the hydrocarbon becomes cleaner as it progresses downstream, it is sometimes possible to cascade wash water.
Wash water injected at the wet gas compressor intercoolers will accumulate downstream. This water could be pumped to the overhead water wash system upstream of the main fractionator overhead condensers.
HYDROCRACKER
Water injection may be used downstream of the reactor to remove ammonia salts (ammonium chloride and ammonium hydrosulfide). Contaminants picked up by this water include hydrocarbon, NH3, and H2S Generally, the water used is steam condensate or deaerated boiler feedwater.
HYDROTREATER
Wash water may be injected into the reactor effluent upstream of the overhead condensers to control ammonium chloride. Steam condensate or deaerated boiler feedwater are frequently used.
Stripped sour water is a possible source of wash water, depending on the source of the water fed to the stripper tower and tower operation.
HYDROGEN UNIT
Wash water systems may be used on hydrogen units.
DELAYED COKING UNIT
A great deal of water and steam may be used on the delayed coking unit: quench water, cutting water, and stripping steam. Water used for quench can be oily, but must be low in salts (the salt level depends on the grade of coke produced). Salts increase the amount of shot coke produced.
Frequently, cutting water is recycled within the unit. Sometimes, oily waste treatment effluent water is used as makeup to the recycle system.
Excess water from the recycle system is sent to the waste treatment plant, where it can cause emulsification. Sour water stripper bottoms are often a good source of makeup to the recycle system.
The fractionator at the coking unit is similar to the distillation column at the crude unit or the main fractionator at the FCCU. Steam is used for stripping and a water wash system is used on the overhead system, The contaminants in this water are also similar to those of the crude unit and FCCU main fractionator.
SOUR WATER STRIPPER
Stripped sour water is frequently used in the desalter as total or partial makeup. Most of the H2S and NH3 have been removed. Phenol removal is poor in the sour water stripper and can lead to problems at the waste treatment plant if that is where the bottoms are treated next.
When sour water stripper bottoms are used in the desalter, the phenols are reabsorbed in the crude. This reabsorption is beneficial when the desalter effluent is sent to the waste treatment plant.
LUBE OIL/WAX
A deasphalter process water wash can be included in lube oil and wax production.
MTBE UNIT
A water wash system can be used to clean up methyl tertiary butyl ether (MTBE) raffinate. This water is contaminated by methanol and oxygenates.
STORM WATER
In some areas of the U.S. storm water can comprise as much as 50%, on average, of the water being treated at a refinery's waste treatment plant. because rainfall is not a constant, this can present considerable problems in waste treatment plant operation.
Holding ponds and equalization basins are necessary for stabilizing flows. If the rain can be segregated in areas of the refinery that are not particularly oily, this water may be reused with minimum treatment.
COOLING SYSTEM WATER
In many refineries, open recirculating cooling comprises about 50% of the total demand for makeup water and, consequently, is the single largest water user in the refinery. Water sent to the cooling system does not need to be as contaminant free as water sent to many other refinery water users, such as boilers and process stream water washes.
For these reasons, the cooling system is the easiest and most likely place to send reuse water. ft is important to recognize that the contaminants in these reuse streams have an effect on the operation of the cooling system as a whole.
Some cooling systems are receiving makeup water containing small amounts of reuse water in the form of boiler blowdown and reboiler condensate. The effect of the contaminants in these streams on corrosion, scaling, fouling, and microbiological growth in the cooling system is often overlooked.
In most systems, a small amount of reuse water, such as boiler blowdown or reboiler condensate, can be handled by practically the same chemical treatment program that would be used to treat the system without these recycle streams.
However, as reuse streams begin to comprise a larger percentage of the makeup, a greater number of contaminants, at higher levels, will enter the system, thereby having a more significant effect on system characteristics. The chemical treatment program must be adjusted accordingly.
To reuse streams most cost-effectively, it is important to understand the following:
- How each contaminant affects corrosion, scaling, fouling, and microbiological growth in the cooling system
- At what level of contaminant the effect becomes significant
- How the various contaminants interact
- How to treat the system to minimize the effects of these contaminants
- At what point treatment is no longer a viable, economical alternative.
Table 1 contains a list of possible contaminants found in refinery process water streams and parameters on their acceptability in cooling systems. Its contents can be used as a starting point in determining the practicality of recycling water from a refinery process to a cooling system.
Parameters on reuse water are meaningful only when applied in context. Process wash water with a light hydrocarbon content of 100 ppm might be acceptable as 10% of the makeup water to a cooling system operating at 4 cycles of concentration. (At 4 cycles of concentration, blowdown has four times the solids concentration of the makeup; i.e., 10% x 4 x 100 ppm = 40 ppm, which is less than the 50 ppm maximum.)
However, the same process wash water stream containing the same level of contaminant would not be acceptable as 50% of the makeup water to the same cooling system operating at 8 cycles (50% x 8 x 100 ppm = 400 ppm).
In Table 1, reuse parameters are given in terms of the component as theoretically cycled in the system. This allows cycles and makeup streams to be combined.
Once cycles are determined, the total makeup water contaminant limit is calculated by a simple formula: the limit divided by the number of cycles. The level of contaminant allowed in the recycle stream is then simply a weighted average the smaller the percentage of the total makeup, the larger the allowable level of contaminant.
Alternatively, once average makeup water quality is determined, cooling system cycles can be determined by how many times the most limiting component can be cycled theoretically before reaching its limit.
Table 1 summarizes the levels of various contaminants typically found in cooling systems and maximum levels that can be successfully treated. The effects the contaminant will have on the cooling system (corrosion, scale, fouling, microbiological growth) are noted along with the type of cooling treatment required and additional comments, where applicable.
If more than one contaminant is close to the limit, the cooling system is more difficult to treat. Because of variabilities in cooling systems, water, and contaminant interactions, caution must be observed when using these guidelines.
For typical makeup water, such as well, surface, or city water, most contaminants are present at levels of < 0.1 ppm or levels are not measured, because in "normal" makeup waters, the contaminant level does not significantly affect the cooling system or treatment program.
Normally, water treaters are concerned primarily with contaminants such as calcium, magnesium, silica, conductivity, alkalinity, pH, suspended solids, phosphate, iron, and manganese. Familiarity can make it easy to forget that these are contaminants, especially in cases such as calcium and alkalinity, where a minimum level of contaminant actually benefits cooling system treatment.
The maximum level noted for each contaminant is the level that can be successfully controlled, based on field experience and lab data. In some cases, such as H2S and benzene, the level that can be controlled is likely to be higher than levels that the refinery safety or regulatory department would deem acceptable. Safety and regulatory considerations must take precedence.
In some cases, under some very restrictive conditions, even higher levels of that contaminant can be successfully treated. For example, at low calcium levels, sulfate levels higher than 5,000 ppm can be controlled.
Potential problems in cooling systems are loosely grouped into three areas: corrosion, fouling (including scale), and microbiological growth. The areas most likely to be affected by each contaminant are listed in the table.
The general types of chemical treatment programs that address these problem areas are: inhibitors (to minimize corrosion), dispersants (to control scale and fouling), and biocides (to minimize microbiological growth).
In the case of some contaminants, nonoxidizing biocides are specified because of their greater efficacy in systems with this type of contaminant. Surfactants are recommended in some cases to enhance the effect of biocides and minimize the effect of hydrocarbon fouling.
The following points should also be noted in reference to Table 1:
- The limits are in terms of what each component would be in the system, if it cycled up theoretically. The fact that a component may not actually cycle in the system as it would cycle theoretically does not raise the limit. For example, the limit for light hydrocarbon is 50 ppm, as theoretically cycled in the system. If the system is at 5 cycles, the limit for the total makeup to the system would be 10 ppm light hydrocarbon. The fact that 50 ppm may never be measured does not affect the limit.
- The limits do not take into account any safety considerations. They also do not take into account any air pollution restrictions, water discharge restrictions, or restrictions on drift. The limits are strictly based on the ability to successfully minimize corrosion, scaling, fouling, and microbiological growth.
- The effects are cumulative. Frequently, systems can be successfully treated when more than one contaminant is at the maximum. However, this is not always the case. A system with 5,000 ppm chlorides and 50 ppm light hydrocarbon (both contaminants at their limit) can be successfully treated. But on the other hand, although a system with 1,500 ppm calcium can be successfully treated, and a system with 50 ppm orthophosphate can be successfully treated, a system with both 1,500 ppm calcium and 50 ppm orthophosphate would be extremely difficult and expensive (if not impossible) to treat.
Some contaminants may limit the type of chemical treatment program that will be appropriate. For example, the orthophosphate limit is listed as 050 ppm. In a system with 50 ppm orthophosphate, an alkaline program (pH 8) may not be appropriate.
The concluding article in this series will describe commonly used treatment options for removing specific process water contaminants.
REFERENCES
- NUS Corp., Manual on Disposal of Refinery Waste, Chapter on Water Use Optimization, Pittsburgh, 1986.
- Puckorius, P.R., and Hess, R.T., "Wastewater Reuse for Industrial Cooling Water Systems," Industrial Water Treatment, 1991.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
