New trends in oil refinery wastewater reclamation

Six of the 10 largest companies in the world are oil companies. This fact would support the argument that there may be no industry more competitive than the oil industry. Within oil companies, the toughest challenges lie within the refining divisions. Extreme pressures exist to expand capacities, produce products which generate market advantages, improve upon environmental considerations and, last but not least, to reduce or optimize operating costs. In reviewing recent SEC Quarterly Reports (10-Q) for several of the large oil companies -- as well as studying historic gas price trends compared to crude oil prices -- it appears at least at present that the oil industry is one of the major industries least affected by the current U.S. economic recession. If downward economic trends continue, however, sooner or later it will be significantly impacted.

Considering the above conditions, perhaps there's no better time for oil refineries to step up investment into capital projects capable of reducing operating costs with attractive return on investment schedules. Steadily accelerating operational costs in an oil refinery in the environmental areas are highly visible on management’s radar screen for target profit opportunities. Unfortunately, environmental departments are cost centers in refineries -- they aren't profit centers. As such, these departments frequently don't have sufficient resources to really produce step improvement with significant impact on the financial bottom line. A great example would be centralized wastewater treatment operations in a typical refinery. These departments often struggle trying to get financial, manpower, and interdepartmental support from the finished product department profit centers. Meanwhile, the profit centers such as the Crude Unit, Vacuum Unit, Cat Cracker, Hydrotreating, Coker, etc., are incurring massive expenditures on the supply water side. Supply of desalter wash water, cooling tower makeup, and boiler feedwater alone -- with related pretreatment costs -- represent major budgetary requirements.

Consider these numbers:

The average refinery requires 2.5 gallons of water for every gallon of crude oil processed.
A typical large refinery will process 8-10 MGD of centrally collected “dirty” process wastewater. Usually this doesn't include stormwater and sanitary sewer, which further add to the treatment burden.
All things considered combining the supply and discharge water sides, a large refinery will incur in excess of $100,000 per day for its total water related expenditures.

Over the years, a number of refineries have examined the possibility of localized capture of generated wastewater at the sources, i.e., at the finished product units themselves, such as the Crude Unit Fractionating Column Overhead Water Accumulator. They've looked at the concepts of potential decentralized wastewater treatment for improved efficiency, as well as the possibility of reclamation and reuse of the wastewater as supply water. In general, based on available technologies at the time, most refineries considered these options to be cost prohibitive. In particular, the idea of having personnel trained in producing finished product, trying to operate difficult wastewater process control at a location such as the Crude Unit, seemed to add significantly to operational costs, let alone considering the potential for disaster under wastewater upset conditions which are quite common due to the varying nature of refinery contaminant profiles.

The wastewater originating at the Crude Unit Overhead is one of the larger volumes of oil refinery process wastewater generated. The major difficult to treat contaminants in this stream are petroleum hydrocarbons, ammonia, and organic sulfur. Other than these three, the characteristics of this stream resemble a highly desirable supply water profile, similar to steam condensate. Historically, conventional treatments for this stream of wastewater have been cost prohibitive with respect to reclamation and reuse as process supply water. Innovative process engineering in conjunction with technological advances in process control of biological treatment of these three major contaminants have made zero discharge of Crude Unit Overhead water cost effective for reuse with internal utilities. Especially in these tough economic times, overall plant operating cost reductions, utilizing innovative technologies such as this, build market competitiveness. The resulting reduced discharge load to the environment is a most welcome windfall.

In the last seven years, several oil refineries have had success in localized capture and reuse of refinery process water with designs similar to the illustrated process in the flow diagram in Figure 1. This particular design was first used by Petrobras of Brazil.

Table 1 is an illustration of the typical characterization of wastewater generated from the Crude Unit Overhead.

The obvious focus of this process falls on the three-stage biological removal reactors. In the first stage, the most successful type of refinery wastewater biological technology, complete mix activated sludge, is utilized for removal of the oil and grease and related petroleum hydrocarbons. The choice of a sequential batch reactor (SBR), with its automated cycles controlled by online effluent contaminant level instrumentation, fulfills the Crude Unit’s personnel needs of a system that does not require intensive expertise in biological wastewater process control. A battery of two SBRs, each alternating between discharge and fill modes, satisfies the requirement of not starving out the downstream Bioprocesses in terms of substrate. Newer technology membrane bioreactors (MBR) are probably not a good choice for this application due to the potential for high maintenance related to membrane fouling from the insoluble oil and grease.

In the second stage, a submerged fixed film biological reactor is utilized. The primary focus of this stage goes after the organic sulfur compounds such as mercaptans, by employing a sulfur oxidizing bacteria population in conjunction with hydrocarbon degraders. This reactor design has no moving parts outside of the diffused air supply. Thus, a most operator friendly bioreactor is realized. Although some sulfur oxidation does occur in the SBR, it generally would be insufficient to produce effluent quality required for the goals of this resulting stream. Of significant note, some additional hydrocarbon oxidation does occur in the second stage, particularly in the category of longer chain hydrocarbons, double- and triple-bonded hydrocarbons, and additional families of hydrocarbons with difficult to degrade functional groups besides sulfur, which require additional retention time for complete degradation.

Finally, in the third stage, standard submerged fixed film biological reactor ammonia removal is achieved via nitrification. Although considerable ammonia removal will occur in the first stage reactor via the nutrient uptake mechanisms inherent during hydrocarbon substrate utilization by the carbon degrading bacteria, it wouldn't be advisable to try to nitrify there, since the required mean cell residence time (MCRT) for efficient nitrification would conflict with that needed for efficient hydrocarbon oxidation, especially in an automated batch reactor configuration. Because of the high ammonia load associated with this particular type of influent, additional ammonia removal is required, and nitrification in this third-stage reactor serves the overall process well, since the major nitrification inhibitors have already been removed in the first and second stages. Although this nitrification design by itself has been well documented, the extra protection of the delicate nitrifier bacterial population afforded to this overall process on the front end by stages 1 and 2 insures a consistently smooth running process. This type of front end Nitrifier protection in fact has not been standard in most refinery wastewater treatment plants, and arguably is the root cause of the most frequent types of upsets in refinery wastewater biological plants.

As far as the end of the entire treatment process goes, the most efficient and cost effective uses of the successfully treated effluent would be, by priority, for low pressure boiler feedwater, followed by cooling tower makeup, followed by high pressure boiler feedwater. These standard options are depicted in the process flow diagram above. Any off-specification effluent can easily be captured, diverted, and used for desalter wash water supply with this process design.

So, what about cost justification and return on investment? Obviously the basis for these financial calculations will vary considerably due to the number of permutations that exist. For example:

Supply Water: Availability of well or freshwater vs. purchased treated municipal water.
Crude Feedstock Contaminant Loading: West Texas Intermediate vs. Dubai Fateh.
Existing Oil Processing Units Configuration: Gasoline producer vs. lubricants producer.
Water Pre-Treatment Systems Already in Place: Boiler feedwater ion exchange vs. reverse osmosis.
Existing Wastewater Treatment Configuration: NPDES permit vs. surcharged dumping to municipality.

The key financial parameter in looking at the metrics of potential success for this investment will ultimately come down to this: The cost of operation of the three-stage biological reactors vs. any combination of substitute technologies capable of producing the same effluent quality. In following this financial investigative pathway, we have found the following general numbers to be representative in the case of most large refineries producing the full spectrum of finished products:

$0.017 per 1,000 gallons treated water using the three-stage biological reactors (total in-use treatment cost).
$0.944 per 1,000 gallons treated water using next best available technologies meeting same treatment criteria as the three-stage bioreactors.

-- Not including cost of activated carbon disposal or regeneration, replacement chemical consumables such as in permanganate, peroxide, breakpoint chlorination, or alkaline stripping treatments, etc.

Utilizing the above numbers in conjunction with an average operational profile from an intermediate or large size refinery, a good estimate for recouping the total investment cost for a treatment system such as this would be in the neighborhood of under three years. This figure would not include the additional savings justifications also realized from the partial capture and containment of the inherent energy (BTUs) inputs from the oil processing unit itself, in this case, the heat required for Crude Unit distillation.

Based upon qualitative analysis, how did this particular refinery benefit from use of this innovative treatment approach?

~300 GPM of water was removed from the centralized wastewater plant, thus reducing disposal treatment costs and discharge load to the environment.
~250 GPM of water of steam condensate quality was captured for use as low pressure boiler feedwater and cooling water makeup for condensers and heat exchangers, thus reducing supply water treatment costs and improving the in-use cost effectiveness of the boiler and cooling water operations.
Significant energy savings were realized by internally containing some of the BTU value associated with the heat transfer to the distillation column water. Instead of simply disposing of the BTU input to the water by routing it to the wastewater plant, this energy was partially recovered at the inlet point to the boiler feedwater in terms of reduced deaerator heating requirement fuel costs.

NOTE: A large U.S. Refinery is currently modifying this existing treatment system design by replacing the cooling water at the equalization tank heat exchanger with effluent from the end-of-the-pipe in this treatment system, which is on its way to the boiler feedwater influent blend. This innovation will capture substantial energy savings in an amount equating to the BTUs associated with an approximate 40°F (4.4°C) delta T. This improvement alone could accelerate the return on investment by more than 30%.

This innovative wastewater reclamation process not only creates a zero-discharge treatment alternative, but also creates very large operational savings for both water processing and energy conservation. Both could prove most beneficial to a number of oil refineries. In addition to the reclamation of Crude Unit Overhead water, several other sources of wastewater originating at various refinery unit processes also have potentially attractive profiles for this same treatment approach as well. Those processes most notably would include vacuum unit overhead water and sour water stripper bottoms water. Even water sources containing cyanides, such as those originating at the fluid Catalytic Cracker and the delayed Coker, have potential for successful treatment with this system design. In this latter case, the bioreactor configuration can easily be modified to biodegrade the simple cyanides. The remaining complexed cyanides could subsequently be removed via the existing boiler pretreatment ion exchange system. It's likely that a cyanide removal step in this design would in fact not require additional capital equipment. At any rate, the starting point for any refinery interested in water reclamation projects would be to conduct a refinery-wide "Material and Heat Balance" assessment on all water streams including financial profiles on each stream. The result of such a study would clearly identify the most bang-for-the-buck for the refinery’s global water management program.

Acknowledgment: The author wishes to thank Petrobras Refining, of Brazil, for its cooperation with this treatment project.

About the Author: Dave Kujawski, of Refinery Water Engineering & Associates, of Corona, CA, has more than over 30 years of industrial water treatment experience. He has worked on refinery water projects in 40-plus facilities around the world, including experience on the inside of refineries as a process engineer as well as an engineering consultant from the outside under contract. Specific areas of refinery expertise include wastewater, supply water, boiler water, cooling water, and process water treatments, as well as remediation of contaminated soils and groundwater. Kujawski has an undergraduate degree in environmental engineering and an advanced degree in marketing. He has worked for several of the largest water treatment companies in technical field support, sales & marketing, business development, and personnel management. Contact: 800-934-0881 or [email protected]