Laurence R. Poche
BP Oil Co. Belle
Chasse, La.
Richard E. Derby, David R. Wagner
BP Research
Warrensville, Ohio
Solvent extraction for treating refinery sludge has been successfully demonstrated on a commercial scale at BP Oil Co.'s Alliance refinery at Belle Chasse, La. The BP Oil solvent extraction process was developed to provide a cost-effective oily waste recycling process as an alternative to incineration.
Data from this operation were submitted to the U.S. Environmental Protection Agency and became a factor in EPA's decision to designate solvent extraction as a best demonstrated available technology (BDAT) treatment option for refinery sludge.
Modified versions of the solvent extraction process also may be suitable for treating nonrefining oily wastes such as exploration and production wastes and contaminated soils.
DRIVING FORCES
Oily wastewater streams are generated at a number of refinery processing units and discharged to a wastewater treatment system where oily solids streams are generated by gravity separation, dissolved air flotation (DAF), and other primary treatment processes. These oily solids, along with slop oil emulsion solids, heat exchanger bundle cleaning solids, and leaded tank bottoms, are regulated under the Resource, Conservation, and Recovery Act (RCRA) as listed hazardous wastes.
As authorized under RCRA, BP Oil Co. refineries currently deliquefy these wastes and either land dispose or land treat the residual solids. However, the 1984 Amendments to RCRA required EPA to identify and promulgate pretreatment criteria based on BDAT before land disposal of hazardous wastes.
Congress initially required the pretreatment standards for refinery oily wastes to be in effect by Aug. 8, 1988. But due to a lack of available treatment capacity, compliance was postponed until Aug. 8, 1990.
EPA revised the BDAT standards on May 8, 1990, and compliance was further postponed to Nov. 8, 1990. BP's five U.S. refineries and the rest of the refining industry must comply with these regulations.
A typical 150,000 b/d refinery generates 15,000-20,000 gal/day of oily wastes. As generated, these wastes are usually 4-6% solids and 1520 wt % oil with the remainder water.
Particle size of the solids can range from sub-micron to 1/8 in., although most particles are in the 1-1 00 m range. The composition of the oil depends on the crude oil slate, refinery configuration, and refinery operation. The composition of the sludge can vary with time.
Deliquefication followed by incineration was expected to be the primary oily waste disposal technology identified by EPA. BP Oil recognized that while incineration was the leading technology at the time, it was burdened by a bad public image and permitting difficulties. Investigations were therefore initiated into alternatives to incineration of oily waste.
Alternative processes examined included fixation of deliquefied waste, land treatment, wet oxidation, coking, and solvent extraction.
BP Oil was looking for a cost-effective technology that could potentially treat oily wastes to meet limits that would keep them off of EPA's list and be used as a benchmark in establishing BDAT standards. The technology would need to be capable of handling fine solids and be robust toward variability in the feed hydrocarbon composition.
Waste reduction or recycling processes were preferred to disposal processes, but recycle could not have an adverse effect on the quality of the refined products. Because BP Oil has a strong preference to maintain complete control of its hazardous wastes, an on site process that easily could be integrated into refinery operations and minimize permitting problems was desirable.
PROMISING TECHNOLOGY
An in-house solvent extraction process, which in 1985 was advancing to the pilot plant stage, was identified as the alternative most likely to meet BP Oil's criteria. Two extraction schemes were being investigated: single-cycle extraction, which contacts the solids with a light hydrocarbon solvent, and multi-cycle extraction, which employs two or three successive extractions with different solvents.
Pilot testing and process evaluations indicated that the BP solvent extraction process would reliably handle the fine solids and feed variability, producing treated solids which could meet delisting and BDAT limits expected to be set by the EPA.
Economic evaluations indicated that the process would have capital and operating costs lower than incineration. BP Oil, therefore, decided to build a full size demonstration plant at the Alliance refinery.
THE PROCESS
A block flow diagram of the Alliance refinery wastewater and oily sludge treatment systems is shown in Fig. 1.
The main sources of oily sludge are the API separator, the DAF unit, and the slop oil recovery system. The refinery has storage tanks to accumulate oily sludge prior to treatment.
BP Oil's solvent extraction process is a union of three unit operations: mechanical treatment for deliquefication, solvent extraction for residual hydrocarbon recovery, and chemical fixation for immobilization of metals.
MECHANICAL TREATMENT
The first step in the process is deliquefication mechanical treatment that removes most of the oil and water from the oily waste. Mechanical treatment produces a solid cake that is processed in the solvent extraction step.
Many refineries use mechanical treatment facilities such as filter presses, vacuum filters, belt presses, or centrifuges. Most of BP Oil's pilot and demonstration-scale extraction work has been done on cake prepared in a recessed plate filter press, but work is currently under way to demonstrate that extractable cake can also be produced by other mechanical treatment methods.
The objective is to utilize existing facilities wherever possible.
In a typical filter press operation, oily waste contained in the refinery's temporary storage facilities is pumped to intermediate tankage where filtration aids are added and the mixture is homogenized. The conditioned sludge mixture is then filtered in a recessed plate filter press.
The maximum operating pressure of the press is 150 psig, and the operation is conducted at temperatures ranging from 100 to 150 F.
During filtration, solids accumulate in the filter press chambers while the oil and water removed from the system are decanted. The recovered oil is returned to the refinery for reprocessing, and the filtrate water is returned to the API separator for further treatment.
Removal of excess water by mechanical treatment prior to solvent extraction eliminates the need for a separate wastewater stream downstream of the extraction unit and simplifies permitting. Filter press operation has been automated and integrated with the extraction unit to minimize turnaround and operator time.
SOLVENT EXTRACTION
The central operation of the process is a batch extraction. This process is designed to remove residual oil by contacting the deliquefied sludge with hydrocarbon solvents.
The solvents are intermediate or product streams produced by most refineries. Proprietary design features of the extraction vessel ensure efficient contacting, minimal solids entrainment in the extract stream, and simple and effective removal of de-oiled solids from the extraction vessel. Design provisions have also been made which effectively handle upset or unusual conditions.
The single-cycle extraction process utilizes a light hydrocarbon such as propane as the solvent. Single-cycle extraction with propane provides excellent bulk oil removal, but is somewhat less effective than the multi-cycle process for removal of heavy organics like polynuclear aromatics (PNAs).
The multi-cycle extraction process utilizes a series of extractions with different solvents. The processing sequence depends on sludge characteristics, solvent availability, recycling considerations, and treatment requirements.
In a typical three-cycle process, the first cycle removes the bulk of the oil from the sludge with a light hydrocarbon solvent. The second cycle employs a heavier solvent to remove polynuclear aromatics.
The third cycle uses a light hydrocarbon to remove residual second cycle solvent and provide final polishing. A two-cycle process which omits the first light hydrocarbon extraction has also been demonstrated.
Most of BP Oil's solvent extraction experience has been with propane as the light solvent and reformate as the heavy solvent. But good pilot plant results have been attained with other refinery streams as well.
After extraction, the deoiled solids are slurried with water and pumped to the fixation step.
FIXATION
The third unit operation of the process is fixation, which reduces the leachability of metals in the extracted sludge. Fixation agents are added to the extracted sludge slurry, and the mixture is dewatered in a conventional filter press.
Dewatering is accompanied by pozzalonic reactions which immobilize metals and other constituents in the product residue. The primary differences between the filter press operations for metal fixation and initial deliquefication are the type and amount of additives employed. The extracted solids de-water very easily.
The filtrate water is recycled to the extraction unit for re-use in slurrying solids from the extractor. Downstream of mechanical treatment, the process is a net water user, thus avoiding the problems associated with disposal of a wastewater stream.
SOLVENT RECYCLE OPTIONS
Used solvents can be recycled directly to the refinery, or facilities can be added to recover and recycle them within the extraction system. Process designs have been developed for a flash system to recycle light hydrocarbons, and a distillation system to recycle the heavier solvent.
Integration of solvent recovery with the refinery can reduce costs significantly and improve process economics. Each installation needs to be optimized on a case-by-case basis.
DEMONSTRATION PLANT
A commercial-scale demonstration plant with a capacity of 7.5 tons/day of filter cake was constructed and operated at BP's Alliance refinery. The demonstration plant included a full-size extraction vessel, but did not include automation, solids handling equipment, or solvent recycle facilities desirable for full-time operation. The unit will be upgraded for full-time operation after the land disposal ban goes into effect.
The demonstration plant was constructed as five mod-ules: feed preparation, extractor vessel, hydrocarbon solvent storage, solvent pumping, and water storage and pumping. The modular design allowed rapid field construction and commissioning.
The final three units arrived at the plant site Nov. 11, 1987, and construction was completed by Dec. 10, 1987. The plant was fully commissioned by Dec. 23, and the first solvent extraction test was completed Dec. 29, 1987.
The Alliance demonstration plant was designed and constructed to normal refinery specifications, and passed a refinery safety "what-if" review prior to commencing operations. The extraction unit was started up and operated by refinery operators under the supervision of a process engineer.
A contractor supplied and operated recessed plate filters for mechanical treatment and fixation.
Three oily hazardous waste materials account for more than 90% of the sludge produced at the Alliance refinery: API separator sludge (KO51 on EPA's list), DAF float and bottoms (KO48), and slop oil emulsion solids (KO49). The remaining 10% of the sludge generated is comprised of intermittent streams including leaded tank bottoms (KO52), heat exchanger bundle cleaning solids (KO50), and miscellaneous oily sludges (nonleaded tank bottoms).
The sludge extraction unit was operated in both the single-cycle and multi-cycle modes. The light hydrocarbon solvent used in the demonstration plant was propane, and the second solvent used in multi-cycle runs was reformate.
Other refinery streams have subsequently been tested as solvents in the pilot plant with good results. The oil extracted from the sludge was successfully recycled to the refinery along with used solvent.
The recovered extract oil was of good quality, typically with less than 0.25% water by volume and 100 ppm solids. Twenty-three batch runs were made in the 1988 demonstration plant campaign. Both the single-cycle and multi-cycle modes were tested.
Forty-seven multi-cycle runs were completed in the 1989 campaign. In all, 220 tons of dewatered feed cake (derived from 1,534 tons of raw sludge) were processed.
In 1988, the raw sludge feed was drawn entirely from pond inventories. The raw sludge feed for the 1989 campaign was drawn from both pond inventory and from "fresh" sludge generated by the refinery.
The demonstration plant is being upgraded for full-time operation, and treatment will resume after the land disposal ban deadline.
PROCESS PERFORMANCE, REGULATORY POSITION
Evolution of EPA's BDAT standards for oily sludge treatment is shown in Table 1. Performance standards initially were set by EPA in August 1988.
Results from BP's first demonstration run contributed to acceptance by EPA of solvent extraction (in addition to incineration) as BDAT treatment for refinery sludge. The initial standards set low levels for polynuclear aromatics, but did not address xylenes and naphthalene.
Neither solvent extraction nor incineration could consistently achieve the initial BDAT standards for leachable metals.
EPA revised the BDAT standards for refinery sludge in May 1990. The new regulations added standards for xylenes and naphthalene but relaxed the standards for leachable metals and some of the PNAS.
The Alliance refinery demonstration plant performance is compared to the current BDAT standards in Table 2. Note that the demonstration runs were completed before the new BDAT standards were announced.
Both the single-cycle and multi-cycle demonstration results met the current standards for PNAs and leachable metals. However, the new xylene and naphthalene standards set by EPA were somewhat lower than the levels achieved by the demonstration plant.
Process improvements directed toward compliance with the xylene and naphthalene standards have been successfully proven in the pilot plant. Pilot plant results for the modified process are shown in Table 3.
Meeting the BDAT standard for total cyanides has been a challenge with Alliance sludge. Because of the configuration of upstream equipment, the cyanides in Alliance sludge are higher than those at most of BP's other refineries.
Cyanide reduction is being addressed on two fronts. First, modifications to reduce cyanides at their sources within the refinery are being implemented. Second, a post-treatment step to remove cyanides from solvent extracted sludge is being developed.
The Louisiana Department of Environmental Quality (DEQ) has reacted favorably to operation of the process at the Alliance refinery. The Hazardous Waste Division of the DEQ approved solvent extraction as part of the refinery's hazardous waste Part B permit. DEQ also approved an air permit exemption for the Alliance solvent extraction unit.
COMPARISON WITH INCINERATION
Economic comparisons between on site incineration and BP single-cycle solvent extraction show a significant advantage for solvent extraction. The costs shown in Table 4 are based on units with capacity to treat 21,000 tons/year of raw refinery sludge (5 wt % solids).
The BP Oil solvent extraction process has several additional advantages relative to incineration which are difficult to translate into monetary terms. The BP process recovers and recycles oil rather than destroying it.
There is no flue gas stream, so the process can qualify for an air permit exemption. These factors combine to simplify both the administrative and public relations aspects of environmental permitting.
The process is easily integrated into refinery operations. It can usually be adapted to existing deliquefication systems, and the solvents are produced in and recycled to the refinery. Ownership of the unit gives control of waste treatment operations to the refiner.
In addition to refinery sludges, the oil industry has a number of other oily wastes which sometimes pose disposal problems. These materials include exploration and production wastes and materials which result from crude oil and refined product spills.
The BP solvent extraction process can be tailored to treatment of some of these materials. Engineering and evaluation of these applications is proceeding on a case-by-case basis.
When data from the Alliance demonstration plant were submitted for consideration as BDAT, BP Oil assured the EPA that the process would be made available through licensing to others.
ACKNOWLEDGMENTS
The authors thank the operations department and operators; the engineering, design, and construction department; the maintenance department; and the laboratory at the Alliance refinery. They also thank the environmental technologies and technology assessment departments at BP's Warrensville, Ohio, research center.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
