Anne Rhodes
Refining/Petrochemical
Editor
A new process uses chemically assisted, thermal flashing to break difficult emulsions and recover oil for reprocessing. The process is best suited for refinery waste management and slop oil systems, where it can process streams with high oil content to recover high-quality oil.
Recent testing of a full-scale, commercial prototype unit on slop oil emulsions at a major Gulf Coast refinery resulted in:
- 97.9% recovery of oil with 99.3-99.6%
- 99.5% recovery of water with 99+% purity
- A centrifuge cake containing 49-60% solids, 23-30% oil, and 17-22% water.
BACKGROUND
The "Ohsol" process is an emulsion-breaking and separation technology that provides a continuous, low-cost means of completely and permanently separating stable emulsions generated by refinery process operations.
Specifically, the process is designed to break the persistent affinity of solids for oil, and keep the solids from sticking together and forming small "cages" around the oil droplets, says process inventor Dr. Ernest Ohsol. These cages resist strong centrifugal forces and retain unacceptably high levels of occluded hydrocarbons.
UniPure Corp. developed the process, which can recover clean, dry oil, and reduce the volume of solids that must be disposed of. And if a refiner wants to process the recovered solids in a coker, the unit can produce a suitable slurry from the centrifuge cake.
The process is applicable to a wide range of refinery waste materials, including slop oils, desalter "rag" layer, dissolved air flotation float, API separator sludge, crude tank sludges, and spent lubricating oils.
PROCESS DESCRIPTION
A simplified flow diagram of the Ohsol process is shown in Fig. 1.
The process breaks strong emulsions using a two-fold attack:
- The addition, under pressure, of heat (and water, if none exists) to the system to raise the temperature and pressure of the system to at least 1000 C. and 150 psig.
- A quick pressure drop, which flashes off part of the aqueous phase.
The feed emulsions are mixed in a static mixer with an effective amount of surfactant, which acts as a demulsifier. At a system pressure of 150 psi, water is added to the mixture, which is then heated to more than 3000 F. Heating can be either direct, by steam injection, or indirect, by heat exchange, says Ohsol.
After high-temperature, high-pressure blending, a flocculating agent is added using static mixers. The use of a flocculent is particularly desirable for emulsions containing inorganic solids.
The pressure is released suddenly by passing the fluid through a Venturi nozzle, which flashes it and cools the mixture. This process of pressurized heating, followed by flashing, ruptures the small cages that stabilize the emulsion and produces an atomized mist of tiny, evenly divided oil and water droplets.
The temperature drop caused by the flashing also interrupts degradation of the polyamide, or other surfactant, added to enhance solids removal.
This mist is an ideal environment for phase transition. The surfactants promote separation of the phases, and the polymers coat the solids before they can recombine with the Oil. Once the emulsion mixture has been flashed, the oil, water, and solids are separated easily using conventional means, such as a three-phase, Tricanter-type centrifuge. The flashed vapors are condensed, and they separate spontaneously into water and light naphtha.
The process eliminates the need to send slop oils back to the desalter.
Small, stabilized, slop-oil rag layers are prone to sudden expansion in the desalter vessel, requiring rejection with the brine to avoid carry-over to the heat exchangers, crude charge heater, and fractionator. Rejection with the brine protects these facilities, but transfers the problem to the process sewer system and API separators, where emulsion formation is exacerbated.
As an added benefit, benzene and other volatiles are flashed from the incoming feed stream and condensed for recovery. The oily waste stream discharged from the process therefore contains only low concentrations of benzene. The small amount of condensed water may contain regulated contaminants, but this stream can be stripped of these compounds easily, or sent to complying sewer lines.
Because volatile organic compounds such as benzene are removed in the flash step and recovered, the cost of complying with the benzene waste operations requirements (national emissions standards for hazardous air pollutants, or Neshaps) and future hazardous organic Neshaps regulations, is built into the process.
The high flow capability of the process (
PERF STUDY
The Petroleum Environmental Research Forum (PERF) is a group of oil companies that conducts cooperative research under the auspices of the 1986 National Cooperative Research Act. Ten oil companies joined for PERF Project 91-14 to evaluate technologies to minimize the environmental impact of crude oil desalter operations.
Treatment options for the brine discharge and desalter mud wash were evaluated. BP Oil Co.'s environmental technology division was selected as the prime contract researcher, and Mobil Research & Development Corp. provided contract coordination.
In the project's initial phase, "paper" studies of the performance and economics of eight treating technologies were conducted. A generic, 100,000 b/d refinery was used as the basis, as were average desalter stream properties, as determined by a survey of the PERF participants.
In late 1992, a committee comprising a representative from each participating company selected three complementary technologies on which to conduct field and laboratory trials. UniPure's Ohsol process was one of the technologies tested.
In the third quarter of 1993, UniPure conducted field trials on desalter emulsion and slop oil at one participant's refinery--a major Gulf Coast facility. A summary of the results of the PERF tests is shown in Table 1. The purity of the recovered oil was greater than 99%.
The successful trials led to modifications that give the process greater ability to handle varying stream quality and flow rates.
SECOND FIELD TRIAL
In the second quarter of this year, the modified unit was demonstrated on slop oil and desalter rag-layer emulsions at another major Gulf Coast refinery. The feed streams ranged from 5% bs&w to 60% bs&w, with the solids content varying from 2.5 to 17% (Table 2).
Typical results from this test were:
- Oil--
- Water--1.3% oil and grease (no emulsion), with
- Solids--65-71% dry solids, with 14-18% water.
The recovered oil, light ends, and water discharged from the unit were remixed in a common header system and pumped to a recovered-products tank. The refiner tested the tank contents intermittently and verified that the oil and water did not emulsify again.
COSTS
Remediation Technologies Inc., Mandeville, La., performed a study of the economics of the Ohsol process. A 100,000 b/d refinery that does not send solid streams to a coker is projected to save about $2.3 million/year using the process. A refinery of the same size that sends 100% of its solid streams to its coker would save about $780,000/year.
These cost savings stem from three major factors:
- Enhanced recovery of higher-value oil
- Volume reduction of solids for disposal
- Breaking of intractable emulsions, thus avoiding prolonged storage, disposal, or both.
The process has been patented in the U.S. and Europe.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
