WO1994016033A1

WO1994016033A1 - Desalter solvent extraction system

Info

Publication number: WO1994016033A1
Application number: PCT/US1994/000347
Authority: WO
Inventors: Christopher Shallice; Lester R. Young
Original assignee: Cf Systems Corporation
Priority date: 1993-01-08
Filing date: 1994-01-10
Publication date: 1994-07-21
Also published as: CA2148704A1

Abstract

An improvement in crude oil refining is disclosed wherein a desalter (50) is operated so as to produce an organics phase (12) of reduced salts content ready for feeding to a distillation step and an aqueous phase of high salts content. By withdrawing the organics phase (14) from a higher level relative to the rag layer in said desalter (50) and after a relatively longer residence time than in conventional desalting operations, substantially little if any of the rag layer's content from the desalter (50) is included in the withdrawn organics phase (12) and the latter is suitable for feed to a distillation step (64). The resulting heavily organics-contaminated aqueous phase is mixed with an extractant comprising a gas converted to a solvent-condition fluid state under critical or near-critical temperature and pressure conditions to provide substantially organics-free waste water (26) suitable for discharge and to form a fluid extract of organics (32) from which benzene and other oil fractions can be recovered.

Description

DESALTER SOLVENT EXTRACTION SYSTEM

The present invention relates generally to a process for treating crude oil as part of an oil refining process and, more particularly, for improving the desalting operation by employing a secondary processing step wherein the aqueous phase withdrawn from the desalter is processed to remove substantially all organics prior to discharge as waste water. It is well known in the oil refining art to employ a desalting step for separating certain solids and water soluble components, especially brine, from the crude oil. Such a desalting process is described, for example, in U.S. Pat. Nos. 3,798,153 (Arndt et al.) and 4,684,457 (McKechnie et al.). Typically in the desalting operation, fresh water or a low-salinity brine is added to a stream of crude oil, the combined stream is mixed and heated to a temperature of about 250°-300° F, and the heated mixture is fed to a desalter tank. By action of gravity, sometimes aided by electrical means, the mixture in the desalter tank separates into a lighter, oil layer with a reduced salt content and a heavier, aqueous layer contaminated with salts, solids and hydrocarbon components. The oil phase can be continuously withdrawn from the upper region of the desalter tank to maintain steady-state conditions while the contaminated aqueous phase is continuously bled from the lower region of the desalter. As noted in the aforementioned Arndt et al. patent, the desalting step is typically carried out at an elevated temperature to increase the efficiency of the separation of brine from oil, while heat-exchanger fouling is reduced by removing salts before the oil is heated much above 300° F. The oil and aqueous phases in the desalter, however, do not form a clean, sharp boundary. Instead, the two phases are typically separated by so-called "rag" layers comprising brine and solids emulsified with oil. Moreover, these rag layers do not remain stationary but have an undesirable tendency to wander, thereby reducing the efficiency and effectiveness of the desalting operation. To insure good separation and to avoid contaminating the oil phase with contents of the rag layers, the oil phase must be drawn from a point well above the rag layers and at a controlled rate. In conventional processing, some of the desalter rag layers are

SUBSTITUTE SHEET either treated by high chemical addition or are passed along with the aqueous phase or the oil phase, thereby requiring expensive downstream processing. Another problem with the conventional desalting operation is that the aqueous phase withdrawn from the bottom of the desalter typically contains a significant proportion of dissolved or entrained organic components. These organic components may include both certain lighter petroleum fractions, such as benzene, as well as heavier hydrocarbons. Recovery of these organic components is desirable not only because of their economic value but, in addition, because discharge or disposal of such contaminated water is ecologically detrimental and may be illegal. Thus, it is conventional to subject the aqueous phase withdrawn from the desalter to downstream treatment to remove most of the organic components and to render the brine suitable either for disposal or recycling. The aforementioned McKechnie et al. patent, for example, describes a cross-flow membrane separator process for treating the oil-containing brine withdrawn from a desalter. While it is known, as taught by U.S. Patent No. 4,568,447 (Pujado) to employ a solvent in the supercritical state for the removal of trace quantities of organic compounds from an aqueous stream, such a process has not been used for downstream processing of the aqueous phase coming from a desalter, nor would such treatment, by itself, solve the problems of an inefficient desalter operation and a partially contaminated oil phase feedstock from the desalter. Another problem with the conventional desalting operation is that, in order to achieve reasonable cost-effectiveness, the rag layer is maintained at a relatively fixed level and the oil phase is withdrawn at such a rate that the oil phase still contains enough remaining brine to cause downstream processing problems unless that brine level is further reduced by additional treatment. Accordingly, it is well known to subject the oil phase coming from the desalter to additional processing prior to the flashing, furnace and distillation steps of a conventional oil refining operation. The aforementioned Arndt et al. patent, for example, describes heating partially desalted oil by injecting a hot fluid into the partially desalted oil immediately prior to the flashing step. This process substitutes for heating the oil phase by the more conventional means of a heat exchanger, which would be subject to heavy fouling by the salts remaining in the partially desalted oil. While¹ it is known, as taught by U.S. Pat. Nos. 4,522,707 (Kriegel et al.) and 4,797,198 (Wetzel et al.) to treat used or salvage oil with a gas under supercritical conditions as part of a purification and reconditioning operation, such a process has not been used for downstream processing of the oil phase from a desalter, nor would such treatment, by itself, solve the problems of an inefficient desalter operation and a contaminated aqueous phase raffinate. These and other problems with and limitations of the prior art desalting and refining operations are overcome with the desalter solvent extraction system of this invention. Specifically, the system of this invention improves the efficiency of the desalter operation, eliminates the need for secondary treatment of the oil phase feedstock prior to distillation, and provides a less-contaminated aqueous phase at lower cost than conventional secondary treatment for the aqueous phase raffinate from a desalter. Accordingly, it is a general object of this invention to provide a more efficient and effective crude oil refining process. Another general object of this invention is to improve the efficiency and effectiveness of the desalter operation in crude oil refining. Another object of this invention is to provide from a desalter an oil phase sufficiently free of brine and other contaminants so that deposition of such contaminants in the crude preheating tower is minimized. It is also an object of this invention to provide an oil phase from the desalter sufficiently free of brine and other contaminants that it can be fed directly to a distillation step without secondary processing. A further object of this invention is to reduce the loss of organic components in the aqueous phase raffinate from the desalter. Still another object of this invention is to provide an aqueous phase raffinate sufficiently free of contaminants that it can be discharged as waste water. Specifically, it is an object of this invention to provide a desalter treatment process so efficient and effective as to provide an aqueous phase withdrawn from the desalter leaving a substantially salt and brine-free oil phase feedstock. These and other objects and advantages of this invention will be better understood from the following description, which is to be read together with the accompanying drawing wherein there is an illustrative flow diagram of a desalter train incorporating the desalter solvent extraction system of this invention. Generally, the present invention comprises a method of processing a feedstock of crude oil containing water and salts, and involves the usual prior art step of desalting the feedstock by mixing with water followed by separation into an organic component-containing phase of reduced salts content and an aqueous phase containing a major portion of the salts. The aqueous phase is then mixed with an exttactant fluid that is a gas under standard ambient conditions of temperature and pressure, but which is under such conditions of temperature and pressure as to render it a fluid solvent for the organic components, but substantially less for water, thus forming a fluid extract of the organics in the exttactant fluid and a raffinate comprising water and salts. The fluid extract is then separated from the raffinate to leave an aqueous salt solution substantially free of the organic components. Referring now to the drawing, there is shown a typical embodiment of apparatus for effecting the method of the present invention where, as in the conventional desalting operation, inlet stream of raw crude oil in conduit 10 is mixed with a water stream in conduit 34 and fed to desalter tank 50. To facilitate separation, the crude oil feed is pre-tteated by adding additional water/brine, or by heating to about 250°-300°F, or both, as described in U.S. Pat. No.3,798,153 (Arndt et al.). An oil phase stream is continuously withdrawn through conduit 12 from the upper region of desalter 50, and an aqueous phase stream is continuously withdrawn through conduit 14 from the lower region of the desalter. Compared with conventional desalting processes, however, for purposes of this invention the rag layer (shown schematically as a dashed line in tank 50) is maintained at a relatively lower level, thereby avoiding drawing substantially any of the rag layers content into the oil phase stream, the rag layer being drawn out separately through conduit 36 into the aqueous phase or together with the aqueous phase. In the preferred embodiment of this invention, the rag layer level and withdrawal rate are selected to achieve an oil phase that can be sufficiently free of brine, solids and other contaminants so as to provide a suitable feedstock to be fed directly to a distillation step, for example carried out in distillation column 64, without secondary purification, as required for example in the Arndt et al. patent. Similar to any unit operation, there is a trade-off between the quality of oil phase stteam in conduit 12 and contamination of aqueous phase stteam in conduit 14. More particularly, by withdrawing the oil phase stteam from desalter 50 at a relatively lower rag layer level and following a longer residence time as compared with conventional desalting processes, the requirement of steady-state conditions necessitates withdrawing the aqueous phase stteam through conduit 14 following a relatively shorter residence time. Just as the relatively longer residence time of the oil phase 12 leads to a less-contaminated oil phase, the relatively shorter residence time of the aqueous phase stteam from the desalter leads to a more contaminated aqueous phase, perhaps including some or all of the so-called "rag" layers. In other words, in accordance with the present invention, the desalter is intentionally operated to optimize the quality of the oil phase feedstock at the expense of wastewater quality. Conduit 14 is connected to feed extractor 52 through valve 15 for secondary treatment. Alternatively, a rag layer stteam is withdrawn through conduit 36 separately from the aqueous stteam and is sent to solvent extraction system 52 through valve 37. The aqueous phase withdrawn from tank 50, being substantially cleaner, can then be sent to conventional downstream treatment by operating valve 15 to divert the stteam to conduit 17. Extractor 52 comprises a sealed, pressurized mixing tank, preferably a liquid- liquid contacting tower, of suitable construction to withstand processing conditions. Inside extractor 52, the aqueous stteam from conduit 14 is mixed with a stteam, introduced through conduit 16, of a suitable extractant fluid that is a gas under standard ambient conditions of temperature and pressure. The exttactant fluid, however, is under such conditions of temperature and pressure as to render it a fluid solvent for the organic components carried over in the aqueous phase stteam, but substantially less for water. Treatment with this exttactant fluid forms a fluid extract of the organic components and a brine raffinate. The stteam of the exttactant fluid may comprise a combination of recycled extractant, as hereinafter described, and a stteam of make-up extractant as needed carried along conduit 20 from exttactant make-up pump tank 54 and pumped into conduit 16. As discussed in U.S. Pat. No. 4,147,624 (Modell) and No. 4,349,415 (DeFilippi et al.), which are incorporated herein by reference, a large number of gaseous compounds have been recognized to have solvent properties when converted to a fluid or fluid-like state. Such compounds, which are gases at ambient temperature and pressure, but which can be converted to a solvent-condition fluid state, include: hydrocarbons such as methane, ethane, propane, butane, ethylene, and propylene; halogenated hydrocarbons such as halomethanes and haloethanes; and inorganics such as carbon dioxide, ammonia, sulfur dioxide, nitrous oxide, hydrogen chloride, and hydrogen sulfide; and chemically compatible mixtures of two or more of the foregoing compounds. Extractor 52 may be operated in any way that insures thorough mixing of aqueous phase stteam from conduit 14 and exttactant stream from conduit 16, for example a countercurrent process, such that substantially all of the oil and other organic components of the aqueous phase are dissolved in the exttactant and a two- phase system is formed. The aqueous phase in extractor 52, comprising water, undissolved solids and some extractant fluid, is continuously withdrawn from the extractor through conduit 22 through a pressure-reducing valve 55. The pressure- reduced stteam in conduit 22 is directed into water separator 56 where, because of a reduced pressure, residual extractant forms a vapor phase which is bled off through line 24 and fed to vapor tank 58. A substantially clean waste water stteam in line 26, suitable for discharge, is withdrawn from separator 56. The organic extractor phase, comprising extractant fluid and dissolved organics, is continuously withdrawn from extractor 52 through conduit 28 through pressure-reducing valve 60. The pressure-reduced stteam in conduit 28 is directed into organics separator 62 where, because of a reduced pressure and heat, the extractant fluid is flashed off to form a vapor phase which, in turn, is bled off through line 30 to vapor tank 56. The separated organic components form a liquid phase in separator 62, which liquid phase is withdrawn through line 32 and typically can be returned to join the crude oil feedstock conduit 10 to desalter tank 50. The extractant vapors in vapor tank 58 are condensed and the extractant is then recycled through line 18 to join line 16 feeding extractor 52. The extraction system of this invention results in numerous improvements and efficiencies as compared with conventional refining operations. Some of the major advantages that are realized by incorporating the extraction system of this invention into a refinery desalter operation include the following: (1) It allows desalter operations to be optimized on crude quality alone; this results in improved quality of desalted crude, and increased crude utilization. (2) It produces wastewater with an oil content in the 5-10 ppm range and a benzene content in the 10-500 ppb range. (3) A major process source of both oil and solids to the waste treatment system is nearly eliminated, thus reducing the treatment and disposal costs as well as reducing long-term liabilities associated with disposal of oily solids. (4) It will increase refinery on-line time by reducing fouling of equipment. (5) The solvent extraction system has the capability of treating other pumpable refinery wastes, such as slop oil, to recover organics, minimize chemical consumption and eliminate some prior art processes. Since certain changes may be made in the above-described apparatuses and processes without depaiting from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted in an illustrative and not in a limiting sense.

Claims

What is claimed is: 1. Method of processing a feedstock of crude oil, water and salts, wherein said feedstock is separated in a desalter into an oil phase containing primarily organic components with a reduced salts content, and an aqueous phase containing a major portion of said salts and a portion of said organic components, together with a rag layer, said method comprising the steps of: separately withdrawing said oil phase and said aqueous phase from said desalter; processing said aqueous phase with an extractant fluid that is a gas under standard ambient conditions of temperature and pressure, but which is under such conditions of temperature and pressure as to render said exttactant fluid a solvent for the organic components in said aqueous phase, but substantially less for water, so as to form a fluid extract of said organic components from said aqueous phase in said extractant fluid and a raffinate comprising primarily water and said salts; and separating said fluid extract from said raffinate to leave an aqueous solution substantially free of said organic components.

2. Method as defined in claim 1 wherein said step of processing includes process said aqueous phase and said rag layer.

3. Method as defined in claim 1 wherein the rag layer level in said desalter is substantially lowered so as to avoid drawing substantially any of the content of said rag layer from said desalter into the withdrawn oil phase.

4. Method as defined in claim 3 wherein said level and rate of withdrawal are selected to provide a withdrawn oil phase sufficiently free of said water and salts as to provide a suitable feedstock for direct feed to a distillation step.

5. Method as defined in claim 1 wherein said aqueous phase is withdrawn from said desalter without drawing substantial portions of said oil phase from said desalter into the withdrawn aqueous phase.

6. Method as defined in claim 1 wherein said rag layer is withdrawn from said desalter either with or separately from said aqueous phase.

7. Method as defined in claim 1 further including the step of flashing off said extractant fluid from said fluid extract to recover said organic components from said extract.

8. Method as defined in claim 7 further including the step of recycling said extractant fluid from the flashing step to the step of treating.

9. Method as defined in claim 7 further including the step of providing said first phase as feedstock to a distillation system.

10. Method as defined in claim 9 including the step of adding said recovered organic components to said feedstock.

11. Method as defined in claim 1 including the steps of adding other refinery waste stteams to said withdrawn aqueous phase so that the combined aqueous phase and refinery waste stteams are treated together with said extractant fluid.

12. Method as defined in claim 1 wherein said extractant fluid is selected from the group consisting essentially of hydrocarbons, halogenated hydrocarbons, carbon dioxide, ammonia, sulfur dioxide, nittous oxide, hydrogen chloride, hydrogen sulfide, and chemically compatible mixtures thereof.

13. In a process of refining crude oil in a desalter in which a crude oil feedstock is separated into an organic phase containing primarily organic components and having a reduced salts content, a first aqueous phase containing a major portion of said salts and a portion of said organic components, and a rag layer being positioned between said organic phase and first aqueous phase, the improvement comprising: so withdrawing said organic phase from said desalter as to avoid including substantially any of the contents of said rag layer in the withdrawn organic phase; withdrawing said first aqueous phase from said desalter; and treating said first aqueous phase with an exttactant fluid that is a gas under standard ambient conditions of temperature and pressure, but which is under such conditions of temperature and pressure as to render said extractant fluid a solvent for the organic components in said first aqueous phase, but substantially less for water, so as to form a fluid extract of said organic components from said first aqueous phase in said extractant fluid, and a raffinate comprising primarily water and said salts and being substantially free of said organic components.

14. In a process of refining crude oil in a desalter as defined in claim 13 including the step of withdrawing said rag layer either separately or together with said aqueous phase from said desalter.