Note: Descriptions are shown in the official language in which they were submitted.
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- DESALTER SOLVENT EXTRACTION SYSTEM
The present invention relates generally to a process for treating crude oil as
2 part of an oil refining p~ocess and, more particularly, for improving the desalting
3 operation by employing a secondary processing step wherein the aqueous phase
4 withdrawn from the desalter is processed tO remove substantially all organics prior
5 to discharge as wastewater.
6 It is well known in the oil refining art to employ a desalting step for
7 separating certain solids and water soluble components, especially brine, from the
8 crude oil. Such a desalting process is described, for example, in U.S. Pat. Nos.
9 3,798,153 (Arndt e~ al.) and 4,684,457 (McKechnie et al.). Typically in the
10 desalting operation, fresh water or a low-salinity brine is added to a stream of crude
11 oil, the combined stream is mixed and heated to a temperature of about 250-300
12 F, and the heated mixture is fed to a desalter tank. By action of gravity, sometimes
13 aided by electrical means, the mixture in the desalter tank separates into a lighter,
14 oil layer with a reduced salt content and a heavier, aqueous layer contaminated with
15 salts, solids and hydrocarbon components. The oil phase can be continuously
16 withdrawn from the upper region of the desalter tank to maintain steady-state
17 conditions while the contaminated aqueous phase is continuously bled from the lower
18 region of the desalter. As noted in the aforementioned Arndt et al. patent, the
19 desalting step is typically carried out at an elevated temperature to increase the
20 efficiency of the separation of brine from oil, while heat-exchanger fouling is
21 red~lced by removing salts before the oil is heated much above 300 F.
22 The oil and aqueous phases in the desalter, however, do not form a clean,
23 sharp boundary. Instead, the two phases are typically separated by so-called "rag"
24 layers comprising brine and solids emulsified with oil. Moreover, these rag layers
2s do not remain stationary but have an undesirable tendency to wander, thereby
2 6 reducing the efficiency and effectiveness of the desalting operation. To insure good
27 separation and to avoid contaminating the oil phase with contents of the rag layers,
28 the oil phase must be drawn from a point well above the rag layers and at a
29 controlled rate. In conventional processing, some of the desalter rag layers are
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either treated by high chemical addition or are passed along with the aqueous phase
2 or the oil phase, thereby requiring expensive downstream processing.
3 Another problem with the conventional desalting operation is that the aqueous
4 phase withdrawn from the bottom of the desalter typically contains a significant
proportion of dissolved or entrained organic components. These organic components
- 6 may include boih certain lighter petroleunl fractions, such as benzene, as well a~
7 heavier hydrocarbons. Recovery of these organic components is desirable not only
8 because of their economic value but, in addition, because discharge or disposal of
g such contaminated water is ecologically detrimental and may be illegal.
Thus, it is conventional to subject the aqueous phase withdrawn from the
11 desalter to downstream treatment to remove most of the organic components and to
12 render the brine suitable either for disposal or recycling. The aforementioned
13 McKechnie et al. patent, for example, describes a cross-flow membrane separator
14 process for treating the oil-containing brine withdrawn from a desalter. While it is
15 known, as taught by U.S. Patent No. 4,568,447 (Pujado) to employ a solvent in the
6 supercritical state for the removal of trace quantities of organic compounds from an
17 aqueous stream, such a process has not been used for downstream processing of the
18 aqueous phase coming from a desalter, nor would such treatment, by itself, solve the
19 problems of an inefficient desalter operation and a partially contaminated oil phase
2 o feedstock from the desalter.
21 Another problem with the conventional desalting operation is that, in order
22 to achieve reasonable cost-effectiveness, the rag layer is maintained at a relatively
23 fixed level and the oil phase is withdrawn at such a rate that the oil phase still
24 contains enough remaining brine to cause downstream processing problems unless
25 that brine level is further reduced by additional treatment. Accordingly, it is well
26 known to subject the oil phase coming from the desalter to addidonal processing
27 prior to the flashing, furnace and disdllation steps of a conventional oil refining
28 operation. The aforementioned Arndt et al. patent, for example, describes heating
2 9 pa~tially desalted oil by injecting a hot fluid into the partially desalted oil
3 o immediately prior to the flashing step. This process substitutes for heating the oil
31 phase by the more conventional means of a heat exchanger, which would be subject
. 32 to heavy fouling by the salts remaining in the partially desalted oil. While it is
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known, as taught by U.S. Pat. Nos. 4,522,707 (Kriegel et al.) and 4,797,198
2 (Wetzel et al.) to treat used or salvage oil with a gas under supercri~ical conditions
3 as part of a purification and reconditioning operation, such a process has not been
4 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
6 contaminated aqueous phase raffinate.
7 These and other problems with and limitations of the prior art desalting and
8 refining operations are overcome ~,vith the desalter solvent extraction system of this
g invention. Specifically, the system of this invention improves the efficiency of the
10 desalter operation, eliminates the need for secondary treatment of the oil phase
feedstock prior to distillation, and provides a less-contaminated aqueous phase at
12 lower cost than conventional secondary treatment for the aqueous phase raffinate
13 from a desalter.
14 Accordingly, it is a general object of this invention to provide a more15 efficient and effective crude oil refining process.
6 Another general object of this invention is to improve the efficiency and
7 effectiveness of the desalter operation in crude oil refining.
8 Another object of this invention is to provide from a desalter an oil phase
19 sufficiently free of brine and other contaminants so that deposition of such
2 o contaminants in the crude preheating tower is minimized.
21 It is also an object of this invention to provide an oil phase from the desalter
22 sufficiently free of brine and other contaminants that it can be fed directly to a
2 3 distillation step without secondary processing.
2 4 A further object of this invention is to reduce the loss of organic components
in the aqueous phase raffinate from the desalter.
2 6 Still another object of this invention is to provide an aqueous phase raffinate
2 7 sufficiently free of contaminants that it can be discharged as waste water.
28 Specifically, it is an object of this invention to provide a desalter treatment
2 9 process so efficient and effective as to provide an aqueous phase withdrawn from the
3 0 desalter leaving a substantially salt and brine-free oil phase feedstock.
31 These and other objects and advantages of this invention will be better32 understood from the following description, which is to be read together with the
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accompanying drawing wherein there is an illustrative flow diagram of a desalter2 train incorporating the desalter solvent extraction system of this invention.3 Generally, the present invention comprises a method of processing a
4 feedstock of crude oil containing water and salts, and involves the usual prior art
s step of desalting the feedstock by mixing with water followed by separation into an
- 6 organic component-c~ntaining phase of reduc~ salts content and an aqueous phase
7 containing a major portion of the salts. The aqueous phase is then mixed with an
8 extractant fluid that is a gas under standard ambient conditions of temperature and
g pressure, but which is under such conditions of temperature and pressure as tolO render it a fluid solvent for the organic components, but substantially less for water,
thus forming a fluid extract of the organics in the extractant fluid and a raffinate
12 comprising water and salts. The fluid extract is then separated from the raffinate to
13 leave an aqueous salt solution substantially free of the organic components.
14 Referring now to the drawing, there is shown a typical embodiment of
15 apparatus for effecting the method of the present invenbon where, as in the
16 conventional desalting operation, inlet stream of raw crudc oil in conduit lO is mixed
7 with a water stream in conduit 34 and fed to desalter tank S0. To facilitate
18 sq~ on, the crude oil feed is pre-treated by adding additional watertbrine, or by
19 heabng to about 250-300F, or both, as described in U.S. Pat. No.3,798,153
20 (Arndt et al.). An oil phase stream is continuously withdrawn through conduit l2
21 from the upper region of desalter 50, and an aqueous phase stream is continuously
2 2 withdrawn through conduit 14 from the lower region of the desalter. Compared with
23 conventional desalting processes, however, for purposes of this invention the rag
24 layer (shown schematically as a dashed line in tank 50) is maintained at a relatively
2s lower level, thereby avoiding drawing substantially any of the rag layers content into
26 the oil phase stream, the rag layer being drawn out separately through conduit 36
2 7 into the aqueous phase or together with the aqueous phase.
28 In the preferred embodiment of this invention, the rag layer level and29 withdrawal rate are selected to achieve an oil phase that can be sufficiently free of
3 o brine, solids and otber contaminants so as to provide a suitable feedstock to be fed
31 directly to a distillation step, for exarnple carried out in distillation column 64,
3 2 without secondary purification, as required for example in the Arndt et al. patent.
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Similar to any unit operation~ there is a trade-off between the quality of oi]
2 phase stream in conduit 12 and contamination of aqueous phase stream in conduil 14.
3 More particularly, by withdrawing the oil phase stream from desalter 50 at a
4 relatively lower rag layer level and following a longer residence time as compared
5 with conventional desalting processes, the requirement of steady-state conditions
6 necessitates withdrawing the aqueous phase stream through conduit 14 following a
7 relatively shorter residence time. Just as the relatively longer residence time of the
8 oil phase 12 leads to a less-contaminated oil phase, the relatively shorter residence
g time of the aqueous phase stream from the desalter leads to a more contaminated
0 aqueous phase, perhaps including some or all of the so-called "rag" layers. ln other
11 words, in accordance with the present invention, the desalter is intentionally operated
12 to optimize the quality of the oil phase feedstock at the expense of wastewater
13 quality. Conduit 14 is connected to feed extractor 52 through valve 15 for secondary
14 treatment. Alternatively, a rag layer stream is withdrawn through conduit 36
15 separately from the aqueous stream and is sent to solvent extraction system 52
16 through valve 37. The aqueous phase withdrawn from tank 50, being substantially
17 cleaner, can then be sent to conventional downstream treatment by operating valve
18 15 to divert the stream to conduit 17.
19 ~ctractor 52 comprises a sealed, pressurized mixing tank, preferably a liquid-
2 o liquid contacting tower, of suitable construction to withstand processing conditions.
21 lnside extractor 52, the aqueous stream from conduit 14 is mixed with a stream,
22 introduced through conduit 16, of a suitable extractant fluid that is a gas under
23 standard ambient conditions of temperature and pressure. The extractant fluid,
24 however, is under such conditions of temperature and pressure as to render it a fluid
25 solvent for the organic components carried over in the aqueous phase stream, but
2 6 substantially less for water. Treatment with this extractant fluid forms a fluid extract
27- of the organic components and a brine raffinate. The stream of the extractant fluid
28 may comprise a combination of recycled extractant, as hereinafter described, and a
29 stream of make-up extractant as needed carried along conduit 20 from extractant
3 o make-up pump tank S4 and pumped into conduit 16.
31 As discussed in U.S. Pat. No. 4,147,624 (Modell) and No. 4,349,415
32 (DeFilippi et al.), which are incorporated herein by reference, a large number of
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gaseous compounds have been recognized to have solvent properties when converted2 to a fluid or fluid-like state. Such compounds, which are gases at ambient
3 temperature and pressure, but which can be converted to a solvent-condition fluid
4 state, include: hydrocarbons such as methane, ethane, propane, butane, ethylene, and
s propylene; halogenated hydrocarbons such as halomethanes and haloethanes; and
6 inorganics such as carbon dioxide, ammonia, sulfur dioxide, nitrous oxide, hydrogen
7 chloride, and hydrogen sulfide; and chemically compatible mixtures of two or more
8 of the foregoing compounds.
g Bxtractor 52 may be operated in any way that insures thorough mixing of
lo aqueous phase stream from conduit 14 and extractant stream from conduit 16, for
11 example a countercurrent process, such that substantially all of the oil and other
12 organic components of the aqueous phase are dissolved in the extractant and a two-
13 phase system is formed. The aqueous phase in extractor 52, comprising water,
undissolved solids and some extractant fluid, is continuously withdrawn from the5 extractor through conduit 22 through a pressure-reducing valve 5S. The pressure-
16 reduced stream in conduit 22 is directed into water separator 56 where, because of
17 a reduced pressure, residual extractant forms a vapor phase which is bled off through
18 line 24 and fed to vapor tank 58. A substantially clean waste water stream in line
19 26, suitable for discharge, is withdrawn from separator 56.
2 o The organic extractor `phase, comprising extractant fluid and dissolved
21 organics, is continuously withdrawn from extractor 52 through conduit 28 through
22 pressure-reducing valve 60. The pressure-reduced stream in conduit 28 is directed
23 into organics separator 62 where, because of a reduced pressure and heat, the24 extractant fluid is flashed off to forrn a vapor phase which, in turn, is bled off
25 through line 30 to vapor tank 56. The separated organic components form a liquid
26 phase in separator 62, which liquid phase is withdrawn through line 32 and typically
27 can be returned to join the crude oil feedstock conduit 10 to desalter tank 50. The
28 extractant vapors in vapor tank 58 are condensed and the extractant is then recycled
29 through line 18 to join line 16 feeding extractor 52.
3 o The extraction system of this invention results in numerous improvements and
31 efficiencies as compared with conventional refining operations. Some of the major
3 2 advantages that are realized by incorporating the extraction system of this invention
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into a refinery desalter opera~ion include the following: (1) It allows desalter2 operations to be optimized on crude quality alone; this results in improved quality
3 of desalted crude, and increased crude utilization. (2) It produces wastewater with
4 an oil content in the 5-10 ppm range and a benzene content in the 10-500 ppb range.
5 (3) A major process source of both oil and solids to the waste treatment system is
6 nearly eiiminated, thus reducing the treatment and disposal costs as well as reducing
7 long-term liabilities associated with disposal of oily solids~ (4) It will increase
8 refinery on-line time by reducing fouling of equipment. (5) The solvent extraction
g system has the capability of treatmg other pumpable refinery wastes, such as slop
10 oil, to recover organics, minimi~e chemical consumption and eliminate some prior
11 art processes.
12 Since certain changes may be made in the above-described apparatuses and
13 processes without deparhng from the scope of the invention herein involved, it is
14 intended that all matter contained in the above description shall be interpreted in an
15 illustrative and not in a limiting sense.
16
