US2121325A - Method for solvent extraction of oil - Google Patents

Description

June 21v, 1,938.

D. R.l MERRILL ,-3 Treazzg Siage 14 l 125 J, 'fr ,l5 l 4 -Treczmg 'zge @Liked-NAH, A,"f 2,' 20 Treazzzng Siege T 7) (Cooler 42 '17 I0 "511 (19 -fa l Sepczzczorf" 6 l IN VEN TOR.

TORNEY.

Patented June 21V, 1938 UNITED sTATts- 2,121,325 METHOD FOR SLVENTEXTRACTION V01"" OIL David R. Merrill, Long Beach, Calif., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application April 281, 1936, Serial No. 76,776 2 claims. (cries-eis) This invention relates to a process for the separation of oil into fractions by means of solvents and is a continuation in part of my .copending application Serial No. 686,941 filed 5 August 26, 1933 and my application Serial No.

51597 filed January 14, 1935.

Most crude oils are complex mixtures of hydrocarbons and contain Varying proportions of paraiinic, naphthene, aromatic and unsaturated l0 hydrocarbons. In the production of lubricating oil for internal combustion motors it is desirable to produce a finished oil that exhibits a low temperature viscosity susceptibility, i. e., one which shows a minimum change in viscosity for a given change in temperature. Theparaiiinic hydrocarbons are characterized by a relatively ,low temperature Viscosity susceptibility, whereas aromatic and highly unsaturated hydrocarbons are characterized by a relatively high temperature viscosity susceptibility. The term viscosity gravity constant is a means of expressing the' temperature viscosity susceptibility of an oil.

This constant is described in a Journal of In-` dustrial &` Engineering Chemistry, volume 20, page 41, of 1928, by Hill and Coates. A high value represents a high degree of non-paraffinicity or a high temperature viscosity susceptibility, while low values indicate vrelatively greater parainicity or a lower temperature visi cosity susceptibility. Lubricating oils from nat- Iural crudes range from .903 for extreme Gulf Coast type to .807 for an extreme Pennsylvania type.

For purposes of identification in this applica-` 354 tion, oil fractions which exhibit a low viscosity gravity constant will be referred to as paraiinic oil fractions and oil fractions which possess a high viscosity gravity constant will be referred to as non-paraiiinic oil fractions. 4:0` In the conventional method of employing solvents to separate paraffinic and non-paraffinic hydrocarbons of petroleum oils a solvent is chosen which exercises a preferential solvent action for the non-paraflinic hydrocarbons present 45' in the oil. These solvents include aniline, nitrobenzene, furfural, beta beta dichlorethyl ether, phenol, cresylic acid, chloraniline, chlorphenol, sulfur dioxide, or sulfur dioxide modified with benzene. lSuch preferential solvents are mixed 50 with the oil and the mixtures maintained at such temperatures that separation into two phases occurs. The phase containing the more parafnic fractions is known as a ralnate phase and the phase containing the less parafiin- 55 fic fractions is known as the extract phase.

The separation of the hydrocarbon components by this process is never sharp however, and the extract phase will always contain more or less of the parainic fractions and the raffinate phase will correspondingly contain more or less of the 6 non-paraflinic fractions.

Among other factors` governing the quantity of parafiinlcIv fractions present in the extract phase is the composition of the raffinate phase with which the extract phase is in equilibrium. 10 In the production of a relatively high paraflinic raflinate phase a relatively high concentration ofthe parainic hydrocarbons will be found in the extract phase. Thus in the production of a highly paraiiinic raliinate there will be a rela- 15 tively high loss of paraflnic oil fractions in the extract phase.

In my copending application Serial No. 686,941 filed August 26, 1933, I have described this phenomena in detail. scribed a process wherein an oil was extracted with a selective solvent, a high grade raffinate was removed from the extraction system and an extract phase containing more or less valuable paraflinic oil fractions was (a) cooled for the 25 removal of valuable parafnic oil fractions which were returned to the extraction system and (b) extracted to recover valuable paraflinic oil fractions from the extract phase which were returned to the feed.

A similar process has also been described in my copending application Serial No. 1597 led January 14, 1935. In the latter application extract phase produced by contacting oil with a selective solvent of the cla/ss described above was cooled for the separation of the more paraiiinic fractions present in the extract phase. The more parainic oil fractions recovered by cooling the extract phase were then returned to the oil feed stream and this mixture was then contacted with a selective solvent.

In the present application I am employing the principle which I disclosed in my two prior led applications described above. In the present process, a raw oil is contacted with one of the selective solvents described above to produce a raffinate phase and an extract phase. The more paraiiinic oil fractions are then recovered from the extract phase and are commingled with the raw oil entering the extraction system in a regulated amount. By returning the parafnic oil fractions recovered from the extract phase continuously and in a regulated amount, I am able to establish equilibrium throughout the entire extraction process.

In that application I de- 2li The inventilon therefore resides in recovering a second quality railiniate from the extract phase which contains valuable paraffinic fractions and returning it to the feed stock. The rate of addition of this second quality raffinate to the feed stock must however be continuous or controlled since otherwise the oil entering the extraction system will be fluctuating in composition and will not admit the establishment of that steady state which is requisite for the realization of high efficiency in the extraction. The invention therefore resides in maintaining a constant composition of the blend of feed stock and second quality raffinate which is recovered from the extract phase. rI'he invention further resides in blending second quality raffinate obtained from the feed stock inr controlled amounts so that the feed stock entering the extraction system is of constant quality and extracting this constant quality oil with a constant quantity of selective solvent under carefully controlled temperature conditions, that is, a non-fluctuating temperature.

My invention will be understood by referring Ato the accompanying drawing. In the figure the raw oil is countercurrently extracted with one of the foregoing selective solvents thereby producing a final raiiinate and a nal extract, the raffinate comprising the more paraflinic oil fractions of the feed and the extract comprising the relatively less paraffinic oil fractions of the feed. Referring more specifically to the drawing, selective solvent is fed into the extraction system through line I. Raw oil is introduced through line 2. A final raffinate is withdrawn through line 8 and a final extract is withdrawn through line 9. Zones 3, 4 and 5 are employed to permit the separation of the solvent and the oil into phases. From the upper section of each of these zones a raffinate phase is withdrawn and from the lower section of each of these zones an extract phase is withdrawn. The extract phase withdrawn from zone 5 passes through a cooler 'I where the temperature of the solution of oil dissolved in the selective solvent is lowered sufficiently to cause the more paraiiinic oil fractions contained in the oil solvent solution to be forced out of the solution. The cooled mixture from the cooler 1 passes by means of line I8 into separator 6 where the more paraflinic oil fractions forced out of solution by the lowered temperature from an upper second raffinate phase in zone 6 are removed through line I9. The final extract phase, that is the lower phase in zone 6, is removed through line 9. Thus the process shown in the figure consists of an extraction system in which oil is countercurrently extracted with a selective solvent and in which the more paraiiinic oil fractions are recovered from the extract phase and returned to the oil feed in a regulated amount so that the oil entering the extraction system is of constant quality, thereby permitting equilibrium to be established throughout the countercurrent extraction system.

Considering the figure in more detail, selective solvent is introduced into the system through line I, the oil to be extracted, that is the oil feed, is introduced through line 2. Extract phase recovered from separating zone 4 through line I6 is commingled with the oil feed and this mixture passes into separating zone 5 where a raffinate phase and an extract phase are formed. The raiiinate phase in zone 5 is continuously drawn off through line I5 where it is commingled with the extract phase withdrawn from separating zone 3 through line I4. This mixture passs into separating zone 4 where it is permitted to stratify into an upper raffinate phase and lower extract phase. The upper rafnate phase is continuously withdrawn through line I3 and mixed with fresh solvent in line I after which this mixture passes into separating zone 3 where it is permitted to separate into a final raffinate phase or upper layer, which is withdrawn through line 8 and a lower extract phase which is withdrawn through line I4.

The extract phase formed in separating zone 5, that is the lower layer, is continuously withdrawn through lineV I'I and passes through cooler I where the temperature of the soil solvent solution is cooled sufficiently to cause rejection from solution of the more paraflinic oil fractions contained therein. VThe cooled mixture then passes from cooler I by means of layer I8 into separating zone 6 where it is permitted to stratify into a lower layer which is the final extract phase produced and into an upper layer which is a second quality raffinate. The upper layer of the second quality raffinate is withdrawn from separating zone 6 through line I9. The second quality raffinate withdrawn through line I9 consists of oil fractions which have a different viscosity gravity constant or viscosity index than the oil feed or stock. In some instances the Viscosity gravity constant of the fractions recovered through line I9 is lower than that of the oil feed and in some instances it may be higher than that of the oil feed, depending upon the degree of cooling of the extract phase in cooler 'I. Since the oil recovered through line I9, that is the second raffinate phase, is of different quality than the oil feed introduced through line 2, it is very important that the rate at which the oil returning through line I9 to the feed be maintained constant in order that the oil in line will be of constant quality. By maintaining the oil at a constant quality in line 20 the equilibrium of the extraction system is never upset. By feeding in a regulated amount of solvent through line I, a regulated amount of oil through line 2, a regulated amount of second grade raffinate through line I9 into the feed in line 2, maintaining constant temperatures in zones 3, 4 and 5 and a constant temperature in cooler 'I and zone 6, it is possible to produce a constant quality raffinate through line 8 and a constant quality extract from line 9.

In some instances more second quality raffinate is produced in separator 6 than can conveniently be vreturned to the feed through line I9. When this condition prevails, a portion of the second quality raiiinate is withdrawn from line I9 through valve II and line I0.

While I have shown in the drawing the recovery of second quality raffinate from the extract phase by means of cooling, I do not wish to limit myself to this method of operation since other methods of recovering second quality raffinate may also be employed. For example, when phenol is used as a selective solvent, water may be injected into the extract phase recovered in line I'I, thereby lowering the solvent power of the selective solvent for the more parafflnie fractions and causing them to be forced out of solution. In other words, addition of water to the extract phase consisting of phenol and dissolved fractions in an equivalent of the cooling means shown in the drawing.

Another method of obtaining the second quality raffinate consists in contacting the extract phase in line II with liquid propane or butane Which dissolves the more paraiiinic oil fractions contained therein. The liquid propane containing the dissolved more paraiiinic fractions of the extract phase is returned to the oil feed in the manner described above through line I9 in a continuous and constant amount so that the oil in line 20 remains of constant quality, that is, the same viscosity gravity constant. Where propane is used to recover the more paranic oil fractions from the extract phase, it is sometimes desirable to remove the propane from the oil before it is returned through line I9 into the oil feed in line 2. Furthermore, if cresylic acid has been used as the selective solvent, liquid ammonia may be injected into the extract phase produced in line I'I, thereby lowering the solvent power of the cresylic acid for the more paraiiinic oil fractions contained in the extract phase. After settlement of this mixture to which liquid ammonia has been added in Zone 6 the more parafnic fractions, forced out of solution by the addition of liquid ammonia, rise to the top of this separating zone and are withdrawn and returned through line I9 to the feed in line 2.

The following is an example of the method of carrying out my process: 100 volumes of raw stock was introduced through line 2. This stock had a viscosity gravity constant of 0.880 and an A. P. I. gravity at F. of 19.6. Into line I 0f the extraction system 850 volume percent of a mixture consisting of 20% benzene and 80% sulfur dioxide was introduced. Through line 8 of the extraction system 44.0 volumes of rannate were recovered having a viscosity gravity constant of 0.807. The temperature of the oil and solvent in zones 3, 4 and 5 including the mixing zones in lines I, I5 and 20 Was 125 F. The temperature of cooler I and separator 6 was 55 F. From .the bottom of separator 6 there was withdrawn 56 volumes of extract having a viscosity gravity constant of 0.937. From the top of separator 6 there was withdrawn a raflinate phase which was returned via line I9 to the feed in line 2 at a rate to maintain the quality of the feed stock entering zone 5 constant (i. e. same viscosity gravity constant). Thus in carrying out the process, the temperature of all of the extraction stages, that is, stages 3, i and 5, are kept constant, that is, temperature in these stages is not permitted to fluctuate. The temperature in the rejection stage 6 is also kept at a constant value, thereby producing a rejected oil in line I9 which is of constant composition. The rejected oil in line I9'is fed back into the oil feed in line 2 at a constant rate so that blend of oil in line 20 is of constant quality or constant composition and since the amount of solvent introduced through line I is constant both in composition and in quantity and since the raw feed introduced through line 2 is constant in quantity, I am able by the process described above to produce a final rainnate in line 8 of constant'quality and a nal extract in line 9 of constant quality.

The invention therefore resides in extracting an oil containing parainic and non-paraflnic oil fractions with a selective solvent capable of resolving the oil into a raffinate phase comprising the more paraiinic constituents of the feed stock and an'extract phase comprising the bulk of the selective solvent, the major portion of the nonparaiiinic oid fractions of the feed stock and some of the relatively paraffinic oil fractions which have been dissolved in the extract phase, separating the more parafiinic oil fractions contained in the extract phase either by changing the composition of the extract phase, cooling the extract phase, or by the addition of modifying agents which cause the more paraiinic oil fractions contained therein to be forced out of solution and then returning the more paraffinic oil fractions forced out of solution in the extract phase to the feed stock entering the extraction system in regulated amounts so that the oil entering the extraction systems is of constant quality. By maintaining a constant composition and quantity of feed to the extraction system together with a constant quantity of selective solvent and constant temperatures in the extraction system, it is possible to recover a final rafrinate and a final extract, each having a constant quality (i. e. constant viscosity gravity constant).

The above example is not to be taken as limiting but is merely illustrative of the invention Which I claim.

1. A process for the separation of oil into fractions which comprises feeding oil into an extraction system, countercurrently extracting said oil with a selective solvent and thereby forming a railinate phase and an extract phase, separating said phases, recovering oil fractions from said extract phase which have a viscosity gravity constant different from the viscosity gravity constant of the feed introduced into said extraction system, and continuously mixing said oil fractions recovered from said extract phase with the oil feed in a regulated amount so that the composition of the oil entering the extraction system will be substantially of constant quality.

2. In a continuous countercurrent process of separating oil into fractions relatively more paraiiinic and relatively less paraninic in character by means of a selective solvent wherein a reflinate phase and an extract phase are produced and oil fractions are recovered from the extract phase having a different viscosity gravity constant from the oil feed passing to extraction system, the steps of continuously mixing the oil fractions recovered from the extract phase with the Oil feed in a regulated amount whereby an oil mixture.

is produced and later extracted which oil mixture continuously possesses substantially the same viscosity gravity constant.

DAVID R. MERRILL.

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