US2865852A - Solvent extraction process - Google Patents

Description

2,865,852 SOLVENT EXTRACTION PROCESS James A. Davies, Scarsdale, N. Y., assignor to The Texas Company, New York, N. Y., a corporation of Delaware Application May 21, 1954, Serial No. 431,363

2 Claims. (Cl. 208-327) This invention relates to the solvent extraction of liquid mixtures wherein a liquid mixture is separated into its components by solvent extraction. More particularly, this invention relates to the liquid-liquid extraction of mixtures by solvent extraction wherein two separate liquid phases are flowed countercurrently with respect to each other in an extraction zone.

In countercurrent liquid-liquid extraction wherein two phases are produced, a so-called raflinate phase and a socalled extract phase, these separate phases being produced at opposite ends of an extraction zone, it has been observed in some instances that one of the phases, such as the raffinate phase, tends to become entrained and produced along with the other phase, such as the extract phase. It is believed that when this condition obtains the density or specific gravity dilference between the two separate phases within the extraction zone is insufficient to permit a reasonably and su'fiiciently rapid and efiicient separation of the phases, one from the other, with the result that one phase tends to become entrained and pro duced along with the other phase. By way of explanation and assuming the simple case wherein the liquid mixture, such as a petroleum fraction, is contacted with a single selective solvent so as to produce a raflinate phase and an extract phase, although the specific gravity diiference between the petroleum fraction being treated and the selective solvent employed originally may be rather high, e. g., about 0.3 and greater, the selective solvent employed having a higher specific gravity than the petroleum fraction being treated, as the selective solvent flows countercurrently in contact with the petroleum fraction some of the components of the petroleum fraction are taken up into the selective solvent to form an extract phase having a specific gravity substantially less than the specific gravity of the selective solvent itself. Also, some of the selective solvent becomes dissolved in the petroleum fraction being treated to form what may be called the raflinate phase with the result that the raflinate phase has a specific gravity substantially greater than the petroleum fraction being treated. his apparent therefore that the specific gravity difference between the extract and rafiinate phases will be considerably smaller than the difference between the specific gravity of the original selective solvent and the original petroleum fraction.

It has been observed during the furfural refining of a petroleum fraction, such as a heavy cycle gas oil, a considerable amount of the oil or rafiinate phase is entrained and. produced along with the extract phase leaving the bottom of the extraction tower. This condition more frequently occurs when the petroleum fraction is treated with a relatively small solvent dosage, that is, about one volume of furfural per volume of petroleum fraction being treated. To prevent and eliminate this entrainment or carryunder of the oil phase along with the extract phase, the solvent dosage of furfural has been substantially increased in order to increase the specific gravity difference between the raffinate or oil phase and the extract or furfural phase. Unfortunately when higher solvent dosages atetf 2,865,852 Patented Dec. 23, 1958 of selective solvent are employed the particular petroleum fraction being treated is over-refined, resulting in decrease yields of raflinate phase or refined oil. Additionally, substantially larger quantities of extract phase (solvent) must be treated, handled and recovered.

It is an object of this invention to provide an improved liquid-liquid extraction process. f

It is another object of this invention to provide an improved liquid-liquid extraction process where two separate liquid phases are flowed countercurrently with respect to each other.

It is still another object of this invention to provide a liquid-liquid countercurrent extraction process wherein entrainment of one liquid phase, such as a raffinate phase, in another liquid phase, such as the extract phase, is substantially eliminated in the produced ratlinate and extract phases. y

In atleast one embodiment of this invention at least one of the foregoing objects will be obtained. How these and other objects of this invention are accomplished will are present in the extraction zone and wherein the liquid mixture to be separated is introduced into the extraction zone intermediate the ends thereof or at said one end and wherein a partially miscible llquid selective solvent is introduced at said other end of the extraction zone, im-.

proved results are obtained by introducing additional selective solvent into said extraction zone at a point downstream from said other end of'the extraction zone in the direction of flow of said selective solvent through said extraction zone. Accordingly, in a liquid-liquid countercurrent extraction operation wherein two separate liquid phases are in intimate contact with each other and wherein said separate liquid phases tend to approach substantially the same specific gravity with the result that a portion of one of the liquid phases, such as the raflinate phase, tends to become entrained in and produced along with the other liquid phase, such as the extract phase, the separation of the separate liquid phases within the extraction zone is enhanced and improved by injecting an additional amount of selective solvent into the extraction zone so as to increase the density or specific gravity differential between the produced rafiinate and extract phases within the extraction zone. In the practice of this invention it is desirable that sufiicient additional selective solvent be injected into the extraction zone so that the minimum specific gravity differential between the separate liquid phases within the extraction zone is maintained at a value not less than 0.1, preferably at least 0.2.

Referring now to the drawing which schematically illustrates the practice of my invention as applied to a vertical or upright extraction zone, the extraction zone is indicated at 11. The extraction zone 11 may be considered as made up of a contacting or refining zone 11a superimposed upon a settling zone 11b. The practice of this invention is particularly applicable to an isothermal operation of extraction Zone 11, i. e., wherein substantially the same temperature prevails throughout the extraction zone. It is realized, however, that a temperature gradient may also be employed in the operation of extraction zone 11. The liquid mixture to be fractionated into its respective components is introduced into the extraction zone 11 via conduit 12 and distributor 12a. In the embodiment illustrated in the drawing, a selective solvent which is more dense than and partially miscible with the feed mixture 'is introduced into the upper end of the extraction zone via'conduit Maud-distributor 14a. In operation, the less dense feed material tends to rise within the extraction zone countercurrently with respect to the downwardlyflowing moredense selective solvent. Contacting masses with the extraction zone 11 are schematically illustrated at 15a, 15b, 15c and 15d, respectively. These contact masses serve to promote and facilitate liquid-liquid mixing and contact between the downwardly flowing solvent and the upwardly flowing feed mixture. These contact masses may comprise a porous mass of granular material, or a mass of -Berl saddles, Raschig rings or the'like. Thesecontact masses may be replaced by a rotating disc co-ntactor.

Two separate liquid phases are present within the extraction zone, a liquid phase comprising predominantly selective solvent together with a portion of the components selectively extracted from the feed mixture undergoing fractionation and other liquid phase comprising predominantly substantially the original feed mixture (certain of the components originally present having been removed) together with a small amount of selective solvent dissolved therein. These separate-phases are separately recovered at opposite ends of the extraction zone 11. A raffinate phase is withdrawn from'the upper end of the extraction zone via conduit 16. This raffinate phase will comprise certain of the components originally present in the feed mixture together with an amount of selective solvent dissolved therein. An extract phase is withdrawn from the lower end of the extraction zone via conduit 18. This extract phase will comprise mainly selective solvent together with certain components originally present in the feed mixture.

Separation of the separate liquid phases within the extraction zone is improved and enhanced by introducing thereinto selective solvent at a point downstream from the pointof injection of selective solvent via conduit 14. The additional selective solvent is injected into extraction zone 11 at the most suitable point to'increase the specific gravity differential between the two separate liquid phases contained therein. Accordingly, this additional selective solvent may be introduced into the extraction zone 11 at about the middle or lower portion of the contacting zone (refining section) of the extraction Zone, above the point of introduction of the feed mixture to be separated, for example, the additional selective solvent may be introduced into extraction zone 11 via valve conduit and distributor 20a". Preferably this additional selective solvent is introduced into the settling section of the extraction zone below the point of introduction of the feed mixture via valve conduit 21 and distributor 21a. Under normal and satisfactory operating conditions obtained in the practice of my invention, a liquid-liquid interface 22 is established in the settling section of the extraction zone.

Exemplary of the advantages to be obtained in the practice of my invention as applied to the furfural solvent refining of a heavy cycle gas oil, a considerable amount of carryunder or entrainment of the oil phase along with the extract phase leaving the bottom. of the extraction tower was experienced during conventional operation wherein all the furfural was introduced into the top of the tower. The solvent dosage of furfural to the oil being treated was increased in order to improve phase separation and to avoid carryunder. By increasing the solvent dosage, however, the gas oil was refined with the .result that the yield of refined oil was substantially reduced. However in accordance with this invention in order to prevent carryunder or entrainment of the raffinate phase, additional selective solvent was introduced into the extraction zone at the lower portion thereof, for example, into the settling section of the extraction zone. At the same time it was found that the overall solvent feed ratio was reduced by about half and still produced a satisfactorily refined oil. There was recovered as rat finate a refined gas-oil fraction in substantially increased yield. The results are set forth in Table l herewith.

1 Solvent dosage may be defined as the volume ratio of solvent to charge 011 and in the practice of this invention the total solvent dosage may have a val e in the range 25200%, more or less, depending upon the properties of the charge oil.

The practice of this invention is broadly applicable to any liquid-liquid solvent extraction process wherein two separate liquid phases are flowed countercurrently with respect-to each other. Accordingly, this invention is suitable not only for those liquid-liquid countercurrent solvent extraction operations employing one selective solvent in contact with a liquid mixture to be fractionated but is also applicable to the so-called Duo Sol solvent extraction process wherein the liquid mixture to be separated is introduced at some intermediate point of the extraction zone wherein two immiscible solvents of different densities are flowing countercurrently. The practice of this invention is particularly applicable to the petroleum industry for the solvent refining of various petroleum fractions, such as gasolines, naphthas, kerosenes, gas oils, e. g., catalytic cycle gas oils and similar boiling virgin distillates, lubricating oils, and the like. Selective solvents which may be used in the practice of this invention may be more dense or less dense than the feed mixture to be separated and may comprise such well-known selective solvents as nitrobenzene, liquid propane and sulfur dioxide, fl,,B-dichloroethyl ether (Chlorex), phenols (Selecto), furfural, liquid hydrogen fluoride and the various other well-known organic and inorganic selective solvents.

In the practice of this invention, the ordinary temperatures and pressures usually employed in conventional solvent extraction processes are employed. It 1s mentioned that the additional selective solvent introduced into the extraction zone to enhance and improve the phase separation of the separate liquid phases therein may be supplemented and/or replaced entirely by the extract phase recycled directly into the extraction zone in the manner in accordance with this invention and without any intervening treatment or change in composition.

Many variations, modifications and substitutions are possible in the practice of this invention without departing from the spirit and scope thereof.

I claim:

1. In the liquid-liquid countercurrent solvent extraction of a catalytic cycle gas oil to be fractionated to produce at one end of an extraction zone an extract phase and at the other end of said extraction zone a raffinate phase, said extraction zone comprising two superposed contiguous sections, a refining section ad acent said other end of said extraction zone and a settling section adjacent said one end of said extraction zone and wherein furfural, a partially miscible selective sol vent, is introduced into said extraction zone at said refining section and said catalytic cycle gas oil is introduced into said extraction zone at about that position where said refining section and said settling section meet, the specific gravity differential between said catalytic cycle gas oil and said furfural selective solvent being substantial, said selective solvent having a higher specific gravity than said catalytic cycle gas oil, and wherein two contiguous continuous separate liquid phases, said raffinate phase and said extract phase, are present within said settling section and tend to have substantially the same specific gravity, the interface between said liquid phases being located in said extraction zone within said settling section, the improvement which comprises introducing additional furfural selective solvent into said extraction zone below the point of introduction of said catalytic cycle gas oil thereinto at said settling section directly into said raffinate phase therein to maintain the specific gravity diflerential between said separate liquid phases in said settling section greater than 0.1.

2. In the liquid-liquid countercurrent solvent extraction of a catalytic cycle gas oil to be fractionated to produce at the lower end of a vertically extending extraction zone an extract phase and at the upper end of said extraction zone a rafiinate phase, said extraction zone comprising two superposed contiguous sections, a refining section adjacent said upper end of said extraction zone and a settling section adjacent said lower end of said extraction zone, wherein furfural, a partially miscible selective solvent, is introduced into said extraction zone at said refining section and said catalytic cycle gas oil is introduced into said extraction zone at about the position therein where said refining section and said settling section meet, the specific gravity differential between said catalytic cycle gas oil and said furfural selective solvent being substantial, said furfural selective solvent having a higher specific gravity than said catalytic cycle gas oil, and wherein two contiguous continuous separate liquid phases, said raffinate phase and said extract phase, are present within the said settling section and tend to have substantially the same specific gravity, the interface between said liquid phases being located within said settling section, the improvement which comprises introducing additional furfural selective solvent into said extraction zone below the point of introduction of said catalytic cycle gas oil thereinto at said settling section directly into said rafiinate phase therein to maintain the specific gravity diiferential between said separate liquid phases in said settling section greater than about 0.2.

References Cited in the file of this patent UNITED STATES PATENTS 2,037,318 Fenske et al Apr. 14, 1936 2,054,295 Merrill Sept. 15, 1936 2,149,574 Brown Mar. 7, 1939 2,346,491 Kiersted Apr. 11, 1944 2,525,813 Fragen Oct. 17, 1950 2,564,071 Lien et al Aug. 14, 1951 2,626,230 Clarke Jan. 20, 1953 2,692,222 Packie Oct. 19, 1954

Claims (1)

1. IN THE LIQUID-LIQUID COUNTERCURRENT SOLVENT EXTRACTION OF A CATALYTIC CYCLE GAS OIL TO BE FRACTIONATED TO PRODUCE AT ONE END OF AN EXTRACTION ZONE AN EXTRACT PHASE AND AT THE OTHER END OF SAID EXTRACTION ZONE A RAFFINATE PHASE, SAID EXTRACTION ZONE COMPRISING TWO SUPERPOSED CONTIGUOUS SECTIONS, A REFINING SECTION ADJACENT SAID OTHER END OF SAID EXTRACTION ZONE AND A SETTLING SECTION ADJACENT SAID ONE END OF SAID EXTRACTION ZONE AND WHEREIN FURFURAL, A PARTIALLY MISCIBLE SELECTIVE SOLVENT, IS INTRODUCED INTO SAID EXTRACTION ZONE AR SAID REFINING SECTION AND SAID CATALYTIC CYCLE GAS OIL IS INTRODUCED INTO SAID EXTRACTION ZONE TO ABOUT THAT POSITION WHERE SAID REFINING SECTION AND SAID SETTLING SECTION MEET, THE SPECIFIC GRAVITY DIFFERENTIAL BETWEEN SAID CATALYTIC CYCLE GAS OIL AND SAID FURFURAL SELECTIVE SOLVENT BEING SUBSTANTIAL, SAID SELECTIVE SOLVENT HAVING A HIGHER SPECIFIC GRAVITY THAN SAID CATALYTIC CYCLE GAS OIL, AND WHEREIN TWO CONTIGUOUS CONTINUOUS SEPARATE LIQUID PHASES, SAID RAFFINATE PHASE AND SAID EXTRACT PHASE, ARE PRESENT WITHIN SAID SETTLING SECTION AND TEND TO HAVE SUBSTANTIALLY THE SAME SPECIFIC GRAVITY, THE INTERFACE BETWEEN SAID LIQUID PHASES BEING LOCATED IN SAID EXTRACTION ZONE WITHIN SAID SETTLING SECTION, THE IMPROVEMENT WHICH COMPRISES INTRODUCING ADDITIONAL FURFURAL SELECTIVE SOLVENT INTO SAID EXTRACTION ZONE BELOW THE POINT OF INTRODUCING OF SAID CATALYTIC CYCLE GAS OIL THEREINTO AT SAID SETTLING SECTION DIRECTLY INTO SAID RAFFINATE PHASE THEREIN TO MAINTAIN THE SPECIFIC GRAVITY DIFFERENTIAL BETWEEN SAID SEPARATE LIQUID PHASES IN SAID SETTLING SECTION GREATER THAN 0.1.

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