US2999892A - Solvent extraction process - Google Patents

Description

Sept. 12, 1961 M. N. PAPADOPOULOS ETAL SOLVENT EXTRACTION PROCESS Filed Sept. 2, 1958 24 COALESCER a COOLER 2 Sheets-Sheet 1 N In HEPTANE FLASHER EXTRACTOR INVENTORSI BYIJ MICHAE L N. PAPADOPOU LOS CAR L H. DEAL THEIR ATTORNEY FIG.

Sept. 12, 1961 Filed Sept. 2, 1958 DISTRIBUTION COEFFICIENT K M. N. PAPADOPOULOS EI'AL 2,999,892

SOLVENT EXTRACTION PROCESS 2 Sheets-Sheet 2 ALKYL NAPHTHALENE SLOPE B ALKYL BENZENES CARBON NUMBER OF on. COMPONENTS FIG. 2

INVENTORSZ MICHAEL N. PAPADOPOULOS CARL H. DEAL THEIR ATTORNEY United States Patent 2,999,892 SOLVENT EXTRACTION PROCESS Michael N. Papadopoulos, Albany, and Carl H. Deal, Jr.,

Orinda, Calif., assignors to Shell Oil Company, a corperation of Delaware Filed Sept. 2, 1958, Ser. No. 758,342 6 Claims. (Cl. 260674) This invention relates to the recovery of naphthalenes by extraction With a sulfolane-type selective solvent from a hydrocarbon fiaction also containing appreciable amounts of alkyl benzenes that normally interfere with the separation of the naphthalenes.

Historically, naphthalene and its homologs have been supplied by the coal tar industry, even though large quantities of these condensed aromatic hydrocarbons are present in various petroleum derived streams. A typical catalytically cracked gas oil fraction will contain a high proportion of aromatics and condensed aromatics, in the range of 30 to 50% or more. The condensed aromatics of a light cracked gas oil fraction are predominantly naphthalene and its homologs, usually making up from about to 20% or more of the fraction. Another petroleum derived stream containing sizable proportions of aromatic materials is coker gas oil. This latter stream will generally have even more naphthalenes than the catalytically cracked gas oil.

Despite the abundance of condensed aromatics in various petroleum fractions, the difficulty of eflectively and economically separating naphthalene and its homologs from other aromatics, particularly the alkyl benzenes, has slowed the development of the oil industry as a source for naphthalenes. -In a large part, this failure of the petroleum industry to compete with the coal tar industry as a source of naphthalenes may be attributed to the lack of a suitable solvent and process for the recovery of these materials from the complex hydrocarbon mixtures in which they are found. Recently, interest in the prospects of hydrogenating naphthalene and its homologs to provide a high density fuel has emphasized again the need for a practical and efiicient method for the separation of the naphthalenes from hydrocarbon streams.

A principal obstacle to the development of a suitable process for the recovery of the naphthalenes has been the failure to find a suitable solvent for the separation of these materials from a wide spectrum hydrocarbon stream of varying carbon numbers and types. There are solvents available which have a high type selectivity for alkyl naphthalenes as opposed to alkyl benzenes of the same carbon number. Unfortunately, invariably the solvents which have been investigated have molecular weight selectivities which interfere with their use as solvents for the separation of the alkyl naphthalenes from a hydrocarbon stream containing alkyl benzenes. It is clear that for efficient type separation of actual oil fractions, for example, kerosene, straight run or cracked gas oils and the like, which comprise materials having a fair range of molecular Weights, say 5 or even 10 or more carbon numbers, the solvent chosen should be effective throughout the range of the oil fraction. The solvents heretofore considered While perhaps having acceptable type selectivities at any given carbon number are generally relatively ineffective because their type separation efliciencies decrease rapidly with an increase in the molecular weight range of the oil fraction. The reason for this rapid decrease in effective type separation may be attributed to the molecular weight sensitivities of the solvents, that is to say, their molecular weight selectivities are unduly large so as to interfere with an efiicient type separation of the naphthalene and its homologs from a wide spectrum stock.

An examination of the various naphthalenes and alkyl ice benzenes will clarify the problem involved in effecting a type separation of these two aromatic types. Both of these aromatic types (excepting naphthalene itself) contain alkyl chains which make these materials subject to molecular weight separation as well as aromatic type separation in the presence of a solvent having both characteristics where the hydrocarbon fraction contains the two aromatic types in a fair range of molecular weights. It is known, for example, that B,/3'-im-inodipropionitrile which provides acceptable type separation for say naphthalene and C alkyl benzene or methyl naphthalene and C alkyl benzene or any other naphthalene and alkyl benzene of like carbon number becomes ineffective as a type solvent where it is employed to separate napthalenes of a broad range of molecular weights from alkyl benzenes of the comparable carbon number range. This may be explained by the sensitivity of 3,B'-iminodipropylnitrile to molecular weights. As a result, when this solvent is employed, the C alkyl benzenes will extract to the same degree as the C alkyl naphthalenes; that is, there will be no separtion of these two materials and they will appear in the extract phase and the rafiinate phase in the same proportions as found originally in the feed.

A particularly desirable solvent for the separation of alkyl naphthalenes from alkyl benzenes of a broad carbon range hydrocarbon fraction, therefore, must have a relatively high type selectivity for the naphthalenes and exhibit a low molecular weight selectivity. Additionally, a suitable solvent should have a high solubility for the naphthalenes and a specific gravity of at least 0.2 unit above that of the feed or say 0.3 unit below that of the feed. A relatively -wide difference in specific gravities facilitates the separation of the rafiinate and the extract phase in the extraction zone. The solvent must be stable, non-corrosive, and should have a low viscosity.

It is an object of this invention to provide a suitable solvent and process permitting the efiective separation of naphthalene and its homologs from a hydrocarbon stream containing alkyl benzenes. Other objects and advantages of the process of the invention will become more apparent from the following description and in the drawings wherein:

FIG. 1 is a schematic representation of a preferred embodiment of a system suitable for the practice of the process; and

FIG. 2 is a plot for a hypothetical solvent illustrating a desirable property for the type separation of alkyl naphthalenes from alkyl benzenes. 7

It has now been found that there is a suitable solvent for the foregoing separation far superior to earlier considered solvents. "In the process of the invention it is proposed to recover naphthalenes of differing carbon numbers from a hydrocarbon fraction containing alkyl benzenes of a like carbon number spread by subjecting the hydrocarbon fraction to a liquid-liquid extraction with a sulfolane solvent, thereby obtaining an extract-solvent phase rich in naphthalenes and a raffinate phase enriched in alkyl benzenes. The two phases are separted and thereafter the naphthalenes are recovered from the sulfolane solvent. In a preferred embodiment, the naphthalene extract is separated from the sulfolane solvent through a second liquid-liquid extraction, here With a nonpolar solvent, such as heptane, which forms a second extract phase containing the naphthalenes. The naphthalenes are conveniently recovered from this second extract phase by flashing to remove the non-polar solvent overhead, leaving the naphthaleues as the bottoms of the distillation.

The napthalenes may be recovered from'various diaromatic rich streams; for example, coker gas oil or a cycle gas oil stream from a catalytic cracking unit. Preferably the naphthalene containing stream having a boiling range of approximately 320*600" F. is passed to a distillation column provided with a trap out tray, intermediate of its height, permitting the removal of a side out. The hydrocarbon stream is preferably split into an overhead vapor stream having a boiling range of circa 320- 400 F., a side cut with a boiling range of approximately 380-450 F. and a bottoms which will normally com prise roughly 70% of the feed. The side out will likely make up about 25% of the feed and be composed of, for example, 10% naphthalenes, 10% mono-aromatics or alkyl benzenes and 80% saturates. The overhead fraction may be expected to comprise about 80% saturates and 20% mono-aromatics. The naphthalene containing side out is passed to an intermediate section of a conventional liquid-liquid extraction zone, at the opposite ends of which there are introduced respectively a sulfolane solvent stream and a stream comprising the condensed overhead fraction from the earlier distillation. These two latter streams flow countercurrently and assist in the formation of a rafdnate made up principally of alkyl benzenes and saturated hydrocarbons and an extract phase containing naphthalene itself and its homologs. Generally, the naphthalene extract comprises among others, naphthalene itself, methyl and dimethyl naphthalenes, ethyl naphthalene, propyl naphthalene and possibly other homologs. The rafiiuate is removed from the top of the extraction zone and the extract phase from the bottom. The overhead fraction from the earlier distillation zone which is introduced to the bottom of the extraction zone backwashes the exiting extract phase from that zone, serving to further concentrate its naphthalenes content.

The data of the following table clearly demonstrate the advantage to be had in the use of simple sulfolane, H C SO as a solvent for the extraction of naphthalene and its homologs from an alkyl benzene containing hydrocarbon fraction. There it will be seen that B,B-iminodipropionitrile and sulfolane are both equally selective with respect'to hydrocarbon types at the same carbon number. Solvent type selectivity appears in the table as the log of B Reference to FIG. 2 which is a plot of an unusually good solvent (an imaginary solvent) will more clearly define the meaning of the data in the table.

The distribution coefiicient K of FIG. 2 is a measure of the ease with which the material is dissolved by the solvent and is defined as the percent weight of a component in the extract phase divided by the percent weight of that same component in theraffinate. For instance, a distribution coefiicient of 10 indicates that the particular material is 10 times more prevalent in the extract than in the ratfinate phase. In the plot of FIG. 2 the distribution coefiicient is plotted on a logarithmic scale as the ordinate against the carbon number of the oil components as the abscissa. The upper curve of the plot (FIG. 2) depicts alkyl naphthalenes and the lower curve pertains to the alkyl benzenes. In the plot of the solvent illustrated, naphthalene which has a carbon number of 10 extracts ten times more readily than an alkyl benzene of a like carbon number and an alkyl naphthalene of say 18 carbo'n'numbers may be extracted with considerably more ease than an alkyl benzene of a comparable carbon number, but in this latter instance it will be seen that the alkyl benzene of 10 carbon atoms has the same distribution coefii cient K as the C alkyl naphthalene. This means that there will be no separation of the latter two materials with the use of the imaginary solvent plotted.

Molecular weight selectivity for the imaginary solvent of FIG. 2 is measured by the slope B of the two parallel lines. This slope expressed as a number for each particular solvent appears in the second data column of the table as B. With reference to sulfolane and B,fl-oxydipropionitrile (see the table) it will be seen that the latter material is much more sensitive to molecular weight changes as evidenced by its larger molecular weight selectivity value B. While the two solvents are equally selective with respect to hydrocarbon type at any carbon number, the loss of selectivity with increase in carbon number is much greater for irninodipropionitrile, approximately twice than for sulfolane. For example, in the case of extracting a hydrocarbon feed in the range of C C (kerosene range) consisting of various hydrocarbon types, among which are alkyl benzenes and alkyl naphthalenes with first say iminodipropionitrile as a solvent, the C alkyl benzene will extract to the same degree as the C alkyl naphthalenes, that is, no separation of these two materials will be achieved. The slope of the type lines (curves) of an iminodipropionitrile plot would be much greater than the slopes of the type lines of FIG. 2. If sulfolane is used as the solvent for this type separation of the C -C kerosene range, the C alkyl naphthalenes would be extracted with a selectivity of 2.5 over the C alkyl benzene, This example illustrates the superiority of sulfolane as a solvent for the separation of diaromatics and mono-aromatics in a typical light kerosene feed. The third data column of the table under the heading B pertains to such a separation from a C C range light kerosene feed.

It is evident from the foregoing kerosene example that the suitability of a particular solvent for a type separation of a series of compounds from a wide spectrum feed cannot be merely determined from the value of its type selectivity. In selecting a suitable solvent for this purpose one must look (see FIG. 2) for the proper correlation of both a high type selectivity (vertical separation B) and for a low molecular weight selectivity (low slope B). Mere review of either type selectivity or molecular weight selectivity to the exclusion of the other is not enough. What is more important is the value of the effective end separation 13, which is the measure of the ease of separation of the several naphthalenes from the alkyl benzenes of an oil fraction. For instance, in the table, the effective end separation data B shows that the highest carbon number alkyl naphthalene of the kerosene range is 2.52 times more readily separated than the alkyl benzene (i.e. the C alkyl benzene) having 5 carbons less than the naphthalene in the case of the solvent sulfolane while for the solvent, fi,fl-oxydipropionitrile the two materials separate with nearly the same case, actually, the C alkyl benzene dissolves somewhat more readily than the C alkyl naphthalene as shown by the 0.96 B value. With the use of the solvent ethylene glycol for this same separation, it develops that there is approximately a four times greater concentration of the C alkyl benzene in the extract compared to the C alkyl naphthalene. The data found in the third data column of the table are derived for a light kerosene fraction 5 carbon numbers Wide.

Thus, it can be seen from the data of the table that for a 5 carbon wide feed, the normally ditficult separation of the end components of the alkyl benzene from the alkyl naphthalene types can be easily accomplished with sulfolane, for there the factor B is 2.52, and hence a few plates in the extractor are sufficient to efiect quantitative separations of the two types, On the other end, the end components of the two aromatic types will not separate in the desired fashion where B is unity or less than unity. Even the dipropionitrile solvents which are highly efiective type selective solvents are inefiicient for the treatment of a broad spectrum feed because of their high molecular weight type" selectivity B. '4 Simple sulfolane is capable of distinguishing between an alkyl naphthalene and an alkyl benzene of 8 less carbon atoms than the naphthalene. This characteristic of sulfolane combined With its high solubility makes it especially desirable for the solvent extraction of hydrocarbon fractions.

Sulfolane has a number of other highly desirable properties. Of these mention should be made of its high density (1.27 at 30 C.) which helps to decrease settling time, thereby improving extractor efficiency. Its unusually high solubility for diaromatics (several-fold as great as the solubility of iminodipropionitrile) means that the size of the extractor for a sulfolane process will be considerably smaller than that required for iminodipropionitrile. Sulfolane is also suitably inert.

The process according to this invention may be applied to mixtures of hydrocarbons with any of the numerous sulfolane solvents. The simple sulfolane has the formula Its derivatives are compounds wherein one or more of the hydrogen atoms are replaced by an organic radical which may contain a polar grouping and, more specifically, may contain oxygen, nitrogen, sulfur and/or halide atoms. In hydrocarbon-substituted sulfolanes the hydrocarbon radicals are most commonly alkyl radicals. The process is applicable to any of the sulfolane derivatives of this type, such derivatives are more completely enumerated in the US. Patents Nos. 2,360,859, 2,360,860 and 2,360,861 the disclosures of which patents are incorporated herein by reference.

The process is particularly applicable to sulfolane itself and to hydrocarbon-substituted sulfolanes wherein one or more of the hydrogen atoms is replaced by an alkyl radical having from one to ten carbon atoms, inclusive, i.e., to sulfolane solvents containing more than three and less than fifteen carbon atoms, inclusive such as 3-methyl sulfolane, 2,4-dimethyl sulfolane, and 2-isopropyl sulfolane. Other examples of such sulfolane solvents are enumerated in US. Patent No.'2,360,861.

In the preferred embodiment of the process described earlier there is a transfer of the naphthalene extract from the sulfolane solvent to a non-polar solvent in a second liquid-liquid extraction. The two solvents should be substantially immiscible in each other and the second nonpolar solvent should preferably display a more favorable distribution coeflicient for the hydrocarbon extract than the first solvent. The non-polar solvent is preferably a paraflin. C7-C10 paraflins are contemplated for use, with heptane in particular and octane being the preferred materials. The non-polar solvent has a lower boiling point than the naphthalene extract and is stable at and above its normal boiling temperature. The separation of the nonpolar solvent and naphthalene extract is conveniently achieved by flashing with the non-polar solvent being vaporized and removed overhead.

'The process of the invention comprises the use of a diluent with the sulfolane solvent, for example water or an organic material, to adjust the selectivity of or the solubility of the solutes in the primary solvent. In the embodiment of FIG. 1 the sulfolane solvent contains 10% Water.

The naphthalene extract product of the process may be readily hydrogenated to provide a good high density fuel or the extract may be used as a source of naphthalenes since the naphthalene concentration is many fold greater than in the original feed stock. Crystallization is the conventional procedure used for the separation of naphthalene from its homologs.

In the preferred embodiment of the improved process as illustrated in FIG. 1 a stream of coker gas oil having a boilingrange of approximately 320-600 F. enters an intermediate section of a distillation column 10 through a line 11. The column illustrated is supplied with a trap out tray 13 for the removal of a side cut fraction via a line 14. The distillation column is operated to supply an overhead vapor stream which passes in a line 15 to a condenser 16. This top stream will normally make up approximately 5% of the feed and have a boiling range of 320400 F. A portion of the overhead condensate is refluxed to the tower in line 18. The side cut fraction removed from the trap out tray 13 in line 14 comprises roughly 25% of the feed and has a boiling range of 380- 450 F. The bottoms product makes up approximately 70% of the feed and leaves the column through a line 19.

The side cut fraction has a composition generally of 10% naphthalenes, 10% mono-aromatics and the rest saturated compounds. The top fraction can be expected to be around saturates and 20% mono-aromatics. The side cut fraction of line 14 is introduced to a central section of a conventional liquid-liquid extraction column 21. The lean sulfolane stream containing 10% water enters the top of this column in a line 24 passing downwardly therethrough in countercurrent flow to a second stream made up of the top condensate which enters the bottom of the extraction column through a line 23. The extract phase enriched in naphthalene and its homologs after being backwashed by the condensate stream leaves the bottom of the extractor in a line 26 and is passed to the top portion of a second liquid-liquid extraction column 29. The raflinate phase of the first extraction column leaves the top of that column via line 31.

The extract phase introduced to this second extraction zone passes in countercurrent flow to a lean heptane stream introduced to the bottom of the zone through a line 32. The naphthalene extract transfers to the lean heptane forming a second extract phase which leaves the top of the second extractor by a line 34. Lean sulfolane solvent is removed from the bottom of the second extractor through the aforementioned line 24 and is recycled to the first extractor. The second extract phase or fat heptane flowing in the line 34 passes to a combination cm alescer and cooler 35 in the bottom of which the en trained sulfolane solvent carried by the fat heptane separates. The sulfolane solvent collecting in the bottom of the coalescer leaves that vessel via a line 36. The eflfluent fat heptane stream from the coalescer moves through a line 39 to a water washing tower 42 which is supplied with water to its top section by a line 43. There is a phase separation within this tower, providing a now sulfolane free fat heptane which leaves the tower in a line 46 opening into a central section of a flasher 47. The other liquid phase from the Water wash tower comprising water and sulfolane is removed from the bottom of that tower in a line 49 which opens into the top section of a second wash tower 51.

The heat needed for the operation of the flasher 47 is provided by a reboiler 52. This flasher achieves a separation of the heptane from the naphthalenes, with the heptane being removed overhead as vapors in a line 54 from the top of the flashing zone to a water cooled condenser 55. The liquid heptane condensate is recycled in the aforementioned line 32 to the bottom of the second extractor. The naphthalene extract is removed from the bottom of the flasher in a line 59. The naphthalene product contains around naphthalene and naphthalene homologs along with approximately 5% alkyl benzenes and a trace of saturates.

The solvent to hydrocarbon ratio for both the sulfolane extractor and the heptane extractor in this example is approximately 0.7 on a weight basis. Generally speaking, the sulfolane-hydrocarbon ratio will be in the range of 0.2 to 2.0 and the heptane-hydrocarbon ratio is in the same range.

The raflinate out of the first extractor 21 flowing in the line 31 enters a coalescer and cooler unit 61 wherein a portion of the entrained sulfolane solvent separates. The

separated solvent is removed from the coalescer via a line 63 which joins with the like line 36 from the other coalescer and cooler 35. The united lines form a common conduit 65 and the combined sulfolane solvent stream flows therethrough to a conduit 67 from the bottom of the Water wash tower 51. The solvent free rafiinate from this latter mentioned wash tower is removed in a line 69. The rafiinate stream is composed almost exclusively of mono-aromatics and saturated compounds. This raffinate is an excellent jet fuel material for commercial airplanes, being devoid of such smoke 'formersas naphthalenes, and, in addition, being of suitably low total aromatic content.

The combined stream of line 67 composed of water and sulfolane is introduced to a central section of a water flashing tower 70. Here the water is removed overhead and recycled via the aforementioned line 43 back to the first water wash tower 42. The water flasher is operated to provide areconcentrated sulfolane solvent stream of the same water content as the lean solvent. The reconcentrated sulfolane stream is returned through a line 72 to the sulfolane recycle line '24. The two extractors may be operated at the same temperature .of approximately 160 F. and the two coalesce'rs and coolers ata somewhat reduced temperature of 100 F.

We claim as our invention:

1. A process for recovering naphthalenes of at least a five carbon number spread from a hydrocarbon fraction containing alkyl benzenes of a like carbon number spread, the steps comprising subjecting said fraction to a liquidliquid extraction with s'ulfolanesolvent to obtain anextract-solvent phase rich in said naphthalenes and a rafiinate phase rich in said alkyl benzenes, separating the extract-solvent phase from the rafiinate and thereafter recovering the naphthalene extract from the sulfolane solvent.

2. A process for recovering a naphthalene series of at least a five carbon number spread from a hydrocarbon fraction containing alkyl benzenes of a like carbon number spread, the steps comprising subjecting said fraction to a liquid-liquid extraction with sulfolane solvent to obtain an extract-solvent phase rich in said. naphthalenes and a rafiinate phase rich in said alkyl benzenes, separating the two phases, thereafter subjecting the separated extract-solvent phase to a second liquid-liquid extraction with a non-polar solvent substantially immiscible with the sulfolane solvent and having a boiling point lower than that of the naphthalene extract to form a second solvent extract phase containing the non-polar solvent and the naphthalenes, and subsequently separating the naphthalenes from the non-polar solvent.

' 3. A process in accordance with claim 2 wherein the non-polar solvent is 'heptane and the materials of the second extract phase are separated by distilling the heptane overhead and recovering the naphthalenes as distillation bottoms.

4. A process in accordance with claim 1 for recovering naphthalenes of from about a live carbon number spread to about an eight carbon number spread from a hydrocarbon fraction containing alkyl benzenes of a like carbon number spread.

5. The method of recovering high yields of a series of naphthalenes of at least a five carbon number spread in a relatively high state of purity from a high boiling range hydrocarbon feed stock containing saturated hydrocarbons and alkyl benzenes having a carbon number range comparable to that of the naphthalenes, the steps comprising distilling the feed stock to separate a fraction containing the desired naphthalenes along with some alkyl benzenes and a second fraction of a lower boiling range than the naphthalene containing fraction and having a substantially higher saturate content than said naphthalene fraction, subjecting the naphthalene-containing fraction to a liquid-liquid extraction with sulfolane solvent to obtain an extract solvent phase rich in said naphthalenes and a ratfinate phase rich in said alkyl benzenes, backwashing the extract-solvent-phase with said second fraction to further enrich the naphthalene content of said extract phase, and thereafter subjecting the extract-solvent phase to a second liquid-liquid extraction with a non-polar solvent immiscible with the sulfolane and having a boiling point lower than that of the naphthalenes to form a second extract phase containing the non-polar solvent and the naphthalenes and subsequently separating the naphthalenes from the non-polar solvent.

6. A process in accordance with claim 5 wherein the non-polar solvent is heptane.

References Cited in the file ofthis patent UNITED STATES PATENTS 2,360,861 Pierotti et a1. Oct. 24, 1944 2,690,417 Shalit et a1. Sept. 28, 1954 2,773,918 Stephens Dec. 11, 1956 2,802,888 Stuart Aug. 13, 1957 2,837,585 Murray et a1. June 3, 1958 2,878,261 Broughton Mar. 17, 1959

Claims (1)

1. A PROCESS FOR RECOVERING NAPHTHALENES OF AT LEAST A FIVE CARBON NUMBER SPREAD FROM A HYDROCARBON FRACTION CONTAINING ALKYL BENZENES OF A LIKE CARBON NUMBER SPREAD, THE STEPS COMPRISING SUBJECTING SAID FRACTION TO A LIQUIDLIQUID EXTRACTION WITH SULFOLANE SOLVENT TO OBTAIN AN EXTRACT-SOLVENT PHASE RICH IN SAID NAPHTHALENES AND A RAFFINATE PHASE RICH IN SAID ALKYL BENZENES, SEPARATING THE EXTRACT-SOLVENT PHASE FROM THE RAFFINATE AND THEREAFTER RECOVERING THE NAPHTHALENE EXTRACT FROM THE SULFOLANE SOLVENT.

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