US3316318A - Process for recovery of aromatics from cracked gasoline fractions - Google Patents

Description

AWM 25, T967 H VOETTER ETAL 3,316,31@

PROCESS FOR RECOVERY OF AROMATICS FROM CRACKED GASOLINE FRACTIONS Filed March 9, 1962 FRACNONANHG HYDROGEHATION ZONE (sELEcm/E INVENTORS.'

HEINZ VOETTER WERNER RIDDERIKHOFF' THEIR ATTORNEY United States Patent O 3,316,318 PROCESS FUR RECVERY @F AROMATICS BRGM CRACKED GASLIINE FRACTIONS Heinz Voetter and Werner Ridderikhoff, Amsterdam, Netherlands, assignors to Shell Oil Company, New York, NSY., a corporation of Delaware Filed Mar. 9, 1962, Ser. No. 178,615 Claims priority, application Netherlands, Mar. 22, 1961, 262,671 6 Claims. (Cl. 260-674) The invention relates to a process for recovering aromatic hydrocarbons in a pure, or almost pure, state from a hydrocarbon mixture containing alkenes and more highly unsaturated compounds in addition to aromatic compounds.

IIt is known that aromatic hydrocarbons can be recovered by extracting hydrocarbon oil fractions. In many cases the sole object is to obtain extracts wit-h a higher content of aromfatics, for use as produced, or after distillation, as high octane components of motor fuel.

It .is also known to separate aromatic hydrocarbons, such as benzene, toluene and xylene, in a pure, or almost pure, state by extraction from petroleum fractions. Such separation cannot, as a rule, be effected in one single process step, particularly in those cases when hydrocarbon oil fractions produced by cracking are used as the starting material. It appears that the presence of unsaturated compounds in these cracked products leads to complications during extraction, such as, for instance, the occurrence of polymerization and/or condensation reactions, in which gum-like products are formed which may cause clogging of the extraction apparatus. Moreover, it is found that certain unsaturated compounds cannot be separated from the aromatic compounds by extraction, or are only partially separated.

The above difficulties can be avoided by subjecting unsaturated cracked fractions to a conventional hydrogenation treatment to saturate unsaturated hydrocarbons before extraction. However, conventional hydrogenation is generally undesirable since, with the exception of the aromatic compounds, not only are the more highly unsaturated compounds, such as the alkadienes and alkynes, satura-ted but also the alkenes are completely or largely saturated. This is disadvantageous since large amounts of costly and often scarce hydrogen are required. Moreover, the alkenes are valuable as a chemical starting material or as a high octane motor fuel component. It has now been found that the above-mentioned intensive hydrogenation, hitherto considered necessary, can be omitted and that pure, or almost pure, aromatic compounds can be recovered from a hydrocarbon mixture containing in addition to aromatic compounds alkenes and more highly unsaturated compounds, by subjec-ting the hydrocarbon mixture to select-ive hydrogenation prior to extraction.

The invention therefore relates to aA process for the .recovery of aromatic hydrocarbons in a pure, or almost pure, state by hydrogenating and subsequently extracting a hydrocarbon mixture containing alkenes and more highly unsaturated compounds in addition to aromatic compounds. The process is characterized in that the hydrocarbon mixture is subjected to a selective hydrogenation, wherein the more highly unsaturated compounds are hydrogenated, and the selectively hydrogenated product is then subjected to extraction.

lBy selective hydrogenation it is meant a hyd-rogenation by which preferably 95% or more of the more highly unsaturated compounds are hydrogenated, while less than 25% and preferably less than :15% of the alkenes originally present or formed from the more highly unsaturated compounds Iare converted into saturated compounds.

The present invention is advantageous in that not only is considerable hydrogen saved, but also in that by extraction of the selectively hydrogenated product, a rainate phase is obtained .that contains most of the Ialkenes present in the base material as well as: any alkenes formed from the more highly unsaturated compounds during the selective hydrogenation. These alkenes in the raffinate phase constitute valuable gasoline components owing to their relatively high octane number. Moreover, the rallinate is more stable since little or no alkadienes are contained therein.

Starting materials suitable for the present process are hydrocarbon mixtures con-taining, in addition to aromatic compounds, alkenes and more highly unsaturated cornpounds, such as alkynes and alkadienes, either in a cyclic structure or not. IFor example, particularly suitable starting materials lare the hydrocarbon mixtures having a tinal boiling point of not more than 375 C. (ASTM) which are produced by thermal and/ or catalytic cracking.

Especially suitable starting materials are gasolines or fractions thereof obtained by thermal cracking hydrocarbons in the presence of steam and/or oxygen. These gasolines are of a special nature in that they have a relatively very high content of highly unsaturated hydrocarbons and a high percentage of valuable aromatic compounds iand alkenes.

Thermal cracking in the presence of steam is preferably conducted with aliphatic hydrocarbon oils having a final boiling point of not more than 230 C. Steam cracking is generally carried out at temperatures between about 550 C. and about 900 C., preferably between 750 C. and 800 C. and preferably under a pressure below about 5 atm. abs. The amount of steam applied is generally 0.110 parts by weight, preferably about one part by weight to one part by weight of starting material. This steam cracking is applied mainly for preparing lower alkenes, in particular ethene and propene, which are used as base materials in the chemical industry. During the cracking treatment usually more than 50% W. of the hydrocarbon oil used as starting material is converted into compounds with four or fewer carbon atoms in the molecule. Gasoline produced as la byproduct of the steam cracking process as a rule contains more than 30% w. and often more than `60% W. of aromatic compounds. In addition the gasoline contains a considerable amount of alkenes as well as more highly unsaturated compounds, mainly dienes, such as the cyclopentadiene type. A fur-ther `advantage is that these gasolines are generally practically free of sulfur.

To obtain pure aromatic compounds it is, according to the present invention, generally advantageous to use starting materials containing no sulfur compounds, or only a small percent-age of sulfur compounds, tha-t is to say, less than 0.1% w. (calculated as elemental sulfur).

The selective hydrogenation is carried out in the presence of free hydrogen and .in the presence of one or more catalysts known for this purpose. Such catalysts contain, for ins-tance, one or more metals of Group VIII of lthe periodic system of elements. Particularly selective catalysts are nickel and/ or cobalt, supported on a carrier containing one or more water-insoluble salts of alkalineearth metals, such as waterinsoluble alkaline-earth metal sulfates, eg., barium sulfate. Other selective catalysts for .this purpose are suliided nickel and sulded platinum, either as such or supported on a carrier.

The selective hydrogenation `is preferably carried out in such a manner that the hydrocarbon mixture is, at least partially, present in the liquid Iphase, and is generally effected at average catalyst temperatures of not more than C. Preferably the treatment is carried out at average catalyst temperatures between 70 C. and 130 C.

l`he fact that selective hydrogenation can be effected at yuch low temperatures provides a considerable economic idvantage compared with complete hydrogenation, which s generally carried out at much higher temperatures. .or example, complete hydrogenation Iin the presence of :obalt-moly'bdenum catalysts is effected at temperatures Jetween 300 C. and 400 C. It is clear that under these :onditions a much more costly plant is required than at :he relatively very low temperatures of the selective hyirogenation according to the present process.

To ensure satisfactory hydrogenation, the amount of iydrogen employed is at least equal to the theoretical imount required for the complete conversion into alkenes 3f the dienes and other highly unsaturated compounds )resent in the starting mixture. Preferably, however, :wice to five times the theoretical amount of hydrogen is used. Owing to this excess of hydrogen, the catalyst activity is generally maintained for a longer period, without, however, leading to further hydrogenation of the alkenes.

Hydrogen can tbe used as such or in the form of a gas mixture containing hydrogen, for instance, a mixture of hydrogen and light hydrocarbons. When usin-g an excess of hydrogen it is advantageous to recirculate the remaining hydrogen, if desired after previously removing undesirable constituents. The gases applied should preferably contain more than 60% v. of hydrogen. Very suitable are, for example, the hydrogen-containing gases obtained in the catalytic reforming of gasoline fractions.

Although the selective hydrogenation catalysts can `be used `in the uidized or suspended state, they are preferably used in the form of a xed bed. Since, as a result of the low hydrogenation temperatures, it is possible for the hydrocarbon mixture to be present at least partially, i.e., to an extent of more than 50% w. and preferably more than 75% w., in the liquid phase during the selective hydrogenation, without it being necessary to apply excessively high pressures. The so-called trickle technique is particularly suitable for this purpose. According to this technique, the starting material, which is present partly in the liquid phase and partly in the vapor phase, is allowed to flow downwards in the presence of hydrogen or of a hydrogen-containing gas over a xed bed of catalyst, the unvaporized part of the starting material owing over the catalyst particles in the form of a thin lm.

Hydrogenation in the liquid phase at a temperature of not more than 160 C. is usually conducted at pressures between and 60 atm. abs. and preferably between 20 and atm. aibs. `Owing to the exothermic nature of the hydrogenation reactions, a certain increase in temperature may occur in the catalyst bed. For this reason, too, it is advantageous to operate in the liquid phase, as, owing to the higher specific heat of the liquid, smaller rises in temperature will generally occur than when operating in the gaseous phase.

The space velocity is generally 0.5 to 5 kg. and preferably 1-3 kg. of hydrocarbon mixture per hour per liter of catalyst. The gas-hydrocarbon ratio is generally between and 300 liters (measured at standard conditions) of gas per kilogram of hydrocarbon mixture.

The present invention is particularly suitable for the recovery of aromatics, e.g., benzene, toluene, xylene and/ or ethyl benzene, for use in the chemical industry. Especially desirable starting materials are hydrocarbon mixtures of which at least v. has a boiling point lower than 200 C. (ASTM). Particularly suitable are the light boiling fractions of the thermally cracked gasolines referred to above, which generally have a relatively high content of highly unsaturated compounds, including cyclic dienes, such as cyclopentadiene, methyl cyclopentadiene and cyclohexadiene, as well as dimers.

It has been found that these gasolines sometimes still contain small quantities of dimers, even after the selective hydrogenation, since under the conditions of the selective hydrogenation, the dimers are only partially hydrogenated. When, in the subsequent extraction or upon removal of the solvent from the extract phase, temperatures of more than C. are applied, the residual dimers partially decompose to the monomers cyclopentadiene, cyclohexadiene and similar compounds. These monomers contaminate the aromatic compounds during further processing and, moreover, they may cause fouling of the plant owing to polymer formation. It was found that this trouble can be obviated by simple means by subjecting the starting material containing the dimer, before extraction, to fractional distillation in such a manner that the fraction to be extracted has a nal boiling point of not more than 160 C. (atmospheric pressure). In order to avoid conversion of the dimers into monomers during distillation, this distillation is preferably carried out under reduced pressure, in such a way that the bottom temperature does not rise above 100 C.

It is essential that the distillation referred to above is carried out after the selective hydrogenation. For, upon distillation, though the dimers remain behind in the bottom product, monomers present in the non-hydrogenated starting material pass over the top. At lower temperatures, such as occur in condensers and lines downstream of the distillation column, the monomers tend to again cause dimer formation, as a result of which the effect of the distillation would `be largely undone.

As a consequence of the selective hydrogenation, extraction of aromatics by means of a selective solvent is accomplished more easily than is possible with unhydrogenated material. The relative order of solubility in a given solvent depends upon hydrocarbon type. For example, the distribution coefficients, expressed as a logarithm, at infinite dilution between diethylene glycol and heptane at C. is approximately as follows for C6 hydrocarbons: Aromatics, 1.61; alkadienes, 2.02; alkenes, 2.18; and alkanes, 2.31. Since selectivity between two components is the ratio between their distribution coefficients, it can `be seen that select-ive hydrogenation of alkadienes improves selectivity.

The selective hydrogenation and/ or extraction is preferably applied to a hydrocarbon mixture from which the components having a boiling point lower than that of benzene are partially or completely removed. As a consequence, the plant is less heavily loaded and a saving of hydrogen may be entailed. Therefore, preferably, the starting material is a hydrocarbon mixture from which the components having a final boiling point of 60 C. or less are removed, for example, by distillation.

The extraction of the aromatic Ihydrocarbons from the selectively hydrogenated product .is effected in a multistage counter-current extraction system, for which a column, for example, filled with packing material or fitted with perforated trays, is suitable. Preferably, however, a column is used -in which a shaft with discs is rotatably mounted as described, for example, in U.S. Patent 2,601,- 674 to G. H. Reman.

In general, suitable extraction agents are the polar solvents known to be selective for aromatics. Preferably a solvent, or mixture of solvents, is chosen with a boiling point higher than that of the starting material to be extracted. These solvents are used as such or in the presence of water in a quantity preferably less than 10% v. Particularly suitable selective solvents are diethylene glycol, dipropylene glycol and/ or other alkylene glycols, as well as sulfolane and/ or substituted sulfolanes, such as methyl and/or dimethyl sulfolane, or mixtures thereof.

In the extraction of the selectively hydrogenated hydrocarbon stream, using solvents of the alkylene glycol type, the ratio by weight of solvent to the product to be extracted is generally between 3:1 and 101:1, and preferably between 411 and 6:1. When u-Sing solvents of the sulfolane type these ratios are generally between 1:1 and 4:1, and preferably lbetween 1.5:1 and 2.511.

The extract phase withdrawn from the extraction zone is passed into a stripper column to remove non-aromatic compounds. In the stripper column a 'bottom temperature of not more than 170 C. and a pressure lower than that in the extraction system are preferably maintained. The vaporous material from the top of the stripper, consisting of non-aromatic hydrocarbons and part of the aromatic hydrocarbons, is condensed and recycled to the extraction system, where it is introduced near the end at which the extract phase is withdrawn.

The amount of material is to be recycled from4 the stripper to the extraction zone is chosen so as to ensure that the extract phase withdrawn from the bottom of the stripper has the required degree of purity, that is to say that is preferably contains less of 0.1% W. of non-aromatic components. When alkylene glycols are used as the selective solvent, the ratio by weight of the material to be recycled from the stripper to the feed to be extracted is generally between 0.6:1 and 2:1, and preferably 'between 0.9:1 and 1.5 :1. When sulfolane or substituted sulfolanes are used as the selective solvents, this ratio by weight is generally between 0.311 and 1.5:1, and preferably *between 0.5 :1 and 1:1.

The extraction and recovery of light aromatic hydrocarbons with diethylene glycol and/ or dipropylene glycol as solvent is preferably carried out as follows. A selectively hydrogenated hydrocarbon mixture containing benzene, toluene, xylene and/ or ethyl benzene, is introduced into a multistage counter-current extraction system held at a temperature of not more than 150 C., and preferably :between 100 and 120 C., and at a pressure sufficient to keep the various streams in the liquid state. At one end of the extraction system diethylene glycol and/ or dipropylene glycol is introduced in a quantity of 4 to 6 parts by weight of product to be extracted and at the same end of the system the raffinate with only a slight content of aromatic hydrocarbons is withdrawn. At `the other end of the extraction system an extract phase with a high content of aromatics is discharged, which is passed, preferably without cooling, into a stripper column with at least four theoretical stages, where a bottom temperature between 40 and 160 C. and a pressure lower than that in the extraction system and preferably between 1 and 2 atm. abs. are maintained. Vapors from the stripper are condensed and, if necessary, any water phase formed can be separated and withdrawn. The Condensed vapors are recycled to the extraction zone where they are introduced at a point near the end at which the extract phase is withdrawn, preferably between the first and second theoretical stages. The extract phase withdrawn from the bottom of the stripper is passed into a distillation column where a pressure lower than that in the stripper and preferably between 0.3 and 0.5 atm. abs. and a bottom temperature of less than 200 C. and preferably :between 150 C. and 180 C. are maintained. The :diethylene glycol and/or dipropylene glycol withdrawn from the bottom of this column is returned, after cooling, to the extraction system. The aromatic compoundfs) passing over the top of the distillation column is (are) i-f so desired further processed in one or more fractionating columns.

The extraction and recovery of light aromatic hydrocarbons with the aid of sulfolane as the solvent is preferably carried out in the same way as described above for diethylene glycol and/ or dipropylene glycol, except that in the extraction system temperatures between 90 C. and 110 C. are preferably applied, while 1.5-2.5 parts by weight of sulfolane per part 'by weight of material to be extracted is introduced. Moreover, a stripper column with at least 3 theoretical stages is generally applied.

The extraction process can be improved further by using a wash liquor (a counter solvent) at the same time. This counter solvent can be introduced into the extraction system either together with the top product from the stripper column, or separately at a point near the end at which the extract phase is withdrawn. In general, `suitable counter solvents are paratiinic hydrocarbons or hydrocarbon mixtures containing at least 30% v. of paraflinic hydrocarbons. When a counter solvent of this kind is used, a smaller amount recycle from the stripper column to the extraction system is sufficient. This amount depends on, among other things, the composition of the counter solvent, and in particular on its aromatic content.

The use of a counter solvent as referred to above has in general the drawback that it has to be separated again from the raffinate, for instance by distillation. It has now been found that this distillation can be omitted if the appropriate counter solvent is used. Thus, for instance, if a gasoline fraction is used as counter solvent the ratlinate mixture withdrawn from the extraction system can be used, for example, as motor fuel. Examples of suitable counter solvents are gasoline fractions with a boiling range between C. and 180 C., such as a platformate fraction, or a naphtha obtained .by direct distillation with a content of at least 30% v. of paratlinic hydrocarbons. In the event of a counter solvent containing aromatic hydrocarbons, these are recovered, as an additional advantage, together with the aromatics from the starting material.

As already indicated above, the load on the extraction system and the subsequent stripper and distillaton columns can be reduced by removing from the selectively hydrogenated product all or part of the components that have a higher and/or lower boiling point than the aromatic hydrocarbon (s) to be recovered. If so desired, the narrow fraction thus obtained can be subjected to a complete hydrogenation before extraction, in order to convert the relatively -small amount of alkenes present in this fraction into alkanes. This hydrogenation can be carried out in a known way, for example with a cobalt-molybdenum catalyst under a pressure of 20 atm. abs., a temperature of 300 C., a space velocity of 1 kg. per liter per hour and a molar ratio of hydrogen to hydrocarbon of 4.

When using diethylene glycol or similar products as the selective solvent which tend to decompose at high temperatures such as those encountered in distillation, a lsmall amount, e.g. ybetween 0.05 and 1% w. of phenothiazine or a substituted phenothiazine, can be added to the solvent.

The invention is further illustrated yby reference to the drawing which is` a schematic diagram of the process of the invention and by four examples wherein the same selectively hydrogenated product is used as the base material in order to facilitate a comparison of results.

Examples I and Il refer to the pdeparation of pure benzene and of pure benzene, toluene, xylene and ethyl benzene, respectively, using diethylene glycol as the selective solvent.4 Example lll illustrates the recovery of pure benzene with diethylene glycol as the selective solvent, applying a counter solvent. Example IV describes the recovery of pure benzene, extraction being elfected with sulfolane as the selective solvent. Compositions of the various streams, as occurring in Examples I-IV, are summarized in Tables I-IV, the columns of the tables being numbered in the same Way as the corresponding lines in the figure; the compositions of the various streams being based on 100 parts by weight of the starting material.

It is to be observed that the laromatic compounds produced according to the present invention are substantially pure, i.e. above 99% purity. Thus, for example, a benzene with a purity of 99.9% or higher anda melting point higher than 5.4 C. can be obtained. In some cases, however, the product does not yet completely fulfill the socalled acid wash color test (ASTM D 848-47). However, after a treatment with activated earth, for instance activated terrana, at a temperature preferably between C. and 180 C., for instance 150 C., the product J )mplies with all the requirements for ASTM test D 83 5- for nitration grade products.

Example l This example illustrates the recovery of pure benzene fom a gasoline obtained as a byproduct in the preparation f ethylene and propylene by steam cracking a straightun hydrocarbon oil having a linal boiling point of 230 I. The composition of this gasoline is listed in column 1 f Table I. Other properties of the gasoline were: boilig range 26 C.-160 C. (ASTM), sulfur content 15 upm., bromine value 50 g./100 (Mcllhiney method), maaic anhydride number 130 mg./g. (Ellis and Jones methd), and hydrogen consumption upon complete hydrogenaion of all unsaturates 90 liters (STN/kilogram.

Referring to the drawing, this gasoline was fed, prin- :ipally in the liquid state and in the presence of 175 liters STP) of hydrogen per kg. of this gasoline, through line l int-o reaction Zone A. The reactor contained a nickel :atalyst supported on alumina as carrier, in the form of l x 3 mm. pellets, which had `been sulfided at a temperaure of 100 C. 'with hydrogen sulfide. Other conditions or the selective hydrogenation were: average catalyst ternvJerature 100 C., pressure 40 atm. abs. and a space velocity 3f 2 kg. of gasoline per hour per liter of catalyst.

The selectivity hydrogenated liquid product withdrawn via line 2 had the composition listed in column 2 of Table l. This product was fed into fractionating column B wherein most of the components with a boiling point lower than that of benzene were `removed under a pressure of 1 atm. abs. Composition of the light tops is given in extraction column D via line 10. A pressure of 2 atm. abs. and a bottom temperature of 140 C. were maintained in stripping column E. Extract phase was withdrawn, via line 12 from the bottom of stripping column E, and then passed, without coo-ling, into distillation column F at a point halfway between the top `and the bottom trays of the column. Column F, having a total of eight theoretical stages, was operated at a pressure of 0.6 atm. abs. and at a bottom temperature of 160 C. Under these conditions, the temperature at the top of the column was 65 C. An amount of steam corresponding to 2% w. calculated on solvent was introduced at the bottom of column F via line 13 to remove traces of aromatic compounds. Solvent withdrawn from the bottom of the column was recycled, after cooling, to extraction column D via line 9. Benzene, together with small amounts of higher boiling aromatic compounds, were passed overhead from column F and condensed. After separation of water formed as the second phase, the benzene was distilled in column G to remove higher boiling components. The benzene thus produced had a purity of 99.9 and `a melting point higher than 5.4 C. In order to bring the product up to nitration grade standards it was passed in the liquid phase at 150 C. over a bed of activated earth at a rate of 1 liter per kg. of earth per hour. The benzene thus treated had an acid wash color (ASTM D 848-47) of less than 1.

Compositions of the various streams are listed in Table I. The numerical headings in the table refer to the corresponding stream shown in the drawing. The compositions are expressed as parts by weight based on 100 parts by weight of original steam cracked gasoline feed.

TABLE I Alkanes Alkencs Alkadienes. Cycloalkancs Cycloalkenes Cycloalkadienes Benzene Toluene. Xylenes Ethylbenzen Styrene. Dimers Total Solvent Water column 3 of Table I. The bottom product was discharged from column B via line 4 and introduced into distillation column C wherein the maior -portion of the components boiling at a higher temperature than benzene was separated as a bottom product at 1 atrn. abs. and a bottom temperature of 100 C. The benzene-containing distillate fraction thus obtained, with a boiling range 65-95 C., was next passed via line 6, in the liquid state, under a pressure of 10 atm. abs. and at a temperature of 110 C., into extraction column D having 13 theoretical stages. The benzene-containing feed was introduced continuously .at a place corresponding to 3 theoretical stages from the bottom of the column. Diethylene glycol was introduced as selective solvent via `line 9, in a quantity of 4 parts by weight per part by weight of feed. A raffinate phase containing no more than 0.5 part by weight of benzene was withdrawn from the top of the extraction column through line 8. At the bottom of column D, through line 10, `an extract phase was withdrawn and introduced, without cooling, into the top of stripper column E, having eight theoretical stages, to remove light non-aromatic hydrocarbons present in the extract phase. In the stripper column, these light non-aromatic hydrocarbons, together with some of the benzene, were removed overhead and recycled, after condensation, to a point near the bottom of Example Il The same selectively hydrogenated product used as a starting material as described in Example I is used to obtain pure benzene, toluene, xylene and ethyl benzene.

Again with reference to the drawing, after the cornponents having a boiling point lower than benzene had been removed from the selectively hydrogenated gasoline in column B, the bottom product withdrawn from B through line 4 (for compositions see Table II) was passed to distillation column C under a pressure of 0.3 atm. abs. and at a bottom temperature of C. A. fraction containing benzene, toluene, xylene and ethylene benzene was removed overhead. The components having a boiling point higher than xylene and ethylbenzene, such as dimers, were removed as a bottoms product from C via line 5. Extraction was effected in extraction column D, with 13 theoretical stages, the benzene-containing feed being introduced at a point corresponding to three theoretical stages from the bottom of column D. The extraction was carried out with diethylene glycol as the selective solvent with a ratio of six parts by weight of solvent per part by weight of feed, a pressure of 10 atom. abs. and at a temperature of 100 C. The extract phase removed through line 10 was separated from light non-aromatic hydrocarbons in stripper column E at a pressure at 1.2 atm. abs. and a Ibottom temperature of 145 C. After removal of the diethylene glycol in distillation column F, at a pressure of 0.4 atom. abs. and at a bottom temperature of 160 C. and a top temperature of 68 C., the mixture of aromatic hydrocarbons removed overhead through line 14 was condensed and separated from water `which formed as a second phase. The mixture was then fractionated in column B to recover benzene overhead via line while the mixture of toluene, xylene and ethyl benzene removed as bottoms was further fractionated in column H to recover the toluene as an overhead product and the C8 aromatics as a bottom product. Composition of the various streams are given in Table II.

10 quantity of 0.5 part by weight per part by weight of feed. Stripper column E was operated under a pressure of 1 atm. abs. with a bottom temperature of 135 C. and a top temperature of 100 C. The top product was discharged and recycled to the extraction column. The extract phase withdrawn through line 12 at the bottom of column E was fractionated in column F under a pressure of 0.5 atm. abs. with a bottom temperature of 160 C. and a top temperature of 70 C. The distillate discharged through line 14 was passed to column G where pure benzene was obtained over the top. Through line 16, a bottom product was removed that contained, among other compound, C7 and C8 hydrocarbons from the counter solvent. This bottom product was mixed with the rainate stream withdrawn through line 8 and used as a TABLE II.-OOMPOSITIONS ARE BASED ON 100 PARTS BY WEIGHT OF STARTING MATERIAL Alkanes Alkenes. Alkadicnes. Cycloalkanes Cycloalkenes Cycloalkadienes. Benzene".

Dimers Hydrodrners Example III For the recovery of pure benzene, the same selectively hydrogenated product described as the starting material. of the components with a boiling point lower or higher than that of benzene was effected in distillation columns B and C in the same way as described in Example I. Extraction of the benzene-containing mixture discharged in Example I was used TABLE IIL-COMPOSITIONS ARE BASED The removal of the major part motor gasoline component. It is observed that in consequence of the application of the counter solvent a saving of heat is effected, because smaller quantities of material from the top of stripper column E are returned to extraction column D than in the process according to Example I, the amounts being 32 parts by weight as against 51.6 parts by weight. Compositions of the various streams are given in Table III.

ON 100 PARTS BY WEIGHT OF STARTING MATERIAL Alkanes 27 Alkenes 5 Alkadienes- 3 Cyeloalkanes. 8 Cycloalkenes 3 Cycloalkadienes 5 Benzene 22 To1uene 13 Xylenes 2 Ethylbenzene 1 Styrene 4 Dimers 7 Hydrodimers Total 100 Solvent Water through line 6 was effected in extraction column D, with 14 theoretical stages, the feed being introduced at a place corresponding to four theoretical stages from the bottom of the column. The extraction was carried out with diethylene glycol as the selective solvent at a ratio of 4.5 parts by weight per part by weight of feed under a pressure of 8 atm. abs. at 100 C. A plattormate fraction with a boiling range of from 95-130 C. (ASTM) was used as counter solvent, which was introduced through line 7 near the bottom of extraction column D in a Example l V las passed to extraction column D, with eight theoetical stages, at a point corresponding to two theoretical tages, from the bottom of the column. Extraction was ffected at a temperature of 90 C. and a pressure of 1 atm. abs. Stripper column E was operated at a pres- ;ure of 2.6 atm. abs. and at a bottom temperature of 50 C. and a top temperature of 100 C., while in :olumn F a pressure of 0.35 atm. abs., a bottom tem- Jerature of 160 C. and a top temperature of 50 C. were naintained. The benzene recovered through line 14 was Eractionated further in column G.

carbon having from 6 through 8 carbon atoms per molecule from a cracked gasoline containing said aromatic hydrocarbon, alkenes, and alkadienes which comprises passing said gasoline together with 50 to 300 liters of hydrogen per kilogram of gasoline over a selective hydrogenation catalyst at a temperature in the range from about 70 to 130 C. and a pressure of about 10 to 60 atmospheres to hydrogenate at least about 95% of the alkadienes while converting less than of the alkenes to alkanes, fractionating the hydrogenated gasoline to obtain a light fraction having a nal boiling point not more TABLE IV.-COMPOSITIONS ARE BADSED ON 100 PARTS BY WEIGHT OF STARTING Alkanes Alkenes Alkadienes Cycloalkanes. Cycloalkenes. Cycloalkadienes Benzene Styreue Dimers Hydrodimers ATE RIAL Total Solvent The results summarized in Tables I-IV, inclusive, clearly show that sulfolane offers important advantages as an extraction solvent compared with diethylene glycol. With comparable extraction results, only 57 parts by weight of sulfolane are required, as against 150 parts by weight of diethylene glycol. Furthermore, when sulfolane is used, only 26.5 parts by weight have to be recycled from the stripper column to the extraction system as against about double that amount (51.6 parts by weight) when diethylene glycol is employed. This signies a relatively considerable saving of heat.

We claim as our invention:

1. A process for the recovery of an aromatic hydrocarbon having from 6 through 8 carbon atoms per molecule from a cracked gasoline containing said aromatic hydrocarbon, alkenes and alkadienes which comprises passing said gasoline together with hydrogen over a selective hydrogenation catalyst under selective hydrogenation conditions to hydrogenate at least 95% of the alkadienes while converting less than 25% of the alkenes to alkanes, fractionating the hydrogenated gasoline to obtain a light fraction having a nal boiling point not more than 160 C., said fractionation being carried out under reduced pressure and with a bottom temperature below C., and extracting said aromatic hydrocarbon from 55 than C., said fractionation being carried out under reduced pressure and with a bottom temperature below 100 C., and extracting said aromatic hydrocarbon from the light fraction by means of a solvent selective for aromatics.

3. The process according to claim 2 wherein the selective hydrogenation catalyst comprises sulfided nickel.

4. The process according to claim 2 wherein the selective hydrogenation catalyst comprises nickel supported on a water-insoluble alkaline earth metal sulfate.

5. The process according to claim 3 wherein the selective solvent is sulfolane.

6. The process according to claim 2 wherein the cracked gasoline is a steam cracked gasoline.

References Cited by the Examiner UNITED STATES PATENTS 2.694,671 11/1954 Baumgarten et al. 208-144 2,717,861 9/1955 Baumgarten et al. 208-144 2,799,627 7 1957 Haensel 208-96 2,870,226 l/1959 Deanesly 208-96 2,952,717 9/1960 Fleck et al. 260-677 3,030,299 4/ 1962 Plummer 208-96 DELBERT E. GANTZ, Primary Examiner. ALPHONSO D. SULLIVAN, Examiner.

H. LEVINE, C. E. SPRESSER, Assistant Examiners.

Claims (1)

1. A PROCESS FOR THE RECOVERY OF AN AROMATIC HYDROCARBON HAVING FROM 6 TGHROUGH 8 CARBON ATOMS PER MOLECULE FROM A CRACKED GASOLINE CONTAINING SAID AROMATIC HYDROCARBON, ALKENES AND ALKADIENES WHICH COMPRISES PASSING SAID GASOLINE TOGETHER WITH HYDROGEN OVER A SELECTIVE HYDROGENATION CATALYST UNDER SELECTIVE HYDROGENATION CONDITIONS TGO HYDROGENATE AT LEAST 95% OF THE ALKADIENES WHILE CONVERTING LESS THAN 25% OF THE ALKENES TO ALKANES, FRACTIONATING THE HYDROGENATED GASOLINE TO OBTAIN A LIGHT FRACTION HAVING A FINAL BOILING POINT NOT MORE THAN 160*C., SAID FRACTIONATION BEING CARRIED OUT UNDER REDUCED PRESSURE AND WITH A BOTTOM TEMPERATURE BELOW 100*C., AND EXTRACTING SAID AROMATIC HYDROCARBON FROM THE LIGHT FRACTION BY MEANS OF A SOLVENT SELECTIVE FOR AROMATICS.

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