Dec. 10, 1968 KARL-Heinz Eisenlohr ET AL 3,415,742 RECOWERY OF AROMATICS BY EXTRACTION WITH SOLVENT MIXTURE OF N-METHYL-PYRROLIDONE AND GLYCEROL Filed Dec. 20, 1967 3. Sheets-Sheet l

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United States Patent Office Patented Dec. 10, 1968 1. 2 3,415,742 On the other hand, the following disadvantages of the RECOVERY OF AROMATICS BY EXTRACTION addition of water must be taken into consideration: WITH SOLVENT MIXTURE OF N-METHYL (1) The solvent capacity for aromatics is reduced. TPYRROL DONE AND GLYCEROL (2) The water is vaporized azeotropically with the Karl-Heinz Eisenlohr, Buchschlag, and Eckart Miller, aromatics to be separated from the solvent which causes Bergen-Enkheim, Germany, assignors to Metallgesell Substantial increase in the heat energy required. schaft Aktiengesellschaft, Frankfurt am Main, Ger many (3) In many instances the water causes undesired Continuation-in-part of application Ser. No. 490,818, chemical reactions, especially hydrolysis. Sept. 28, 1965. This application Dec. 20, 1967, Ser. No. A number of non-aqueous solvent mixtures have also 692,019 O become known, such as, for example, mixtures of SO2, Claims priority, application Germany, Sept. 29, 1964, ethylene glycol and formamide or mixtures which con M 62,610 sist of a "primary solvent” which contains glycol deriva 1 Claim. (C. 208-323) tives and a "secondary solvent' such as methanol, etha nol and acetone. 5 These solvent mixtures, however, have the disadvan tage that a portion thereof has a lower boiling point than ABSTRACT OF THE DISCLOSUIRE the aromatics to be recovered whereas the other portion Aromatic hydrocarbons are extracted with solvent from has a higher boiling point. The separation of the aromatics hydrocarbon mixtures containing both aromatic and non from the solvent therefor requires substantial plant in aromatic hydrocarbon components using a mixture of N 20 vestment as well as high operation costs which in general methyl pyrrolidone and glycerol as the solvent. The ex are not compensated for by the advantages of the solvent tractions are conducted by liquid-liquid extraction. combinations. Further proposals for solvents include, for example, Hammunw"...wlu. (1) the combination of various glycol derivatives, (2) Cross-reference to related applications 25 mixtures of two solvents of which the first contains 1 This application is a continuation in part of copending or 2 hydroxyl groups and the second contains 2 or more application Ser. No. 490,818, entitled "Recovery of Aro hydroxyl groups, (3) ethylene carbonate with additions matics by Extraction or Extractive Distillation with Sol of glycerol, ethylene glycol, pentaerythritol, formamide, vent Mixtures,' and filed Sept. 28, 1965, now Patent formic acid, ethanol amine, monochlorohydrin, acetamide, 3,366,568, in the names of Karl-Heinz Eisenlohr and 30 resorcinol or hydroquinone as "diluent,” (4) alkane dini Eckart Muller. triles, dimethyl hydantoin, N-alkylpyrrolidones, B-buty rolactone, cyan ethers of diethylene glycol, tri- or tetra Background of the invention ethylene glycols or any desired mixtures of these solvents Field of the invention.--The present invention provides which are selective for aromatics or olefins. an improved process for the recovery of aromatic hydro 35 However, without exception, no advantages over the carbons, especially benzene, toluene and xylene, from use of a single solvent have been proved for these and hydrocarbon mixtures containing the same, by extraction many other proposals for the use of mixtures of various with water free solvent mixtures selective for aromatics Selective Solvents boiling above the aromatics to be re which as a whole boil over the boiling point range of the covered for the liquid-liquid extraction of aromatics, nor aromatics to be recovered. The extraction with the solvent 40 have any indications been given as to what quantity and mixtures concerned is by liquid-liquid extraction. in what proportions that various components of the Description of the prior art-The recovery of aromatic mixture should be used in order to attain advantages over hydrocarbons from hydrocarbon mixtures by liquid-liquid the use of single solvents. extraction with selective solvents has been practiced on a The state of the art concerning the possibilities of use commercial scale for several decades. However, from the of solvent combinations for the liquid-liquid extraction fact that again and again new solvents are Suggested and of aromatics can be summarized in that the effect of the patented for this purpose, it can be concluded that the addition of water to a solvent is known qualitatively, and ideal solvent has as yet not been found. that something is known concerning the properties of In addition to pure solvents, in many instances organic various combinations of glycol derivatives. However, solvent mixtures have been proposed as the selective 50 from which points of view, solvents may otherwise be solvents. Furthermore, widely spread practice is to add combined in order that they give good properties for certain quantities of water to the solvent. The addition liquid-liquid extraction of aromatics has neither been of water offers the following advantages: described in scientific literature nor in the patent literature. (1) The selectivity of the solvent is increased. 55 Summary of the invention.-The object of the present (2) The separation, especially of the higher aromatics, invention is to provide a process for the recovery of from the extract obtained is easily accomplished even aromatic hydrocarbons from hydrocarbon mixtures con with a small difference between the boiling point of the taining the same by means of a liquid-liquid extraction solvent and the aromatics because the aromatics distill azeotropically with the water at temperatures under process. 100° C. 60 The essence of the present invention lies in the use, (3) The sump temperature in the distillation column as the liquid extracting medium, of a water free solvent employed for separation of the aromatics from the sol mixture which boils above the boiling point range of the vent extract is lowered so that not only are losses en aromatics to be recovered and which consists of a mix gendered by thermal decompositions reduced but also the ture of N-methyl pyrrolidone and glycerol. heat energy required for heating the solvent to the sump 65 Brief description of the drawings.-In the accompany temperature is lowered. ing drawings: (4) With regard to liquid-liquid extractions a further FIG. 1 is a diagram showing the maximum quantity advantage is that the field of the existence of two phases of benzene in the heavy phase (solvent phase) with re is increased, so that larger quantities of aromatics may be 70 spect to the composition of the solvent mixture N-methyl contained in the feed stock without leaving the two pyrrolidone (hereinafter referred to as NMP) and glyc phase field. erol; 3,415,742 3. 4. FIG. 2 is a diagram showing the dependency of the sulfoxide with aromatics ranges between 35 and 50% selectivity of the composition of solvent mixture NMP especially are illustrative of the group of solvents with and glycerol; low aromatics range. FIG. 3 is a diagram in which the selectivity is plotted The solvents of group B with a high aromatic range, against the partition-coefficient in dependence on the 5 that is, solvents with an aromatic range of at least 50% composition of solvent mixture NMP and glycerol; in the maximum of the two-phase system belong to the FIG. 4 is a phase diagram for the 3 component Sys second group. They are therefore near the Solubility tem. NMP-glycerol-benzene; limit with aromatics, for example, so that they are mis FIG. 5 is a diagram showing the dependency of the cible in all proportions with benzene but not with Xylene. viscosity at 50° C. of solvent mixture NMP and glycerol O There are solvents of this group which have very high on its composition; and selectivity with good capacity, the latter, nevertheless, FIG. 6 schematically shows an apparatus for carrying being lower than that of the solvents of group A. As a out a liquid-liquid extraction according to the invention. consequence, solvents of group B would be the ideal Sol Description of the preferred embodiment.-According vents for the extraction of aromatics insofar as only to the invention, it was unexpectedly found that despite the selectivity and capacity is taken into consideration, the large number of solvents known for these purposes but remarkably they without exception possess other and despite their very different types of chemical struc physical properties of such unfavorable values that they ture, certain rules exist, the application of Which renders must be considered substantial disadvantages. Above all, it possible to select from the boundless large number of representatives of this group all have the disadvantages possible combinations, those which can be expected to 20 that they are no longer stable at their boiling point and have optimal properties. have a high viscosity. In addition, their melting points in Above all the following physical properties are desired some instances are above normal temperature. in a selective solvent for the recovery of aromatics by Compounds with double bonded oxygen and dinitriles, liquid-liquid extraction. such as, for example, Sulfolane, ethylene and propylene (1) High selectivity. 25 carbonate, oxy-, thio- and imino-dipropionitrile, mono (2) High capacity. methyl formamide and tetraethylene glycol especially are (3) High aromatics range, that is, the formation of illustrative of group B solvents. a two-phase system even at high aromatic concentrations. The solvents of the third group C have a miscibility (In the following, the highest aromatics concentration gap with aromatics, that means, they are not miscible in in the 3 substance system benzene-n-heptane-Solvent at 30 all proportions with aromatic hydrocarbons. Representa which a two-phase system is still present and above which tives of this group which dissolve less than 25% of separation into two phases no longer occurs is indicated benzene at 20 C. are preferably employed. Their capac as the "aromatics range'). ity is less than that of the solvents of both other groups. (4) Low solubility of the solvent in the hydrocarbon Their selectivity is better than that of solvents of the first phase. group. Almost all have a high viscosity. Thermal stabil (5) Low viscosity. ity at the boiling point is sometimes given. This group (6) High density. mostly contains chain compounds with one or more (7) Boiling point above the boiling point range of the hydroxyl groups, often also compounds with amino aromatics to be recovered. groups, such as, for example, ethylene glycol, diethylene (8) Thermal and chemical stability at boiling point. 40 glycol, propylene glycol, glycerol, mono-, di- and tri (9) Melting point below ambient temperature. ethanol amine, 1.4-cyclohexane dimethanol, diglycol It was ascertained that these various physical proper amine, formamide, malondinitrile, hydrazine and others. ties do not occur with complete irregularity in the in According to the invention it was found that solvent dividual solvents but rather that certain conformities mixtures could be obtained which have substantially bet to rules can be observed which are of special significance ter properties than could be expected from the rule of for the selection of a favorable solvent combination. mixtures by mixing a solvent having a low aromatic range Of the properties given above, the third, namely, the belonging to the first group (A) with a solvent having aromatics range is especially suited as the classifying a miscibility gap with aromatics and belonging to the principle for dividing the solvent into certain groups. If third group (C). In each instance the selectivity is better the solvents are arranged according to increasing aro 50 than could be expected from the rule of mixtures. The matics range they can be divided into three groups: ability to take up aromatics with solvents of group A is (A) Solvents with low aromatics range. limited by their aromatics range, and with solvents of (B) Solvents with high aromatics range and group C by their low solvent capacity for aromatics. The (C) Solvents which do not have unlimited miscibility combination of a solvent of group A with a solvent of with aromatics. group C results in a mixture having an ability to take A low aromatics range is one of less than 50% by up aromatics which is always better than that of each weight and preferably less than 35% by weight is the component and which often exceeds that of the com maximum of the two-phase region. ponent with the best take up ability by 50% and more. The solvents of the first group A are characterized An undesirable characteristic of solvents of group C by their high capacity, low selectivity and high solubility 60 is their high viscosity. In this respect, an unexpected of the solvent in the hydrocarbon phase. In view of their advantage also is found in the combinations according to low selectivity the solvents of this group normally are the invention as the viscosity of the solvent mixture is only employed in combination with water whereby the always less than could be expected from the rule of above-mentioned disadvantages must be accepted. mixtures. It has now been found that most of the solvents of In addition, it was found that the best mixing solu this group possess two other important and advantageous tion for the various solvent mixtures can quickly be found properties, namely, low viscosity and good thermal stabil according to a very definite method: ity at the boiling point. Compounds with heterocyclic One investigates the three component system, solvent ring systems, such as, morpholine, N-methyl pyrrolidone, A-solvent C-benzene, determines the solubility limits and furfural, keto dioxane, and also dimethyl formamide, then the critical point ("plait point”), that is, the point aniline, ethylene diamine, nitromethane, ethylene glycol where at the solubility limit the volumes of the light and mono-methyl ether, diethylene glycol mono-methyl ether, heavier phases are equal. The composition at this point is and others, all of which have an aromatics range below the most favorable composition for the liquid-liquid ex 35%, and diethylene triamine, triethylene tetramine, traction of aromatics. tetraethylene pentamine, butyrolactone, and dimethyl The critical point for a particular composition of a 3,415,742 5 6 solvent A and solvent C, with the prerequisite that solvent Preferred mixtures of solvents of the group A with A and benzene and solvent A and solvent C are com solvents of the group C are as follows: pletely miscible and that solvent C and benzene have a miscibility gap, is determined as follows: Equal parts by volume of benzene and solvent C are 5 Solvent Weight placed in a vessel. Two phases are formed, one of which mixture Components percent predominantly contains benzene-generally the lighter phase-in the following designated as the HC-phase (hy I a v- w (SEEidoteEthylene glycol- 45,55, 0O drocarbon phase) and one which predominantly contains II (Siretylpyrrollidon 58.99 Solvent C-generally the heavy phase-designated in the a -nethylpyrrollidone.----Alioviion a : following as the S-phase (solvent phase). Then small in O III------{REa - a - a a - a - a ------66.8 crements of solvents A are added while observing how IV as a (SSEYEidone:Diglycolamine- 25.14.9 both phases behave or change with respect to each other. W W w W (EEacetone.Glycerol-...--- 20,79.8 2 When a certain quantity of solvent A has been added the 2(2-amino ethoxy) ethanol or oxethoxyethylamine. phase separation line will suddenly disappear and only 5 one phase will be present. If by chance the volumes of the phases have been equal just before disappearance of These relationships are more fully explained in the the phase separation line the critical point would already following with reference to FIGS. 1-6 with the mixture have been determined. The critical point lies between the of N-methyl pyrrolidone (abrreviated as NMP) and composition where two phases are last observed and the 20 glycerol as example. As can be seen from FIG. 1 the composition at which the phase separation line disap the ability to take up aromatics of a mixture of a solvent pears. Usually, however, the volumes of the phases will of group A (NMP) and a solvent of group C (glycerol) be different and shortly before disappearance of the phase is better than that which corresponds to the rule of mix separation line the phase present in the larger quantity tures and is represented by the broken line A-C. In gen will be a multiple of the other phase. The procedure 25 eral, the ability of a solvent mixture of group A with a which follows under such circumstances depends upon solvent of group C to take up aromatics is greater than which of the two phases was the larger just before dis that of each solvent individually. In the illustration given appearance of the phase separation line. If the HC-phase the maximum take up is achieved with a mixture of 41% is the smaller one, small increments of benzene are added of glycerol and 59% of NMP and amounts to 45 weight to the mixture until the mixture becomes cloudy and 30 percent of benzene in the heavy phase. again separates into two phases. If at this point the HC The dependency of the selectivity upon the solvent phase is still the smaller one, then a mixture of equal mixture composition, once for 10% benzene and once for parts of benzene and solvent A are added in small in 40% benzene in the light phase, is shown in FIG. 2. Both crements until the phase separation line again disappears curves run above the tie line between the points of the so that only one phase is present. Thereafter this pro 35 pure solvent compound. cedure is repeated, that is, so much benzene is added The manner in which the optimum between selectivity until clouding and phase separation occurs and then so and capacity can be determined is shown in FIG. 3. In much of an equal mixture of benzene and solvent A is Such figure the capacities (partition coefficients) and the added until the phase separation line disappears, at some selectivities for different mixing ratios of NMP and glyc point the condition will occur in which the S-phase rather 40 erol are respectively plotted as abscissa and ordinates. than the HC-phase is the smaller. At this point the critical All data are based on a 3 component mixture of solvent point has already been exceeded and one can usually de benzene-heptane at a ratio of 70:20:10 parts by weight in termine its position by interpolating between the last and order to have comparable results. A logarithmic scale second last measuring point. However, if this is not be Was chosen since the relative change of the two charac lieved sufficiently accurate, because the quantities added 45 teristics is more important than their absolute change. It were selected too large, several mixtures are prepared can be seen that the points of the individual mixtures lie which are near the critical point and which still just above and to the right from the line A-C and therefore separate into two phases and a mixture of equal quanti towards values of higher capacity and higher selectivity. ties of benzene and solvent A is added thereto in small It furthermore can be seen that the range which extends 50 furthest up and right and therefore the optimal economic increments. The mixture in which the two phases are of range lies between 35 and 45% glycerol. equal volume just before the phase separation line dis This value of about 41% glycerol is also found when appears corresponds to the composition of the critical the three component diagram of both solvents with ben point. Zene is investigated and the critical point is determined as If, on the other hand, the S-phase is smaller just be 55 described above. FIG. 4 gives the diagram for the system fore disappearance of the phase separation line during the NMP-glycerol-benzene. At the critical point P the ratio first addition of solvent A the procedure is analogous of glycerol to NMP is 41:59. but the benzene added to cause phase separation is re It also can be seen from FIG. 5 that the viscosity at placed by solvent C and the mixture of equal parts of 50° C. of a mixture of NMP and glycerol, above all in the benzene and solvent A added to cause disappearance of 60 middle range, is significantly lower than was to be ex the phase Separation line is replaced by a mixture of equal pected from the rule of mixtures. parts of solvent C and solvent A and the procedure re The present invention is generally suited for the re peated until the S-phase just becomes the larger phase covery of aromatics from hydrocarbon mixtures contain and the critical point interpolated or determined anal ing aromatics by liquid-liquid extraction and also for Ogously to the above procedures. 65 the recovery of extracts in which the concentration of aromatics in the hydrocarbon extracts has only been in When the feed stock from which the aromatics are creased over that in the starting mixture. An especial ad to be recovered is one in which the non-aromatics pre vantage of the process according to the invention is that dominantly are paraffins or if a paraffinic antisolvent is it also is adapted for the recovery of high purity aro used the quantity of solvent C in the mixture can be 5 70 matics, which in recent times have assumed considerable to 10% less than previously defined. If, on the other hand, significance as starting materials for chemical syntheses. the non-aromatics in the feed stock contains large quanti The process according to the invention renders it possi ties of olefins and/or naphthenes it often is expedient ble to recover aromatics whose non-aromatics content is to increase the quantity of solvent C in the mixture about below 0.1% and even under 0.01% without difficulty. 5 to 10%. 75 Even higher purity requirements such as absence of non 3,415,742 7 8 aromatics in quantities capable of chromatographic detec supplied to solvent stripper 8 which contained 20 actual tion can be achieved according to the invention. trays and was operated with a reflux ratio of 1:1. 1000 The invention is further illustrated by the following kg./h. of pure aromatics of the composition example and FIG. 6 which diagramatically shows the ap paratus employed in such example. Weight percent B ------5 EXAMPLE T ------30 1500 kg/h. of a feed stock of the following composi X ------40 tion: Co aromatics ------25 Weight percent O B (benzene) ------3.3 were taken off overhead through conduit 9. The Sump T (toluene) ------20.3 product (4500 kg./h.), which was practically pure sol X (Xylene) ------28.0 vent mixture, was recycled to extractor 2 through line 10. Co aromatics ------25.0 The aromatics mixture taken off overhead from strip Nonaromatic hydrocarbons ------23.4 5 per 8 was distilled in a known manner to separate it into its components. The benzene recovered had a melt were supplied through conduit 1 into the 12th stage of a ing point of 5.50° C., the toluene recovered had a refrac 24 stage countercurrent extractor 2 (a mixer settler bat tive index n=1.4967 and no non-aromatics were dis tery) in which it was extracted with 4500 kg./h. of a cernible gas chromatographically in the xylene fraction. solvent mixture consisting of 40 weight percent of glyc We claim: erol and 60 weight percent of NMP. 6250 kg./h. of an 1. A process for separating a hydrocarbon mixture extract of the composition containing aromatic and non-aromatic hydrocarbons into Weight percent fractions of different degrees of aromaticity by solvent B ------5.1 extraction with a solvent mixture and recovering a more T ------5.3 concentrated aromatics fraction from said mixture which X ------6.4 comprises contacting the mixture with a water free sol Co aromatics ------4.0 vent mixture which as a whole boils above the aromatics Nonaromatic hydrocarbons ------7.2 to be recovered, said solvent mixture consisting of sol Solvent ------72.O. vent (A), N-methyl-pyrrollidone, and solvent (C), glyc were withdrawn from the bottom of extractor 2 and Sup 30 erol, the ratio of the quantity of said solvent A to the plied to the preliminary distillation column 4 over con quantity of said solvent C in said solvent mixture being duit 3. Column 4 had 40 actual trays and was operated from X:Y to X:Y-10 weight percent, the ratio X:Y with a reflux ratio of 0.5:1. 750 kg./h. of the head prod being the ratio of solvents A and C at the critical point uct which consisted of in the three component system solvent A, solvent C and Weight percent 35 benzene, said solvent mixture being a mixture of 31 to B ------36 51% of glycerol and 69 to 49 weight percent of N-methyl T ------4 pyrrollidone, effecting phase separation of the phases thus Nonaromatic hydrocarbons ------60 formed, an extract phase in which the hydrocarbons ex was recycled to extractor 2 as reflux over line 5. This tracted from said mixture has a higher aromatic concen head product contained all of the non-aromatics orig 40 tration than said mixture and a raffinate phase less aro inally contained in the extract. 500 kg/h. of raftinate of matic than said mixture and distilling an aromatics rich the composition hydrocarbon fraction from the solvent mixture in said Weight percent extract phase. Toluene ------1. 45 No references cited. Xylene ------4 Co aromatics ------25 DELBERT E. GANTZ, Primary Examiner. Nonaromatic hydrocarbons ------70 H. LEVINE, Assistant Examiner. were withdrawn from the extractor through line 6. 5500 kg./h. of a non-aromatics free extract were with 50 U.S. C. X.R. drawn from the sump of column 4 through line 7 and 260-674