Method for separating thiophene from mixtures thereof with benzene
United States Patent [1 1 Wehner et a1.  
 METHOD FOR SEPARATING THIOPHENE FROM MIXTURES THEREOF WITH BENZENE Inventors: Klaus Wehner, Leuna: Werner Kisan, Halle-Neustadt: Gerald Kunz, Merseburg-West. all of Germany Assignee: Veb Leuna-Werke Walter Ulbricht&#34;, Leuna. Germany Filed: Oct. 23, 1973 Appl. N0.: 408,892  
 US. Cl. 203/53; 203/51; 203/56; 203/58; 203/59; 203/60; 260/332.8; 260/674 R; 260/674 SE Int. Cl B0ld 3/40; C07c 7/08; C07c 7/10 Field of Search 203/51, 56, 54, 55, 58, 203/60, 59; 260/3328. 674 SE, 674 R Apr. 22, 1975 Primary E.raminerWilbur L. Bascomb. Jr. Attorney. Agent, or Firm-Nolte and Nolte [57] ABSTRACT In the extraction, particularly the extractive distillation. of thiophene from a mixture thereof with benzene, a N-alkyl-ecaprolactam, particularly in which the alkyl is of 1-4 carbon atoms, is used as the solvent which is selective for thiophene.  
 8 Claims, No Drawings METHOD FOR SEPARATING THIOPHENE FROM MIXTURES THEREOF WITH BENZENE This invention concerns a method for separating thiophene from mixtures thereof with benzene. particularly from technical fractions which contain thiophene and benzene as well as possibly other hydrocarbons. using selective solvents.  
  Benzene is an important starting product of the chemical industry. It is obtained today mostly from petroleum. but the production of benzene from coal has gained considerable importance. The crude benzene obtained in the technical production is always contaminated with sulfur compounds. particularly crude benzene fractions obtained by processing coal contain relatively high proportions of thiophene.  
  Because of the very high purity requirements for benzene used for further processing. it is necessary to remove the thiophene to a great extent from the crude benzene.  
  It is known to remove the thiophene impurities chemically from benzene. The most important technically used methods of this type are washing&#39;with concentrated sulfuric acid and catalytic hydrorefining.  
  The disadvantage of these methods is that losses in the benzene yield are unavoidable, and that the thiophene becomes unusuable since it is transformed into resinous waste products during the washing with sulfuric acid. while it is completely destroyed in catalytic hydrorefining.  
  Thiophene is a valuable starting product which is in great demand for chemical processing. Among the purposes for which it can be used are the production of dyes, plastics. special resins, vulcanization accelerators. antioxidants, perfume products. special solvents. growth stimulators. herbicides. defoliants and biologically active substances. Accordingly, the development of methods for separating thiophene from technical crude benzene without chemical changes is of great importance.  
  lt has already been suggested to separate thiophene and benzene by azeotropic distillation with the use of methanol or dioxolane as azeotrope formers. The disadvantages of this procedure are the poor yields of the end product (dioxolane difficult separation due to the proximity of the boiling point of the azeotrope of thiophene and benzene to that of the separating agent (methanol), as well as the technical difficulty in recovering the separating fluid from the azeotrope.  
  It is also known to free benzene of its thiophene content by liquid-liquid extraction. A suitable solvent is. for example. dimethyl sulfoxide in mixture with water. Due to the low thermal stability of dimethyl sulfoxide, there are considerable difficulties, however. in the obtaining of the extracts and in the regeneration of the solvent.  
  The same disadvantages also exist with the use of diethylene glycol as a liquid-liquid extractant.  
  It has also been suggested to separate benzene and thiophane by rectification. However, because of the small difference of the boiling points. the separation with sufficient purities and yields of both components is extremely difficult. It requires the use of a highly effective rectification apparatus, is very costly. and can therefore hardly be realized on a technical scale. Though distillation processes for the production of lowsulfur crude benzene fractions and thiophene-enriched fractions are of great importance in the coke chemicals industry. the production of highly concentrated thiophene fractions requires very high energy expenditures and results in great yield losses. while the production of pure thiophene from coking products by distillation is not possible because of the inseparable content of nonaromatic hydrocarbons.  
  Finally. it is known to achieve the separation ofthiophene and benzene by extractive distillation. using selected solvents. This principle has advantages. due to its great effectiveness. The following solvents have been suggested so far: aliphatic glycols. such as ethylene glycol. diand triethylene glycol; glycol ethers; alkanol amines. such as mono-, diand triethanol amine: aliphatic diamines, such as ethylene diamine; aromatic amines. such as N.N-dimethyl-aniline. oand mphenylene diamine; dimethyl formamide; pyrrolidone- 2; N-methyl-pyrrolidone-Z; tricresyl phosphate; propylene carbonate; and l,l-dioxytetrahydrothiophene (sulfolane).  
  A solvent for separating thiophene and benzene by extractive distillation must meet various requirements. The most important are:  
 high selectivity. as much as possible in wide concentration and temperature ranges:  
 high dissolving power at the operating temperature;  
 sufficiently great boiling point difference relative to the components to be separated;  
 high thermal stability;  
 high chemical stability;  
 low melting point;  
 safe to handle;  
 easy technical production:  
 low price.  
  All the solvents used so far have disadvantages as far as some of these requirements are concerned. Thus. for example. tricresyl phosphate and N.N-dimethylamine have a low selectivity. Pyrrolidone-Z. sulfolane and particularly oand m-phenylene diamine are not suitable for technical use because of their high melting points. Ethylene glycol and its derivatives. alkanol amine and ethylenediamine have relatively low dissolving powers. Propylene carbonate and derivatives of ethylene glycol. have disadvantages regarding thermal or chemical stability.  
  The object ofthe invention is to provide a method for separating thiophene from mixtures with benzene. particularly from technical fractions which contain thiophene and benzene as well as possibly other hydrocarbons, by using selective solvents and which is more economical than the known methods.  
  The problem is thus to find solvents for the separation of thiophene from mixtures with benzene and possibly other hydrocarbons which have a high dissolving power, high selectivity. very good thermal and chemi cal stability, a low melting point, and low viscosity values, are easy and safe to handle and of low toxicity, and are technically easy to produce, at a low price.  
  This problem is solved according to the invention in that a N-alkyl- -caprolactam or a mixture containing N-alkyl-e-caprolactam is used as the selective solvent.  
  Preferably, a N-alkyl-e-caprolactam is used the alkyl group of which contains 1 to 4 carbon atoms.  
  Of particular advantage is the use of N-methyl-ecaprolactam. It is advisable that the N alkyl-ecaprolactam be used in mixture with added components, particularly with ethylene glycol and/or diethylene glycol and/or phenylene diamine.  
  Of particular advantage is the separation of thiophene by extractive distillation.  
  The solvents or solvent mixtures according to the invention can be used similarly to the prior art solvents for the selective separation of thiophene from mixtures with benzene. Since these solvents are also suitable for the selective separation of aromatic and non-aromatic hydrocarbons. the non-aromatic hydrocarbons contained in technical benzene-thiophene mixtures can likewise be separated at the same time in the benzenethiophene separation.  
  By producing mixtures of the solvents according to the invention with added components, it is possible to vary the dissolving power and selectivity of the solvent system and to adapt it to different problems.  
  The N-alkyl-e-caprolactams, particularly N-methyl-ecaprolactam. exhibit a favorable selectivity behavior in the thiophene/benzene system (see Table 1). This is surprising insofar as the values of the dissolving power are very high. too. This shows. among other things, the superiority of the solvents according to the invention over dimethyl formamide and N-methyl-pyrrolidone-Z.  
  The solvents according to the invention have. furthermore. a high thermal stability. Thus, for example, N-methyl-e-caprolactam showed no signs of destruction on prolonged heating to boiling temperature at normal pressure.  
  Furthermore, the high chemical stability of the solvents according to the invention, particularly against hydrolytic influences in neutral aqueous media, is very noteworthy. This results in an extremely low corroding effect. which is highly advantageous for technical uses. Selective solvents used on a technical scale are employed in many cases in mixture with water to increase their selectivity. in addition to the reduced dissolving power, these solvent mixtures have a considerable corroding effect which can cause unpleasant disturbances and considerable operating costs. The corroding effects of aqueous mixtures of N-methyl-e-caprolactam, dimethylformamide, N-methyl-pyrrolidone-2 and other solvents were, therefore, compared with each other by determining the linear corrosion rate to be expected per year by extrapolating the linear corrosion of carbon steel subjected to the action of the solvent mixtures for 28 days at 100C. The following values, in which the proportions of water are by weight, were obtained:  
 corrosion in mm/ year Remarks N-methyl-e-caprolactam/ 0.01  
 257: water slight punctiform corrosion The great thermal stability. the great chemical stability, and related thereto, the extremely low corrosion effect represent substantial advantages of the solvents according to the invention, particularly N-methyl-ecaprolactam.  
  The substances used according to the invention have relatively low viscosity values, and are easy to handle since their crystallization point is below 10C and they exhibit a great tolerance for supercooling.  
  The toxicity of the solvents according to the invention is low.  
  Another advantage is that the solvents according to the invention can be synthesized from starting products which are technically available in large quantities.  
  The invention described above will be explained more fully in the following examples.  
 EXAMPLE 1 for thiophene is expressed as the reciprocal value of the limiting activity coefficient at infinite dilution. For comparison, the corresponding values for some of the known solvents tested under the same conditions have been tabulated.  
 Table l Dissolving Power Selectivity Temperature ll-yThio- (7Benzene/ Solvent (C) phene) -yThiophene) N-Methyl-scaprolactam 30 1.48 1.66  
  1 .59 1 .63 N-ethyl-e-caprolactam 30 1.88 1.55  
  60 1 .9 1 1 .5 3 N-hutyl-e-caprolactam 30 1.95 1.48  
  60 1 .97 1 .46 Ethyleneglycol 30 0.04 1.53  
  60 0.04 1 .50 Diethyleneglycol 30 0.19 1.50  
  60 0.2 l l .45 Triethyleneglycol 30 0.32 1 .49  
  60 0.34 l .44 Sulfolane 30 0.71 1.63  
  60 0.78 1 .60 Monoethanolamine 30 0.05 1.5 8  
  60 0.07 1 .55 m-Phenylenediamine 0.27 1.71 Dimethylformamide 30 0.88 1.50  
  60 0.98 1 .46 N-Methylpyrrolidone-Z 30 1.22 1.61  
  The foregoing values show the particularly great dissolving power of the solvents according to the invention and at the same time their high selectivity. particularly in the case of N-methyl-e-caprolactam.  
 column. In the upper part of the column was added preheated N-methyl-e-caprolactam in a 12.5 fold volume relative to the test mixture. With a reflux ratio of 3:1 to 3.521. by volume. a distillate was removed from the EXAMPLE 5 head of the column which contained practically quantitatively the amounts of benzene and cyclohexane con- Slm&#39;larly to F compllailon of dam i Example tained in the original mixture. as well as 0.25% by l was p l conFemmg h behavior ofN&#39;meth&#39; weight thiophene. The solvent obtained in the sump of yl-e-caprolactam 1n mixture with added components. m the column which was laden with the extracted thi0 a d ll l gph l i l ly I&#34; phene. was introduced into the center part of a second g g ldmmej fi fi f Table m packed column and distilled. A head product was ob- W 1 e PWPOYIIOM t e w are tained which contained 98.7% thiophene. The Shh-em pressed by weight.  
 Table 2 Dissolving Selectivity Temperature power (-yBenzene/ Solvent Mixture (C) (7Thiophene) 7Thiophene) N-methyl-e-caprolactam 30 0.25 1.55 ethyleneglycol 80% N-methyl-Ecaprolactam 30 0.33 1.58 507r/ ethyleneglycol 50% 60 0.39 1.52 N-methyl-s-caprolactam 30 0.95 1.63 75%/ ethyleneglycol N-methyl-e-caprolactam 60 1.17 1.59 80%/ ethyleneglycol 20% N-methy -e-caprolaetam 0.36 1.53 20%! diethyleneglycol 80% N-methyl-e-caprolactam 30 0.61 1.57 diethyleneglycol 50% 0.75 1.53 N-methyl-e-caprolactam 30 l. 10 1.61 diethyleneglycol 25% 60 1.18 1.58 N-methyl-e-caprolactam 30 0.45 1.67 50%} m-phenylenediamine 50% 60 0.62 1.65 N-methyl-s-caprolactam 30 1.80 1.64 N-ethyl-e-caprolactam 60 1.85 l .61 20% N-methyl-e-caprolactam 30 1.67 1.61  
  0% N-ethyl-e-caprolactam 60 1.75 1.58 50% The values show that the dissolving power of ethyl- N-methyl-e-caprolactam was recovered in the sump of ene glycol and diethylene glycol is considerably imthe second column in practically pure form and it could proved by the addition of N-methyl-e-caprolactam. In 45 be returned to the extractive distillation column for furthe N-methyl-e-caprolactam/mphenylene diamine ther use. mixture the dissolving power of the mixture exceeds What is claimed is: that of pure m-phenylene diamine. At the same time it 1. In the method of separating thiophene from a mixis of advantage that the very unfavorable high crystalliture thereof with benzene in which the mixture is subzation point of m-phenylene diamine (628C) is con- 50 jected to extraction with a solvent selective for the thiosiderably reduced by the production of a mixture with N-methyl-e-caprolactam. thereby providing substantially greater ease of handling.  
 The applicability of the solvents according to the invention in mixtures with added components is not limited to the exemplary such components and mixing ratios indicated above. In principle. the solvents according to the invention can be used in combination with all substances mentioned in the prior art as solvents for separating thiophene and benzene.  
 EXAMPLE 3 phene and the benzene and thiophene are recovered. the improvement in which the solvent comprises a N- alkyl-e-caprolactam.  
  2. In the method of claim 1, in the improvement in which the alkyl is of l-4 carbon atoms.  
  3. In the method of claim 2, in the improvement in which the solvent comprises N-methyl-e-caprolactam.  
  4. In the method of claim 1, in the improvement in which the solvent further comprises at least one of ethylene glycol, diethylene glycol and phenylene diamine.  
  5. The method of claim 1, in which the extraction is extractive distillation.  
  6. The method of claim I, in which the extraction is liquid-liquid extraction.  
  7. In the method of claim 1, in the improvement in which the solvent is in mixture with water.  
 8. In the method of claim 3, in the improvement in which the solvent is in mixture with water.