Method for separating cyclohexene

Abstract: A method for separating cyclohexene from a mixture comprising cyclohexene and at least one member selected from cyclohexane and benzene, which comprises subjecting the mixture to extractive distillation to obtain a fraction rich in cyclohexene, wherein a nitrogen-containing compound of the following formula (1), or a mixed liquid of N-methyl-2-pyrrolidone and water, is used as an extraction solvent for the extractive distillation: wherein each of R 1 and R 2 is a C 1-10 alkyl group or hydrogen, and n is an integer of from 2 to 4.

Description: The present invention relates to a method for separating cyclohexene from a mixture comprising cyclohexene and at least one member selected from cyclohexane and benzene. In recent years, an attention has been drawn to a method for producing cyclohexanol by hydrating cyclohexene obtainable by partial hydrogenation of benzene. The partial hydrogenation of benzene is carried out usually in the presence of a ruthenium catalyst and water, whereby a reaction mixture comprising cyclohexene, unreacted benzene and by-product cyclohexane, is usually obtained (e.g. Japanese Examined Patent Publications No. 5370/1991 and No. 19098/1990 and Japanese Unexamined Patent Publication No. 074141/1992). In order to obtain cyclohexene of high purity from the reaction mixture of such partial hydrogenation of benzene, a method for separating cyclohexene from such a mixture becomes critical. However, boiling points of the three components i.e. cyclohexene, benzene and cyclohexane, are very close to one another, and it. is difficult to separate them by usual distillation. Accordingly, it is common to employ extractive distillation as a method for separating such a mixture. For such extractive distillation, methods have been proposed in which various extracting media, such as N,N-dimethylacetamide, adiponitrile, sulforan, dimethyl malonate and dimethyl succinate, are used (e.g. Japanese Unexamined Patent Publications No. 1645724/1983, No. 1645725/1983, No. 172323/1983, No. 295311/1987 and No. 41441/1992). In such conventional extractive distillation employing various extracting media, the separation efficiency is not necessarily adequate, and when an expensive extraction solvent is used, such extractive distillation is economically disadvantageous. Further, as mentioned in the above-mentioned Japanese Unexamined Patent Publication No. 41441/1992, a decomposition product of an extraction solvent sometimes brings about corrosion of the distillation apparatus or acts as a catalyst poison in various reactions, as an impurity in cyclohexene. The present inventors have conducted extensive studies on a method for separating cyclohexene and as a result, have found it possible to solve all of the above-mentioned problems by extractive distillation employing a certain specific nitrogen-containing compound as an extraction solvent. The present invention has been accomplished on the basis of this discovery. Thus, the present invention provides a method for separating cyclohexene from a mixture comprising cyclohexene and at least one member selected from cyclohexane and benzene, which comprises subjecting the mixture to extractive distillation to obtain a fraction rich in cyclohexene, wherein a nitrogen-containing compound of the following formula (1) is used as an extraction solvent for the extractive distillation: wherein each of R 1 and R 2 is a C 1-10 alkyl group or hydrogen, and n is an integer of from 2 to 4. In the accompanying drawings: Figure 1 is a flow sheet illustrating an embodiment of the present invention which corresponds to the method 3 ○ as described hereinafter. Figure 2 is a flow sheet illustrating another embodiment of the present invention which corresponds to the method 4 ○ as described hereinafter. Now, the present invention will be described in detail with reference to the preferred embodiments. In the present invention, the mixture which is subjected to extractive distillation is a mixture comprising cyclohexene and at least one member selected from the cyclohexane and benzene. A reaction mixture obtained by partial hydrogenation of benzene, may be mentioned as such a mixture to which the present invention is applicable. The nitrogen-containing compound of the above formula (1) is preferably the one wherein each of R 1 and R 2 is a C 1-3 alkyl group, and n is 2. Specifically, such a compound may, for example, be 1,3-dimethyl-2-imidazolidinone or 1,3-diethyl-2-imidazolidinone, preferably 1,3-dimethyl-2-imidazolidinone. 1,3-Dimethyl-2-imidazolidinone is not only excellent as an extracting agent for separation useful in the present invention, but also relatively easily available for industrial purpose, thermally stable and free from corrosiveness, and it has a characteristic such that the toxicity is low. The nitrogen-containing compound of the formula (1) will hereinafter be referred to as "the nitrogen-containing compound (1)". The following methods 1 ○ to 4 ○ may be mentioned as typical embodiments for separating cyclohexene from a mixture containing cyclohexene by extractive distillation employing the nitrogen-containing compound (1) as an extraction solvent. 1 ○ A method wherein a mixture of cyclohexene and cyclohexane is subjected to extractive distillation by means of the nitrogen-containing compound (1) as the extraction solvent, so that cyclohexane is distilled from the top of the distillation column and a mixed liquid of cyclohexene and the nitrogen-containing compound (1), is withdrawn from the bottom of the column, and the mixed liquid is subjected to usual distillation to separate cyclohexene. 2 ○ A method wherein a mixture of cyclohexene and benzene is subjected to extractive distillation by means of the nitrogen-containing compound (1) as the extraction solvent, so that a mixed liquid of benzene and the nitrogen-containing compound (1), is withdrawn from the bottom of the distillation column, and cyclohexene is distilled from the top of the column to separate cyclohexene. 3 ○ A method wherein a mixture of cyclohexene, cyclohexane and benzene, is subjected to first extractive distillation by means of the nitrogen-containing compound (1) as the extraction solvent, so that a mixed liquid of benzene and the nitrogen-containing compound (1), is withdrawn from the bottom of the distillation column, and a mixed liquid of cyclohexene and cyclohexane, is distilled from the top of the column, the distillate liquid is subjected to second extractive distillation by means of the nitrogen-containing compound (1) as the extraction solvent, so that cyclohexane is distilled from the top of the distillation column, and a mixed liquid of cyclohexene and the nitrogen-containing compound (1), is withdrawn from the bottom of the column, and the mixed liquid is subjected to distillation to separate cyclohexene. 4 ○ A method wherein a mixture of cyclohexene, cyclohexane and benzene, is subjected to first extractive distillation by means of the nitrogen-containing compound (1) as the extraction solvent, so that cyclohexane is distilled from the top of the distillation column, and a mixed liquid of cyclohexene, benzene and the nitrogen-containing compound (1), is withdrawn from the bottom of the column, the mixed liquid is subjected to second extractive distillation, so that benzene and the nitrogen-containing compound (1) are withdrawn from the bottom of the distillation column, and cyclohexene is distilled from the top of the column to separate cyclohexene. When extractive distillation is carried out in the above methods 1 ○ to 4 ○, a distillation column is used which usually has a theoretical plate number of at least 10, preferably at least 20. The reflux ratio is usually from 0.5 to 30. If the reflux ratio is too small or too large, the separation efficiency deteriorates. If the reflux ratio is too large, the column system is obliged to be large. Usually, the nitrogen-containing compound (1) as the extraction solvent is supplied to an upper portion of the distillation column, and the mixture is supplied to a portion lower than the intermediate portion of the distillation column. At the upper portion, about 5 plates are usually provided as recovery plates. In each of the methods 1 ○ to 4 ○, the extraction solvent is used usually in an amount of at least equivalent to the mixture, and the pressure at the top of the column is usually from 0.02 to 0.3 MPa. In the second extractive distillation in the method 4 ○, the nitrogen-containing compound (1) as the extraction solvent is already contained in the mixed liquid supplied to the distillation step, but the nitrogen-containing compound (1) may be supplemented depending upon the composition or the amount of the components to be separated. Now, more specific embodiments of the present invention will be described with reference to the drawings with respect to a method for separating and recovering cyclohexene from a mixture comprising three different compounds of cyclohexane, cyclohexene and benzene. In the drawings, the nitrogen-containing compound (1) is represented by "NNDIN" as an abbreviation. Figure 1 illustrates the above-mentioned method 3 ○ which comprises firstly separating benzene from the mixture comprising cyclohexene, benzene and cyclohexane, and then separating cyclohexene and cyclohexane. Referring to Figure 1, the mixture comprising three compounds of cyclohexane, cyclohexene and benzene, is supplied by a line 1 to a distillation column D1, and the nitrogen-containing compound (1) as an extraction solvent is supplied by a line 2 to the distillation column D1. In the distillation column D1, extractive distillation is carried out, whereby a mixture of cyclohexene and cyclohexane is withdrawn from the top of the column and transported by a line 3 to a distillation column D3, and from the bottom of the column, a mixture of benzene and the nitrogen-containing compound (1), is withdrawn and sent by a line 4 to a distillation column D2. In the distillation column D2, benzene and the nitrogen-containing compound (1) are separated by distillation, whereby from the top of the column, benzene is withdrawn by a line 5, and from the bottom of the column, the nitrogen-containing compound (1) is withdrawn by a line 6. On the other hand, to the distillation column D3, the mixture of cyclohexene and cyclohexane is supplied by the line 3, and the nitrogen-containing compound (1) is supplied by a line 7, to carry out extractive distillation. From the top of the column, cyclohexane is withdrawn by a line 8. From the bottom of the column, a mixture comprising cyclohexene and the nitrogen-containing compound (1), is withdrawn and sent to a distillation column D4 by a line 9. In the distillation column D4, cyclohexene and the nitrogen-containing compound (1) are separated by distillation, whereby from the top of the column, cyclohexene is withdrawn by a line 10, and from the bottom of the column, the nitrogen-containing compound (1) is withdrawn by a line 11. Figure 2 illustrates the above method 4 ○ which comprises firstly removing cyclohexane from the mixture comprising cyclohexene, benzene and cyclohexane and then separating cyclohexene and benzene. Referring to Figure 2, the mixture comprising cyclohexene, benzene and cyclohexane, is sent by a line 1 to a distillation column D1, and the nitrogen-containing compound (1) is sent by a line 2 to the distillation column D1. In the distillation column D1, extractive distillation is carried out, whereby from the top of the column, cyclohexane is withdrawn by a line 3, and from the bottom of the column, a mixture comprising benzene, cyclohexene and the nitrogen-containing compound (1), is withdrawn by a line 4, and the mixture is supplied to a distillation column D2. To the distillation column D2, the nitrogen-containing compound (1) is supplied also from a line 5. Also in the distillation column D2, extractive distillation is carried out, whereby from the top of the column, cyclohexene is withdrawn by a line 6, and from the bottom of the column, a mixture of benzene and the nitrogen-containing compound (1), is withdrawn by a line 7 and supplied to a distillation column D3. In the distillation column D3, benzene and the nitrogen-containing compound (1) are separated by distillation, whereby from the top of the column, benzene is withdrawn by a line 8, and from the bottom of the column, the nitrogen-containing compound (1) is withdrawn by a line 9. Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples. In the following Examples, "parts" and "%" means "parts by weight" and "% by weight" unless otherwise specified. EXAMPLE 1 Extractive distillation was carried out at a reflux ratio of 20 under a column top pressure of 0.1 MPa, while introducing 25 parts/hr of cyclohexane, 45 parts/hr of cyclohexene and 38 parts/hr of benzene to a position at 1360 mm from the top of a packing type distillation column of a 25  SULZER LABO PACKING packed column having a packing height of 2.8 m (HETP = 65 mm, distillation column theoretical plate number = 43 plates) and introducing 320 parts/hr of 1,3-dimethyl-2-imidazolidinone to a position at 330 mm from the top of the column. As a result, from the top of the column, cyclohexane having a purity of 98.6% was obtained at a rate of 23 parts/hr, and from the bottom of the column, 405 parts of a mixture comprising cyclohexene, benzene and 1,3-dimethyl-2-imidazolidinone, was obtained. COMPARATIVE EXAMPLE 1 Extractive distillation was carried out in the same manner as in Example 1 except that as the extraction solvent, N,N-dimethylacetamide was used instead of 1,3-dimethyl-2-imidazolidinone. As a result, from the top of the column, cyclohexane having a purity of 95.5% was obtained at a rate of 23 parts/hr. EXAMPLE 2 Using the same distillation column as used in Example 1, extractive distillation was carried out at a reflux ratio of 5 under a column top pressure of 0.1 MPa, while introducing a mixture comprising cyclohexene and benzene obtained in the same manner as in Example 1 to a position at 2100 mm from the top of the column at a rate of 52.8 parts/hr of cyclohexene and 32.4 parts/hr of benzene and introducing 320 parts/hr of 1,3-dimethyl-2-imidazolidinone to a position at 330 mm from the top of the column. As a result, from the top of the column, cyclohexene having a purity of 98.2% was obtained at rate of 48.7 parts/hr, and from the bottom of the column, a very small amount of cyclohexene and 367 parts of a mixture of benzene and 1,3-dimethyl-2-imidazolidinone, were obtained. EXAMPLE 3 Using the same packing type distillation column as used in Example 1, extractive distillation was carried out at a reflux ratio of 10 under a column top pressure of 0.1 MPa, while introducing a mixture comprising cyclohexene and benzene obtained in the same manner as in Example 1 to a position at 2100 mm from the top of the column at a rate of 52.8 parts/hr of cyclohexene and 32.4 parts/hr of benzene and introducing 320 parts/hr of 1,3-dimethyl-2-imidazolidinone to a position at 330 mm from the top of the column. As a result, from the top of the column, cyclohexene having a purity of 93.2% was obtained at a rate of 51.1 parts/hr, and from the bottom of the column, 354 parts/hr of a mixture of benzene and 1,3-dimethyl-2-imidazolidinone, was obtained. EXAMPLE 4 Using the same distillation column as used in Example 1, extractive distillation was carried out at a reflux ratio of 5 under a column top pressure of 0.1 MPa, while introducing a mixture comprising cyclohexene and benzene obtained in the same manner as in Example 1 to a position at 2100 mm from the top of the column at a rate of 52.3 parts/hr of cyclohexene and 32.4 parts/hr of benzene and introducing 320 parts/hr of 1,3-dimethyl-2-imidazolidinone to a position at 330 mm from the top of the column. As a result, from the top of the column, cyclohexene having a purity of 98.2% was obtained at a rate of 49.0 parts/hr, and from the bottom of the column, a very small amount of cyclohexene and 356 parts of a mixture of benzene and 1,3-dimethyl-2-imidazolidinone, were obtained. EXAMPLE 5 Using the same distillation column as used in Example 1, extractive distillation was carried out at a reflux ratio of 5 under a column top pressure of 0.1 MPa, while introducing a mixture comprising cyclohexene and benzene obtained in the same manner as in Example 1, to a position at 2100 mm from the top of the column at a rate of 52.1 parts/hr of cyclohexene and 37.7 parts/hr of benzene and introducing 423 parts/hr of 1,3-dimethyl-2-imidazolidinone to a position at 330 mm from the top of the column. As a result, from the top of the column, cyclohexene having a purity of 99.7% was obtained at a rate of 48.7 parts/hr, and from the bottom of the column, a very small amount of cyclohexene and 464 parts of a mixture comprising benzene and 1,3-dimethyl-2-imidazolidinone, were obtained. EXAMPLE 6 Distillation simulation was carried out using static simulator soft ASPEN, manufactured by ASPENTEC Co. (the physical property model for the simulation was the one calculated as a homogeneous system by correction UNIFAC, and the same applies to the results of distillation simulation given hereinafter). As a result, extractive distillation is carried out at a reflux ratio of 8 under a column top pressure of 0.1 MPa, while introducing 750 parts/hr of cyclohexane and 3000 parts/hr of cyclohexene and 2500 parts/hr of benzene to the 12th plate from the top of the distillation column having a theoretical plate number of 22 plates (each of a condenser at the top and a reboiler at the bottom is counted as one plate) and introducing 3000 parts/hr of 1,3-dimethyl-2-imidazolidinone to the 4th plate from the top of the column. As a result, from the top of the column, a mixture of cyclohexane and cyclohexene (19.96% of cyclohexane and 77.45% of cyclohexene) containing 2.58% of benzene, is obtained. EXAMPLE 7 Distillation simulation was carried out using static simulator soft ASPEN, manufactured by ASPENTEC Co. As a result, extractive distillation is carried out at a reflux ratio of 20 under a column top pressure of 0.1 MPa, while introducing 750 parts/hr of cyclohexane and 3000 parts/hr of cyclohexene to the 50th plate from the top of a distillation column having a theoretical plate number of 62 plates (each of a condenser at the top and a reboiler at the bottom is counted as one plate) and introducing 20000 parts/hr of 1,3-dimethyl-2-imidazolidinone to the 5th plate from the top of the column. As a result, from the top of the column, cyclohexane containing 30.0% of cyclohexene is obtained, and from the bottom of the column, a mixture of cyclohexene and 1,3-dimethyl-2-imidazolidinone (12.9% of cyclohexene and 87.0% of 1,3-dimethyl-2-imidazolidinone) containing 0.1% of cyclohexane, is obtained. According to the present invention, cyclohexene of high purity can be obtained efficiently from a mixture comprising cyclohexene and at least one member selected from cyclohexane and benzene. The specific nitrogen-containing compound used as an extraction solvent in extractive distillation of the present invention, is chemically stable and relatively inexpensive. Accordingly, the present invention is expected to be a very advantageous method for producing cyclohexene of high purity on an industrial scale over a long period of time.

Claims: A method for separating cyclohexene from a mixture comprising cyclohexene and at least one member selected from cyclohexane and benzene, which comprises subjecting the mixture to extractive distillation to obtain a fraction rich in cyclohexene, wherein a nitrogen-containing compound of the following formula (1) is used as an extraction solvent for the extractive distillation: wherein each of R 1 and R 2 is a C 1-10 alkyl group or hydrogen, and n is an integer of from 2 to 4. The method for separating cyclohexene according to Claim 1, wherein in the formula (1), each of R 1 and R 2 is a C 1-3 alkyl group, and n is 2. The method for separating cyclohexene according to Claim 1, wherein the nitrogen-containing compound of the formula (1) is 1,3-dimethyl-2-imidazolidinone. The method for separating cyclohexene according to Claim 1, wherein a mixture of cyclohexene and cyclohexane is subjected to extractive distillation by means of the nitrogen-containing compound of the formula (1) as the extraction solvent, so that cyclohexane is distilled from the top of the distillation column, and a mixed liquid of cyclohexene and the nitrogen-containing compound of the formula (1) , is withdrawn from the bottom of the column, and the mixed liquid is subjected to distillation to separate cyclohexene. The method for separating cyclohexene according to Claim 1, wherein a mixture of cyclohexene and benzene is subjected to extractive distillation by means of the nitrogen-containing compound of the formula (1) as the extraction solvent, so that cyclohexene is distilled from the top of the distillation column, and a mixed liquid of benzene and the nitrogen-containing compound of the formula (1), is withdrawn from the bottom of the column. The method for separating cyclohexene according to Claim 1, wherein a mixture of cyclohexene, cyclohexane and benzene, is subjected to first extractive distillation by means of the nitrogen-containing compound of the formula (1) as the extraction solvent, so that a mixed liquid of benzene and the nitrogen-containing compound of the formula (1), is withdrawn from the bottom of the distillation column, and a mixed liquid of cyclohexene and cyclohexane, is distilled from the top of the column, the distillate liquid is subjected to second extractive distillation by means of the nitrogen-containing compound of the formula (1) as the extraction solvent, so that cyclohexane is distilled from the top of the column, and a mixed liquid of cyclohexene and the nitrogen-containing compound of the formula (1), is withdrawn from the bottom of the column, and the mixed liquid is subjected to distillation to separate cyclohexene. The method for separating cyclohexene according to Claim 1, wherein a mixture of cyclohexene, cyclohexane and benzene is subjected to extractive distillation by means of the nitrogen-containing compound of the formula (1) as the extraction solvent, so that cyclohexane is distilled from the top of the distillation column, and a mixed liquid of cyclohexene, benzene and the nitrogen-containing compound of the formula (1), is withdrawn from the bottom of the column, and the mixed liquid is subjected to distillation, so that cyclohexene is distilled from the top of the column, and benzene and the nitrogen-containing compound of the formula (1), are withdrawn from the bottom of the column.

Inventors: ISHIKAWA TAKESHI, KANDA YU, TSUBOI AKIO, UCHIBORI TOSHIO, ISHIKAWA, TAKESHI, KANDA, YU, TSUBOI, AKIO, UCHIBORI, TOSHIO, Ishikawa, Takeshi, Mizushima Plant, Kanda, Yu, Kurosaki Plant, Mitsubishi Chem. Corp., Tsuboi, Akio, Mizushima Plant, Uchibori, Toshio, Kurosaki Plant, Mitsubishi Chem.
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1996
Main classification: C07C7
IPC codes: C07C7