||||||||||||III USOO5292.917A United States Patent (19) (1) Patent Number: 5,292,917 Nishihira et al. 45 Date of Patent: Mar. 8, 1994 54 PROCESS FOR PURIFYING DIMETHYL 58) Field of Search ......................................... 558/277 CARBONATE 56 References Cited 75 Inventors: Keigo Nishihira; Shinichi Yoshida; Shuji Tanaka, all of Ube, Japan U.S. PATENT DOCUMENTS 4,467,109 8/1984 Tahara et al. ....................... 560/193 73) Assignee: Ube Industries, Ltd., Yamaguchi, Japan 4,663,477 5/1987 Crandall et al. .................... 560/204 Primary Examiner-Mary C. Lee 21 Appl. No.: 56,891 Assistant Examiner-Michael Ambrose 22 Filed: May 5, 1993 Attorney, Agent, or Firn-Finnegan, Henderson, Farabow, Garrett & Dunner Related U.S. Application Data 57 ABSTRACT 63 Continuation of Ser. No. 837,881, Feb. 9, 1992. Disclosed a process for purifying dimethyl carbonate (30) Foreign Application Priority Data which comprises distillating a mixture of dimethyl car bonate and methanol in the presence of dimethyl oxa Feb. 26, 1991 (JP Japan .................................. 2-053148 late to separate and remove methanol. 51) int. Cli.............................................. CO7C 69/96 52 U.S.C. .................................................... 558/277 4 Claims, 1 Drawing Sheet U.S. Patent Mar. 8, 1994 5,292,917 F. G. ... O O.9 | | | | | | U-2 O. 8 | | | | I-4-31 O.7 | || 424 y O.6 41 || 4 || O.5 4 || 4 || | O. 4 /YA/P | 14 | ||| | ||| | | O.3 // 1 | | | | | | O.2 //1 || | | | | | O. 4 || | | | | || O O O. O.2 O.3 O.4 O5 O.6 O.7 O.8 O.9 1.O X o Without addition of dimethyl oxalate O Addition of dimethyl oxalate F. G. 2 5,292,917 1. 2 azeotropic phenomenon of dimethyl carbonate and PROCESS FOR PURIFYING DEMETHYL methanol, and distilling and separating methanol easily. CARBONATE The present inventors have intensively investigated, and consequently have found that dimethyl carbonate This is a continuation of application Ser. No. 5 and methanol do not constitute azeotropic composition 07/837,881, filed Feb. 20, 1992, pending. in the three-component system of dimethyl oxalate, dimethyl carbonate and methanol whereby accom BACKGROUND OF THE INVENTION plished the present invention. This invention relates to a process for purifying di That is, the present invention is a process for purify methyl carbonate which is useful as a synthesis starting 10 ing dimethyl carbonate which comprises distilling a material for an aromatic polycarbonate and various mixture of dimethyl carbonate and methanol in the chemical products and as a solvent. More particularly, presence of dimethyl oxalate to separate and remove it relates to a process for purifying dimethyl carbonate methanol from the mixture. which comprises distillating and separating methanol to BRIEF DESCRIPTION OF THE DRAWINGS purify dimethyl carbonate by co-presenting dimethyl 15 oxalate in order to prevent reacting carbon monoxide FIG. 1 is a graph showing the measured results of and an ester of nitrous acid in the presence of a specific gas-liquid equilibrium of dinnethyl carbonate and metha catalyst with high yield. nol, and As a process for preparing dimethyl carbonate, there FIG. 2 shows a processing diagram of the continuous have been practiced by, for example, the method of 20 preparation process. reacting carbon monoxide, methanol and an acid by DESCRIPTION OF THE PREFERRED using copper chloride (Japanese Patent Publications EMBODIMENTS No. 11129/1970 and No. 45655/1980), the method of interesterifying a cyclic carbonate such as ethylene In the following, the present invention is to be de 25 scribed in more detail. carbonate with methanol in the presence of a catalyst An amount of dimethyl carbonate and methanol is (U.S. Pat. Nos. 3,642,858 and 3,803,201, and Japanese not particularly limitative. An amount of dimethyl oxa Patent Publication No. 27658/1985), and the method of late to be added is preferably 0.3 or more, particularly subjecting to vapor reaction of carbon monoxide and preferably 0.5 or more in terms of a mole fraction of nitrite in the presence of a catalyst (Japanese Unexam dimethyl oxalate in the three components in order to ined Patent Publications No. 27486/1989 and No. enhance separation efficiency. In view of separation 201146/1990). However, in either of the methods, di efficiency, there is no upper limit thereof, and it should methyl carbonate can be obtained as a mixture with be considered the range which can be industrially prac methanol so that separation from methanol is indispens ticed, e.g. 0.8 to 0.9 in terms of a mole fraction of di able for purifying dimethyl methyl carbonate. Dimethyl 35 methyl oxalate in the three components. carbonate and methanol constitute azeotropic mixture In the three component system of dimethyl carbon in a composition ratio of 30:70 (weight ratio), and thus, ate, dimethyl oxalate and methanol, dimethyl carbonate it is difficult to separate the mixture by distillation under and methanol do not cause azeotropic phenomenon, so normal pressure. that methanol can be separated by distillation under Thus, many investigations have been carried out normal pressure. Next, the mixture ofdimethyl carbon about the method for purifying dimethyl carbonate ate and dimethyl oxalate remained can be separated by from the mixture of the both components, and various the normal distillation to give dimethyl carbonate sepa proposals have been made. For example, there has been rated. Dimethyl oxalate remained can be used again by proposed the method of obtaining crystalline product circulating the system. enriched in dimethyl carbonate by cooling as disclosed 45 An industrial process of the present invention can be in U.S. Pat. No. 3,803,201, the method of subjecting to practiced by using a continuous apparatus as shown in separation of the mixture by distillation by breaking FIG. 2. In FIG. 2, a crude reaction mixture 1 which is azeotrope with pressurization as disclosed in Japanese a mixture of dimethyl carbonate and methanol is contin Patent Publication No. 3463/1984, and the method of uously fed to a first column can A and also a solution of subjecting to separation by distillation by adding a hy SO dimethyl oxalate is fed continuously. The mixture is drocarbon such as hexane and heptane as disclosed in refluxed by heating the bottom of the column to effect Japanese Unexamined Patent Publication No. continuous distillation and methanol is taken out from 41820/1979. However, these methods cannot be said to the top of the first column distilled solution 3. A first be industrially satisfied. That is, the method of using column can solution 2 of the bottom is a mixture of extraction and distillation with water is not economical 55 dimethyl carbonate and dimethyl oxalate and it is fed to since dimethyl carbonate dissolves well in water and a second column can B and refluxed to effect continu easily saponified so that causing many loss. An appara ous distillation whereby dimethyl carbonate is taken out tus to be used in the method of distillation under pres from the top thereof as a second column distilled solu sure becomes extremely high cost and operations tion 5. A second tower can solution 4 at the bottom thereof are difficult. Also, the method of adding hydro thereof is dimethyl oxalate as a distillation residue and it carbon is complicated in operations and disadvanta is returned to the first column can A to use again by geous in energy. Further, the method of precipitation circulation whereby continuous operation can be done. by cooling is industrially not practical. When the process of the cresent invention is effected, if water exist in the distillation system, dimethyl oxalate SUMMARY OF THE INVENTION is hydrolyzed so that water should previously be re An object of the present invention is to provide a moved, if necessary. In the processes for preparing process for purifying dimethyl carbonate which can dimethyl carbonate as mentioned above, water is sub give high quality dimethyl carbonate by cancelling stantially not contained in the reaction product obtained 5,292,917 3 4. by the method prepared from a cyclic carbonate or the methyl oxalate to be fed was made 400 g per hour. As method using a nitrite. Also, in the latter method, di the results, a flow amount of the distilled solution at the methyl oxalate is contained as a by-product in a little top was 26 g/hr and the composition thereof was 97.7% amount so that the reaction product prepared by the by weight of methanol and 2.3% by weight of dimethyl method can be applied to the purifying process of the 5 carbonate. Also, a taken out amount of the bottom solu present invention directly. By the reason as mentioned tion was 493 g/hr and the composition thereof was above, dimethyl carbonate can be prepared advanta 0.04% by weight of methanol, 19.0% by weight of geously by the latter method as compared to the reac dimethyl carbonate and 81.0% by weight of dimethyl tion product obtained by the other method. oxalate. In the three-component system comprising dimethyl 10 oxalate, dimethyl carbonate and methanol according to Comparative example 1 (the case that dimethyl oxalate the process of the present invention, dimethyl carbonate is not added) and methanol do not constitute azeotropic material. A solution of 70% by weight of methanol and 30% by FIG. 1 is a graph of the measured results of gas-liquid weight of dimethyl carsonate was continuously fed equilibrium showing the matter, and it shows that by 5 without feed of dimethyl oxalate as in Example l, and addition of dimethyl oxalate, separation by distillation then the bottom was heated. The continuous operation of dimethyl carbonate and methanol becomes easy.