Continuous Process for Preparing Dimethyl Carbonate

Europa,schesP_ MM M M M Ml 1 1 Ml Ml M M M I M J European Patent Office i- #>o _ © Publication number: 0 523 728 B1 Office_„. europeen des brevets

@ Date of filing: 17.07.92

© Priority: 19.07.91 JP 269950/91 1978-10, Oaza Kogushi 19.07.91 JP 269951/91 Ube-shi, Yamaguchi-ken (JP) @ Date of publication of application: Inventor: Tanaka, Shuji, c/o Ube Chemical 20.01.93 Bulletin 93/03 Factory Ube Industries, Ltd., © Publication of the grant of the patent: 1978-10, Oaza Kogushi 27.12.95 Bulletin 95/52 Ube-shi, Yamaguchi-ken (JP) © Designated Contracting States: BE DE ES FR GB IT NL © Representative: Hansen, Bernd, Dr.rer.nat. et © References cited: al EP-A- 0 425 197 Hoffmann, Eitle & Partner Patentanwalte © Proprietor: UBE INDUSTRIES, LTD. Postfach 81 04 20 12-32, Nishihonmachi 1-chome D-81904 Munchen (DE) Ube-shi, Yamaguchi-ken 755 (JP)

@ Inventor: Nlshihira, Keigo, c/o Ube Chemical Factory Ube Industries, Ltd., 1978-10, Oaza Kogushi 00 Ube-shi, 00 Yamaguchi-ken (JP) CM Inventor: Yoshlda, Shin-ichi, c/o Ube Chemi- IV cal Factory 00 Ube Industries, Ltd., CM m Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid (Art. 99(1) European patent convention). Rank Xerox (UK) Business Services (3. 10/3.09/3.3.3) 1 EP 0 523 728 B1 2

Description nitrogen gas, and in addition to dimethyl carbonate, contains carbon monoxide, methyl nitrite, nitrogen This invention relates to a process for continu- monoxide, methanol and a small amount of ously preparing dimethyl carbonate which is useful dimethyl oxalate. as a synthesis starting material for an aromatic 5 In order to separate dimethyl carbonate and polycarbonate and various chemical products. dimethyl oxalate from the mixed gas, the above More particularly, it relates to a process for prepar- mentioned methods are used, and of these meth- ing dimethyl carbonate from carbon monoxide and ods, an absorption separation using methanol has methyl nitrite in a vapor phase in the presence of a most commonly used. In fact, a certain extent of platinum group metal solid catalyst which can be io separation efficiency can be obtained by the meth- carried out industrially advantageously and sepa- od, However, in order to heighten separation effi- rated continuously. Dimethyl carbonate is a com- ciency to a level sufficient for an industrial scale, pound extremely useful as a starting material for there are inappropriate aspects that cooling is con- organic syntheses of medicines and agricultural ducted at a significantly low temperature, a separa- chemicals, and for syntheses of aromatic polycar- 75 tion tower having high step number is required or a bonates and urethanes. separation system should be made significantly In the prior art, as a process for preparing high pressure. dimethyl carbonate by contacting carbon monoxide Also, when separating dimethyl carbonate by and methyl nitrite to a platinum group metal solid distillation, it forms azeotropic composition with catalyst in a vapor phase, the present inventors 20 methanol and thus it should be finally separated have proposed, for example, in Japanese Provi- from methanol. Considering this fact, it is clear that sional Patent Publication No. 141243/1991 (which a methanol concentration in a separated material corresponds to U.S. Patent Application Serial No. from a reaction gas mixture should preferably be 07/599, 134 now allowed or European Patent Ap- low in view of energy burden. Thus, it cannot be plication No. 90 31 1 469.2) or Japanese Provisional 25 considered that it is appropriate to absorb and Patent Publication No. 139152/1992. The reaction separate dimethyl carbonate in the mixed gas by itself disclosed therein is extremely excellent as a using methanol. process for preparing dimethyl carbonate. How- ever, in order to apply the reaction to the industries SUMMARY OF THE INVENTION and to effect preparation of dimethyl carbonate to 30 an industrial scale, it is required to develop a An object of the present invention is to provide process which can conduct the reaction, separation an industrially advantageous continuous process and purification continuously and effectively. for preparing dimethyl carbonate from carbon mon- As a process for preparing dimethyl carbonate oxide and methyl nitrite. from carbon monoxide and methyl nitrite, in addi- 35 Another object of the present invention is to tion to the process proposed by the present in- provide a process for separating dimethyl carbon- ventors, there has been proposed a process as ate in a reaction gas mixture effectively and easily. disclosed in Japanese Provisional Patent Publica- The present inventors have studied intensively tion No. 181051/1985, but the disclosure is insuffi- in order to solve the above problems involved in cient for using the process as an industrially con- 40 the conventional separation methods, and as the tinuous process. results, they have established an industrially novel As a conventional technique, in the process for continuous process whereby accomplished the separating a desired component from a mixed gas, present invention. there may be mentioned an absorption method That is, the present invention is a continuous using a solvent, a condensation method by cooling, 45 process for preparing dimethyl carbonate which a method of using an absorbant and a combination comprises the steps that: of these methods when the desired component has a first step of introducing a gas containing a high boiling point as in the present invention. In carbon monoxide and methyl nitrite into a reactor addition, there is a specific method such as a filled with a solid catalyst carried thereon at least membrane separation, but in order to prepare 50 one of a platinum group metal and a compound dimethyl carbonate which should be prepared by a thereof, or at least one of a platinum group metal mass production with inexpensive using an indus- and a compound thereof and a co-catalyst to effect trial apparatus, it is required that the industrial catalytic reaction in a vapor phase to form a reac- apparatus should be simple, advantageous in en- tion product containing dimethyl carbonate; ergy efficiency and economically excellent. 55 a second step of separating the reaction prod- A mixed gas obtained by the reaction dis- uct formed in the first step to a non-condensed gas closed in the aforesaid Japanese Provisional Patent containing nitrogen monoxide and a solution con- Publication No. 141243/1991 mainly comprises a taining dimethyl carbonate by introducing the reac-

2 3 EP 0 523 728 B1 4 tion product into an absorption column and adding and/or a compound thereof, or a platinum group dimethyl oxalate as an absorption solvent; metal and/or a compound thereof and a co-catalyst a third step of introducing the non-condensed to effect catalytic reaction in a vapor phase. As the gas of the second step into a regenerating column reactor, single tubular system or multi-tubular sys- to contact it with a molecular state oxygen-contain- 5 tern catalyst-filled column is effective. A contact ing gas and methanol whereby regenerating nitro- time of the platinum group metal solid catalyst and gen monoxide in the non-condensed gas to methyl starting gases is preferably set 10 seconds or nitrite, so as to contain 2 to 7 % by volume of shorter, more preferably 0.2 to 5 seconds. As the nitrogen monoxide in a non-absorbed gas at an platinum group metal solid catalyst, effective are a outlet of the column, which in turn is introduced in io catalyst in which platinum group metal compounds the reactor of the first step; described in Japanese Provisional Patent Publica- a fourth step of extracting, distilling and sepa- tion No. 141243/1991 are carried on a carrier, a rating dimethyl carbonate from the solution contain- catalyst in which a cat ion of the platinum group ing dimethyl carbonate, methanol and dimethyl ox- metal is carried on a carrier, and a catalyst in alate obtained in the second step by further adding is which a salt of copper or iron is added to the dimethyl oxalate to remove methanol; and above catalyst as a co-catalyst or promoter. a fifth step of distilling and separating dimethyl As the carrier, activated charcoal, silica, dia- carbonate from a mixed solution containing tomaceous earth, zeolite and clay mining are used. dimethyl carbonate and dimethyl oxalate obtained Also, a gas containing carbon monoxide and meth- in the fourth step to remove dimethyl oxalate and 20 yl nitrite, which is a starting gas, is generally dilut- recycling the removed dimethyl oxalate to the ed by a gas inert to the reaction such as nitrogen fourth step. or carbon dioxide gas. Also, the present invention relates to a process The reaction proceeds at a low temperature for preparing ing dimethyl carbonate which com- sufficiently and smoothly, and less side reaction prises synthesizing dimethyl carbonate from car- 25 occurs at a lower reaction temperature. Thus, the bon monoxide and methyl nitrite in a vapor phase reaction is preferably carried out at a temperature reaction in a reactor, characterized in that dimethyl as low as possible so long as a desired space time oxalate is added to a column as an absorption yield (STY) can be maintained, i.e. at 50 to 200 solvent to absorb dimethyl carbonate for separating °C, more preferably at 80 to 150 °C. Also, the dimethyl carbonate from the reaction gas issueing 30 reaction is preferably carried out at a pressure of a from the reactor containing (1) a high boiling point normal pressure (0 MPa (0 kg/cm2) (gauge pres- reaction product mainly comprising dimethyl car- sure)) to 0.981 MPa (10 kg/cm2) (gauge pressure), bonate, and (2) a gas containing unreacted carbon more preferably a normal pressure to 0.49 MPa (5 monoxide, methyl nitrite and nitrogen monoxide. kg/cm2) (gauge pressure), and some instances, the In the present invention, by contacting a mixed 35 pressure may be slightly lower than the normal gas containing dimethyl carbonate with dimethyl pressure. oxalate, dimethyl carbonate can be effectively ab- A concentration of methyl nitrite in the starting sorbed to dimethyl oxalate under mild conditions gas may vary with a wide range, and in order to while depressing methanol concentration at a low obtain satisfactory reaction rate, it is necessary to degree. 40 set the concentration to 1 % by volume or more. However, a high concentration of methyl nitrite is BRIEF DESCRIPTION OF THE DRAWINGS not preferred in view of safety since methyl nitrite is an explosive compound so that the concentration Fig. 1 is a processing diagram of the continu- range is preferably 3 to 25 % by volume based on ous preparation process showing one embodiment 45 the starting gas. A concentration of carbon monox- of the invention. ide in the starting gas may also vary with a wide range and suitably be selected in the range of 10 DESCRIPTION OF THE PREFERRED EMBODI- to 90 % by volume based on the starting gas. In a MENTS continuous process, since part of a circulating gas 50 is purged from the system in order to maintain the In the following, the present invention is to be concentration of an inert gas constantly, when the described in more detail. concentration is made high, loss of the starting gas out of the system becomes remarkable so that it is First step: economically preferred to effect the reaction with 55 the carbon monoxide concentration of 5 to 30 % by A gas containing carbon monoxide and methyl volume based on the starting gas. nitrite is introduced into a reactor filled with a solid catalyst carried thereon a platinum group metal

3 5 EP 0 523 728 B1 6

Second step: tower, a spray column and a plate column may be used. The reaction product formed in the first step is The non-condensed gas and the molecular led to a bottom portion of an absorption column at state oxygen-containing gas to be contacted with which gas separation is carried out, and simulta- 5 methanol may be introduced in the regenerating neously, at the top portion of the absorption col- column separately or a mixed state. In the regener- umn, dimethyl oxalate is fed to absorb and sepa- ating column, part of nitrogen monoxide is oxidized rate dimethyl carbonate in the reaction gas by to nitrogen dioxide by the molecular state oxygen- dimethyl oxalate. In a gas from which dimethyl containing gas, and they are absorbed to and reac- carbonate is separated, a small amount of dimethyl io ted with methanol to regenerate methyl nitrite. As carbonate and dimethyl oxalate are accompanied the molecular state oxygen-containing gas, pure by and they are all becoming loss by hydrolysis at oxygen gas or oxygen diluted by an inert gas may the third step for regenerating nitrogen monoxide. be used, and it is fed so as to become nitrogen Thus, in order to recover dimethyl carbonate and monoxide concentration in the regenerated gas be- dimethyl oxalate to be accompanied by, it is pre- 15 ing 2 to 7 % by volume. When the regenerated gas ferred to feed a small amount of methanol from the is used by circulating to the reactor of the first top portion of the absorption column. step, if the concentration of nitrogen monoxide An amount of dimethyl oxalate to be flown may exceeds 7 % by volume, it remarkably inhibits the vary depending on an amount of dimethyl car- reaction, whereas if it is less than 2 % by volume, bonate to be incorporated into the absorption col- 20 significant amounts of oxygen and nitrogen dioxide umn, but suitably 3 to 10-fold weight, more prefer- are contained in the regenerated gas, which lower ably 4 to 6-fold weight based on the weight of activity of the catalyst. dimethyl carbonate. Also, an amount of methanol to Thus, it is preferred that the molecular state be fed from the top portion of the column is prefer- oxygen-containing gas is supplied to in an amount ably as little as possible since methanol itself 25 of 0.08 to 0.2 mole per mole of nitrogen monoxide should be removed at the fourth step, but it is too in the gas to be introduced into the regenerating little, losses of dimethyl carbonate and dimethyl column and these gases are contacted with metha- oxalate become remarkable. nol at a temperature of 60 °C or lower with a Thus, it is suitably 5 to 30 % by weight, more contact time of preferably 0.5 to 2 seconds. An preferably 10 to 20 % by weight based on the 30 amount of methanol to be used is an amount amount of dimethyl carbonate in the reaction gas. necessary for completely absorb and react nitrogen An operation temperature of the absorption col- dioxide formed and substantially the equivalent umn is preferably as low as possible in order to mole of nitrogen monoxide, or more. Methanol is carry out absorption of dimethyl carbonate effec- generally and preferably used in an amount of 2 to tively, but it is too low, dimethyl oxalate is solidified 35 5 mole per mole of nitrogen monoxide in the gas to and it is disadvantageous in view of energy burden. be introduced into the regenerating column. Thus, it is suitably carried out in the range of 0 ° C Since the present invention is a continuous to 80 •C, more preferably 10 °C to 50 °C. A process, methyl nitrite is expelled out of the sys- mixed solution of dimethyl carbonate and dimethyl tem by dissolving in the absorption solution in the oxalate separated by dimethyl oxalate is trans- 40 absorption column or a can solution in the regener- ferred to the fourth step in order to remove metha- ating column or part of the circulating gas is nol and a small amount of a low boiling point purged so that nitrogen oxide components are lost. compound such as methyl formate formed by the Therefore, methyl nitrite may be supplemented to reaction. On the other hand, the non-condensed the reactor of the first step, or nitrogen oxide such gas contains, in addition to nitrogen monoxide 45 as nitrogen monoxide, nitrogen dioxide, dinitrogen formed by the catalytic reaction at the first step, trioxide and dinitrogen tetroxide or nitric acid is unreacted carbon monoxide and methyl nitrite, and supplemented to the regenerating column at the transferred to the third step. third step. Also, when a content of nitrogen monoxide in Third step: 50 the non-condensed gas at the second step is much and methyl nitrite with a larger amount than that of The non-condensed gas of the second step is required is obtained at converting nitrogen monox- introduced into a regenerating column to contact it ide into a nitrite, part thereof may be directly cir- with a molecular state oxygen-containing gas and culated to the reactor of the first step without methanol whereby regenerating nitrogen monoxide 55 leading to the whole amount of the non-condensed in the gas to methyl nitrite. As the regenerating gas. A solution led out from the regenerating col- column at this step, usual vapor-liquid contacting umn is a methanol solution containing water which apparatuses such as a filled column, a bubble is by-produced by the regenerating reaction so that

4 7 EP 0 523 728 B1 8 it is industrially advantageous to use again at the ponents of dimethyl carbonate and dimethyl ox- second step or the third step after purifying it to alate. the water content in methanol of suitably 2 % by volume or less, more preferably 0.2 % by volume Fifth step: or less. 5 A solution from which low boiling point pro- Fourth step: ducts such as methanol and others are removed in the fourth step is transferred into a distillation col- A mixed solution of dimethyl carbonate, metha- umn of the fifth step, at which dimethyl carbonate nol and dimethyl oxalate led out from the second io is distilled and separated therefrom and high qual- step is fed to the bottom portion of an extraction ity dimethyl carbonate can be taken out continu- and distillation column in order to remove metha- ously. An operational pressure is not particularly nol. At the same time, to the extraction and distilla- limited and the operation can be carried out with a tion column is newly fed dimethyl oxalate from the wide range from a reduced pressure to under pres- top portion of the column in order to prevent 15 sure. On the other hand, from a can solution of the azeotropic phenomenon of methanol and dimethyl distillation column of this fifth step, a significantly carbonate. An amount of dimethyl oxalate to be fed high purity dimethyl oxalate can be obtained and it is suitably 0.1 to 2-fold mole based on the total can be supplied as such to the second step and mole of dimethyl carbonate and methanol to be fed the third step. Dimethyl oxalate formed as a by- to the column, preferably 0.5 to 1.5-fold mole. 20 product by the reaction at the first step is taken out Dimethyl oxalate has a role of an absorbing agent as a by-produced product. Since dimethyl oxalate which is important for preventing substantial loss of itself has various uses, it may be further distilled dimethyl carbonate to a distillation side, and an and purified, if necessary, whereas it has high amount of dimethyl oxalate is defined by a number purity in the state of the can solution. The distilla- of steps of the distillation column for separation 25 tion column used in the fourth step and the fifth and an amount of energy to be used at separation. step may be used conventional ones such as a That is, if an amount of dimethyl oxalate is too filling column and plate column. small, loss of dimethyl carbonate to the distillation In one of the preferred embodiments of the side becomes remarkable whereby an yield will present invention, a process for separating worsen and number of steps of the absorption 30 dimethyl carbonate of the second step is described column required to be high. To the contrary, even in detail below. if too much amount of dimethyl oxalate is flown, an A process for separating dimethyl carbonate effect of preventing loss of dimethyl carbonate to from the reaction mixture of carbon monoxide and the distillation side cannot be obtained in propor- methyl nitrite should be considered from a global tion to the amount added and energy loss will be 35 view point including treatment of methanol since, caused since the loss of dimethyl carbonate sub- after separation, further separation from methanol, stantially stops when a predetermined amount of and distillation and purification should be effected dimethyl oxalate is added. as described above. That is, as for the process for An operational pressure is not particularly limit- separating methanol from a mixture of dimethyl ed and the operation can be carried out with a wide 40 carbonate and methanol, there are a method in range from a reduced pressure to under pressure which extraction and distillation are carried out by but preferably in the range of a normal pressure (0 using dimethyl oxalate as disclosed in Japanese MPa (0 kg/cm2G) to 0.196 MPa (2 kg/cm2G) (guage Patent Application No. 57696/1991 (which corre- pressure). Methanol distilled out from the extraction sponds to European Patent Application No. 92 103 and distillation column is, as an industrial process, 45 454.2), a method of using water as disclosed in preferably used again at the second step or the Japanese Patent Publication No. 17333/1981, a third step. However, methyl formate or methylal method of using benzene or hydrocarbon, and a which are by-produced in the reaction at the first method of removing azeotropic phenomenon by step with a small amount is contained the methanol pressurizing. However, even when either of the distilled, so that the methanol is preferably used 50 method is used, it is advantageous if an amount of again after removing these by-produced com- methanol to be taken in is as little as possible in pounds by distillation. Incidentally, distillation resi- view of energy burden. Accordingly, it is necessary dues such as methyl formate and methylal are to reduce a methanol concentration after separation disposed by burning up, but they can recover as as well as improving recovery of dimethyl car- methanol by decomposing with an alkali. A can 55 bonate. solution remained at the bottom of the extraction On the other hand, in order to separate and distillation column is transferred to the fifth dimethyl carbonate from the reaction mixture only step in the state of a solution containing two com- by using methanol, a significant amount of metha-

5 9 EP 0 523 728 B1 10 nol is required in order to heighten recovery of Also, as for pressure, it is not particularly limit- dimethyl carbonate and also high step number and ed in view of extracting effect of dimethyl oxalate, cooling are required. For example, if one wishes to but it is preferably close or the same with the make a recovery of dimethyl carbonate from the reaction pressure since the reactor is a series of an reaction mixture 98 % or more, the obtained mix- 5 apparatus of the process. It is preferably carried ture of dimethyl carbonate and methanol is re- out at slightly pressurized condition of 0.098 to quired to have a methanol concentration of 50 % 0.491 MPa (1 to 5 kg/cm2G), more preferably 0.098 by weight or more. to 0.294 MPa (1 to 3 kg/cm2G). To the contrary, when dimethyl oxalate is used Next, the process of the present invention is as an absorbing solvent as in the present invention, io explained in detail by referring to a flow sheet chart it is extremely easy to make a ratio of methanol to shown in Fig. 1 which shows one embodiment of dimethyl carbonate 20 % or less with a recovery of the present invention. dimethyl carbonate being 99 % or more. Also, A gas containing carbon monoxide, methyl ni- dimethyl oxalate itself is circulated through separa- trite and nitrogen monoxide is introduced into the tion from methanol in the next step and through the is upper portion of a multi-tubular system reactor 1 in distillation and purification step, but usually exists which a platinum series metal solid catalyst is filled at a can solution side whereby energy loss is in a reactor through a pipe 22 by pressurizing with extremely little so that the method is, as a whole, a gas circulating machine (not shown) provided at a advantageous one in view of energy burden. pipe 20. Catalytic reaction in vapor phase is carried As an apparatus for separating dimethyl car- 20 out at the reactor 1 , and a gas passed through the bonate from the reaction mixture, usual filling col- catalytic layer and formed by the reaction is taken umn or plate column may be used, and they may out from the bottom portion of the reactor 1 and be 5 to 7 plates or so in number of theoretical introduced into an absorption column 2 through a plates which may vary depending on a reaction gas pipe 1 1 . composition and a flow amount thereof. 25 At the absorption column 2, dimethyl carbonate As operational conditions, the reaction gas is in the gas formed by the reaction is contacted with fed from the upper portion to the lower plate of the methanol and dimethyl oxalate provided from pipes absorption column, dimethyl oxalate is fed from 2 13 and 14, respectively, to absorb it to dimethyl to 3 plates from the top and a little amount of oxalate for separation, and a solution comprising methanol is fed from the top portion in order to 30 dimethyl carbonate, dimethyl oxalate and methanol prevent loss of dimethyl oxalate accompanied by a is introduced into an extraction and distillation col- gas. A flow amount of dimethyl oxalate may vary umn 4 from the bottom portion of the column 2 depending on the operational temperature and through a pipe 15. On the other hand, a non- pressure, and on a desired recovery of dimethyl condensed gas containing unreacted carbon mon- carbonate, but preferably 3-fold weight or more 35 oxide and methyl nitrite, as well as a by-produced based on the amount of dimethyl carbonate in the nitrogen monoxide and others is introduced to the reaction gas, more preferably 4-fold weight or bottom portion of a regenerating column 3 from the more. Here, an amount of 3-fold weight gives a upper portion of the column 2 through a pipe 12. recovery of 98 % or more and that of 4-fold weight At the regenerating column 3, the non-con- gives a recovery of 99 % or more. Even when the 40 densed gas is mixed with a molecular state oxy- amount is made too large, it is not effective and gen-containing gas which is introduced to the bot- loss of dimethyl oxalate into the gas increases so tom portion of the regenerating column 3 through a that 4 to 6-fold weight is particularly preferred. pipe 16 and the mixture is reacting with methanol Also, a flow amount of methanol is preferably introduced from the upper portion of the regenerat- as little as possible, but it is too little, a recovery of 45 ing column 3 through a pipe 19 by countercurrent dimethyl carbonate decreases and loss of dimethyl contact reaction to form methyl nitrite. In the regen- oxalate into the separated gas increases. Thus, it is erating column 3, subsequent to oxidation reaction preferably 5 to 30 % by weight, more preferably 10 of nitrogen monoxide to nitrogen dioxide, absorp- to 20 % by weight. tion reaction into methanol occurs and methyl ni- An operational temperature of the column 50 trite is formed. When a nitrogen source sufficient largely affects to the recovery of dimethyl car- for forming methyl nitrite is lacked, nitrogen oxide- bonate and it is preferably as low as possible. In (s) may be mixed through a pipe 17. the operation under the above conditions, only the A non-absorbed gas containing methyl nitrite top portion of the column may be slightly cooled formed at the regenerating column 3 is in turn by circulation to the column top temperature pref- 55 supplied to the reactor 1 by circulation with newly erably in the range of 0 to 50 ° C, more preferably supplied carbon monoxide from a pipe 21 through 10 to 30 °C. pipes 20 and 22. On the other hand, water by- produced at the regenerating column 3 is taken out

6 11 EP 0 523 728 B1 12 from the bottom portion of the column 3 in the form diaphragm type gas circulating pump to 0.245 MPa of an aqueous methanol solution through a pipe 18. (2.5 kg/cm2) (gauge pressure) with a rate of 6.9 This aqueous methanol solution can be circulated Nm3/hr, and a temperature at the center portion of and utilized again as a methanol supplied to the the catalyst layer was maintained at about 120 °C absorption column 2 or the regenerating column 3 5 by passing a hot water through a shell side of the through the above pipes 13 or 19 after removing reactor. The reaction rate of forming dimethyl car- water content in the solution by operations such as bonate by the reaction was STY of 430 kg/m3hr. distillation. The gas passed through the catalyst layer was At the extraction and distillation column 4, ex- introduced into the bottom portion of a Raschig traction only of dimethyl carbonate is carried out io ring filled type vapor-liquid contact absorption col- by countercurrent contact with dimethyl oxalate in- umn having an inner diameter of 100 mm and a troduced thereinto through a pipe 25 to separate it height of 1300 mm, and methanol was introduced from methanol. The separated methanol is intro- therein at the top of the column with a rate of 0.21 duced into a distillation column 6 at which metha- Jl/hr and dimethyl oxalate was introduced therein at nol purification is carried out from the upper portion is the portion 200 mm below the top of the same with of the column 4 through a pipe 24. After effecting a rate of 2.65 kg/hr to effect countercurrent contact purification, methanol is circulated and utilized with a top column temperature of 5 °C and a again as a methanol supplied to the absorption bottom column temperature of 20 °C. From the column 2 and the regenerating column 3 through bottom of the absorber, 2.8 kg/hr of an absorbed the above pipes 13 and 19, respectively. A mixed 20 solution (composition: 76.7 % by weight of solution of dimethyl carbonate and dimethyl oxalate dimethyl oxalate, 19.6 % by weight of dimethyl from which methanol is removed is led to a distilla- carbonate, 3.7 % by weight of methanol and 0.1 % tion column 5 through a pipe 23. by weight of methyl formate) was obtained. On the At the distillation column 5, dimethyl carbonate other hand, from the top Of the column, 6.8 Nm3/hr is obtained as a final product from the upper por- 25 of a non-condensed gas (composition: 12.8 % by tion of the column 5 through a pipe 27. The re- volume of carbon monoxide, 10.3 % by volume of maining can solution is dimethyl oxalate with high methyl nitrite, 8.7 % by volume of nitrogen monox- purity and part thereof can be obtained as a by- ide, 1.9 % by volume of methanol, 2.2 % by weight product through a pipe 28, but the remaining are of carbonic acid gas and 64.0 % by volume of supplied to the absorption column 2 and the dis- 30 nitrogen) was obtained. tillation column 4 through pipes 26, 14 and 25, After the non-condensed gas was mixed with respectively. 87.2 N-l/hr of oxygen and 7.5 N-l/hr of a nitrogen gas containing 14.0 % by weight of nitrogen mon- EXAMPLES oxide, and then introduced into a vapor-liquid con- 35 tact type regenerating column from the bottom In the following, the process of the present portion thereof. From the top portion thereof, invention is explained specifically by referring to methanol was introduced with a rate of 5.0 Jl/hr to Examples, but they are one of the embodiments of effect countercurrent contact with a top column the present invention and the present invention is temperature of 30 °C and a bottom column tem- not limited by these Examples. 40 perature of 20 ° C. 6.6 Nm3/hr of a regenerated gas (composition: 12.8 % by volume of carbon monox- Example 1 ide, 15.4 % by volume of methyl nitrite, 3.7 % by volume of nitrogen monoxide, 1 .9 % by volume of In a tube of a multi-piping reactor made of methanol, 2.3 % by volume of carbonic acid gas stainless steel having 6 tubes, an inner diameter of 45 and 64.1 % by volume of nitrogen) at the regener- 26.1 mm and a height of 500 mm was filled 780 g ating column was supplied to the above gas cir- (1.73 I) of a catalyst in which palladium is carried culating pump and condensed. Then, 0.2 Nm3/hr of on an activated charcoal (available from K.K. carbon monoxide was supplied to the compresseed Takeda, Japan, Shirasagi 4mm<#> x 6 mm, trade regenerated gas and the mixed gas was introduced name) as disclosed in Japanese Patent Application 50 into the reactor. On the other hand, 4.0 Jl/hr of No. 257042/1990. To the upper portion of the cata- methanol containing 2.2 % by weight of water led lyst layer was supplied a starting gas (composition: out from the regenerating column was used again 15.0 % by volume of carbon monoxide, 15.0 % by as a methanol source in said column after remov- volume of methyl nitrite, 3.5 % by volume of nitro- ing water by distillation. gen monoxide, 1.8 % by volume of methanol, 2.2 55 The absorbed solution in an amount of 3.5 % by volume of carbonic acid gas and 62.5 % by kg/hr led out from the above absorption column volume of nitrogen) previously preheated to about was led to the middle step of a distillation column 90 ° C by a heat-exchanger and compressed by a having an inner diameter of 50 m and a height of

7 13 EP 0 523 728 B1 14

2500 mm, and 1.4 kg/hr of liquid dimethyl oxalate nol, 0.13 % by volume of dimethyl oxalate, 11.32 was introduced therein at the position 300 mm % by volume of methyl nitrite, 1 1 .74 % by volume below from the top of the column and distillation of carbon monoxide, 7.60 % by volume of nitrogen was carried out with a top column temperature of monoxide, 0.10 % by volume of methyl formate, 64 °C and a bottom column temperature of 146 5 0.50 % by volume of carbon dioxide and 63.00 % °C. From the bottom of the column, 4.78 kg/hr of a by volume of nitrogen. mixed solution containing 14.3 % of dimethyl car- A liquid was taken out so as to become a liquid bonate and 87.5 % by weight of dimethyl oxalate surface of the bottom flask constant (524 g/hr), and was obtained. On the other hand, from the top of the operation was continued until inside of the the column, 0.13 kg/hr of a distilled solution com- io column and bottom become a steady state. When prising 94.5 % by weight of methanol, 5.2 % by the state becomes steady, a gas led out from the weight of methyl formate and 0.3 % by weight of top of the column and the bottom solution were dimethyl carbonate was obtained. The distilled so- sampled and analyzed by using a gas chromatog- lution was circulated and used again in the above raphy. As the results, the composition of a gas led regenerating column and the absorption column is out from the top portion of the column was 0.01 % after purification of methanol in a distillation col- by volume of dimethyl carbonate and 0.00 % by umn. volume of dimethyl oxalate, and that of the bottom The mixed solution of dimethyl carbonate and solution was 18.70 % by weight of dimethyl car- dimethyl oxalate led out from the above distillation bonate, 3.17 % by weight of methanol, and 78.05 column was led to a filling column having an inner 20 % by weight of dimethyl oxalate. diameter of 65 mm and a height of 1600 mm, and A recovery of dimethyl carbonate was 99.5 % distilled with a top column temperature of 90 °C and a ratio of methanol to dimethyl carbonate in a and a bottom column temperature of 163 °C. From taken out solution was 0.17 (weight ratio). the top of the column, 0.68 kg/hr of dimethyl carbonate with a purity of 99.4 % was obtained. Also, 25 Example 3 from the bottom of the column, 4.69 kg/hr of dimethyl oxalate with a purity of substantially 100 In the same manner as in Example 2 except for % was taken out, and 4.05 kg/hr thereof was cir- changing a feeding amount of dimethyl oxalate to culated and supplied to the absorption column and 300 g/hr. As the results, a recovery of dimethyl the extraction and distillation column. As the re- 30 carbonate was 98.4 %, and a ratio of methanol to sults, from dimethyl carbonate formed by the reac- dimethyl carbonate in the extracted solution was tion, high purity dimethyl carbonate can be ob- 0.17 (weight ratio). tained continuously with a formation yield of 98 %. Example 4 Example 2 35 In the same manner as in Example 2 except for At the bottom of an oldershow having an inner changing a feeding amount of methanol to 5 g/hr. diameter of 32 mm and number of steps of 10, a As the results, a recovery of dimethyl carbonate one liter of a flask having was equipped and was 98.0 %, and a ratio of methanol to dimethyl dimethyl oxalate was charged previously in the 40 carbonate in the extracted solution was 0.06 bottom flask by dissolving under heating. A reac- (weight ratio). tion gas mixture was fed from a nozzle attached to the top of the flask with a rate of 1400 N-i/hr and Comparative example 1 400 g/hr of dimethyl oxalate dissolved solution was fed continuously from the fourth step from the top 45 In the apparatus used in Example 2, a liquid portion of the column. Also, from the top portion of circulation for cooling was changed that a liquid the column, 15 g/hr of methanol was fed and a was taken out from the bottom solution and the liquid was taken out from the third step from the ninth step from the upper portion of the column to top portion of the column and circulated to the top circulate to the seventh step, and the reaction gas portion of the column through a cooler whereby the 50 mixture was fed in the same manner as in Example top portion of the column was cooled to 20 ° C. 2. From the top portion of the column, 100 g/hr of A reaction gas containing dimethyl carbonate methanol was fed and the column was cooled so was obtained by passing carbon monoxide and as to the bottom temperature of 20 °C and the methyl nitrite through a solid catalyst bed as shown ninth step temperature of 0 °C, respectively. As in Japanese Provisional Patent Publication No. 55 the results, a recovery of dimethyl carbonate was 141243/1991, and the composition of which is, at a 98.2 % and a ratio of methanol to dimethyl car- temperature of 105 °C, 1.75 % by volume of bonate in the extracted solution was 1.02 (weight dimethyl carbonate, 4.00 % by volume of metha- ratio).

8 15 EP 0 523 728 B1 16

Comparative example 2 dimethyl carbonate from a mixed solution containing dimethyl carbonate and dimethyl ox- In the same manner as in Comparative exam- alate obtained in the fourth step to remove ple 1 except for changing a feeding amount of dimethyl oxalate and recycling the removed methanol to 24 g/hr. As the results, a recovery of 5 dimethyl oxalate to the fourth step. dimethyl carbonate was 95.6 %, and a ratio of methanol to dimethyl carbonate in the extracted 2. The process according to Claim 1, wherein solution was 0.24 (weight ratio). dimethyl oxalate is added in the second step According to the process of the present inven- in an amount of 3 to 10-fold weight per weight tion, when separating dimethyl carbonate from a io of dimethyl carbonate. mixed gas containing dimethyl carbonate formed by the reaction, by contacting the mixed gas with 3. The process according to Claim 1, wherein dimethyl oxalate, dimethyl carbonate can be effec- dimethyl oxalate is added in the second step tively separated under mild conditions and ob- in an amount of 4 to 6-fold weight per weight tained extremely economically as a whole while an is of dimethyl carbonate. amount of methanol to be co-presented can be depressed as little as possible. Thus, the process 4. The process according to Claim 1, wherein of the present invention can provide dimethyl car- methanol is further added to the second step bonate continuously which is industrially extremely in an amount of 5 to 30 % by weight based on advantageous. 20 the amount of dimethyl carbonate in the reaction product. Claims 5. The process according to Claim 4, wherein the 1. A continuous process for preparing dimethyl second step is carried out at a temperature of carbonate which comprises: 25 0 ° C to 80 ° C. a first step of introducing a gas containing carbon monoxide and methyl nitrite into a re- 6. The process according to Claim 1 , wherein the actor filled with a solid catalyst carried thereon catalytic reaction at the first step is carried out at least one of a platinum group metal and a at a temperature of 50 to 200 °C and at a compound thereof, or at least one of a plati- 30 pressure of normal pressure to 0.981 MPa (10 num group metal and a compound thereof and kg/cm2) (gauge pressure). a co-catalyst to effect catalytic reaction in a vapor phase to form a reaction product con- 7. The process according to Claim 6, wherein the taining dimethyl carbonate; gas introduced in the first step comprises 3 to a second step of separating the reaction 35 25 % by volume of methyl nitrite, 5 to 30 % product formed in the first step to a non- by volume of carbon monoxide and an inert condensed gas containing nitrogen monoxide gas as the reminder. and a solution containing dimethyl carbonate by introducing the reaction product into an 8. The process according to Claim 1 , wherein the absorption column and adding dimethyl oxalate 40 molecular state oxygen-containing gas is ad- as an absorption solvent; ded in the third step in an amount of 0.08 to a third step of introducing the non-con- 0.2 mole in terms of oxygen per mole of nitro- densed gas of the second step into a regener- gen monoxide and contacted with the non- ating column to contact it with a molecular condensed gas and methanol for 0.5 to 2 sec- state oxygen-containing gas and methanol 45 onds. whereby regenerating nitrogen monoxide in the non-condensed gas to methyl nitrite, so as to 9. The process according to Claim 8, wherein contain 2 to 7 % by volume of nitrogen mon- methanol is added in the third step in an oxide in a non-absorbed gas at an outlet of the amount of 2 to 5 mole per mole of nitrogen column, which in turn is introduced in the 50 monoxide in the non-condensed gas. reactor of the first step; a fourth step of extracting, distilling and 10. The process according to Claim 1, wherein separating dimethyl carbonate from the solu- dimethyl oxalate is added in the fourth step in tion containing dimethyl carbonate, methanol an amount of 0.1 to 2-fold mole per total molar and dimethyl oxalate obtained in the second 55 number of dimethyl carbonate and methanol. step by further adding dimethyl oxalate to remove methanol; and 11. The process according to Claim 1, wherein a fifth step of distilling and separating dimethyl oxalate is added in the fourth step in

9 17 EP 0 523 728 B1 18

an amount of 0.5 to 1.5-fold mole per total Regenerierungssaule einfuhrt und es dort mit molar number of dimethyl carbonate and einem Sauerstoff im Molekularzustand enthal- methanol. tenden Gas und Methanol in Kontakt bringt, wodurch das Stickstoffmonoxid in dem nicht- 12. A process for preparing dimethyl carbonate 5 kondensierten Gas zu Methylnitrit regeneriert which comprises synthesizing dimethyl car- wird, wobei 2 bis 7 Volumenprozent Stickstoff- bonate from carbon monoxide and methyl ni- monoxid in dem nichtabsorbierten Gas am trite in a vapor phase reaction in a reactor AuslaB der Kolonne enthalten sind, und wieder- characterized in that dimethyl oxalate is added um in den Reaktor der ersten Stufe eingefuhrt to a column as an absorption solvent to absorb io werden; dimethyl carbonate for separating dimethyl eine vierte Stufe des Extrahierens und Destil- carbonate from the reaction gas issueing from lierens und Abtrennens von Dimethylcarbonat the reactor containing (1) a high boiling point aus der Dimethylcarbonat, Methanol und Dime- reaction product mainly comprising dimethyl thyloxalat enthaltenden Losung, die erhalten carbonate, and (2) a gas containing unreacted is wurde in der zweiten Stufe durch weitere Zu- carbon monoxide, methyl nitrite and nitrogen gabe von Dimethyloxalat unter Entfernung von monoxide. Methanol; und eine funfte Stufe des Destillierens und Abtren- 13. The process according to Claim 12, wherein nens von Dimethylcarbonat aus einer Mischlo- dimethyl oxalate is added in an amount of 4 to 20 sung, enthaltend Dimethylcarbonat und Dime- 6-fold weight per weight of dimethyl carbonate. thyloxalat, welches in der viereten Stufe erhalten wurde, unter Entfernung von Dimethyloxa- 14. The process according to Claim 12, wherein lat und Zuruckfuhren des entfernten Dimethy- methanol is further added in an amount of 5 to loxalats in die vierte Stufe. 30 % by weight based on the amount of 25 dimethyl carbonate in the reaction gas. 2. Verfahren gemaB Anspruch 1, bei dem Dime- thyloxalat in der zweiten Stufe in einer Menge, 15. The process according to Claim 12, wherein die der drei- bis zehnfachen Gewichtsmenge absorption is carried out at a temperature at des Gewichtes von Dimethylcarbonat ent- the top of the column of 0 to 50 °C and a 30 spricht, zugegeben wird. pressure in the column of 0.0981 to 0.491 MPa (1 to 5 kg/cm2G). 3. Verfahren gemaB Anspruch 1, bei dem Dime- thyloxalat in der zweiten Stufe in einer Menge, Patentanspruche die dem vier- bis sechsfachen Gewicht des 35 Dimethylcarbonats entspricht, zugegeben wird. 1. Kontinuierliches Verfahren zur Herstellung zur Herstellung von Dimethylcarbonat, umfassend: 4. Verfahren gemaB Anspruch 1, bei dem Metha- eine erste Stufe zum Einfuhren eines Kohlen- nol weiterhin in der zweiten Stufe in einer monoxid enthaltendenden Gases und Methylni- Menge von 5 bis 30 Gew.-%, bezogen auf die trit in einen Reaktor, der mit einem festen 40 Menge des Dimethylcarbonats im Reaktions- Katalysator gefullt ist, auf dem wenigstens ein produkt zugegeben wird. Platingruppenmetall und/oder eine Verbindung davon oder wenigstens ein Platingruppenmetall 5. Verfahren gemaB Anspruch 4, bei dem die und/oder eine Verbindung davon und ein Co- zweite Stufe bei einer Temperatur von 0 ° C bis Katalysator die katalytische Umsetzung in der 45 80 ° C durchgefuhrt wird. Dampfphase unter Bildung eines Dimethylcar- bonat enthaltenden Produktes bewirt, abge- 6. Verfahren gemaB Anspruch 1, bei dem die schieden ist; katalytische Umsetzung in der ersten Stufe bei eine zweite Stufe, bei der das in der ersten einer Temperatur von 50 bis 200 ° C und einem Stufe gebildete Reaktionsprodukt in ein nicht- 50 Druck von Normaldruck bis 0,981 MPa (10 kondensiertes Gas enthaltendes Stickstoffmon- kg/cm2) (gauge pressure) durchgefuhrt wird. oxid und eine Losung, die Dimethylcarbonat enthalt, getrennt wird, indem man das Reak- 7. Verfahren gemaB Anspruch 6, bei dem das in tionsprodukt in eine Absorptionssaule einfuhrt der ersten Stufe eingefuhrte Gas 3 bis 25 Vol- und Dimethyloxalat als Absorptionslosungsmit- 55 % Methylnitrit, 5 bis 30 Vol-% Kohlenmonoxid tel zugibt; und einem Inertgas als Rest umfaBt. eine dritte Stufe, bei welcher man das nicht- kondensierte Gas aus der zweiten Stufe in eine

10 19 EP 0 523 728 B1 20

8. Verfahren gemaB Anspruch 1, bei dem das Revendicatlons Sauerstoff im Molekuarzustand enthaltene Gas in der dritten Stufe in einer Menge von 0,08 1. Procede continu de preparation du carbonate bis 0,2 mol, ausgedruckt als Sauerstoff pro mol de dimethyle comprenant : Stickstoffmonoxid zugegeben wird, und mit 5 une premiere etape consistant a introduire dem nichtkondensierten Gas und Methanol 0,5 un gaz contenant du monoxyde de carbone et bis 2 Sekunden kontaktiert wird. du nitrite de methyle dans un reacteur rempli d'un catalyseur solide portant au moins un 9. Verfahren gemaB Anspruch 8, bei dem Metha- metal du groupe du platine et un compose de nol in der dritten Stufe in einer Menge von 2 io celui-ci, ou au moins un metal du groupe du bis 5 mol pro mol Stickstoffmonoxid in dem platine et un compose de celui-ci et un co- nichtkondensierten Gas zugegeben wird. catalyseur pour produire une reaction catalytique en phase vapeur afin de former un produit 10. Verfahren gemaB Anspruch 1, bei dem Dime- de reaction contenant du carbonate de dimethyloxalat in der vierten Stufe in der 0,1- bis 15 thyle ; 2,5-fachen Molmenge pro Gesamtmolzahl an une deuxieme etape consistant a separer Dimethylcarbonat und Methanol zugegeben le produit de reaction forme lors de la premie- wird. re etape en un gaz non-condense contenant du monoxyde d'azote et une solution conte- 11. Verfahren gemaB Anspruch 1, bei dem Dime- 20 nant du carbonate de dimethyle en introduisant thyloxalat in der vierten Stufe in der 0,5- bis le produit de reaction dans une colonne d'ab- 1,5-fachen Molmenge Gesamtmolzahl an Di- sorption et en ajoutant de I'oxalate de dimethy- methylcarbonat und Methanol zugegeben wird. le en tant que solvant d'absorption ; une troisieme etape consistant a introduire 12. Verfahren zur Herstellung von Dimethylcarbo- 25 le gaz non-condense de la deuxieme etape nat, umfassend das Synthetisieren von Dime- dans une colonne de regeneration pour le met- thylcarbonat aus Kohlenmonoxid und Methylni- tre en contact avec un gaz contenant de I'oxy- trit in einer Dampfphasenreaktion in einem Re- gene moleculaire et du methanol, regenerant aktor, dadurch gekennzeichnet, daB Dimethy- ainsi le monoxyde d'azote dans le gaz noncon- loxalat in eine Saule als Absorptionslosungs- 30 dense en nitrite de methyle, de maniere a mittel zum Ansorbieren von Dimethylcarbonat contenir, a un orifice de sortie de la colonne, zum Abtrennen von Dimethylcarbonat aus dem de 2 a 7 % en volume de monoxyde d'azote Reaktionsgas, das aus dem Reaktor heraus- dans un gaz non-absorbe, qui, a son tour, est kommt, enthaltend (1) ein hochsiedendes Re- introduit dans le reacteur de la premiere etape aktionsprodukt, das sich hauptsachlich aus Di- 35 ; methylcarbonat zusammensetzt, und (2) ein une quatrieme etape consistant a extraire, Gas, das nichtumgesetztes Kohlenmonoxid, distiller et separer du carbonate de dimethyle Methylnitrit und Stickstoffmonoxid enthalt, zu- de la solution contenant du carbonate de dime- gegeben wird. thyle, du methanol et de I'oxalate de dimethyle 40 obtenue lors de la deuxieme etape, en ajoutant 13. Verfahren gemaB Anspruch 12, bei dem Dime- en outre de I'oxalate de dimethyle pour enlever thyloxalat in einer Menge, die dem vier- bis le methanol ; et sechsfachen Gewicht pro Gewicht des Dime- une cinquieme etape consistant a distiller thylcarbonats entspricht, zugegeben wird. et a separer du carbonate de dimethyle d'une 45 solution melangee contenant du carbonate de 14. Verfahren gemaB Anspruch 12, bei dem Me- dimethyle et de I'oxalate de dimethyle obtenue thanol weiterhin in einer Menge von 5 bis 30 lors de la quatrieme etape pour enlever I'oxala- Gew.-%, bezogen auf das Gewicht von Dime- te de dimethyle et recycler I'oxalate de dimethylcarbonat in dem Reaktionsgas, zugegeben thyle enleve, de la quatrieme etape. wird. 50 2. Procede selon la revendication 1, dans lequel 15. Verfahren gemaB Anspruch 12, bei dem die de I'oxalate de dimethyle est ajoute lors de la Absorption bei einer Temperatur am Kolonnen- deuxieme etape dans une quantite de 3 a 10 kopf bei 0 bis 50 °C und einem Druck in der fois, en poids, par poids de carbonate de di- Kolonne von 0,0981 bis 0,491 MPa (1 bis 5 55 methyle. kg/cm2G) vorgenommen wird. 3. Procede selon la revendication 1, dans lequel de I'oxalate de dimethyle est ajoute lors de la

11 21 EP 0 523 728 B1 22

deuxieme etape dans une quantite de 4 a 6 ce que de I'oxalate de dimethyle est ajoute a fois, en poids, par poids de carbonate de di- une colonne en tant que solvant d'absorption methyle. pour absorber du carbonate de dimethyle afin de separer du carbonate de dimethyle du gaz 4. Procede selon la revendication 1, dans lequel 5 de reaction sortant du reacteur contenant (1) on ajoute en outre du methanol lors de la un produit de reaction a point d'ebullition eleve deuxieme etape dans une quantite de 5 a 30 comprenant principalement du carbonate de % en poids base sur la quantite de carbonate dimethyle, et (2) un gaz contenant du monoxyde dimethyle dans le produit de reaction. de de carbone n'ayant pas reagi, du nitrite de io methyle et du monoxyde d'azote. 5. Procede selon la revendication 4, dans lequel la deuxieme etape est effectuee a une tempe- 13. Procede selon la revendication 12, dans lequel rature de 0 ° C a 80 ° C. de I'oxalate de dimethyle est ajoute dans une quantite de 4 a 6 fois, en poids, par poids de 6. Procede selon la revendication 1, dans lequel is carbonate de dimethyle. la reaction catalytique lors de la premiere etape est effectuee a une temperature de 50 a 14. Procede selon la revendication 12, dans lequel 200 °C et a une pression de la pression nor- on ajoute en outre du methanol dans une male de 0,981 M (10 kg/cm2) (pression de quantite de 5 a 30 % en poids base sur la jauge). 20 quantite de carbonate de dimethyle dans le gaz de reaction. 7. Procede selon la revendication 6, dans lequel le gaz introduit lors de la premiere etape com- 15. Procede selon la revendication 12, dans lequel prend 3 a 25 % en volume de nitrite de I'absorption s'effectue a une temperature en methyle, 5 a 30 % en volume de monoxyde 25 haut de la colonne de 0 a 50 ° C et une pres- de carbone, le reste etant un gaz inerte. sion dans la colonne de 0,0981 a 0,491 MFe (1 a 5 kg/cm2G). 8. Procede selon la revendication 1, dans lequel le gaz contenant de I'oxygene moleculaire est ajoute lors de la troisieme etape dans une 30 quantite de 0,08 a 0,2 mole en termes d'oxygene par mole de monoxyde d'azote et mis en contact avec le gaz non-condense et le methanol pendant 0,5 a 2 secondes. 35 9. Procede selon la revendication 8, dans lequel du methanol est ajoute lors de la troisieme etape dans une quantite de 2 a 5 moles par mole de monoxyde d'azote dans le gaz non- condense. 40

10. Procede selon la revendication 1, dans lequel de I'oxalate de dimethyle est ajoute lors de la quatrieme etape dans une quantite de 0,1 a 2 fois en moles, par nombre molaire total de 45 carbonate de dimethyle et de methanol.

11. Procede selon la revendication 1, dans lequel de I'oxalate de dimethyle est ajoute lors de la quatrieme etape dans une quantite de 0,5 a 50 1 ,5 fois en moles, par nombre molaire total de carbonate de dimethyle et de methanol.

12. Procede de preparation de carbonate de dimethyle comprenant la synthese de carbonate de 55 dimethyle a partir de monoxyde de carbone et de nitrite de methyle dans une reaction en phase vapeur dans un reacteur, caracterise en

12 EP 0 523 728 B1

Fig. 1