Process for producing dimethyl carbonate

Abstract: The reaction between methanol, carbon monoxide and oxygen for preparing dimethyl carbonate, in the presence of cuprous chloride as the catalyst, is carried out in a special reactor constituted by two parallel, vertical tubes, at the basis of one of which the fresh gas and the recycle gas are fed; the reaction products are separated by partially condensing the recirculate vapours.

Description: The present invention relates to an improved process for producing dimethyl carbonate (referred to as DMC in the following) by starting from CO, O₂ and methanol, in the presence of CuCl as the catalyst, a process according to which the reaction is carried out in a heterogeneous-phase system containing substances in the gas, liquid and solid phases, in a special reactor constituted by two parallel, vertical tubes, at the basis of one of which the fresh gas and the recycle gas are fed, so as to obtain a recirculation of the liquid phase thanks to the difference in density which is generated between the upwards tube, containing the gas, and the downwards tube, in which a lower amount of gas is contained. The esters of carbonic acid are known, which find use as solvents and as agents of polymerization by transesterification of glycols and bis-phenols in the production of polycarbonates. It is known as well that the most common methods for preparing such compounds have been based for long time on the reaction between an alcohol and phosgene or chloroformates, carried out in the presence of suitably selected bases, methods by now blamed from many viewpoints, due to the inherent riskfulness of the system. Processes for preparing esters of carbonic acid are known as well, which do not use phosgene, but which, however, not always lead to satisfactory results when the relevant technology is transferred to the commercial level. For example, Japanese Patent No. 11,129 discloses the preparation of carbonate esters by reaction of an alcohol with carbon monoxide in the presence of cupric ions: the yields of carbonate are extremely low and furthermore, as the reduction proceeds, considerable amounts are formed of alkyl halide, ether and halogen-containing acid, with an increased acidification of the system being caused. The same Applicant controls a large number of patents relevant to the oxidative carbonylation of alcohols with carbon monoxide and oxygen: for example, Italian Patent No. 898,077, relating to a process for preparing esters of carbonic acids, which consists in the reaction between an alcohol, carbon monoxide and oxygen, carried out in the presence of a catalytic system consisting of complexes of metals capable of existing in two different valency states; or Italian Patent No. 1,070,574, which discloses a preparation of carbonic acid esters by means of the reaction of an alcohol with oxygen and carbon monoxide in the presence of a catalyst preferably constituted by a salt of cuprous copper; or Italian Patent No. 1,127,270 which, by referring to the reaction as per the prior Patent, discloses the feasibility thereof by starting from a gas mixture constituted by hydrogen, carbon monoxide and oxygen. The presently most widely used route is based on the use of a catalyst consisting of cuprous chloride, and is essentially based on the following reaction 2 CH₃OH + CO + 1/2O₂ → (CH₃O)₂CO + H₂O The reaction proceeds through the two steps of oxidation and reduction: without wishing to enter in detail into the actual mechanism of reaction, the Applicant thinks that in the first step cuprous chloride reacts with methanol and oxygen to form a cupric methoxy chloride which, in the second step, would be reduced by carbon monoxide, with dimethyl carbonate being produced and cuprous chloride being regenerated 2CuCl +2CH₃OH + 1/2O₂ → 2Cu(OCH₃)Cl + H₂O 2Cu(OCH₃)Cl + CO → (CH₃O)₂CO + 2CuCl The oxidative carboxylation is carried out in the liquid phase through two reactors, with the effluents from the bottom of the first reactor being fed to the top of the second reactor. The present Applicant has found now an operating solution which makes it possible the above process to be carried out with a particularly simple separation of the liquid reaction products from the catalyst, without the latter having to be removed from the reactor. Such a solution, which is the subject-matter of the present invention, is based on the reaction for preparing dimethyl carbonate from CO, O₂ and methanol being carried out in a reactor essentially comprising two parallel, vertical tubes, connected with each other at their top ends and at their bottom ends, at the basis of one of which the fresh gas and the recycle gas are fed, so that a recirculation and consequent mixing of the liquid is obtained, thanks to the difference in density which is generated between the upwards tube, which contains the gas and the downwards tube which contains a smaller amount of gas. Under the operating conditions, the recycle gas is saturated with the vapours coming from the reaction liquid; methanol is the most volatile component, followed by DMC and water; inasmuch as the reaction produces one water molecule per each DMC molecule, the removal of water from the reaction system is the controlling parameter in order to keep steady-state reaction conditions, i.e., a constant-composition reaction liquid. By partially condensing the vapours contained in the recycle gas, a liquid is obtained, whose composition is close to the composition of the reaction liquid. From said liquid, water to be discharged, produced DMC and a stream of methanol to be recycled to the reaction together with an aliquot of DMC -- which normally is extracted in excess over water --as it will result more clearly from the following examples, are then separated by means of techniques known. The proposed solution can be implemented of course according to other operating solutions as regards the realization of reactors which, meeting the principles exposed hereinabove, must be regarded as fully equivalent to the reactor disclosed in the following and therefore falling within the scope of the present invention. The reactor for practicing the process according to the present invention is constituted by two tubes provided with an inner enamel lining and an outer jacket (1 and 2 in Figure 1). The gas is fed to the bottom of one of said tubes by means of a suitable gas distributor. At their bottom end, the tubes are smoothly connected with each other by a bent pipe portion (3) provided with an inner enamel lining; at their top end, they are both connected with the vessel (4) also provided with an internal enamel lining, and namely, the tube (1) by means of the 90°-bent fitting (5), and the tube (2) directly through a hole provided at the bottom end of the vessel (4). When the process is started, the liquid with the catalyst is charged to the tubes, until it comes to lap the bottom portion of the bent pipe (5), and the gas feed is then started; said gas feed causes the liquid to be recirculated, with the circulation flowrate of the liquid being the higher, the higher the volume flowrate of the gas. For the tubes (1) and (2), diameters comprised within the range of from 0.025 to 1 m are adopted, with their length being comprised within the range of from 1.5 to 20 metres; the vessel (4) as a diameter which is from 2 to 4 times as large as the diameter of the tubes (1) and (2), and a height of from 2 to 10 times as large as the diameter. The diameter of (4) must be larger than the diameter of tubes (1) and (2), so as to enable the separation between the gas and the liquid to occur. The operating conditions are as follows: the operating temperatures are comprised within the range of from 80°C to 200°C, and preferably of from 100°C to 180°C. The pressure is comprised within the range of from 10 to 50 bar. The gas flowrate per each unit of surface area of the cross-section of the tubes is comprised within the range of from 100 to 1000 m³/m². In Figure 2 a possible operating cycle is reported, still for exemplifying purposes, which makes it possible an operating cycle to be carried out, which enables the process for preparing dimethyl carbonate according to the principles of the present invention to be practiced. In order to operate correctly, the reactor disclosed requires a high gas flowrate, which gas reacts to only a low percentage per each pass, with the unreacted portion being recycled to the bottom end of the reactor. The recycle gas may have a very variable composition, also as a function of the operating conditions adopted; it is preferable to operate with an excess of CO and a less-than-stoichiometric amount of oxygen, so as to remain outside the concentration limit of the explosive mixtures. The composition of the gas may vary within the range of from 99% CO to 25% CO, oxygen within the range of from 1 to 15%, with the balance to 100 being N₂, Ar, CH₂, H₂ and CO₂ or other gases which are inert in the reaction system. During the reaction, oxygen and CO are consumed in the stoichiometric ratio of 1 : 2 by mol. Inasmuch as also CO₂ is produced, some gas should be vented off from the system, also if pure gases are fed; the best point for this vent (3) resulted to be at the top end of the reactor, with a reflux condenser being provided, in order that the vapours are condensed and the escape of products or reaction solvent is prevented. The composition of the reaction solvent may be very variable; it may range from nearly pure methanol, up to mixtures containing up to 10-25% of water by weight and/or up to 50-60% of DMC by weight, with the balance to 100% being methanol. To this liquid, an amount of CuCl is added, which is comprised within the range of from 1% by weight to 20% by weight. The reaction temperature is comprised within the range of from 80°C to 200°C, and the operating pressure is comprised within the range of from 10 bar to 50 bar. The recycle gas leaves the reactor saturated with vapours at the operating temperature. The level of vapours in the gas may vary with the composition of the reaction liquid, the total pressure and temperature; in any case, it will be a significant percentage of the recirculated gas phase. By means of a cooling of a few degrees (of from 3 to 30°C), a partial condensation can be accomplished; the condensate liquid is richer in water and DMC than the vapour phase, with a composition similar to the composition of the reaction liquid (from 10 to 70% of vapours present). This liquid phase should be kept circulating with a flowrate at which water and DMC produced by the reaction leave the reactor; in case an excess is drawn, the reaction liquid will be depleted of water and DMC and will be enriched with methanol; in case a too small amount is drawn, the liquid, and hence the equilibrium vapour, will be enriched with water and DMC. Therefore, the system is capable of self-regulating within the limits of the kinetics of the formation of DMC. After the separation of the condensate liquid, the gas is compressed back to the reaction pressure, by the compressor K1, and is fed back to the reactor. To the gas recycle stream, fresh oxygen and CO are added, streams (4) and (5), in amounts equivalent to the amounts which react, plus the amount leaving the system, as said, with the stream (3). Downstream of the compressor, to the reactor methanol is fed in an amount which corresponds to the methanol amount which will react, plus the methanol amount extracted in excess by means of stream (2), and the possible excess of DMC removed (stream (6)). The addition of liquid should be such as to keep constant the liquid level inside the reactor, with the volume changes of the reaction medium being compensated for. Further details of the process according to the present invention will be clear from the reading of the following examples: in such examples, obviously reported for merely illustrative purposes, only the reactor was used, the description of which is reported in the following. The reactor is totally constituted by enamel-coated steel, to prevent corrosions. The reactor is constituted by two tubes of 25 mm of inner diameter, provided with an internal enamel lining and an external jacket; both tubes are connected with each other at their bottom and top ends. In the top portion of the reactor, a tube of 75 mm of diameter is installed; thanks to the decrease in velocity, the gas is separated from the recirculated liquid. The gas is fed to the bottom portion of the reactor through a pipe (Figure 4). The available volume for the liquid phase is of approximately 5 litres. The flow scheme of the process is shown in Figure 3. The gas stream leaving the reactor (1) goes to the condenser E1, in which a portion of vapours is condensed; the condensate liquid (3) containing water, DMC and methanol under level control, goes to the separation. The gas, containing the non-condensed vapours (4) goes to the compressor K1 which feeds it to the bottom portion of the reactor. Carbon monoxide and oxygen fed as raw materials to the reactor are supplied from cylinders under flowrate control, to the main gas stream (5 and 6). Methanol and DMC contained in V2 are fed (7) to the reactor by the pump P1, through the heat exchanger E2, with the necessary enthalpy level in order not to have to supply or remove heat to/from the reactor. The vapours contained in the discharged overhead stream (2) are condensed in E3, collected in V3 and, through the pump P3, they are back sent to the reactor together with the stream (7) (owing to problems of overall dimensions, a reflux condenser was not used). Example No. 1 The process was carried out at 30 bar and at 150°C. A total volume of 5000 cc of a liquid having the molar composition * Methanol 60% * DMC 25% * Water 15% was charged to the reactor. 0.5 kg of CuCl was added. The system was heated up to the required operating temperature (150°C) and the reactor was pressurized with CO (30 bar), with the pressure control on the gas loop (9) being enabled, and the compressor K1 was started up, with a recycle flowrate of 250 l/h. The feed of CO was started up at the required flowrate of 10.5 gram-moles/h (5), with the necessary amount for maintaining a constant pressure being discharged through the line 2. As soon as the feed of oxygen (6) was started up, the reaction began, and the flowrate was adjusted to the planned value of 2.5 mol/h within approximately 1 hour. The condensation of the product was started with a temperature change of 6°C between the line upstream and the line downstream of the condenser E1. As soon as inside the collection vessel the liquid reached the desired level, the condensate from the gas vent (8) was fed to the reactor. Simultaneously, the feed of azeotropic mixture MeOH/DMC and fresh methanol (7) was started, so as to keep constant the liquid level inside reactor R1. After approximately 1 hour, the system had reached its steady-state operating conditions, with the flowrate shown in the following, reported in weight units. The run was continued for about four hours, and 1000 g of DMC was produced. At the end of the test, the composition of the liquid resulted to be substantially the same as of the charged liquid. Example No. 2 The process was carried out at 20 bar and at 120°C. A total volume of 5000 cc of a liquid having the molar composition * Methanol 70% * DMC 20% * Water 10% was charged to the reactor. 250 g of CuCl was added. The reaction was started by the same procedure as of Example 1. The process was carried out with a flowrate of recycle gas of 200 l/h at 20 bar and 120°C, with 50% of vapours contained in the recycle stream being condensed. During the test time of 6 hours under steady-state conditions a production rate of 135 g/h of DMC was obtained, which corresponds to 800 g. Example No. 3 The test run was carried out at 25 bar and 130°C. To the reactor a total volume of 5000 cc was charged of a mixture having the molar composition * Methanol 70% * DMC 20% * Water 10% to which 1000 g of CuCl was added. A stream of CO of 80%, with the balance to 100% being hydrogen, was fed, still from a cylinder (a cylinder charged with the mixture was used). The same start up procedure of Example 1 was used, with a recycle flowrate of 380 l/h being maintained by the compressor K1, and the CO-H₂ mixture was fed at the flowrate of 16.25 mol/h (5), with the pressure inside the system being maintained by means of the pressure control and the vent 2. As soon as the feed of oxygen was started up, the reaction began, and the flowrate of oxygen supply was adjusted to the planned value of 1.7 mol/h within approximately 1 hour. The condensation of the product was started with a temperature change of 6°C between the line upstream and the line downstream of the condenser E1. As soon as inside the collection vessel V1 the liquid reached the desired level, the condensate obtained from the gas vent (8) was fed to the reactor. Simultaneously, the feed of azeotropic mixture MeOH/DMC and fresh methanol (7) was started so as to keep constant the liquid level inside the reactor R1. After approximately 2 hours, the system had reached steady-state operating conditions, with the flowrate shown in the following, reported in weight units. The run was continued for about four hours, and 500 g of DMC was produced. At the end of the test, the composition of the liquid resulted to be substantially the same as of the charged liquid.

Claims: Process for producing dimethyl carbonate, based on the reaction between methanol, carbon monoxide and oxygen in the presence of cuprous chloride as the catalyst, characterized in that said reaction is carried out in a reactor constituted by two parallel, vertical tubes, connected with each other at their top ends and at their bottom ends, at the basis of one of which tubes the fresh gas and the recycle gas are fed and in which a mixing of the liquid is accomplished, thanks to the difference in density which is generated between the upwards tube, which contains the gas, and the downwards tube, which contains a smaller amount of gas. Process for preparing dimethyl carbonate according to claim 1, characterized in that the reactor is operated at a temperature comprised within the range of from 80°C to 200°C. Process for preparing dimethyl carbonate according to claim 1, characterized in that the reactor is operated at a pressure comprised within the range of from 10 to 50 bar. Process for preparing dimethyl carbonate according to claim 1, further characterized in that dimethyl carbonate and water produced are removed by means of a partial condensation of the vapours contained in the recirculated gases, with a concentration of water and DMC being obtained, which is higher than the concentration of said substances which would be obtained with a total condensation.

Inventors: DONATI GIANNI, GHIRARDINI MAURIZIO, PARET GIANCARLO, DONATI, GIANNI, PARET, GIANCARLO
Applicants: ENICHEM SINTESI, ENICHEM SYNTHESIS S.P.A.
Application year: 1991
Main classification: C07C68
IPC codes: B01J19, C07C69, C07C68, C07B61, B01J27