Process for the preparation of a mixed dialkyl carbonate

Abstract

The invention relates to a continuous process for the preparation of a mixed dialkyl carbonate of formula R1O(C=O)OR2 by reacting a first dialkyl carbonate of formula R1O(C=O)OR1 and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein R1 and R2 are different alkyl groups and R2 contains more carbon atoms than R1, said process comprising: (a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates; (b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) and recycling the separated unconverted first and second dialkyl carbonates to step (a), wherein the total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol% and has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1.

Description

[0001] PROCESS FOR THE PREPARATION OF A MIXED DIALKYL CARBONATE

[0002] Field of the invention

[0003] The present invention relates to a process for the preparation of a mixed dialkyl carbonate.

[0004] Background of the invention

[0005] Mixed dialkyl carbonates are well-known and are of formula RIO (C=O) OR2, wherein Ri and R2 are different alkyl groups. An example of a mixed dialkyl carbonate is ethyl methyl carbonate (EMC) which is of formula RIO (C=O) OR2 wherein Ri is methyl and R2 is ethyl. EMC is known for its use as a solvent and as an organic synthesis intermediate. In specific, EMC has become an important solvent in electrolytes for lithium batteries in recent years. Due to the rapid development of lithium batteries, the demand for ethyl methyl carbonate has also been expanding.

[0006] Ethyl methyl carbonate may be produced by transesterification of dimethyl carbonate with ethanol. Further, ethyl methyl carbonate may be produced by disproportionation (transesterification) of dimethyl carbonate with diethyl carbonate.

[0007] An object of the present invention is to provide a process for the preparation of a mixed dialkyl carbonate from different dialkyl carbonates, wherein the mixed dialkyl carbonate can be recovered at a high yield and in a high purity, and which process is efficient and affordable, and in particular has relatively low operating expenses, relatively low capital expenditure and relatively low energy demand.

[0008] Summary of the invention

[0009] Surprisingly, it was found that the above object may be achieved by a process for the preparation of a mixed dialkyl carbonate by reacting different first and second dialkyl carbonates, the second dialkyl carbonate containing more carbon atoms than the first dialkyl carbonate, wherein the total feed stream to such preparation process:

[0010] (i) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol% and / or

[0011] (ii) has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1.

[0012] Accordingly, the present invention relates to a continuous process for the preparation of a mixed dialkyl carbonate of formula RIO (C=O) OR2 by reacting a first dialkyl carbonate of formula RiO(C=O)ORi and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups and R2 contains more carbon atoms than Ri, said process comprising:

[0013] (a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates ;

[0014] (b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) and recycling the separated unconverted first and second dialkyl carbonates to step (a) , wherein the total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol% and / or has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1.

[0015] Further, the present invention relates to a process for preparing a battery electrolyte solution comprising combining a mixed dialkyl carbonate with one or more alkali metal salts, wherein the mixed dialkyl carbonate has been prepared by the above-mentioned process.

[0016] Detailed description of the invention The process of the present invention comprises steps (a) and (b) , as described hereinbelow. Said process may comprise one or more intermediate steps between steps (a) and (b) . Further, said process may comprise one or more additional steps preceding step (a) and / or following step (b) .

[0017] While the process of the present invention and the stream(s) , catalyst or composition ( s ) used or produced in said process are described in terms of "comprising", "containing" or "including" one or more various described steps and components, respectively, they can also "consist essentially of" or "consist of" said one or more various described steps and components, respectively.

[0018] In the context of the present invention, in a case where a stream, catalyst or composition comprises two or more components, these components are to be selected in an overall amount not to exceed 100%.

[0019] Further, where upper and lower limits are quoted for a property then a range of values defined by a combination of any of the upper limits with any of the lower limits is also implied .

[0020] Within the present specification, "substantially no" means that no detectible amount of the component in question is present in the catalyst or composition.

[0021] Unless indicated otherwise, where in the present specification reference is made to a boiling point this means the boiling point at 760 mm Hg pressure.

[0022] In the process of the present invention, a mixed dialkyl carbonate is prepared from different dialkyl carbonates. That is to say, a mixed dialkyl carbonate of formula RIO (C=O) OR2, wherein Ri and R2 are different alkyl groups, is prepared by reacting a dialkyl carbonate of formula RiO (C=O) ORi and a dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups. Further, R2 contains more carbon atoms than Ri .

[0023] Said mixed dialkyl carbonate of formula RiO (C=0) OR2 may also be referred to as an "asymmetric dialkyl carbonate", whereas the dialkyl carbonate of formula RiO(C=O)ORi and the dialkyl carbonate of formula R2O(C=O)OR2 are "symmetric dialkyl carbonates". In the present specification, the dialkyl carbonate of formula RiO(C=O)ORi is also referred to as the "first dialkyl carbonate", and the dialkyl carbonate of formula R2O(C=O)OR2 is also referred to as the "second dialkyl carbonate".

[0024] In above-mentioned dialkyl carbonates of formulas RIO (C=O) OR2, RIO (C=O) ORI and R2O(C=O)OR2, Ri and R2are different alkyl groups and R2 contains more carbon atoms than Ri . Preferably, Ri and R2 are C1-C4 alkyl groups, more preferably C1-C3 alkyl groups, most preferably C1-C2 alkyl groups. The C1-C4 alkyl groups comprise methyl, ethyl, n- propyl, isopropyl, n-butyl, sec-butyl and isobutyl groups. Most preferably, in the present invention, Ri is methyl and R2 is ethyl, in which case the mixed dialkyl carbonate is ethyl methyl carbonate (EMC) , as prepared by reacting diethyl carbonate (DEC) and dimethyl carbonate (DMC) . Other suitable examples of mixed dialkyl carbonates that can be prepared in the present process, are isopropyl n-butyl carbonate, n- propyl n-butyl carbonate and ethyl n-propyl carbonate.

[0025] Step (a) of the present process comprises reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates.

[0026] Further, above-mentioned product stream may comprise light components. In the present invention, the light components may comprise one or more components selected from the group consisting of carbon dioxide (CO2) , an alkanol of formula RiOH, an alkanol of formula R2OH, an ether of formula R1OR1, an ether of formula R2OR2 and an ether of formula R1OR2, wherein Ri and R2 have the same meanings as described above. Said alcohols and ethers may also be referred to as "light ends" .

[0027] The process of the present invention is a continuous process for the preparation of a mixed dialkyl carbonate from above-mentioned first and second dialkyl carbonates. In the present process, unconverted first and second dialkyl carbonates are recycled to the reactor in step (a) , via step (b) , which dialkyl carbonates may also be referred to as "recycle dialkyl carbonates". Further, fresh first and second dialkyl carbonates are fed to the present continuous process. Within the present specification, "fresh dialkyl carbonate" means dialkyl carbonate which has not been fed to the present process before. This is e.g. different from "recycle dialkyl carbonate" which has already been fed to step (a) of the present process before and which is recycled to step (a) .

[0028] The feed stream or feed streams to the reactor in step (a) of the present process comprises or comprise the first and second dialkyl carbonates. Said feed stream(s) to the reactor comprise the unconverted first and second dialkyl carbonates that are recycled in step (b) of the present process. Further, it is preferred that said feed stream(s) to the reactor comprise fresh first dialkyl carbonate and / or fresh second dialkyl carbonate. Further, it is preferred that one feed stream comprising the first and second dialkyl carbonates is fed to the reactor in step (a) . Said one feed stream comprises the unconverted first and second dialkyl carbonates that are recycled in step (b) of the present process. Preferably, said one feed stream also comprises fresh first dialkyl carbonate and / or fresh second dialkyl carbonate . In the present invention, it is preferred that multiple feed streams to the reactor in step (a) are first combined and then fed as one feed stream to said reactor. Said streams may be combined and mixed in a mixing vessel, with or without the aid of a mixing device, such as a stirrer or an eductor. Further, said streams may be combined and mixed by a static mixer .

[0029] The total feed stream that is fed to step (a) may comprise the mixed dialkyl carbonate. Said "total feed stream" is either a single feed stream that is fed to step (a) or the combination of multiple feed streams that are fed to step (a) . Said mixed dialkyl carbonate product in the total feed stream to step (a) originates from step (b) . The amount of the mixed dialkyl carbonate in the total feed stream to step (a) may be higher than 0.1 mol% or higher than 0.3 mol% or higher than 0.5 mol% . Further, said amount may be of from 0.2 to 5 mol% or of from 0.5 to 5 mol% or of from 0.5 to 3 mol% or of from 1 to 2 mol% . By allowing a relatively high amount of the mixed dialkyl carbonate product in the total feed stream to step (a) , energy may advantageously be saved in the recovery of the mixed dialkyl carbonate from the product stream resulting from step (a) , which recovery includes the separation of the unconverted first and second dialkyl carbonates from said product stream, as in step (b) , that is to say in the purification of the product stream from step (a) . Generally, since the transesterification reaction of different dialkyl carbonates into a mixed dialkyl carbonate is an equilibrium-limited reaction, a skilled person would keep the amount of target product in a recycle stream as low as possible in order to maximise conversion of starting materials in such reaction.

[0030] Further, the total feed stream that is fed to step (a) may have a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 5:1 or lower than 3:1 or lower than 2:1. Further, said molar ratio may be of from 0.1 to 5 or of from 0.1 to 3 or of from 0.1 to 1.8 or of from 0.5 to 2 or of from 0.5 to 1.5 or of from 0.8 to 1.2. Still further, said molar ratio may be about 1. Said "total feed stream" is either a single feed stream that is fed to step (a) or the combination of multiple feed streams that are fed to step (a) . By having a relatively low molar ratio of the second dialkyl carbonate to the first dialkyl carbonate in the total feed stream to step (a) , energy may advantageously be saved in the present process. Generally, since the transesterification reaction of different dialkyl carbonates into a mixed dialkyl carbonate is an equilibriumlimited reaction, a skilled person would keep the molar ratio of one starting material to the other starting material as high as possible in order to maximise the conversion of starting materials in such reaction, especially in a case wherein the former starting material stays liquid (and hence is not evaporated) in the entire recovery and recycle process and the latter starting material does not stay liquid.

[0031] Preferably, in step (a) of the present process, the first and second dialkyl carbonates are reacted in the presence of a catalyst. The nature of the catalyst is not essential. Any catalyst which catalyses the transesterification reaction of different dialkyl carbonates into a mixed dialkyl carbonate may be used. For example, aluminum phosphate may be used as a catalyst in step (a) . The following articles [1] and [2] disclose the preparation of ethyl methyl carbonate (EMC) by reacting dimethyl carbonate (DMC) and diethyl carbonate (DEC) in the presence of an aluminophosphate (A1PO) catalyst. Suitably, in step (a) of the present process an A1PO catalyst as disclosed in said articles may be used as a catalyst. [1] "Amorphous mesoporous aluminophosphate as highly efficient heterogeneous catalysts for transesterification of diethyl carbonate with dimethyl carbonate" by Jinghui Shi et al. in Catalysis Communications, 12, 2011, pages 721-725.

[0032] [2] "Amorphous magnesium substituted mesoporous aluminophosphate: An acid-base sites synergistic catalysis for transesterification of diethyl carbonate and dimethyl carbonate in fixed-bed reactor" by Wang Hefang et al. , in Microporous and Mesoporous Materials, 292, 2020, 109757.

[0033] The conditions in step (a) of the present process may include a temperature of from 10 to 200 °C. Further, they may include a pressure of from 0.5 to 50 bara (5xl04to 5xl06N / m2) . Preferably, said pressure ranges from 1 to 20 bar, more preferably 1.5 to 20 bar, most preferably 2 to 15 bar. Further, preferably said temperature ranges from ambient temperature to 200 °C, more preferably 20 to 200 °C, more preferably 30 to 200 °C, more preferably 40 to 170 °C, more preferably 50 to 140 °C, most preferably 60 to 120 °C.

[0034] Further, the weight hourly space velocity (WHSV) in step (a) of the present process may suitably range of from 0.5 to 50 kg / kgcat.hr ("kgcat" refers to the catalyst amount) , more suitably 1 to 20 kg / kgcat . hr , more suitably 1 to 10 kg / kgcat . hr . Still further, the liquid hourly space velocity (LHSV) in step (a) of the present process may suitably range of from 0.5 to 50 ml / gcat.hr ("gcat" refers to the catalyst amount) , more suitably 1 to 20 ml / gcat.hr, more suitably 1 to 10 ml / gcat.hr.

[0035] Preferably, step (a) of the present process is conducted in a co-current manner. A suitable way to operate step (a) is in a reactor with only liquids. A suitable reaction zone of this type is a pipe-type reaction zone wherein the reaction is conducted in a plug flow manner. For example, step (a) may be carried out in one plug flow reactor or in a series of two or more plug flow reactors. This will enable the reaction to approach equilibrium. Suitably, the pipe-type reactor is a fixed-bed reactor.

[0036] A further possibility is to conduct step (a) of the present process in a continuously stirred tank reactor (CSTR) . In the latter case the effluent from the CSTR is preferably subjected to a post-reaction in a plug flow reactor so that the reaction can approach equilibrium.

[0037] Optionally, before step (b) of the present process, the product stream resulting from step (a) may be pre-heated and / or passed through a gas-liquid separator removing carbon dioxide from the liquid, wherein the resulting pre-heated product stream or separated liquid is sent to step (b) .

[0038] Step (b) of the present process comprises separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) . Such separation may be performed in any way and may comprise one or more steps, and one way of such separation is described in the Examples hereinbelow. In the present invention, the separation ( s ) in step (b) is (are) preferably carried out by distillation. Said distillation is carried out in a distillation column. Said distillation column may comprise one or more distillation sections, preferably one distillation section.

[0039] Step (b) of the present process also comprises recycling the separated unconverted first and second dialkyl carbonates to step (a) . In this way, the unconverted first and second dialkyl carbonates are recycled to step (a) for further conversion. Part of the recycle stream (s) containing the separated first and / or second dialkyl carbonate (s) may be bled off and discarded to remove impurities from the process. Further, said recycle stream (s) may comprise the mixed dialkyl carbonate. The amount of the mixed dialkyl carbonate in said recycle stream(s) may be higher than 0.1 wt . % or higher than 0.3 wt . % or higher than 0.5 wt . % . Further, said amount may be of 0.5 to 10 wt . % or of from 0.5 to 8 wt . % or of from 1 to 5 wt . % .

[0040] Further, the present invention relates to a continuous process for the preparation of a mixed dialkyl carbonate of formula RIO (C=O) OR2 by reacting a first dialkyl carbonate of formula RiO(C=O)ORi and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups and R2 contains more carbon atoms than Ri, said process comprising :

[0041] (a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates ;

[0042] (b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) and recycling the separated unconverted first and second dialkyl carbonates to step (a) , wherein the total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol%.

[0043] Still further, the present invention relates to a continuous process for the preparation of a mixed dialkyl carbonate of formula RIO (C=O) OR2 by reacting a first dialkyl carbonate of formula RiO(C=O)ORi and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups and R2 contains more carbon atoms than Ri, said process comprising:

[0044] (a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates ;

[0045] (b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) and recycling the separated unconverted first and second dialkyl carbonates to step (a) , wherein the total feed stream to step (a) has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1.

[0046] The above description of the process wherein the total feed stream to step (a) both (i) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol% and (ii) has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1, equally applies to the 2 above-mentioned processes wherein either said requirement (i) or said requirement (ii) is met and wherein said requirement (ii) or said requirement (i) , respectively, is optionally met.

[0047] Finally, the present invention relates to a process for preparing a battery electrolyte solution comprising combining a mixed dialkyl carbonate with one or more alkali metal salts, wherein the mixed dialkyl carbonate has been prepared by any one of the above-described processes . Said combining may comprise blending. Said salts may be dissolved in the carbonate. Preferably, the mixed dialkyl carbonate is combined with the one or more alkali metal salts, one or more other carbonate compounds and optionally additives. Said one or more other carbonate compounds may comprise symmetric dialkyl carbonates, including diethyl carbonate and dimethyl carbonate, and alkylene carbonates, including ethylene carbonate. Further, said one or more alkali metal salts may comprise lithium salts and / or sodium salts.

[0048] The invention is further illustrated by the following Examples .

[0049] Examples

[0050] Aspen modelling of production and recovery of ethyl methyl carbonate Example Al (comparison)

[0051] A fresh diethyl carbonate (DEC) feed stream, a fresh dimethyl carbonate (DMC) feed stream, a recycle DEC feed stream and a recycle DMC feed stream are combined and mixed in a mixing vessel and fed as one feed stream to a reactor. The molar DEC / DMC ratio in the total feed stream (combination of said 4 feed streams) is 2. Further, the ethyl methyl carbonate (EMC) content in said total feed stream is 0.3 wt . % (0.3 mol%) .

[0052] The product stream from the reactor has a temperature of 60-80 °C and comprises 9.8 wt . % DMC, 48.9 wt . % DEC and 41.1 wt . % EMC, with the remainder being light ends and other impurities. The light ends comprise alcohols and ethers including ethanol (EtOH) , methanol (MeOH) , diethyl ether (DEE) , dimethyl ether (DME) and ethyl methyl ether (EME) . The other impurities comprise carbon dioxide.

[0053] The product stream is fed to a first distillation column to separate DMC and light ends from EMC, DEC and heavier components. The first distillation column is operated at 1.8 bara with a bottom temperature of 140 °C. The overheads section includes a partial condenser operated at 58 °C with liquid and vapor distillates.

[0054] The following streams are withdrawn in the above first distillation step: (i) a top vapor stream containing CO2 (82.3 wt.%) , DMC (11.7 wt.%) and alcohols and ethers, which stream is vented; (ii) a top liquid stream containing DMC (97.5 wt.%) , EMC (0.5 wt.%) and alcohols and ethers, which is sent to a second distillation column; and (iii) a bottom stream containing DEC (54.4 wt.%) , EMC (45.6 wt.%) , DMC (37 ppmw) and heavy components, which is sent to a third distillation column.

[0055] The second distillation column removes the light ends from DMC. It is operated at 1.8 bara with a bottom temperature of 110 °C. The overheads section includes a partial condenser operated at 65 °C with vapor-only distillate. All the condensed liquid is returned to the column as reflux.

[0056] The following streams are withdrawn in the above second distillation step: (i) a top vapor stream containing DMC (32 wt.%) , CO2 (27 wt.%) and ethers and alcohols, which stream is vented; and (ii) a bottom stream containing DMC (99.5 wt.%) , EMC (0.5 wt.%) and light by-products, which stream is recycled to the reactor. A small part of the latter stream is diverted to waste as a bleed to remove process impurities.

[0057] The third distillation column separates the EMC product from DEC and purifies the EMC product to ultra-pure grade. It is operated at 1.2 bara with a bottom temperature of 140 °C. The overheads section includes a partial condenser operated at 110 °C with virtually only liquid, wherein a vapor stream is included to allow venting of non-condensable compounds.

[0058] The following streams are withdrawn in the above third distillation step: (i) a top stream comprising ultra-pure EMC product (99.99 wt.%) , which stream is sent to a product storage tank; and (ii) a bottom stream containing DEC (99.5 wt.%) , EMC (0.4 wt.%) and heavy by-products, which stream is recycled to the reactor. A small part of the latter stream is diverted to waste as a bleed to remove process impurities.

[0059] Example A2 (invention)

[0060] A fresh diethyl carbonate (DEC) feed stream, a fresh dimethyl carbonate (DMC) feed stream, a recycle DEC feed stream and a recycle DMC feed stream are combined and mixed in a mixing vessel and fed as one feed stream to a reactor. The molar DEC / DMC ratio in the total feed stream (combination of said 4 feed streams) is 1. Further, the ethyl methyl carbonate (EMC) content in said total feed stream is 1.6 wt.%

[0061] (1.6 mol%) . The product stream from the reactor has a temperature of 60-80 °C and comprises 22.1 wt . % DMC, 29.1 wt . % DEC and 47.8 wt . % EMC, with the remainder being light ends and other impurities. The light ends comprise alcohols and ethers including ethanol (EtOH) , methanol (MeOH) , diethyl ether (DEE) , dimethyl ether (DME) and ethyl methyl ether (EME) . The other impurities comprise carbon dioxide.

[0062] The product stream is fed to a first distillation column to separate DMC and light ends from EMC, DEC and heavier components. The first distillation column is operated at 1.66 bara with a bottom temperature of 140 °C. The overheads section includes a partial condenser operated at 59 °C with liquid and vapor distillates.

[0063] The following streams are withdrawn in the above first distillation step: (i) a top vapor stream containing CO2 (59.4 wt.%) , DMC (32.7 wt.%) and alcohols and ethers, which stream is vented; (ii) a top liquid stream containing DMC (92.4 wt.%) , EMC (4.5 wt.%) and alcohols and ethers, which is sent to a second distillation column; and (iii) a bottom stream containing DEC (37.8 wt.%) , EMC (61.9 wt.%) , DMC (30 ppmw) and heavy components, which is sent to a third distillation column.

[0064] The second distillation column removes the light ends from DMC. It is operated at 1.7 bara with a bottom temperature of 115 °C. The overheads section includes a partial condenser operated at 75 °C with vapor-only distillate. All the condensed liquid is returned to the column as reflux.

[0065] The following streams are withdrawn in the above second distillation step: (i) a top vapor stream containing DMC (50 wt.%) , CO2 (21 wt.%) and ethers and alcohols, which stream is vented; and (ii) a bottom stream containing DMC (93.2 wt.%) , EMC (4.5 wt.%) and light by-products, which stream is recycled to the reactor. A small part of the latter stream is diverted to waste as a bleed to remove process impurities.

[0066] The third distillation column separates the EMC product from DEC and purifies the EMC product to ultra-pure grade. It is operated at 1.5 bara with a bottom temperature of 150 °C. Impurities lighter than EMC are removed from the EMC product via a pasteurization section at the top of the third distillation column. The 99.99 wt . % pure EMC product is withdrawn as a side-draw stream from the top of the third distillation column, at a point which is located below the point at which the former top stream is withdrawn from the pasteurization section. The overheads section includes a partial condenser operated at 120 °C with virtually only liquid, wherein a vapor stream is included to allow venting of non-condensable compounds.

[0067] The following streams are withdrawn in the above third distillation step: (i) a top stream (from the pasteurization section) containing DMC (0.04 wt.%) and EMC (99.96 wt.%) , which stream is recycled to the first distillation column, wherein a small part of the latter stream is diverted to waste as a bleed to remove process impurities; (ii) a top stream (side-draw stream) comprising ultra-pure EMC product (99.99 wt.%) , which stream is sent to a product storage tank; and (iii) a bottom stream containing DEC (97.7 wt.%) , EMC (1.8 wt.%) and heavy by-products, which stream is recycled to the reactor. A small part of the latter stream is diverted to waste as a bleed to remove process impurities.

[0068] Energy consumption in Examples Al and A2

[0069] Example A2 is in accordance with the present invention, but Example Al is not (comparison) . Advantageously, the overall energy consumption in the 3 distillation columns is lower in Example A2 as compared to Example Al, wherein for Example Al the EMC content in the total feed stream to the reactor is 0.3 mol% and the DEC / DMC ratio in the total feed stream to the reactor is 2 mol / mol, whereas for Example A2 the EMC content in the total feed stream to the reactor is 1.6 mol% and the DEC / DMC ratio in the total feed stream to the reactor is 1 mol / mol. The table below shows said energy consumption (in normalized energy units) .

[0070] (*) = not in accordance with the present invention

Claims

C L A I M S1. A continuous process for the preparation of a mixed dialkyl carbonate of formula RI0 (C=0) 0R2 by reacting a first dialkyl carbonate of formula RiO(C=O)ORi and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups and R2 contains more carbon atoms than Ri, said process comprising:(a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates ;(b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) and recycling the separated unconverted first and second dialkyl carbonates to step (a) , wherein the total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol% and has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1.

2. The process according to claim 1, wherein the total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of from 0.5 to 5 mol% .

3. The process according to claim 1 or 2, wherein the total feed stream to step (a) has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate of from 0.1 to 1.8.

4. The process according to any one of claims 1 to 3, wherein Ri and R2 are C1-C4 alkyl groups.

5. The process according to claim 4, wherein Ri is methyl and R2 is ethyl.

6. A continuous process for the preparation of a mixed dialkyl carbonate of formula RI0 (C=0) 0R2 by reacting a first dialkyl carbonate of formula RiO(C=O)ORi and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups and R2 contains more carbon atoms than Ri, said process comprising:(a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates ;(b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a) and recycling the separated unconverted first and second dialkyl carbonates to step (a) , wherein the total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of higher than 0.3 mol%.

7. A continuous process for the preparation of a mixed dialkyl carbonate of formula RIO (C=O) OR2 by reacting a first dialkyl carbonate of formula RiO(C=O)ORi and a second dialkyl carbonate of formula R2O(C=O)OR2, wherein Ri and R2 are different alkyl groups and R2 contains more carbon atoms than Ri, said process comprising:(a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate and unconverted first and second dialkyl carbonates ;(b) separating the unconverted first and second dialkyl carbonates from the product stream resulting from step (a)and recycling the separated unconverted first and second dialkyl carbonates to step (a) , wherein the total feed stream to step (a) has a molar ratio of the second dialkyl carbonate to the first dialkyl carbonate which is lower than 2:1.

8. A process for preparing a battery electrolyte solution comprising combining a mixed dialkyl carbonate with one or more alkali metal salts, wherein the mixed dialkyl carbonate has been prepared by the process according to any one of claims 1 to 7.

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