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, unconverted first and second dialkyl carbonates and light components; (b) separating the product stream resulting from step (a) into a top stream comprising the unconverted first dialkyl carbonate and the light components and a bottom stream comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate; (c) separating the top stream resulting from step (b) into a top stream comprising the light components and a bottom stream comprising the unconverted first dialkyl carbonate; (d) recycling the bottom stream resulting from step (c) to step (a); (e) separating the bottom stream resulting from step (b) into a top stream comprising the mixed dialkyl carbonate and a bottom stream comprising the unconverted second dialkyl carbonate; (f) recycling the bottom stream resulting from step (e) to step (a).

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 the process of the present invention as described hereinbelow . The present invention relates to a continuous process for the preparation of a mixed dialkyl carbonate of formula RiO (C=0) OR2 by reacting a first dialkyl carbonate of formula RiO (C=0) 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 :

[0010] (a) reacting the first and second dialkyl carbonates in a reactor resulting in a product stream comprising the mixed dialkyl carbonate, unconverted first and second dialkyl carbonates and light components;

[0011] (b) separating the product stream resulting from step (a) into a top stream comprising the unconverted first dialkyl carbonate and the light components and a bottom stream comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate;

[0012] (c) separating the top stream resulting from step (b) into a top stream comprising the light components and a bottom stream comprising the unconverted first dialkyl carbonate ;

[0013] (d) recycling the bottom stream resulting from step (c) to step (a) ;

[0014] (e) separating the bottom stream resulting from step (b) into a top stream comprising the mixed dialkyl carbonate and a bottom stream comprising the unconverted second dialkyl carbonate ;

[0015] (f) recycling the bottom stream resulting from step (e) to step (a) .

[0016] 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. Brief description of the drawings

[0017] Figures 1, 2A, 2B and 2C depict line-ups for processes for producing and recovering ethyl methyl carbonate which are in accordance with the present invention.

[0018] Detailed description of the invention

[0019] The process of the present invention comprises steps (a) to (f ) , as described hereinbelow. Said process may comprise one or more intermediate steps between steps (a) and (b) , between steps (b) and (c) , and between steps (c) and (d) , and between steps (d) and (e) , and between steps (e) and (f) .

[0020] Further, said process may comprise one or more additional steps preceding step (a) and / or following step (f) .

[0021] 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.

[0022] 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%.

[0023] 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 .

[0024] Within the present specification, "substantially no" means that no detectible amount of the component in question is present in the catalyst or composition.

[0025] 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. 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 RI0 (C=0) 0R2, wherein Ri and R2 are different alkyl groups, is prepared by reacting a dialkyl carbonate of formula RiO (C=0) 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 .

[0026] 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".

[0027] 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.

[0028] 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, unconverted first and second dialkyl carbonates and light components .

[0029] 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" .

[0030] 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 steps (d) and (f) , respectively, 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) .

[0031] 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 dialkyl carbonate that is recycled in step (d) of the present process and the unconverted second dialkyl carbonate that is recycled in step (f) 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, fresh first dialkyl carbonate may be fed to step (b) and subsequently be recycled in step (d) . Still further, fresh second dialkyl carbonate may be fed to step (b) and subsequently be recycled in step (f) . 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 dialkyl carbonate that is recycled in step (d) and the unconverted second dialkyl carbonate that is recycled in step (f) . Preferably, said one feed stream also comprises fresh first dialkyl carbonate and / or fresh second dialkyl carbonate.

[0032] 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 .

[0033] 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 recycle step (d) and / or recycle step (f) . 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 step (b) , step (c) and / or step (e) of the present process, 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 equilibriumlimited 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 .

[0034] 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.

[0035] 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 (A1P0) catalyst.

[0036] Suitably, in step (a) of the present process an A1P0 catalyst as disclosed in said articles may be used as a catalyst.

[0037] [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.

[0038] [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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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) .

[0044] In the present invention, the separation in steps (b) , (c) and (e) is 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.

[0045] Within the present specification, by "top stream" or ''bottom stream" from a column, or from a section of a column, reference is made to a stream which exits the column or section at a position, which is between 0% and 30%, more suitably between 0% and 20%, even more suitably between 0% and 10%, based on the total column or section length, from the top of the column or section or the bottom of the column or section, respectively.

[0046] In one embodiment of the present invention, each of steps

[0047] (b) , (c) and (e) is carried out in a separate distillation column, referred to below as first, second and third distillation columns, respectively. Each of said distillation columns may comprise one distillation section.

[0048] In accordance with above-mentioned embodiment of the present invention wherein a separate distillation column is used in each of steps (b) , (c) and (e) , steps (b) to (f ) may be as follows :

[0049] (b) separating the product stream resulting from step (a) , in a first distillation column, into a top stream comprising the unconverted first dialkyl carbonate and the light components and a bottom stream comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate ;

[0050] (c) separating the top stream resulting from step (b) , in a second distillation column, into a top stream comprising the light components and a bottom stream comprising the unconverted first dialkyl carbonate;

[0051] (d) recycling the bottom stream resulting from step (c) to step (a) ;

[0052] (e) separating the bottom stream resulting from step (b) , in a third distillation column, into a top stream comprising the mixed dialkyl carbonate and a bottom stream comprising the unconverted second dialkyl carbonate;

[0053] (f) recycling the bottom stream resulting from step (e) to step (a) .

[0054] Each of steps (b) to (f) is further described below with reference to above-mentioned embodiment of the present invention wherein a separate distillation column is used in each of steps (b) , (c) and (e) . Step (b) of the present process comprises separating the product stream resulting from step (a) , in a first distillation column, into a top stream comprising the unconverted first dialkyl carbonate and the light components and a bottom stream comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate.

[0055] In addition to above-mentioned product stream, fresh first dialkyl carbonate may also be fed to the first distillation column, preferably at a point above the feed point of said product stream. Further, in addition to said product stream, fresh second dialkyl carbonate may also be fed to the first distillation column, preferably at a point below the feed point of said product stream. Advantageously, by feeding fresh first and / or second dialkyl carbonates to the first distillation column, any impurities from these fresh dialkyl carbonates may be removed before feeding the dialkyl carbonates to reaction step (a) , via recycle step (d) and / or recycle step (f) .

[0056] Further, in addition to above-mentioned product stream, a top stream resulting from below-described step (e) and comprising the mixed dialkyl carbonate and the unconverted first dialkyl carbonate, may also be fed to the first distillation column, preferably at a point below the feed point of said product stream.

[0057] In step (b) of the present process, the product stream resulting from step (a) is subjected to distillation in the first distillation column. The first distillation column may have of from 30 to 75 theoretical stages. Further, the first distillation column may be operated at a pressure of from 0.5 to 5 bara, suitably 1.3 to 3 bara. Further, the first distillation column may be operated at a condenser temperature of from 30 to 70 °C. Further, the first distillation column may be operated at a reflux ratio of from 3 to 7. Within the present specification, by said "reflux ratio", reference is made to the ratio of the molar flow rate of the "reflux stream" which is that part of the stream that leaves the condenser at the top of the distillation column which is sent back to that column, to the molar flow rate of the "distillate stream" which is that part of the stream that leaves the condenser at the top of the distillation column which is not sent back to that column.

[0058] Step (c) of the present process comprises separating the top stream resulting from step (b) , in a second distillation column, into a top stream comprising the light components and a bottom stream comprising the unconverted first dialkyl carbonate .

[0059] In step (c) of the present process, the top stream resulting from step (b) is subjected to distillation in the second distillation column. The second distillation column may have of from 5 to 40 theoretical stages. Further, the second distillation column may be operated at a pressure of from 0.5 to 7 bara . Further, the second distillation column may be operated at a condenser temperature of from 30 to 90 °C. Further, the second distillation column may be operated at a reflux ratio of from 4 to 12.

[0060] In addition to a top stream comprising the unconverted first dialkyl carbonate and the light components, another stream comprising the first dialkyl carbonate may be withdrawn from the top of the second distillation column at a point which is below the point at which the former top stream is withdrawn. The latter top stream may also be referred to as a "side-draw stream" and contains the first dialkyl carbonate in a high purity.

[0061] Step (d) of the present process comprises recycling the bottom stream resulting from step (c) to step (a) . In this way, the unconverted first dialkyl carbonate is recycled to step (a) for further conversion. Part of said bottom stream may be bled off and discarded to remove impurities from the process. Further, the bottom stream resulting from step (c) that is recycled to step (a) may comprise the mixed dialkyl carbonate. The amount of the mixed dialkyl carbonate in said bottom stream 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 . % .

[0062] Step (e) of the present process comprises separating the bottom stream resulting from step (b) , in a third distillation column, into a top stream comprising the mixed dialkyl carbonate and a bottom stream comprising the unconverted second dialkyl carbonate.

[0063] In step (e) of the present process, the bottom stream resulting from step (b) is subjected to distillation in the third distillation column. The third distillation column may have of from 40 to 85 theoretical stages. Further, the third distillation column may be operated at a pressure of at most 5 bara, suitably of from 0.5 to 3 bara. Further, the third distillation column may be operated at a condenser temperature of from 60 to 150 °C. Further, the third distillation column may be operated at a ref lux-to-product ratio of from 1 to 10. Within the present specification, by said "ref lux-to-product ratio", reference is made to the ratio of the molar flow rate of the "reflux stream" which is that part of the stream that leaves the condenser at the top of the distillation column which is sent back to that column, to the molar flow rate of the "product stream" which is (i) that part of the stream that leaves the condenser at the top of the distillation column which is not sent back to that column (the "distillate stream") or (ii) a "side-draw stream" as described above and below. In addition to a top stream comprising the mixed dialkyl carbonate and the unconverted first dialkyl carbonate, another stream comprising the mixed dialkyl carbonate may be withdrawn from the top of the third distillation column at a point which is below the point at which the former top stream is withdrawn. The latter top stream may also be referred to as a "side-draw stream" and contains the mixed dialkyl carbonate in a higher purity than the former top stream. Said side-draw stream may be withdrawn at a point which is at of from 2 to 10 stages below the top of the third distillation column. The former top stream comprising the mixed dialkyl carbonate and the unconverted first dialkyl carbonate may be fed to the first distillation column, preferably at a point below the feed point of the product stream resulting from step (a) . Further, said former top stream may be fed to the reactor in step (a) . Still further, said former top stream may be discarded, in part or completely, to remove impurities from the process.

[0064] Step (f) of the present process comprises recycling the bottom stream resulting from step (e) to step (a) . In this way, the unconverted second dialkyl carbonate is recycled to step (a) for further conversion. Part of said bottom stream may be bled off and discarded to remove impurities from the process. Further, the bottom stream resulting from step (e) that is recycled to step (a) may comprise the mixed dialkyl carbonate. The amount of the mixed dialkyl carbonate in said bottom stream 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 . % .

[0065] Optionally, the condenser of any one of the above- mentioned first, second and third distillation columns can be used to generate steam. The invention in accordance with the above-described embodiment wherein a separate distillation column is used in each of steps (b) , (c) and (e) , is further illustrated by Figure 1 which depicts a line-up for a process for producing and recovering ethyl methyl carbonate which is in accordance with the present invention.

[0066] In the process of Figure 1, a feed stream 2 comprising fresh first dialkyl carbonate (e.g. dimethyl carbonate) and a feed stream 3 comprising fresh second dialkyl carbonate (e.g. diethyl carbonate) are fed to and reacted in a reactor 1 containing a catalyst. A product stream 4 is withdrawn from reactor 1 and comprises a mixed dialkyl carbonate (e.g. ethyl methyl carbonate) , unconverted first and second dialkyl carbonates and light components.

[0067] Product stream 4 is fed to a first distillation column 5 and separated into a top stream 6 comprising the unconverted first dialkyl carbonate and the light components and a bottom stream 7 comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate. Optionally, a feed stream 8 comprising fresh first dialkyl carbonate is fed to first distillation column 5.

[0068] Top stream 6 from first distillation column 5 is fed to a second distillation column 9 and separated into a top stream 10 comprising the light components and a bottom stream 11 comprising the unconverted first dialkyl carbonate which is recycled to reactor 1.

[0069] Bottom stream 7 from first distillation column 5 is fed to a third distillation column 12 and separated into a top stream 13 comprising the mixed dialkyl carbonate and a bottom stream 14 comprising the unconverted second dialkyl carbonate which is recycled to reactor 1. Optionally, in case bottom stream 7 from first distillation column 5 also comprises the unconverted first dialkyl carbonate, an additional top stream 15 comprising the mixed dialkyl carbonate and the unconverted first dialkyl carbonate is withdrawn from the third distillation column. Stream 15 may be fed to the first distillation column.

[0070] In another embodiment of the present invention, two or all of steps (b) , (c) and (e) are carried out in separate distillation sections of a divided-wall column. In the latter embodiment, two or three of the above-described first, second and third distillation columns are combined into one divided- wall column. A divided-wall column is a distillation column which comprises two or more, preferably two or three, distillation sections separated by one or more internal walls .

[0071] In above-mentioned embodiment of the present invention wherein two or all of steps (b) , (c) and (e) are carried out in separate distillation sections of a divided-wall column, it is preferred that:

[0072] (i) step (b) is carried out in a first distillation section of the divided-wall column, step (c) is carried out in a second distillation section of the divided-wall column, and step (e) is carried out in a distillation column which is not a divided-wall column; or

[0073] (ii) step (b) is carried out in a first distillation section of the divided-wall column, step (c) is carried out in a distillation column which is not a divided-wall column, and step (e) is carried out in a second distillation section of the divided-wall column; or

[0074] (iii) steps (b) , (c) and (e) are carried out in first, second and third distillation sections, respectively, of the divided-wall column.

[0075] The above description of the embodiment of the present invention wherein no divided-wall column is used but wherein a separate distillation column is used in each of steps (b) , (c) and (e) , equally applies to the above embodiments wherein a divided-wall column is used.

[0076] In the present invention, when using a divided-wall column, the following embodiments are envisaged:

[0077] (i) the first and second distillation columns are combined into one divided-wall column wherein the abovedescribed third distillation column remains as a distillation column which is not a divided-wall column;

[0078] (ii) the first and third distillation columns are combined into one divided-wall column wherein the abovedescribed second distillation column remains as a distillation column which is not a divided-wall column; and

[0079] (iii) the first, second and third distillation columns are combined into one divided-wall column.

[0080] Said 3 embodiments (i) , (ii) and (iii) wherein a divided- wall column is used, are further described hereinbelow.

[0081] In accordance with above-mentioned embodiment (i) of the present invention wherein a divided-wall column is used, steps (b) to (f) may be as follows:

[0082] (b) separating the product stream resulting from step (a) , in a first distillation section of a divided-wall column, into a top stream from the first distillation section comprising the unconverted first dialkyl carbonate and the light components and a bottom stream from the first distillation section comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate;

[0083] (c) separating the top stream resulting from step (b) , in a second distillation section of the divided-wall column, into a top stream from the divided-wall column comprising the light components and a bottom stream from the second distillation section comprising the unconverted first dialkyl carbonate ; (d) recycling the bottom stream resulting from step (c) to step (a) ;

[0084] (e) separating the bottom stream resulting from step (b) , in a distillation column which is not a divided-wall column, into a top stream comprising the mixed dialkyl carbonate and a bottom stream comprising the unconverted second dialkyl carbonate ;

[0085] (f) recycling the bottom stream resulting from step (e) to step (a) .

[0086] The above description of the embodiment of the present invention wherein no divided-wall column is used, equally applies to said embodiment (i) wherein a divided-wall column is used. In said embodiment (i) , the first distillation section of the divided-wall column corresponds with the above-described first distillation column; the second distillation section of the divided-wall column corresponds with the above-described second distillation column; and the distillation column which is not a divided-wall column corresponds with the above-described third distillation column .

[0087] In said embodiment (i) , the divided-wall column may comprise an internal vertical wall which separates the first distillation section from the second distillation section and which extends to the bottom and not to the top of the divided-wall column.

[0088] The invention in accordance with said embodiment (i) wherein a divided-wall column is used in steps (b) and (c) , is further illustrated by Figure 2A. Figure 2A depicts a divided-wall column (i) comprising first and second distillation sections 5 and 9 replacing the first and second distillation columns 5 and 9 in the line-up depicted in Figure 1. Each of the streams shown in Fig. 2A corresponds with the stream with the same number shown in Figure 1. The unchanged part of the line-up of Figure 1 is not shown in Figure 2A.

[0089] In accordance with above-mentioned embodiment (ii) of the present invention wherein a divided-wall column is used, steps (b) to (f) may be as follows:

[0090] (b) separating the product stream resulting from step (a) , in a first distillation section of a divided-wall column, into a top stream from the first distillation section comprising the unconverted first dialkyl carbonate and the light components and a bottom stream from the first distillation section comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate;

[0091] (c) separating the top stream resulting from step (b) , in a distillation column which is not a divided-wall column, into a top stream comprising the light components and a bottom stream comprising the unconverted first dialkyl carbonate ;

[0092] (d) recycling the bottom stream resulting from step (c) to step (a) ;

[0093] (e) separating the bottom stream resulting from step (b) , in a second distillation section of the divided-wall column, into a top stream from the second distillation section comprising the mixed dialkyl carbonate and a bottom stream from the divided-wall column comprising the unconverted second dialkyl carbonate;

[0094] (f) recycling the bottom stream resulting from step (e) to step (a) .

[0095] The above description of the embodiment of the present invention wherein no divided-wall column is used, equally applies to said embodiment (ii) wherein a divided-wall column is used. In said embodiment (ii) , the first distillation section of the divided-wall column corresponds with the above-described first distillation column; the second distillation section of the divided-wall column corresponds with the above-described third distillation column; and the distillation column which is not a divided-wall column corresponds with the above-described second distillation column .

[0096] In said embodiment (ii) , the divided-wall column may comprise an internal vertical wall which separates the first distillation section from the second distillation section and which extends to the top and not to the bottom of the divided-wall column.

[0097] The invention in accordance with said embodiment (ii) wherein a divided-wall column is used in steps (b) and (e) , is further illustrated by Figure 2B. Figure 2B depicts a divided-wall column (ii) comprising first and second distillation sections 5 and 12 replacing the first and third distillation columns 5 and 12 in the line-up depicted in Figure 1. Each of the streams shown in Fig. 2B corresponds with the stream with the same number shown in Figure 1. The unchanged part of the line-up of Figure 1 is not shown in Figure 2B.

[0098] In accordance with above-mentioned embodiment (iii) of the present invention wherein a divided-wall column is used, steps (b) to (f) may be as follows:

[0099] (b) separating the product stream resulting from step (a) , in a first distillation section of a divided-wall column, into a top stream from the first distillation section comprising the unconverted first dialkyl carbonate and the light components and a bottom stream from the first distillation section comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate;

[0100] (c) separating the top stream resulting from step (b) , in a second distillation section of the divided-wall column, into a top stream from the divided-wall column comprising the light components and a bottom stream from the second distillation section comprising the unconverted first dialkyl carbonate ;

[0101] (d) recycling the bottom stream resulting from step (c) to step (a) ;

[0102] (e) separating the bottom stream resulting from step (b) , in a third distillation section of the divided-wall column, into a top stream from the third distillation section comprising the mixed dialkyl carbonate and a bottom stream from the divided-wall column comprising the unconverted second dialkyl carbonate;

[0103] (f) recycling the bottom stream resulting from step (e) to step (a) .

[0104] The above description of the embodiment of the present invention wherein no divided-wall column is used, equally applies to said embodiment (iii) wherein a divided-wall column is used. In said embodiment (iii) , the first, second and third distillation sections of the divided-wall column correspond with the above-described first, second and third distillation columns, respectively.

[0105] In said embodiment (iii) , the divided-wall column may comprise an internal vertical wall which separates the first distillation section from the second and third distillation sections, which does not extend to the top of the divided- wall column and which does not extend to the bottom of the divided-wall column, and an internal horizontal wall which separates the second distillation section from the third distillation section wherein the second distillation section is located above the third distillation section.

[0106] The invention in accordance with said embodiment (iii) wherein a divided-wall column is used in steps (b) , (c) and (e) , is further illustrated by Figure 2C. Figure 2C depicts a divided-wall column (iii) comprising first, second and third distillation sections 5, 9 and 12 replacing the first, second and third distillation columns 5, 9 and 12 in the line-up depicted in Figure 1. Each of the streams shown in Fig. 2C corresponds with the stream with the same number shown in Figure 1. The unchanged part of the line-up of Figure 1 is not shown in Figure 2C.

[0107] 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-described process. 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.

[0108] The invention is further illustrated by the following Examples .

[0109] Examples

[0110] A) Production and recovery of ethyl methyl carbonate in accordance with the present invention

[0111] In these Examples, ethyl methyl carbonate (EMC) was produced, by reacting dimethyl carbonate (DMC) with diethyl carbonate (DEC) , and subsequently recovered in a continuous process in accordance with the present invention.

[0112] A stainless-steel reactor (internal diameter = 3.3 cm) was loaded with about 100 g of inert glass beads, followed by 120 g of a catalyst and 1,000 g of inert glass beads. The catalyst used was the catalyst as prepared in a way as disclosed in W02019016126, in specific as described under A) in the Examples thereof.

[0113] The reactor was electrically heated and was operated in an up-flow mode at 60 or 70 °C and at 1 bara. The reactor received its feed from a feed vessel, where the DMC and DEC recycle streams were mixed with fresh DMC and DEC feed streams, maintaining a DEC / DMC molar ratio of 1 in the combined feed stream to the reactor. The reactor effluent comprising EMC, unconverted DMC and DEC, and light components, said light components including ethanol (EtOH) , methanol (MeOH) , diethyl ether (DEE) , dimethyl ether (DME) and ethyl methyl ether (EME) , was passed to a buffer vessel, from which off-gas (nitrogen and carbon dioxide) was removed, and then further sent to a first distillation column.

[0114] The first distillation column was an Oldershaw column with 20 physical trays in the stripping section and 20 physical trays in the rectification section. The first distillation column was operated at 1.1 bara and a reboiler temperature of 110-120 °C, with a reflux ratio between 4 and 6. In the first distillation column, a top stream comprising the unconverted DMC and light components and a bottom stream comprising the unconverted DEC and EMC were separated.

[0115] The top stream from the first distillation column was fed to a second distillation column, which was an Oldershaw column with 5 physical trays in the stripping section and 15 physical trays in the rectification section. The second distillation column was operated at 1.1 bar and a reboiler temperature of 90-95 °C, with a reflux to feed ratio (by weight) between 0.3 and 0.6. In the second distillation column, a top stream comprising the light components and a bottom stream comprising the unconverted DMC were separated. The bottom stream comprising the unconverted DMC was recycled to the reactor, via the feed vessel. The bottom stream from the first distillation column was fed to a third distillation column, which was an Oldershaw column with 10 physical trays in the stripping section and 20 physical trays in the rectification section. The third distillation column was operated at 1.1 bara and a reboiler temperature of 125-130 °C, with a reflux ratio between 3.5 and 4.5. In the third distillation column, a top stream comprising recovered EMC and a bottom stream comprising the unconverted DEC were separated. The bottom stream comprising the unconverted DEC was recycled to the reactor, via the feed vessel. Before such recycle, part of said stream was split off in order to bleed heavy components from the process.

[0116] The table below mentions some operating conditions which were varied, as well as the results, for Examples 1-4.

[0117] WHSV = weight hourly space velocity

[0118] Advantageously, the purity of the recovered EMC (top stream from third distillation column) , as produced and recovered in accordance with the present invention, was relatively high, which purity ranged from 98.4 to 99.3 wt . % (in Examples 1-4) . The recovered EMC product from Example 1 (EMC purity of 98.4 wt.%) was further subjected to a batch distillation in an Oldershaw column with 30 physical trays at atmospheric pressure. The distillation products were collected in 27 fractions, which allows determination of vapor-liquid equilibria needed to design the recovery section of a large- scale process to recover high-purity EMC. The DMC from the recovered EMC product was concentrated in the first 11 fractions, while the DEC and heavy components from the recovered EMC product were concentrated in the residue. The collected 16 EMC fractions had an EMC purity of 99.97 wt.%, with traces of DMC and DEC.

[0119] The latter batch distillation experiment demonstrated that high-purity EMC can advantageously be obtained from the reactor product by a recovery section comprising distillation columns in accordance with the present invention. This is further substantiated under B) below, by Aspen modelling.

[0120] B) Aspen modelling of production and recovery of ethyl methyl carbonate in accordance with the present invention

[0121] Example Bl (invention)

[0122] 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%) .

[0123] 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.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] Example B2 (invention)

[0131] 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.% (1.6 mol%) .

[0132] 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.

[0133] 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. 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] Energy consumption in Examples Bl and B2

[0139] Examples Bl and B2 are both in accordance with the present invention. Advantageously, the overall energy consumption in the 3 distillation columns is lower in Example B2 as compared to Example Bl, wherein for Example Bl 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 B2 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) .

[0140] Example B3 (comparison) 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%) .

[0141] 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.

[0142] The product stream is fed to a first distillation column to separate light ends from DMC and to separate DMC from EMC, DEC and heavier components. This first distillation column is operated at 1.60 bara with a bottom temperature of 140 °C. The overheads section includes a partial condenser operated at 60 °C with vapor distillate. Impurities lighter than DMC are removed from the unconverted DMC product via a pasteurization section at the top of the first distillation column. Unconverted DMC is withdrawn as a side-draw stream from the top of the first distillation column, at a point which is located below the point at which the former top stream is withdrawn from the pasteurization section.

[0143] The following streams are withdrawn in the above first distillation step: (i) a top vapor stream containing CO2 (27.2 wt.%) , DMC (31.3 wt.%) and alcohols and ethers, which stream is vented; (ii) a top stream (side-draw stream) comprising DMC (98.5 wt.%) and light by-products, which stream is recycled to the reactor, wherein a small part of the latter stream is diverted to waste as a bleed to remove process impurities; and (iii) a bottom stream containing EMC (46.0 wt.%) , DEC (54.0 wt.%) , DMC (26 ppmw) and heavy byproducts, which is sent to a second distillation column.

[0144] The second 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. 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 second 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 110 °C with virtually only liquid, wherein a vapor stream is included to allow venting of non-condensable compounds.

[0145] The following streams are withdrawn in the above second distillation step: (i) a top stream (from the pasteurization section) containing EMC (99.97 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 (99.6 wt.%) , EMC (0.3 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.

[0146] Energy consumption in Examples Bl and B3

[0147] Example Bl is in accordance with the present invention, and Example B3 is not. Advantageously, the overall energy consumption is lower in Example Bl (using three distillation columns) as compared to Example B3 (using two distillation columns) . The table below shows said energy consumption (in normalized energy units) .

[0148] (*) = 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, unconverted first and second dialkyl carbonates and light components;(b) separating the product stream resulting from step (a) into a top stream comprising the unconverted first dialkyl carbonate and the light components and a bottom stream comprising the unconverted second dialkyl carbonate and the mixed dialkyl carbonate;(c) separating the top stream resulting from step (b) into a top stream comprising the light components and a bottom stream comprising the unconverted first dialkyl carbonate ;(d) recycling the bottom stream resulting from step (c) to step (a) ;(e) separating the bottom stream resulting from step (b) into a top stream comprising the mixed dialkyl carbonate and a bottom stream comprising the unconverted second dialkyl carbonate ;(f) recycling the bottom stream resulting from step (e) to step (a) .

2. The process according to claim 1, wherein a total feed stream to step (a) comprises the mixed dialkyl carbonate in an amount of from 0.2 to 5 mol% .

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

4. The process according to any one of claims 1 to 3, wherein a 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 5.

5. The process according to claim 4, 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.

6. The process according to any one of claims 1 to 5, wherein Ri and R2 are C1-C4 alkyl groups.

7. The process according to claim 6, wherein Ri is methyl and R2 is ethyl.

8. The process according to any one of claims 1 to 7, wherein each of steps (b) , (c) and (e) is carried out in a separate distillation column.

9. The process according to any one of claims 1 to 7, wherein two or all of steps (b) , (c) and (e) are carried out in separate distillation sections of a divided-wall column.

10. 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 9.

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