Method for producing oligoethylene glycol methyl ether borate

Abstract

The present invention relates to a method for producing oligoethylene glycol methyl ether borate, the method comprising the steps of: a) feeding boric acid and oligoethylene glycol monomethyl ether to a reactor (10) and reacting the resulting mixture to obtain a crude product comprising oligoethylene glycol methyl ether borate, water, and unreacted boric acid and oligoethylene glycol monomethyl ether; b) feeding the crude product to a reactive distillation apparatus (30) and reacting the boric acid with the oligoethylene glycol monomethyl ether until complete conversion of the boric acid; c) transferring a distillation stream containing water from the top of the reactive distillation apparatus (30) to a condenser (34) and recycling the condensed liquid stream to the top of the reactive distillation apparatus (30); and d) withdrawing a bottoms product stream containing oligoethylene glycol methyl ether borate from the reactive distillation apparatus (30), transferring a portion of the bottoms product stream to a reboiler (33), and recycling the resulting vapor stream to the bottom section of the reactive distillation apparatus (30).

Description

[Technical field]

[0001] The present invention relates to a process for producing oligoethylene glycol methyl ether borate by reacting boric acid and oligoethylene glycol monomethyl ether in a reactive distillation apparatus. [Background technology]

[0002] Triethylene glycol methyl ether borate is the main component of brake fluids due to its excellent performance, characterized by very good thermal stability and low viscosity. This borate is produced by the esterification reaction of boric acid with glycol ether. Generally, the reaction mixture contains the product triethylene glycol methyl ether borate, by-product water, reactant triethylene glycol monomethyl ether, as well as heavy boiling components and metal impurities. In the conventional manufacturing process, the reaction mixture is separated by distillation using either a series of distillation columns or a single distillation column with multiple side draws.

[0003] Document CN104447829(A) discloses a process for the production of triethylene glycol methyl ether borate in a single distillation column. The desired product is recovered through a side draw from the stripping section of the distillation column. This distillation column design allows the production of very pure triethylene glycol methyl ether borate, since impurities and heavies are separated as bottom products. Triethylene glycol monomethyl ether is withdrawn through a side draw in the rectification section, while water is obtained as distillate. However, a high number of theoretical stages is required in the distillation column to achieve the required product specifications. In addition, a high reflux ratio is also required, resulting in high energy demand for the process. The energy demand is further increased by the evaporation of not only unconverted triethylene glycol monomethyl ether, but also the evaporation of the entire borate product stream. To meet the high energy demand, the process is of the internal heat exchange type in several stages. The recovered triethylene glycol monomethyl ether is used to preheat the reaction mixture. In addition, the reaction mixture is preheated by withdrawn triethylene glycol methyl ether borate. The triethylene glycol methyl ether borate is then cooled by condensed water recovered at the top of the column. Although the borate can be obtained in high purity, this process is characterized by high energy demand, high equipment cost, and difficult process control strategy due to the complex process behavior caused by internal heat exchange.

[0004] The document EP 1 661 901 A1 discloses a process for the preparation of boric acid esters from boric acid or boron trioxide by reaction with alcohols in a reactive distillation column, where distillation is at least partially overlapped with extraction inside the column. The specific alcohols disclosed are methanol, ethanol and propanol. The corresponding borates as products of this process form low-boiling azeotropes with the alcohols, which necessitates the design of a reactive distillation column with overlapping extractions and an extractant as an additional substance. The borates are withdrawn from the top of the column and the extractant is withdrawn from the bottom of the column. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Chinese Patent No. 104447829(A) [Patent Document 2] European Patent Application Publication No. 1661901(A1) Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the present invention was to provide a process for producing high purity oligoethylene glycol methyl ether borate, which is characterized by low overall energy demand, low capital costs, and a smooth operating range. [Means for solving the problem]

[0007] This object is achieved according to the invention by a process for the preparation of oligoethylene glycol methyl ether borate according to claim 1. Advantageous embodiments and further developments of this process are presented in the dependent claims 2-15. [Brief description of the drawings]

[0008] [Figure 1]FIG. 1 is a process flow sheet for producing triethylene glycol methyl ether borate as a first embodiment according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The first subject of the present invention is a process for producing an oligoethylene glycol methyl ether borate, comprising the steps of: a) feeding boric acid and oligoethylene glycol monomethyl ether to a reactor and reacting the resulting mixture to obtain a crude product comprising oligoethylene glycol methyl ether borate, water, and unreacted boric acid and oligoethylene glycol monomethyl ether; b) feeding the crude product to a reactive distillation apparatus and reacting boric acid with oligoethylene glycol monomethyl ether until complete conversion of boric acid; c) transferring a water-containing distillation stream from the top of the reactive distillation apparatus to a condenser and recycling the condensed liquid stream to the top of the reactive distillation apparatus; d) withdrawing a bottoms product stream containing oligoethylene glycol methyl ether borate from the reactive distillation apparatus, transferring a portion of the bottoms product stream to a reboiler, and recycling the resulting vapor stream to the bottoms section of the reactive distillation apparatus. The method includes:

[0010] Within the scope of the present invention, the term "oligo" means "di", "tri" and "tetra". Thus, oligoethylene glycol monomethyl ether may be diethylene glycol monomethyl ether, triethylene glycol monomethyl ether or tetraethylene glycol monomethyl ether. Correspondingly, oligoethylene glycol methyl ether borate, hereinafter sometimes also referred to as "borate", may be diethylene glycol methyl ether borate, triethylene glycol methyl ether borate or tetraethylene glycol methyl ether borate.

[0011] In a first embodiment of the first subject of the present invention, the oligoethylene glycol monomethyl ether is diethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is diethylene glycol methyl ether borate. The method for producing diethylene glycol methyl ether borate comprises the steps of: a) feeding boric acid and diethylene glycol monomethyl ether to a reactor and reacting the resulting mixture to obtain a crude product comprising diethylene glycol methyl ether borate, water, and unreacted boric acid and diethylene glycol monomethyl ether; b) feeding the crude product to a reactive distillation apparatus and reacting boric acid with diethylene glycol monomethyl ether until complete conversion of boric acid; c) transferring a water-containing distillation stream from the top of the reactive distillation apparatus to a condenser and recycling the condensed liquid stream to the top of the reactive distillation apparatus; d) withdrawing a bottoms product stream containing diethylene glycol methyl ether borate from the reactive distillation apparatus, transferring a portion of the bottoms product stream to a reboiler, and recycling the resulting vapor stream to the bottoms section of the reactive distillation apparatus. Includes.

[0012] In a second embodiment of the first subject of the present invention, the oligoethylene glycol monomethyl ether is triethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is triethylene glycol methyl ether borate. The method for producing triethylene glycol methyl ether borate comprises the steps of: a) feeding boric acid and triethylene glycol monomethyl ether to a reactor and reacting the resulting mixture to obtain a crude product comprising triethylene glycol methyl ether borate, water, and unreacted boric acid and triethylene glycol monomethyl ether; b) feeding the crude product to a reactive distillation apparatus and reacting boric acid with triethylene glycol monomethyl ether until complete conversion of boric acid; c) transferring a water-containing distillation stream from the top of the reactive distillation apparatus to a condenser and recycling the condensed liquid stream to the top of the reactive distillation apparatus; d) withdrawing a bottoms product stream containing triethylene glycol methyl ether borate from the reactive distillation apparatus, transferring a portion of the bottoms product stream to a reboiler, and recycling the resulting vapor stream to the bottoms section of the reactive distillation apparatus. Includes.

[0013] In a third embodiment of the first subject matter of the present invention, the oligoethylene glycol monomethyl ether is tetraethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is tetraethylene glycol methyl ether borate. The method for producing tetraethylene glycol methyl ether borate comprises the steps of: a) feeding boric acid and tetraethylene glycol monomethyl ether to a reactor and reacting the resulting mixture to obtain a crude product comprising tetraethylene glycol methyl ether borate, water, and unreacted boric acid and tetraethylene glycol monomethyl ether; b) feeding the crude product to a reactive distillation apparatus and reacting boric acid with tetraethylene glycol monomethyl ether until complete conversion of boric acid; c) transferring a water-containing distillation stream from the top of the reactive distillation apparatus to a condenser and recycling the condensed liquid stream to the top of the reactive distillation apparatus; d) withdrawing a bottoms product stream containing tetraethylene glycol methyl ether borate from the reactive distillation apparatus, transferring a portion of the bottoms product stream to a reboiler, and recycling the resulting vapor stream to the bottoms section of the reactive distillation apparatus. Includes.

[0014] In step (a), the reactants boric acid and oligoethylene glycol monomethyl ether are fed into a reactor. Preferably, the solid boric acid is completely dissolved in the liquid oligoethylene glycol monomethyl ether and mixed thoroughly. The mixing and dissolving may be carried out entirely in the reactor, for example by providing a corresponding mixing device such as a stirrer. The mixing and dissolving may also be carried out partially or entirely in a device other than the reactor, for example a premixer such as a mixing pump.

[0015] In the esterification reaction, the desired product oligoethylene glycol methyl ether borate is formed. Water is formed as a by-product. The esterification reaction is equilibrium limited, which means that in conventional processes, a significant excess of boric acid is required to shift the equilibrium towards the desired borate. However, according to the present invention, only a part of the reaction takes place in the reactor in step (a), while the remaining part of the reaction to completely convert boric acid takes place in the reactive distillation unit in step (b). Therefore, the method according to the present invention provides flexibility in terms of reaction management in the reactor and reactive distillation unit.

[0016] In a preferred embodiment, the reaction inside the reactor is carried out to an equilibrium state resulting from the molar ratio of the reactants fed to the reactor.

[0017] In a preferred embodiment of the process according to the invention, the reactor is fed with a small excess of alcohol. The molar ratio of oligoethylene glycol monomethyl ether to boric acid fed to the reactor in step (a) is preferably 3.01:1 to 3.4:1, more preferably 3.25:1 to 3.35:1. One advantage of the excess of alcohol is that it shifts the chemical equilibrium towards the desired borate, which can be used to produce more borate in the reactor and thus in the feed to the reactive distillation unit. A further advantage is that the excess of alcohol results in a lower bottom temperature of the reactive distillation column, since the alcohol has a lower boiling point than the product borate and unreacted alcohol is included in the bottom product stream.

[0018] The temperature in the reactor is preferably from 60° C. to 90° C., more preferably from 75° C. to 85° C. The reactor may be equipped with, for example, a heating jacket, internal coils, or a heat exchanger in an external loop to maintain the temperature within the desired range.

[0019] The esterification reaction inside the reactor is preferably carried out at ambient pressure or at a slightly elevated pressure. The pressure in the reactor is preferably between 1.0 bar (absolute) and 2.0 bar (absolute), preferably between 1.1 bar (absolute) and 1.3 bar (absolute).

[0020] Furthermore, the residence time of the mixture in the reactor is preferably 2 to 5 hours.

[0021] Preferably, the above mentioned parameters for the esterification reaction inside the reactor are selected so that the conversion of boric acid in the reactor is between 30% and 90%. Depending on the oligoethylene glycol monomethyl ether present in the reactor, the preferred range of conversion translates into the preferred range of water content of the crude product at the end of the residence time.

[0022] When the oligoethylene glycol monomethyl ether is diethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is diethylene glycol methyl ether borate, the water content of the crude product at the end of the residence time is preferably 3.5% by weight to 11.5% by weight.

[0023] When the oligoethylene glycol monomethyl ether is triethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is triethylene glycol methyl ether borate, the water content of the crude product at the end of the residence time is preferably 2.6% by weight to 8.8% by weight.

[0024] When the oligoethylene glycol monomethyl ether is tetraethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is tetraethylene glycol methyl ether borate, the water content of the crude product at the end of the residence time is preferably 2.1% by weight to 7.1% by weight.

[0025] After completion of the reaction in the reactor of step (a), the crude product is fed to a reactive distillation apparatus, which reacts boric acid with oligoethylene glycol monomethyl ether until complete conversion of boric acid. The reactor contents may be fed directly to the reactive distillation apparatus. Preferably, the reactor contents are discharged to a buffer tank, and the reactive distillation apparatus is fed from the buffer tank. In this case, the reactor is available for further use immediately after the production of the crude product. Separation and purification of the crude product and a new reaction to form a further crude product batch may be carried out simultaneously to save time.

[0026] In a preferred embodiment of the process according to the invention, the reaction in step (a) is carried out discontinuously, while steps (b) to (d) are carried out continuously using a feed stream of the crude product produced in step (a). More preferably, the reaction in step (a) is carried out discontinuously and the resulting crude product is buffered in a tank. (Buffering)Steps (b) to (d) are carried out continuously using a feed stream of the crude product taken out of the tank.

[0027] According to the present invention, the reactive distillation apparatus includes at least one condenser and one reboiler, which allows a distillation stream containing water to be transferred from the top of the reactive distillation apparatus to the condenser and a condensed liquid stream to be recycled to the top of the reactive distillation apparatus, a bottoms product stream containing oligoethylene glycol methyl ether borate to be withdrawn from the reactive distillation apparatus, a portion of the bottoms product stream to the reboiler and the resulting vapor stream to be recycled to the bottom section of the reactive distillation apparatus.

[0028] In the reactive distillation apparatus, unreacted boric acid reacts with unreacted oligoethylene glycol monomethyl ether to form the desired product borate and by-product water. By combining reaction and distillation, by-product water is continuously removed from the reaction mixture. This shifts the reaction equilibrium towards the desired product borate and allows for complete conversion of the reactant boric acid. Water is obtained at the top of the reactive distillation apparatus, while product borate is obtained at the bottom.

[0029] The reactive distillation apparatus comprises internals that facilitate mass exchange between the liquid and gas phases. The internals may be, for example, trays, random packings, or structured packings. Preferably, structured packings are used as the internals.

[0030] In a preferred embodiment, the rectification section of the reactive distillation apparatus between the feed tray and the top of the column is equipped with internals equivalent to 1 to 3 theoretical trays, which ensures adequate separation of the ether and water.

[0031] In a further preferred embodiment, the stripping section between the feed tray and the bottom of the reactive distillation apparatus comprises internals corresponding to 2 to 15 theoretical trays, more preferably 5 to 10. The number of theoretical trays is preferably selected to ensure sufficient residence time for complete conversion of the reactant boric acid.

[0032] The ratio of the number of theoretical stages in the rectification section to the number of theoretical stages in the stripping section of the reactive distillation apparatus is preferably 1:2 to 1:4, more preferably 1:2.5 to 1:3.5. It has been shown that a ratio within the preferred range ensures both sufficient residence time for complete conversion of the reaction in the stripping section and removal of by-product water in the rectification section.

[0033] According to the invention, the distillation stream, which mainly contains water, is transferred from the top of the reactive distillation apparatus to a condenser (step (c)). The distillation stream is at least partially condensed and the condensed liquid stream is recycled to the top of the reactive distillation apparatus as a reflux stream. In a preferred embodiment of the process according to the invention, the reflux ratio, i.e. the mass ratio of the recycled condensed liquid stream to the distillation stream withdrawn from the top of the reactive distillation apparatus, is between 0.2 and 0.5 by mass. This low reflux ratio results in a significant reduction in energy consumption compared to processes known in the art, without compromising the quality of the desired borate product.

[0034] In a preferred variant of this embodiment, the reactive distillation apparatus comprises a further condenser. In the first condenser, hereafter called "reflux condenser", the distillation stream from the top of the reactive distillation apparatus is partially condensed. The condensed liquid stream is rich in organic compounds and is recycled as reflux stream to the top of the reactive distillation apparatus. The uncondensed vapour stream is rich in water and is transferred to a further condenser, in which the water-rich vapour stream is at least partially condensed, allowing the water to be withdrawn as liquid condensate. This two-stage condenser idea allows the minimisation of alcohol (ether) losses and the TOC (total organic carbon) content of the wastewater withdrawn from the process.

[0035] According to the present invention, a bottoms product stream containing primarily oligoethylene glycol methyl ether borate is withdrawn from the reactive distillation apparatus (step (d)). A portion of the bottoms product stream is transferred to a reboiler, vaporized, and recycled to the bottom section of the reactive distillation apparatus. The bottoms product may contain unconverted oligoethylene glycol monomethyl ether.

[0036] Preferably, the bottoms of the reactive distillation unit are designed to minimize residence time of the bottoms product, for example, by minimizing bottoms sump holdup.

[0037] In a preferred embodiment, the reactive distillation unit is operated at a top pressure of 10 mbar (absolute) to 30 mbar (absolute), more preferably 10 mbar (absolute) to 20 mbar (absolute). The lower pressure at the top of the reactive distillation unit leads to a lower temperature of the borate product stream obtained from the bottom of the reactive distillation unit, which has the favorable effect of a higher quality in terms of the color of the final product.

[0038] In a further preferred embodiment, the reactive distillation apparatus is operated at a top temperature between 7°C and 25°C, more preferably between 10°C and 15°C.

[0039] In a further preferred embodiment, the reactive distillation apparatus is operated at a bottom temperature of 150° C. to 200° C. Depending on the oligoethylene glycol methyl ether borate to be produced, the preferred ranges are as follows:

[0040] In the method for producing diethylene glycol methyl ether borate, the reactive distillation apparatus is preferably operated at a column bottom temperature of 150°C to 180°C.

[0041] In the method for producing triethylene glycol methyl ether borate, the reactive distillation apparatus is operated at a column bottom temperature of preferably 160°C to 190°C, more preferably 170°C to 190°C.

[0042] In the method for producing tetraethylene glycol methyl ether borate, the reactive distillation apparatus is preferably operated at a column bottom temperature of 170°C to 200°C.

[0043] In a preferred embodiment of the process according to the invention, the mass fraction of oligoethylene glycol monomethyl ether in the bottom product stream of step (d) is between 0% and 12% by weight, more preferably between 1% and 9% by weight, which can be achieved by appropriately selecting the molar ratio of oligoethylene glycol monomethyl ether and boric acid fed to the reactor in step (a) and the operating parameters of the reactive distillation apparatus.

[0044] In a further preferred embodiment of the process according to the invention, the mass fraction of water in the bottom product stream of step (d) is less than 0.05% by weight, preferably less than 0.02% by weight. This can be achieved by appropriately selecting the molar ratio of oligoethylene glycol monomethyl ether and boric acid fed to the reactor in step (a) and the operating parameters of the reactive distillation apparatus.

[0045] In a further preferred embodiment of the process according to the invention, the APHA color number (ASTM D1209-05(2019)) of the bottoms product stream is less than 50, preferably less than 20. This can be achieved by appropriate selection of the operating parameters of the reactive distillation apparatus to provide a mild reaction and separation process that avoids long residence times of the borate at high temperatures.

[0046] The process according to the present invention has several advantages over the conventional processes known in the art, mainly due to the division of the reaction into two parts by forming a crude product in a reactor and then completely converting the reactant boric acid in a reactive distillation apparatus. - The mild process conditions minimize thermal stress on the product borate, resulting in a product with less discoloration, avoiding the formation of high boiling impurities and allowing the product borate to be withdrawn as a bottoms product. - Efficient continuous separation of by-product water in the reactive distillation unit allows for very low water content in the final product, resulting in high quality product borate. The process is characterized by low energy consumption due to the low reflux ratio and the withdrawal of the desired product borate as bottom product. - No side draw is required to minimize impurities compared to conventional distillation.

[0047] The invention will be explained in more detail below with reference to the drawings, which should be interpreted as illustrating the principles and do not constitute any limitation of the invention, for example with regard to specific dimensions or design variants. FIG. 1 shows a process flow sheet for producing triethylene glycol methyl ether borate as a first embodiment according to the present invention. EXAMPLES

[0048] [Example 1] FIG. 1 shows a process flow sheet for producing triethylene glycol methyl ether borate as a first embodiment according to the present invention.

[0049] Boric acid and triethylene glycol monomethyl ether were fed to the reactor 10 in a molar ratio of 3.3:1. The reactor 10 was equipped with a heating jacket through which low pressure steam flowed as a heat exchange medium to maintain the temperature of the mixture at 80° C. The reactor 10 was operated at a pressure of 1.2 bar (absolute). After a residence time of 2.5 hours, the reaction mixture contained triethylene glycol methyl ether borate, 8 wt. % water, unreacted boric acid, and unreacted triethylene glycol monomethyl ether. The conversion of boric acid in the reactor was 89%. The crude product was discharged from the reactor 10 to a buffer tank 20.

[0050] The crude product was continuously fed from the buffer tank 20 to the reactive distillation apparatus 30 at a flow rate of 3 t / h. The reactive distillation apparatus was equipped with structured packing. The packing bed of the rectification section 31 was 2 m high, corresponding to 5 theoretical plates. The packing bed of the stripping section 32 was 6 m high, corresponding to 15 theoretical plates. The ratio of the number of theoretical plates of the rectification section 31 to the number of theoretical plates of the stripping section 32 was therefore 1:3. The crude product was fed to the feed stage between the rectification section 31 and the stripping section 32. In the reactive distillation apparatus 30, the unreacted boric acid and triethylene glycol monomethyl ether were reacted until complete conversion of the boric acid.

[0051] The reactive distillation apparatus 30 was equipped with a bottom reboiler 33, a reflux condenser 34 and a further condenser 35. The water-containing distillation stream was transferred from the top of the reactive distillation apparatus 30 to the reflux condenser 34, which was operated with cold water as the cooling medium. The condensed liquid stream was recycled from the reflux condenser 34 to the top of the reactive distillation apparatus 30. The uncondensed vapor phase was transferred from the reflux condenser 34 to the further condenser 35, which was operated with cold water as the cooling medium. The liquid condensate stream withdrawn from the further condenser 35 contained almost pure water with a TOC content of 1700 ppm. Most of the triethylene glycol monomethyl ether was therefore condensed in the reflux condenser 34. The pressure at the top of the reactive distillation apparatus 30 was 15 mbar (absolute). The condensation temperature of the reflux condenser 34 was about 12° C. and that of the further condenser 35 was about 10° C. The pressure drop across the column packing was about 50 mbar.

[0052] A bottom product stream containing triethylene glycol methyl ether borate was withdrawn from the reactive distillation apparatus 30. A portion of the bottom product stream was transferred to a bottom reboiler 33 heated with steam at a pressure of 16 bar. The resulting vapor stream was recycled to the bottom section of the reactive distillation apparatus 30. The temperature of the reactive distillation apparatus sump was 164° C. The bottom product stream mainly contained triethylene glycol methyl ether borate. A further component was unreacted excess triethylene glycol monomethyl ether in an amount of 9 wt. %. The analyzed water content was 0.01 wt. %. The APHA color number was 15.

[0053] [Example 2] As a second embodiment according to the present invention, a method for producing diethylene glycol methyl ether borate was simulated using a steady-state flowsheet simulation tool. The simulation results show that the method for producing diethylene glycol methyl ether borate according to the present invention is feasible.

[0054] This process flow sheet corresponds to that of Figure 1. Boric acid and diethylene glycol monomethyl ether are fed in a molar ratio of 3.1:1 to reactor 10. Reactor 10 is equipped with a heating jacket to maintain the temperature of the mixture at 80°C. The reactants are reacted to chemical equilibrium to form diethylene glycol methyl ether borate as the desired product and water as a by-product. This crude product is fed to reactive distillation unit 30.

[0055] In order to achieve complete conversion of boric acid, the reactive distillation apparatus was equipped with 8 theoretical stages, of which 2 theoretical stages were in the rectification section 31 and 6 theoretical stages in the stripping section 32. The ratio of the number of theoretical stages in the rectification section 31 to the number of theoretical stages in the stripping section 32 was therefore 1:3. In practice, the theoretical stages can easily be realized as trays or packings. The pressure at the top of the reactive distillation apparatus 30 was set at 30 mbar (absolute), which results in a distillate stream temperature of about 23°C.

[0056] The reactive distillation apparatus 30 comprises a bottom reboiler 33, a reflux condenser 34, and a further condenser 35. A distillate stream containing water is transferred from the top of the reactive distillation apparatus 30 to the reflux condenser 34. A condensed liquid stream containing 7 wt. % diethylene glycol monomethyl ether is recycled from the reflux condenser 34 to the top of the reactive distillation apparatus 30. The uncondensed vapor phase is transferred from the reflux condenser 34 to the further condenser 35. The liquid condensate stream withdrawn from the further condenser 35 comprises almost pure water.

[0057] A bottom product stream containing diethylene glycol methyl ether borate is withdrawn from the reactive distillation apparatus 30. A portion of the bottom product stream is transferred to a bottom reboiler 33. The resulting vapor stream is recycled to the bottom section of the reactive distillation apparatus 30. The temperature of the reactive distillation apparatus sump is 150° C. The bottom product stream mainly comprises diethylene glycol methyl ether borate. A further component is unreacted excess diethylene glycol monomethyl ether. The water content is 0.01% by weight. [Explanation of symbols]

[0058] 10. Reactor 20 Buffer Tank 30 Reactive distillation apparatus 31 Rectification Section 32 Stripping Section 33 Reboiler 34 Reflux condenser 35 More Condensers

Claims

1. 1. A method for producing an oligoethylene glycol methyl ether borate, comprising the steps of: a) feeding boric acid and oligoethylene glycol monomethyl ether to a reactor (10) and reacting the resulting mixture to obtain a crude product comprising oligoethylene glycol methyl ether borate, water, and unreacted boric acid and oligoethylene glycol monomethyl ether; b) feeding the crude product to a reactive distillation apparatus (30) to react the boric acid with oligoethylene glycol monomethyl ether until complete conversion of the boric acid; c) transferring a water-containing distillation stream from the top of the reactive distillation unit (30) to a condenser (34) and recycling the condensed liquid stream to the top of the reactive distillation unit (30); d) withdrawing a bottoms product stream containing oligoethylene glycol methyl ether borate from the reactive distillation unit (30), transferring a portion of the bottoms product stream to a reboiler (33), and recycling the resulting vapor stream to the bottoms section of the reactive distillation unit (30); A method comprising:

2. 2. The method of claim 1, wherein the oligoethylene glycol monomethyl ether is triethylene glycol monomethyl ether and the oligoethylene glycol methyl ether borate is triethylene glycol methyl ether borate.

3. 3. The process according to claim 1 or 2, wherein the molar ratio of oligoethylene glycol monomethyl ether to boric acid fed to the reactor (10) in step (a) is from 3.01:1 to 3.4:

1.

4. 4. The process according to any one of claims 1 to 3, wherein the temperature of the reactor (10) is between 60°C and 90°C and the pressure of the reactor (10) is between 1.0 bar (absolute) and 2.0 bar (absolute).

5. 5. The process according to any one of claims 1 to 4, wherein the residence time of the mixture in the reactor (10) of step (a) is between 2 hours and 5 hours.

6. The process according to any one of claims 1 to 5, wherein the conversion of boric acid in the reactor is between 30% and 90%.

7. 7. The process according to any one of claims 1 to 6, wherein the reaction of step (a) is carried out discontinuously and the resulting crude product is buffered in tank (20), and steps (b) to (d) are carried out continuously using a feed stream of the crude product removed from tank (20).

8. 8. The process according to any one of claims 1 to 7, wherein the mass fraction of oligoethylene glycol monomethyl ether in the bottom product stream of step (d) is from 0 wt.% to 12 wt.%.

9. 9. The process according to any one of claims 1 to 8, wherein the mass fraction of water in the bottom product stream of step (d) is less than 0.05 wt.%.

10. 10. The process of any one of claims 1 to 9, wherein the bottoms product stream has an APHA color number of less than 50.

11. 11. The process according to any one of claims 1 to 10, wherein the reactive distillation unit (30) is operated at a top pressure of between 10 mbar (absolute) and 30 mbar (absolute) and a bottom temperature of between 150°C and 200°C.

12. 12. The process according to any one of claims 1 to 11, wherein the reflux ratio between the recycled condensed liquid stream and the distillate stream withdrawn from the top of the reactive distillation unit (30) is from 0.2 to 0.5 by weight.

13. 13. The process according to claim 1, wherein the ratio of the number of theoretical stages of the rectification section (31) to the number of theoretical stages of the stripping section (32) of the reactive distillation apparatus (30) is from 1:2 to 1:

4.

14. 14. The process according to claim 1, wherein the number of theoretical stages in the stripping section (32) of the reactive distillation apparatus is between 2 and 15.

15. 15. The method according to any one of the preceding claims, wherein in step (c), the uncondensed vapour stream is transferred from the condenser (34) to a further condenser (35) from which water is withdrawn as liquid condensate.

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