Process for the preparation of oligomeric ethylene glycol methyl ether borate esters

Abstract

The invention relates to a process for the preparation of oligomeric ethylene glycol methyl ether borate comprising the steps of: a) feeding boric acid and oligomeric ethylene glycol monomethyl ether into a reactor (10) and allowing the resulting mixture to react to obtain a crude product comprising oligomeric ethylene glycol methyl ether borate, water and unreacted boric acid and oligomeric ethylene glycol monomethyl ether; b) feeding the crude product into a reactive distillation apparatus (30) and allowing the boric acid to react with the oligomeric ethylene glycol monomethyl ether to complete conversion of the boric acid; c) transferring a distillate stream containing water from the top of the reactive distillation apparatus (30) to a condenser (34) and recirculating the condensed liquid stream to the top of the reactive distillation apparatus (30); and d) withdrawing a bottom product stream containing oligomeric ethylene glycol methyl ether borate from the reactive distillation apparatus (30), transferring a portion of the bottom product stream to a reboiler (33) and recirculating the resulting vapor stream to the bottom zone of the reactive distillation apparatus (30).

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for the preparation of oligomeric glycol methyl ether borate esters by reacting boric acid and oligomeric glycol monomethyl ether in a reactive distillation apparatus. BACKGROUND

[0002] Due to its excellent properties, triethylene glycol methyl ether borate ester is a main component of brake fluids, which have very good thermal stability and low viscosity. The borate ester is produced in an esterification reaction of boric acid with glycol ethers. Typically, the reaction mixture comprises the product triethylene glycol methyl ether borate ester, the by-product water, the reactant triethylene glycol monomethyl ether, as well as heavy boiling point components and metal impurities. In conventional production processes, the reaction mixture is separated by distillation using a series of distillation columns or a single distillation column with multiple side draws.

[0003] Document CN 104447829 A discloses a process for the preparation of triethylene glycol methyl ether borate ester in a single distillation column. The desired product is recovered by a side draw from the stripping section of the column. With this column design, a very pure triethylene glycol methyl ether borate ester can be produced, since impurities and heavy components are separated as column bottoms. Triethylene glycol monomethyl ether is discharged from a side draw in the rectifying section, while water is obtained as distillate. However, to achieve the desired product specifications, a large number of theoretical stages is required in the distillation column. Furthermore, a high reflux ratio is required, leading to a high energy demand of the process. The energy demand is further increased not only due to the evaporation of unconverted triethylene glycol monomethyl ether, but also due to the evaporation of the total borate ester product stream. To counteract the high energy demand, the process is heat integrated in several stages. The recovered triethylene glycol monomethyl ether is used to preheat the reaction mixture. Furthermore, the reaction mixture is preheated by the withdrawn triethylene glycol methyl ether borate ester. Subsequently, this triethylene glycol methyl ether borate ester is cooled by the condensed water recovered at the top of the column. Although the borate ester can be obtained in high purity, this process is characterized by a high energy demand, high capital costs, and a complex process behavior due to the challenging process control strategy required for the heat integration.

[0004] Document EP 1661901 A1 discloses a process for the production of borate esters by reacting boric acid or diboron trioxide with alcohols in a reactive distillation column, wherein the distillation is at least partially repeated by extraction within the column. The specific alcohols disclosed are methanol, ethanol and propanol. The respective borate esters as products of the process form low boiling azeotropes with the alcohols, which requires the design of a reactive distillation column with repeated extraction and an extractant as further substance. The borate ester is discharged from the top of the column, and the extractant is discharged from the bottom of the column. SUMMARY

[0005] It is an object of the present invention to provide a process for the production of oligomeric ethylene glycol methyl ether borate which has the features of low overall energy demand, low capital cost and a smooth operating envelope.

[0006] The first subject matter of the present invention is a process for the production of oligomeric ethylene glycol methyl ether borate comprising the following steps:

[0007] a) feeding boric acid and oligomeric ethylene glycol monomethyl ether into a reactor and allowing the resulting mixture to react to obtain a crude product comprising oligomeric ethylene glycol methyl ether borate, water and unreacted boric acid and oligomeric ethylene glycol monomethyl ether;

[0008] b) feeding the crude product into a reactive distillation apparatus and allowing boric acid to react with oligomeric ethylene glycol monomethyl ether until boric acid is completely converted;

[0009] c) transferring a distillate containing water from the top of the reactive distillation apparatus to a condenser and recirculating the condensed liquid stream to the top of the reactive distillation apparatus; and

[0010] d) withdrawing a bottom product stream containing oligomeric ethylene glycol methyl ether borate from the reactive distillation apparatus, transferring a portion of the bottom product stream to a reboiler and recirculating the resulting vapor stream to the bottom zone of the reactive distillation apparatus.

[0011] Within the scope of the present invention, the term "oligomeric" refers to "di", "tri" and "tetra". Thus, oligomeric ethylene glycol monomethyl ether can be diethylene glycol monomethyl ether, triethylene glycol monomethyl ether or tetraethylene glycol monomethyl ether. Correspondingly, oligomeric ethylene glycol methyl ether borate, hereinafter also referred to as "borate", can be diethylene glycol methyl ether borate, triethylene glycol methyl ether borate or tetraethylene glycol methyl ether borate.

[0012] In a first embodiment of the first subject matter of the present invention, the oligomeric ethylene glycol monomethyl ether is diethylene glycol monomethyl ether and the oligomeric ethylene glycol methyl ether borate is diethylene glycol methyl ether borate. The process for the production of diethylene glycol methyl ether borate comprises the following steps:

[0013] a) feeding boric acid and diethylene glycol monomethyl ether into a reactor and allowing the resulting mixture to react to obtain a crude product comprising diethylene glycol methyl ether borate, water and unreacted boric acid and diethylene glycol monomethyl ether;

[0014] b) feeding the crude product into a reactive distillation apparatus and allowing boric acid to react with diethylene glycol monomethyl ether until boric acid is completely converted;

[0015] c) transferring a distillate containing water from the top of the reactive distillation apparatus to a condenser and recirculating the condensed liquid stream to the top of the reactive distillation apparatus; and

[0016] d) withdrawing a bottoms stream containing diethylene glycol methyl ether borate from the reaction rectification apparatus, transferring a portion of the bottoms stream to a reboiler and recycling the resulting vapor stream to a bottom zone of the reaction rectification apparatus.

[0017] In a second embodiment of the first subject matter of the present application, 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 of making triethylene glycol methyl ether borate comprises the following steps:

[0018] 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;

[0019] b) feeding the crude product to a reaction rectification apparatus and reacting the boric acid with the triethylene glycol monomethyl ether to complete conversion of the boric acid;

[0020] c) transferring a distillate containing water from a top of the reaction rectification apparatus to a condenser and recycling the condensed liquid stream to the top of the reaction rectification apparatus; and

[0021] d) withdrawing a bottoms stream containing triethylene glycol methyl ether borate from the reaction rectification apparatus, transferring a portion of the bottoms stream to a reboiler and recycling the resulting vapor stream to a bottom zone of the reaction rectification apparatus.

[0022] In a third embodiment of the first subject matter of the present application, 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 of making tetraethylene glycol methyl ether borate comprises the following steps:

[0023] 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;

[0024] b) feeding the crude product to a reaction rectification apparatus and reacting the boric acid with the tetraethylene glycol monomethyl ether to complete conversion of the boric acid;

[0025] c) transferring a distillate containing water from a top of the reaction rectification apparatus to a condenser and recycling the condensed liquid stream to the top of the reaction rectification apparatus; and

[0026] d) withdrawing a bottoms stream containing tetraethylene glycol methyl ether borate from the reaction rectification apparatus, transferring a portion of the bottoms stream to a reboiler and recycling the resulting vapor stream to a bottom zone of the reaction rectification apparatus. DETAILED DESCRIPTION

[0027] In step (a), the reactants boric acid and oligomeric ethylene glycol monomethyl ether are fed to the reactor. Preferably, the solid boric acid is completely dissolved in and mixed with the liquid oligomeric ethylene glycol monomethyl ether. The mixing and dissolving can be performed completely in the reactor, for example by providing a corresponding mixing device such as a stirrer. The mixing and dissolving can also be performed partially or completely in a device outside the reactor, for example a premixer such as a mixing pump.

[0028] In the esterification reaction, the target product oligomeric ethylene 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 ester. According to the present application, however, only a part of the reaction is performed in the reactor of step (a), while the remaining part of the reaction to complete conversion of the boric acid is performed in the reactive distillation device of step (b). Thus, the reaction scheme in the reactor and the reactive distillation device gives freedom to the process according to the present application.

[0029] In a preferred embodiment, the reaction in the reactor is performed towards equilibrium conditions, which are created by the molar ratio of the reactants fed to the reactor.

[0030] In a preferred embodiment of the process according to the present application, a low excess of alcohol is provided in the reactor. In step (a), the molar ratio of oligomeric ethylene glycol monomethyl ether to boric acid fed to the reactor is preferably 3.01 : 1 to 3.4 : 1, more preferably 3.25 : 1 to 3.35 : 1. One advantage of the alcohol excess is that the chemical equilibrium is shifted towards the desired borate ester, which can be used to produce more borate ester in the reactor to be fed to the reactive distillation device. Another advantage is that the excess alcohol leads to a lower bottom temperature of the reactive distillation column, since the alcohol has a lower boiling point than the borate ester product and the unreacted alcohol is contained in the bottom product stream.

[0031] The temperature in the reactor is preferably 60 °C to 90 °C, more preferably 75 °C to 85 °C. The reactor can be equipped with a heating jacket, an inner coil or a heat exchanger, for example in an external circuit, to maintain the temperature in the desired range.

[0032] The esterification reaction in the reactor is preferably performed at ambient pressure or slightly elevated pressure. The pressure in the reactor is preferably 1.0 bar (absolute) to 2.0 bar (absolute), preferably 1.1 bar (absolute) to 1.3 bar (absolute).

[0033] It is further preferred that the residence time of the mixture in the reactor is 2 hours to 5 hours.

[0034] Preferably, the above parameters of the esterification reaction in the reactor are chosen such that the conversion of boric acid in the reactor is between 30% and 90%. The preferred conversion range results in a preferred range of water content of the crude product at the end of the residence time, depending on the oligomeric ethylene glycol monomethyl ether present in the reactor.

[0035] In case the oligomeric ethylene glycol monomethyl ether is diethylene glycol monomethyl ether and the oligomeric ethylene 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 between 3.5 wt.% and 11.5 wt.%.

[0036] In case the oligomeric ethylene glycol monomethyl ether is triethylene glycol monomethyl ether and the oligomeric ethylene 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 between 2.6 wt.% and 8.8 wt.%.

[0037] In case the oligomeric ethylene glycol monomethyl ether is tetraethylene glycol monomethyl ether and the oligomeric ethylene 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 between 2.1 wt.% and 7.1 wt.%.

[0038] After the reaction in the reactor of step (a) is completed, the crude product is fed to a reactive distillation device, whereby boric acid is reacted with oligomeric ethylene glycol monomethyl ether to convert the boric acid completely. The content of the reactor can be fed directly to the reactive distillation device. Preferably, the content of the reactor is discharged to a buffer tank and fed from the buffer tank to the reactive distillation device. In this case, the reactor can be used further immediately after production of the crude product. The separation and purification of the crude product and the new reaction to form another batch of crude product can be carried out simultaneously, which saves time.

[0039] In a preferred embodiment of the process according to the application, 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, the resulting crude product is buffered in a tank, and steps (b) to (d) are carried out continuously using a feed stream of the crude product taken from the tank.

[0040] According to the application, the reactive distillation device comprises at least one condenser and one reboiler, and the configuration is such that a distillate containing water is transferred from the top of the reactive distillation device to the condenser and the condensed liquid stream is recirculated to the top of the reactive distillation device, a bottom product stream containing oligomeric ethylene glycol methyl ether borate is discharged from the reactive distillation device, a part of the bottom product stream is transferred to the reboiler and the resulting vapor stream is recirculated to the bottom zone of the reactive distillation device.

[0041] In the reaction rectification apparatus, unreacted boric acid reacts with unreacted oligomeric ethylene glycol monomethyl ether to form the desired borate ester product and water as a by-product. Due to the combination of reaction and distillation, the by-product water is continuously removed from the reaction mixture. This shifts the reaction equilibrium towards the desired borate ester product and allows for the complete conversion of the reactant boric acid. Water is obtained at the top of the reaction rectification apparatus, while the borate ester product is obtained at its bottom.

[0042] The reaction rectification apparatus is equipped with internals that facilitate the mass exchange between the liquid phase and the gas phase. The internals can be, for example, trays, random packing or structured packing. Preferably, structured packing is used as internals.

[0043] In a preferred embodiment, the rectifying section between the feed location and the top of the reaction rectification apparatus is equipped with internals corresponding to one to three theoretical stages. This ensures proper separation of the ether and water.

[0044] In another preferred embodiment, the stripping section between the feed location and the bottom of the reaction rectification apparatus is equipped with internals corresponding to 2 to 15 theoretical stages, more preferably 5 to 10 theoretical stages. Preferably, the number of theoretical stages is chosen to ensure sufficient residence time for the complete conversion of the reactant boric acid.

[0045] The ratio of the number of theoretical stages of the rectifying section to the number of theoretical stages of the stripping section of the reaction rectification apparatus is preferably 1 :2 to 1 :4, more preferably 1 :2.5 to 1 :3.5. A ratio within the preferred range shows that both the complete conversion of the reaction in the stripping section and the removal of the by-product water in the rectifying section ensure sufficient residence time.

[0046] According to the present application, the distillate, which mainly contains water, is transferred from the top of the reaction rectification apparatus to a condenser (step (c)). The distillate is at least partially condensed, and the condensed liquid stream is recycled to the top of the reaction rectification apparatus as a reflux stream. In a preferred embodiment of the process according to the present application, the reflux ratio, i.e. the mass ratio of the recycled condensed liquid stream to the distillate withdrawn from the top of the reaction rectification apparatus, is 0.2 to 0.5 by mass. This low reflux ratio leads to a significantly reduced energy consumption compared to processes known in the art, without compromising the quality of the desired borate ester product.

[0047] In a preferred variant of this embodiment, the reaction rectification apparatus comprises an additional condenser. In a first condenser (hereinafter denoted "reflux condenser"), the distillate from the top of the reaction rectification apparatus is partially condensed. The condensed liquid stream is rich in organic compounds and is recycled to the top of the reaction rectification apparatus as a reflux stream. The non-condensed vapor stream is rich in water and is transferred to another condenser. In this additional condenser, the vapor stream rich in water is at least partially condensed, so that water can be withdrawn as a liquid condensate. This two-stage condenser concept allows to minimize alcohol (ether) losses and to minimize the TOC (total organic carbon) content in the waste water withdrawn from the process.

[0048] According to the present application, a bottom product stream, which is mainly composed of oligomeric ethylene glycol methyl ether borate, is withdrawn from the reaction rectification apparatus (step (d)). Part of the bottom product stream is transferred to a reboiler, vaporized and recycled to the bottom zone of the reaction rectification apparatus. The bottom product can contain unconverted oligomeric ethylene glycol monomethyl ether.

[0049] Preferably, the bottom of the reaction rectification apparatus is designed for a minimum residence time of the bottom product, for example by minimizing the holdup of the bottom tank.

[0050] In a preferred embodiment, the reaction rectification apparatus 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 reaction rectification apparatus leads to a low temperature of the borate product stream obtained from the bottom of the reaction rectification apparatus, which has a positive influence on the color quality of the final product.

[0051] In another preferred embodiment, the reaction rectification apparatus is operated at a top temperature of 7 °C to 25 °C, more preferably 10 °C to 15 °C.

[0052] In another preferred embodiment, the reaction rectification apparatus is operated at a bottom temperature of 150 °C to 200 °C. Depending on the oligomeric ethylene glycol methyl ether borate to be prepared, the preferred range is as follows:

[0053] In the process for preparing diethylene glycol methyl ether borate, the reaction rectification apparatus is preferably operated at a bottom temperature of 150 °C to 180 °C.

[0054] In the process for preparing triethylene glycol methyl ether borate, the reaction rectification apparatus is preferably operated at a bottom temperature of 160 °C to 190 °C, more preferably 170 °C to 190 °C.

[0055] In the process for preparing tetraethylene glycol methyl ether borate, the reaction rectification apparatus is preferably operated at a bottom temperature of 170 °C to 200 °C.

[0056] In a preferred embodiment of the process according to the application, the mass fraction of oligomeric ethylene glycol monomethyl ether in the bottom product stream in step (d) is from 0 to 12 wt.-%, more preferably from 1 to 9 wt.-%. This can be achieved by appropriate selection of the molar ratio of oligomeric ethylene glycol monomethyl ether to boric acid fed to the reactor in step (a) and the operating parameters of the reactive rectification apparatus.

[0057] In another preferred embodiment of the process according to the application, the mass fraction of water in the bottom product stream in step (d) is below 0.05 wt.-%, preferably below 0.02 wt.-%. This can be achieved by appropriate selection of the molar ratio of oligomeric ethylene glycol monomethyl ether to boric acid fed to the reactor in step (a) and the operating parameters of the reactive rectification apparatus.

[0058] In another preferred embodiment of the process according to the application, the APHA color number (ASTM D1209-05 (2019)) of the bottom product stream is below 50, preferably below 20. This can be achieved by appropriate selection of the operating parameters of the reactive rectification apparatus, resulting in a mild reaction and separation process, avoiding high residence times of the borate ester at high temperatures.

[0059] Compared to conventional processes known in the art, the process according to the application has several advantages mainly due to the separation of the reaction into two parts by forming the crude product in the reactor and the complete conversion of the reactant boric acid in the subsequent reactive rectification apparatus:

[0060] - reasonable process conditions minimize thermal stress of the borate ester product and result in a product with low coloration. Furthermore, the formation of high-boiling impurities is avoided, which leads to the borate ester product being discharged as a bottom product.

[0061] - very low water content in the final product is achieved by the efficient continuous separation of the by-product water in the reactive rectification apparatus, resulting in a high-quality borate ester product.

[0062] - the process is characterized by low energy consumption due to the low reflux ratio and the discharge of the desired borate ester product as a bottom product.

[0063] - no side draw is required to minimize impurities compared to conventional distillation processes. BRIEF DESCRIPTION OF DRAWINGS

[0064] The application is explained in more detail below with reference to the drawings. The drawings are to be interpreted as schematic representations. They do not constitute any limitation on the application, for example with regard to specific dimensions or design variants. In the drawings:

[0065] Figure 1 shows a flow scheme as a process for the preparation of triethylene glycol monomethyl ether borate ester according to a first embodiment of the application.

[0066] List of used reference signs:

[0067] 10... reactor

[0068] 20... buffer tank

[0069] 30... reactive distillation apparatus

[0070] 31... rectifying section

[0071] 32... stripping section

[0072] 33... reboiler

[0073] 34... reflux condenser

[0074] 35... additional condenser

[0075] Example 1

[0076] Figure 1 A flow diagram of a process for the preparation of triethylene glycol methyl ether borate as a first embodiment according to the present application is shown.

[0077] 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 was flowing as heat exchange medium to keep the temperature of the mixture at 80°C. The reactor 10 was operated at a pressure of 1.2 bar (absolute pressure). After a residence time of 2.5 hours the reaction mixture contained triethylene glycol methyl ether borate, 8 wt.% of 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 the buffer tank 20.

[0078] The crude product was continuously fed from the buffer tank 20 to the reactive distillation apparatus 30 at a flow rate of 3 tons per hour. The reactive distillation apparatus was equipped with structured packing. The height of the packing bed in the rectifying section 31 was 2 m, corresponding to 5 theoretical stages. The height of the packing bed in the stripping section 32 was 6 m, corresponding to 15 theoretical stages. Thus, the ratio of the number of theoretical stages of the rectifying section 31 to the number of theoretical stages of the stripping section 32 was 1 :3. The crude product was fed to a feed position between the rectifying section 31 and the stripping section 32. In the reactive distillation apparatus 30 the unreacted boric acid and triethylene glycol monomethyl ether were reacted to complete conversion of the boric acid.

[0079] The reaction rectifier 30 is equipped with a bottom reboiler 33, a reflux condenser 34 and an additional condenser 35. The distillate containing water is transferred from the top of the reaction rectifier 30 to the reflux condenser 34, which is operated with cold water as cooling medium. The condensed liquid stream is recycled from the reflux condenser 34 to the top of the reaction rectifier 30. The gaseous phase, which is not condensed, is transferred from the reflux condenser 34 to the additional condenser 35, which is operated with cold water as cooling medium. The liquid condensate withdrawn from the additional condenser 35 contains almost pure water with a TOC content of 1700 ppm. Thus, most of the triethylene glycol monomethyl ether has been condensed in the reflux condenser 34. The pressure at the top of the reaction rectifier 30 is 15 mbar (absolute pressure). The condensation temperature of the reflux condenser 34 is about 12°C and the condensation temperature of the additional condenser 35 is about 10°C. The pressure drop through the column packing is about 50 mbar.

[0080] A bottom product stream containing triethylene glycol methyl ether borate is withdrawn from the reaction rectifier 30. A part of the bottom product stream is transferred to the bottom reboiler 33, which is heated by water vapor at a pressure of 16 bar. The resulting vapor stream is recycled to the bottom zone of the reaction rectifier 30. The temperature in the tank of the reaction rectifier is 164°C. The bottom product stream contains mainly triethylene glycol methyl ether borate. The other components are 9 wt.% of unreacted excess triethylene glycol monomethyl ether. The water content analyzed is 0.01 wt.%. The APHA color number is 15.

[0081] Example 2

[0082] A steady-state flow sheet simulation tool has been used to simulate a process for the preparation of diethylene glycol methyl ether borate as a second embodiment according to the present application. The simulation results show that the process for the preparation of diethylene glycol methyl ether borate according to the present application is feasible.

[0083] The process flow diagram corresponds to the process flow diagram in Figure 1 The boronic acid and diethylene glycol monomethyl ether are fed to the reactor 10 in a molar ratio of 3.1 : 1. The reactor 10 is equipped with a heating jacket to keep the temperature of the mixture at 80°C. The reactants react up to the chemical equilibrium, forming diethylene glycol methyl ether borate as the desired product and water as a by-product. The crude product is fed to the reaction rectifier 30.

[0084] To achieve complete conversion of boric acid, the reaction rectification apparatus is equipped with eight theoretical stages, of which two theoretical stages are in the rectifying section 31 and six theoretical stages are in the stripping section 32. Thus, the ratio of the number of theoretical stages of the rectifying section 31 to the number of theoretical stages of the stripping section 32 is 1 :3. In practice, the theoretical stages can easily be realized as trays or packing. The pressure at the top of the reaction rectification apparatus 30 is set to 30 mbar (absolute), which results in a temperature of the distillate stream of about 23 °C.

[0085] The reaction rectification apparatus 30 is equipped with a bottom reboiler 33, a reflux condenser 34 and an additional condenser 35. The distillate stream containing water is transferred from the top of the reaction rectification apparatus 30 to the reflux condenser 34. The condensate stream containing 7 wt% diethylene glycol monomethyl ether is recycled from the reflux condenser 34 to the top of the reaction rectification apparatus 30. The non-condensed gas phase is transferred from the reflux condenser 34 to the additional condenser 35. The liquid condensate stream discharged from the additional condenser 35 comprises almost pure water.

[0086] The bottom product stream containing diethylene glycol methyl ether borate is discharged from the reaction rectification apparatus 30. A part of the bottom product stream is transferred to the bottom reboiler 33. The resulting vapor stream is recycled to the bottom zone of the reaction rectification apparatus 30. The temperature in the sump of the reaction rectification apparatus is 150 °C. The bottom product stream mainly comprises diethylene glycol methyl ether borate. Other components are unreacted excess diethylene glycol monomethyl ether. The water content is 0.01 wt%.

Claims

1. A method for preparing oligoethylene glycol methyl ether borate, comprising the following steps: a) feeding boric acid and oligoethylene glycol monomethyl ether into 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 into a reactive distillation unit (30) and reacting boric acid with oligoethylene glycol monomethyl ether until the boric acid is completely converted; c) transferring the distillate containing water from the top of the reactive distillation device (30) to a condenser (34), and recycling the condensed liquid stream to the top of the reactive distillation device (30); and d) discharging a bottom product stream containing oligoethylene glycol methyl ether borate from the reactive distillation unit (30), transferring part of the bottom product stream to a reboiler (33) and recycling the resulting vapor stream to the bottom zone of the reactive distillation unit (30), wherein the temperature in the reactor (10) is from 60°C to 90°C, and the absolute pressure in the reactor (10) is from 1.0 bar to 2.0 bar, wherein the reactive distillation unit (30) is operated at a top absolute pressure of 10 mbar to 30 mbar and a bottom temperature of 150° C. to 200° C., The oligoethylene glycol monomethyl ether is diethylene glycol monomethyl ether, triethylene glycol monomethyl ether or tetraethylene glycol monomethyl ether, and the oligoethylene glycol methyl ether borate is the corresponding diethylene glycol methyl ether borate, triethylene glycol methyl ether borate or tetraethylene glycol methyl ether borate. 2 . The method according to 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. 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 3.01:1 to 3.4:

1.

4. The process according to claim 3, wherein the molar ratio of oligoethylene glycol monomethyl ether to boric acid fed to the reactor (10) in step (a) is 3.25:1 to 3.35:

1.

5. The process according to claim 1 or 2, wherein the temperature in the reactor (10) is from 75°C to 85°C, and the absolute pressure in the reactor (10) is from 1.1 bar to 1.3 bar.

6. The process according to claim 1 or 2, wherein the residence time of the mixture in the reactor (10) in step (a) is 2 to 5 hours.

7. The method according to claim 1 or 2, wherein the conversion rate of boric acid in the reactor is 30% to 90%.

8. The process according to claim 1 or 2, wherein in step (a) the reaction is carried out discontinuously, the resulting crude product is buffered in a tank (20), and steps (b) to (d) are carried out continuously using a feed stream of the crude product taken out of the tank (20).

9. The process according to claim 1 or 2, wherein the mass fraction of oligoethylene glycol monomethyl ether in the bottom product stream in step (d) is 0% by weight to 12% by weight.

10. The method according to claim 9, wherein the mass fraction of oligoethylene glycol monomethyl ether in the bottom product stream in step (d) is 1% by weight to 9% by weight.

11. The process according to claim 1 or 2, wherein the mass fraction of water in the bottom product stream in step (d) is less than 0.05 wt%.

12. The process according to claim 11, wherein the mass fraction of water in the bottom product stream in step (d) is less than 0.02% by weight.

13. The process according to claim 1 or 2, wherein the APHA color number of the bottom product stream is less than 50.

14. The process of claim 13, wherein the bottoms stream has an APHA color number of less than 20.

15. The process according to claim 1 or 2, wherein the reflux ratio of the recycled condensed liquid stream to the distillate withdrawn from the top of the reactive distillation unit (30) is 0.2 to 0.5 by mass.

16. The method according to claim 1 or 2, wherein the ratio of the theoretical number of stages of the rectifying section (31) to the theoretical number of stages of the stripping section (32) of the reactive distillation device (30) is 1:2 to 1:

4.

17. The method according to claim 16, wherein the ratio of the theoretical number of stages of the rectifying section (31) to the theoretical number of stages of the stripping section (32) of the reactive distillation device (30) is 1:2.5 to 1:3.

5.

18. The method according to claim 1 or 2, wherein the theoretical number of stages of the stripping section (32) of the reactive distillation apparatus is 2 to 15.

19. The method according to claim 18, wherein the theoretical number of stages of the stripping section (32) of the reactive distillation apparatus is 5 to 10.

20. The method according to claim 1 or 2, wherein in step (c) the uncondensed steam stream is transferred from the condenser (34) to an additional condenser (35) in which water is discharged as liquid condensate.

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