RU2807294C1 - Method of obtaining methyl lactate - Google Patents

Abstract

FIELD: organic synthesis.

SUBSTANCE: invention relates to the production of lactic acid ester - methyl lactate, which can be used as a raw material for the synthesis of lactide and as a “green” solvent in the production of varnishes and additives, as well as a plasticizer in the production of polymer materials. The method of producing methyl lactate involves the continuous esterification of lactic acid with methanol in a flow-type reactor with a fixed bed of a heterogeneous catalyst and purification of methyl lactate from the reaction mass in two distillation columns. A mixture of an aqueous solution of lactic acid with methanol in a molar ratio of 1:15 is fed into a mixer, where it is mixed with argon at a flow rate of 0.3 ml/min. The resulting gas-liquid mixture is sent to a heat exchanger with the temperature maintained at 180°C for evaporation of lactic acid and methanol. The heated vapors of the reaction mass are sent to a flow-type reactor with a fixed layer of silica gel at a temperature of 100–180°C. The resulting vapor products are condensed in a direct refrigerator, and the condensate is collected in a container. In the reactor, heat exchanger, direct cooler and the container, a pressure of 30–160 mbar is maintained, and from the container the reaction products are sequentially separated and purified in two distillation columns, and in the first distillation column high-boiling compounds and lactic acid are separated, in the second distillation column the mixture is purified, and the finished product is obtained.

EFFECT: creation of a method for gas-phase synthesis of methyl lactate from a commercial solution of lactic acid, which makes it possible to reduce the duration of the process.

1 cl, 1 dwg, 6 ex

Description

The invention relates to organic synthesis, namely to the production of lactic acid ester - methyl lactate, which can be used as a raw material for the synthesis of lactide and as a “green” solvent in the production of varnishes and additives, as well as a plasticizer in the production of polymer materials.

A known method for producing lactic acid ester [EP 2321251 B1, IPC (2006.01) C07C67/08 C07C67/54, C07C69/68 , publ. 02/29/2012], including the esterification of lactic acid in a capacitive reactor without a catalyst at a temperature of 110 - 200 ° C and a pressure of 15-300 kPa, continuous selection of the reaction mass and its supply to two successive distillation columns for the purification of lactic acid ester.

The selectivity for the formation of methyl lactate is 99.09% after 24 hours of synthesis, and the yield is more than 80%.

The disadvantage of this method is the duration of the process and the process without a catalyst.

There is a known method for the continuous esterification of alcohol with lactic acid, catalyzed by a solid acid [CN 109942358 A, IPC (2006.01) C07B41/12, C07C67/08, C07C67/48, C07C69/00 , publ. 06.28.2019], including the esterification of lactic acid with methanol at a ratio of 2:1 – 8:1 in a distillation column, the temperature is 65-120°C. Esterification occurs on a solid acid catalyst. The reaction products are separated in a distillation column.

There is a known method for the continuous esterification of alcohol with lactic acid [CN 111943849 A, IPC (2006.01) C07C67/08, C07C67/54, C07C69 / 68 , publ. 11/17/2020] including the esterification of lactic acid with methanol at a ratio of 2 - 4:1 in a distillation column, the pressure in the column is 40-101 kPa. The reaction products are separated in a distillation column.

The yield of methyl lactate in these methods due to the recycling of lactic acid is 98%.

The disadvantages of methods for producing lactic acid esters in distillation columns are: the duration of the process, the dependence of the ether yield on the reflux ratio, the frequency of the process, metal consumption and energy consumption.

A known method for the continuous production of methyl lactate [WO 2008098581 A1, IPC (2006.01) C07C67/08, C07C69/68 , publ. 08/21/2008], which includes supplying a mixture of lactic acid, oligomers and polymers, its alkyl esters, methanol and a homogeneous catalyst (tetra-n-butoxytitanium) into a vessel, from which the mixture is then circulated through a jet reactor at a temperature of 170°C for 7 o'clock. Then the vaporous ether is condensed and purified by rectification. The yield of methyl lactate is 99.8%.

The disadvantages of this method of producing methyl lactate in a jet reactor are the use of a mixture of lactic acid, its oligomers and polymers as a raw material, the periodicity of this process, the metal consumption, the long duration of synthesis, as well as the presence of a highly flammable catalyst, the flash point of which in an open crucible is 78° C.

Known, adopted as a prototype, is a method for the production of methyl lactate [CN 113603585 B, IPC (2006.01) C07C67/08, C07C67/54, C07C69/68 , publ. 11.22.2022], which includes preliminary concentration of lactic acid in a distillation column to remove water from the lactic acid solution, resulting in a lactic acid content of 85% and a polylactic acid content of 13%. Next, the resulting mixture of lactic acid and polylactic acid is mixed with methanol in a ratio of 1:4 and fed into a reactor with a fixed bed of catalyst. An ion exchange resin is used as a catalyst. The reaction temperature is 70°C, and the residence time of the mixture in the reactor is 30 minutes. After the reactor, the mixture is separated in five successive distillation columns from lactic acid, methanol, water, impurities and remaining moisture. Lactic acid is returned to the distillation column for concentration, and methanol is returned to the reactor. As a result, methyl lactate is obtained with a purity of 99% with a yield of 99.91%.

The disadvantages of this method for producing methyl lactate are: the need to pre-concentrate the lactic acid solution, a long residence time of the reaction mass in the reactor, and a large number of steps for isolating and purifying methyl lactate.

The technical result of the proposed invention is the creation of a method for the gas-phase synthesis of methyl lactate from a commercial lactic acid solution, which makes it possible to reduce the duration of the process and eliminate the stage of preliminary concentration of the lactic acid solution.

The proposed method for producing methyl lactate, as in the prototype, includes continuous esterification of lactic acid with methanol in a flow-type reactor with a fixed bed of a heterogeneous catalyst and purification of methyl lactate from the reaction mass in two distillation columns.

According to the invention, a mixture of an aqueous solution of lactic acid with methanol in a molar ratio of 1:15 is fed into a mixer, where it is mixed with argon at a flow rate of 0.3 ml/min. The resulting gas-liquid mixture is sent to a heat exchanger, which is maintained at 180°C to evaporate lactic acid and methanol. The heated vapors of the reaction mass are directed into a flow-type reactor with a fixed layer of silica gel at a temperature of 100-180°C. The resulting vapor products are condensed in a direct refrigerator, and the condensate is collected in a container. In the reactor, heat exchanger, direct cooler and in the container, a pressure of 30 - 160 mbar is maintained, and from the container the reaction products are sequentially separated and purified in two distillation columns, and in the first distillation column high-boiling compounds and lactic acid are separated, in the second distillation column the mixture is purified, receiving the finished product.

Carrying out the reaction in the gas phase makes it possible to reduce the residence time of the reaction mass in the methyl lactate reactor to 0.11-0.64 s. The process is also simplified by reducing the number of purification steps.

Thus, in comparison with the prototype, carrying out the reaction in the gas phase makes it possible to reduce the residence time in the reactor, eliminate the stage of preliminary concentration of lactic acid, and also makes it possible to achieve high conversion of lactic acid in the gas phase process using a catalyst that is safe for the health of personnel and the environment.

Figure 1 shows a technological diagram of the installation for gas-phase synthesis of methyl lactate.

Example 1

A mixture was prepared consisting of 4 g of an aqueous solution of lactic acid with a concentration of 80% and 17.1 g of methanol (in a molar ratio of 1:15). The resulting solution was placed in container 1, from where, using a peristaltic pump 3, the solution was fed into mixer 2, where it was mixed with argon, which was supplied from a cylinder at a flow rate of 0.3 ml/min. The resulting gas-liquid mixture was fed into coil heat exchanger 4, where the reaction mass transitioned from liquid to gas phase at a temperature of 180°C and the reaction mass vapor was heated at a temperature of 180°C. Next, the vapors of the reaction mass entered flow-type reactor 5 with a fixed bed of catalyst. The temperature in the catalytic layer of reactor 5 was maintained at 140°C. The temperature of the catalytic layer was monitored with a thermocouple placed in it. A heterogeneous silicon-containing catalyst, silica gel, weighing 15 g was used as a catalyst. From reactor 5, vapors consisting of products and initial reagents entered direct refrigerator 6, where condensation of the vapors occurred. The resulting condensate was collected in a collection container 7. Moreover, a pressure of 130 mbar was maintained in the reactor 5, heat exchanger 4, direct cooler 6 and in the collection container 7. Next, methyl lactate was isolated and purified in distillation columns 8 and 9 at a temperature of 60°C and a pressure of 300-30 mbar. In distillation column 8, the reaction mixture was purified from high-boiling by-products and lactic acid, and in distillation column 9, methyl lactate was isolated. As a result, the purity of methyl lactate was 99.98%.

The selectivity for the formation of methyl lactate was 84%, the yield was 44%, and the residence time in the catalytic zone of reactor 5 was 0.52 s.

Example 2

The production of methyl lactate was carried out similarly to example 1, but the pressure in heat exchanger 4, reactor 5, direct cooler 6 and in collecting tank 7 was maintained at 30 mbar. As a result, the selectivity for the formation of methyl lactate was 22% and the yield was 14%, and the residence time in the catalytic zone of reactor 5 was 0.12 s.

Example 3

The production of methyl lactate was carried out similarly to example 1, but the pressure in heat exchanger 4, reactor 5, direct cooler 6 and in collecting tank 7 was maintained at 160 mbar. As a result, the selectivity for the formation of methyl lactate was 62% and the yield was 34%, and the residence time in the catalytic zone of reactor 5 was 0.64 s.

Example 4

The production of methyl lactate was carried out similarly to example 1, but the pressure in heat exchanger 4, reactor 5, direct cooler 6 and in the collection vessel 7 was maintained at 30 mbar, and the temperature of the catalytic zone of reactor 5 was 100°C. As a result, the selectivity for the formation of methyl lactate was 30% and the yield was 18%, the residence time in the catalytic zone of reactor 5 was 0.18 s, but a by-product was formed in the form of lactide, the selectivity of which was 16%.

Example 5

The production of methyl lactate was carried out similarly to example 1, but the pressure in heat exchanger 4, reactor 5, direct cooler 6 and in the collection vessel 7 was maintained at 30 mbar, and the temperature of the catalytic zone of reactor 5 was 140°C. As a result, the selectivity for the formation of methyl lactate was 43% and the yield was 25%, the residence time in the catalytic zone of reactor 5 was 0.12 s, but a by-product was formed in the form of lactide, the selectivity of which was 6%.

Example 6

The production of methyl lactate was carried out similarly to example 1, but the pressure in heat exchanger 4, reactor 5, direct cooler 6 and in the collection vessel 7 was maintained at 30 mbar, and the temperature of the catalytic zone of the reactor was 180°C. As a result, the selectivity for the formation of methyl lactate was 23% and the yield was 14%, the residence time in the catalytic zone of reactor 5 was 0.11 s, but a by-product was formed in the form of lactide, the selectivity of which was 39%.

Claims (1)

A method for producing methyl lactate, including continuous esterification of lactic acid with methanol in a flow-type reactor with a fixed bed of a heterogeneous catalyst and purification of methyl lactate from the reaction mass in two distillation columns, characterized in that a mixture of an aqueous solution of lactic acid with methanol in a molar ratio of 1:15 is fed into the mixer , where it is mixed with argon at a flow rate of 0.3 ml/min, the resulting gas-liquid mixture is sent to a heat exchanger in which 180°C is maintained to evaporate lactic acid and methanol, the heated vapors of the reaction mass are sent to a flow-type reactor with a fixed bed of silica gel at 100-180°C, the resulting vaporous products are condensed in a direct refrigerator, the condensate is collected in a container, and a pressure of 30-160 mbar is maintained in the reactor, heat exchanger, direct refrigerator and in the container, and from the container the reaction products are sequentially separated and purified in two distillation columns , while high-boiling compounds and lactic acid are separated in the first distillation column, and the mixture is purified in the second distillation column, obtaining the finished product.