阅读说明：本技术 一种超高纯乳酸甲酯的连续生产工艺 (Continuous production process of ultra-pure methyl lactate ) 是由 黄智贤 陈宇 钱贵菊 邱挺 刘征 叶长燊 钱俊 王红星 喻军 于 2021-09-23 设计创作，主要内容包括：本发明涉及一种超高纯乳酸甲酯的连续生产工艺,包括乳酸预处理、乳酸甲酯合成、轻重组分分离、甲醇分离及回收、除水、除杂以及精制等步骤,使用固定床反应器和精馏分离相结合技术生产超高纯乳酸甲酯,产品纯度＞99.99%,并且未反应的乳酸及甲醇均可循环利用。本发明解决了现有酯化反应工艺中转化率低、设备腐蚀和环境污染等诸多问题,且实现了乳酸甲酯的连续化生产,不仅转化率高、反应物利用率高,而且无污染、产品纯度高。(The invention relates to a continuous production process of ultra-high purity methyl lactate, which comprises the steps of lactic acid pretreatment, methyl lactate synthesis, light and heavy component separation, methanol separation and recovery, water removal, impurity removal, refining and the like, wherein the ultra-high purity methyl lactate is produced by using a fixed bed reactor and rectification separation combined technology, the purity of the product is more than 99.99 percent, and unreacted lactic acid and methanol can be recycled. The invention solves the problems of low conversion rate, equipment corrosion, environmental pollution and the like in the existing esterification reaction process, realizes the continuous production of methyl lactate, and has the advantages of high conversion rate, high utilization rate of reactants, no pollution and high product purity.)

1. A continuous production process of ultra-high purity methyl lactate is characterized in that: the method comprises the following steps:

step A, lactic acid pretreatment: introducing raw material lactic acid into a pretreatment tower, extracting moisture contained in the raw material from the top of the pretreatment tower in a gaseous state through a reduced pressure rectification operation, condensing the moisture by a condenser at the top of the pretreatment tower, and discharging the condensed moisture; the dehydrated lactic acid is discharged from the tower kettle of the pretreatment tower;

step B, methyl lactate synthesis: b, feeding the lactic acid and the methanol discharged after dehydration in the step A into a first fixed bed reactor filled with a catalyst in proportion, and carrying out esterification reaction on the lactic acid and the methanol under the catalytic action of the catalyst to generate methyl lactate and water;

step C, separating light and heavy components: feeding the material obtained at the outlet end of the first fixed bed reactor into a lactic acid recovery tower, and performing reduced pressure rectification operation on the material to obtain light component materials in a tower top extraction system of the lactic acid recovery tower; heavy component materials are extracted from a tower kettle of the lactic acid recovery tower; unreacted lactic acid is extracted from the side line of the lactic acid recovery tower and is sent back to the first fixed bed reactor for cyclic utilization;

step D, methanol separation and recovery: the material extracted from the top of the lactic acid recovery tower is sent to a methanol recovery tower for further separation, methanol is extracted from the top of the methanol recovery tower and sent back to the first fixed bed reactor for recycling, and the material discharged from the bottom of the methanol recovery tower is sent to a subsequent dehydration tower for further separation;

step E, dewatering: in the dehydration tower, azeotrope of water and methyl lactate is extracted from the top of the dehydration tower, the azeotrope is sent to a pretreatment tower, and the azeotropic composition of the water and the methyl lactate is broken by utilizing the extraction effect of lactic acid; or the azeotrope is sent into a second fixed bed reactor filled with a catalyst, so that methyl lactate is subjected to hydrolysis reaction to obtain lactic acid, and then the hydrolyzed material is sent into a pretreatment tower for separation;

step F, removing impurities: feeding the material extracted from the tower kettle of the dehydrating tower into an impurity removing tower for further separation, extracting light component material from the tower top of the impurity removing tower, extracting high-boiling heavy component material from the tower kettle of the impurity removing tower, and extracting crude methyl lactate from the side line of the impurity removing tower and sending the crude methyl lactate to a subsequent refining tower;

step G, refining: in the refining tower, the low-boiling-point substances such as trace moisture and the like remained in the materials are extracted from the top of the refining tower and returned to the impurity removing tower, the ultra-high-purity methyl lactate product is extracted from the side line of the refining tower, and a small amount of methyl lactate and high-boiling-point heavy components are extracted from the tower kettle of the refining tower and returned to the impurity removing tower.

2. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in the step C, the lactic acid recovery tower is a packed tower or a plate tower, the number of theoretical plates of the lactic acid recovery tower is 30-80, the operation pressure of the top of the tower is 1-10 KPa, the reflux ratio is 3-10, a feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, a side extraction opening is arranged at a position close to 1/4-1/2 of the top of the tower, the temperature of the top of the tower is lower than 22.8 ℃, the temperature of the side extraction opening is lower than 206.0 ℃, and the temperature of the kettle is lower than 218.7 ℃.

3. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in the step D, the methanol recovery tower is a packed tower or a plate tower, the number of theoretical plates of the methanol recovery tower is 15-50, the operation pressure at the top of the tower is 25-50 kPa, the reflux ratio is 3-8, the temperature at the top of the tower is lower than 47.6 ℃, and the temperature at the bottom of the tower is lower than 94.5 ℃.

4. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in the step E, the dehydration tower is a packed tower or a plate tower, the number of theoretical plates of the dehydration tower is 15-50, the operation pressure at the top of the tower is 1-10 KPa, the reflux ratio is 0.5-10, the temperature at the top of the tower is lower than 45.8 ℃, and the temperature at the kettle is lower than 85.8 ℃.

5. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in step E, when the material taken out from the top of the dehydrating tower is sent to the pretreating tower, feeding is carried out from the middle lower part of the pretreating tower.

6. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in the step E, when the material extracted from the top of the dehydrating tower is firstly fed into the second fixed bed reactor filled with the catalyst, and the second fixed bed reactor feeds the hydrolyzed material into the pretreatment tower for separation, the material is fed from the middle upper part of the pretreatment tower.

7. The continuous process for producing ultra-high purity methyl lactate according to claim 1 or 6, wherein: and the catalyst filled in the second fixed bed reactor is an acidic ion exchange resin catalyst.

8. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in the step F, the impurity removal tower is a packed tower or a plate tower, the number of theoretical plates of the impurity removal tower is 40-100, the operation pressure of the top of the tower is 1-10 kPa, the reflux ratio is 10-30, a feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, a side extraction opening is arranged at a position close to 1/4-1/2 of the top of the tower, the temperature of the top of the tower is lower than 81.6 ℃, the temperature of the side extraction opening is lower than 83.7 ℃, and the temperature of the kettle is lower than 97.9 ℃.

9. The continuous production process of ultra-high purity methyl lactate according to claim 1, wherein: in the step G, the refining tower is a packed tower or a plate tower, the number of theoretical plates of the refining tower is 40-100, the operating pressure of the top of the tower is 1-10 KPa, the reflux ratio is 5-30, a feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, a side mining position is arranged at a position close to 1/4-1/2 of the top of the tower, the temperature of the top of the tower is lower than 81.6 ℃, the temperature of a side mining opening is lower than 81.6 ℃, and the temperature of the kettle is lower than 82.3 ℃.

The technical field is as follows:

the invention belongs to the technical field of organic chemical industry, and particularly relates to a continuous production process of ultra-high-purity methyl lactate.

Background art:

methyl lactate is a hydroxycarboxylic acid ester compound, is a colorless liquid, has the characteristics of no toxicity, difficult volatilization, good solubility and the like, has biodegradability, is a green solvent with development value and application prospect, and is widely applied to the fields of medicines, resin coatings, adhesives, cleaning agents, dry cleaning fluids, printing ink and the like. Methyl lactate can be used independently or as a formula of a cleaning solvent, is used for cleaning various precision machines and cleaning before surface coating, is a substitute of an ODS (ozone depletion substrate) solvent type cleaning agent, is widely applied to the electronic industry, and can be used for cleaning various optical disks, liquid crystal displays, magnetic heads, chips, circuit board bases and modules; it can also be used for cleaning optical lens, printing screen, ink and metal.

The current methyl lactate synthesis process in the market is H₂SO₄The catalyst is subjected to esterification reaction in a batch reaction kettle, methanol and lactic acid are catalyzed and esterified to synthesize methyl lactate, and unreacted acid and catalyst H are removed through alkali neutralization₂SO₄Finally, pass throughThe methyl lactate with higher purity (the purity is more than 99%) is obtained by series of rectification operations, but because the methyl lactate and water in a reaction system can form an azeotrope (the water accounts for 0.9461 and the methyl lactate accounts for 0.0539), the azeotropic temperature is 100.01 ℃, benzene is usually adopted as a water-carrying agent to separate the methyl lactate, and the final product has low purity and contains a small amount of benzene. In addition, the intermittent process causes complex operation, the H2SO4 is used as a catalyst, equipment corrosion is easily caused, the utilization rate of lactic acid is low, the lactic acid cannot be recycled, and the defects of serious environmental pollution, low yield, generation of a large amount of three wastes, benzene-containing products and the like exist.

In order to overcome the defects of the traditional process, the patent with the publication numbers of CN105949053, CN112062673, CN105924354, CN106316839 and the like discloses a method for preparing methyl lactate by directly converting saccharides or biomass glycerol. Although H is avoided₂SO₄The reaction is carried out in a high-pressure reactor, but is not suitable for industrial mass production from the economical point of view.

Chinese patent publication No. CN101914022 discloses a two-step method for producing methyl lactate with high content and high optical purity, in which lactic acid is first polycondensed to obtain lactic acid oligomer, then catalyst is added to the lactic acid oligomer for depolymerization reaction to obtain crude lactide, and finally refined lactide is reacted with anhydrous methanol to obtain methyl lactate. The content of methyl lactate obtained by the method is more than 99.5 percent, the optical purity can reach more than 99 percent, but the process relates to lactic acid polycondensation and depolymerization, the reaction process is complex, and the utilization rate of lactic acid is low.

In view of this, there is a need for a simple and direct process for the synthesis of methyl lactate.

The invention content is as follows:

the invention is improved aiming at the problems existing in the prior art, namely the technical problem to be solved by the invention is to provide a continuous production process of ultra-high-purity methyl lactate.

In order to achieve the purpose, the invention adopts the technical scheme that: a continuous production process of ultra-high purity methyl lactate comprises the following steps:

step A, lactic acid pretreatment: introducing raw material lactic acid into a pretreatment tower, extracting moisture contained in the raw material from the top of the pretreatment tower in a gaseous state through a reduced pressure rectification operation, condensing the moisture by a condenser at the top of the pretreatment tower, and discharging the condensed moisture; the dehydrated lactic acid is discharged from the tower kettle of the pretreatment tower;

step B, methyl lactate synthesis: b, feeding the lactic acid and the methanol discharged after dehydration in the step A into a first fixed bed reactor filled with a catalyst in proportion, and carrying out esterification reaction on the lactic acid and the methanol under the catalytic action of the catalyst to generate methyl lactate and water;

step C, separating light and heavy components: feeding the material obtained at the outlet end of the first fixed bed reactor into a lactic acid recovery tower, and performing reduced pressure rectification operation on the material to obtain light component materials in a tower top extraction system of the lactic acid recovery tower; heavy component materials are extracted from a tower kettle of the lactic acid recovery tower; unreacted lactic acid is extracted from the side line of the lactic acid recovery tower and is sent back to the first fixed bed reactor for cyclic utilization;

step D, methanol separation and recovery: the material extracted from the top of the lactic acid recovery tower is sent to a methanol recovery tower for further separation, methanol is extracted from the top of the methanol recovery tower and sent back to the first fixed bed reactor for recycling, and the material discharged from the bottom of the methanol recovery tower is sent to a subsequent dehydration tower for further separation;

step E, dewatering: in the dehydration tower, azeotrope of water and methyl lactate is extracted from the top of the dehydration tower, the azeotrope is sent to a pretreatment tower, and the azeotropic composition of the water and the methyl lactate is broken by utilizing the extraction effect of lactic acid; or the azeotrope is sent into a second fixed bed reactor filled with a catalyst, so that methyl lactate is subjected to hydrolysis reaction to obtain lactic acid, and then the hydrolyzed material is sent into a pretreatment tower for separation;

step F, removing impurities: feeding the material extracted from the tower kettle of the dehydrating tower into an impurity removing tower for further separation, extracting light component material from the tower top of the impurity removing tower, extracting high-boiling heavy component material from the tower kettle of the impurity removing tower, and extracting crude methyl lactate from the side line of the impurity removing tower and sending the crude methyl lactate to a subsequent refining tower;

step G, refining: in the refining tower, the low-boiling-point substances such as trace moisture and the like remained in the materials are extracted from the top of the refining tower and returned to the impurity removing tower, the ultra-high-purity methyl lactate product is extracted from the side line of the refining tower, and a small amount of methyl lactate and high-boiling-point heavy components are extracted from the tower kettle of the refining tower and returned to the impurity removing tower.

Further, in the step C, the lactic acid recovery tower is a packed tower or a plate tower, the number of theoretical plates of the lactic acid recovery tower is 30-80, the operation pressure of the tower top is 1-10 KPa, the reflux ratio is 3-10, a feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, a side mining opening is arranged at a position close to 1/4-1/2 of the tower top, the temperature of the tower top is lower than 22.8 ℃, the temperature of the side mining opening is lower than 206.0 ℃, and the temperature of the kettle is lower than 218.7 ℃.

Further, in the step D, the methanol recovery tower is a packed tower or a plate tower, the number of theoretical plates of the methanol recovery tower is 15-50, the operation pressure at the top of the tower is 25-50 kPa, the reflux ratio is 3-8, the temperature at the top of the tower is lower than 47.6 ℃, and the temperature at the kettle is lower than 94.5 ℃.

Further, in the step E, the dehydration tower is a packed tower or a plate tower, the number of theoretical plates of the dehydration tower is 15-50, the operation pressure at the top of the tower is 1-10 KPa, the reflux ratio is 0.5-10, the temperature at the top of the tower is lower than 45.8 ℃, and the temperature of the kettle is lower than 85.8 ℃.

Further, in step E, when the material taken out from the top of the dehydrating tower is sent to the pretreating tower, feeding is carried out from the lower middle part of the pretreating tower.

Further, in the step E, when the material extracted from the top of the dehydrating tower is firstly fed into the second fixed bed reactor filled with the catalyst, and the second fixed bed reactor feeds the hydrolyzed material into the pretreatment tower for separation, the material is fed from the middle upper part of the pretreatment tower.

Further, the catalyst filled in the second fixed bed reactor is an acidic ion exchange resin catalyst.

Further, in the step F, the impurity removing tower is a packed tower or a plate tower, the number of theoretical plates of the impurity removing tower is 40-100, the operation pressure of the tower top is 1-10 kPa, the reflux ratio is 10-30, a feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, a side extraction opening is arranged at a position close to 1/4-1/2 of the tower top, the temperature of the tower top is lower than 81.6 ℃, the temperature of the side extraction opening is lower than 83.7 ℃, and the temperature of the kettle is lower than 97.9 ℃.

Further, in the step G, the refining tower is a packed tower or a plate tower, the number of theoretical plates of the refining tower is 40-100, the operation pressure of the tower top is 1-10 KPa, the reflux ratio is 5-30, a feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, a side mining position is arranged at a position close to 1/4-1/2 of the tower top, the temperature of the tower top is lower than 81.6 ℃, the temperature of a side mining opening is lower than 81.6 ℃, and the temperature of the kettle is lower than 82.3 ℃.

Compared with the prior art, the invention has the following effects: the invention has reasonable design, solves the problems of low conversion rate, equipment corrosion, environmental pollution and the like in the existing esterification reaction process, realizes the continuous production of methyl lactate, and has high conversion rate, high utilization rate of reactants, no pollution and high product purity.

Description of the drawings:

FIG. 1 is a schematic process flow diagram of a first embodiment of the present invention;

FIG. 2 is a schematic process flow diagram of a second embodiment of the present invention.

In fig. 1:

1-a pretreatment tower; 2-a first fixed bed reactor; a 3-lactic acid recovery column; 4-methanol recovery column; 5-a dehydration column; 6-impurity removal tower; 7-refining tower;

in fig. 2:

8-a pretreatment tower; 9-a first fixed bed reactor; a 10-lactic acid recovery column; 11-methanol recovery column; 12-a dehydration column; 13-a depurating tower; 14-a second fixed bed reactor; 15-refining tower.

The specific implementation mode is as follows:

the present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

In the description of the present invention, it is to be understood that the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, are merely for convenience of description of the present invention, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention.

The invention relates to a continuous production process of ultra-high purity methyl lactate, which comprises the following steps:

step A, lactic acid pretreatment: introducing raw material lactic acid into a pretreatment tower, extracting moisture contained in the raw material from the top of the pretreatment tower in a gaseous state through a reduced pressure rectification operation, condensing the moisture by a condenser at the top of the pretreatment tower, and discharging the condensed moisture; discharging the dehydrated lactic acid from the tower kettle of the pretreatment tower, and conveying the lactic acid to the first fixed bed reactor to participate in reaction;

the raw material lactic acid is subjected to vacuum rectification operation by a pretreatment tower, so that 15-17% of water contained in the raw material lactic acid is removed, and the water content of the dehydrated lactic acid in a tower kettle is less than 2%, so that the inhibition effect of the esterification reaction process on the reaction due to the presence of water is reduced, and the conversion rate of the lactic acid is greatly increased;

step B, methyl lactate synthesis: b, feeding the lactic acid and the methanol discharged after dehydration in the step A into a first fixed bed reactor filled with a resin catalyst according to a certain proportion, and carrying out esterification reaction on the lactic acid and the methanol under the catalytic action of the resin catalyst to generate methyl lactate and water;

step C, separating light and heavy components: feeding the material obtained at the outlet end of the first fixed bed reactor into a lactic acid recovery tower, and performing reduced pressure rectification operation to obtain light component materials in a tower top extraction system of the lactic acid recovery tower, wherein the light component materials mainly comprise methanol, water and methyl lactate; heavy component materials are extracted from a tower kettle of the lactic acid recovery tower, and mainly comprise polylactic acid; unreacted lactic acid is extracted from the side line of the lactic acid recovery tower and is sent back to the first fixed bed reactor for cyclic utilization;

step D, methanol separation and recovery: the material extracted from the top of the lactic acid recovery tower is sent to a methanol recovery tower for further separation, methanol is extracted from the top of the methanol recovery tower and sent back to the first fixed bed reactor for recycling, and the material discharged from the bottom of the methanol recovery tower is sent to a subsequent dehydration tower for further separation;

step E, dewatering: in the dehydration column, water generated in the esterification reaction is extracted from the top of the dehydration column, and the water extracted from the top of the dehydration column contains a small amount of methyl lactate because water and methyl lactate form a homogeneous azeotrope. In order to improve the conversion rate of lactic acid, two schemes are provided for recycling methyl lactate in water extracted from the top of the dehydrating tower: one is that the material extracted from the top of the dehydrating tower is sent to the lower part of the pretreatment tower, the azeotropic composition of water and methyl lactate is broken by the extraction effect of lactic acid, the water is extracted from the top of the pretreatment tower, and the methyl lactate and the lactic acid are extracted from the bottom of the pretreatment tower; the other is that the material extracted from the top of the dehydrating tower is sent into a second fixed bed reactor filled with catalyst, so that methyl lactate is subjected to hydrolysis reaction to obtain lactic acid, and then the hydrolyzed material is sent into a pretreatment tower for separation;

step F, removing impurities: feeding the material extracted from the tower kettle of the dehydrating tower into an impurity removing tower for further separation, extracting light component material from the tower top of the impurity removing tower, extracting high-boiling heavy component material from the tower kettle of the impurity removing tower, and extracting crude methyl lactate from the side line of the impurity removing tower and sending the crude methyl lactate to a subsequent refining tower;

step G, refining: in the refining tower, the low-boiling-point substances such as trace moisture and the like remained in the materials are extracted from the top of the refining tower and returned to the impurity removing tower for circular separation, the ultra-high-purity methyl lactate product (with the purity of 99.99%) is extracted from the side line of the refining tower, and a small amount of methyl lactate and high-boiling-point heavy components are extracted from the tower kettle of the refining tower and returned to the impurity removing tower for circular separation.

In the embodiment, in the step C, the lactic acid recovery tower is a packed tower or a plate tower, and the number of theoretical plates of the lactic acid recovery tower is 30 to 80, preferably 40 to 60; the operation pressure of the tower top is 1-10 KPa, preferably 1-5 KPa; the reflux ratio is 3-10, preferably 3-6; the feed inlet is arranged at a position close to 1/4-1/3 of the tower kettle, the side mining outlet is arranged at a position close to 1/4-1/2 of the tower top, the temperature of the tower top is lower than 22.8 ℃, the temperature of the side mining outlet is lower than 206.0 ℃, and the temperature of the kettle is lower than 218.7 ℃.

In the embodiment, in the step D, the methanol recovery tower is a packed tower or a plate tower, and the number of theoretical plates of the methanol recovery tower is 15 to 50, preferably 15 to 30; the operation pressure at the tower top is 25-50 kPa, and preferably 30-40 Kpa; the reflux ratio is 3-8, preferably 4-8; the temperature at the top of the tower is lower than 47.6 ℃, and the temperature at the bottom of the kettle is lower than 94.5 ℃.

In the embodiment, in the step E, the dehydration tower is a packed tower or a plate tower, and the number of theoretical plates of the dehydration tower is 15 to 50, preferably 30 to 50; the operation pressure of the tower top is 1-10 KPa, preferably 1-5 KPa; the reflux ratio is 0.5-10, preferably 0.5-3; the temperature at the top of the tower is lower than 45.8 ℃, and the temperature at the bottom of the kettle is lower than 85.8 ℃.

In this example, in step E, when the material taken out from the top of the dehydrating tower was sent to the pretreating tower, the material was fed from the lower middle portion of the pretreating tower.

In this embodiment, in step E, when the material extracted from the top of the dehydrating tower is first fed into the second fixed bed reactor filled with the catalyst, and the second fixed bed reactor feeds the hydrolyzed material into the pretreating tower for separation, the material is fed from the middle upper part of the pretreating tower.

In this embodiment, the catalyst filled in the second fixed bed reactor is an acidic ion exchange resin catalyst.

In the embodiment, in the step F, the impurity removing tower is a packed tower or a plate tower, and the number of theoretical plates of the impurity removing tower is 40 to 100, preferably 45 to 80; the operation pressure at the tower top is 1-10 kPa, and the preferable pressure is 1-5 Kpa; the reflux ratio is 10-30, preferably 5-20; the feed inlet is arranged at a position close to 1/4-1/3 of a tower kettle, the side mining outlet is arranged at a position close to 1/4-1/2 of the tower top, the temperature of the tower top is lower than 81.6 ℃, the temperature of the side mining outlet is lower than 83.7 ℃, and the temperature of the kettle is lower than 97.9 ℃.

In the embodiment, in the step G, the refining tower is a packed tower or a plate tower, and the number of theoretical plates of the refining tower is 40 to 100, preferably 45 to 80; the operation pressure at the tower top is 1-10 KPa, preferably 1-5 kPa; the reflux ratio is 5-30, preferably 10-20; the feed inlet is arranged at a position close to 1/4-1/3 of the tower kettle, the side mining position is arranged at a position close to 1/4-1/2 of the tower top, the temperature of the tower top is lower than 81.6 ℃, the temperature of the side mining opening is lower than 81.6 ℃, and the temperature of the kettle is lower than 82.3 ℃.

In the production process, unreacted lactic acid and methanol can be returned to the fixed bed reactor for recycling after rectification and separation; the polylactic acid obtained from the tower bottom of the lactic acid recovery tower can be collected as a second product; the material extracted from the top of the dehydrating tower can be returned to the pretreatment tower for cyclic separation, and the lactic acid can also be recycled through the hydrolysis process of the fixed bed reactor; the materials extracted from the top and bottom of the refining tower can be returned to the impurity removing tower for circular separation.

The first embodiment is as follows: as shown in fig. 1, the production process comprises the following steps:

1. feeding raw material lactic acid into a pretreatment tower 1 at a rate of 1750kg/h for vacuum rectification, wherein the raw material lactic acid contains 72.03% of lactic acid, 17.07% of water and 10.9% of polylactic acid; the pretreatment tower 1 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 30, the pressure at the top of the tower is 2 KPa, the temperature is 17.5 ℃, the reflux ratio is 4, and the number of the theoretical plates at the feeding position away from the tower kettle is 15; the dehydrated lactic acid (with the extraction rate of 1480.89 kg/h) extracted from the tower kettle of the pretreatment tower, wherein the water content is 2 percent, the lactic acid content is 85.12 percent, and the polylactic acid content is 12.88 percent, is sent into the fixed bed reactor 2.

2. Methanol is introduced into a first fixed bed reactor 2 filled with acidic ion exchange resin HND-8 at the flow rate of 1792kg/h, the ratio of the alcohol to the acid is 4:1, the reaction temperature is 70 ℃, the retention time is 30 min, methyl lactate and water are generated by esterification, and the material obtained at the outlet of the first fixed bed reactor (the rate of 3272.89 kg/h) is sent into a lactic acid recovery tower 3.

3. The lactic acid recovery tower 3 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 60, the reflux ratio is 3, the pressure at the top of the tower is 4 KPa, the temperature is 7 ℃, and the number of theoretical plates at the feeding position away from the tower kettle is 30; materials (mainly methanol, water and methyl lactate) extracted from the top of the lactic acid recovery tower are sent to a methanol recovery tower 4 at the speed of 3031.82kg/h for further separation; unreacted lactic acid is extracted from the side line of the lactic acid recovery tower 3 (the extraction rate is 50.32 kg/h) and is sent back to the first fixed bed reactor for recycling; the polylactic acid is extracted from the tower bottom of the lactic acid recovery tower (the extraction rate is 172.89 kg/h).

4. The methanol recovery tower 4 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 30, the reflux ratio is 4, the pressure at the top of the tower is 30KPa, the temperature is 36.5 ℃, and the number of the theoretical plates at the feeding position away from the tower kettle is 15; methanol is extracted from the top of the methanol recovery tower (the extraction rate is 1361.55 kg/h) and is sent back to the fixed bed reactor for recycling; heavy components (with the speed of 1670.17 kg/h) extracted from the tower bottom of the methanol recovery tower are sent to a subsequent dehydration tower 5 for further separation.

5. The dehydration tower 5 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 50, the reflux ratio is 3, the pressure at the top of the tower is 5KPa, the temperature is 32.9 ℃, and the number of theoretical plates at the feeding position away from the tower kettle is 35. Extracting materials (the extraction rate is 274 kg/h) from the top of the dehydrating tower, conveying the materials to a feed inlet at the lower part of the pretreatment tower 1, and breaking the azeotropic composition of water and methyl lactate by using the extraction effect of lactic acid; methyl lactate and a small amount of heavy components (the extraction rate is 1396.17 kg/h) are extracted from the tower bottom and are sent to a subsequent impurity removal tower 6 for separation;

6. the impurity removing tower 6 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 60, the operation reflux ratio is 15, the tower top pressure is 5KPa, the temperature is 66.87 ℃, and the number of the theoretical plates of the feeding position from the tower bottom is 20. Light component materials (methanol and water) are extracted from the top of the impurity removing tower at the speed of 1 kg/h, and heavy component materials (lactic acid and polylactic acid) are extracted from the bottom of the tower at the speed of 0.17 kg/h. Crude methyl lactate product (extraction rate 1395 kg/h) is extracted from the liquid phase side at the middle lower part of the tower, the number of theoretical plates at the extraction position from the tower bottom is 30, and the crude methyl lactate product is sent to a refining tower 7 for refining.

7. The refining column 7 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 60, the reflux ratio is 20, the overhead pressure is 5KPa, the temperature is 66.87 ℃, and the number of theoretical plates at the feeding position away from the column bottom is 20. Methyl lactate containing trace water, methanol and other low-boiling-point substances is extracted from the top of the refining tower, and methyl lactate containing trace lactic acid, polylactic acid and other high-boiling-point substances is extracted from the tower bottom and sent to an impurity removal tower for cyclic separation. Ultra-high purity methyl lactate is extracted from the liquid phase at the middle-lower part of the tower at the side line (the extraction rate is 1394.98kg/h, and the number of theoretical plates at the extraction position from the tower bottom is 30).

The methyl lactate (with the mass content of 99.994%) is obtained by the process, the conversion rate of the lactic acid is 99.9%, and the yield of the methyl lactate is 99.91%.

Example two: as shown in fig. 2, the production process comprises the following steps:

1. raw material lactic acid is sent into a pretreatment tower 8 at the rate of 1750kg/h for vacuum rectification, wherein the raw material lactic acid contains 72.03% of lactic acid, 17.07% of water and 10.9% of polylactic acid. The pretreatment tower 8 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 30, the pressure at the top of the tower is 2 KPa, the temperature is 17.5 ℃, the reflux ratio is 4, and the number of the theoretical plates at the feeding position away from the tower kettle is 15. The dehydrated lactic acid (with the extraction rate of 1480.89 kg/h) extracted from the tower bottom of the pretreatment tower, wherein the water content is 2%, the lactic acid content is 85.12%, and the polylactic acid content is 12.88%, is fed into a first fixed bed reactor 9.

2. Methanol is introduced into a first fixed bed reactor 9 filled with acidic ion exchange resin HND-8 at the flow rate of 1792kg/h, the ratio of alcohol to acid is 4:1, the reaction temperature is 70 ℃, the retention time is 30 min, methyl lactate and water are generated by esterification, and the material obtained at the outlet of the first fixed bed reactor (the extraction rate is 3272.89 kg/h) is sent into a lactic acid recovery tower 10.

3. The lactic acid recovery tower 10 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 60, the reflux ratio is 3, the pressure at the top of the tower is 4 KPa, the temperature is 7 ℃, and the number of theoretical plates at the feeding position away from the tower kettle is 30. Materials (mainly methanol, water and methyl lactate) extracted from the top of the lactic acid recovery tower are sent into a methanol recovery tower 11 for further separation in the amount of 3031.82 kg/h; unreacted lactic acid is extracted from the side line of the lactic acid recovery tower 10 (the extraction rate is 50.32 kg/h) and is sent back to the fixed bed reactor for cyclic utilization; the polylactic acid is extracted from the tower 10 of the lactic acid recovery tower (the extraction rate is 172.89 kg/h).

4. The methanol recovery tower 11 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 30, the reflux ratio is 4, the overhead pressure is 30KPa, the temperature is 36.5 ℃, and the number of theoretical plates at the feeding position away from the tower bottom is 15. Methanol is extracted from the top of the methanol recovery tower (the extraction rate is 1361.55 kg/h) and is sent back to the fixed bed reactor for recycling; the heavy components extracted from the tower bottom of the methanol recovery tower are sent to a subsequent dehydration tower 12 for further separation in the amount of 1670.17 kg/h.

5. The dehydration column 12 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 50, the reflux ratio is 3, the pressure at the top of the column is 5KPa, the temperature is 32.9 ℃, and the number of theoretical plates at the feeding position away from the bottom of the column is 35. 274kg/h of material is extracted from the top of the dehydration tower and is sent to a second fixed bed reactor 14 filled with acidic ion exchange resin HND-8, so that the azeotropic composition of water and methyl lactate generates hydrolysis reaction in the second fixed bed reactor, the temperature of the hydrolysis reaction is 75 ℃, and the retention time is 30 min; methyl lactate and a small amount of heavy components (the extraction rate is 1396.17 kg/h) are extracted from the tower bottom of the dehydration tower and are sent to a subsequent impurity removal tower 13 for separation;

6. the impurity removing tower 13 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 60, the operation reflux ratio is 15, the tower top pressure is 5KPa, the temperature is 66.87 ℃, and the number of the theoretical plates of the feeding position from the tower bottom is 20. Light component materials (methanol and water) are extracted from the top of the impurity removing tower at the rate of 1 kg/h, and heavy component materials (lactic acid and polylactic acid) are extracted from the bottom of the tower at the rate of 0.17 kg/h. The crude methyl lactate product (the extraction rate is 1395 kg/h) is extracted from the liquid phase side at the middle lower part of the tower, the number of theoretical plates at the extraction position from the tower bottom is 30, and the crude methyl lactate product is sent to a refining tower 15 for refining.

7. The refining column 15 adopts Sulzer Mellapak structured packing, the number of theoretical plates is 60, the reflux ratio is 20, the pressure at the top of the column is 5KPa, the temperature is 66.87 ℃, and the number of theoretical plates at the feeding position away from the bottom of the column is 20. Methyl lactate containing trace water, methanol and other low-boiling-point substances is extracted from the top of the refining tower, and methyl lactate containing trace lactic acid, polylactic acid and other high-boiling-point substances is extracted from the tower bottom and sent to an impurity removal tower for cyclic separation. Ultra-high purity methyl lactate is extracted from the liquid phase at the middle-lower part of the tower at the side line (the extraction rate is 1394.98kg/h, and the number of theoretical plates at the extraction position from the tower bottom is 30).

The methyl lactate (with the mass content of 99.993%) is obtained by the process, the conversion rate of the lactic acid is 99.99%, and the yield of the methyl lactate is 99.74%.

The invention has the advantages that:

(1) in the prior art, the treatment mode of unreacted lactic acid is to add alkali for neutralization, so that the utilization rate of lactic acid is reduced; the unreacted lactic acid in the fixed bed reactor is recycled after being separated through the vacuum rectification operation, so that the utilization rate of raw materials and the conversion rate of the lactic acid in the esterification process are greatly increased;

(2) the lactic acid recovery tower adopts the pressure reduction side mining operation, so that the temperature in the tower is reduced, the residence time of the lactic acid in the tower is also reduced, the occurrence of the self-polymerization reaction of the lactic acid is effectively avoided, and the conversion rate of the lactic acid is improved (more than 98 percent); the obtained recombinant component of the lactic acid recovery tower is polylactic acid which can be collected and sold as a product;

(3) because water and methyl lactate can form an azeotrope and are not easy to separate, the prior art separates the methyl lactate by adding a water-carrying agent (benzene), so that the purity of the product methyl lactate is not high, and the invention does not add the water-carrying agent, but provides two solutions: (1) one proposal is that the water extracted from the top of the dehydration tower and the trace amount of methyl lactate contained in the water are introduced into the lower part of the pretreatment tower, and the azeotropic point of the water and the methyl lactate is broken by utilizing the extraction function of the fed lactic acid, so as to realize the separation of the water and the methyl lactate; (2) the other scheme is that the material extracted from the top of the dewatering tower is fed into a fixed bed reactor filled with an acidic resin catalyst to hydrolyze methyl lactate to obtain lactic acid, and the hydrolyzed material is fed into a pretreatment tower to be separated. The two treatment methods can improve the yield of the product and the conversion rate of the lactic acid, and new impurities are not introduced in the process, so that the purity of the product is ensured;

(4) the methyl lactate adopts double-tower bilateral rectification operation, the purity of the methyl lactate is high (more than 99.99 percent), and the yield of the methyl lactate is high (more than 98 percent).

If the invention discloses or relates to parts or structures which are fixedly connected to each other, the fixedly connected parts can be understood as follows, unless otherwise stated: a detachable fixed connection (for example using bolts or screws) is also understood as: non-detachable fixed connections (e.g. riveting, welding), but of course, fixed connections to each other may also be replaced by one-piece structures (e.g. manufactured integrally using a casting process) (unless it is obviously impossible to use an integral forming process).

In addition, terms used in any technical solutions disclosed in the present invention to indicate positional relationships or shapes include approximate, similar or approximate states or shapes unless otherwise stated.

Any part provided by the invention can be assembled by a plurality of independent components or can be manufactured by an integral forming process.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention and not to limit it; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art will understand that: modifications to the specific embodiments of the invention or equivalent substitutions for parts of the technical features may be made; without departing from the spirit of the present invention, it is intended to cover all aspects of the invention as defined by the appended claims.