System and method for continuously producing lactic acid

Abstract

The invention provides a system and a method for continuously producing lactic acid. The system comprises a reaction kettle, a methanol flash tank, an ion exchange tank, a rectifying tower and a membrane filter which are connected in sequence, wherein the reaction kettle is respectively connected with a reactant methanol feeding pipeline, an ethylene glycol feeding pipeline and a basic catalyst solution feeding pipeline; the basic catalyst solution is a solution containing a catalyst and solid alkali. According to the invention, after the reaction product is subjected to flash evaporation, ion exchange is firstly carried out, and then rectification is carried out, so that methanol in the ion exchange stage can be retained to the greatest extent, the fluidity of lactic acid-base meets the expectation, the ion exchange effect and the energy consumption of the whole system are finally facilitated, and the condition that a large amount of methanol is separated after rectification is carried out is avoided; and therefore, the problems of poor lactic acid-base flowability, large pipeline resistance, difficulty in type selection of fluid conveying equipment and high energy consumption of the device are caused.

Description

Technical Field

[0001] This invention belongs to the field of lactic acid production technology, specifically relating to a system and method for continuous production of lactic acid. Background Technology

[0002] Lactic acid, also known as 2-hydroxypropionic acid, is one of the three major organic acids and has wide applications in the food and pharmaceutical industries. It is also used as a raw material in the chemical industry for producing various products. For example, lactic acid oxidative dehydrogenation produces pyruvic acid, dehydration produces acrylic acid, and the alkyl lactate produced by esterification with alcohols can be used as a green solvent. Furthermore, polylactic acid (PLA), produced by the polymerization of lactic acid, is biocompatible and suitable for medical applications such as sutures, pharmaceutical packaging, and prostheses. Currently, there are two main methods for preparing lactic acid: bio-fermentation and chemical conversion. Chemical synthesis typically uses sulfuric acid catalysis, which has the advantages of a short production cycle and a simple process; however, it has the disadvantage of using toxic materials such as hydrogen cyanide, hydrofluoric acid, N₂O₄, and CO as raw materials.

[0003] Henan Jindan Lactic Acid Technology Co., Ltd. published an article titled "Production Technology and R&D Status of Lactic Acid in my country" in *Henan Chemical Industry*. The article describes the current production technology and R&D status of my country's lactic acid industry, summarizes the main production methods of lactic acid (chemical synthesis, enzymatic synthesis, and fermentation), and comments on the advantages and disadvantages of each method. Jiangsu Ruixiang Chemical Co., Ltd. published an article titled "Research Progress in Lactic Acid Preparation" in *Contemporary Chemical Research*. The article mentions that "fermentation, as the earliest commercially available large-scale method for producing lactic acid, remains the main method for commercial lactic acid production worldwide. However, it has a long cycle, and the fermentation preparation conditions for high-purity lactic acid are stringent and complex. Although the chemical method for preparing lactic acid has a shorter cycle, it usually only produces racemic lactic acid, and subsequent separation is difficult."

[0004] Therefore, developing a system for the continuous production of lactic acid with a short production cycle, simple process, high efficiency, green and safe operation has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to solve the problems existing in the prior art and to provide a system and method for continuous production of lactic acid.

[0006] One of the objectives of this invention is to provide a system for continuous production of lactic acid, comprising a reaction vessel, a methanol flash tank, an ion exchange tank, a distillation column, and a membrane filter connected in sequence; wherein the reaction vessel is connected to a methanol feed pipe, an ethylene glycol feed pipe, and an alkaline catalyst solution feed pipe; the alkaline catalyst solution is a solution containing a catalyst and a solid alkali.

[0007] In a preferred embodiment of the present invention

[0008] The methanol flash tank inlet is connected to the bottom outlet of the reactor; and / or, the ion exchange tank inlet is connected to the bottom outlet of the methanol flash tank; and / or, the distillation column inlet is connected to the outlet of the ion exchange tank; and / or, the membrane filter inlet is connected to the bottom outlet of the distillation column; and / or, the reactor is equipped with a heating device; and / or, the reactor is equipped with a top gas phase outlet.

[0009] In a preferred embodiment of the present invention

[0010] The solvent for the alkaline catalyst solution is an alcohol; and / or,

[0011] An alkaline catalyst solution preparation tank is provided upstream of the alkaline catalyst solution feed pipe; preferably, it includes one, or two or more alkaline catalyst solution preparation tanks connected in parallel; more preferably, it is a first alkaline catalyst solution preparation tank and / or a second alkaline catalyst solution preparation tank; and / or,

[0012] An alkaline catalyst solution feed pump is provided on the alkaline catalyst solution feed pipeline;

[0013] Preferred,

[0014] The first alkaline catalyst solution preparation tank and / or the second alkaline catalyst solution preparation tank are both equipped with a catalyst feed port, a solid alkali feed port, and a methanol feed port;

[0015] More preferably, the solid base is potassium hydroxide; and / or,

[0016] The alcohol is methanol;

[0017] The catalyst may be selected from existing catalysts used in the preparation of lactic acid from methanol and ethylene glycol, and is preferably at least one of nitrogen heterocyclic carbene metal coordination compounds. More preferably, it is the "nitrogen heterocyclic carbene metal coordination compound" described in Chinese Patent CN110357770A as a catalyst used in the selective catalytic conversion of ethylene glycol to lactic acid. The definition and description of the catalyst nitrogen heterocyclic carbene metal coordination compound in this application are the same as those in Chinese Patent CN110357770A, which is incorporated herein by reference in its entirety.

[0018] In a preferred embodiment of the present invention

[0019] The top vapor outlet of the methanol flash tank is connected to the flash vapor discharge pipeline; and / or

[0020] The bottom outlet of the methanol flash evaporator is connected to the inlet of the ion exchange tank via a flash liquid phase outlet pipeline.

[0021] Preferred,

[0022] A flash vapor phase condenser is installed on the flash vapor phase discharge line, and the flash vapor phase condenser is connected to the molecular sieve tower line; and / or

[0023] A flash liquid phase cooler is provided on the flash liquid phase discharge line; and / or a flash liquid phase discharge pump is provided on the flash liquid phase discharge line.

[0024] More preferably,

[0025] A flash vapor phase condensate tank is provided between the flash vapor phase condenser and the molecular sieve tower.

[0026] In a preferred embodiment of the present invention

[0027] The ion exchange tank is equipped with an ion exchange resin, which is a hydrogen-form ion exchange resin; and / or...

[0028] The outlet of the ion exchange tank is connected to the distillation column via a post-ion exchange material pipeline; a post-ion exchange material discharge pump is provided on the post-ion exchange material pipeline.

[0029] Preferred,

[0030] The inlet of the ion exchange tank is connected to the cleaning waste liquid pipeline and the resin regeneration liquid pipeline, respectively; and / or,

[0031] The outlet of the ion exchange tank is connected to the cleaning water pipeline and the resin regeneration waste liquid pipeline, respectively.

[0032] In a preferred embodiment of the present invention

[0033] The distillation column is provided with a top outlet, which is connected to the top outlet pipeline of the distillation column; and / or, the distillation column is provided with a middle outlet, which is connected to an ethylene glycol tank.

[0034] Preferably, a top condenser is provided at the top discharge pipeline connection of the distillation column, and the top condenser is connected to the molecular sieve column;

[0035] More preferably, a reflux tank is provided between the top condenser and the molecular sieve column; and / or,

[0036] The membrane filter contains a microporous membrane;

[0037] The best option,

[0038] The microporous membrane is a nanofiltration membrane resistant to organic solvents; and / or,

[0039] The microporous membrane has a molecular weight cutoff of 100–300 Da; and / or,

[0040] The microporous membrane has a processing capacity of 50–200 kg / h.

[0041] In a preferred embodiment of the present invention

[0042] The molecular sieve tower is provided with a top feed inlet, which is connected to the reaction vessel; and / or

[0043] The bottom feed inlet of the molecular sieve tower is connected to the wastewater pipeline;

[0044] Preferred,

[0045] A methanol tank is provided between the molecular sieve tower and the reactor; the methanol tank and the reactor are connected by a methanol recovery and reuse pipeline; a methanol discharge pump is provided on the methanol recovery and reuse pipeline;

[0046] More preferably,

[0047] The feed inlet at the top of the molecular sieve tower is connected to a nitrogen pipeline; and / or

[0048] The bottom feed inlet of the molecular sieve tower is connected to the purging exhaust gas pipeline;

[0049] The best option,

[0050] The nitrogen pipeline includes a first nitrogen pipeline and a second nitrogen pipeline; a nitrogen heater is provided on the second nitrogen pipeline.

[0051] A second objective of this invention is to provide a method for continuous production of lactic acid, which is carried out using the continuous lactic acid production system described in one objective of this invention.

[0052] Preferably, the method sequentially includes a reaction section, a flash evaporation section, an ion exchange section, a purification section, and a lactic acid acquisition and catalyst recovery section;

[0053] More preferably, the method further includes a methanol recovery section and / or an ethylene glycol recovery section.

[0054] In a preferred embodiment of the present invention

[0055] The reaction section includes: reacting methanol and ethylene glycol in the presence of a catalyst, preferably including feeding the catalyst and solid base into a solvent to obtain an alkaline catalyst solution; adding methanol, ethylene glycol, and the alkaline catalyst solution into a reaction vessel to react and obtain a crude reaction product; and / or

[0056] The flash evaporation section includes: the crude reaction product entering a methanol flash tank for adiabatic flash evaporation at atmospheric pressure to obtain a flash vapor phase product and a flash liquid phase product; and / or

[0057] The ion exchange section includes: the flash evaporation liquid phase product entering an ion exchange tank for cation exchange to obtain the ion-exchanged material; and / or

[0058] The purification section includes: the ion-exchange material enters a distillation column for distillation to obtain distillation gas products, ethylene glycol waste liquid, lactic acid and catalyst mixture;

[0059] The lactic acid acquisition and catalyst recovery section includes: the mixture of lactic acid and catalyst enters a membrane filter for separation, and lactic acid and catalyst are obtained separately;

[0060] Preferred,

[0061] The methanol recovery section includes condensing the flash vapor phase product into a flash vapor phase condensate, dehydrating it through a molecular sieve tower, and then recycling the resulting methanol into the reactor; and / or, it includes condensing the distillation vapor phase product into a distillation vapor phase condensate, dehydrating it through a molecular sieve tower, and then recycling the resulting methanol into the reactor.

[0062] The ethylene glycol recovery section includes feeding the ethylene glycol waste liquid obtained from the purification section into an ethylene glycol tank.

[0063] In a preferred embodiment of the present invention

[0064] In the reaction section, the reaction temperature is 100–140℃; and / or, the reaction pressure is 0.8–1.1 MPa; and / or, the reaction time is 4–8 h; and / or

[0065] In the ion exchange section, the ion exchange resin is a hydrogen-form ion exchange resin; and / or

[0066] In the purification section, the operating pressure at the top of the distillation column is 20–80 kPaA; and / or, the operating pressure at the bottom of the distillation column is 40–50 kPaA; and / or, the operating temperature at the top of the distillation column is 50–80 °C; and / or, the operating temperature at the bottom of the distillation column is 190–230 °C; and / or, the number of theoretical plates in the distillation column is 20–60; and / or, the reflux ratio in the distillation column is 15–40; and / or

[0067] In the lactic acid acquisition and catalyst recovery section, the membrane filter contains a microporous membrane; preferably, the microporous membrane is a nanofiltration membrane resistant to organic solvents; and / or, the molecular weight cutoff of the microporous membrane is 100-300 Da; and / or, the processing capacity of the microporous membrane is 50-200 kg / h.

[0068] Compared with the prior art, the beneficial effects of the present invention are:

[0069] 1. The continuous lactic acid production system of the present invention performs ion exchange followed by distillation after flash evaporation of the reaction products. This allows methanol to be retained to the maximum extent during the ion exchange stage, thereby ensuring the fluidity of the lactic acid base meets expectations. Ultimately, this benefits the ion exchange effect and reduces the energy consumption of the entire system. It avoids the problems of poor lactic acid base fluidity, high pipeline resistance, difficulty in selecting fluid transport equipment, and high energy consumption caused by the large amount of methanol separated after distillation.

[0070] 2. The continuous lactic acid production system of the present invention can recover and reuse all of the methanol as reactant, methanol as solvent, ethylene glycol as reactant, and catalyst, resulting in good process economy.

[0071] 3. The continuous production method of lactic acid of the present invention uses methanol and ethylene glycol as reactants and employs an alkali to provide an alkaline environment for the reaction. Firstly, the entire production process does not involve the addition of toxic materials, solving the problem of the use of highly hazardous substances in existing chemical synthesis processes, making the process green and safe. Secondly, the entire production process does not require the addition of enzyme catalysts, resulting in a simple process flow, short production cycle, and low production cost. Thirdly, the product produced using the method of the present invention not only has a high concentration but is also easy to purify. Attached Figure Description

[0072] Figure 1 This is a schematic diagram of the continuous lactic acid production system of the present invention;

[0073] In the diagram, 1-Methanol feed pipe for reactants, 2-Ethylene glycol feed pipe, 3-Catalyst feed port, 4-Methanol feed pipe for solvent, 5-Solid alkali feed port, 6-Hydrogen pipeline, 7-Alkaline catalyst solution feed pipe, 8-Crude product discharge line, 9-Flash vapor phase discharge line, 10-Flash liquid phase discharge line, 11-Exhaust line, 12-Flash vapor phase condensate pipeline, 13-Washing waste liquid pipeline, 14-Washing water pipeline, 15-Resin regeneration waste liquid pipeline, 1 6-Resin regeneration liquid pipeline, 17-Nitrogen pipeline, 18-Post-ion exchange material pipeline, 19-Distillation column bottom discharge pipeline, 20-Lactic acid discharge pipeline, 21-Lactic acid finished product pipeline, 22-Distillation column top discharge pipeline, 23-Reflux pipeline, 24-Distillation column middle discharge pipeline, 25-Distillation column top condensate pipeline, 26-Vacuum line, 27-Middle section waste liquid pipeline, 28-Wastewater pipeline, 29-Purge exhaust gas pipeline, 30-Methanol recovery and reuse pipeline.

[0074] R101A - First alkaline catalyst solution preparation tank, R101B - Second alkaline catalyst solution preparation tank, P101 - Alkaline catalyst solution feed pump, R102 - Reactor, E102 - Hydrogen condenser, V201 - Methanol flash tank, E201 - Flash liquid phase cooler, P201 - Flash liquid phase discharge pump, E202 - Flash vapor phase condenser, V202 - Flash vapor phase condensate tank, P202 - Flash vapor phase condensate discharge pump, C301 - Ion exchange tank, P301 - [Missing information - likely a typo, should be "Ion exchange tank"]. Substrate exchange material discharge pump, T401-distillation column, E401-top condenser, V401-reflux tank, P401-reflux pump, E402-reboiler at the bottom of the column, P402-lactic acid and catalyst mixture discharge pump, F401-membrane filter, V403-lactic acid tank, P403-lactic acid discharge pump, V404-ethylene glycol tank, P404-ethylene glycol discharge pump, T501-molecular sieve column, E501-nitrogen heater, V501-methanol tank, P501-methanol discharge pump. Detailed Implementation

[0075] The present invention will now be described in further detail with reference to the accompanying drawings:

[0076] like Figure 1 As shown, this invention provides a system for continuous production of lactic acid, comprising a reactor R102, a methanol flash evaporator V201, an ion exchange tank C301, a distillation column T401, and a membrane filter F401 connected in sequence. The reactor R102 is provided with a reactant inlet, which is connected to a methanol inlet pipe 1, an ethylene glycol inlet pipe 2, and an alkaline catalyst solution inlet pipe 7. The reactant inlet is preferably located on the top surface of the reactor R102.

[0077] In a preferred embodiment of the present invention, a first alkaline catalyst solution preparation tank R101A and / or a second alkaline catalyst solution preparation tank R101B are provided upstream of the alkaline catalyst solution feed pipe 7. The alkaline catalyst solution feed pipe 7 is connected to both the first alkaline catalyst solution preparation tank R101A and the second alkaline catalyst solution preparation tank R101B. Preferably, the first alkaline catalyst solution preparation tank R101A and the second alkaline catalyst solution preparation tank R101B are provided upstream of the alkaline catalyst solution feed pipe 7. Since the reaction of the present invention is a continuous homogeneous reaction, the catalyst needs to be continuously replenished. Furthermore, since catalyst dissolution takes time, using two alkaline catalyst solution preparation tanks achieves continuous operation. More preferably, an alkaline catalyst solution feed pump P101 is provided on the alkaline catalyst solution feed pipe 7 to pump the alkaline catalyst solution in the alkaline catalyst solution feed pipe 7 into the reaction vessel R102.

[0078] Taking the first alkaline catalyst solution preparation tank R101A as an example, the first alkaline catalyst solution preparation tank R101A is provided with a catalyst feeding port 3, through which the catalyst is added to the first alkaline catalyst solution preparation tank R101A. The first alkaline catalyst solution preparation tank R101A is also provided with a solid alkali feeding port 5, through which a solid alkali is added to the first alkaline catalyst solution preparation tank R101A. Similarly, the second alkaline catalyst solution preparation tank R101B is also provided with a catalyst feeding port 3 and a solid alkali feeding port 5. Preferably, the solid alkali is potassium hydroxide.

[0079] Taking the first alkaline catalyst solution preparation tank R101A as an example, the first alkaline catalyst solution preparation tank R101A is equipped with a methanol inlet, which is connected to the solvent methanol inlet pipe 4, so that methanol is added into the first alkaline catalyst solution preparation tank R101A through the methanol inlet, thereby dissolving the catalyst and solid alkali to form an alkaline catalyst solution. Similarly, the second alkaline catalyst solution preparation tank R101B is also equipped with a methanol inlet and a solvent methanol inlet pipe 4.

[0080] The reactor R102 is provided with a top gas phase outlet, which is connected to the hydrogen pipeline 6 to transfer the hydrogen gas inside the reactor R102 out. Preferably, a hydrogen condenser E102 is provided on the hydrogen pipeline 6 to condense the hydrogen gas inside the hydrogen pipeline 6. The reactor R102 is provided with a bottom discharge port, which is connected to the crude reaction product discharge line 8 to transfer the crude reaction product inside the reactor R102 out.

[0081] The reactor R102 is equipped with a heating device. For example, the heating device can be a jacket, which is connected to the heat medium inlet pipeline and the heat medium outlet pipeline to achieve temperature regulation of the reactor R102.

[0082] The reactor R102 is connected to the methanol flash tank V201. Specifically, the crude product discharge line 8 is connected to the methanol flash tank V201. The methanol flash tank V201 is provided with a reaction product inlet, which is connected to the crude product discharge line 8. The crude product in the reactor R102 enters the methanol flash tank V201 via the crude product discharge line 8 and the reaction product inlet of the methanol flash tank V201.

[0083] The methanol flash evaporator V201 is equipped with a top vapor phase outlet, which is connected to the flash vapor phase discharge pipeline 9 to transfer the flash vapor phase product from the methanol flash evaporator V201. Preferably, a flash vapor phase condenser E202 is provided on the flash vapor phase discharge pipeline 9 to condense the flash vapor phase product in the flash vapor phase discharge pipeline 9 to obtain flash vapor phase condensate; the flash vapor phase condenser E202 is connected to the molecular sieve tower T501 so that the flash vapor phase condensate enters the molecular sieve tower T501 for dehydration, thereby recovering and reusing methanol.

[0084] In a preferred embodiment of the present invention, a flash vapor phase condensate tank V202 is provided between the flash vapor phase condenser E202 and the molecular sieve column T501. Specifically, in this embodiment, a flash vapor phase condensate tank V202 is provided below the flash vapor phase condenser E202. The flash vapor phase product is condensed in the flash vapor phase condenser E202 to obtain flash vapor phase condensate, which is then transferred to the flash vapor phase condensate tank V202. The flash vapor phase condensate tank V202 is connected to a flash vapor phase condensate pipeline 12. A flash vapor phase condensate discharge pump P202 is provided on the flash vapor phase condensate pipeline 12 to transfer the flash vapor phase condensate in the flash vapor phase condensate tank V202 out. Downstream of the flash vapor phase condensate pipeline 12, it is connected to the condensate pipeline 25 at the top of the distillation column, and then to the molecular sieve column T501, so that the flash vapor phase condensate in the flash vapor phase condensate tank V202 enters the molecular sieve column T501. The flash vapor phase condensate tank V202 is also connected to the exhaust pipeline 11. It should be noted that the flash vapor phase products cannot be completely condensed, therefore, the present invention sets up the exhaust pipeline 11 so that non-condensable gases such as hydrogen and nitrogen mixed in can be discharged and then directly sent to the tail gas treatment.

[0085] The methanol flash evaporator V201 is equipped with a bottom outlet, which is connected to the flash liquid phase discharge line 10 to transfer the flash liquid phase product from the methanol flash evaporator V201. A flash liquid phase discharge pump P201 is installed on the flash liquid phase discharge line 10 to provide power for the transfer of the flash liquid phase product from the methanol flash evaporator V201. A flash liquid phase cooler E201 is also installed on the flash liquid phase discharge line 10 to cool the flash liquid phase product within the flash liquid phase discharge line 10, thereby meeting the temperature requirements of the downstream ion exchange tank C301.

[0086] The methanol flash evaporator V201 is connected to the ion exchange tank C301. Specifically, the flash liquid phase discharge line 10 is connected to the ion exchange tank C301. The ion exchange tank C301 is provided with an inlet, which is connected to the flash liquid phase discharge line 10. The flash liquid phase product in the methanol flash evaporator V201 enters the ion exchange tank C301 through the flash liquid phase discharge line 10 and the inlet of the ion exchange tank C301. The ion exchange tank C301 contains an ion exchange resin, which is a hydrogen-form ion exchange resin. The flash liquid phase product undergoes ion exchange in the ion exchange tank C301 to obtain the ion-exchanged material; wherein, the lactic acid in the flash liquid phase product is converted to lactic acid. The ion-exchanged material includes lactic acid, methanol, ethylene glycol, water, and catalyst.

[0087] In a preferred embodiment of the present invention, the inlet of the ion exchange tank C301 is also connected to the cleaning waste liquid pipeline 13 and the resin regeneration liquid pipeline 16, respectively realizing the discharge of cleaning waste liquid and the feeding of resin regeneration liquid. The ion exchange tank C301 is also provided with an outlet, which is connected to the cleaning water pipeline 14 and the resin regeneration waste liquid pipeline 15, respectively realizing the feeding of cleaning water and the discharge of resin regeneration waste liquid. The outlet of the ion exchange tank C301 is also connected to the post-ion exchange material pipeline 18 to transfer the post-ion exchange material in the ion exchange tank C301 out. An ion exchange material discharge pump P301 is provided on the post-ion exchange material pipeline 18 to provide power for the transfer of post-ion exchange material in the ion exchange tank C301.

[0088] The ion exchange tank C301 is connected to the distillation column T401. Specifically, the post-ion exchange material pipeline 18 is connected to the distillation column T401. The distillation column T401 is provided with a post-ion exchange material inlet, which is connected to the post-ion exchange material pipeline 18. Preferably, the post-ion exchange material inlet is located on the side wall of the distillation column T401. The post-ion exchange material in the ion exchange tank C301 enters the distillation column T401 via the post-ion exchange material pipeline 18 and the post-ion exchange material inlet of the distillation column T401.

[0089] The distillation column T401 has a discharge port at the top, middle, and bottom of the column. These three discharge ports are connected to the top discharge pipeline 22, the middle discharge pipeline 24, and the bottom discharge pipeline 19 of the distillation column, respectively, to realize the collection of the gaseous product at the top of the column, the liquid product in the middle of the column (i.e., ethylene glycol waste liquid), and the liquid product in the bottom of the column (i.e., the mixture of lactic acid and catalyst).

[0090] In a preferred embodiment of the present invention, a top condenser E401 is provided on the top discharge line 22 of the distillation column to condense the top gaseous product and obtain a top gaseous product condensate. The top condenser E401 is connected to the molecular sieve column T501 so that the top gaseous product condensate enters the molecular sieve column T501 for dehydration to obtain methanol for recovery and reuse. More preferably, a reflux tank V401 is provided between the top condenser E401 and the molecular sieve column T501. Specifically, in this embodiment, a reflux tank V401 is also provided on the top discharge line 22 of the distillation column. The reflux tank V401 is connected to the molecular sieve column T501 through a distillation column top condensate line 25 so that the top gaseous product condensate condensed by the top condenser E401 enters the reflux tank V401 and is then transferred to the molecular sieve column T501 via the distillation column top condensate line 25.

[0091] In a preferred embodiment of the present invention, the reflux tank V401 is provided with a reflux liquid outlet, and the distillation column T401 is also provided with a reflux liquid inlet. A reflux pipeline 23 is provided between the reflux liquid inlet and the reflux liquid outlet of the reflux tank V401 to allow the condensate of the overhead vapor product in the reflux tank V401 to reflux into the distillation column T401. The production quality of the target product can then be controlled by adjusting the reflux ratio. A reflux pump P401 is provided on the reflux pipeline 23 to provide power for the reflux of the overhead vapor product condensate into the distillation column T401. More preferably, the reflux tank V401 is also connected to a vacuum line 26 to achieve vacuum treatment of the distillation column T401. It should be noted that the present invention preferably uses vacuum distillation, therefore vacuum treatment is required. The vacuum point of the distillation column T401 is located on the reflux tank V401, which is a conventional technique in the art and will not be described further here.

[0092] The continuous lactic acid production system of this invention, after flash evaporation of the reaction products, first undergoes ion exchange followed by distillation. This maximizes methanol retention during the ion exchange stage, ensuring the lactic acid-base flowability meets expectations. Ultimately, this benefits the ion exchange effect and reduces the overall system energy consumption, avoiding the problems of poor lactic acid-base flowability, high pipeline resistance, difficulty in selecting fluid transport equipment, and high energy consumption that occur with pre-distillation where large amounts of methanol are separated. Furthermore, the pre-ion exchange followed by distillation simplifies subsequent purification stages and reduces equipment investment. This is because if distillation is performed first, some methanol must be retained in the distillation column to prevent the lactic acid-base from automatically precipitating due to reduced methanol content, resulting in poor flowability. Poor flowability prevents the subsequent ion exchange stage from achieving the desired effect. Preliminary distillation followed by ion exchange converts the lactic acid-base into lactic acid. After conversion, lactic acid still contains some methanol; therefore, to improve the purity of the lactic acid product, a light-light-removal column is needed to remove the methanol used as a solvent. This invention first performs ion exchange and then distillation, requiring only one distillation column to remove methanol, simplifying subsequent purification processes and reducing equipment investment.

[0093] The distillation column T401 is connected to the molecular sieve column T501 via a reflux tank V401 and the distillation column top condensate line 25. Specifically, in this embodiment, the molecular sieve column T501 has a bottom feed inlet. The liquid phase in the distillation column T401 enters the molecular sieve column T501 via the distillation column top condensate line 25 and the bottom feed inlet of the molecular sieve column T501. The main components of the distillation column top condensate line 25 are methanol and water. It should be noted that, as mentioned above, "the downstream of the flash vapor phase condensate line 12 is connected to the distillation column top condensate line 25." Therefore, the methanol in the distillation column top condensate line 25 originates not only from the distillation column T401 but also from the flash vapor phase condensate tank V202.

[0094] The molecular sieve tower T501 is also equipped with a top feed inlet, which is connected to a dehydrated methanol pipeline. The dehydrated methanol pipeline is further connected to a methanol tank V501, preferably to the top inlet of the methanol tank V501, so that the dehydrated methanol obtained after dehydration by the molecular sieve tower T501 is transferred into the methanol tank V501. The bottom feed inlet of the molecular sieve tower T501 is connected to a wastewater pipeline 28, so that the water removed by the molecular sieve T501 is discharged. A wastewater shut-off valve is installed on the wastewater pipeline 28.

[0095] In a preferred embodiment of the present invention, the molecular sieve tower T501 is further provided with a top feed port, which is connected to the nitrogen pipeline 17; the bottom feed port of the molecular sieve tower T501 is also connected to the purge exhaust gas pipeline 29 to activate the molecular sieve tower T501. The purge exhaust gas pipeline 29 is equipped with purge exhaust gas shut-off valves. More preferably, the nitrogen pipeline 17 includes a first nitrogen pipeline and a second nitrogen pipeline, both of which are equipped with shut-off valves; a nitrogen heater E501 is provided on the second nitrogen pipeline to control the temperature of the nitrogen gas about to enter the molecular sieve tower T501.

[0096] The methanol tank V501 is also equipped with a bottom outlet, which is connected to the methanol recovery and reuse pipeline 30 and further connected to the reactant inlet of the reactor R102. Therefore, the molecular sieve tower T501 is equipped with a top feed port, which is indirectly connected to the reactor R102. A methanol discharge pump P501 is installed on the methanol recovery and reuse pipeline 30 to control whether the dehydrated methanol in the methanol tank V501 passes through the bottom outlet of the methanol tank V501, the methanol recovery and reuse pipeline 30, and the reactant inlet of the reactor R102, ultimately entering the reactor R102 to achieve methanol recovery and reuse.

[0097] Those skilled in the art will know that after the molecular sieve tower T501 has been running for a period of time, the molecular sieve inside it will reach adsorption saturation, at which point it needs to be activated. Specifically, in this embodiment, the activation method is as follows: first, nitrogen gas is introduced into the molecular sieve tower T501 through the first nitrogen pipeline to pressurize the residual waste liquid in the molecular sieve into the wastewater pipeline 28, which eventually goes to the wastewater treatment system, until the residual waste liquid in the molecular sieve is discharged; then, the shut-off valve of the first nitrogen pipeline is closed, and the shut-off valve of the second nitrogen pipeline and the nitrogen heater E501 are opened. Nitrogen gas reaching a preset temperature is introduced into the molecular sieve tower T501 through the second nitrogen pipeline for purging and drying. The purging exhaust gas is sent to the waste gas treatment system through the purging exhaust gas pipeline 29.

[0098] An ethylene glycol tank V404 is installed on the discharge line 24 of the distillation column to store the ethylene glycol waste liquid collected from the column, thereby realizing the recovery and reuse of ethylene glycol. An ethylene glycol discharge pump P404 is also installed on the discharge line 24 of the distillation column. The other end of the discharge line 24 is connected to the waste liquid line 27 in the middle section of the column to transfer the ethylene glycol waste liquid stored in the ethylene glycol tank V404 into the waste liquid line 27 for further processing.

[0099] A pump P402 for discharging a mixture of lactic acid and catalyst is installed on the bottom discharge line 19 of the distillation column. The other end of the bottom discharge line 19 is connected to a membrane filter F401 to transfer the target product and catalyst in the bottom discharge line 19 into the membrane filter F401 for separation. Preferably, the distillation column T401 is also equipped with a reboiler E402 to further improve the separation effect of the substances at the top, middle, and bottom of the column. The catalyst separated by the membrane filter can be recycled, specifically, it can be returned to the reactor after passing through an alkaline catalyst solution preparation tank.

[0100] In a preferred embodiment of the present invention, the membrane filter F401 contains a microporous membrane; more preferably, the microporous membrane is a nanofiltration membrane resistant to organic solvents; and / or, the molecular weight cutoff of the microporous membrane is 100–300 Da; and / or the processing capacity of the microporous membrane is 50–200 kg / h. The membrane filter F401 is provided with an outlet connected to a lactic acid discharge pipeline 20 to transfer lactic acid. The other end of the lactic acid discharge pipeline 20 is connected to a lactic acid tank V403 to store lactic acid. The lactic acid tank V403 is provided with a lactic acid discharge port connected to a lactic acid finished product pipeline 21. A lactic acid discharge pump P403 is provided on the lactic acid finished product pipeline 21 to provide power for transferring lactic acid from the lactic acid tank V403 to the lactic acid finished product pipeline 21.

[0101] This invention also provides a method for continuous production of lactic acid, which is implemented using the aforementioned system for continuous lactic acid production. Preferably, the method sequentially includes a reaction section, a flash evaporation section, an ion exchange section, a purification section, and a lactic acid acquisition and catalyst recovery section. More preferably, the method further includes a methanol recovery section and / or an ethylene glycol recovery section.

[0102] Specifically, the steps include the following:

[0103] (1) Reaction Section

[0104] 1) Feed the solvent into the alkaline catalyst solution preparation tank R101A, and add the solid catalyst and solid alkali into the alkaline catalyst solution preparation tank R101A containing the solvent to dissolve the solid catalyst and solid alkali and obtain the alkaline catalyst solution.

[0105] 2) Add methanol, ethylene glycol, and the alkaline catalyst solution from the preparation tank R101A to the reactor R102, and carry out the reaction to obtain the crude product. Preferably, the reaction temperature is 100-140℃, the reaction pressure is 0.8-1.1 MPa, and the reaction time is 4-8 h.

[0106] The gaseous product in reactor R102 is hydrogen, which enters hydrogen pipeline 6 and is cooled by hydrogen condenser E102 before being discharged. The liquid product in reactor R102 is the crude reaction product, which includes methanol (excess), ethylene glycol (unreacted), alkali hydroxide (excess), lactic acid (generated from the reaction), and catalyst. This crude reaction product is transferred to crude reaction product discharge line 8, and then enters methanol flash tank V201. It should be noted that methanol is in excess because it is not only a reactant but also a solvent; alkali hydroxide is in excess because it is not only a reactant but also a pH adjuster, as the entire reaction requires an alkaline environment; furthermore, since the reaction conversion rate will not be 100%, there will inevitably be unreacted ethylene glycol.

[0107] (2) Flash Evaporation Section:

[0108] The crude product from the reaction enters a methanol flash evaporator V201, where it undergoes adiabatic flash evaporation at atmospheric pressure to obtain a flash vapor phase product and a flash liquid phase product. The flash liquid phase product in the methanol flash evaporator V201 consists of methanol, ethylene glycol, alkali hydroxide, lactic acid, and a catalyst, and its temperature is approximately 50–80°C. The flash liquid phase product from the methanol flash evaporator V201 enters the flash liquid phase discharge line, and after being cooled (30–50°C) by the flash liquid phase cooler E201, it enters the ion exchange tank C301.

[0109] (3) Ion exchange section:

[0110] Considering that the methanol content may affect the fluidity of the lactic acid base (or lactic acid), ion exchange is performed first. In ion exchange tank C301, the flash-evaporated liquid phase product enters ion exchange tank C301 for cation exchange, yielding the ion-exchanged material (including methanol, ethylene glycol, water, lactic acid, and catalyst). Specifically, in this embodiment, the ion exchange resin is a hydrogen-form ion exchange resin, preferably an ion exchange resin with a functional group of sulfonic acid group (i.e., -SO3H hydrogen form).

[0111] After the exchange is completed, the ion exchange tank C301 can be rinsed with water, and the ion exchange resin in the ion exchange tank C301 can be regenerated with resin regeneration solution.

[0112] (4) Purification section:

[0113] The ion-exchange material enters distillation column T401 for distillation, yielding distilled gaseous products, ethylene glycol waste liquid, lactic acid, and a catalyst mixture. Preferably, the operating pressure at the top of distillation column T401 is 20–80 kPaA, the operating pressure at the bottom of distillation column T401 is 40–50 kPaA, the operating temperature at the top of distillation column T401 is 50–80°C, the operating temperature at the bottom of distillation column T401 is 190–230°C, the number of theoretical plates in distillation column T401 is 20–60, and the reflux ratio is 15–40. The lactic acid and catalyst mixture collected from the bottom of distillation column T401 enters membrane filter F401 via distillation column bottom outlet pipeline 19.

[0114] (5) Lactic acid acquisition and catalyst recovery section:

[0115] The mixture of lactic acid and catalyst enters membrane filter F401 for separation, yielding lactic acid and catalyst separately. The catalyst is retained within membrane filter F401 and can be recycled; the lactic acid is filtered out and then flows through lactic acid discharge pipeline 20 into lactic acid tank V403. Alternatively, it can be transferred to lactic acid finished product pipeline 21 by lactic acid discharge pump P403. Preferably, the membrane filter F401 contains a microporous membrane, preferably a nanofiltration membrane resistant to organic solvents. The microporous membrane in membrane filter F401 can filter based on molecular weight, with a preferred molecular weight cutoff of 100–300 Da and a preferred processing capacity of 50–200 kg / h.

[0116] In a preferred embodiment of the present invention, in the methanol recovery section, the flash vapor product of the methanol flash tank V201 is condensed into a flash vapor condensate, which is then dehydrated by the molecular sieve tower T501 and enters the reactor; the distillation vapor product of the distillation tower T401 is condensed into a distillation vapor condensate, which is then dehydrated by the molecular sieve tower T501 and enters the reactor.

[0117] Specifically, in this embodiment, the flash vapor products in the methanol flash tank V201 are methanol and water vapor. After exiting the methanol flash tank V201, the methanol and water vapor enter the flash vapor discharge line 9, and are then condensed into flash vapor condensate in the flash vapor condenser E202, ultimately entering the flash vapor condensate tank V202. The flash vapor condensate in the flash vapor condenser E202 is methanol and water, which enter the molecular sieve tower T501 via the distillation column top condensate line 25 and the bottom feed port of the molecular sieve tower T501. After dehydration in the molecular sieve tower T501, the dehydrated methanol enters the methanol tank V501 via the dehydrated methanol line and the feed port of the methanol tank V501. The dehydrated methanol in methanol tank V501 is discharged from the bottom outlet of methanol tank V501, methanol recovery and reuse pipeline 30, and reactant inlet of reactor R102, and finally enters reactor R102 to realize the recovery and reuse of methanol.

[0118] Specifically, in this embodiment, the distillation vapor product (an azeotrope of methanol and water) is collected from the top of distillation column T401. This vapor product then flows through the top condensate line 25 of the distillation column and the bottom feed port of the molecular sieve column T501, ultimately entering the molecular sieve column T501. After dehydration within the molecular sieve column T501, the dehydrated methanol flows through the dehydrated methanol line and the inlet of methanol tank V501, ultimately entering methanol tank V501. The dehydrated methanol in methanol tank V501 then flows through the bottom outlet of methanol tank V501, the methanol recovery and reuse line 30, and the reactant inlet of reactor R102, ultimately entering reactor R102, thus achieving methanol recovery and reuse.

[0119] In a preferred embodiment of the present invention, in the ethylene glycol recovery section, the ethylene glycol waste liquid enters the ethylene glycol tank V404. Specifically, ethylene glycol waste liquid is collected from the middle section of the distillation column T401, which enters the ethylene glycol tank V404 via the distillation column discharge pipeline 24, and is finally transferred to the waste liquid pipeline in the middle section of the column, thereby realizing the recovery and reuse of ethylene glycol.

[0120] In this invention, based on the system of this invention, the automated operation of the vacuum pump unit is achieved. Specifically, this is achieved by controlling the reaction pressure, reaction time, and timely removal of the generated gas phase (i.e., hydrogen gas) to improve the conversion rate and yield of lactic acid. The reaction of this invention is a volume-increasing reaction; in principle, the lower the pressure, the easier it is for the reaction to proceed in the forward direction. However, because the reaction temperature is relatively high, too low a pressure can lead to the vaporization of the raw materials. Therefore, by optimizing the reaction pressure and timely removing the generated gas phase, the reaction proceeds in the forward direction, overcoming the problem of low conversion rate and yield of lactic acid when potassium hydroxide is used as a solid alkali. Furthermore, using potassium hydroxide as a solid alkali only requires ion exchange of the potassium lactate produced in the reaction to obtain lactic acid, avoiding the problem of needing to add centrifugal filtration equipment to separate barium salt precipitate and lactic acid, and the limitation to intermittent production, as is required when using barium hydroxide as a solid alkali.

[0121] Example 1

[0122] An illustrative embodiment of the present invention, such as Figure 1 As shown, a process apparatus for preparing lactic acid from ethylene glycol and methanol includes:

[0123] (1) Reaction Section

[0124] 1) The solid catalyst and potassium hydroxide were added to an alkaline catalyst solution preparation tank R101A containing methanol to dissolve them, thereby obtaining a potassium hydroxide solution; wherein, the solid catalyst used in this embodiment is the nitrogen heterocyclic carbene iridium catalyst 3b prepared in Example 6 of CN110357770A.

[0125] 2) The reactor used is a continuous reactor R102. Methanol, ethylene glycol, and a prepared catalyst and potassium hydroxide solution are added to the reactor for reaction. The reaction temperature is 140℃ and the pressure is 1.1 MPa.

[0126] 3) The gaseous product of the reactor is hydrogen, and the liquid products are methanol (excess), ethylene glycol (unreacted), potassium hydroxide (excess), potassium lactate (generated from the reaction) and catalyst, which are the crude products of the reaction.

[0127] (2) Flash Evaporation Section:

[0128] The crude product from the reaction is discharged into flash tank V201 for adiabatic flash evaporation under atmospheric pressure. The flash vapor phase product (methanol and a small amount of ethylene glycol) is condensed and dehydrated before being returned to reactor R102 for reuse. The flash liquid phase product is still methanol, ethylene glycol, potassium hydroxide, potassium lactate, and catalyst. The temperature of the flash liquid phase product is 440℃, which is then cooled to 50℃.

[0129] (3) Ion exchange section:

[0130] Considering that the methanol content of the solvent may affect the flowability of potassium lactate (or lactic acid), ion exchange is performed first. The flash evaporation liquid phase product undergoes cation exchange through the ion exchange resin in ion exchange tank C301, yielding methanol, ethylene glycol, water, lactic acid, and catalyst, which are the ion-exchanged materials.

[0131] (4) Purification section:

[0132] After ion exchange, the material enters distillation column T401 for distillation. The operating pressure at the top of the distillation column is 30 kPaA; and / or, the operating pressure at the bottom of the distillation column is 50 kPaA; and / or, the operating temperature at the top of the distillation column is 60℃; and / or, the operating temperature at the bottom of the distillation column is 200℃; and / or, the number of theoretical plates in the distillation column is 30; and / or, the reflux ratio of the distillation column is 20.

[0133] 1) The distillation vapor product at the top of the column is an azeotrope of methanol and water. After being dehydrated by molecular sieve T501, the dehydrated methanol is sent back to the reactor for reuse, and the water is discharged as waste liquid.

[0134] 2) Ethylene glycol waste liquid collected from the middle section of the tower is treated and then returned to the reactor for reuse;

[0135] 3) A mixture of lactic acid and catalyst is extracted from the bottom section of the tower;

[0136] (5) Lactic acid acquisition and catalyst recovery section:

[0137] The mixture of lactic acid and catalyst in the bottom of the distillation column is filtered through membrane filter F401 to retain large molecular catalysts for recycling. The lactic acid passing through membrane filter F401 is then fed into lactic acid product tank V403.

[0138] A fine chemical production plant, using the lactic acid production method of this invention, obtained the following data:

[0139] Operation time: Continuous

[0140] Production capacity: 500t / a

[0141] Workshop staffing: 2 operators.

[0142] During operation, the process is simple, efficient, green, safe, highly automated, and has low labor costs.

[0143] This embodiment solves the problems of cumbersome operation process, easy operation error and low level of automation in the prior art, realizes the automated operation of vacuum pump unit and improves production efficiency.

[0144] Comparative Example 1

[0145] A fine chemical production plant, using traditional chemical production methods, obtained the following data:

[0146] Operation time: intermittent

[0147] Production capacity: 200t / a

[0148] Workshop staffing: 13 operators.

[0149] The operation process is as follows:

[0150] 1. Acetaldehyde and hydrogen cyanide were continuously added to the reactor. After the reaction was completed, the reaction solution was pumped into the hydrolysis vessel. The reaction conversion rate was 87.5%.

[0151] 2. Under acidic conditions, lactic acid hydrolyzes into crude lactic acid and ammonium bisulfate at a temperature of 90°C.

[0152] 3. Then, crude lactic acid is injected into an esterification reactor containing methanol using a pump. Crude methyl lactate is synthesized in the esterification reactor, and ammonium bisulfate is further separated.

[0153] 4. Crude methyl lactate is distilled, thermally decomposed, and concentrated to obtain refined lactic acid.

[0154] The operation requires multiple people, all wearing protective suits and protecting each other, making it quite cumbersome. Furthermore, the subsequent hazardous media treatment equipment also requires dedicated operators, making the process complex and prone to errors.

[0155]

[0156] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0157] In the description of this invention, unless otherwise stated, the terms "upper," "lower," "left," "right," "inner," "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0158] The above technical solution is only one embodiment of the present invention. For those skilled in the art, based on the principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the technical solutions described in the specific embodiments of the present invention. Therefore, the foregoing description is only a preferred option and is not restrictive.

Claims

1. A system for continuous production of lactic acid, characterized in that, The system includes a reaction vessel, a methanol flash tank, an ion exchange tank, a distillation column, and a membrane filter connected in sequence; wherein the reaction vessel is connected to a methanol feed pipe, an ethylene glycol feed pipe, and an alkaline catalyst solution feed pipe, respectively; the alkaline catalyst solution is a solution containing a catalyst and a solid alkali.

2. The system according to claim 1, characterized in that, The methanol flash tank inlet is connected to the bottom outlet of the reactor; and / or, the ion exchange tank inlet is connected to the bottom outlet of the methanol flash tank; and / or, the distillation column inlet is connected to the outlet of the ion exchange tank; and / or, the membrane filter inlet is connected to the bottom outlet of the distillation column; and / or, the reactor is equipped with a heating device; and / or, the reactor is equipped with a top gas phase outlet.

3. The system according to claim 1, characterized in that, The solvent for the alkaline catalyst solution is an alcohol; and / or, An alkaline catalyst solution preparation tank is provided upstream of the alkaline catalyst solution feed pipe; preferably, it includes one, or two or more alkaline catalyst solution preparation tanks connected in parallel; more preferably, it is a first alkaline catalyst solution preparation tank and / or a second alkaline catalyst solution preparation tank; and / or, An alkaline catalyst solution feed pump is provided on the alkaline catalyst solution feed pipeline; Preferred, The first alkaline catalyst solution preparation tank and / or the second alkaline catalyst solution preparation tank are both equipped with a catalyst feed port, a solid alkali feed port, and a methanol feed port; More preferably, the solid base is potassium hydroxide; and / or, The alcohol is methanol; The catalyst is at least one of nitrogen heterocyclic carbene metal coordination compounds.

4. The system according to claim 1, characterized in that, The top vapor outlet of the methanol flash tank is connected to the flash vapor discharge pipeline; and / or The bottom outlet of the methanol flash evaporator is connected to the inlet of the ion exchange tank via a flash liquid phase outlet pipeline. Preferred, A flash vapor phase condenser is installed on the flash vapor phase discharge line, and the flash vapor phase condenser is connected to the molecular sieve tower line; and / or A flash liquid phase cooler is provided on the flash liquid phase discharge line; and / or a flash liquid phase discharge pump is provided on the flash liquid phase discharge line. More preferably, A flash vapor phase condensate tank is provided between the flash vapor phase condenser and the molecular sieve tower.

5. The system according to claim 1, characterized in that, The ion exchange tank is equipped with an ion exchange resin, which is a hydrogen-form ion exchange resin; and / or... The outlet of the ion exchange tank is connected to the distillation column via a post-ion exchange material pipeline; a post-ion exchange material discharge pump is provided on the post-ion exchange material pipeline. Preferred, The inlet of the ion exchange tank is connected to the cleaning waste liquid pipeline and the resin regeneration liquid pipeline, respectively; and / or, The outlet of the ion exchange tank is connected to the cleaning water pipeline and the resin regeneration waste liquid pipeline, respectively.

6. The system according to claim 1, characterized in that, The distillation column is provided with a top outlet, which is connected to the top outlet pipeline of the distillation column; and / or, the distillation column is provided with a middle outlet, which is connected to an ethylene glycol tank. Preferably, a top condenser is provided at the top discharge pipeline connection of the distillation column, and the top condenser is connected to the molecular sieve column; More preferably, a reflux tank is provided between the top condenser and the molecular sieve column; and / or, The membrane filter contains a microporous membrane; The best option, The microporous membrane is a nanofiltration membrane resistant to organic solvents; and / or, The microporous membrane has a molecular weight cutoff of 100–300 Da; and / or, The microporous membrane has a processing capacity of 50–200 kg / h.

7. The system according to claim 4 or 6, characterized in that, The molecular sieve tower is provided with a top feed inlet, which is connected to the reaction vessel; and / or The bottom feed inlet of the molecular sieve tower is connected to the wastewater pipeline; Preferred, A methanol tank is provided between the molecular sieve tower and the reactor; the methanol tank and the reactor are connected by a methanol recovery and reuse pipeline; a methanol discharge pump is provided on the methanol recovery and reuse pipeline; More preferably, The feed inlet at the top of the molecular sieve tower is connected to a nitrogen pipeline; and / or The bottom feed inlet of the molecular sieve tower is connected to the purging exhaust gas pipeline; The best option, The nitrogen pipeline includes a first nitrogen pipeline and a second nitrogen pipeline; a nitrogen heater is provided on the second nitrogen pipeline.

8. A method for continuous production of lactic acid, characterized in that, The process is carried out using the continuous lactic acid production system according to any one of claims 1 to 7; Preferably, the method sequentially includes a reaction section, a flash evaporation section, an ion exchange section, a purification section, and a lactic acid acquisition and catalyst recovery section; More preferably, the method further includes a methanol recovery section and / or an ethylene glycol recovery section.

9. The method according to claim 8, characterized in that, The reaction section includes: reacting methanol and ethylene glycol in the presence of a catalyst, preferably including feeding the catalyst and solid base into a solvent to obtain an alkaline catalyst solution; adding methanol, ethylene glycol, and the alkaline catalyst solution into a reaction vessel to react and obtain a crude reaction product; and / or The flash evaporation section includes: the crude reaction product entering a methanol flash tank for adiabatic flash evaporation at atmospheric pressure to obtain a flash vapor phase product and a flash liquid phase product; and / or The ion exchange section includes: the flash evaporation liquid phase product entering an ion exchange tank for cation exchange to obtain the ion-exchanged material; and / or The purification section includes: the ion-exchange material enters a distillation column for distillation to obtain distillation gas products, ethylene glycol waste liquid, lactic acid and catalyst mixture; The lactic acid acquisition and catalyst recovery section includes: the mixture of lactic acid and catalyst enters a membrane filter for separation, and lactic acid and catalyst are obtained separately; Preferred, The methanol recovery section includes condensing the flash vapor phase product into a flash vapor phase condensate, dehydrating it through a molecular sieve tower, and then recycling the resulting methanol into the reactor; and / or, it includes condensing the distillation vapor phase product into a distillation vapor phase condensate, dehydrating it through a molecular sieve tower, and then recycling the resulting methanol into the reactor. The ethylene glycol recovery section includes feeding the ethylene glycol waste liquid obtained from the purification section into an ethylene glycol tank.

10. The method according to claim 8, characterized in that, In the reaction section, the reaction temperature is 100–140℃; and / or, the reaction pressure is 0.8–1.1 MPa; and / or, the reaction time is 4–8 h; and / or In the ion exchange section, the ion exchange resin is a hydrogen-form ion exchange resin; and / or In the purification section, the operating pressure at the top of the distillation column is 20–80 kPaA; and / or, the operating pressure at the bottom of the distillation column is 40–50 kPaA; and / or, the operating temperature at the top of the distillation column is 50–80 °C; and / or, the operating temperature at the bottom of the distillation column is 190–230 °C; and / or, the number of theoretical plates in the distillation column is 20–60; and / or, the reflux ratio in the distillation column is 15–40; and / or In the lactic acid acquisition and catalyst recovery section, the membrane filter contains a microporous membrane; preferably, the microporous membrane is a nanofiltration membrane resistant to organic solvents; and / or, the molecular weight cutoff of the microporous membrane is 100-300 Da. And / or, the processing capacity of the microporous membrane is 50-200 kg / h.

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