Process for synthesizing methane chloride from PVA byproduct methyl acetate

Abstract

The invention discloses a process for synthesizing methane chloride from PVA (polyvinyl alcohol) byproduct methyl acetate. The process comprises a catalytic reactor, a first rectifying tower, a second rectifying tower and the like. A PVA byproduct methyl acetate is used as a raw material, a methyl acetate hydrolysis endothermic reaction and an exothermic reaction for synthesizing methane chloride from methanol and hydrogen chloride gas are simultaneously completed in a reactor, a hydrolysis reaction product methanol and a synthesis reaction product water are mutually reaction raw materials, the consumption of the reaction products promotes chemical equilibrium to be carried out in a positive direction, and the reaction degree is more thorough. The problem that azeotrope formed by methyl acetate hydrogenation and methanol is difficult to separate can be avoided. The method not only has the characteristics of novelty, simple process, easiness in separation of reaction products, energy conservation, consumption reduction and the like, but also provides a new method and way for deep development and application of the PVA byproduct methyl acetate.

Description

Technical Field

[0001] The invention belongs to the field of in-depth development and application of PVA by-product methyl acetate. Background Art

[0002] Polyvinyl alcohol (PVA) is an important polymer material. A large amount of methyl acetate is produced as a by-product during the production process, which is about 1.7 times the weight of PVA. Its composition is generally 17% methanol and 83% methyl acetate. After refining, methyl acetate with a purity of 99.99% can be obtained. Due to its small industrial use, it is generally refined into high-purity methyl acetate before in-depth development and application. From the current literature, there are two main aspects: one is to hydrolyze the by-product methyl acetate of PVA into methanol and acetic acid, and recycle it as PVA raw materials. Since the current PVA production process mostly uses petroleum product vinyl acetate (VAC) as raw material to produce PVA, the development significance of this approach no longer exists; the second is to directly use methyl acetate as raw material to synthesize methanol and ethanol through catalytic hydrogenation. There are many domestic patents and papers in this regard. The patents include "A method for preparing methanol and ethanol by hydrogenating methyl acetate" invented by Li Meizhou et al. (CN 103288594 A, 2013); "A process and system for hydrogenating crude methyl acetate produced as a by-product in the production of polyvinyl alcohol" invented by Wang Xunwen et al. (CN103570492 A, 2014-02); "A method and device for separating a mixed solution of methyl acetate-methanol-ethanol-benzene-water by combining double extraction distillation towers" invented by Gu Zhenggui et al. (CN 103641680 A, 2014-03); "A method for separating ethanol from methyl acetate and co-production of methanol" invented by Cao Jun et al. (CN 105367385 A, 2016). Representative papers on this approach include Yao Ji's master's thesis "Research on the reaction and process simulation of methyl acetate hydrogenation to ethanol". Although the products of this pathway, methanol and ethanol, have wide industrial application value, there are many constant boiling systems in the reaction products of the process, such as methyl acetate-methanol, ethanol-water, etc. This not only makes it difficult to separate the mixture, but also a large amount of methanol and methyl ester circulates in the reaction system, and energy consumption is also a problem worthy of attention.

[0003] Methyl chloride (CH 3Cl) is an organic compound, and there are four main synthesis processes. The methanol method is the main method for producing chloromethane at home and abroad. Its process principle is to react methanol with hydrogen chloride gas under the action of a catalyst to produce chloromethane and water. This method has the advantages of easy raw materials, simple operation, and mild reaction conditions, but it also has problems such as easy catalyst deactivation and equipment corrosion. The acetic acid method is a new method for producing chloromethane. Its process principle is to react acetic acid with hydrogen chloride gas under the action of a catalyst to produce chloromethane and acetic anhydride. This method has the advantages of easy raw materials, mild reaction conditions, and high catalyst activity, and is one of the current research hotspots. It is not difficult to see from the synthesis route of chloromethane that if the two raw materials of methanol and acetic acid are used, the economic efficiency of producing chloromethane is too poor, and it is urgent to find a process for producing chloromethane using by-products.

[0004] As the mainstream monochloromethane production process, the most critical technical indicator of the methanol method is the molar ratio of hydrogen chloride gas to methanol, which generally needs to reach 1.1~1.25. If the ratio is too low, a large amount of by-products will be produced, reducing the methanol yield and affecting product quality. Unreacted methanol will enter the by-product hydrochloric acid, which is very troublesome to handle. If the ratio is too high, the water generated by the synthesis reaction of unreacted hydrogen chloride gas and monochloromethane will form a large amount of hydrochloric acid, and the subsequent refining of monochloromethane still needs to add water to remove it, resulting in a large amount of hydrochloric acid produced and a low concentration. Summary of the invention

[0005] The present invention aims to provide a process for synthesizing methyl chloride from methyl acetate produced as a byproduct of PVA. Figure 1 .

[0006] The technical solution of the present invention includes two aspects: (1) The process flow involved in the present invention comprises: the methyl acetate solution from the PVA device is gasified and heated in a methyl acetate preheater, the hydrogen chloride gas from the hydrogen chloride synthesis furnace of the chlor-alkali device and the circulating hydrogen chloride from the top condenser of the second distillation tower are heated in a hydrogen chloride preheater, the two enter the gas phase mixer through the discharge pipes of their respective preheaters to be evenly mixed, and enter the catalytic reactor filled with zinc chloride aqueous solution through the discharge pipe of the mixer to react. The gas mixture generated by the reaction enters the compressor through the discharge pipe of the reactor for pressurization, and the pressurized gas enters the flash tank through the discharge pipe of the compressor for flash evaporation, the gas phase of the flash tank is added from the upper part of the first distillation tower through the gas phase feeding pipe of the first distillation tower, and the liquid phase of the flash tank is added from the lower part of the first distillation tower through the liquid phase feeding pipe of the first distillation tower. The mixed solution of methyl chloride, water, a trace amount of hydrogen chloride and acetic acid is condensed from the first distillation tower condenser at the top of the first distillation tower, and is used as the liquid phase feed of the second distillation tower through the feed pipe of the second distillation tower. The uncondensed hydrogen chloride gas from the first distillation tower condenser at the top of the tower is returned through the hydrogen chloride reflux pipe and incorporated into the hydrogen chloride feed pipe, where it is further preheated to participate in the reaction and used as circulating hydrogen chloride. The high-concentration acetic acid solution obtained from the bottom of the first distillation tower can be comprehensively recycled and treated, and is sent to the reboiler of the second distillation tower through the discharge pipe of the first distillation tower bottom for indirect heat exchange to achieve comprehensive energy utilization. Methyl chloride, trace hydrogen chloride and water are directly extracted from the gas phase from the top of the second distillation tower and enter subsequent conventional refining treatment. Trace acetic acid aqueous solution is separated from the bottom of the second distillation tower for recycling and treatment. The process flow of producing methyl chloride by catalysis of zinc chloride aqueous solution using PVA as a byproduct is shown in the attached Figure 1 As shown; (2) The process parameters involved in the present invention include: zinc chloride aqueous solution, zinc chloride mass proportion of 60~90%. Raw material mass ratio HCl: MeAc: MeOH = 49~62: 80~99.99: 0.01~20, feed molar ratio HCl: MeOH=1:1; reaction mass ratio HCl: MeAc: MeOH = 75~95: 80~99.99: 0.01~20, reaction molar ratio HCl: MeOH=1.3~1.5: 1. Reaction temperature is 135~145℃, reaction pressure is 0.3~0.4MPa. The top temperature of the first distillation tower is 14~15℃, the pressure is 0.89~0.91MPa, the bottom temperature is 195~200℃, and the pressure is 0.90~0.91MPa. The number of theoretical plates is 35~40, the liquid phase feed plate is at the 25th~28th plate, the gas phase feed plate is at the 9th~11th plate, and the reflux ratio is 1.1~1.3. The top temperature of the second distillation tower is 9.5~10℃, the pressure is 0.09~0.11MPa, the bottom temperature is 120~125℃, and the pressure is 0.2~0.25MPa. The number of theoretical plates is 35~40, the feed plate is at the 28th~32nd theoretical plate, and the reflux ratio is 1.1~1.3.

[0007] The chemical principle involved in the present invention: (1) The by-product methyl acetate of PVA undergoes hydrolysis in an acidic solution to generate acetic acid and methanol:

[0008] (2) The hydrolysis product methanol continues to react with hydrogen chloride in the zinc chloride aqueous solution to produce methyl chloride and water:

[0009] The beneficial effects of the present invention are as follows: (1) The present invention uses the liquid phase catalytic reaction of PVA by-product methyl acetate and hydrogen chloride to produce monochloromethane, which can not only completely hydrolyze methyl acetate, but also generate methanol that can completely react with hydrogen chloride. No azeotropic products will appear in the entire process, and there is no need to separate methyl acetate and methanol. There is no high temperature and high pressure of hydrogenation reaction and large flow of hydrogen mixed gas circulation, and the expansion and accumulation of by-products are also eliminated. Because the two reaction products are reacted quickly, the two reaction equilibria proceed in the positive direction, the reaction efficiency is higher, and the reaction degree is higher. Because the water generated by methanol and hydrogen chloride is consumed by the hydrolysis reaction, the content of the catalyst zinc chloride aqueous solution is relatively constant, which further stabilizes the reaction; (2) The present invention conducts the hydrolysis of methyl acetate and the synthesis of methyl chloride at the same time and space. Under the conditions of gas phase feeding and gas phase discharging, the overall reaction is a slightly endothermic reaction. The heat of the exothermic reaction of the synthesis of methyl chloride in the catalytic reactor is directly supplied to the endothermic hydrolysis of methyl acetate, which is superior to indirect heat exchange with the outside, reduces the equipment for heating and heat removal, and reduces the energy consumption of frequent heat exchange. (3) The present invention adopts two-tower distillation. The first distillation tower uses pressurization, with a high temperature of 195-200°C at the bottom of the tower and a low temperature of 14-15°C at the top of the tower. Most of the water and acetic acid are separated, and the first distillation tower kettle obtains high-concentration acetic acid with a concentration greater than 85%. It can continue to synthesize monochloromethane through the acetic acid method, and at the same time produce acetic anhydride, a high value-added product; the temperature of the second distillation tower kettle is controlled at 120-124°C to obtain an azeotrope of acetic acid and water, and the trace acetic acid in the monochloromethane mixed gas is carried by water, thereby ensuring that the gas phase extracted from the top of the second distillation tower does not contain acetic acid. The high-temperature acetic acid solution from the first distillation tower kettle is used as the heat source of the reboiler of the atmospheric pressure second distillation tower, so that the work and heat of the pressurization operation can be reused; (4) The present invention adopts a hydrogen chloride circulation reaction method. The uncondensed hydrogen chloride gas in the first distillation tower fractionator can be returned to the reactor. Under the condition of a feed molar ratio of HCl: MeOH = 1: 1, a reaction molar ratio of HCl: MeOH = 1.3-1.5: 1 is achieved, which strongly inhibits the side reaction of methanol and hydrogen chloride synthesis, improves the purity of the monochloromethane product, and reduces the generation of dilute hydrochloric acid. The hydrogen chloride at the top of the first distillation tower is insoluble in acetic acid and monochloromethane, has a boiling point of -85°C, and can be circulated through its own pressure difference. The increase in the circulation amount of hydrogen chloride also improves the water removal efficiency of the first distillation tower and reduces the water entrainment of the gas phase at the top of the tower; (5) The present invention adopts flash evaporation operation to separate the outlet material of the pressurized catalytic reactor into gas phase material and liquid phase material, which enter the pressurized first distillation tower respectively, thereby reducing the energy consumption of the first distillation tower; (6) The water produced in the methyl chloride synthesis of the present invention is consumed by the hydrolysis reaction of methyl acetate. The water content in the methyl chloride product is low, and the entrained hydrogen chloride is less, which is also convenient for the circulation reaction of hydrogen chloride. The amount of by-product hydrochloric acid generated in the quencher is small, which reduces the difficulty of handling the by-product. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 The invention discloses a process flow chart for synthesizing monochloromethane from methyl acetate as a byproduct of PVA, wherein the accompanying drawings are marked as follows: 1-hydrogen chloride preheater, 2-methyl acetate preheater, 3-gas phase mixer, 4-catalytic reactor, 5-compressor, 6-flash tank, 7-first distillation tower, 8-first distillation tower decondenser, 9-second distillation tower, 10-second distillation tower condenser, 11-first distillation tower reboiler, 12-second distillation tower reboiler, 13-hydrogen chloride feed pipe, 14-methyl acetate feed pipe, 15-hydrogen chloride preheater discharge pipe, 16-methyl acetate preheater discharge pipe, 17-gas phase mixer discharge pipe, 18-reactor discharge pipe, 19-compressor The invention relates to a compressor discharge pipe, 20-a first distillation tower gas phase feed pipe, 21-a first distillation tower liquid phase feed pipe, 22-a first distillation tower top gas phase pipe, 23-a second distillation tower feed pipe, 24-a second distillation tower top gas phase pipe, 25-a second distillation tower gas phase extraction pipe, 26-a first distillation tower kettle discharge pipe, 27-a second distillation tower kettle discharge pipe, 28-a hydrogen chloride reflux pipe, 29-a raw material methyl acetate flow meter, 30-a raw material hydrogen chloride flow meter, 31-a circulating hydrogen chloride flow meter, 32-a raw material methyl acetate regulating valve, 33-a raw material hydrogen chloride regulating valve, 34-a circulating hydrogen chloride regulating valve, 35-a second distillation tower pressure reducing valve, and 36-a flow automatic control system. DETAILED DESCRIPTION

[0011] The technical solution and implementation methods of the present invention are described below by way of examples, but the present invention is not limited to the following examples.

[0012] Example 1 According to the embodiment of the present invention, the feed rate of hydrogen chloride (HCl) is 55.2 kg / h and the feed rate of methyl acetate is 100 kg / h. Figure 1 The process is as follows: the circulating hydrogen chloride regulating valve is closed to control the circulating hydrogen chloride gas flow rate to 0, and the feed and reaction molar ratio HCl:CH 3 OH=0.91. The corresponding products and their compositions are shown in Table 1.

[0013] Table 1 Raw materials, products and their composition in Example 1

[0014] Example 1 In the molar ratio of HCl:CH 3 When OH=0.91 does not reach 1.0~1.1, a large amount of unreacted methanol remains, and by-products such as acetone and dimethyl ether are produced in large quantities. In a complex system, there is even methyl chloride in the discharge from the bottom of the second distillation tower.

[0015] Example 2 According to the embodiment of the present invention, the feed rate of hydrogen chloride (HCl) is 68.2 kg / h and the feed rate of methyl acetate is 100 kg / h. Figure 1 The process is as follows: the circulating hydrogen chloride regulating valve is closed to control the circulating hydrogen chloride gas flow rate to 0, and the feed and reaction molar ratio HCl:CH 3 OH=1.13. The corresponding products and their compositions are shown in Table 2.

[0016] Table 2 Raw materials, products and their composition in Example 2

[0017] In Example 2, when the molar ratio reaches the normal process control index, methanol is completely reacted and no by-products are produced, which fully demonstrates that the hydrolysis and synthesis reactions of the present invention simultaneously suppress side reactions and achieve the separation of acetic acid. The only drawback is that the methyl chloride product extracted from the top gas phase of the second distillation tower contains a large amount of water and hydrogen chloride gas, which increases the difficulty of subsequent treatment. This is also the pain point of the existing methanol method for synthesizing methyl chloride.

[0018] Example 3 According to the embodiment of the present invention, the feed rate of hydrogen chloride (HCl) is 60.3 kg / h and the feed rate of methyl acetate is 100 kg / h. Figure 1 The process uses the flow automatic control system to control the circulating hydrogen chloride regulating valve to control the circulating hydrogen chloride gas flow rate to 30.5 kg / h, and the feed molar ratio of HCl:CH 3 OH=1.0, and the reaction molar ratio HCl:CH 3OH = 1.5. The corresponding products and their compositions are shown in Table 3.

[0019] Table 3 Raw materials, products and their composition in Example 3

[0020] It can be seen that the water removal efficiency of the first distillation tower is significantly increased after the increase of hydrogen chloride gas, and most of the water and acetic acid generated by the reaction are taken away from the discharge pipe of the first distillation tower bottom. The water removal load of the second distillation tower is greatly reduced, and there is only 0.53 kg of aqueous solution containing trace acetic acid in the bottom of the tower; relying on the difference in boiling points of methyl chloride and hydrogen chloride under high pressure conditions, the circulation of hydrogen chloride is realized through the dephlegmator of the first distillation tower, so that the water and hydrogen chloride gas in the gas phase output of the second distillation tower are rapidly reduced, reducing the difficulty of subsequent refining treatment of methyl chloride.

Claims

1. A process for synthesizing methyl chloride from PVA by-product methyl acetate, characterized in that: (1) A production equipment for the process of synthesizing chloromethane from methyl acetate as a byproduct of PVA is composed of a catalytic reactor, a compressor, a first distillation tower, and a second distillation tower; The methyl acetate solution from the PVA device is gasified and heated in the methyl acetate preheater, and the hydrogen chloride gas from the hydrogen chloride synthesis furnace of the chlor-alkali device and the circulating hydrogen chloride from the top condenser of the second distillation tower are heated in the hydrogen chloride preheater together, and the two enter the gas phase mixer through the discharge pipes of their respective preheaters to be evenly mixed, and enter the catalytic reactor filled with zinc chloride aqueous solution through the discharge pipe of the mixer to react; the gas mixture generated by the reaction enters the compressor through the discharge pipe of the reactor for pressurization, and the pressurized gas enters the flash tank for flash evaporation through the discharge pipe of the compressor, and the gas phase of the flash tank is added from the upper part of the first distillation tower through the gas phase feeding pipe of the first distillation tower, and the liquid phase of the flash tank is added from the lower part of the first distillation tower through the liquid phase feeding pipe of the first distillation tower; The mixed solution of methyl chloride, water, trace amounts of hydrogen chloride and acetic acid is condensed in the first distillation tower fractional condenser at the top, and is used as the liquid phase feed of the second distillation tower through the feed pipe of the second distillation tower; the uncondensed hydrogen chloride gas in the first distillation tower fractional condenser at the top of the tower is returned through the hydrogen chloride reflux pipe and is then incorporated into the hydrogen chloride feed pipe and then continues to be preheated to participate in the reaction and is used as circulating hydrogen chloride; the high-concentration acetic acid solution obtained from the bottom of the first distillation tower can be comprehensively recycled and processed, and is sent to the reboiler of the second distillation tower through the discharge pipe of the first distillation tower bottom for indirect heat exchange to realize comprehensive utilization of energy; methyl chloride, trace amounts of hydrogen chloride and water are directly extracted from the gas phase at the top of the second distillation tower and enter the subsequent conventional refining treatment; a trace amount of acetic acid aqueous solution is separated from the bottom of the second distillation tower for recycling and processing; (2) The catalyst in the catalytic reactor is a zinc chloride aqueous solution, wherein the mass ratio of zinc chloride is 60-90%. The raw materials are added through a hydrogen chloride feed pipe and a methyl acetate feed pipe according to a raw material mass ratio of HCl: MeAc: MeOH = 49-62: 80-99.99: 0.01-20, and a flow rate automatic control system is used to ensure that the feed molar ratio of HCl: MeOH is 1:

1. The circulating hydrogen chloride from the fractional condenser of the second distillation tower is introduced into the hydrogen chloride feed pipe through a hydrogen chloride reflux pipe and mixed to form a reaction mass ratio of HCl: MeAc: MeOH = 75-95: 80-99.99: 0.01-20, and a flow rate automatic control system is used to ensure that the reaction molar ratio of HCl: MeOH is 1.3-1.5:

1. After the hydrogen chloride gas is preheated and the methyl acetate solution is gasified and heated, the reaction temperature of the catalytic reactor is maintained at 135-145°C and the reaction pressure is 0.3-0.4MPa. The mass ratio of the discharge of the catalytic reactor is HCl: H2O: HAc: CH3Cl = 13-33: 0.01-11: 65-81: 68-86. The hydrolysis of methyl acetate is an endothermic reaction, and the synthesis of methyl chloride is an exothermic reaction. Under the conditions of gas phase feeding and gas phase discharging, the reaction system is slightly endothermic. The reactor is provided with a steam heating jacket. When the reactor is started or the reaction temperature is less than 135°C, steam is introduced into the jacket for heating. (3) The first distillation tower is used to preliminarily separate acetic acid and water in the catalytic reactor discharge by pressurized distillation; the outlet gas of the catalytic reactor is pressurized to 0.89-0.91 MPa by a compressor, and the liquid phase and gas phase separated by the flash tank are respectively added to the first distillation tower; the top temperature of the first distillation tower is 14-15°C, the pressure is 0.89-0.91 MPa, the bottom temperature is 195-200°C, and the pressure is 0.90-0.91 MPa. Under the conditions of high temperature and high pressure, acetic acid and water form an azeotrope and are separated from other low-boiling substances, and high-concentration acetic acid with a content of >85% is obtained in the bottom of the tower for comprehensive utilization; the first distillation tower has 35-40 theoretical plates, the liquid phase feed plate is at the 25th to 28th plate, the gas phase feed plate is at the 9th to 11th plate, the reflux ratio is 1.1-1.3, and the mass ratio of the gas phase at the top of the tower is HCl:H2O:HAc:CH3Cl = 13-33: 0.001~2.5 : 0.005~0.015 : 68~86, the uncondensed hydrogen chloride gas in the first distillation tower fractional condenser is merged into the hydrogen chloride feed pipe through the hydrogen chloride reflux pipe, the mass ratio of the liquid phase condensed and extracted from the first distillation tower fractional condenser is HCl : H2O : HAc : CH3Cl = 0~0.1 : 0.001~2.5 : 0.005~0.015 : 68~86, the mass ratio of the first distillation tower bottom discharge is H2O : HAc = 0.01~15 : 85~99.99, the concentration of acetic acid in the bottom is determined by the methanol content in the methyl acetate feed, and the lower the methanol content, the higher the acetic acid concentration; (4) The second distillation tower is used to separate the remaining water and trace acetic acid in the liquid phase extracted from the top of the first distillation tower by distillation. The temperature at the top of the second distillation tower is 9.5-10°C, the pressure is 0.09-0.11 MPa, the temperature at the bottom of the tower is 120-125°C, and the pressure is 0.2-0.25 MPa. Under this temperature and pressure condition, acetic acid and methyl chloride product are completely separated, and the aqueous solution containing trace acetic acid in the bottom of the tower is recovered and treated. The number of theoretical plates at the top of the second distillation tower is 35-40, the feed plate is located at the 28th-32nd theoretical plate, the reflux ratio is 1.1-1.3, the gas phase is directly extracted from the top of the tower for subsequent refining treatment, the mass ratio of the gas phase is HCl:H2O:CH3Cl = 0.05-0.1: 0.05-0.1: 99.8-99.9, and the mass ratio of the bottom of the second distillation tower is H2O: HAc=99.5~99.95: 0.05~0.

5.

2. A process for synthesizing chloroform from methyl acetate as a by-product of PVA according to claim 1, characterized in that: Before starting, first add hydrogen chloride gas separately through the hydrogen chloride feed pipe, pass through the circulating hydrogen chloride regulating valve, hydrogen chloride preheater, gas phase mixer, catalytic reactor, compressor, flash evaporator, first distillation tower, first distillation tower decondenser, and finally return to the hydrogen chloride preheater to form a cycle, at which time the second distillation tower feed pressure reducing valve is in a closed state; during starting, the methyl acetate feed mass flow rate is set through the methyl acetate feed regulating valve, and the flow automatic control system automatically controls the mass flow rate of raw hydrogen chloride gas added through the hydrogen chloride feed regulating valve according to the feed molar ratio of HCl: MeOH=1:1, and the raw hydrogen chloride gas mass flow rate is automatically controlled by the circulating hydrogen chloride according to the reaction molar ratio of HCl: MeOH=1.3~1.5:1 through the circulating hydrogen chloride; MeOH in the ratio is the sum of methanol in the feed methyl acetate solution and methanol generated after hydrolysis of methyl acetate in the feed methyl acetate solution, and the raw material feed molar ratio of HCl: MeOH=1: 1 ensures that both hydrogen chloride and methanol react completely, and after the hydrogen chloride is circulated, the reaction molar ratio of HCl: MeOH=1.3~1.5: 1 ensures that hydrogen chloride is always in excess during the reaction.

3. The process for synthesizing chloroform from methyl acetate as a byproduct of PVA according to claim 1, characterized in that: The automatic control system is based on the established components and molecular weight of methyl acetate solution, and converts the feed and reaction molar ratios into mass ratios, thereby controlling the raw hydrogen chloride gas mass flow rate and the circulating hydrogen chloride gas mass flow rate through the regulating valve. The mass flow rate of methyl acetate solution is manually given and also determines the production load. In order to ensure that the feed molar ratio HCl: MeOH=1:1 is accurate, the composition of the raw methyl acetate solution should remain relatively stable, and the hydrogen chloride circulation can play a buffering role to a certain extent. If the hydrogen chloride feed continues to be excessive, the amount of by-product hydrochloric acid will increase during the subsequent monochloromethane refining.

4. A process for synthesizing methyl chloride from methyl acetate as a by-product of PVA according to claim 1, characterized in that: The top pressure of the first distillation tower is controlled by a compressor to be 0.89~0.91MPa. Under this pressure, the boiling point of methyl monochloroethane is 45°C, the boiling point of hydrogen chloride is -32°C, and the top temperature of the first distillation tower is 14~15°C. Therefore, the cooling medium of the first distillation tower condenser must be lower than 15°C, so it is easy to obtain liquid methyl monochloroethane, thereby separating the hydrogen chloride; the separation of hydrogen chloride gas and methyl chloride liquid at the top of the first distillation tower needs to be achieved under anhydrous conditions. The bottom pressure of the first distillation tower is 0.90~0.91MPa, and the bottom temperature is 195~200°C. By utilizing the azeotrope formed by acetic acid and water, acetic acid acts as a water-carrying agent to carry out most of the water generated by the synthesis reaction in the catalytic reactor from the bottom of the first distillation tower.

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