Tandem reaction system for generating methyl methacrylate through gas phase condensation and method for synthesizing methyl methacrylate

Abstract

The invention provides a tandem reaction system for generating methyl methacrylate through gas phase condensation and a method for synthesizing methyl methacrylate, and relates to the technical field of organic synthesis. According to the method, a gas phase condensation reaction is carried out through a multi-stage cascade reaction system under proper process conditions, so that methyl methacrylate is synthesized, and operation, switching and regeneration of a new reaction system and an old reaction system are periodically carried out according to corresponding rules, so that the fluctuation of the content of the product methyl methacrylate (MMA) in each reaction period process is favorably reduced, and the yield of methyl methacrylate is improved. The MMA content is kept basically stable, and stable raw material conditions are provided for further rectification and purification of MMA in a subsequent separation section; in conclusion, the technical problem that the MMA content in the product fluctuates greatly is solved, and the technical effects of effectively reducing the MMA content fluctuation, improving the operation stability of the subsequent separation and rectification section and prolonging the single activity period of the catalyst are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of organic synthesis, in particular to a series reaction system for generating methyl methacrylate by gas phase condensation and a method for synthesizing methyl methacrylate. Background Art

[0002] Methyl methacrylate (MMA) is an important organic chemical raw material, mainly used in the production of organic glass (PMMA), polyvinyl chloride additives (ACR) and methyl methacrylate-styrene-butadiene copolymer (MBS). It can also be used as resin, adhesive, coating, ion exchange resin, paper polish, textile printing and dyeing auxiliaries, leather treatment agent, lubricant additive, crude oil depressant, impregnant for wood and cork, impregnant for motor coil, insulation potting material and plasticizer for plastic emulsion, etc. It has a wide range of uses.

[0003] The main production process routes of MMA products are: ethylene process (C2 process) for C2 raw material route, acetone cyanohydrin process (ACH process) for C3 raw material route and isobutylene process (C4 process) for C4 raw material route. At present, the ACH process is mostly used in the industry, and the devices using this process technology account for 60.6% of the total MMA production capacity. European, American and domestic enterprises mainly use this process route; C4 process is the second largest process route in the industry, and the devices using this process technology account for 31.7% of the total MMA production capacity, which is concentrated in Japan and parts of Asia; C2 process accounts for 7.7% of the total MMA production capacity, and the proportion is gradually increasing.

[0004] The traditional ACH method is mature and stable in operation, but the reaction process requires highly toxic and corrosive raw materials such as hydrocyanic acid and concentrated sulfuric acid, which will produce a large amount of ammonium bisulfate by-products, posing great pressure on environmental protection and safety. The C4 method is an oxidation and esterification process using high-purity isobutylene as raw material. The raw material source is relatively abundant and there are many sets of equipment in operation abroad. The technology is relatively reliable and domestic technology has achieved breakthroughs, but it requires high-purity isobutylene as raw material, and there are many by-products in the oxidation section, so the selectivity is not high, the process flow is long and the overall investment of the device is high. The C2 method includes the propionaldehyde route and the methyl propionate route. The process flow of the propionaldehyde route is very long, requires four steps of reaction, and the production cost is relatively high; the process of the methyl propionate route can obtain MMA through only two steps of reaction. The first step is to synthesize methyl propionate through carbonylation of ethylene, carbon monoxide and methanol, and the second step is to generate MMA through aldol condensation reaction of methyl propionate and formaldehyde. The raw materials of the methyl propionate route are widely available, cheap, easy to obtain, highly safe, have an extremely short process flow, a green and environmentally friendly production process, and high product selectivity. It is recognized as the most economical and promising process route.

[0005] The main technical difficulty of the methyl propionate route lies in the second step reaction, that is, the aldol condensation reaction of methyl propionate and formaldehyde. This reaction is generally carried out in a fixed bed with an alkaline oxide as a catalyst and requires a high reaction temperature. Since the aldol condensation reaction produces water, the water will undergo a hydrolysis reaction with the methyl propionate source to generate propionic acid, which is adsorbed on the alkaline active center of the catalyst, causing the catalyst to be chemically deactivated and the activity to continue to decrease; at the same time, there may be a decarbonization reaction and polycondensation reaction of formaldehyde in the reactor, and the carbon deposits or macromolecular organic matter produced will adhere to the catalyst bed, which will also cause the catalytic activity to continue to decrease. Under the influence of the above factors, the service life of the catalyst in the second step reaction is generally short, and it needs to face periodic repeated burning to restore the catalytic activity, which is a large workload; the most disadvantageous thing is that for a single reactor, as the reaction time increases, the MMA content in the product continues to decrease, and the unstable MMA composition brings great difficulties to the further distillation and purification in the subsequent separation section.

[0006] Prior art CN111574369 A and CN212532806 U provide a method for preparing methyl methacrylate by aldol condensation of methyl propionate and formaldehyde and a multi-stage reaction system. The method attempts to reduce the variation range of the reaction product composition at the initial and final stages of the reaction by connecting multiple parallel reactors. The method adopts time-sharing and segmented opening of each stage of the reactor. After the activity of the first stage reactor decreases to a certain level, the second stage reactor is opened to compensate for the decrease in the MMA content in the product. Although this method has a certain effect on the stability of the MMA content in the product compared to a single reactor, there is still a problem of large fluctuations in the MMA content.

[0007] Prior art CN212549490 U provides an integrated reactor, which includes three relatively independent catalyst beds and has a compact and reasonable structure. However, the catalyst beds are still substantially connected in parallel, and there is still a problem of large fluctuations in the MMA content in the product.

[0008] Prior art CN102775302 B provides a method for preparing methyl methacrylate from methyl propionate and formaldehyde. The method effectively solves the problem of rapid catalyst deactivation by coupling a fluidized bed reactor and a catalyst regenerator, achieves production continuity, and reduces the volatility of MMA product concentration. However, the fluidized bed reactor used in the method itself has many disadvantages, such as high strength requirements for gas-phase condensation catalysts, and active components are easily pulverized and lost during the continuous circulation process, resulting in poor economic efficiency.

[0009] Prior art CN109232247 A provides a method for preparing methyl methacrylate by aldol condensation of methyl acetate, methyl propionate and formaldehyde. The method adopts a fluidized bed reactor, utilizes the characteristics of the inner circulation movement of the bed material in the fluidized state, introduces a small amount of oxygen to form a local oxygen-containing zone in the lower part of the fluidized bed to periodically regenerate the catalytic bed material, and can avoid serious side reactions in the presence of a large amount of oxygen and insufficient and uneven regeneration of the fluidized bed material, realizes catalyst regeneration in the aldol condensation, maintains the long-term high activity of the catalyst, and solves the problem that the catalyst is easily deactivated by carbon deposition and has a short lifespan; however, the method adopts a fluidized bed reactor, has very high requirements on the strength of the gas phase condensation catalyst, and the catalyst is seriously damaged and pulverized, resulting in a high production cost.

[0010] In view of this, the present invention is proposed. Summary of the invention

[0011] One of the purposes of the present invention is to provide a series reaction system for producing methyl methacrylate by gas phase condensation, which is beneficial to maintaining the basic stability of MMA content and improving the single activity cycle of the catalyst.

[0012] The second object of the present invention is to provide a method for synthesizing methyl methacrylate, which solves the technical problem of large fluctuations in the MMA content in the product, and achieves the technical effects of effectively reducing the volatility of the MMA content, improving the operational stability of the subsequent separation and distillation sections, and increasing the single activity cycle of the catalyst.

[0013] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0014] In a first aspect, a series reaction system for producing methyl methacrylate by gas phase condensation comprises n series reactors, wherein n≥3;

[0015] The series reaction system is provided with a raw gas main pipe for distributing the raw gas to each level of reactors for gas phase condensation reaction;

[0016] The raw material feed pipelines of each stage of the reactor are independently provided with a polymerization inhibitor injection system for injecting the polymerization inhibitor into the reactor;

[0017] The series reaction system is provided with a reaction product main pipe, and the reaction product outlets of the reactors at each stage are connected to the feed inlet of the next reactor and to the reaction product main pipe;

[0018] The series reactors for gas phase condensation reaction in the series reaction system are i-stage, wherein 2≤i<n, and there is at least one reactor in a regeneration or standby state.

[0019] Furthermore, the series reaction system is also provided with a regeneration gas main pipe for distributing the regeneration gas to each level of reactors for in-situ regeneration of the catalyst.

[0020] Furthermore, the series reaction system is also provided with a regeneration tail gas main pipe for collecting the regeneration tail gas from each stage of the reactor.

[0021] Further, the reactor comprises a vertical cylindrical adiabatic fixed bed type reactor;

[0022] The adiabatic fixed bed type reactor comprises, from top to bottom, a feed distribution area, a reaction isothermal area and a discharge collection area.

[0023] Furthermore, the feed distribution area is provided with an inlet distributor, a distribution tray, a support beam and a support grid, and an inert filler is filled between the support grid and the distribution tray.

[0024] Furthermore, the reaction isothermal zone is filled with a gas phase condensation reaction catalyst.

[0025] Furthermore, the discharge collection area is provided with an outlet distribution tray, a support beam, a support grid and an outlet collector, and an inert filler is filled between the support grid and the outlet distribution tray.

[0026] In a second aspect, a method for synthesizing methyl methacrylate using the tandem reaction system described in any one of the above items comprises the following steps:

[0027] A raw material gas mixed with a formaldehyde source and a methyl propionate source is fed into the series reaction system, and a polymerization inhibitor is injected into the reactor to make the raw material gas undergo a gas phase condensation reaction in the series reaction system to obtain methyl methacrylate.

[0028] Furthermore, the method comprises the following steps:

[0029] Feeding the first raw gas into the first stage reactor for gas phase condensation reaction, feeding the gas phase condensation product obtained from the first stage reactor and the second raw gas into the second stage reactor for gas phase condensation reaction, feeding the gas phase condensation product obtained from the second stage reactor and the third raw gas into the third stage reactor for gas phase condensation reaction, and so on, feeding the gas phase condensation product obtained from the j stage reactor and the j+1th raw gas into the j+1th stage reactor for gas phase condensation reaction, wherein 1≤j<i, until the gas phase condensation product obtained from the last stage reactor enters the reaction product main pipe to obtain methyl methacrylate;

[0030] Preferably, the pressure condition of the gas phase condensation reaction is 0.05MPa-1.0MPa, the temperature condition is 300℃-400℃, and the volume space velocity is 0.1h -1~10h -1 ;

[0031] Preferably, the temperature and pressure of the first-stage reactor are not lower than those of the second-stage reactor, the temperature and pressure of the second-stage reactor are not lower than those of the third-stage reactor, and the temperature and pressure of the j-th stage reactor are not lower than those of the j+1-th stage reactor;

[0032] Preferably, the temperature of the j-th reactor is not lower than that of the j+1-th reactor and ranges from 5°C to 60°C;

[0033] Preferably, the pressure of the j-th reactor is not lower than that of the j+1-th reactor and ranges from 0.01 MPa to 1.0 MPa.

[0034] Furthermore, the method further comprises the step of periodically switching the new and old reaction systems;

[0035] The periodic switching method comprises the following steps:

[0036] During the operation of the i-stage series reactor, when the periodic switching standard of the new and old reaction systems is reached, the first-stage reactor is removed from the original reaction system and regenerated in situ, the second-stage reactor in the original reaction system is used as the first-stage reactor in the new reaction system, the third-stage reactor in the original reaction system is used as the second-stage reactor in the new reaction system, and so on, the j-th stage reactor in the original reaction system is used as the j-1-th stage reactor in the new reaction system, where 2≤j≤i, and the next-stage reactor of the i-th stage reactor in the original reaction system is used as the i-th stage reactor in the new reaction system;

[0037] Preferably, the periodic switching standard of the new and old reaction systems is that the methyl methacrylate content at the reaction product main pipe is reduced to 80% to 85% of the initial value, and formaldehyde begins to appear in the sample;

[0038] Preferably, the in situ regeneration method comprises the following steps:

[0039] The deactivated catalyst is regenerated in situ using a mixture of nitrogen and oxygen as the regeneration gas;

[0040] Preferably, the pressure condition of the in-situ regeneration is 0-1.0 MPa, the temperature condition is 300°C-600°C, and the volume space velocity is 100h -1 ~3000h -1 .

[0041] Compared with the prior art, the present invention has at least the following beneficial effects:

[0042] The series reaction system for generating methyl methacrylate by gas phase condensation provided by the present invention is a reaction system composed of n-stage reactors (n≥3) connected in series, wherein the series reactors participating in the condensation reaction are i-stage (2≤i<n), and at least one reactor is in a regeneration or standby state; the series reaction system uses a methyl propionate source and a formaldehyde source to perform gas phase condensation, i.e., aldol condensation, to generate methyl methacrylate (MMA); in a normal production process, the fresh raw gas in the raw gas main pipe can enter each stage of the reactor according to a certain flow distribution ratio, and gas phase condensation occurs in each stage of the reactor. The condensation reaction generates MMA, and the gas phase condensation product of each reactor can be mixed with the fresh raw gas of the next reactor as the feed of the next reactor, and the gas phase condensation product of the last reactor can enter the reaction product main pipe and flow into the next distillation section; at the same time, the new and old reaction systems can be periodically operated, switched and regenerated according to the analysis results of regular sampling at the reaction product main pipe. For example, when it is found that the MMA content is reduced to 80% to 85% of the initial value and trace formaldehyde begins to appear in the sample, the new and old reaction systems can be switched and regenerated. In short, the reaction system of the present invention cooperates with the corresponding operation mode, and when the alkaline oxide is used as the catalyst for the gas phase condensation reaction, it can effectively ensure the basic stability of the MMA content during the entire reaction cycle, create good raw material conditions for further distillation and purification of MMA in the subsequent separation section, and can effectively improve the stability of the operation of the separation and distillation section, and at the same time, can increase the single activity cycle of the catalyst to about 240h.

[0043] The method for synthesizing methyl methacrylate provided by the present invention solves the technical problem of large fluctuation of MMA content in the product, and achieves the technical effects of effectively reducing the volatility of MMA content, improving the operational stability of subsequent separation and distillation sections, and increasing the single activity cycle of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0045] Figure 1 A schematic diagram of a multi-stage series reaction system provided for one embodiment of the present invention.

[0046] Icons: 1-first raw material tank; 2-second raw material tank; 3-preheating furnace; 4-raw material gas main pipe; 5-inhibitor; 6-first reactor; 7-second reactor; 8-third reactor; 9-i-th reactor; 10-n-th reactor; 11-reaction product main pipe; 12-regeneration tail gas main pipe; 13-regeneration gas main pipe. DETAILED DESCRIPTION

[0047] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] According to a first aspect of the present invention, there is provided a series reaction system for producing methyl methacrylate by gas phase condensation, comprising n series reactors, wherein n≥3;

[0049] The series reaction system is provided with a raw gas main pipe for distributing the raw gas to each level of reactors for gas phase condensation reaction;

[0050] At the same time, the raw material feed pipelines of each level of reactor are independently provided with an inhibitor injection system for injecting the inhibitor into the reactor;

[0051] The series reaction system is provided with a reaction product main pipe, and the reaction product outlet of each stage of the reactor is connected to the feed inlet of the next stage of the reactor and the reaction product main pipe;

[0052] The series reactors for gas phase condensation reaction in the series reaction system are i-stage, 2≤i<n, and at least one reactor is in regeneration or standby state.

[0053] In the present invention, a multi-stage series reaction system utilizes a methyl propionate source and a formaldehyde source to perform gas phase condensation, i.e., aldol condensation, to generate methyl methacrylate (MMA). In a normal production process, fresh raw gas in a raw gas main pipe can enter each stage of the reactor according to a certain flow distribution ratio, and a gas phase condensation reaction occurs in each stage of the reactor to generate MMA. The gas phase condensation product of each stage of the reactor can be mixed with the fresh raw gas of the next stage of the reactor as the feed of the next stage of the reactor. The gas phase condensation product of the last stage of the reactor can enter the reaction product main pipe and can flow into the next distillation section. At the same time, the new and old reaction systems can be periodically operated, switched, and regenerated according to the analysis results of regular sampling at the reaction product main pipe. For example, when it is found that the MMA content is reduced to 80%-85% of the initial value and a trace amount of formaldehyde begins to appear in the sample, the new and old reaction systems can be switched and regenerated.

[0054] In summary, when alkaline oxides are used as gas-phase condensation reaction catalysts, the present invention can effectively ensure the basic stability of the MMA content during the entire reaction cycle, create good raw material conditions for further distillation and purification of MMA in the subsequent separation section, effectively improve the stability of the separation and distillation section operation, and at the same time increase the single activity cycle of the catalyst to about 240 hours.

[0055] In the present invention, a facility system for injecting an inhibitor solution may be provided on the raw material pipeline before entering each stage of the reactor; wherein the inhibitor may be one or more of phenols, quinones, ethers, aromatic amines and nitro compounds, or other compounds capable of inhibiting polymerization, and may be further preferably at least one of phenols, quinones and aromatic amines, and further preferably phenols; the content of the inhibitor added to the reaction system may be controlled within the range of 0.001wt% to 1wt%, and may be further preferably within the range of 0.005wt% to 0.5wt%, and further preferably within the range of 0.01wt% to 0.1wt%.

[0056] In a preferred embodiment, a regeneration gas main pipe may be arranged in the multi-stage series reaction system of the present invention to distribute the regeneration gas to each stage of the reactor for in-situ regeneration of the catalyst; at the same time, a regeneration exhaust gas main pipe may be arranged in the multi-stage series reaction system to collect the regeneration exhaust gas from each stage of the reactor.

[0057] In a multi-stage series reaction system, see Figure 1 , the solutions in the first raw material tank 1 and the second raw material tank 2 can be pumped into the preheating furnace 3, and after being heated in the preheating furnace, they are mixed together and enter the reaction system, and the inhibitor 5 is injected into the reaction system; each stage of the reaction system can lead out its own raw gas feed pipeline system from the raw gas main pipe 4, and each stage of the reactor can lead out its own regeneration gas pipeline system from the regeneration gas main pipe 13, and the regeneration tail gas of each stage of the reactor can enter the regeneration tail gas main pipe 12, and each stage of the reactor has a corresponding next stage reactor, and the next stage reactor of the numbered 1 reactor (the first reactor) is the numbered 2 reactor (the second reactor), and the next stage reactor of the numbered 2 reactor is the numbered 3 reactor (the third reactor), and so on, the next stage reactor of the numbered n-1 reactor is the numbered n reactor (the nth reactor), and the next stage reactor of the numbered n reactor is the numbered 1 reactor; at the same time, the reaction product outlet of each stage of the reactor can be connected to two process flows: one is connected to the feed port of the next stage reactor, and the other is connected to the reaction product main pipe 11.

[0058] In the present invention, a main valve and a main flow meter may be provided on the raw gas main pipe; a heater, a valve, a nitrogen flow regulating valve and a flow meter, an oxygen flow regulating valve and a flow meter may be provided on the regeneration gas main pipe to facilitate temperature and flow control during regeneration of the deactivated catalyst; in addition, a separate shut-off valve, a flow regulating valve and a flow meter may be provided on the fresh raw gas feed pipeline of each stage of the reactor to facilitate separate control and metering of the fresh raw gas entering each stage of the reactor.

[0059] In a preferred embodiment, each reactor in the multi-stage series reaction system can be a vertical cylindrical adiabatic fixed bed type reactor with the same structure. The reactor is divided into three areas, the top can be a feed distribution area, the middle can be a reaction isothermal area, and the bottom can be a discharge collection area.

[0060] In a preferred embodiment, the feed distribution area may be provided with an inlet distributor, a distribution tray, a support beam and a support grid, and inert ceramic balls or other fillers may be filled between the support grid and the distribution tray.

[0061] In the present invention, the reaction isothermal zone may be filled with a gas phase condensation reaction catalyst.

[0062] In a preferred embodiment, the discharge collection area may be provided with an outlet distribution tray, a support beam, a support grid and an outlet collector, and inert ceramic balls or other fillers may be filled between the support grid and the outlet distribution tray.

[0063] According to a second aspect of the present invention, there is provided a method for synthesizing methyl methacrylate using any of the above-mentioned tandem reaction systems, comprising the following steps:

[0064] A raw material gas mixed with a formaldehyde source and a methyl propionate source is fed into a series reaction system, and a polymerization inhibitor is injected into the reactor to make the raw material gas undergo a gas phase condensation reaction in the series reaction system to obtain methyl methacrylate.

[0065] The method for synthesizing methyl methacrylate provided by the present invention solves the technical problem of large fluctuation of MMA content in the product, and achieves the technical effects of effectively reducing the volatility of MMA content, improving the operational stability of subsequent separation and distillation sections, and increasing the single activity cycle of the catalyst.

[0066] In the present invention, the formaldehyde source may be one or more of formaldehyde, trioxymethylene, polyoxymethylene, methylal, polyoxymethylene and other formaldehyde sources that can depolymerize or react to generate formaldehyde. It may be further preferred that trioxymethylene is supplied in situ as the formaldehyde raw material for gas phase condensation after depolymerization reaction. It may be further preferred that the anhydrous formaldehyde is prepared in situ by oxidative dehydrogenation of methanol.

[0067] Among them, the solvent for dissolving the formaldehyde source can be one or more of alcohols, alkanes, benzene rings and other solvents that can dissolve the formaldehyde source, and can be further preferably at least one of methanol, ethanol, propanol, butanol and 1,3-propylene glycol, and is further preferably methanol; the molar ratio of the formaldehyde source to the solvent can be 1:2 to 20, and can be further preferably 1:3 to 10, and is further preferably 1:4 to 6.

[0068] In the present invention, the methyl propionate source can be one or more of methyl propionate, a mixture of methyl propionate and an alcohol solvent, a mixture of methyl propionate and an alkane solvent, a mixture of methyl propionate and a benzene ring-containing solvent, and a mixture of methyl propionate and other solvents that do not participate in the gas phase condensation reaction. It can be further preferably a mixture of methyl propionate and an alcohol, and further preferably high-purity methyl propionate (purity not less than 99.9wt%).

[0069] The feed molar ratio of the methyl propionate source to the formaldehyde source may be 0.1 to 10:1, more preferably 0.5 to 6:1, and even more preferably 1 to 3:1.

[0070] In a preferred embodiment, the method for synthesizing methyl methacrylate of the present invention comprises the following steps:

[0071] The first raw gas is fed into the first stage reactor for gas phase condensation reaction, the gas phase condensation product obtained from the first stage reactor is fed together with the second raw gas into the second stage reactor for gas phase condensation reaction, the gas phase condensation product obtained from the second stage reactor is fed together with the third raw gas into the third stage reactor for gas phase condensation reaction, and so on, the gas phase condensation product obtained from the j stage reactor is fed together with the j+1th raw gas into the j+1th stage reactor for gas phase condensation reaction, wherein 1≤j<i, until the gas phase condensation product obtained from the last stage reactor enters the reaction product main pipe to obtain methyl methacrylate.

[0072] In a normal production process, a multi-stage series reaction system has i-stage series reactors (2≤i<n) for gas-phase condensation reaction to synthesize methyl methacrylate, and each stage of the reactor is in a series working state; the metered fresh raw gas enters from the reactor numbered j as the feed of the first stage reactor; the gas-phase condensation product of the reactor numbered j is mixed with the fresh raw gas separately metered in the next stage reactor (numbered j+1, if j+1 is greater than n, the reactor number here automatically becomes 1, the same below) and serves as the feed of the reactor numbered j+1, that is, the feed of the second stage reactor; the gas-phase condensation product of the reactor numbered j+1 is mixed with the fresh raw gas separately metered in the next stage reactor (numbered j+2) and serves as the feed of the reactor numbered j+2, that is, the feed of the third stage reactor; and so on, the multi-stage series reaction system consisting of reactors numbered j, j+1, j+2... performs gas-phase condensation reaction to synthesize methyl methacrylate, and the gas-phase condensation product of the last stage reactor enters the reaction product main pipe.

[0073] In the present invention, the process conditions of the gas phase condensation reaction are as follows: the reaction pressure may be 0.05 MPa to 1.0 MPa, preferably 0.1 MPa to 0.5 MPa, and more preferably 0.2 MPa to 0.4 MPa; the reaction temperature may be 300° C. to 400° C., preferably 320° C. to 380° C., and more preferably 330° C. to 360° C.; the volume space velocity may be 0.1 h -1 ~10h -1 , which may be further preferably 0.5h -1 ~8h -1 , more preferably 1h -1 ~4h -1 .

[0074] During the production operation, the temperature and pressure of each reactor of the multi-stage series reaction system have the following control relationship: the first-stage reactor is not lower than the second-stage reactor, the second-stage reactor is not lower than the third-stage reactor, and the j-th reactor is not lower than the j+1-th reactor (where 1≤j<i<n);

[0075] The temperature range of the j-th reactor not lower than that of the j+1-th reactor (where 1≤j<i<n) may be 0-60°C, more preferably 5°C-30°C, and even more preferably 8°C-15°C;

[0076] The pressure range of the j-th reactor which is not lower than that of the j+1-th reactor (wherein 1≤j<i<n) may be 0.01MPa~1.0MPa, and may be further preferably 0.03MPa~0.5MPa, and further preferably 0.05MPa~0.1MPa.

[0077] In a preferred embodiment, the method for synthesizing methyl methacrylate of the present invention further comprises the step of periodically switching the new and old reaction systems.

[0078] In the present invention, the method for periodic switching includes the following steps:

[0079] During the operation of the i-stage series reactor, when the periodic switching standard between the old and new reaction systems is reached, the first-stage reactor is removed from the original reaction system and regenerated in situ, and the second-stage reactor in the original reaction system is used as the first-stage reactor in the new reaction system, and the third-stage reactor in the original reaction system is used as the second-stage reactor in the new reaction system, and so on. The j-th reactor in the original reaction system is used as the j-1-th reactor in the new reaction system, where 2≤j≤i, and the next-stage reactor of the i-th reactor in the original reaction system is used as the i-th reactor in the new reaction system.

[0080] Specifically, the operation and switching of the new and old reaction systems in the multi-stage series reaction system can be based on the following rules: during the operation of the i-stage series reactor, when the switching standard of the new and old reaction systems is reached, since the first-stage reactor has the largest fresh feed gas feed amount and the highest reaction temperature, its activity decrease is the largest among all the i operating reactors. Therefore, it is necessary to remove the first-stage reactor from the original reaction system and regenerate it in situ, and use the second-stage reactor of the original reaction system as the first-stage reactor of the new reaction system, and adjust it according to the process conditions such as the temperature, pressure and raw material amount of the first-stage reactor of the original reaction system; The third-stage reactor of the original reaction system is used as the second-stage reactor of the new reaction system, and is adjusted according to the process conditions such as temperature, pressure and raw material quantity of the second-stage reactor of the original reaction system; by analogy, the j-th reactor of the original reaction system is used as the j-1-th reactor (2≤j≤i) of the new reaction system, and is adjusted according to the process conditions such as temperature, pressure and raw material quantity of the j-1-th reactor of the original reaction system; the next-stage reactor of the i-th reactor of the original reaction system is used as the i-th reactor of the new reaction system, and is controlled and adjusted according to the process conditions such as temperature, pressure and raw material quantity of the i-th reactor of the original reaction system.

[0081] In a preferred embodiment, the switching standard between the old and new reaction systems is that when regular sampling and analysis are performed at the reaction product main pipe, it is found that the methyl methacrylate content drops to 80%-85% of the initial value, and a trace amount of formaldehyde begins to appear in the sample.

[0082] In the present invention, when the catalyst is regenerated in situ, a certain ratio of nitrogen and oxygen can be used as regeneration gas to regenerate the deactivated catalyst in situ.

[0083] The process conditions for in-situ regeneration of the catalyst are as follows: the regeneration pressure may be 0-1.0 MPa, preferably 0.02 MPa-0.5 MPa, and more preferably 0.05 MPa-0.1 MPa; the regeneration temperature may be 300°C-600°C, preferably 360°C-500°C, and more preferably 400°C-460°C; the volume space velocity may be 100 h -1 ~3000h -1 , which may be further preferably 300h -1 ~2000h -1 , more preferably 500h -1 ~1000h -1 .

[0084] The present invention is further described below by way of examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or directly purchased from the market.

[0085] Example 1

[0086] A series reaction system for producing methyl methacrylate by gas phase condensation, see Figure 1 , comprising n series reactors, wherein n ≥ 3;

[0087] The series reaction system is provided with a raw gas main pipe 4, and each stage of the reactor leads out its own raw gas feed pipeline system from the raw gas main pipe 4, so as to distribute the raw gas to each stage of the reactor for gas phase condensation reaction;

[0088] At the same time, the raw material feed pipelines of each level of reactor are independently provided with an inhibitor injection system for injecting the inhibitor into the reactor;

[0089] The series reaction system is provided with a regeneration gas main pipe 13, and each stage of the reactor leads out its own regeneration gas pipeline system from the regeneration gas main pipe 13, so as to distribute the regeneration gas to each stage of the reactor for in-situ regeneration of the catalyst;

[0090] In addition, the series reaction system is provided with a regeneration tail gas main pipe 12 for collecting the regeneration tail gas from each stage of the reactor;

[0091] Each reactor has a corresponding next reactor. The next reactor of reactor No. 1 (first reactor) is reactor No. 2 (second reactor), the next reactor of reactor No. 2 is reactor No. 3 (third reactor), and so on. The next reactor of reactor No. n-1 is reactor No. n (nth reactor), and the next reactor of reactor No. n is reactor No. 1.

[0092] The series reaction system is provided with a reaction product main pipe 11, and the reaction product outlet of each stage of the reactor is connected to the feed inlet of the next stage of the reactor and the reaction product main pipe 11;

[0093] Among them, each level of reactor is a vertical cylindrical adiabatic fixed bed type reactor with the same structure, which is divided into three areas: the top is the feed distribution area, the middle is the reaction isothermal area, and the bottom is the discharge collection area;

[0094] An inlet distributor, a distribution tray, a support beam and a support grid are provided in the feed distribution area, and an inert filler is filled between the support grid and the distribution tray;

[0095] A gas phase condensation reaction catalyst is loaded in the reaction isothermal zone;

[0096] An outlet distribution tray, a support beam, a support grid and an outlet collector are provided in the discharge collection area, and an inert filler is filled between the support grid and the outlet distribution tray;

[0097] In this embodiment, the series reactors for performing gas phase condensation reaction in the series reaction system are i-stage, wherein 2≤i<n, and there is at least one reactor in a regeneration or standby state.

[0098] A method for synthesizing methyl methacrylate (MMA) using the above-mentioned tandem reaction system comprises the following steps:

[0099] [1] Weigh 2.5 kg of trioxymethylene and dissolve it in 10 kg of methanol solvent. Stir evenly to form a 20% by mass trioxymethylene methanol solution. Add the trioxymethylene methanol solution into the first raw material tank;

[0100] 【2】Weigh 10 kg of high-purity methyl propionate (≥99.9 wt%) and add it to the second raw material tank;

[0101] 【3】Pump the solution in the first raw material tank into the preheating furnace and control the flow rate here to be 100g / h;

[0102] 【4】Pump the solution in the second raw material tank into the preheating furnace and control the flow rate here to be 176 g / h;

[0103] 【5】The temperature of the preheating furnace is controlled at 150℃~300℃. The raw material solutions are heated and mixed together and enter the reaction system. At this time, the total flow rate of fresh raw materials is 276g / h;

[0104] 【6】The flow distribution ratio of fresh raw materials entering reactor No. 1 and reactor No. 2 is set to 7:3, that is, the flow rate of fresh raw materials entering reactor No. 1 is controlled at 193 g / h by a regulating valve and a flow meter, the temperature of reactor No. 1 is controlled at 350°C, and the pressure is controlled at 0.25 MPa; methyl propionate and formaldehyde undergo a gas phase condensation reaction in reactor No. 1 to generate MMA, and all the reaction products flow into reactor No. 2 to continue to participate in the reaction;

[0105] 【7】The flow rate of fresh raw material entering the No. 2 reactor is controlled at 83 g / h by a regulating valve and a flow meter, and is mixed with all the reaction products of the No. 1 reactor to serve as the feed of the No. 2 reactor. The temperature of the No. 2 reactor is controlled at 340°C and the pressure is controlled at 0.2 MPa. Methyl propionate and formaldehyde continue to undergo gas phase condensation reaction in the No. 2 reactor to generate MMA. All the reaction products flow into the reaction product main pipe and are sent to the next distillation section.

[0106] [8] At this time, the status of each reactor in the reaction system is: Reactor No. 1 is the first-stage reactor in operation, Reactor No. 2 is the second-stage reactor in operation, the product of the first-stage reactor enters the second-stage reactor in series, and the product of the second-stage reactor enters the reaction product main pipe; Reactor No. 3 is in standby status;

[0107] 【9】After running for about 240 hours, when regular sampling and analysis are performed at the reaction product main pipe, it is found that the MMA content has dropped to 80% to 85% of the initial value, and a small amount of formaldehyde begins to appear in the sample. At this time, it is necessary to switch the old and new reaction systems, remove reactor No. 1, and put reactor No. 3 into use;

[0108]

[10] Reactor No. 1, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials, and its activity decrease is the largest. Therefore, Reactor No. 1 needs to be removed and isolated for regeneration. At this time, Reactor No. 2 will serve as the first-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate will be increased from 83 g / h to 193 g / h, the reaction temperature will be increased from 340°C to 350°C, and the reaction pressure will be increased from 0.2 MPa to 0.25 MPa. The newly added Reactor No. 3 will serve as the second-stage reactor of the new reaction system and will be controlled according to the process conditions of the original second-stage reactor: the fresh raw material flow rate is 83 g / h, the reaction temperature is 340°C, and the reaction pressure is 0.2 MPa.

[0109]

[11] At this time, the states of the reactors in the new reaction system after switching are: Reactor No. 2 is used as the first-stage reactor in the new reaction system, Reactor No. 3 is used as the second-stage reactor in the new reaction system, the product of the first-stage reactor is connected in series to the second-stage reactor, and the product of the second-stage reactor is connected to the reaction product main pipe; Reactor No. 1 is in a state of preparing for regeneration;

[0110]

[12] The reactor No. 1 is regenerated in situ, and the regeneration gas is a certain ratio of nitrogen and air. The temperature of the reactor is controlled at about 400°C. After the temperature stabilizes, air is introduced, and the total feed of air and nitrogen is controlled to be 200L / h. The air volume is gradually increased until the maximum air volume is 50L / h. During the whole regeneration process, the reactor bed temperature is controlled to be no more than 500°C; when the upper and lower bed temperatures of the reactor do not change, and after the exhaust gas analysis confirms that there is no oxygen consumption, the regeneration is stopped, and the temperature of the reactor No. 1 is cooled and kept for standby;

[0111]

[13] After running for about 240 hours again, regular sampling and analysis at the reaction product main pipe revealed that the MMA content dropped to 80% to 85% of the initial value, and trace amounts of formaldehyde began to appear in the samples. At this time, it was necessary to switch the old and new reaction systems again, remove reactor No. 2, and put reactor No. 1 into use;

[0112]

[14] Reactor No. 2, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials, and its activity decrease is the largest. Therefore, Reactor No. 2 needs to be removed and isolated for regeneration. At this time, Reactor No. 3 will be used as the first-stage reactor of the new reaction system and adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate is increased from 83 g / h to 193 g / h, the reaction temperature is increased from 340°C to 350°C, and the reaction pressure is increased from 0.2 MPa to 0.25 MPa. Reactor No. 1 will be used as the second-stage reactor of the new reaction system and controlled according to the process conditions of the original second-stage reactor: the fresh raw material flow rate is 83 g / h, the reaction temperature is 340°C, and the reaction pressure is 0.2 MPa.

[0113]

[15] At this time, the states of the reactors in the new reaction system after switching are as follows: Reactor No. 3 is used as the first-stage reactor in the new reaction system, Reactor No. 1 is used as the second-stage reactor in the new reaction system, the product of the first-stage reactor is connected in series to the second-stage reactor, and the product of the second-stage reactor is connected to the reaction product main pipe; Reactor No. 2 is in a state of preparing for regeneration;

[0114]

[16] The reactor No. 2 is regenerated in situ, and the regeneration gas is a certain ratio of nitrogen and air. The temperature of the reactor is controlled at about 400°C. After the temperature stabilizes, air is introduced, and the total feed of air and nitrogen is controlled to be 200L / h. The air volume is gradually increased until the maximum air volume is 50L / h. During the whole regeneration process, the reactor bed temperature is controlled to be no more than 500°C; when the upper and lower bed temperatures of the reactor do not change, and after the exhaust gas analysis confirms that there is no oxygen consumption, the regeneration is stopped, and the temperature of the reactor No. 2 is cooled and kept for standby;

[0115]

[17] After running for about 240 hours again, regular sampling and analysis at the reaction product main pipe revealed that the MMA content dropped to 80% to 85% of the initial value, and trace amounts of formaldehyde began to appear in the samples. At this time, it was necessary to switch the old and new reaction systems again, removing reactor No. 3 and putting reactor No. 2 into use.

[0116]

[18] Reactor No. 3, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials, and its activity decrease is the largest. Therefore, Reactor No. 3 needs to be removed and isolated for regeneration. At this time, Reactor No. 1 will be used as the first-stage reactor of the new reaction system and adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate is increased from 83 g / h to 193 g / h, the reaction temperature is increased from 340°C to 350°C, and the reaction pressure is increased from 0.2 MPa to 0.25 MPa. Reactor No. 2 will be used as the second-stage reactor of the new reaction system and controlled according to the process conditions of the original second-stage reactor: the fresh raw material flow rate is 83 g / h, the reaction temperature is 340°C, and the reaction pressure is 0.2 MPa.

[0117]

[19] At this time, the states of the reactors in the new reaction system after switching are: Reactor No. 1 is used as the first-stage reactor in the new reaction system, Reactor No. 2 is used as the second-stage reactor in the new reaction system, the product of the first-stage reactor enters the second-stage reactor in series, and the product of the second-stage reactor enters the reaction product main pipe; Reactor No. 3 is in a state of preparing for regeneration;

[0118]

[20] The reactor No. 3 was regenerated in situ, and the regeneration gas was a certain ratio of nitrogen and air. The reactor temperature was controlled at about 400°C. After the temperature stabilized, air was introduced, and the total feed of air and nitrogen was controlled to be 200L / h. The air volume was gradually increased until the maximum air volume was 50L / h. During the whole regeneration process, the reactor bed temperature was controlled to be no more than 500°C. When the upper and lower bed temperatures of the reactor did not change and the exhaust gas analysis confirmed that there was no oxygen consumption, the regeneration was stopped and the reactor No. 3 was cooled down for standby use.

[0119]

[21] At this time, the state of the two-stage series reaction system is exactly the same as that of [8]. The subsequent reaction system can be cycled and switched for regeneration according to the steps determined from [8] to

[20] ;

[0120] The product composition in the two-stage series reaction system is shown in Table 1.

[0121] Example 2

[0122] This embodiment provides a method for synthesizing methyl methacrylate (MMA) using a tandem reaction system, which is different from Embodiment 1 only in that the concentration of the methanol solution of trioxymethylene is increased to 30%, the temperature of the first-stage reactor is increased to 360° C., and the temperature of the second-stage reactor is increased to 350° C.;

[0123] If other process conditions remain unchanged, the reaction system needs to be switched after running for about 190 h. The product composition in the two-stage series reaction system at this time is shown in Table 1.

[0124] Example 3

[0125] This embodiment provides a method for synthesizing methyl methacrylate (MMA) using a tandem reaction system, which is different from Embodiment 1 only in that the flow rate of the methyl propionate raw material is increased to 352 g / h;

[0126] If other process conditions remain unchanged, the reaction system needs to be switched after running for about 290 h. The product composition in the two-stage series reaction system at this time is shown in Table 1.

[0127] Table 1

[0128]

[0129] Example 4

[0130] A method for synthesizing methyl methacrylate (MMA) using a tandem reaction system (the tandem reaction system is the same as in Example 1), comprising the following steps:

[0131] [1] Weigh 2.5 kg of trioxymethylene and dissolve it in 10 kg of methanol solvent. Stir evenly to form a 20% by mass trioxymethylene methanol solution. Add the trioxymethylene methanol solution into the first raw material tank;

[0132] 【2】Weigh 10 kg of high-purity methyl propionate (≥99.9 wt%) and add it to the second raw material tank;

[0133] 【3】Pump the solution in the first raw material tank into the preheating furnace and control the flow rate here to be 120 g / h;

[0134] 【4】Pump the solution in the second raw material tank into the preheating furnace and control the flow rate here to be 210 g / h;

[0135] 【5】The temperature of the preheating furnace is controlled at 150℃~300℃. The raw material solutions are heated and mixed together and enter the reaction system. At this time, the total flow rate of fresh raw materials is 330g / h;

[0136] 【6】The flow distribution ratio of fresh raw materials entering reactors No. 1, 2, and 3 is 5:3:2, that is, the flow rate of fresh raw materials entering reactor No. 1 is controlled at 165 g / h by a regulating valve and a flow meter, the temperature of reactor No. 1 is controlled at 350°C, and the pressure is controlled at 0.25 MPa; methyl propionate and formaldehyde undergo a gas phase condensation reaction in reactor No. 1 to generate MMA, and all the reaction products flow into reactor No. 2 to continue to participate in the reaction;

[0137] [7] The flow rate of fresh raw materials entering the No. 2 reactor is controlled at 100 g / h by a regulating valve and a flow meter, and is mixed with all the reaction products of the No. 1 reactor and used as the feed of the No. 2 reactor. The temperature of the No. 2 reactor is controlled at 340° C. and the pressure is controlled at 0.2 MPa. Methyl propionate and formaldehyde continue to undergo gas phase condensation reaction in the No. 2 reactor to generate MMA, and all the reaction products flow into the No. 3 reactor to continue to participate in the reaction.

[0138] [8] The flow rate of fresh raw materials entering the No. 3 reactor is controlled at 65 g / h by a regulating valve and a flow meter, and is mixed with all the reaction products of the No. 2 reactor to serve as the feed of the No. 3 reactor. The temperature of the No. 3 reactor is controlled at 330° C. and the pressure is controlled at 0.15 MPa. Methyl propionate and formaldehyde continue to undergo gas phase condensation reaction in the No. 3 reactor to generate MMA. All the reaction products flow into the reaction product main pipe and are sent to the next distillation section.

[0139] [9] At this time, the status of each reactor in the reaction system is: Reactor No. 1 is used as the first-stage reactor in operation, Reactor No. 2 is used as the second-stage reactor in operation, Reactor No. 3 is used as the third-stage reactor in operation, the product of the first-stage reactor is connected in series to the second-stage reactor, the product of the second-stage reactor is connected in series to the third-stage reactor, and the product of the third-stage reactor enters the reaction product main pipe; Reactor No. 4 is in standby state;

[0140]

[10] After running for about 240 hours, regular sampling and analysis at the reaction product main pipe revealed that the MMA content dropped to 80% to 85% of the initial value, and trace amounts of formaldehyde began to appear in the samples. At this time, the old and new reaction systems needed to be switched, reactor No. 1 was removed, and reactor No. 4 was put into use.

[0141]

[11] Reactor No. 1, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials. Its activity decrease is the largest. Therefore, Reactor No. 1 needs to be removed and isolated for regeneration. At this time, Reactor No. 2 will serve as the first-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate is increased from 100 g / h to 165 g / h, the reaction temperature is increased from 340 °C to 350 °C, and the reaction pressure is increased from 0.2 MPa to 0.25 MPa. At this time, Reactor No. 3 The reactor will be used as the second-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original second-stage reactor: the fresh feed flow rate will be increased from 65g / h to 100g / h, the reaction temperature will be increased from 330℃ to 340℃, and the reaction pressure will be increased from 0.15MPa to 0.2MPa; the newly put into use No. 4 reactor will be used as the third-stage reactor of the new reaction system and will be controlled according to the process conditions of the original third-stage reactor: the fresh feed flow rate is 65g / h, the reaction temperature is 330℃, and the reaction pressure is 0.15MPa;

[0142]

[12] At this time, the status of each reactor in the new reaction system after switching is: Reactor No. 2 is used as the first-stage reactor in the new reaction system, Reactor No. 3 is used as the second-stage reactor in the new reaction system, Reactor No. 4 is used as the third-stage reactor in the new reaction system, the product of the first-stage reactor is connected in series to the second-stage reactor, the product of the second-stage reactor is connected in series to the third-stage reactor, and the product of the third-stage reactor enters the reaction product main pipe; Reactor No. 1 is in a state of preparing for regeneration.

[0143]

[13] The reactor No. 1 is regenerated in situ, and the regeneration gas is a certain ratio of nitrogen and air. The reactor temperature is controlled at about 400°C. After the temperature stabilizes, air is introduced, and the total feed of air and nitrogen is controlled to be 200L / h. The air volume is gradually increased until the maximum air volume is 50L / h. During the whole regeneration process, the reactor bed temperature is controlled to be no more than 500°C; when the upper and lower bed temperatures of the reactor do not change, and after the exhaust gas analysis confirms that there is no oxygen consumption, the regeneration is stopped, and the reactor No. 1 is cooled and kept for standby;

[0144]

[14] After running for about 240 hours again, regular sampling and analysis at the reaction product main pipe revealed that the MMA content dropped to 80% to 85% of the initial value, and trace amounts of formaldehyde began to appear in the samples. At this time, it was necessary to switch the old and new reaction systems again, remove reactor No. 2, and put reactor No. 1 into use;

[0145]

[15] Reactor No. 2, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials. Its activity decrease is the largest. Therefore, Reactor No. 2 needs to be removed and isolated for regeneration. At this time, Reactor No. 3 will be used as the first-stage reactor of the new reaction system and adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate is increased from 100 g / h to 165 g / h, the reaction temperature is increased from 340 °C to 350 °C, and the reaction pressure is increased from 0.2 MPa to 0.25 MPa. At this time, Reactor No. 4 The reactor will be used as the second-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original second-stage reactor: the fresh feed flow rate will be increased from 65g / h to 100g / h, the reaction temperature will be increased from 330℃ to 340℃, and the reaction pressure will be increased from 0.15MPa to 0.2MPa; the newly put into use No. 1 reactor will be used as the third-stage reactor of the new reaction system and will be controlled according to the process conditions of the original third-stage reactor: the fresh feed flow rate is 65g / h, the reaction temperature is 330℃, and the reaction pressure is 0.15MPa;

[0146]

[16] At this time, the states of the reactors in the new reaction system after switching are: Reactor No. 3 is used as the first-stage reactor in the new reaction system, Reactor No. 4 is used as the second-stage reactor in the new reaction system, Reactor No. 1 is used as the third-stage reactor in the new reaction system, the product of the first-stage reactor is connected in series to the second-stage reactor, the product of the second-stage reactor is connected in series to the third-stage reactor, and the product of the third-stage reactor enters the reaction product main pipe; Reactor No. 2 is in a state of preparing for regeneration;

[0147]

[17] The reactor No. 2 was regenerated in situ, and the regeneration gas was a certain ratio of nitrogen and air. The reactor temperature was controlled at about 400°C. After the temperature stabilized, air was introduced, and the total feed of air and nitrogen was controlled to be 200L / h. The air volume was gradually increased until the maximum air volume was 50L / h. During the whole regeneration process, the reactor bed temperature was controlled to be no more than 500°C. When the upper and lower bed temperatures of the reactor did not change and the exhaust gas analysis confirmed that there was no oxygen consumption, the regeneration was stopped and the reactor No. 2 was cooled down for standby use.

[0148]

[18] After running for about 240 hours again, regular sampling and analysis at the reaction product main pipe revealed that the MMA content dropped to 80% to 85% of the initial value, and trace amounts of formaldehyde began to appear in the samples. At this time, it was necessary to switch the old and new reaction systems again, removing reactor No. 3 and putting reactor No. 2 into use.

[0149]

[19] Reactor No. 3, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials. Its activity decrease is the largest. Therefore, Reactor No. 3 needs to be removed and isolated for regeneration. At this time, Reactor No. 4 will be used as the first-stage reactor of the new reaction system and adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate is increased from 100 g / h to 165 g / h, the reaction temperature is increased from 340 °C to 350 °C, and the reaction pressure is increased from 0.2 MPa to 0.25 MPa. At this time, Reactor No. 1 The reactor will be used as the second-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original second-stage reactor: the fresh feed flow rate will be increased from 65g / h to 100g / h, the reaction temperature will be increased from 330℃ to 340℃, and the reaction pressure will be increased from 0.15MPa to 0.2MPa; the newly put into use No. 2 reactor will be used as the third-stage reactor of the new reaction system and will be controlled according to the process conditions of the original third-stage reactor: the fresh feed flow rate is 65g / h, the reaction temperature is 330℃, and the reaction pressure is 0.15MPa;

[0150]

[20] At this time, the states of the reactors in the new reaction system after switching are as follows: Reactor No. 4 is used as the first-stage reactor in the new reaction system, Reactor No. 1 is used as the second-stage reactor in the new reaction system, Reactor No. 2 is used as the third-stage reactor in the new reaction system, the product of the first-stage reactor is connected in series to the second-stage reactor, the product of the second-stage reactor is connected in series to the third-stage reactor, and the product of the third-stage reactor enters the reaction product main pipe; Reactor No. 3 is in a state of preparing for regeneration;

[0151]

[21] The reactor No. 3 was regenerated in situ, and the regeneration gas was a certain ratio of nitrogen and air. The reactor temperature was controlled at about 400°C. After the temperature stabilized, air was introduced, and the total feed of air and nitrogen was controlled to be 200L / h. The air volume was gradually increased until the maximum air volume was 50L / h. During the whole regeneration process, the reactor bed temperature was controlled to be no more than 500°C. When the upper and lower bed temperatures of the reactor did not change and the exhaust gas analysis confirmed that there was no oxygen consumption, the regeneration was stopped and the reactor No. 3 was cooled down for standby use.

[0152]

[22] After running for about 240 hours again, regular sampling and analysis at the reaction product main pipe revealed that the MMA content dropped to 80% to 85% of the initial value, and trace amounts of formaldehyde began to appear in the samples. At this time, it was necessary to switch the old and new reaction systems again, removing reactor No. 4 and putting reactor No. 3 into use.

[0153]

[23] Reactor No. 4, as the first-stage reactor, has the highest reaction temperature and processes the largest amount of fresh raw materials. Its activity decrease is the largest. Therefore, Reactor No. 4 needs to be removed and isolated for regeneration. At this time, Reactor No. 1 will serve as the first-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original first-stage reactor: the fresh raw material flow rate is increased from 100 g / h to 165 g / h, the reaction temperature is increased from 340 °C to 350 °C, and the reaction pressure is increased from 0.2 MPa to 0.25 MPa. At this time, Reactor No. 2 The reactor will be used as the second-stage reactor of the new reaction system and will be adjusted according to the process conditions of the original second-stage reactor: the fresh feed flow rate will be increased from 65g / h to 100g / h, the reaction temperature will be increased from 330℃ to 340℃, and the reaction pressure will be increased from 0.15MPa to 0.2MPa; the newly put into use No. 3 reactor will be used as the third-stage reactor of the new reaction system and will be controlled according to the process conditions of the original third-stage reactor: the fresh feed flow rate is 65g / h, the reaction temperature is 330℃, and the reaction pressure is 0.15MPa;

[0154]

[24] At this time, the status of each reactor in the new reaction system after switching is: Reactor No. 1 is used as the first-stage reactor in the new reaction system, Reactor No. 2 is used as the second-stage reactor in the new reaction system, Reactor No. 3 is used as the third-stage reactor in the new reaction system, the product of the first-stage reactor is connected in series to the second-stage reactor, the product of the second-stage reactor is connected in series to the third-stage reactor, and the product of the third-stage reactor enters the reaction product main pipe; Reactor No. 4 is in a state of preparing for regeneration.

[0155]

[25] Reactor No. 4 was regenerated in situ, with the regeneration gas being a certain ratio of nitrogen and air. The reactor temperature was controlled at about 400°C. After the temperature stabilized, air was introduced, and the total feed of air and nitrogen was controlled at 200 L / h. The air volume was gradually increased until the maximum air volume was 50 L / h. During the entire regeneration process, the reactor bed temperature was controlled to be no greater than 500°C. When the upper and lower bed temperatures of the reactor did not change and no oxygen consumption was confirmed by tail gas analysis, the regeneration was stopped and Reactor No. 4 was cooled down for standby use.

[0156]

[26] At this time, the state of the three-stage series reaction system is exactly the same as that of [9]. The subsequent reaction system can be cycled and switched for regeneration according to the steps determined from [9] to

[25] ;

[0157] The product composition in the three-stage cascade reaction system is shown in Table 2.

[0158] Example 5

[0159] This embodiment provides a method for synthesizing methyl methacrylate (MMA) using a tandem reaction system, which is different from Embodiment 4 only in that the concentration of the methanol solution of trioxymethylene is increased to 30%, the temperature of the first-stage reactor is increased to 360° C., the temperature of the second-stage reactor is increased to 350° C., and the temperature of the third-stage reactor is increased to 340° C.;

[0160] If other process conditions remain unchanged, the reaction system needs to be switched after running for about 190 h. The product composition in the three-stage series reaction system at this time is shown in Table 2.

[0161] Example 6

[0162] This embodiment provides a method for synthesizing methyl methacrylate (MMA) using a tandem reaction system, which is different from Embodiment 4 only in that the flow rate of the methyl propionate raw material is increased to 420 g / h;

[0163] If other process conditions remain unchanged, the reaction system needs to be switched after running for about 290 hours. The product composition in the three-stage series reaction system at this time is shown in Table 2.

[0164] Table 2

[0165]

[0166] The multi-stage series reaction system for preparing methyl methacrylate by gas phase condensation of a methyl propionate source and a formaldehyde source of the present invention is not limited to the series connection of two-stage and three-stage reactors, but can be the series connection of any i-stage reactors (i≥2). However, no matter how many stages of the series reaction system there are, its basic raw material flow, product flow, process control range and switching rules of the new and old reaction systems still follow the paradigm of the above-mentioned embodiment.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A series reaction system for producing methyl methacrylate by gas phase condensation, characterized in that: It comprises n series reactors, wherein n≥3; The series reaction system is provided with a raw gas main pipe for distributing the raw gas to each level of reactors for gas phase condensation reaction; The raw material feed pipelines of each stage of the reactor are independently provided with a polymerization inhibitor injection system for injecting the polymerization inhibitor into the reactor; The series reaction system is provided with a reaction product main pipe, and the reaction product outlets of the reactors at each stage are connected to the feed inlet of the next stage reactor and to the reaction product main pipe; In the series reaction system, the series reactors for gas phase condensation reaction are i-stage, wherein 2≤i<n, and there is at least one reactor in a regeneration or standby state.

2. The tandem reaction system according to claim 1, characterized in that: The series reaction system is also provided with a regeneration gas main pipe for distributing the regeneration gas to each level of reactors for in-situ regeneration of the catalyst.

3. The tandem reaction system according to claim 2, characterized in that: The series reaction system is also provided with a regeneration tail gas main pipe for collecting the regeneration tail gas from each stage of the reactor.

4. The tandem reaction system according to any one of claims 1 to 3, characterized in that: The reactor comprises a vertical cylindrical adiabatic fixed bed type reactor; The adiabatic fixed bed type reactor comprises, from top to bottom, a feed distribution area, a reaction isothermal area and a discharge collection area.

5. The tandem reaction system according to claim 4, characterized in that: The feed distribution area is provided with an inlet distributor, a distribution tray, a support beam and a support grid, and an inert filler is filled between the support grid and the distribution tray.

6. The tandem reaction system according to claim 4, characterized in that: The reaction isothermal zone is filled with a gas phase condensation reaction catalyst.

7. The tandem reaction system according to claim 4, characterized in that: The material discharging and collecting area is provided with an outlet distribution tray, a support beam, a support grid and an outlet collector, and an inert filler is filled between the support grid and the outlet distribution tray.

8. A method for synthesizing methyl methacrylate using the tandem reaction system according to any one of claims 1 to 7, characterized in that: The following steps are involved: A raw material gas mixed with a formaldehyde source and a methyl propionate source is fed into the series reaction system, and a polymerization inhibitor is injected into the reactor to make the raw material gas undergo a gas phase condensation reaction in the series reaction system to obtain methyl methacrylate.

9. The method according to claim 8, characterized in that The method comprises the following steps: Feeding the first raw gas into the first stage reactor for gas phase condensation reaction, feeding the gas phase condensation product obtained from the first stage reactor and the second raw gas into the second stage reactor for gas phase condensation reaction, feeding the gas phase condensation product obtained from the second stage reactor and the third raw gas into the third stage reactor for gas phase condensation reaction, and so on, feeding the gas phase condensation product obtained from the j stage reactor and the j+1th raw gas into the j+1th stage reactor for gas phase condensation reaction, wherein 1≤j<i, until the gas phase condensation product obtained from the last stage reactor enters the reaction product main pipe to obtain methyl methacrylate; Preferably, the pressure condition of the gas phase condensation reaction is 0.05MPa-1.0MPa, the temperature condition is 300℃-400℃, and the volume space velocity is 0.1h -1 ~10h -1 ; Preferably, the temperature and pressure of the first-stage reactor are not lower than those of the second-stage reactor, the temperature and pressure of the second-stage reactor are not lower than those of the third-stage reactor, and the temperature and pressure of the j-th stage reactor are not lower than those of the j+1-th stage reactor; Preferably, the temperature of the j-th reactor is not lower than that of the j+1-th reactor and ranges from 5°C to 60°C; Preferably, the pressure of the j-th reactor is not lower than that of the j+1-th reactor and ranges from 0.01 MPa to 1.0 MPa.

10. The method according to claim 9, characterized in that The method further comprises the step of periodically switching the new and old reaction systems; The periodic switching method comprises the following steps: During the operation of the i-stage series reactor, when the periodic switching standard of the new and old reaction systems is reached, the first-stage reactor is removed from the original reaction system and regenerated in situ, the second-stage reactor in the original reaction system is used as the first-stage reactor in the new reaction system, the third-stage reactor in the original reaction system is used as the second-stage reactor in the new reaction system, and so on, the j-th stage reactor in the original reaction system is used as the j-1-th stage reactor in the new reaction system, where 2≤j≤i, and the next-stage reactor of the i-th stage reactor in the original reaction system is used as the i-th stage reactor in the new reaction system; Preferably, the periodic switching standard of the new and old reaction systems is that the methyl methacrylate content at the reaction product main pipe is reduced to 80% to 85% of the initial value, and formaldehyde begins to appear in the sample; Preferably, the in situ regeneration method comprises the following steps: The deactivated catalyst is regenerated in situ using a mixture of nitrogen and oxygen as the regeneration gas; Preferably, the pressure condition of the in-situ regeneration is 0-1.0 MPa, the temperature condition is 300°C-600°C, and the volume space velocity is 100h -1 ~3000h -1 .

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