A system and process for preparing vinyl acetate by oxidation of ethylene

Abstract

The present invention relates to the technical field of vinyl acetate production, and specifically to a system and process for preparing vinyl acetate by ethylene oxidation, the system comprising two-stage reaction units or more than two-stage reaction units; wherein the two-stage reaction units comprise an initial reaction unit and a final reaction unit sequentially arranged along the material feeding direction; or, the more than two-stage reaction units comprise an initial reaction unit, at least one intermediate reaction unit and a final reaction unit sequentially arranged along the material feeding direction; the initial reaction unit, the intermediate reaction unit and the final reaction unit all comprise an evaporator, a reactor and a gas-liquid separator sequentially arranged along the material feeding direction. Compared with the prior art, the present invention adopts series and parallel processes, significantly improves conversion rate and economic benefits, effectively reduces equipment size, reduces equipment manufacturing difficulty, and reduces device investment.

Description

Technical Field

[0001] The invention relates to the technical field of vinyl acetate production, and in particular to a system and process for preparing vinyl acetate by ethylene oxidation. Background Art

[0002] Vinyl acetate is an important organic chemical raw material, and its market demand continues to grow. Vinyl acetate can usually be prepared by the acetylene method or the ethylene method.

[0003] With the maturity of domestic MTO technology for preparing olefins and the increasing production capacity, the domestic supply of ethylene is becoming more abundant. Compared with the acetylene gas phase method, the process of producing vinyl acetate by ethylene gas phase method has easy access to raw materials and good economic efficiency. Therefore, the process of producing vinyl acetate by ethylene gas phase method has attracted more and more attention.

[0004] When using the ethylene gas phase method to produce vinyl acetate, in order to ensure the reaction conversion rate and improve economic benefits, a large amount of unreacted raw materials need to be circulated. In addition, after the vinyl acetate production plant is scaled up, the system pressure drop increases and the equipment size increases sharply. Therefore, it is necessary not only to use new equipment suitable for large-scale vinyl acetate production, but also to transform the process flow to achieve the goal of significantly reducing energy consumption in a systematic manner.

[0005] At present, there is no process suitable for large-scale ethylene gas phase production of vinyl acetate. In order to give full play to the advantages of the ethylene gas phase method, an ethylene gas phase method for large-scale production of vinyl acetate is urgently needed. Summary of the invention

[0006] The purpose of the present invention is to provide a system and process for preparing vinyl acetate by gas phase oxidation of ethylene in order to solve the above problems. The series and parallel processes are adopted, which not only significantly improve the conversion rate, but also effectively reduce the equipment size, reduce the difficulty of equipment manufacturing, reduce the investment in equipment, and improve the economic benefits.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] The first aspect of the present invention discloses a system for preparing vinyl acetate by oxidation of ethylene, the system comprising two or more reaction units;

[0009] Wherein, the two-stage reaction unit comprises an initial reaction unit and a final reaction unit arranged in sequence along the material feeding direction; or,

[0010] The two or more reaction units include an initial reaction unit, at least one intermediate reaction unit and a final reaction unit arranged in sequence along the material feeding direction;

[0011] The initial reaction unit, the intermediate reaction unit and the final reaction unit all include an evaporator, a reactor and a gas-liquid separator arranged in sequence along the material feeding direction;

[0012] In the working state, the material containing ethylene, acetic acid and oxygen passes through the initial reaction unit and the final reaction unit in sequence to complete the two-stage oxidation reaction; or,

[0013] The material containing ethylene, acetic acid and oxygen is sequentially passed through an initial reaction unit, at least one intermediate reaction unit and a final reaction unit to complete more than two stages of oxidation reaction.

[0014] Preferably, under working conditions, the material containing ethylene, acetic acid and oxygen is first subjected to an initial oxidation reaction in an initial reaction unit, and the obtained product is subjected to gas-liquid separation, and the liquid phase is collected as the target product, and the gas phase is introduced into the final reaction unit and subjected to a final oxidation reaction with the material containing acetic acid and oxygen entering the final reaction unit, and the obtained product is subjected to gas-liquid separation, and the liquid phase is collected as the target product, and the gas phase is diverted and returned to the initial reaction unit and / or the final reaction unit, and the above process is repeated; or,

[0015] Under working conditions, the material containing ethylene, acetic acid and oxygen is first subjected to an initial oxidation reaction in an initial reaction unit. After the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, and the gas phase is introduced into at least one intermediate reaction unit located downstream of the initial reaction unit, and undergoes at least one intermediate oxidation reaction with the material containing acetic acid and oxygen entering the at least one intermediate reaction unit. After the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, and the gas phase is introduced into the final reaction unit located downstream of the at least one intermediate reaction unit, and undergoes a final oxidation reaction with the material containing acetic acid and oxygen entering the final reaction unit. After the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, and the gas phase is diverted and returned to one or more reaction units, and the above process is repeated.

[0016] Further preferably, a cooler is provided at the front end of the separator. A cooler integrated with the gas-liquid separator or a separate cooler can be used as required, as long as the mixture in the gas-liquid separator can reach the required temperature.

[0017] Preferably, each reaction unit further comprises a preheater disposed between the evaporator and the reactor to preheat the material containing ethylene, acetic acid and oxygen from the evaporator before entering the reactor.

[0018] The preheater preheats the gas phase entering the reactor to prevent the low-temperature gas phase from directly entering the high-temperature reactor, causing a large amount of energy loss and incomplete reaction, thereby promoting the improvement of reaction conversion rate and yield.

[0019] Preferably, each reaction unit further comprises a heat exchanger, wherein the heat source of the heat exchanger is the reaction product, and the cold source is the mixed gas from the evaporator.

[0020] By setting up a heat exchanger and using high-temperature reaction products to preheat low-temperature reaction raw materials, it helps to reduce the overall energy loss of the system and thus reduce production energy consumption.

[0021] Preferably, each reaction unit further comprises a reactor drum, the top extraction of the reactor enters the reactor drum for steam collection, and the obtained liquid phase returns to the reactor, and the temperature of the reaction drum is 145-155°C.

[0022] Equipping the reactor with a corresponding reactor drum can produce low-pressure steam as a by-product during the reaction. By recycling the low-pressure steam, the production cost of the process can be further reduced.

[0023] Preferably, each reaction unit further comprises a reactor circulation pump, the inlet of the reactor circulation pump is connected to the outlet of the reactor drum, the outlet of the reactor circulation pump is connected to the bottom of the reactor, and the reactor circulation pump forcibly circulates the remaining liquid separated from the reactor drum.

[0024] A reactor circulation pump is set between the reactor and the reactor drum, which forms a forced circulation; while no reactor circulation pump is set between the reactor and the reactor drum, which forms a natural circulation. Whether to set it or not is determined according to actual production needs and operating conditions.

[0025] Preferably, the system further comprises a tail gas recovery unit coupled to the final reaction unit, wherein the tail gas recovery unit comprises a compressor, an absorption tower and a membrane separator.

[0026] Under working conditions, the gas phase separated by the gas-liquid separator of the final reaction unit enters the compressor, and the gas phase is split after being pressurized by the compressor. 90-99 vol% of the gas phase returns to the initial unit to mix with the newly added ethylene, and the remaining gas phase enters the absorption tower and the membrane separator in turn, and then returns to the evaporator of at least one intermediate reaction unit and / or the final reaction unit.

[0027] Preferably, the gas phase separated by the gas-liquid separator of the final reaction unit is pressurized to 0.9-1.2 MPaG by a compressor for diversion.

[0028] The inert gas phase can be fully discharged and vented through the tail gas recovery unit, effectively avoiding the accumulation of the inert gas phase (mainly nitrogen and CO2) during the circulation process and affecting the occurrence of the reaction; at the same time, the effective gas phase (such as the remaining ethylene after the reaction) can be returned to the cycle again, avoiding the waste of reaction raw materials.

[0029] Preferably, the bottom of the evaporator in the at least one intermediate reaction unit and the final reaction unit is provided with a regulating port for replenishing ethylene to the evaporator to adjust the load of the corresponding unit and maintain thermal balance. The regulating port is a spare port for replenishing ethylene to the evaporator of the at least one intermediate reaction unit and the final reaction unit as needed, rather than continuously replenishing ethylene; only when the load of the corresponding reaction unit is low and thermal balance cannot be maintained, the regulating port of the corresponding reaction unit will be opened to replenish ethylene to adjust the load of the corresponding unit and maintain thermal balance.

[0030] Preferably, the preheating temperature of the preheater is 110-185°C; the temperature of the evaporator is 110-120°C; and the temperature of the gas-liquid separator is 40-90°C.

[0031] The second aspect of the present invention discloses a process for preparing vinyl acetate by ethylene oxidation, comprising the following steps:

[0032] The material containing ethylene, acetic acid and oxygen is introduced into the initial reaction unit for initial oxidation reaction, the obtained product is subjected to gas-liquid separation, the separated liquid phase is collected as the target product, the gas phase is introduced into the final reaction unit, and it is subjected to final oxidation reaction with the material containing acetic acid and oxygen entering the final reaction unit, the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, the gas phase is diverted and returned to the initial reaction unit and / or the final reaction unit, and the above process is repeated; or,

[0033] The material containing ethylene, acetic acid and oxygen is first subjected to an initial oxidation reaction in an initial reaction unit, the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, the gas phase is introduced into at least one intermediate reaction unit located downstream of the initial reaction unit, so that it undergoes at least one intermediate oxidation reaction with the material containing acetic acid and oxygen entering the at least one intermediate reaction unit, the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, the gas phase is introduced into a final reaction unit located downstream of the at least one intermediate reaction unit, so that it undergoes a final oxidation reaction with the material containing acetic acid and oxygen entering the final reaction unit, the obtained product is subjected to gas-liquid separation, the liquid phase is collected as the target product, the gas phase is diverted and returned to one or more reaction units, and the above process is repeated.

[0034] Preferably, in the oxidation reaction, the molar ratio of ethylene, oxygen and acetic acid is (7-12):1:(1-4), the reaction gauge pressure is 0.70-0.91 MPaG, the reaction temperature is 120-185°C, and the catalyst can be selected from commercially available conventional catalysts, such as palladium catalysts or gold catalysts.

[0035] Preferably, the molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid added to the final reaction unit is (1.2-2):1, and the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen added to the final reaction unit is (1.2-2):1; or,

[0036] The molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid supplemented into at least one intermediate reaction unit located downstream of the initial reaction unit is (1.2-2):1, the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen supplemented into at least one intermediate reaction unit located downstream of the initial reaction unit is (1.2-2):1, the amount of acetic acid in at least one intermediate reaction unit to the amount of acetic acid in the final reaction unit located downstream of the at least one intermediate reaction unit is (1.2-2):1, and the amount of oxygen in at least one intermediate reaction unit to the amount of oxygen in the final reaction unit located downstream of the at least one intermediate reaction unit is (1.2-2):1.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. The process proposed by the present invention comprises two reaction loops. For ethylene, series feeding is adopted, while for acetic acid and oxygen, parallel feeding is adopted. Through such feeding method, acetic acid and oxygen are added to the system in a distributed manner, so that ethylene can be continuously in an excess state.

[0039] 2. The system involved in the present invention can effectively improve the safety of the system and keep the system concentration away from the explosion limit.

[0040] 3. In the system involved in the present invention, ethylene is fed in series, so that the ethylene partial pressure is greatly increased, which is beneficial to increase the reaction rate and further increase the conversion rate of oxygen and acetic acid.

[0041] 4. The system involved in the present invention can effectively reduce the hot spot temperature, make the reaction temperature uniform, effectively inhibit the occurrence of side reactions (ethylene oxidation to generate carbon dioxide), thereby improving the selectivity of the main reaction, reducing the content of by-products, and increasing the yield of the system. The selectivity of the reaction can be increased by 0.5-1% from the original 90%-95% to about 90.5-96%. Since the selectivity of the main reaction is improved, the impurity content in the reaction product is reduced, and then the energy consumption can be effectively reduced in the subsequent separation and purification steps, achieving the purpose of energy saving and consumption reduction.

[0042] 5. In the system involved in the present invention, the reaction loop can share a set of compression processes, thereby reducing equipment investment and energy consumption.

[0043] 6. Use series and parallel processes to effectively reduce equipment size. Under the condition that the total yield target remains unchanged, connecting the reactions in series and parallel can reduce the yield target of a single reactor, thereby reducing the equipment size of a single reactor, avoiding excessive pressure drop in the reactor, reducing the difficulty of equipment manufacturing, and reducing the investment cost of the device.

[0044] 7. The system involved in the present invention is provided with an evaporator, which is beneficial to improving the energy utilization efficiency of the entire system and reducing the energy consumption of the system. The evaporator is used for evaporating acetic acid and mixing it with ethylene in the initial reaction unit, and the evaporators of other reaction units are used for evaporating acetic acid.

[0045] 8. By using the system of the present invention, the hot spot temperature of each reactor can be reduced by 1-3°C, thereby reducing the occurrence of side reactions and reducing the amount of byproduct CO2 generated by about 0.1-0.3%. In addition, because the hot spot temperature is reduced, the catalyst life can be increased by about 2-3 months. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a schematic diagram of the process flow of Example 1 of the present invention;

[0047] Figure 2 This is a schematic diagram of the process flow of Example 4 of the present invention;

[0048] In the figure: 1-a first-stage evaporator; 2-a first-stage reactor; 3-a first-stage gas-liquid separator; 4-a second-stage evaporator; 5-a second-stage reactor; 6-a second-stage gas-liquid separator; 7-a first-stage preheater; 8-a second-stage preheater; 9-a first-stage heat exchanger; 10-a second-stage heat exchanger; 11-a first-stage reactor drum; 12-a second-stage reactor drum; 13-a first-stage reactor circulation pump; 14-a second-stage reactor circulation pump; 15-tail gas recovery unit; 41-a third-stage evaporator; 51-a third-stage reactor; 61-a third-stage gas-liquid separator; 81-a third-stage preheater; 101-a third-stage heat exchanger; 121-a third-stage reactor drum; 141-a third-stage reactor circulation pump; 42-a fourth-stage evaporator; 52-a fourth-stage reactor; 62-a fourth-stage gas-liquid separator; 82-a fourth-stage preheater; 102-a fourth-stage heat exchanger; 122-a fourth-stage reactor drum; 142-a fourth-stage reactor circulation pump. DETAILED DESCRIPTION

[0049] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0050] A system for preparing vinyl acetate by oxidizing ethylene, the system comprising two or more reaction units;

[0051] Wherein, the two-stage reaction unit includes an initial reaction unit and a final reaction unit arranged in sequence along the material feeding direction; or,

[0052] The two or more reaction units include an initial reaction unit, at least one intermediate reaction unit and a final reaction unit arranged in sequence along the material feeding direction;

[0053] The initial reaction unit, the intermediate reaction unit and the final reaction unit all include an evaporator, a reactor and a gas-liquid separator arranged in sequence along the material feeding direction;

[0054] In the working state, the material containing ethylene, acetic acid and oxygen passes through the initial reaction unit and the final reaction unit in sequence to complete the two-stage oxidation reaction; or,

[0055] The material containing ethylene, acetic acid and oxygen is sequentially passed through an initial reaction unit, at least one intermediate reaction unit and a final reaction unit to complete more than two stages of oxidation reaction.

[0056] The two reaction units connected in sequence are composed of an initial reaction unit and a final reaction unit, wherein:

[0057] The initial reaction unit comprises a first-stage evaporator 1, a first-stage reactor 2 and a first-stage gas-liquid separator 3 connected in sequence; the final reaction unit comprises a second-stage evaporator 4, a second-stage reactor 5 and a second-stage gas-liquid separator 6 connected in sequence; the first-stage gas-liquid separator 3 is connected to the second-stage evaporator 4. In the initial reaction unit, a first-stage oxidation reaction is carried out based on ethylene, acetic acid and oxygen, and the reaction products are separated, and the separated target product is output to the outside, and the separated gas phase and the material containing acetic acid and oxygen are subjected to a second-stage oxidation reaction in the final reaction unit, and the reaction products of the second-stage oxidation reaction are separated, and the separated target product is output to the outside, and the separated gas phase is returned to the initial reaction unit and / or the final reaction unit as a circulating gas diversion.

[0058] In addition, each stage reaction unit further includes a preheater, a heat exchanger, a reactor drum and a reactor circulation pump, and a tail gas recovery unit 15 connected to the end of the final reaction unit.

[0059] Acetic acid and ethylene are mixed and gasified in the first evaporator 1, and the gasified mixed gas phase is then heated in the first heat exchanger 9 with the reaction product flowing out of the first reactor 2, and then mixed with oxygen to form a first mixed reaction gas phase. The first mixed reaction gas phase is further preheated in the first preheater 7 and input into the first reactor 2 to undergo a first oxidation reaction. During the reaction, the top product extracted from the top of the first reactor 2 enters the first reactor drum 11, and the separated low-pressure steam is discharged from the first reactor drum 11 and collected, and the remaining liquid is transported back to the bottom of the first reactor 2 through the first reactor circulation pump 13 to continue to participate in the reaction. The reaction product first flows through the first heat exchanger 9 and exchanges heat with the mixed gas phase from the first evaporator 1, and then enters the first gas-liquid separator 3 for gas-liquid separation. The separated liquid phase is collected in the intermediate storage tank, and the gas phase is then continuously sent to the second evaporator 4 to mix and gasify with the added acetic acid.

[0060] The mixed gas phase vaporized in the second stage evaporator 4 is then heated in the second stage heat exchanger 10 with the reaction product flowing out of the second stage reactor 5, and then mixed with oxygen to form a second mixed reaction gas phase. The second mixed reaction gas phase is further preheated in the second stage preheater 8 and input into the second stage reactor 5 for a second stage oxidation reaction. During the reaction, the top product extracted from the top of the second stage reactor 5 enters the second stage reactor drum 12, the separated low-pressure steam is discharged from the second stage reactor drum 12 and collected, and the remaining liquid is transported back to the bottom of the second stage reactor 5 through the second stage reactor circulation pump 14 to continue to participate in the reaction. The reaction product first flows through the second stage heat exchanger 10 and exchanges heat with the mixed gas phase from the second stage evaporator 4, and then enters the second stage gas-liquid separator 6 for gas-liquid separation. The separated liquid phase is collected in the intermediate storage tank, and the gas phase then enters the tail gas recovery unit 15 for further treatment.

[0061] The tail gas recovery unit 15 first inputs the gas phase from the top of the second-stage gas-liquid separator 6 into the compressor for pressure increase. The gas phase after pressure increase is split, and most of it (90-99% (volume ratio)) returns to the initial reaction unit to mix with the newly added ethylene and then enters the first-stage evaporator 1. The remaining gas phase enters the absorption tower and the membrane separator in turn to separate the effective gas phase and the inert gas phase. The effective gas phase is then sent back to the second-stage evaporator 4 of the final reaction unit to participate in the reaction of the reaction unit, forming a cycle, and the inert gas phase is then emptied.

[0062] For a reaction unit with more than two stages, a four-stage reaction unit connected in sequence is taken as an example, which is composed of an initial reaction unit, two intermediate reaction units and a final reaction unit, wherein:

[0063] The initial reaction unit includes a first-stage evaporator 1, a first-stage reactor 2 and a first-stage gas-liquid separator 3 connected in sequence; the second-stage reaction unit includes a second-stage evaporator 4, a second-stage reactor 5 and a second-stage gas-liquid separator 6 connected in sequence; the three-stage reaction unit includes a three-stage evaporator 41, a three-stage reactor 51 and a three-stage gas-liquid separator 61 connected in sequence; the final reaction unit includes a four-stage evaporator 42, a four-stage reactor 52 and a four-stage gas-liquid separator 62 connected in sequence; the first-stage gas-liquid separator 3 is connected to the second-stage evaporator 4, the second-stage gas-liquid separator 6 is connected to the third-stage evaporator 41, and the third-stage gas-liquid separator 61 is connected to the fourth-stage evaporator 42. In the initial reaction unit, a first-stage oxidation reaction is carried out based on ethylene and acetic acid, and the reaction products are separated, and the separated target product is output to the outside; the separated gas phase and the material containing acetic acid and oxygen are subjected to a second-stage oxidation reaction in the second-stage reaction unit, and the reaction products of the second-stage oxidation reaction are separated, and the separated target product is output to the outside; the separated gas phase and the material containing acetic acid and oxygen are subjected to a third-stage oxidation reaction in the third-stage reaction unit, and the reaction products of the third-stage oxidation reaction are separated, and the separated target product is output to the outside; the separated gas phase and the material containing acetic acid and oxygen are subjected to a fourth-stage oxidation reaction in the final reaction unit, and the reaction products of the fourth-stage oxidation reaction are separated, and the separated target product is output to the outside; the separated gas phase is returned to the initial reaction unit and the second-stage reaction unit and / or the third-stage reaction unit and / or the final reaction unit as a circulating gas diversion.

[0064] In addition, each stage reaction unit further includes a preheater, a heat exchanger, a reactor drum and a reactor circulation pump, and a tail gas recovery unit 15 connected to the end of the final reaction unit.

[0065] Acetic acid and ethylene are mixed and gasified in the first evaporator 1, and the gasified mixed gas phase is then heated in the first heat exchanger 9 with the reaction product flowing out of the first reactor 2, and then mixed with oxygen to form a first mixed reaction gas phase. The first mixed reaction gas phase is further preheated in the first preheater 7 and input into the first reactor 2 to undergo a first oxidation reaction. During the reaction, the top product extracted from the top of the first reactor 2 enters the first reactor drum 11, and the separated low-pressure steam is discharged from the first reactor drum 11 and collected, and the remaining liquid is transported back to the bottom of the first reactor 2 through the first reactor circulation pump 13 to continue to participate in the reaction. The reaction product first flows through the first heat exchanger 9 and exchanges heat with the mixed gas phase from the first evaporator 1, and then enters the first gas-liquid separator 3 for gas-liquid separation. The separated liquid phase is collected in the intermediate storage tank, and the gas phase is then continuously sent to the second evaporator 4 to mix and gasify with the added acetic acid.

[0066] The mixed gas phase vaporized in the second stage evaporator 4 is then heated in the second stage heat exchanger 10 with the reaction product flowing out of the second stage reactor 5, and then mixed with oxygen to form a second mixed reaction gas phase. The second mixed reaction gas phase is further preheated in the second stage preheater 8 and input into the second stage reactor 5 for a second stage oxidation reaction. During the reaction, the top product extracted from the top of the second stage reactor 5 enters the second stage reactor drum 12, the separated low-pressure steam is discharged from the second stage reactor drum 12 and collected, and the remaining liquid is transported back to the bottom of the second stage reactor 5 through the second stage reactor circulation pump 14 to continue to participate in the reaction. The reaction product first flows through the second stage heat exchanger 10 and exchanges heat with the mixed gas phase from the second stage evaporator 4, and then enters the second stage gas-liquid separator 6 for gas-liquid separation. The separated liquid phase is collected in the intermediate storage tank, and the gas phase is then continuously sent to the third stage evaporator 41 to mix and gasify with the added acetic acid.

[0067] The mixed gas phase vaporized in the three-stage evaporator 41 is then heated in the three-stage heat exchanger 101 with the reaction product flowing out of the three-stage reactor 51, and then mixed with oxygen to form a third mixed reaction gas phase. The third mixed reaction gas phase is further preheated in the three-stage preheater 81 and then input into the three-stage reactor 51 to undergo a three-stage oxidation reaction. During the reaction, the top product extracted from the top of the three-stage reactor 51 enters the three-stage reactor drum 121, the separated low-pressure steam is discharged from the three-stage reactor drum 121 and collected, and the remaining liquid is transported back to the bottom of the three-stage reactor 51 through the three-stage reactor circulation pump 141 to continue to participate in the reaction. The reaction product first flows through the three-stage heat exchanger 101 and exchanges heat with the mixed gas phase from the three-stage evaporator 41, and then enters the three-stage gas-liquid separator 61 for gas-liquid separation. The separated liquid phase is collected in the intermediate storage tank, and the gas phase is then continuously sent to the four-stage evaporator 42 to be mixed and gasified with the added acetic acid.

[0068] The mixed gas phase vaporized in the four-stage evaporator 42 is then heated in the four-stage heat exchanger 102 with the reaction product flowing out of the four-stage reactor 52, and then mixed with oxygen to form a fourth mixed reaction gas phase. The fourth mixed reaction gas phase is further preheated in the four-stage preheater 82 and input into the four-stage reactor 52 to undergo a four-stage oxidation reaction. During the reaction, the top product extracted from the top of the four-stage reactor 52 enters the four-stage reactor drum 122, the separated low-pressure steam is discharged from the four-stage reactor drum 122 and collected, and the remaining liquid is transported back to the bottom of the four-stage reactor 52 through the four-stage reactor circulation pump 142 to continue to participate in the reaction. The reaction product first flows through the four-stage heat exchanger 102 and exchanges heat with the mixed gas phase from the four-stage evaporator 42, and then enters the four-stage gas-liquid separator 62 for gas-liquid separation. The separated liquid phase is collected in the intermediate storage tank, and the gas phase then enters the tail gas recovery unit 15 for further treatment.

[0069] The tail gas recovery unit 15 first inputs the gas phase from the top of the four-stage gas-liquid separator 62 into the compressor for pressure increase. The gas phase after pressure increase is split, and most of it (90-99% (volume ratio)) returns to the initial reaction unit to mix with the newly added ethylene and then enters the first evaporator 1. The remaining gas phase enters the absorption tower and the membrane separator in turn to separate the effective gas phase and the inert gas phase. The effective gas phase is then sent back to the evaporator of the second-stage reaction unit and / or the third-stage reaction unit and / or the final reaction unit to participate in the reaction of the reaction unit, forming a cycle, and the inert gas phase is then emptied.

[0070] Example 1

[0071] A system for preparing vinyl acetate by oxidation of ethylene, such as Figure 1 As shown, in this embodiment, there are two reaction units:

[0072] Ethylene from outside enters into the first stage evaporator 1 from the bottom of the first stage evaporator 1, and acetic acid from outside enters into the first stage evaporator 1 from the top of the first stage evaporator 1. In the first stage evaporator 1 (115°C, 0.95±0.02MPaG), the acetic acid and ethylene mixture is mixed and gasified (the gas phase enters from the bottom side, and the liquid enters from the top side, which is beneficial to improve the gasification effect). The gasified acetic acid and ethylene mixture is first heated in the first stage heat exchanger 9 with the reaction product of the first stage reactor 2. The heated mixed gas phase is mixed with oxygen to form a first mixed reaction gas phase, which is then sent to a first stage preheater 7 (0.92±0.02MPaG) and preheated to 135°C. The preheated reaction gas phase enters the first stage reactor 2 from the top of the first stage reactor 2 to undergo a first stage oxidation reaction (the catalyst may be a conventional commercially available catalyst, such as a palladium catalyst or a gold catalyst), wherein the raw material molar ratio is ethylene: oxygen: acetic acid = 9: 1: 2.5, the reaction temperature is 175°C, and the reaction pressure is 0.89±0.02MPaG; the reaction products include vinyl acetate, water, carbon dioxide, etc., and unreacted raw materials, such as ethylene and acetic acid. During the reaction process, the top produced by the first stage reactor 2 enters the first stage reactor drum 11 (150°C, 0.5MPaG) from the top of the first stage reactor 2, and the low-pressure steam in the first stage reactor drum 11 is separated and sent out for recovery, and the remaining liquid is sent back to the bottom of the first stage reactor 2 by a forced circulation method through the first stage reactor circulation pump 13.

[0073] The reaction product (including the target product and the incompletely reacted gas phase) generated by the first stage oxidation reaction leaves the first stage reactor 2 from the bottom of the first stage reactor 2 and is used as a heat source for heat recovery in the first stage heat exchanger 9, and then the reaction product is sent to the first stage gas-liquid separator 3 (60°C, 0.8±0.02MPaG). The fluid obtained at the bottom of the first stage gas-liquid separator 3 mainly contains most of the target product vinyl acetate, which is sent to the intermediate storage tank area; and the gas phase obtained from the top contains most of the unreacted ethylene (about 56vol%), which then enters the second stage evaporator 4 for further reaction. In other embodiments, a cooler can be set before the first stage gas-liquid separator 3, and the setting temperature of the cooler can ensure that the reaction product can reach the target temperature after entering the first stage gas-liquid separator 3.

[0074] The gas phase obtained from the top of the first-stage gas-liquid separator 3 enters the second-stage evaporator 4 from the bottom of the second-stage evaporator 4 (115°C, 0.78±0.02MPaG). At the same time, supplementary acetic acid enters the second-stage evaporator 4 from the top of the second-stage evaporator 4 (same as the first-stage evaporator 1, countercurrent helps to improve the gasification effect). The gasified mixed gas phase first passes through the second-stage heat exchanger 10 to exchange heat with the high-temperature reaction product obtained by the reaction of the second-stage reactor 5, and then mixes with oxygen to form a second mixed reaction gas phase. The reaction gas phase is input into the second-stage preheater 8 (0.75±0.02MPaG) to be preheated to 135°C, and then enters the second-stage reactor 5 from the top of the second-stage reactor 5, and a second-stage oxidation reaction is carried out under the action of a catalyst (a conventional commercially available catalyst can be selected, such as a palladium catalyst or a gold catalyst), wherein the raw material molar ratio is ethylene: oxygen: acetic acid = 9:1:4, the reaction temperature is 175°C, and the reaction gauge pressure is 0.72±0.02MPaG. Consistent with the first stage reactor 2, the second stage reactor 5 is also connected to a second stage reactor drum 12 (150°C, 0.5MPaG), the inlet of the second stage reactor drum 12 is connected to the top of the second stage reactor 5, the outlet is connected to the inlet of the second stage reactor circulation pump 14, and the outlet of the second stage reactor circulation pump 14 is connected to the bottom of the second stage reactor 5, forming a small circulation of the reactor part. During the second stage reaction of the second stage reactor 5, the top produced material will enter the second stage reactor drum 12 from the top of the second stage reactor 5 for gas-liquid separation, the low-pressure steam obtained by separation is discharged outside the boundary for recovery, and the remaining liquid is sent back to the bottom of the second stage reactor 5 under the forced circulation of the second stage reactor circulation pump 14.

[0075] The reaction products obtained from the second stage reaction (including the target product and the raw materials that have not been completely reacted) are output from the second stage reactor 5, and after passing through the second stage heat exchanger 10 (as a heat source for heat recovery), they enter the second stage gas-liquid separator 6 (60°C, 0.63±0.02MPaG) for separation. The fluid separated at the bottom of the second stage gas-liquid separator 6 contains most of the target products, which are sent to the intermediate storage tank area for storage; and the gas phase separated at the top of the second stage gas-liquid separator 6 (containing the gas phase that has not been completely reacted, mainly including the ethylene that has not been completely reacted, about 48vol%) is extracted from the top of the second stage gas-liquid separator 6, and then this part of the gas phase is transported to the tail gas recovery unit 15 connected to the end of the final reaction unit.

[0076] The tail gas recovery unit 15 is provided with a compressor, an absorption tower and a membrane separator connected in sequence; the gas phase separated at the top of the second-stage gas-liquid separator 6 is input into the compressor for compression and pressure-raising to 1.1MpaG, and the gas phase after pressure-raising is split, wherein 90-99% (volume ratio) of the gas phase is returned to the initial reaction unit as a circulating gas (circulating ethylene) to mix with the newly added ethylene to form an ethylene mixture, and then the ethylene mixture enters the first-stage evaporator 1 to participate in the reaction of the initial reaction unit to form an ethylene cycle, and the remaining 1-10% (volume ratio) of the gas phase enters the absorption tower and the membrane separator in sequence, and the obtained effective gas phase (including ethylene, oxygen and acetic acid) returns to the second-stage evaporator 4 of the final reaction unit and participates in the reaction of the reaction unit, while the remaining inert gas phase is discharged from the venting point.

[0077] In this embodiment, the molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid added to the final reaction unit is 1.6:1, and the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen added to the final reaction unit is 1.4:1.

[0078] A regulating port is provided at the bottom of the evaporator 4 of the second-stage reaction unit. If the load in the second-stage reaction unit is too low to maintain thermal balance, the regulating port is opened to add a proper amount of supplementary ethylene to adjust the thermal balance and ensure normal operation.

[0079] Comparative Example 1

[0080] The process flow of this comparative example only includes one stage reaction unit (initial reaction unit) in Example 1, that is, only includes the content of one stage oxidation reaction in Example 1, and the parameter setting is the same as that in Example 1. The liquid product separated by the first stage gas-liquid separator 3 is collected, and the gas phase product is discharged after treatment.

[0081] Example 2

[0082] The process flow of this embodiment is basically the same as that of Embodiment 1, except for the temperature and pressure of each tower, and the specific parameters are shown in Table 1. In addition, in this embodiment, the molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid added to the final reaction unit is 1.2:1, and the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen added to the final reaction unit is 1.2:1.

[0083] Example 3

[0084] The process flow of this embodiment is basically the same as that of Embodiment 1, except for the temperature and pressure of each tower, and the specific parameters are shown in Table 1. In addition, in this embodiment, the molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid added to the final reaction unit is 2:1, and the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen added to the final reaction unit is 2:1.

[0085] Table 1 Summary of tower parameters for Examples 1-3

[0086]

[0087] Table 2 Ethylene conversion and vinyl acetate selectivity of Examples 1-3 and Comparative Example 1

[0088]

[0089]

[0090] After calculation, after using the systems and processes of Examples 1-3, relative to the single-stage reaction (Comparative Example 1), as shown in Table 2, the single-pass conversion rate of ethylene can be increased by 5-10%, from the original 10% to 15%-18%; the reaction selectivity is also improved to a certain extent, from the original 90% to 90.5%-96.0%; in addition, the compression power consumption per unit vinyl acetate (per ton of vinyl acetate) of the entire device can also be reduced by 30%, which greatly reduces energy consumption.

[0091] Example 4

[0092] In this embodiment, the system is formed by connecting four reaction units in sequence. Figure 2 As shown, the overall system is similar to the system formed by the two-stage reaction units proposed in Example 1, which is briefly described in this example, and the details can be referred to the contents of Example 1.

[0093] The ethylene from the outside is countercurrently contacted with the acetic acid from the outside in the first evaporator 1 and mixed and gasified. The gas phase mixture formed enters the first heat exchanger 9 to heat exchange with the reaction product obtained from the first reactor 2 to increase the temperature, and then further mixes with the oxygen input from the outside to form the first mixed reaction gas phase. The gas phase enters the first preheater 7 for preheating, and then enters the first reactor 2 for a first oxidation reaction. The top product of the first reactor 2 enters the first reactor drum 11 for low-pressure steam collection, and the remaining liquid phase is forced to circulate back to the first reactor 2 under the action of the first reactor circulation pump 13. The reaction product and the incompletely reacted raw material enter the first heat exchanger 9 as a heat source for heat recovery, and then enter the first gas-liquid separator 3 for gas-liquid separation. The separated liquid phase is sent to the intermediate storage tank area, and the gas phase enters the second evaporator 4 to wait for further reaction.

[0094] The gas phase from the first gas-liquid separator 3 is countercurrently contacted with acetic acid from outside the boundary in the second stage evaporator 4 and mixed and gasified (if the load in the second stage reaction unit is too low to maintain heat balance, an appropriate amount of supplementary ethylene is added from the regulating port set at the bottom of the second stage evaporator 4 to adjust the heat balance and ensure normal operation). The formed gas phase mixture enters the second stage heat exchanger 10 to heat and heat the reaction product obtained by the second stage reactor 5, and then further mixes with the oxygen input from outside the boundary to form a second mixed reaction gas phase. The gas phase enters the second stage preheater 8 for preheating, and then enters the second stage reactor 5 for a second stage oxidation reaction. The top product of the second stage reactor 5 enters the second stage reactor drum 12 for low-pressure steam collection, and the remaining liquid phase is forced to circulate back to the second stage reactor 5 under the action of the second stage reactor circulation pump 14. The reaction products and incompletely reacted raw materials enter the second-stage heat exchanger 10 as heat sources for heat recovery, and then enter the second-stage gas-liquid separator 6 for gas-liquid separation. The separated liquid phase is sent to the intermediate storage tank area, and the gas phase enters the third-stage evaporator 41 to wait for further reaction.

[0095] The gas phase from the second-stage gas-liquid separator 6 is countercurrently contacted with acetic acid from outside the boundary in the third-stage evaporator 41 and mixed and gasified (if the load in the three-stage reaction unit is too low to maintain heat balance, an appropriate amount of supplementary ethylene is added from the regulating port set at the bottom of the three-stage evaporator 41 to adjust the heat balance and ensure normal operation). The formed gas phase mixture enters the three-stage heat exchanger 101 to heat and heat the reaction product obtained by the three-stage reactor 51, and then further mixes with the oxygen input from outside the boundary to form a third mixed reaction gas phase. The gas phase enters the three-stage preheater 81 for preheating, and then enters the three-stage reactor 51 for three-stage oxidation reaction. The top product of the three-stage reactor 51 enters the three-stage reactor drum 141 for low-pressure steam collection, and the liquid phase is forced to circulate back to the three-stage reactor 51 under the action of the three-stage reactor circulation pump 141. The reaction products and incompletely reacted raw materials enter the three-stage heat exchanger 101 as heat sources for heat recovery, and then enter the three-stage gas-liquid separator 61 for gas-liquid separation. The separated liquid phase is sent to the intermediate storage tank area, and the gas phase enters the four-stage evaporator 42 to wait for further reaction.

[0096] The gas phase from the three-stage gas-liquid separator 61 countercurrently contacts with acetic acid from outside the boundary in the four-stage evaporator 42 and is mixed and gasified (if the load in the four-stage reaction unit is too low to maintain heat balance, an appropriate amount of supplementary ethylene is added from the regulating port set at the bottom of the four-stage evaporator 42 to adjust the heat balance and ensure normal operation). The formed gas phase mixture enters the four-stage heat exchanger 102 to heat and heat the reaction product obtained by the four-stage reactor 52, and then further mixes with the oxygen input from outside the boundary to form a fourth mixed reaction gas phase. The gas phase enters the four-stage preheater 82 for preheating, and then enters the four-stage reactor 52 for four-stage oxidation reaction. The top product of the four-stage reactor 52 enters the four-stage reactor drum 122 for low-pressure steam collection, and the remaining liquid phase is forced to circulate back to the four-stage reactor 52 under the action of the circulation pump 142 of the four-stage reactor 52. The reaction products and incompletely reacted raw materials enter the four-stage heat exchanger 102 as heat sources for heat recovery, and then enter the four-stage gas-liquid separator 62 for gas-liquid separation. The separated liquid phase is sent to the intermediate storage tank area, and the gas phase enters the tail gas recovery unit 15 to wait for further separation and recovery.

[0097] The gas phase separated by the four-stage gas-liquid separator 62 enters the compressor, and the gas phase after being pressurized by the compressor is split, of which 90-99% (volume ratio) of the gas phase is returned to the initial reaction unit as circulating gas to mix with the newly added ethylene, and the formed ethylene mixture enters the first evaporator 1 of the initial reaction unit to constitute the circulation of ethylene, and the remaining 1-10% enters the absorption tower and the membrane separator, and the obtained effective gas phase returns to the second evaporator 4 and / or the third evaporator 41 and / or the fourth evaporator 42, and participates in the reaction of the corresponding reaction unit, and the remaining inert gas phase is discharged from the venting point.

[0098] In this embodiment, the molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid supplemented to the second-stage reaction unit is 2:1, the molar ratio of the amount of acetic acid added to the second-stage reaction unit to the amount of acetic acid supplemented to the third-stage reaction unit is 1.6:1, and the molar ratio of the amount of acetic acid added to the third-stage reaction unit to the amount of acetic acid supplemented to the final reaction unit is 1.2:1; the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen supplemented to the second-stage reaction unit is 2:1, the molar ratio of the amount of oxygen added to the second-stage reaction unit to the amount of oxygen supplemented to the third-stage reaction unit is 1.6:1, and the molar ratio of the amount of oxygen added to the third-stage reaction unit to the amount of oxygen supplemented to the final reaction unit is 1.2:1.

[0099] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A process for preparing vinyl acetate by oxidation of ethylene, characterized in that: The system includes two or more reaction units; Wherein, the two-stage reaction unit comprises an initial reaction unit and a final reaction unit arranged in sequence along the material feeding direction; or, The two or more reaction units include an initial reaction unit, at least one intermediate reaction unit and a final reaction unit arranged in sequence along the material feeding direction; The initial reaction unit, the intermediate reaction unit and the final reaction unit all include an evaporator, a reactor and a gas-liquid separator arranged in sequence along the material feeding direction; In the working state, the material containing ethylene, acetic acid and oxygen passes through the initial reaction unit and the final reaction unit in sequence to complete the two-stage oxidation reaction; or, The material containing ethylene, acetic acid and oxygen is sequentially passed through an initial reaction unit, at least one intermediate reaction unit and a final reaction unit to complete more than two stages of oxidation reaction; The system further comprises a tail gas recovery unit coupled to the final reaction unit, wherein the tail gas recovery unit comprises a compressor, an absorption tower and a membrane separator. In the working state, the material containing ethylene, acetic acid and oxygen is first subjected to an initial oxidation reaction in the initial reaction unit, and the obtained product is separated from the gas and liquid, and the liquid phase is collected as the target product, and the gas phase is introduced into the final reaction unit, and the final oxidation reaction is carried out with the material containing acetic acid and oxygen entering the final reaction unit. The obtained product is separated from the gas and liquid, and the liquid phase is collected as the target product, and the gas phase enters the compressor, and the gas phase is split after the compressor increases the pressure, and 90-99 vol% of the gas phase returns to the initial unit to mix with the newly added ethylene, and the remaining gas phase enters the absorption tower and the membrane separator in sequence, and then returns to the evaporator of the final reaction unit, and the above process is repeated; or, Under working conditions, the material containing ethylene, acetic acid and oxygen is first subjected to an initial oxidation reaction in the initial reaction unit. After the gas-liquid separation of the obtained product, the liquid phase is collected as the target product, and the gas phase is introduced into at least one intermediate reaction unit located downstream of the initial reaction unit, and undergoes at least one intermediate oxidation reaction with the material containing acetic acid and oxygen entering the at least one intermediate reaction unit. After the gas-liquid separation of the obtained product, the liquid phase is collected as the target product, and the gas phase is introduced into the final reaction unit located downstream of the at least one intermediate reaction unit, and undergoes a final oxidation reaction with the material containing acetic acid and oxygen entering the final reaction unit. After the gas-liquid separation of the obtained product, the liquid phase is collected as the target product, and the gas phase enters the compressor. The gas phase is split after being pressurized by the compressor, and 90-99 vol% of the gas phase returns to the initial unit to mix with the newly added ethylene. The remaining gas phase enters the absorption tower and the membrane separator in sequence, and then returns to the evaporator of the at least one intermediate reaction unit and / or the final reaction unit, and the above process is repeated.

2. The process for preparing vinyl acetate by oxidation of ethylene according to claim 1, characterized in that: Each reaction unit further comprises a preheater disposed between the evaporator and the reactor to preheat the gas phase containing ethylene, acetic acid and oxygen from the evaporator before entering the reactor.

3. The process for preparing vinyl acetate by oxidation of ethylene according to claim 1, characterized in that: Each reaction unit also includes a reactor drum, and the top extraction of the reactor enters the reactor drum for steam collection, and the obtained liquid phase returns to the reactor. The temperature of the reaction drum is 145-155°C.

4. The process for preparing vinyl acetate by oxidation of ethylene according to claim 1, characterized in that: The gas phase separated by the gas-liquid separator of the final reaction unit is pressurized to 0.9-1.2 MPaG by a compressor for diversion.

5. The process for preparing vinyl acetate by oxidation of ethylene according to claim 1, characterized in that: The bottom of the evaporator in at least one intermediate reaction unit and the final reaction unit is provided with a regulating port for replenishing ethylene to the evaporator to adjust the load of the corresponding unit and maintain thermal balance.

6. The process for preparing vinyl acetate by oxidation of ethylene according to claim 1, characterized in that: In the oxidation reaction, the molar ratio of ethylene, oxygen and acetic acid is (7-12):1:(1-4), the reaction pressure is 0.70-0.91 MPaG, and the reaction temperature is 120-185°C.

7. The process for preparing vinyl acetate by oxidation of ethylene according to claim 1, characterized in that: The molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid added to the final reaction unit is (1.2-2):1, and the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen added to the final reaction unit is (1.2-2):1; or, The molar ratio of the amount of acetic acid added to the initial reaction unit to the amount of acetic acid supplemented into at least one intermediate reaction unit located downstream of the initial reaction unit is (1.2-2):1, the molar ratio of the amount of oxygen added to the initial reaction unit to the amount of oxygen supplemented into at least one intermediate reaction unit located downstream of the initial reaction unit is (1.2-2):1, the amount of acetic acid in at least one intermediate reaction unit to the amount of acetic acid in the final reaction unit located downstream of the at least one intermediate reaction unit is (1.2-2):1, and the amount of oxygen in at least one intermediate reaction unit to the amount of oxygen in the final reaction unit located downstream of the at least one intermediate reaction unit is (1.2-2):1.

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