Process for the production of vinyl acetate monomer

Abstract

A method for producing vinyl acetate monomer includes feeding an inlet stream comprising ethylene, acetic acid, oxygen, and carbon dioxide into a reactor. Ethylene, acetic acid, and oxygen react in the presence of a palladium-containing catalyst to produce a product stream comprising vinyl acetate monomer, ethylene, acetic acid, water, and carbon dioxide. The product stream is passed through a flash separator to provide a vapor stream comprising carbon dioxide and ethylene, and a liquid stream comprising acetic acid and vinyl acetate monomer. At least a portion of the vapor stream is recycled to the inlet stream fed into the reactor, such that the amount of carbon dioxide entering the reactor in the inlet stream is in the range of greater than 5 mol% to 20 mol%, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor. Vinyl acetate monomer is recovered from the liquid stream.

Description

Technical Field

[0001] This invention relates to a method for producing vinyl acetate monomer. Background Technology

[0002] Vinyl acetate, commonly known as vinyl acetate monomer or VAM, is a high-volume chemical used to produce several different polymers.

[0003] VAM is typically prepared continuously in the gas-phase reaction of ethylene with acetic acid and oxygen.

[0004] C2H3 + CH3COOH + ½O2 CH3COOCH=CH2 + H2O

[0005] Carbon dioxide is produced as a byproduct through the oxidation of ethylene.

[0006] C2H4 + 3O2 → 2CO2 + 2H2O

[0007] Because carbon dioxide is considered an unwanted byproduct of the reaction, efforts have been made to reduce the selectivity for carbon dioxide and / or reduce the amount of carbon dioxide present in the reactor.

[0008] U.S. Patent No. 8,029,748 discloses a method for producing VAM in which the selectivity for carbon dioxide is reduced. The method comprises a reaction stream containing a high concentration of reactants in near-stoichiometric amounts, with a relative ratio of ethylene:acetic acid:oxygen of 2.5:2.5:1.

[0009] U.S. Patent No. 7,803,965 discloses a method for producing VAM, wherein carbon dioxide is removed from a recirculation stream to reduce the amount of energy used by the system. The recirculation stream is washed to reduce the amount of carbon dioxide to 1% to 4% by volume.

[0010] VAM is typically prepared in fixed-bed or fluidized-bed reactors using catalysts, which usually consist of palladium and alkali metal salts on a support material. The catalyst may also contain other elements such as gold, rhodium, or cadmium. The activity of the catalyst decreases over time. To counteract this decrease in activity, the temperature in the VAM reactor can be increased at the expense of reduced product selectivity. However, this results in a further decrease in catalyst selectivity and necessitates catalyst replacement.

[0011] Catalyst deactivation remains a problem in VAM production. Due to deactivation, the catalyst must be replaced periodically, resulting in production losses and significant costs.

[0012] Motahari et al. (The Canadian Journal of Chemical Engineering, March 2016, Vol. 94, No. 3, pp. 506-511) explored the deactivation of palladium-gold catalysts in VAM production. In a model developed by Motahari et al., they determined the deactivation rate as a function of time, temperature, and ethylene concentration.

[0013] A method is still needed to reduce catalyst deactivation and / or improve VAM selectivity in VAM production. Summary of the Invention

[0014] According to one aspect of the invention, a method for producing vinyl acetate monomer includes feeding an inlet stream comprising ethylene, acetic acid, oxygen, and carbon dioxide into a reactor. Ethylene, acetic acid, and oxygen react in the presence of a palladium-containing catalyst to produce a product stream comprising vinyl acetate monomer, ethylene, acetic acid, water, and carbon dioxide. The product stream is passed through a flash separator to provide a vapor stream comprising carbon dioxide and ethylene, and a liquid stream comprising acetic acid and vinyl acetate monomer. At least a portion of the vapor stream is recycled to the inlet stream fed into the reactor, such that the amount of carbon dioxide entering the reactor in the inlet stream is in the range of greater than 5 mol% to 20 mol%, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor. Vinyl acetate monomer is recovered from the liquid stream. Detailed Implementation

[0015] As used herein, the terms “a,” “the,” “at least one,” and “a or more” are used interchangeably. The terms “comprising,” “including,” “containing,” and variations thereof, when used in the specification and claims, are not restrictive. Thus, for example, a mixture comprising a polymerization inhibitor can be interpreted as the mixture containing at least one polymerization inhibitor.

[0016] As used herein, a numerical range described by endpoints includes all numbers encompassed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). For the purposes of this invention, it should be understood, consistent with the understanding of those skilled in the art, that a numerical range is intended to include and support all possible subranges included within that range. For example, a range of 1 to 100 is intended to express 1.1 to 100, 1 to 99.99, 1.01 to 99.99, 40 to 6, 1 to 55, etc.

[0017] As used herein, descriptions of numerical ranges and / or values, including such descriptions in the claims, may be read, including the term "about". In such cases, the term "about" refers to numerical ranges and / or values ​​that are substantially the same as those described herein.

[0018] Unless otherwise stated or implied from the context, all parts and percentages are based on weight, and all testing methods are up-to-date as of the date of this application.

[0019] This invention relates to a method for producing vinyl acetate monomer.

[0020] The inventors have surprisingly discovered that, in the production of vinyl acetate monomer, increasing the amount of carbon dioxide present in the reactor extends the catalyst lifetime by reducing the catalyst deactivation rate. In any method for preparing vinyl acetate monomer, the catalyst loses activity over time. To overcome the decrease in activity in conventional vinyl acetate monomer production methods, the temperature in the reactor is increased. However, increasing the reactor temperature results in lower selectivity for vinyl acetate monomer. Therefore, reducing the deactivation rate is a significant improvement, as any reduction in the deactivation rate leads to an increase in selectivity during the catalyst lifetime by enabling the production process to operate at cooler temperatures.

[0021] Vinyl acetate monomer is formed through the acetylation of ethylene in a gas-phase reaction. Ethylene reacts with acetic acid and oxygen to form vinyl acetate monomer. In a side reaction, ethylene also reacts with oxygen to form carbon dioxide.

[0022] By using a recurrent neural network model trained with data accumulated over many years from operating a vinyl acetate monomer production plant, the inventors have discovered that including carbon dioxide in the reactor can significantly extend the catalyst lifetime (i.e., reduce the deactivation rate).

[0023] In this invention, vinyl acetate monomer is prepared by controlling the amount of carbon dioxide entering the reactor. Carbon dioxide enters the reactor as an inlet stream that also contains ethylene, acetic acid, and oxygen. As used herein, the term "inlet stream" refers to the gas entering the reactor. The inlet stream may include one or more gas streams. For example, ethylene may enter the reactor from both a feed stream of fresh ethylene entering the system and a portion of a recirculation stream containing ethylene and at least one other gas.

[0024] In the reactor, ethylene, acetic acid, and oxygen react in the presence of a palladium-containing catalyst to produce a product stream. The product stream contains vinyl acetate monomer as well as unreacted ethylene and acetic acid. The product stream also contains carbon dioxide and water, which are byproducts of the reaction.

[0025] The product stream is passed through a flash separator to provide a vapor stream and a liquid stream. The vapor stream contains carbon dioxide and ethylene. The liquid stream contains acetic acid and vinyl acetate monomer. The vinyl acetate monomer is recovered from the liquid stream. Vinyl acetate monomer can be recovered from the liquid stream using any known method. Acetic acid can also be recovered from the liquid stream and can be recycled back to the reactor.

[0026] At least a portion of the steam stream is recirculated back to the reactor to be added to the inlet stream.

[0027] The inventors have discovered that, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide, an amount of carbon dioxide in the range of greater than 5 mol% to 20 mol% can extend catalyst lifetime by at least 5%, such as at least 10%. In the production of vinyl acetate monomer, even an increase in catalyst lifetime of 1% or 2% would be significant, as catalyst replacement is very time-consuming and expensive.

[0028] The recirculation flow is adjusted such that, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor, the amount of carbon dioxide entering the reactor is greater than 5 mol%, preferably at least 6 mol%, more preferably at least 8 mol%, even more preferably at least 10 mol%, still more preferably at least 12 mol%, and even more preferably at least 14 mol%. Based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor, the amount of carbon dioxide entering the reactor is not greater than 20 mol%, preferably not greater than 19 mol%, even more preferably not greater than 18 mol%, and still more preferably not greater than 17 mol%. As used herein, the phrase "entering the reactor" refers to the total gas (i.e., ethylene, acetic acid, oxygen, and carbon dioxide) at the reactor inlet. The gas may enter the reactor individually or in a combination of one or more streams. For example, the recirculation flow may be combined with the feed flow entering the reactor, or the recirculation flow may enter the reactor separately from the feed flow entering the reactor. The gas entering the reactor is collectively referred to as the inlet flow.

[0029] Additional gases may also be introduced into the reactor. For example, inert gases or diluents may be used. Oxygen may be present in the form of oxygen gas or in the form of oxygen present in the air entering the reactor. Preferably, oxygen gas is present in the air entering the reactor. When oxygen gas is present in the air entering the reactor, the amount of gas is calculated using only the actual amount of oxygen gas.

[0030] Based on the total amount of ethylene, acetic acid, oxygen and carbon dioxide entering the reactor, the amount of ethylene entering the reactor can be in the range of 20 mol% to 60 mol%, preferably 25 mol% to 50 mol%, and even more preferably 30 mol% to 40 mol%.

[0031] Based on the total amount of ethylene, acetic acid, oxygen and carbon dioxide entering the reactor, the amount of acetic acid entering the reactor can be in the range of 15 mol% to 55 mol%, preferably 20 mol% to 45 mol%, more preferably 25 mol% to 35 mol%.

[0032] The molar ratio of ethylene to oxygen entering the reactor can be in the range of 8:1 to 2:1. Preferably, the molar ratio of ethylene to oxygen entering the reactor is in the range of 7.5:1 to 4:1, more preferably in the range of 7:1 to 5:1.

[0033] The molar ratio of acetic acid to oxygen entering the reactor can be in the range of 8:1 to 2:1. Preferably, the molar ratio of acetic acid to oxygen entering the reactor is in the range of 7.5:1 to 4:1, more preferably in the range of 7:1 to 5:1.

[0034] The acetoxylation reactor is preferably operated at pressures above atmospheric pressure, such as, for example, above 50 psig. The reactor can be maintained at a temperature in the range of 100°C to 200°C. The reactor temperature can be varied with changes in catalyst activity. For example, when activity decreases, the reactor temperature can be increased to maintain the production of vinyl acetate monomer.

[0035] The reactor can be a fixed-bed reactor or a fluidized-bed reactor. Preferably, the reactor is a fixed-bed reactor.

[0036] The reactor contains a catalyst for the acetoxylation reaction. The catalyst is a palladium-containing catalyst. Preferably, the catalyst contains palladium in an amount of at least 0.5% by weight based on the total weight of the catalyst. More preferably, the catalyst contains at least 0.7% by weight of palladium based on the total weight of the catalyst.

[0037] The palladium-containing catalyst may contain additional metals selected from gold, cadmium, and rhodium. Preferably, the palladium-containing catalyst contains gold. When using a gold-palladium catalyst, the weight ratio of gold to palladium can be in the range of 0.01 to 0.8 wt / wt. Preferably, the weight ratio of gold to palladium is in the range of 0.1 to 0.7 wt / wt.

[0038] The palladium-containing catalyst may include a support. The support may be selected from silica, alumina, and titanium dioxide. Preferably, the support comprises silica. When present, the support may be present in an amount of at least 80% by weight based on the total weight of the catalyst.

[0039] The palladium-containing catalyst may also contain an alkali metal acetate. Suitable alkali metals include, for example, lithium, sodium, potassium, and cesium. Preferably, the alkali metal acetate comprises potassium acetate. When present, the alkali metal acetate may be present in an amount ranging from 4% to 20% by weight, based on the total weight of the catalyst. Because the alkali metal acetate may be lost from the catalyst over time, additional alkali metal acetate may be added to the reactor as needed.

[0040] Example

[0041] The following examples illustrate the present invention, but are not intended to limit the scope of the invention.

[0042] A recurrent neural network model was trained using MATLAB to model a single tube of a VAM reactor based on the Hagan method, representing a steady-state, pseudo-homogeneous, plug flow reactor. The model captures the relevant mass and energy balances used to describe VAM production and utilizes a laboratory-based kinetic model to capture VAM kinetics. The neural network was used to evaluate how a range of process variables affect catalyst deactivation.

[0043] The input layer of the neural network model consists of nodes for each process variable listed in Table 1 below. Furthermore, the neural network includes hidden layers and an output layer corresponding to the inactivation rate constant.

[0044] Table 1

[0045]

[0046] To avoid gradient vanishing caused by the model forgetting older information in the time series, a Long Short-Term Memory (LSTM) recurrent neural network was used. The neural network was trained using hourly averages collected from the VAM production plant over a 576-day period. 80% of the available data was used to train the model, and the remaining 20% ​​was used to test the model's fit. The overall normalized root mean square error for the training and testing data was 0.4643 and 0.4986, respectively.

[0047] Using the model described above, it was surprisingly found that increasing the flow rate of carbon dioxide into the reactor by 5% resulted in a 14% increase in the amount of VAM during disturbance, meaning that the catalyst deactivation rate was significantly improved.

Claims

1. A method for producing vinyl acetate monomer, the method comprising: The feed to the reactor consists of an inlet stream of ethylene, acetic acid, oxygen, and carbon dioxide; The ethylene, acetic acid, and oxygen are reacted in the presence of a palladium-containing catalyst to produce a product stream comprising vinyl acetate monomer, ethylene, acetic acid, water, and carbon dioxide. The product stream is passed through a flash separator to provide a vapor stream containing carbon dioxide and ethylene, and a liquid stream containing acetic acid and vinyl acetate monomers; At least a portion of the vapor stream is recycled to the inlet stream fed into the reactor, such that the amount of carbon dioxide entering the reactor in the inlet stream is in the range of greater than 5 mol% to 20 mol%, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor; and The vinyl acetate monomer is recovered from the liquid stream.

2. The method according to claim 1, wherein the amount of carbon dioxide entering the reactor in the inlet stream is in the range of 8 mol% to 19 mol% based on the total amount of ethylene, acetic acid, oxygen and carbon dioxide.

3. The method according to claim 2, wherein the amount of ethylene, acetic acid, oxygen and carbon dioxide entering the reactor in the inlet stream is in the range of 10 mol% to 18 mol%.

4. The method according to any one of the preceding claims, wherein the catalyst comprises palladium and gold.

5. The method of claim 4, wherein the ratio of gold to palladium is in the range of 0.01 to 0.8 wt / wt.

6. The method of claim 5, wherein the ratio of gold to palladium is in the range of 0.1 to 0.7 wt / wt.

7. The method according to any one of the preceding claims, wherein the amount of palladium in the catalyst is at least 0.5% by weight based on the total weight of the catalyst.

8. The method of claim 7, wherein the amount of palladium in the catalyst is at least 0.7 wt% based on the total weight of the catalyst.

9. The method according to any one of the preceding claims, wherein the catalyst comprises a support selected from silica, alumina, titanium dioxide and magnesium oxide.

10. The method of claim 9, wherein the carrier comprises silicon dioxide.

11. The method of claim 9 or claim 10, wherein the support is present in an amount of at least 80% based on the total weight of the catalyst.

12. The method according to any one of the preceding claims, wherein the catalyst comprises an alkali metal acetate, and the amount of the alkali metal acetate is in the range of 4% to 20% by weight based on the total weight of the catalyst.

13. The method of claim 12, wherein the alkali metal acetate comprises an alkali metal selected from lithium, sodium, potassium and cesium.

14. The method of claim 13, wherein the alkali metal acetate comprises potassium acetate.

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