Process for producing vinyl acetate monomer

Abstract

A process for producing vinyl acetate monomer comprises feeding a reactor an inlet stream comprising 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 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 feeding the reactor so that the amount of carbon dioxide entering the reactor in the inlet stream ranges from greater than 5 mol% to 20 mol% based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor. The vinyl acetate monomer is recovered from the liquid stream.

Description

PROCESS FOR PRODUCING VINYL ACETATE MONOMERFIELD OF THE INVENTION

[0001] The present invention relates to a process for producing vinyl acetate monomer.BACKGROUND OF THE INVENTION

[0002] Vinyl acetate, commonly referred to as vinyl acetate monomer or VAM, is a high- volume chemical that is used for the production of several different polymers.

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

[0004] Carbon dioxide is produced as a byproduct through the oxidation of ethylene.

[0005] Because carbon dioxide is considered an unwanted byproduct of the reaction, efforts have been made to decrease the selectivity to carbon dioxide and / or to reduce the amount of carbon dioxide present in the reactor.

[0006] U.S. Patent No. 8,029,748 discloses a process for producing VAM in which the selectivity to carbon dioxide is reduced. The process comprises a reactant stream containing high concentrations of reactants in near stoichiometric amounts, The relative amounts of ethylene:acetic acid:oxygen is 2.5:2.5:1 .

[0007] U.S. Patent No. 7,803,965 discloses a process for producing VAM in which carbon dioxide is removed from the recycle stream to reduce the amount of energy used by the system. The recycle stream is scrubbed to reduce the amount of carbon dioxide to 1 to 4% by volume.

[0008] VAM is conventionally prepared in either a fixed bed reactor or a fluidized bed reactor using a catalyst that generally comprises 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 counter the decrease in catalyst activity, the temperature in the VAM reactor may be increased at the cost ofreduced product selectivity. There is a point, however, at which the selectivity of the catalyst decreases and the catalyst must be replaced.

[0009] Catalyst deactivation remains a problem in VAM production. Due to deactivation, the catalyst must be replaced periodically, which results in loss of production and extensive costs.

[0010] Motahari et al. (The Canadian Journal of Chemical Engineering, Vol. 94, Issue 3, March 2016, p. 506-51 1 ) explored the deactivation of palladium-gold catalyst in VAM production. In the model developed by Motahari et al., the deactivation rate law was determined as a function of time, temperature, and ethylene concentration.

[0011] There remains a need for a process that can reduce catalyst deactivation in VAM production and / or increase the selectivity to VAM.SUMMARY OF THE INVENTION

[0012] According to one aspect of the present invention, a process for producing vinyl acetate monomer comprises feeding a reactor an inlet stream comprising 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 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 feeding the reactor so that the amount of carbon dioxide entering the reactor in the inlet stream ranges from greater than 5 mol% to 20 mol% based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor. The vinyl acetate monomer is recovered from the liquid stream.DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0013] As used herein, the terms “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably. The terms “comprises,” “includes,” “contains,” and variations thereof do not have a limiting meaning where these terms appear in the description andclaims. Thus, for example, a mixture that includes a polymerization inhibitor can be interpreted to mean that the mixture comprises at least one polymerization inhibitor.

[0014] As used herein, recitations of numerical ranges by endpoints includes all numbers subsumed in that range (e.g. 1 to 5 includes 1 , 1 .5, 2, 2.75, 3, 3.80, 4, 5, etc.). For the purposes of the invention, it is to be understood, consistent with what one of ordinary skill in the art would understand, that a numerical range is intended to include and support all possible subranges that are included in that range. For example, the range from 1 to 100 is intended to convey from 1 .1 to 100, from 1 to 99.99, from 1 .01 to 99.99, from 40 to 6, from 1 to 55, etc.

[0015] As used herein, the recitations of numerical ranges and / or numerical values, including such recitations in the claims, can be read to include the term “about.” In such instances, the term “about” refers to numerical ranges and / or numerical values that are substantially the same as those recited herein.

[0016] Unless stated to the contrary, or implicit from the context, all parts and percentages are based on weight and all test methods are current as of the filing date of this application.

[0017] The present invention relates to a process for producing vinyl acetate monomer.

[0018] The inventors have surprisingly discovered that increasing the amount of carbon dioxide present in the reactor during the production of vinyl acetate monomer results in extending the lifetime of the catalyst by reducing the rate of deactivation of the catalyst. In any process for producing vinyl acetate monomer, the catalyst will lose activity over time. To counter the reduction in activity in conventional vinyl acetate monomer production processes, the temperature in the reactor is increased. Increasing the reactor temperature, however, results in lower selectivity to vinyl acetate monomer.Therefore, decreasing the rate of deactivation is a significant improvement because any decrease in the rate of deactivation results in an increase in the selectivity over the life of the catalyst by enabling the production process to be run at cooler temperatures.

[0019] Vinyl acetate monomer is formed by the acetoxylation of ethylene in a gas phase reaction. Ethylene is reacted with acetic acid and oxygen to form vinyl acetate monomer. In a side reaction, ethylene also reacts with oxygen to form carbon dioxide.

[0020] Through the use of a recurrent neural network model trained with data accumulated from years of running a vinyl acetate monomer production plant, the present inventors have found that including carbon dioxide in the reactor can significantly extend the lifetime (i.e., decrease the rate of deactivation) of the catalyst.

[0021] In the present invention, vinyl acetate monomer is produced by a process in which the amount of carbon dioxide entering the reactor is controlled. Carbon dioxide enters a reactor as an inlet stream that also comprises ethylene, acetic acid, and oxygen. As used herein, the term “inlet stream” refers to the gases entering the reactor. The inlet stream may comprise one or multiple streams of gases. For example, ethylene may enter the reactor from both a feed stream of new ethylene entering the system, as well as part of a recycle stream that contains ethylene and at least one other gas.

[0022] In the reactor, ethylene, acetic acid, and oxygen are reacted in the presence of a palladium-containing catalyst to produce a product stream. The product stream comprises vinyl acetate monomer, as well as unreacted ethylene and acetic acid. The product stream further comprises carbon dioxide and water, which are byproducts of the reaction.

[0023] The product stream is passed through a flash separator to provide a vapor stream and a liquid stream. The vapor stream comprises carbon dioxide and ethylene. The liquid stream comprises acetic acid and vinyl acetate monomer. The vinyl acetate monomer is recovered from the liquid stream. The vinyl acetate monomer may be recovered from the liquid stream using any known process. The acetic acid may also be recovered from the liquid stream and may be recycled back to the reactor.

[0024] At least a portion of the vapor stream is recycled back to the reactor to join the inlet stream.

[0025] The inventors have found that an amount of carbon dioxide ranging from greater than 5 mol% to 20 mol% based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide may extend the lifetime of the catalyst by at least 5%, such as by at least 10%. In the production of vinyl acetate monomer, even a 1 or 2% increase in the catalyst lifetime would be significant because catalyst replacement costs substantial amounts of time and money.

[0026] The recycle stream is adjusted so that 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 yet 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 no greater than 20 mol%, preferably no greater than 19 mol%, even more preferably no greater than 18 mol%, and still more preferably no greater than 17 mol% based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor. As used herein, the phrase “entering the reactor” means the total gases (i.e., ethylene, acetic acid, oxygen, and carbon dioxide) at the inlet of the reactor. The gases may enter the reactor individually or in combinations of one or more streams. For example, the recycle stream may combine with a feed stream entering the reactor, or the recycle stream may enter the reactor separately from a feed stream entering the reactor. Collectively, the gases entering the reactor are referred to as the inlet stream.

[0027] Additional gases may also enter the reactor. For example, inert gases or diluents may also be used. The oxygen may be present in the form of oxygen gas or as the oxygen present in air that enters the reactor. Preferably, the oxygen is present in air entering the reactor. When the oxygen is present in air entering the reactor, only the actual amount of oxygen is used to calculate the amounts of the gases.

[0028] The amount of ethylene entering the reactor may range from 20 mol% to 60 mol%, preferably from 25 mol% to 50 mol%, and even more preferably from 30 mol% to 40 mol%, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor.

[0029] The amount of acetic acid entering the reactor may range from 15 mol% to 55 mol%, preferably from 20 mol% to 45 mol%, and more preferably from 25 mol% to 35 mol%, based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor.

[0030] The molar ratio of ethylene to oxygen entering the reactor may range from 8:1 to 2:1 . Preferably, the molar ratio of ethylene to oxygen entering the reactor ranges from 7.5:1 to 4:1 , and more preferably from 7:1 to 5:1 .

[0031] The molar ratio of acetic acid to oxygen entering the reactor may range from 8:1 to 2:1 . Preferably, the molar ratio of acetic acid to oxygen entering the reactor ranges from 7.5:1 to 4:1 , and more preferably from 7:1 to 5:1 .

[0032] The acetoxylation reactor is preferably operated at above atmospheric pressure, such as, for example above 50psig. The reactor may be maintained at a temperature ranging from 100 °C to 200 °C. The reactor temperature may be changed as the activity of the catalyst changes. For example, when the activity decreases, the temperature of the reactor may be increased to maintain production of the vinyl acetate monomer.

[0033] The reactor may be a fixed bed reactor or a fluidized bed reactor. Preferably, the reactor is a fixed bed reactor.

[0034] 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 wt% based on the total weight of the catalyst. More preferably, the catalyst contains palladium in an amount of at least 0.7 wt% based on the total weight of the catalyst.

[0035] The palladium-containing catalyst may contain an additional metal selected from gold, cadmium, and rhodium. Preferably, the palladium-containing catalyst comprises gold. When a gold-palladium catalyst is used, the weight ratio of gold to palladium may range from 0.01 to 0.8 wt / wt. Preferably, the weight ratio of gold to palladium ranges from 0.1 to 0.7 wt / wt.

[0036] The palladium-containing catalyst may comprise 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 wt% based on the total weight of the catalyst.

[0037] The palladium-containing catalyst may further comprise 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 wt% to 20 wt% 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.Example

[0038] The following example illustrates the present invention but is not intended to limit the scope of the invention.

[0039] A recurrent neural network model was trained using MATLAB to model a steadystate, pseudo-homogeneous, plug-flow reactor based on the Hagan Method to describe a single tube of the VAM reactors. The model captured relevant mass and energy balances for describing VAM production and utilized laboratory-based kinetic models to capture VAM kinetics. The neural network was used to evaluate how a range of process variables impacted catalyst deactivation.

[0040] The input layer of the neural network model consisted of a node for each of the process variables listed below in Table 1 . In addition, the neural network comprised a hidden layer, and an output layer corresponding to the deactivation rate constants.Table 1

[0041] To avoid vanishing gradients caused when older information in a time series is forgotten by the model, a long short-term memory recurrent neural network was used. The neural network was trained using hourly averages collected from a VAM production plant over a period of 576 days. 80% of the available data was used to train the model, with the additional 20% of data being used to test the fit of the model. The overall normalized root mean square errors for the training and test data was 0.4643 and 0.4986, respectively.

[0042] Using the model described above, it was surprisingly discovered that increasing the flow rate of carbon dioxide entering the reactor by 5% resulted in a 14% increase in the amount of VAM over the perturbation period, i.e., the deactivation rate of the catalyst was significantly improved.

Claims

We claim:1 . A process for producing vinyl acetate monomer, the process comprising: feeding a reactor an inlet stream comprising ethylene, acetic acid, oxygen, and carbon dioxide; reacting the ethylene, acetic acid, and oxygen in the presence of a palladiumcontaining catalyst to produce a product stream comprising vinyl acetate monomer, ethylene, acetic acid, water, and carbon dioxide, passing the product stream 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; recycling at least a portion of the vapor stream to the inlet stream feeding the reactor so that the amount of carbon dioxide entering the reactor in the inlet stream ranges from greater than 5 mol% to 20 mol% based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide entering the reactor; and recovering the vinyl acetate monomer from the liquid stream.

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

3. The process of claim 2, wherein the entering the reactor in the inlet stream ranges from 10 mol% to 18 mol% based on the total amount of ethylene, acetic acid, oxygen, and carbon dioxide.

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

5. The process of claim 4, wherein the ratio of gold to palladium ranges from 0.01 to 0.8 wt / wt.

6. The process of claim 5, wherein the ratio of gold to palladium ranges from 0.1 to 0.7 wt / wt.

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

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

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

10. The process of claim 9, wherein the support comprises silica.1 1. A process 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 process of any one of the preceding claims, wherein the catalyst comprises an alkali metal acetate in an amount ranging from 4 to 20 wt% based on the total weight of the catalyst.

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

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

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