Process for the production of vinyl acetate

Details Process for the production of vinyl acetate Process for the production of vinyl acetate

1999-07-29

2002-01-29

Geist, Gary (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S243000

Reexamination Certificate

active

06342628

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for the production of vinyl acetate.
Vinyl acetate may be produced by the acetoxylation of ethylene in the presence of a palladium-containing catalyst. In addition to vinyl acetate, carbon dioxide is produced as a by-product.
In commercial operation, vinyl acetate product and unreacted ethylene are recovered from the gases exiting the reaction vessel. The unreacted ethylene is recycled, together with fresh ethylene to the reaction vessel. The recycled ethylene contains some carbon dioxide and other by-products as well as some inert gases such as nitrogen and/or argon. The presence of these components limits the concentration of ethylene in the feed to the reaction vessel.
The factors which influence the selectivity of the process towards vinyl acetate are not fully understood. The general view taken by those skilled in the art is that once the concentration of ethylene in the reactant mixture exceeds a threshold value, the rate of formation of vinyl acetate becomes independent of the concentration of ethylene. The rationale behind this view is that even if ethylene is involved in the rate determining step, the rate of formation of vinyl acetate with respect to ethylene will tend to zero once an amount greater than the stoichiometric amount of ethylene is exceeded. In other words, as long as an excess of ethylene is present in the reactant mixture, the magnitude of this excess should not have a bearing on the amount of vinyl acetate produced.
This general view or technical prejudice is supported by experimental data in Davidson et al. (Front. Chem. React. Eng., 1984, (1) 300-313). The data show that in acetoxylation reactions carried out at atmospheric pressure, the amount of vinyl acetate produced remains substantially constant as the ethylene concentration is increased between 31.6 mol % to 47.4 mol %.
The technical prejudice also extends to acetoxylation reactions performed above atmospheric pressure. In Abel et al. (Chem. Eng. Technol. 17 (1994) 112-118), ethylene, acetic acid and oxygen are reacted together at a total pressure of 8 barg. The document recites that the rate of formation of vinyl acetate is independent of the concentration of ethylene in the reaction mixture, provided that the concentration of ethylene exiting the reactor is above 30 mol %. To ensure that the effluent concentration exceeds this value, ethylene feed concentrations of 57 mol % are employed.
R S Shetty and S B Chandalia in Metals and Minerals Review December 1970 35-40 propose that in an industrial process an ethylene concentration of more than about 65% may be chosen to keep outside the explosive limit. However in an experiment using a gas mixture containing a mole ratio of ethylene to oxygen of 69.3:30.7 the catalyst activity changed rapidly with time. Furthermore, the concentration of acetic acid and hence the ethylene concentration is not apparent. Neither is it apparent whether ethylene was recovered and recycled.
Nakamura et al in J. Catal. 17 (1970) 366-374 describes the effect of potassium acetate on the catalytic activity of a palladium catalyst with a feed gas of ethylene: oxygen: acetic acid of 80:10:10 (FIG.
4
). However it is not apparent that ethylene was recycled nor is any beneficial effect of high ethylene concentration described.
Samanos et al in J Catal. (1971) 23 19-30 describe the rate of reaction forming vinyl acetate varying linearly with ethylene partial pressure whilst the rate at which carbon dioxide is formed remains constant. However, it is not apparent the ethylene is recovered and recycled in the experiment.
DESCRIPTION OF THE INVENTION
We have now found that when vinyl acetate is produced above atmospheric pressure, the selectivity towards vinyl acetate increases as the concentration of ethylene in the reactant mixture is increased above 60 mol %. This finding is contrary to the technical prejudice in the art.
Accordingly, the present invention provides a process for the production of vinyl acetate, said process comprising the steps of:
(a) introducing ethylene, acetic acid and an oxygen containing gas into a reactor,
(b) reacting said ethylene, acetic acid, and oxygen-containing gas in the presence of a catalyst material in said reactor to produce vinyl acetate at above atmospheric pressure,
(c) withdrawing from said reactor gases comprising unreacted ethylene, vinyl acetate, carbon dioxide by product and inert gases such as nitrogen and/or argon;
(d) recovering from the gases withdrawn from said reactor, unreacted ethylene optionally together with minor amounts of said carbon dioxide and inert gases, and
(e) introducing the recovered ethylene from step (d) and additional ethylene to said reactor in step (a) wherein the amount of ethylene in the combined feed to the reactor is at least 60 mol % .
Preferably also, the method further comprises the step of recovering the vinyl acetate produced.
The present invention provides a novel and cost effective route for the production of vinyl acetate at above atmospheric pressure, 0 Pa gauge (0 barg). The reaction may be carried out at a pressure of between 5×10
4
Pa gauge (0.5 barg) and 2×10
6
Pa gauge (20 barg), preferably between 6×10
5
Pa gauge (6 barg) and 1.4×10
6
Pa gauge (14 barg) and, most preferably, between 7×10
5
Pa gauge (7 barg) and 1.2×10
6
Pa gauge (12 barg). These pressure conditions are believe to affect the intersection between ethylene and the surface of the catalyst such that an increase in selectivity towards the production of vinyl acetate is observed when the amount of ethylene in the feed is increased above 60 mol%. This effect is surprising: the rate of vinyl acetate production with respect to ethylene is expected to be of zero order once an initial ethylene concentration of at least 57 mol % is employed as taught in Abel et al. (Chem. Eng. Technol. 17 (1994) 112-118).
Whilst we do not wish to be bound to theory, the increase in selectivity may be explained by a combination of two factors. In addition to increasing the amount of vinyl acetate produced, an increase in the amount of ethylene in the feed is also found to reduce the amount of carbon dioxide produced as a by-product. This in turn, can reduce the amount of carbon dioxide which has to be separated from the recovered unreacted ethylene and/or the amount of carbon dioxide recycled to the reactor with the recovered ethylene.
The catalyst of the present invention may be fixed-bed or fluid-bed catalyst. Preferably, a fluid-bed catalyst is employed in a fluid bed reactor.
The catalyst suitably comprises a Group VIII metal and a promoter. Preferably, the catalyst further comprises a co-promoter. These compounds are suitably accommodated on a support.
With regards to the Group VIII metal, the preferred metal is palladium. The metal may be present in a concentration of greater than 0.2% by weight, preferably greater than 0.5% by weight, especially about 1% by weight based upon total weight of catalyst. The metal concentration may be as high as 10% by weight. Suitable sources of palladium include palladium (II) chloride, sodium or potassium tetrachloropalladate (II) (Na
2
PdCI
4
or K
2
PdCI
4
), palladium acetate, palladium (II) nitrate, H
2
PdCl
4
or palladium (II) sulphate.
In addition to the Group VIII metal, the catalyst comprises a promoter. Suitable promoters include gold, copper and/or nickel. A preferred promoter is gold. Suitable sources of gold include gold chloride, tetrachloroauric acid (HAuCl
4
), NaAuCl
4
, KAuCl
4
, dimethyl gold acetate, barium acetoaurate or gold acetate. The preferred gold compound is HAuCl
4
. The promoter metal may be present in an amount of from 0.1 to 10% by weight in the finished catalyst.
The catalyst composition may comprise a co-promoter material. Suitable co-promoters include Group I, Group II, lanthanide or transition metals for example cadmium, barium, potassium, sodium, iron, manganese, nickel, antimony, and/or lanthanum, which are present in the finished catalyst as salts, e.g. an acetat

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