Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
1999-01-22
2001-02-13
Wilson, James O. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reexamination Certificate
active
06187949
ABSTRACT:
The present invention relates to a process for the synthesis of esters by reacting an olefin with a lower carboxylic acid in the presence of an acidic catalyst.
It is well known that olefins can be reacted with lower aliphatic carboxylic acids to form the corresponding esters. One such method is described in GB-A-1259390 in which an ethylenically unsaturated compound is contacted with a liquid medium comprising a carboxylic acid and a free heteropolyacid of molybdenum or tungsten. This process is a homogeneous process in which the heteropolyacid catalyst is unsupported. A further process for producing esters is described in JP-A-05294894 in which a lower fatty acid is esterified with a lower olefin to form a lower fatty acid ester. In this document, the reaction is carried out in the gaseous phase in the presence of a catalyst consisting of at least one heteropolyacid salt of a metal eg Li, Cu, Mg or K, being supported on a carrier. The heteropolyacid used is phosphotungstic acid and the carrier described is silica. One of the problems with this process is that impurities present in the feeds to the reaction whether they be fresh feeds or recycle streams from the process have a tendency to deactivate the acid catalyst. In particular, presence of an aldehyde such as acetaldehyde in amounts at or above 100 ppm in the feed streams are detrimental to the heteropolyacid catalyst.
It has now been found that the process efficiency can be improved significantly by using a gaseous feedstock substantially free of such impurities.
Accordingly the present invention is a process for the production of lower aliphatic esters by bringing together in an addition reaction reactants comprising a lower olefin and a saturated, lower aliphatic, mono-carboxylic acid in the vapour phase into contact with a heteropolyacid catalyst characterised in that the reactants are rendered substantially free of aldehydes impurities prior to being brought into contact with the heteropolyacid catalyst.
By the expression “substantially free of aldehyde impurities is meant here and throughout the specification that the reactants comprise (a) a feedstream (comprising the lower olefin, and a saturated, lower aliphatic, mono-carboxylic acid, and optionally water, which feed stream may optionally contain any ether or alcohol recycled to the feedstream) to the reactor, and (b) contain less than 90 ppm, preferably less than 60 ppm and more preferably less than 55 ppm of aldehyde impurities prior to the feedstream entering the reactor inlet.
The aldehyde impurities in particular are detrimental to the acid catalyst and cause deactivation. A particular example of such an impurity is acetaldehyde. Such impurities may either be present in the fresh feeds to the reaction or may be formed as a by-product in the reactors during reaction and tend to be recycled to the reactor along with light products such as diethyl ether and if not checked quickly, tend to build up to levels far in excess of the tolerance levels specified above.
The aldehyde impurities are believed to cause deactivation of the heteropolyacid catalyst by reaction to form “coke” or unwanted resinification due to oligomerisation which then tend to block the catalyst pores. Whilst this is believed to be one of the mechanisms of such deactivation, it is by no means the only mechanism. The feedstream to the reaction is rendered free of any aldehyde impurities by subjecting the feedstream to a technique selected from:
a. distillation,
b. reaction with an solution of a base such as eg sodium hydroxide,
c. reaction with a borohydride such as eg sodium borohydride,
d. reaction with an acidic or a basic resin, and
e. extraction eg with acid or water.
Of these, distillation is the easiest and most convenient technique, especially to remove any aldehydes contained the ether by-products being recycled. Such distillation is suitably performed in a pressurised column.
The reaction with a base or a borohydride may also be suitably carried out in a distillation column or in a separate vessel. Where a resin is used, the feedstream which may be in the liquid or gas phase, is suitably brought into contact with the acidic or the basic resin.
In the reaction, the olefin reactant used is suitably ethylene, propylene or mixtures thereof Where a mixture of olefins is used, the resultant product will inevitably be a mixture of esters. The mixture of olefins is suitably sourced from a refinery product or a chemical grade olefin which also contains some alkanes admixed therewith.
The saturated, lower aliphatic mono-carboxylic acid reactant is suitably a C1-C4 carboxylic acid and is preferably acetic acid.
The reaction may be carried out in a plurality of reactors set up in series such that gaseous products exiting from a first reactor are fed as a feed (reactant) gas to a second reactor and the gaseous products exiting from the second reactor are fed as a feed gas to the third reactor so on for subsequent reactors, and an aliquot of the reactant monocarboxylic acid is introduced into the feed gas to the second and subsequent reactors so as to maintain the olefin to monocarboxylic acid ratio in the feed gas to each of the second and subsequent reactors within a pre-determined range.
Thus, the mole ratio of olefin to the lower monocarboxylic acid in the gaseous reactants fed to the first reactor is suitably in the range from 1:1 to 18:1, preferably from 10:1 to 14:1. During the reaction, when the gaseous reactants come into contact with the heteropolyacid in a catalyst bed, at least some of the acid is used up to form the ester in an exothermic reaction and the mole ratio of olefin to monocarboxylic acid increases considerably from a starting ratio of 12:1 to about 30:1 in the exit gases from the final reactor. Where the reaction is carried out in a plurality of reactors set up in series, the exit gases from the first reactor are fed as the feed (reactant) gas to the second reactor and the exit gases from the second reactor are fed as the feed gas to the third reactor and so on. When using such a series of reactors, the olefin to monocarboxylic acid mole ratio in the feed gas to the second and subsequent reactors is seriously depleted due to the acid being used up in the formation of the ester. This mole ratio of olefin to monocarboxylic acid is brought to the desired range by injecting further aliquots of the monocarboxylic acid to the feed gas prior to its entry into each of the second and subsequent reactors. In the case of the manufacture of ethyl acetate from ethylene and acetic acid, this range of mole ratios of ethylene to acetic acid in the gaseous reactants fed to the first reactor is suitably in the range from 1:1 to 18:1, preferably from 10:1 to 14:1 and that of the feed gas to the second and subsequent reactors is suitably from 10:1 to 16:1. The addition of further aliquots of the monocarboxylic acid to the feed gas to the second and subsequent reactors should be sufficient to bring the mole ratio of the olefin to acid within this range of 10:1 to 16:1.
The plurality of reactors set up in series referred to above can each be disposed in an axial mode with the feed (reactant) and product gases traversing a substantially axial path within each reactor from entering the top of the reactor until the product gases leave each reactor from the base thereof, the catalyst being positioned somewhere midway between the point of entry of the feed gas and the point of exit of the product gases. However, the reactors need not be set-up in a series where the flow-path of the feed (reactant) and product gases are in a substantially axial direction within each reactor. They could be set-up as a series of radial flow reactors. In such a radial flow set-up, the feed (reactant) gases will enter at the top of a reactor, pass down the middle thereof and then outwards radially over the catalyst in said reactor.
Briefly, each radial flow reactor in the series is of a substantially tubular shape which in a planar view has the appearance of a set of three substantially concentric tubes and wherein the fe
Froom Simon Frederick Thomas
Hodge Stephen Robert
Sharma Bhushan
BP Chemicals Limited
Deemie Robert W.
Nixon & Vanderhye
Wilson James O.
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