Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2001-01-03
2004-09-21
Trinh, Ba K. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S241000, C560S248000
Reexamination Certificate
active
06794535
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.
We have now discovered that metallic or metal compound impurities present in the reactants and any inert gases used in the reaction have a tendency to deactivate the acid catalyst. In particular, the presence of, for example, iron, chromium, molybdenum and nickel arising from the corrosion of equipment and sodium/potassium (if present in significant amounts) or calcium from any water or acetic acid reactant used are detrimental to the heteropolyacid catalyst. These impurities may contaminate the catalyst either by entrainment in the vapour streams or as gas phase acetate salts in the vapour stream.
Accordingly, the present invention is a process for the production of lower aliphatic esters which comprises reacting a lower olefin with a saturated lower aliphatic mono-carboxylic acid in the vapour phase in the presence of a heteropolyacid catalyst, characterised in that a) the reaction is carried out in a plurality of reactors set up in series, and b) the feedstock is rendered substantially free of metallic or metal compound impurities substances prior to being brought into contact with the heteropolyacid catalyst.
By using a gaseous feedstock substantially free of such impurities, the process efficiency can be improved significantly.
By the expression “substantially free of metallic or metal compound impurities” is meant here and throughout the specification that the total feed to the reactor has no more than 0.1 ppm of metals and/or metal compounds, preferably less than 0.01 ppm, prior to being brought into contact with the catalyst so as to enhance acceptable catalyst life. The feedstock to the reactor is made up of fresh and recycled components.
The metallic and metal compound impurities in particular are detrimental to the acid catalyst and cause deactivation. Specific examples of such impurities include the metals iron, chromium, nickel, sodium, potassium and calcium and compounds thereof Impurities such as iron, chromium, molybdenum or nickel usually arise from the corrosion of equipment whereas those of sodium, potassium or calcium result from any water or acetic acid reactant used in the reaction. In particular, these have a tendency to build up in recycle streams, especially in the acid recycle because they are carried over in the vaporiser.
These impurities may be removed from the feed to the reactor using a guard bed or, preferably, a vaporiser. Where a guard bed is used, this could be in the form of a resin which is added to the liquid streams whether they be fresh feeds or recycle streams before these are vaporised. The guard bed suitably contains an ion-exchange resin through which the liquid streams pass so as to entrap the metallic or metal compound impurity present. Other materials which can be used as a guard bed include amorphous aluminosilicates, clays, zeolites, aluminophosphates, silicoaluminophosphates, metalaluminophosphates and supported heteropolyacids. Specific examples of resins are eg Amberlyst® 15H, Purolite® CT 145 and CT 175. Since the impurities are likely to build up above the specified threshold levels in any streams being recycled to the reaction, such recycle streams should also be passed through the guard bed in order to minimise contamination of the catalyst by adventitious entry of metal/metal compound impurities into the reactor.
Where a vaporiser is employed, it may be designed to minimise carry over of these metallic impurities by using demister pads and/or using a heavy ends take-off at the base of the vaporiser where most of the metal salts will be removed. The design of the vaporiser can be such that fresh acid, which is low in heavy metals, can be fed in at the top of the reactor to scrub out metals. This would improve the efficiency of metal removal.
In one embodiment, both a guard bed and a vaporiser are employed to remove metal impurities from the feedstock. The feedstock is first passed through a guard bed as described above, and the liquid exiting the bed (ie the eluate) is introduced into a middle and/or upper region of the vaporiser. For example, where a 5-tray vaporiser is employed, the eluate may be passed to tray 2 (from the top). Ethylene reactant may then be fed into the bottom of the same vaporiser, whereby the acid, and any recycle streams fed thereto are vaporised. The vaporiser suitably contains a liquid demister at or above the top tray to minimise any liquid carry over. Fresh acetic acid is suitably fed above the top tray of the vaporiser to scrub the vapours of recycled acid as it rises up the vaporiser thereby preventing any heavy metal carry over along with the vaporised acid and ethylene.
Ethylene saturated with vaporised acid (and any water) emerging from the vaporiser may be suitably further heated before being fed to the plurality of reactors.
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 source of the olefin reactant used may be a refinery product or a chemical grade olefin which invariably contains some alkanes admixed therewith. The other feedstock such as acid, water and recycle streams, in particular, may contain metal or metal compound impurities which have to be removed as described above prior to being brought into contact with the acid catalyst.
The saturated, lower aliphatic mono-carboxylic acid reactant is suitably a C1-C4 carboxylic acid and is preferably acetic acid.
The reaction is carried out in a plurality of reactors set up in series such that the reactant gases exiting from a first reactor are fed as the feed gas to a second reactor and 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 reactant gases 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 reactant gases 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. As 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 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. 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 th
Froom Simon Frederick Thomas
Hodge Stephen Robert
Pacynko Witold Franciszek
BP Chemicals Limited
Nixon & Vanderhye
Oh Taylor Victor
Trinh Ba K.
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