Process for preparing alkylene oxide

Details Process for preparing alkylene oxide Process for preparing alkylene oxide

2000-06-08

2002-04-02

Trinh, Ba K. (Department: 1625)

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

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C502S020000, C502S029000, C502S030000, C502S031000, C502S033000

Reexamination Certificate

active

06365761

ABSTRACT:

1. FIELD OF THE INVENTION
The present invention relates to a process for the preparation of an alkylene oxide, more specifically, to a process for preparing an alkylene oxide from an alkene wherein the epoxidation reactors used are operated in such a way that the life of the epoxidation catalyst is significantly prolonged. The present invention also relates to a process for re-activating an at least partly deactivated heterogeneous epoxidation catalyst.
2. BACKGROUND OF THE TECHNOLOGY
The epoxidation of an alkene into alkylene oxide by reacting the alkene with an organic hydroperoxide is known in the art.
For instance, in the commonly known method for co-producing propylene oxide and styrene starting from ethylbenzene, the aforementioned epoxidation reaction is applied. In general this co-production process involves the steps of (i) reacting ethylbenzene with oxygen or air to form ethylbenzene hydroperoxide, (ii) reacting the ethylbenzene hydroperoxide thus obtained with propene in the presence of an epoxidation catalyst to yield propylene oxide and 1-phenyl-ethanol, and (iii) converting the 1-phenyl-ethanol into styrene by dehydration using a suitable dehydration catalyst.
Another method for producing alkylene oxide is the coproduction of propylene oxide and methyl tert-butyl ether (MTBE) starting from isobutane and propene. This process is well known in the art and involves similar reaction steps as the styrene/propylene oxide production process described in the previous paragraph. In the epoxidation step tert-butyl hydroperoxide is reacted with propene forming propylene oxide and tert-butanol in the presence of a heterogeneous epoxidation catalyst. Tert-butanol is subsequently etherified with methanol into MTBE, which is used as an additive in motor fuels.
The present invention concerns the epoxidation reaction between an alkene and an organic hydroperoxide, and more in particular the epoxidation reaction, wherein use is made of a bank of serially connected fixed bed reactors, each filled with at least one bed of heterogeneous epoxidation catalyst, thereby particularly addressing the deactivation of this heterogeneous epoxidation catalyst.
Heterogeneous epoxidation catalysts are known in the art. Such catalysts may comprise as the catalytically active metal one or more transition metals, such as vanadium, molybdenum, tungsten, titanium and zirconium. One particularly suitable class of heterogeneous epoxidation catalysts are the titanium-based catalysts. Examples of such catalysts are for instance described in U.S. Pat. No. 4,367,342 and EP-A-0,345,856. U.S. Pat. No. 4,367,342 discloses the use of inorganic oxygen compounds of silicon in chemical composition with at least 0.1% by weight of an oxide or hydroxide of titanium, while EP-A-0,345,856 discloses a titania-on-silica heterogeneous catalyst. According to EP-A-0,345,856 this catalyst is obtainable by impregnating a silicon compound with a stream of gaseous titanium tetrachloride followed by calcination and hydrolysis steps and optionally a silylation step.
When such heterogeneous epoxidation catalysts are used to catalyse the epoxidation of an alkene, deactivation of the catalyst will occur. Without any preventive measures an epoxidation catalyst, which is contacted with a stream containing an alkene and an organic hydroperoxide from the preceding oxidation step, will have a limited lifetime due to deactivation and consequently will have to be replaced.
An increase of the lifetime of the catalyst would be beneficial, as it would result in a higher and more cost effective production of alkylene oxide. It would reduce the costs due to catalyst consumption and the time and costs involved in reloading of the reactors. Furthermore, an increased lifetime of the catalyst is desirable, because in that case the average reaction temperature could be kept lower, thus producing less by-products.
3. SUMMARY OF THE INVENTION
The present invention provides in a first aspect a process for the preparation of an alkylene oxide involving the cyclic operation of a bank of serially connected, fixed bed epoxidation reactors operated under certain conditions which have been found to significantly prolong the catalyst life. The process according to the present invention results in a re-activation of deactivated catalyst and hence in an increased lifetime of the catalyst. This, in return, results in a more cost effective and productive process.
Accordingly, the present invention relates to a process for the preparation of alkylene oxide, which process comprises passing a feed comprising an organic hydroperoxide and alkene through a bank of at least two serially connected reactors all containing a bed of heterogeneous epoxidation catalyst particles and operated in a cyclic mode, optionally followed by at least one additional epoxidation reactor containing a bed of heterogeneous epoxidation catalyst particles, and continuously withdrawing a product stream from the final epoxidation reactor comprising alkylene oxide and an alcohol as reaction products, from which product stream the alkylene oxide end-product is recovered, in which process:
(a) the first reactor of the cyclically operated bank is put in a position further down this bank or in a position directly after any one of the additional reactors, when the activity of the epoxidation catalyst contained therein has decreased to an undesirably low level;
(b) in this position the catalyst with decreased activity is contacted with the effluent from the reactor in the preceding position at a temperature which is at least 5° C. higher than the final temperature at which the catalyst was in use in the first position of the bank and for sufficient time to restore its activity to the desired level.
4. DETAILED DESCRIPTION OF THE INVENTION
A major advantage of the process according to the present invention is that the reactor containing the deactivated catalyst does not have to be taken out of operation each time the level of deactivation has become undesirably high, so that the process need not be interrupted each time the epoxidation catalyst in the first epoxidation reactor has deactivated. Furthermore, while being in its new position, the deactivated catalyst may still continue to contribute to the final alkylene oxide yield. Namely, while its activity is increasing, the amount of alkene converted into alkylene oxide over this specific catalyst bed may also increase. Cyclic operation of a bank of serially connected epoxidation reactors is known in the art. For instance, U.S. Pat. No. 5,849,937 discloses a method for operating a bank of serially connected epoxidation reactors for producing alkylene oxide, wherein the reactor containing the mostly deactivated catalyst is either in the first or in the terminal position and, when taken out of operation, is immediately replaced in respectively the terminal and first position by another reactor containing fresh catalyst. Accordingly, in this method a constant number of epoxidation reactors is continuously in operation while one reactor is in a standby position. Having one reactor on standby is considered undesired from a cost perspective.
In commercial operation the epoxidation reaction is typically carried out at temperatures of 50 to 135° C., suitably 70 to 125° C. and pressures up to 80 bar, suitably 10 to 60 bar, with the reaction medium being in the liquid phase. Normally there is a temperature increase in the reactor as the epoxidation proceeds. Therefore, cooling means are suitably present between each two subsequent epoxidation reactors. In order to compensate for the activity loss of the catalyst due to deactivation, the temperature in the reactor can be raised, e.g. by controlling the amount of cooling applied, so that the conversion in each reactor can be kept at the desired level. The temperature is raised until a temperature is reached above which one would expect the negative side-effects (e.g. the formation of by-products) to become unacceptable and the deactivation to be at such level that replacement of the catalyst is necessary. The ma

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