The skeletal isomerisation of the N–butenes catalyst performance and kinetic investigation

Abstract

With the advent of catalytic converters being placed in the exhaust systems of motor vehicles, in an attempt to reduce pollution, the use of tetra ethyl lead as an anti-knock additive to fuel was no longer acceptable. Lead is a poison for the exhaust catalysts used and so an alternative had to be found. At present the ethers, particularly methyl tertiary butyl ether (MTBE), are being used as a substitute for lead. The limiting factor in the production of the ethers is the availability of isobutene. lsobutene may be produced using a number of complex processes and feedstocks. One route that has not been commercialised is the direct conversion of the linear butenes to isobutene via skeletal isomerisation. In the present study, using a specially constructed bench scale reactor system and a pilot plant, it was shown that an amorphous silica alumina catalyst could be used for the skeletal isomerisation of the n-butenes to isobutene. By investigating the effects of the residence time, n-butene partial pressure, total system pressure, system temperature and water to hydrocarbon molar ratio, a commercially suitable operating and regeneration procedure was developed. The effects of the various feed and product constituents, that build-up in the recycled hydrocarbon and process water streams, such as n-butane, isobutene, pentene, 1 ,3-butadiene and acetone, on the performance of the catalyst were also quantified. The long-term stability of the material, during repeated on-line and regeneration cycles, was determined. To allow the rigorous design of a commercial reactor, a suitable rate equation is required. Hence, a detailed kinetic investigation was conducted using the bench scale reactor system, the suitability of which for such a study was first investigated and confirmed. For the mono-molecular mechanism with either a single step or multiple steps controlling the overall reaction rate, and including the law of mass action, a total of eight cases were considered. Although the overall reaction mechanism could not be identified, a suitable rate equation was developed. The robustness of the rate equation was confirmed by its ability to predict accurately the performance of the pilot plant reactor system.