Process for the preparation of benzene and benzene homologues

摘要

Benzene and its homologues are prepared by subjecting C6 cycloalkanes viz. methyl cyclopentane, cyclohexane and mixtures thereof, to dehydrogenation in the vapour phase at an elevated temperature in the presence of a gas containing hydrogen and a platinum catalyst under pressure to form benzene, separately subjecting cycloalkanes having more than 6 carbon atoms to dehydrogenation in the vapour phase at an elevated temperature in the presence of a platinum catalyst and a gas containing hydrogen to form benzene homologues, the gas containing hydrogen being under such a pressure that the gas separated from the hydrocarbon products after this dehydrogenation is under a pressure higher than that employed in the dehydrogenation of the C6 cycloalkanes and using this separated gas, wholly or in part, in the dehydrogenation of the C6 cycloalkanes. The C6 cycloalkanes may be contained in a hydrocarbon fraction having a boiling range closely approximating to their boiling points. Similarly, the higher cycloalkanes may be a fraction from the same or different source. The C6 hydrocarbons are preferably treated at 450-525 DEG C., under 10-15 atmospheres hydrogen pressure and the higher hydrocarbons at similar temperatures under 20-50 atmospheres. The catalyst is preferably used on an acidic carrier, e.g. alumina-silica or alumina treated with a hydrohalic acid or otherwise containing halogen. The hydrocarbons are usually fed at the rate of 1-10 liquid volumes per volume of catalyst per hour and the hydrogen is used in molar amounts of 2-10 times that of the hydrocarbon. In a process illustrated by a flow sheet (not shown) a petroleum fraction rich in C6-8 cycloalkanes is separated into C6 and C7-8 fractions. The C6 fraction is passed with hydrogen through a series of reactors containing the catalyst under the specified conditions, the mixture being heated prior to entering each reactor and the final products are separated into a benzene-rich fraction and a gas containing hydrogen, part of which is recycled. The C7-8 is likewise treated with hydrogen in a similar series of reactors but at a higher pressure and the products are separated into a toluenexylene fraction and a gas containing hydrogen, part of which is recycled and the remainder fed into the stream of C6 hydrocarbons treated in the lower pressure hydrogenation. If desired, the C7-8 fraction may be separated into C7 and C8 fractions and each separately dehydrogenated as above, the C8 fraction being treated under a still higher pressure and the gas separated from the xylene product being used in the dehydrogenation of the C7 fraction.