GB917962A - Improvements in and relating to the production of aromatic and/or unsaturated acyclic hydrocarbons - Google Patents

Abstract

Aromatic and/or unsaturated acyclic hydrocarbons are obtained from mixtures containing saturated cyclic and/or acyclic hydrocarbons by passing the mixture through a plurality of molecular sieves of differing pore sizes arranged in decreasing order of pore size whereby higher molecular weight hydrocarbons are adsorbed in the earlier stage(s) and lower molecular weight hydrocarbons in the later stage(s), desorbing the saturated hydrocarbons, subjecting them to a cracking or reforming process to form unsaturated or aromatic hydrocarbons and using a noncondensable component of the product to desorb the lower molecular weight hydrocarbons in a subsequent adsorption step and using desorbed lower molecular weight hydrocarbons to desorb higher molecular weight hydrocarbons adsorbed in an earlier sieve stage. In a preferred embodiment the higher molecular weight hydrocarbons are subjected to mild cracking or reforming and lower molecular weight hydrocarbons are subjected to pyrolysis. The later molecular sieve stages are preferably of greater capacity than earlier stages. Inert gases, e.g. paraffin-free waste gases from cracking or hydrogenation processes may also be used to assist in desorption of the lightest fractions. In an example a p light gasoline is passed through two sieves of 5 and 4 pore size to separate firstly C7 and C8 n-paraffins which are subjected to platforming to yield aromatic hydrocarbons and secondly C5 and C6 n-paraffins which are pyrolysed at above 700 DEG C. with steam to yield olefins. Residual gases from the platforming and pyrolysis are used to desorb the 4 sieve and light hydrocarbon desorbate is used to desorb the 5 sieve. Preferably two sets of sieves are used in parallel each alternately on adsorption and desorption.