Patent GB-581904-A (GB)
Title: Isomerizing hydrocarbons
IPC: C07C5/27
Filed: 1943-05-27
Abstract: The isomerization of straight chain paraffins containing at least 6 carbon atoms to multibranched chain isomers is effected by subjecting to such isomerization conditions that single-branched paraffins are predominantly formed, separating these from the straight chain paraffins and subjecting them to further isomerization under more severe conditions to convert them to multi-branched isomers.  Where the initial paraffins are present in mixtures also containing naphthenes of non-hydroaromatic structure, i.e. not having a hexamethylene ring, for example mixtures obtained by fractionation of gasoline, the mixture is first subjected to mild isomerization conditions to convert the said naphthenes into naphthenes of hydroaromatic structure.  Naphthenes are then removed, and the remaining hydrocarbons subjected to the two stage isomerization of paraffins.  The process is preferably effected in the liquid phase. Where naphthenic mixtures containing more than 6 carbon atoms are treated, the hydroaromatic naphthenes produced may be converted into aromatic hydrocarbons which are separated as by solvent extraction. A hexane fraction comprising n- and cyclo-hexane, and methyl cyclopentane, if desired, after fractionating to separate dimethyl butane and cyclohexane, is subjected to mild isomerization to convert the methyl cyclopentane to cyclohexane.  Preferably a catalyst of the molten salt type, such as aluminium and antimony chlorides, whose activity has been reduced by use in the isomerization of hexane in a later stage, is used at temperatures not above 80 DEG  C., the volume of catalyst to hydrocarbon being about 1 : 5-10.  Hydrogen halide may be present in amount below about 0.5 per cent by weight of the hydrocarbon charge.  In examples, catalyst to hydrocarbon ratios of 1 : 5 or 1 : 18, and contact times of 7.5 or 5 mins. at 80 or 75 DEG C. are used.  Other molten salt catalysts referred to comprise AlCl3 with combinations of NaCl, KCl, ZnCl2, and SO2.  Fresh catalysts of restricted activity which are suitable in this stage are aluminium halide-hydrocarbon complexes containing no free halide made, for example, by reacting aluminium chloride with toluene or a cycloolefin.  Temperatures up to 100 DEG  C. may be used.  The reaction products are fractionated, together with isomerized hexanes from later stages, cyclohexane being separated as a final product, and n-hexane fed to a second isomerizing zone, if desired with normal and single branched hexanes from other sources.  Catalyst of high activity such as the fresh molten salt type is used under moderate conditions, e.g. 80-90 DEG  C., catalyst to hydrocarbon ratio 1 : 2-5, contact time 4-15 mins., and hydrogen halide 2-10 per cent. Diluents such as hydrogen or isobutane may be added.  Hexane is converted to methyl pentane without substantial formation of dimethyl butane. In an example, molten aluminium-antimony chlorides are used. The effect on the octane rating of the product of varying proportions of methyl cyclopentane in hexane isomerization feed are exemplified. The products are fractionated to give a hexane, methyl pentane fraction which is fed to the fractionator of the first stage wherein methyl pentane is separated and passed to a third isomerizing stage wherein more severe conditions are maintained to give conversion to dimethyl butane.  In an example, a molten mixture of 92.5 per cent SbCl3 and 7.5 AlCl3 in a ratio of hydrocarbon of 1 : 5 is contacted at 80 DEG  C. for 30 mins.  Hydrogen chloride 2.5 per cent, and benzene 1.1 per cent are added.  Temperature should be about 80-125, preferperferably 80-95 DEG  C.  Aromatic hydrocarbons such as benzene or toluene, for example about 1-3 per cent of the charge, prevent decomposition. The products are separated into dimethyl butane as final product, and a fraction comprising methyl pentane and hexane which is fed back to the fractionator after the first stage.  The spent catalyst from the second and third stages may be used in the first stage until inactive when it may be scrubbed with the hydrocarbon feed to the second stage to remove more soluble components, e.g. SbCl3, before disposal.  When the initial feed is a heptane fraction, it may be given a preliminary fractionation to remove a methyl cyclohexane fraction, the remainder, comprising the heptane and dimethyl cyclopentanes, being subjected to the first isomerizing stage to convert methyl-and ethyl-cyclopentanes to hydroaromatic naphthenes comprising methyl cyclohexanes.  The products are dehydrogenated to yield aromatic hydrocarbons using, for example, catalysts of the nickel-tungsten-sulphur type or chromium or molybdenum oxide on alumina or zirconia, at about 400-550 DEG  C., pressures above about 350 lbs., and with addition of hydrogen. Extractive distillation in the presence of a selective solvent for aromatics, e.g. phenol, alkyl phenols or cresylic acids, gives the aromatic hydrocarbons including toluene as a final product, and paraffins which are fractionated and isomerized as described above.