Patent GB-592213-A (GB)
Title: Isomerizing hydrocarbons
IPC: C07C9/16
Filed: 1943-07-05
Abstract: A mixture of saturated hydrocarbons containing straight chain paraffins of at least 6 carbon atoms and naphthenes is pretreated under mild isomerizing conditions with an isomerization catalyst to produce a preferential isomerization of non-hydroaromatic naphthenes to hydroaromatic naphthenes, the naphthenes are then separated, and the remaining hydrocarbons contacted with an isomerization catalyst under more severe isomerizing conditions to convert paraffins to branched or more highly branched isomers. The feed may be a naphthenic hexane or heptane fraction from gasoline or petroleum or may comprise both C6 and C7 hydrocarbons.  The preferential isomerization of the naphthenes in the first conversion stage is effected by using a catalyst of mild activity or by control of reaction conditions or both. Suitable catalysts are aluminium halide-hydrocarbon complexes, e.g. aluminium chloride - toluene, containing no free aluminium halide, used at about 50-100 DEG , preferably 60-90 DEG  C. and, preferably, with hydrogen halide promoters not exceeding 0.5 per cent of the feed.  Thus, a naphthenic hexane fraction containing methyl cyclopentane, paraffins predominating in n-hexane, and some benzene treated with AlCl3-toluene complex at 65 DEG  C. with a catalyst to hydrocarbon ratio of 1 : 2 shows substantial conversion of methyl cyclopentane to cyclohexane with little isomerization of paraffins.  The preferred catalyst is, however, one of the molten salt type which has been used in the subsequent paraffin isomerization until spent.  Mixtures of aluminium chloride with antimony trichloride or of aluminium chloride or bromide with alkali metal halides are suitable.  The hydroaromatic naphthenes produced in the treatment of a hexane fraction can be separated by fractionation.  With higher fractions such as C7, separation of the naphthenes, such as methyl cyclohexane, from the paraffins, such as n-heptane, is not possible by this method.  The hydroaromatic naphthenes may then be dehydrogenated to aromatic hydrocarbons which can be separated as by solvent extraction.  A hexane fraction comprising n-hexane, methyl cyclopentane and cyclohexane, for example, is fed to the initial isomerization stage. If desired, a preliminary fractionation to remove a light fraction containing methyl pentanes and a heavy fraction predominating in cyclohexane may be effected.  The isomerization product, after separation and recycling of catalyst, is fractionated to separate hydrogen halide and, if desired, a light fraction containing n-hexane and methyl cyclopentane which are returned, from a fraction containing n-hexane and cyclohexane which are separated in a second fractionator.  The n-hexane is fed to the second isomerizing stage where a more active catalyst, higher temperature, longer contact time or higher catalyst ratio is employed.  Molten salt catalysts at 80-125, preferably 85-100 DEG  C. are suitable with hydrogen halide equal to 0.5-10 per cent of the hydrocarbon charge.  The deleterious effects of increasing amounts of methyl cyclopentane on the octane rating of the product and the catalyst life are shown when hexane is isomerized at about 85 DEG  C. with a molten mixture of 92.5 per cent SbCl3 and 7.5 AlCl3 using a contact time of 30 mins., catalyst to hydrocarbon ratio of 1 : 5, and hydrogen chloride equal to 4 per cent of the hydrocarbon feed.  The isomerization product, after catalyst separation, is fractionated to separate hydrogen chloride, and the hydrocarbon fraction further fractionated to give a product containing methyl pentanes and dimethyl butane, and n-hexane which may be recycled.  Spent catalyst from the isomerization of the paraffins may be returned to the initial isomerization stage where a temperature below about 80 DEG  C. and a ratio of molten salt catalyst to hydrocarbon from 1 : 5 to 1 : 10 is suitable.  When its activity for naphthene isomerization is spent, the separated catalyst may be scrubbed with the paraffin charge to the main isomerizer to extract its more soluble components before it is eliminated.  Where a fresh molten salt catalyst is used in the naphthene isomerization, reaction conditions must be mild, for example with AlCl3-SbCl3, a temperature of 75 DEG  C. for 5 mins. with a catalyst ratio of 1 : 18.  In treating a heptane fraction containing n-heptane, dimethyl and ethyl cyclopentanes, and methyl cyclohexane, a preliminary fractionation may be effected to separate a bottom fraction containing methyl cyclohexane before isomerizing the remaining naphthenes to methyl cyclohexanes.  The combined methyl cyclohexanes may then be dehydrogenated at 400-550 DEG  C. and 350 lbs./sq. in. in the presence of hydrogen, preferably using a nickel-tungstensulphur type catalyst.  Extractive distillation with phenolic solvents separates toluene from the paraffins which are sent to the second isomerization zone.