Patent GB-631309-A (GB)
Title: Improvements relating to the production of cyclohexane
IPC: C07C13/18
Filed: 1945-10-19
Abstract: Cyclohexane is produced from a benzene-containing hydrocarbon feed-stock, substantially free of paraffins, by hydrogenating the feed-stock under conditions such that the benzene is incompletely hydrogenated to yield a product in which the cyclohexane/benzene molar ratio is greater than 1 : 1 and fractionating the product for the recovery of an overhead product comprising an azeotrope of benzene and cyclohexane and a bottom product being substantially pure cyclohexane; if desired, the initial feed-stock may be that obtained by fractionating a feed-stock containing paraffinic impurities to produce an overhead product containing the paraffinic impurities and some benzene and a bottoms product consisting of substantially pure benzene which is hydrogenated as above.  In a modification, a benzene-containing feed-stock containing, as paraffinic impurities, paraffins which boil below 77.6 DEG  C., or which form benzene azeotropes boiling below 77.6 DEG  C., and in amount preferably not more than 20 per cent by weight of the feed-stock, is hydrogenated to convert part of the benzene to cyclohexane and yield a product containing sufficient benzene to form azeotropes with all of the paraffinic impurities and part only of the cyclohexane present, and passing the product to a fractionation zone for the removal overhead of the benzene in the form of its azeotropes with the paraffinic impurities and cyclohexane respectively, and for the recovery of substantially pure cyclohexane as bottoms product.  This method may be modified by hydrogenating part of the impure feed-stock to convert part of the benzene therein to cyclohexane, admixing the product with the remainder of the feed-stock to yield a mixture containing sufficient benzene to form azeotropes with the paraffinic impurities and cyclohexane respectively and fractionating as before.  All or part of the benzene-cyclohexane azeotrope may be recycled to the hydrogenation zone, if desired after removal of the cyclohexane therein by solvent extraction or azeotropic or extractive distillation.  The azeotropic mixture of paraffinic impurities, benzene and cyclohexane may also be recycled but should first be treated to remove the paraffins to prevent them building up excessively in the hydrogenation zone.  To this end, the paraffins may be removed by solvent extraction or azeotropic or extractive distillation.  Also the impure feed-stock may be given a preliminary fractionation to remove all or part of the paraffins as azeotropes with benzene before the hydrogenation and these azeotropes may then be treated in the same manner as the previously mentioned paraffinic mixture, and if desired together with it, to remove the paraffins, the residue passing to the hydrogenation zone.  In an example, two portions of a synthetic mixture of benzene and 2 : 2- and 2 : 4-dimethyl pentanes are hydrogenated, one to 80 per cent conversion and the other to completion, and both are fractionated in identical columns, fractions being removed until substantially pure cyclohexane remains; a greater yield of cyclohexane is obtained when hydrogenation is incomplete.  Specifications 585,850, 606,797, U.S.A. Specifications 2,356,240, 2,365,898 and 2,378,808 are p  referred to. According to the Provisional Specification, when paraffinic impurities are present, the fractionation of the hydrogenated product must be carried out in the presence of at least sufficient benzene to ensure formation of benzene azeotropes with the paraffins.ALSO:Cyclohexane is produced from a benzene-containing hydrocarbon feed-stock, substantially free of paraffins, by hydrogenating the feed-stock under conditions such that the benzene is incompletely hydrogenated to yield a product in which the cyclohexane/benzene molar ratio is greater than 1 : 1 and fractionating the product for the recovery of an overhead product comprising an azeotrope of benzene and cyclohexane and a bottoms product being substantially pure cyclohexane; if desired, the initial feed-stock may be that obtained by fractionating a feed-stock containing paraffinic impurities to produce an overhead product containing the paraffinic impurities and some benzene and a bottoms product consisting of substantially pure benzene.  In a modification, a benzene-containing feed-stock containing, as paraffinic impurities, paraffins which boil below 77.6 DEG  C. or which form azeotropes with benzene boiling below 77.6 DEG  C. and in amount preferably not more than 20 per cent by weight of the stock, is hydrogenated to convert part of the benzene to cyclohexane and yield a product containing sufficient benzene to form azeotropes with all of the paraffinic impurities and part only of the cyclohexane present and passing the product to a fractionation zone for the removal overhead of the benzene in the form of its azeotropes with the paraffinic impurities and cyclohexane respectively and for the recovery of substantially pure cyclohexane as bottoms product.  This method may be modified by hydrogenating part of the impure feedstock to convert part of the benzene therein to cyclohexane, admixing the hydrogenated product with the remainder of the feed-stock to yield a mixture containing sufficient benzene to form azeotropes with the paraffinic impurities and fractionating as before.  All or part of the benzene-cyclohexane azeotrope removed in the fractionation may be recycled to the hydrogenation zone, if desired after removal of the cyclohexane therein by solvent extraction or by azeotropic or extractive distillation.  The mixture of paraffins, benzene and cyclohexane obtained when impure feed-stock is used may also be recycled, but should be treated to remove paraffins to avoid excessive build-up of these impurities in the hydrogenation zone. To this end, the recycle mixture may be freed of the paraffins wholly or in part by solvent extraction or azeotropic or extractive distillation.  Also the impure feed-stock may be given a preliminary fractionation to remove all or part of the paraffins as azeotropes with benzene, before the hydrogenation; the paraffin-benzene fraction may then be treated in the same way as the above-mentioned paraffin-benzene-cyclo-hexane mixture and if desired with it, to remove the paraffins and provide pure benzene which may be passed to the hydrogenation zone. Hydrogenation is generally effected at 150-300 DEG  C. and under 5-100 atmospheres pressure, nickel type catalysts such as nickel, nickel/alumina, nickel/kieselguhr and nickel oxide are preferred when sulphur-free feed-stock is used and in other cases catalysts resistant to sulphur poisoning such as a nickel sulphide/tungsten sulphide mixture, cobalt molybdate on alumina and generally, elements of Groups 5 and 7 of the Period Table and compounds thereof.  In an example, two portions of a synthetic mixture of benzene and 2 : 2- and 2 : 4-dimethyl pentanes are hydrogenated, one to an 80 per cent conversion and the other to completion and both are fractionated in identical columns, fractions being removed until substantially pure cyclohexane remains; a greater yield of cyclohexane is achieved when hydrogenation is incomplete. Specifications 585,850 and 606,797, [Group III], and U.S.A. Specifications 2,356,240, 2,365,898 and 2,378,808 are referred to. According to the Provisional Specification when paraffinic impurities are present, the fractionation of the hydrogenated mixture must be carried out in the presence of at least sufficient benzene to ensure formation of benzene azeotropes with the paraffins.