GB793151A - Recovery of aromatic hydrocarbons - Google Patents

Abstract

An aromatic hydrocarbon is prepared from a hydrocarbon fraction containing naphthenic hydrocarbons consisting substantially of a naphthene containing the same number of carbon atoms as the desired aromatic by catalytically <PICT:0793151/IV (b)/1> hydroforming to produce a hydroformed fraction containing the desired aromatic, recovering by distillation an aromatic concentrate and subjecting this to extractive distillation with an aromatic hydrocarbon of higher boiling-point than the desired aromatic (see Group III) which may then be recovered from the solvent. The process can be applied to the production and recovery of benzene, toluene, xylenes, ethylbenzene, mesitylene, propyl benzene and butylbenzenes. The solvent in the extractive distillation may be xylenes or C9 aromatics for toluene and toluene or xylenes for benzene. A naphtha fraction boiling in the range 140-430 DEG F. is charged to a fractional distillation zone 12 from which one of four fractions may be led to the hydroforming zone 20 having the approximate boiling ranges:-line 14, 190-233 DEG F. (for toluene); line 15, 233-265 DEG F. (for xylenes); line 16, 265-300 DEG F.; and line 17, 300-350 DEG F.; bottoms being removed by line 18. The fraction from line 14 is used in this embodiment. The hydroforming zone is conventional and contains a catalyst such as 0.1-50 per cent of an oxide or sulphide of a metal of Groups IVb, Vb, VIb or VIII, such as molybdenum, chromium, tungsten, vanadium, cobalt or nickel, or 0.01-2 per cent of platinum supported on aluminium oxide as bauxite, acid-treated bauxite, aluminium hydrate, alumina gel, activated alumina, peptized alumina, silica-alumina gels and hydrofluoric acid-treated alumina. Operating conditions may be 850-1000 DEG F., slightly above atmospheric pressure-600 p.s.i., an L.H.S.V. of 0.1-5, and a hydrogen or recycle gas rate of 1000-6000 cu. ft./barrel of feed. The hydroformed product is fractionated in tower 23, the bottoms, boiling about the same range as the fraction in line 14, are fed to the extractive distillation zone 27 into which a C9 aromatic hydrocarbon is fed by line 29. Toluene in the solvent is removed by line 30a, non-aromatic compounds leaving by line 30. Toluene is finally recovered in the fractionating zone 31. If desired, this product may be clay treated and re-run.ALSO:An aromatic hydrocarbon contained in a hydroformate, obtained by catalytically hydroforming a hydrocarbon fraction containing a naphthene having the same number of carbon atoms as the desired aromatic hydrocarbon (see Group IV (b)) is recovered by distilling to obtain an aromatic concentrate containing the desired hydrocarbon and extractively distilling the concentrate using as solvent an aromatic hydrocarbon having a higher boiling point than the desired hydrocarbon. Suitable solvents for recovering toluene are C9 aromatics or xylenes and for recovering benzene, xylenes or toluene can be used. The amount of solvent used is preferably such that the mol. fraction of the solvent in the extraction zone is 0.1-0.9. The desired aromatic hydrocarbon is finally recovered from solution in the solvent by distillation. An example indicates that to obtain pure toluene from a mixture containing n-heptane and 40 per cent by volume of toluene without the use of a solvent, 65 plates are required (assuming 75 per cent overall efficiency) whereas using a 0.7 mol. fraction of a C9 aromatic hydrocarbon only 25 plates are needed.