Patent GB-694980-A (GB)
Title: Recovery of para xylene from hydrocarbon mixtures by fractional crystallisation
IPC: B01D9/00
Filed: 1950-12-07
Abstract: <PICT:0694980/III/1> Paraxylene is recovered from a mixture containing the three isomeric xylenes by cooling the mixture to a temperature in the range -75 DEG  to -120 DEG  to obtain a solid phase containing para-xylene, filtering to separate a crystalline phase and a mother liquor, washing the crystal phase with a liquid having a para-xylene content greater than that of the mother liquor, filtering the wash liquid from the crystals and returning a portion of the filtered wash liquid to the cooling zone and utilizing the remainder as wash liquid in a succeeding cycle of operation.  The washing and second-mentioned filtering are conducted at such temperatures that a substantial portion of crystal phase is melted during these steps so that the crystal phase is purified and the wash liquid enriched in para-xylene. The starting material is preferably an equilibrium mixture of xylenes from hydroformed non-aromatic petroleum fractions.  The mixture may contain ethyl benzene and paraffins.  The hydroformed fraction is distilled in one or more columns, Fig. 1, C4 and lighter hydrocarbons being taken overhead, C5, C6, and C7 hydrocarbons being removed as side-streams, and the C8 xylene fraction being withdrawn by line 19.  This fraction, preferably after adjusting the ortho/meta-xylene ratio to enhance recovery of para-xylene, is cooled by mother liquor in heat exchanger 27 and flows to crystal-soaking tank 31 where it is reduced to crystallizing temperature by mixing with previously cooled xylene stock and where it is retained, e.g. for twenty minutes, to allow crystal growth.  The cooling rate is preferably 5 DEG F per minute in the crystallizing temperature range.  The crystallizing temperature is maintained by circulation of a xylene sidestream though chillers 34, 36 and 37.  The degree of cooling is preferably automatically regulated by the temperature in discharge lines 42-44 leading back to tank 31.  After growth the crystal slurry is led by line 47 to centrifuge 48 from which mother liquor is withdrawn to super-fractionator 78.  The crystals are washed in washer 51 with para-xylene-rich liquor from a succeeding separation stage 56 to which the slurry of crystals and wash liquid are fed.  Liquid from stage 56 in excess of that required for washing is returned by line 67 to the crystal soaking tank.  A substantial proportion of crystals are melted in the washing and second separation stage.  The separated crystals are melted at 63 and taken to storage.  In superfractionator 78 ortho-xylene is removed as bottoms and part passed to tank 22 to adjust the ortho/meta-xylene ratio of the initial mixture.  The remainder is taken off as product.  The overheads consist of mixed xylenes and other hydrocarbons.  The non-aromatic hydrocarbon content of the xylene fraction supplied to column 78 is maintained below 15 per cent by weight.  The column 78 should be equivalent to at least 35 theoretical plates, preferably 45-60 plates.  Reflux ratios of from 7 : 1 to 12 : 1 are used and close temperature regulation is important.  The column is preferably operated at a constant feed rate while (1) removing bottoms and overhead distillate at a constant ratio corresponding to the feed rate and in a relative proportion such that the desired purity of ortho-xylene is maintained and (2) maintaining a constant volume of liquid and still bottoms by controlling the rate of heat input thereto.  The Specification gives detailed information as to the influence of factors such as ortho-xylene, meta-exylene, ethyl-benzene and paraffin content in the crystallizer feed on the temperature of crystallization and the theoretical yield and as to the effect on the latter of the temperature of crystallization for various feeds.