Patent GB-677368-A (GB)
Title: Separation of methyl benzenes from hydrocarbon mixtures
IPC: C07C7/14
Filed: 1950-05-16
Abstract: A polymethyl benzene having at least one pair of para-positioned methyl groups is separated from a hydrocarbon mixture by adding a tetrahalogenated methane of molecular weight at least that of carbon tetrachloride, cooling to form a solid phase containing said polymethyl benzene and tetrahalogenated methane, and a mother liquor phase containing other hydrocarbons, and separating said phases. Co-crystallized solids of relatively definite melting-points, density, and composition, viz. equimolar ratios, are formed, for example, with benzene, toluene, p-xylene, and pseudo-cumene but not with cyclohexane, biphenyl, naphthalene, ethyl benzene, styrene, p-tert. butyl toluene, p-bromotoluene, and p-cresol.  Durene but not isodurene forms a complex with dibromdichlormethane while both give a complex with carbon tetrabromide.  Other halogenated methanes used include carbon tetrachloride and tetraiodide, bromtrichlo-, dibromdichlor-, and tribromchlor - methanes. p - Xylene may be separated from its isomers, ethyl benzene, and non-aromatics since the solid eutectic composition is altered, a reduction of p-xylene in the mother liquor from 13 to less than 1 per cent being effected when tetrahalogenated methane is used.  Batch or continuous processes, e.g. in centrifuges or columns, may be used. Thus, the reactants are fed to the middle of a column at almost the crystallization temperature, and pass upwards to a cooled portion at a temperature to form a solid p-xylene complex. Mother liquor leaves at the top and the solid settles to the bottom where it may be withdrawn and centrifuged but, preferably, heated to melting-point and the melt removed at the base, a temperature gradient being maintained in the column.  Hydrocarbons containing up to 50 or more per cent of xylene isomers are the preferred feeds and may be obtained from petroleum, coal tar or other natural sources or by synthesis such as the Fischer-Tropsch process.  For p-xylene separation, a petroleum fraction of about 160-365 DEG  F., i.e. C7-C9, is suitable; for durene, a 350-415 DEG  F., i.e. C9-C11 fraction.  The amount of halogenated methane added is in excess of equimolar.  Thus, a 1.4 molar excess over that required to form the p-xylene or durene complex is suitable.  With high m-xylene content, larger amounts of halogenated methane may be required to prevent its crystallization. A mixture of xylene isomers may be fractionated to separate o-xylene, the overhead mixture treated according to the invention to separate p-xylene, and the mother liquid, after solvent removal, cooled to crystallize m-xylene.  Alternatively, the p-xylene is separated prior to the fractionation and crystallization.  Mother liquid, after solvent removal, may be isomerized and recycled.  The cocrystallized solids may be washed with the halogenated methane itself, isopentane or methyl alcohol, and recrystallized.  They may be used as constant temperature baths and calibration points for thermometers.  Examples describe the treatment of reformed naphtha fractions with CCl4 or CBr4 in the upper part of a Dewar flask divided by a fritted-glass disc into upper and lower chambers, a flow of cooled gas preventing entry to the lower chamber.  Solid CO2 is added or the flask cooled in liquid nitrogen to form a solid phase containing p-xylene complex, gas flow stopped, and a vacuum applied to draw off mother liquor. The solid is then melted and drawn off while the mother liquor is freed from CCl4 and cooled as above to crystallize m-xylene. Reformed naphtha bottoms containing durene or pseudocumene, respectively, are treated in similar manner with CBr2Cl2 or CCl4.ALSO:A polymethyl benzene having at least one pair of para-positioned methyl groups is separated from a hydrocarbon mixture by adding a tetrahalogenated methane of molecular weight at least that of carbon tetrachloride, cooling to form a solid phase containing said polymethyl benzene and tetrahalogenated methane, and a mother liquor phase containing other hydrocarbons, and separating said phases.  Co-crystallized solids of relatively definite melting points, density, and composition, viz. equimolar ratios, are formed, for example, with benzene, toluene, p-xylene, and pseudocumene but not with cyclohexane, biphenyl, naphthalene, ethyl benzene, styrene, p-tert. butyl toluene, p-bromotoluene, and p-cresol.  Durene but not isodurene forms a complex with dibromdichlormethane while both give a complex with carbon tetrabromide.  Other halogenated methanes used include carbon tetrachloride and tetraiodide, bromtrichlor-, dibromdichlor-, and tribromchlor-methanes.  p-Xylene may be separated from its isomers, ethyl benzene, and non-aromatics since the solid eutetic composition is altered, a reduction of p-xylene in the mother liquor from 13 to less than 1 per cent being effected when tetrahalogenated methane is used.  Batch or continuous processes, e.g. in centrifuges or columns, may be used.  Thus, the reactants are fed to the middle of a column at almost the crystallization temperature, and pass upwards to a cooled portion at a temperature to form a solid p-xylene complex.  Mother liquor leaves at the top and the solid settles to the bottom where it may be withdrawn and centrifuged but is, preferably, heated to melting point and the melt removed at the base, a temperature gradient being maintained in the column.  Hydrocarbons containing up to 50 or more per cent of xylene isomers are the preferred feeds and may be obtained from petroleum, coal tar or other natural sources or by synthesis such as the Fischer-Tropsch process.  For p-xylene separation, a petroleum fraction of about 160-365 DEG F, i.e. C7-C9, is suitable; for durene, a 350-415 DEG F, i.e. C9-C11 fraction.  The amount of halogenated methane added is in excess of equimolar.  Thus, a 1.4 molar excess over that required to form the p-xylene or durene complex is suitable.  With high m-xylene content, larger amounts of halogenated methane may be required to prevent its crystallization.  A mixture of xylene isomers may be fractionated to separate o-xylene, the overhead mixture treated according to the invention to separate p-xylene, and the mother liquid, after solvent removal, cooled to crystallize m-xylene. Alternatively, the p-xylene is separated prior to the fractionation and crystallization.  Mother liquid, after solvent removal, may be isomerized and recycled.  The co-crystallized solids may be washed with the halogenated methane itself, isopentane or methyl alcohol, and re-crystallized.  They may be used as constant temperature baths and calibration points for thermometers.  Examples describe the treatment of reformed naphtha fractions with C Cl4 or C Br4 in the upper part of a Dewar flask divided by a fritted-glass disc into upper and lower chambers, a flow of cooled gas preventing entry to the lower chamber.  Solid CO2 is added or the flask cooled in liquid nitrogen to form a solid phase containing p-xylene complex, gas flow stopped, and a vacuum applied to draw off mother liquor. The solid is then melted and drawn off while the mother liquor is freed from C Cl4 and cooled as above to crystallize m-xylene. Reformed naptha bottoms containing durene or pseudocumene respectively are treated in similar manner with C Br2 Cl2 or C Cl4.