Patent GB-656838-A (GB)
Title: Process of producing phthalic anhydride
IPC: C07C51/265
Filed: 1947-08-07
Abstract: Phthalic anhydride is prepared by aromatizing a complex mixture of non-aromatic hydrocarbons, separating from the mixture an aromatic fraction containing a major proportion of phthalic anhydride inconvertible and a minor proportion of anhydride convertible hydrocarbons, treating this fraction non-destructively by physical means to obtain a fraction containing a major proportion of phthalic anhydride convertible dialkyl benzenes and a minor proportion of inconvertible benzenes, and catalytically oxidizing this fraction to give phthalic anhydride and to eliminate the inconvertible alkyl benzenes by ring-rupture at the same time.  Starting-materials specified are naphthenic, paraffinic, or olefinic petroleum hydrocarbon mixtures, which may be mixed with aromatics.  It is preferred to use a mixture of naphthenes containing 6-12 and preferably 6-8 carbon atoms per molecule and having a boiling range of 180-420 DEG  F., and preferably of 180-320 DEG  F.  The aromatization step (e.g. n-octane to o-xylene) is preferably performed at about 1000 DEG  F. using a vanadium oxide or molybdenum oxide-alumina catalyst at about 100-400 lb./sq. inch; a partial pressure of hydrogen (45-150 pounds per square inch) and of hydrogen sulphide (0.5-30 pounds per square inch) is desirably maintained in the apparatus, and gaseous hydrocarbons from later steps are also recycled as diluents; further catalysts are noted.  The non-aromatic content of the aromatization product is lowered to >25 per cent or preferably less than 15 per cent by a distillation in which components boiling above and below the range of 275-295 DEG  F. are removed; if necessary further purification may be effected by solvent extraction, selective cracking, or a further aromatization.  This aromatic fraction, containing a major proportion of phthalic anhydride inconvertible alkyl benzenes, is fractionated on a column of at least 45 plates with a reflux ratio of 7 : 1 to 12 : 1 to give a cut of B.P. 285-295 DEG  F. containing only 3-30 per cent, and preferably about 15 per cent of anhydride inconvertible alkylbenzenes.  This is oxidized under the conditions described in Specification 644,920, whereby the o-dialkyl benzenes form phthalic anhydride and the non-convertible ones are over-oxidized (ring-ruptured) to products which are easily separable from the anhydride.  Suitable apparatus is described and an example of the process given.ALSO:Alkyl benzenes, formed by the aromatization of paraffinic or naphthenic hydrocarbons (see Group IV (b), are suitable blending agents for aviation gasoline.  Preferred blending agents are the m- and p-dialkyl benzenes (which may contain further alkyl groups), which are separated from the aromatization product by fractional distillation (see Group IV (b)).  Aromatization catalysts such as molybdenum oxide-alumina are prepared, for example, by mixing a 6 per cent aqueous ammonium chloride solution with an aqueous solution of ammonium di- or heptamolybdate or ammonium vanadate; the solution should be free from alkali metal ions and be maintained at pH 7.25-8.  The resultant gels are dried, desirably mixed with graphite, and calcined at high temperature.  Another catalyst is prepared by impregnating activated alumina with aqueous chromium trioxide, drying, and reducing with hydrogen.  An oxidation catalyst is prepared by evaporation of an aqueous paste of ammonium metavanadate on granular aluminium, followed by ignition.  Molybdenum and manganese oxides may also be present.  Alkyl benzenes are separated by a continuous fractional distillation using a high efficiency column, where the overhead and the bottoms are removed in a constant ratio corresponding to the feed-rate and where the ratio is chosen dependent on the purity of the fractions required, and where a constant volume of liquid still bottoms is maintained by controlling the rate of heat input.