GB1078660A - Steam sublimation of terephthalic acid - Google Patents

Abstract

1,078,660. Purification of terephthalic add by sublimation. MOBIL OIL CORPORATION. Dec. 14, 1965 [Dec. 15, 1964], No. 53035/65. Heading BIB. Terephthalic acid is purified by entraining finely-divided particles of the impure acid in a gaseous stream containing steam, preferably superheated steam at 600‹F. to effect vaporization of a substantial portion of the acid, heating the stream (if necessary) to vaporize the acid completely, separating non-volatile solid impurites (ash) from the stream, cooling the stream to condense a substantial portion of the acid in the form of freely-flowing solid particles while retaining at least a major proportion of the more volatile material in the gaseous phase, and separating the purified acid particles from the stream. The process is particularly suitable for eliminating p-carboxybenzaldehyde from terephthalic acid produced by the partial oxidation of p-xylene. Ash is separated from the hot gaseous stream, containing vaporized acid, by passing the stream through a cyclone or a filter made of sintered stainless steel. The stream is then cooled in a condenser comprising one or more U-tubes 32 each surrounded by a heating-jacket 39 and provided, at the inlet end, with steam injectors. In operating this primary condenser, the mixture of acid vapour and gaseous carrier enters tube 32 from pipe 30 and is joined by two annular streams of steam (1) superheated steam at 650‹F. supplied through line 52 to the inner annular zone, and (2) saturated steam at 350‹F. supplied through conduit 58 into the outer annular zone. Saturated steam may also be injected into the condenser tube at one or more points downstream. The cooled stream is conveyed from the primary condenser to a cyclone separator maintained at about 500‹F. by an automatic control which governs the admission of saturated steam to the condenser. The partially purified particles of terephthalic acid withdrawn from the cyclone separator are re-sublimed in a second stage, and the desired product is removed from the cyclone separator of the condenser of this second stage. The vapours from the two cyclone separators, following the two primary condensers, are united and passed to a secondary condenser comprising a U-tube, into the inlet end of which demineralized water is sprayed. The amount of sprayed water is controlled to maintain a stream outlet temperature of 380‹F. whereby slightly impure terephthalic acid is condensed and the water is vaporized. The terephthalic acid is separted in a cyclone separator, and is returned to the first sublimation stage. The gaseous stream from the separator of the secondary condenser is passed to a tertiary condenser where a waterspray condenses the residual terephthalic acid. This acid is separated in a bag filter, and the steam passes on to a barometric jet condenser which produces sub-atmospheric pressure in the system. The condensation of the terephthalic acid may also be carried out in horizontal tubes.