Nippon Steel Chemical Co., Ltd.

Masakazu Takeuchi

Kazuyoshi Horibe

Hiroyuki Kuwahara

Nobumasa Noma

Tadanori Hara

Kunio Wada

Yukio Akimaru

Noboru Daito

Yuji Arifuku

Haruki Kawano

This invention relates to a process for purifying crystalline substances by charging the feed crystals containing impurities in a solid state to the top of a tower-type continuous crystallizer equipped with an agitating device, bringing said solid feed crystals into countercurrent contact with the melt formed by melting the descending crystals by a heating device provided in the lower section, taking out the purified crystals as melt from the lower section, and taking out the mother liquor containing the concentrated impurities from the upper section and comprises maintaining the temperature of the mother liquor existing in the upper section in a specified range by either heating the feed crystals or the upper section of the continuous crystallizer or making the average particle diameter of the feed crystals mm or more. This procedure makes it possible to form good crystals and a slurry layer in the crystallization tower and yield a crystalline product of high purity in high yield even when applied to feed crystals of reportedly inferior properties.

Method of refining crystalline material

This invention relates to a process for purifying benzoic acid continuously to a purity of 99.9% or more by introducing crystals of crude benzoic acid with a bulk density of 0.2 g/cm,or more and an average particle diameter of 0.3 mm or more into a continuous crystallization apparatus equipped with a stirrer and a heating device from the upper portion of the apparatus, effecting counter-current contact of the falling crystals of crude benzoic acid with the melt generated by heating with the heating device, withdrawing the purified benzoic acid as melt from the lower portion of the apparatus, and withdrawing the mother liquor from the upper portion of the apparatus. The process makes it possible to prepare high-purity benzoic acid continuously in a relatively simple apparatus at high efficiency.

Method of refining benzoic acid

Pyromellitic dianhydride, useful as raw material for the manufacture of polyimides, is prepared in high yield and purity by the reaction of benzene derivatives such as ethylbenzene, diethylbenzenes, and triethylbenzenes with an ethylating agent in the presence of a Friedel-Crafts catalyst, the separation of tetraethylbenzenes from the reaction products by distillation, the separation of 1,2,4,5-tetraethylbenzene from other tetraethyl benzenes by differential centrifugal precipitation at -10 to -30 degrees Celsius, and the catalytic vapor-phase oxidation of tetraethyl benzenes, using a catalyst containing vanadium pentoxide and titanium dioxide.

Process for producing pyromellitic dianhydride

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Yukihashi, Japan

Masakazu Takeuchi