TECHNOLOGY
Japanese launch xylene separation process Japan Gas-Chemical's plant will handle 100,000 tons of feed per year while cutting C8 losses • PETROLEUM - Next month, Japan Gas-Chemical will start up a new plant at Mizushima, Japan, using the company's new process to separate aromatic C 8 -isomers. The new plant, with an annual feed capacity of 100,000 tons per year of mixed xylenes (including ethylbenzene), will use a process developed to reduce overall utility costs and losses of C 8 's during the isomerization step of the conventional xylene separation processes. ISOMERIZER. J PC's Mizushima plant will be first to use new process, which has also been licensed by Badger Co.
Meanwhile, Badger has licensed the process from Japan Gas-Chemical (C&EN, Sept. 9, page 4 1 ) . In the new process, a mixture of hydrogen fluoride and boron trifluoride (HF-BF 3 ) is used both as an extractant for m-xylene to remove it from the C 8 mixture and as the catalyst for isomerizing m-xylene to o-xylene and p-xylene if desired, Dr. Yukio Igarashi of Japan Gas-Chemical told the Symposium on Foreign Developments in Petrochemicals. The raffinate from the initial extraction is fractionally distilled to give ethylbenzene, p-xylene, and o-xylene, Dr. Igarashi says. Any m-xylene not needed for making isophthalic acid, m-xylene diamine, and so forth, is isomerized as a liquid at low temperature to o- and p-xylenes with minimum formation of ethylbenzene and other by-products. The process has special advantages over other xylene separation processes, according to Dr. Igarashi. In conventional processes, p-xylene is obtained
Japan Gas-Chemical's process cuts Cs losses in xylene separation
50 C&EN SEPT. 16, 1968
EB—ethylbenzene PX~p-xylene OX—o-xylene MX—m-xyiene
by fractional crystallization at temperatures as low as —70° C , if the concentration of p-xylene is low. Then, recrystallization is required to obtain high-purity p-xylene. The ethylbenzene and o-xylene in the mixture are separated by distillation. Processes combining crystallization and fractionation do not separate an m-xylene product with a purity higher than 99%. If there is little need for the m-xylene it can be isomerized as a gas at relatively high temperatures over a solid catalyst. This process gives high losses of the C 8 mixture and appreciable yields of ethylbenzene, a less valuable product than o- or pxylene, compared to Japan Gas-Chemical's new process. The new process has four sections. In the first section, m-xylene in the feed and in the recycle from isomerization is extracted with HF-BF 3 , with which it forms a complex. In a pilot plant operated since May 1966, the extractor has multiple stages and operates at 0° to 10° C , Dr. Igarashi says. The extract stream, dissolved in H F , is split, part going to a decomposer, part to the isomerizer. In the decomposer, the complex separates on heating into m-xylene and HF-BF 3 . Recovered m-xylene ( 9 9 . 5 + % pure) is taken out of the bottom of the decomposer vessel and fed, along with the remainder of the H F solution of the complex, to the isomerizer. HF-BF 3 coming from the top of the decomposer is recycled to the extractor. In the isomerization section, the complex solution acts as the catalyst in the conversion of the recovered mxylene. The effluent from the isomerization unit is recycled. The raffinate from the extractor, containing under 0.03% m-xylene, goes to the fractionation section. Here, ethylbenzene is taken out of the raffinate in the first tower. The bottoms from the first tower are fractionated in a second tower to p-xylene overhead and o-xylene bottoms. Losses of C 8 's during extraction are less than 0.5% of the feed, Dr. Igarashi says. Most of the loss that does occur comes from alkyl group migration. Loss during isomerization is less than 1% of the feed. Side reactions during isomerization, a liquidphase process, can be controlled by controlling such factors as the relative amount of catalyst, residence time, and temperature.
