TECHNOLOGY
Process Lowers Pyromellitic Dianhydride Cost West Germany's Ruhroel can now produce L I million lb. of PMDA per year from durene Continuous oxidation processes promise to open new markets for pyromellitic dianhydride (PMDA). Ruhroel Chemiewerk, Bottrop, West Germany, has developed a continuous oxidation process and can now produce 1.1 million lb. per year of PMDA from durene. In the U.S., Princeton Chemical Research, Inc., Princeton, N.J., has also started up an air oxidation process for making 400,000 lb. per year of the chemical. Ruhroel will sell PMDA initially for $1.70 a pound—about one third of the current U.S. list price. However, Ruhroel says that tariffs based on the "U.S. selling price" make it impossible to sell any of its products in the states. Ruhroel is pilot-planting a process for making a starting material other than durene and expects to cut its introductory price for PMDA nearly 50% "in about a year." A price reduction of this magnitude would kindle interest for PMDA and pyromellitic acid in a number of areas where cost limits use. Aromatic carboxylic acids and their anhydrides are important products for the chemical industry. The list includes such large-volume chemicals as phthalic, maleic, trimellitic fumaric, and naphthenic acids. These materials find uses in the manufacture of a very wide spectrum of products such as saturated and unsaturated polyesters, foods, plasticizers, alkyd resins, paints, catalysts, dyes, and pharmaceuticals. While pyromellitic acid and its dianhydride have yet to show the commercial promise of others in this series, they possess properties that are certain to encourage wider use. Oxidation. Oxidation of durene ( 1,2,4,5-tetramethylbenzene ) gives PMDA in much the same way that oxidation of o-xylene produces phthalic anhydride. The processes are so similar, in fact, that Ruhroel has converted a small phthalic reactor for its 44
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REACTORS. Ruhroel produces pyromellitic dianhydride (which it hopes to sell in the U.S.) at its plant in Bottrop, in a converted phthalic anhydride reactor. The remaining reactors are available for PMDA production should demafid increase PMDA unit. The major difference between phthalic and pyromellitic dianhydride processes is in the catalyst used to speed up oxidation. Both catalysts are based on vanadium pentoxide. But the PMDA catalyst operates at a lower temperature and is specific for the oxidation of tetraalkyl benzenes. Ruhroel will not reveal more about the catalyst's composition. Ruhroel starts by carbureting durene and preheated air. The dureneair mixture then passes through multiple tubes in a single-stage fixed-bed reactor. After product is cooled by heat exchange with incoming air, pyromellitic dianhydride is collected in haybox condensers (two in series). The melting point of PMDA is 280° to 284° C. and it must be removed from the condensers as a solid. Product removal occurs every 24 hr. It isn't necessaiy to shut the unit down to remove PMDA from the condensers. Ruhroel says that high yields and high PMDA purity for its process made it possible to trim the price of the product to $1.70 a pound. One pass through the reactor converts almost all of the durene in the feed stream to PMDA. There is no recycle, and off-gases are vented to the
atmosphere. Product purity is more than 9 8 % , so subsequent distillation of the PMDA is not required, according to the firm. The major impurity is pyromellitic acid, since water is a product of combustion. Price. Further reductions in the price of PMDA hinge almost entirely on obtaining lower-cost starting materials, according to Dr. Guenther Ibing, Ruhroel's works director. Durene is expensive. Sinclair Oil, the only supplier in the U.S., separates it from C 1 0 aromatic streams by freeze crystallization. Enjay Chemical, once a supplier, no longer sells durene. The U.S. price is about 60 cents a pound. Price reduction below 25 cents a pound for durene is unlikely. It's also doubtful if enough durene can be recovered from refinery aromatic streams to satisfy demand should PMDA become a large-volume chemical. Ruhroel's catalyst isn't restricted to the use of durene feeds, according to Dr. Ibing. Any compound made up of a benzene ring with alkyl groups at C-l, C-2, C-4, or C-5 will oxidize to give PMDA with Ruhroel's catalyst. The pilot plant is now producing an alternate feedstock for PMDA manufacture. Starting materials could be
toluene or any of the xylene isomers, Dr. Ibing says. Ruhroel can easily increase its rated capacity beyond 1.1 million lb. a year for PMDA. Production cost won't drop significantly, however, as a result of such a move, the firm says. Additional phthalic reactors are available for conversion to PMDA manufacture at little investment cost. The present unit is almost completely automated, and its depreciation charges are minimal. Little reduction in unit production costs will result from placing more reactors on stream, the firm points out. Producers. Hexagon Laboratories and Du Pont round out the list of U.S. producers of PMDA. Both offer the material commercially. However, most of Du Pont's production is tagged for captive use. Tokyo Kasei Kogyo Co., Ltd., Japan, also makes PMDA for captive use. Both U.S. firms use a wet process (the cookbook routes for making pyromellitic acid are oxidation of durene with nitric acid or condensation of pseudocumene with acetyl chloride and oxidation of the resulting acetopseudocumene). Bergbau-Forschung, G.m.b.H., in Essen, also in West Germany, has developed a process for producing PMDA based on chloromethylation of p-xylene, followed by saponification with methyl alcohol and caustic soda to produce a diester. This diester is oxidized to PMDA with nitric acid. PMDA's two anhydride groups make it more reactive than phthalic anhydride. It cures epoxy resins quickly. Temperature and electrical resistance of resins made with PMDA are high. Such materials as polyimide resins are finding uses in electrical insulation coatings and temperature-resistant lacquers. Plasticizers can be made with PMDA. Uses in dye and pharmaceutical synthesis are also promising. In addition, PMDA and trimellitic anhydride can be used to cross-link nylon 66 and polyester fibers. Ruhroel is western Europe's first PMDA producer. The firm says it is not interested in licensing the process to other companies in western Europe, since the market is still too small to support other entrants. It has licensed other aromatic oxidation processes developed at Bottrop, however. As yet, the company has reached no decision whether to license the newly developed process in countries outside western Europe.
Sekisui Irradiates Acrylonitrile to Produce Strong Heat-Resistant Foam Acrylonitrile, on the move in fibers, rubbers, and plastics, may now invade the foamed insulation market. Sekisui Chemical Co., Ltd., Japan's largest plastics processor, is now test marketing a new acrylic foam that the company feels could capture some of the market now held by polystyrene, polyurethane, polyvinyl chloride, and other foam plastics. Sekisui bills its new product as an irradiated (with beta and gamma rays) acrylic foam. According to the firm, irradiation promotes cross-linking and imparts strength and heat resistance. The material also features high thermal insulation value and ease of fabrication (easy to saw and form, for example). Maximum working temperature of Sekisui's acrylic foam is 150° C. for a material that has a specific gravity of 0.033. That's higher than that of rigid polyurethane foam, whose maximum working temperature is 100° C. at a specific gravity of 0.033 or polystyrene beads, which have a maximum working temperature of 60° C. at a specific gravity of 0.029, according to Sekisui data. Rock wool and diatomaceous earth, by contrast, both exceed acrylic foam in heat resistance, Sekisui says. Acrylic foam's tensile strength— about 255 lb. per sq. in. at a density of
0.05 gram per c c , and about 540 lb. per sq. in. at a density of 0.100 gram per cc—exceeds that of polystyrene or polyurethane foam, Sekisui says. Thermal conductivity of acrylic foam is about 0.03 B.t.u. per ft.-hr. at 60° C. This value is lower than that of rock wool, polystyrene beads, or flexible polyurethane foam. But it is higher than that of rigid polyurethane foam, Sekisui data indicate. The new product, however, doesn't rate quite so high in certain other properties. Its water absorption, for example, is somewhat higher than that of competitive products. And it can't be foamed in place, though it can be molded into any shape before use. Price of acrylic foam in Japan is about $1.25 per pound. That's about the same as that of rigid polyurethane foam and some 1.5 times as much as polystyrene foam. Uses. Main use of acrylic foam is, of course, as a thermal insulator, especially where a high-strength, highheat-resistant, easy-to-fabricate material is needed. But these same properties make the product potentially useful as a mold material in iron casting, where sand molds are now used, Sekisui notes. The Japanese firm's process involves acrylonitrile, acrylamide, and acrylic acid, which are then reacted to
FOAM. Noboru Shimizu, Sekisui Chemical Co., Ltd., inspects a piece of irradiated acrylic foam which could capture some of the foamed insulation market NOV. 22, 1965 C & E N
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