Friday, May 14, 2010

Phthalic Anhydride

Phthalic anhydride is one of the most important intermediates for the plastics industry. The process for the preparation of phthalic anhydride are controlled oxidation of o-xylene or naphthalene.

R-R + 4 ½ O2 -------> R-R-O-(CO)2 + 2 CO2 + 2 H2O

Oxidation is one of the very useful chemical conversation in organic technology. The cheapest agent is air, but oxygen is sometimes employed. For liquid phase reaction a great many oxidizing agents are in industrial use, such as nitric acid, permanganate, pyrolusite, dichromate, chromic anhydride, hypochlorite, chlorites, lead peroxide, and hydrogen peroxide. Water and carbon dioxide and many other oxidized substance are the by-products of the main oxidation. When charcoal or carbon (amorphous) changes to carbon dioxide Δ H = -404 kJ/g-mol, and when hydrogen burns to water (gaseous), Δ H = -242 kJ/g-mol. These energy changes, although the basis of combustion, frequently accompany controlled oxidation, as in the making of phthalic anhydride and maleic acid. In most oxidation, even when the formation of carbon dioxide and water can be repressed, the energy change is exothermic and large. This requires particular care in the design and construction of the equipment to ensure efficient heat transfer and to prevent the controlled oxidation from becoming combustion.

The naphthalene process, with a suitable catalyst, was discovered by Gibbs and Connover. It was also necessary to work out efficient equipment to remove the great amount of heat liberated and to keep the temperature within favourable narrow limits. One of the successful devices was that patented by Downs and depicted in principle. In the Down reactor the temperature is controlled by raising or lowering the boiling point of mercury by raising and lowering the pressure of an inert gas (nitrogen) on the mercury boiling and condensing system. Other heat transfer media have been proposed, such as water, sulphur, diphenyl, diphenyl oxide, mercury amagams, and mixed molten nitrate. The products from the reaction are rapidly cooled to about 125oC (approximately the dew point of phtalic anhydride) and then sublimed or distilled.

Fundamental changes have been made in the manufacture of phthalic anhydride. The first supplemented by developing a purer one from petrochemicals by demethylating methyl naphthalenes. The second changes was to use a fluidized bed of the catalyst V2O2 instead of the long successful fixed bed. The third change was the use of o-xylene in either of the above procedure. A fourth change was to develop other catalyst which would work favourable on either naphthalene or o-xylene. The fifth change involve removing the large heat of reaction by using a molten salt bath. This made it possible to recover some of the heat of reaction to generate steam.

If naphthalene is burned completely to carbon dioxide and water, 41,870 kJ/kg of naphthalene is liberated. For naphthalene conversion to phthalic anhydride, Δ H = -12700 kJ/kg of naphthalene oxidized. During of the actual of this reaction an exothermic reaction of from 14,000 to more than 23,000 kJ/kg of naphthalene occurs, owing to an amount of complete combustion that always take place.

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