Terephthalic acid was first produced commercially by the oxidation of paraxylene (PX) using dilute nitric acid and, later, air. These older processes provided impure terephthalic acid and purification was achieved via esterification to dimethyl terephthalate (DMT).
Modern technologies produce purified terephthalic acid by the catalytic liquid phase oxidation of paraxylene in acetic acid, in the presence of air. The process uses manganese or cobalt acetate as a catalyst. The reaction is exothermic, producing water which is removed in a solvent recovery system. Acetic acid from this is returned to the reactor together with the oxidation catalyst. The resulting PTA is purified in a crystalliser where the unreacted xylene and water are flashed off.
Technology for PTA has been led by BP (Amoco) and Advansa (ICI). BP claims to have simplified the process offering savings in capital and operating costs with the optimal plant size at 700,000 tonnes/year. However, processes have also been developed by Dow (Inca), Mitsubishi, Eastman and Mitsui.
At the beginning of 2000, Eastman Chemical licensed its proprietary PTA technology to Lurgi. The Eastman process is claimed to make polymer-grade, medium quality terephthalic acid - sometimes called qualified terephthalic acid (QTA) - in mild oxidation conditions with less formation of byproducts. Product purification is simplified with no need for hydrogenation while the reactor itself is simple and does not require an agitator. Production cost savings of up to $50/tonne are claimed for a 500,000 tonnes/year plant in western Europe.
Samsung General Chemicals has discovered that carrying out the reaction in an atmosphere containing appreciable amounts of carbon dioxide accelerates the oxidation of paraxylene to PTA by as much as 26%. It also claims that a higher quality crude PTA with less partially oxidised components is produced. This development could offer significant cost savings in both the oxidation and purification sections of a new PTA plant and can also be employed in the revamp of existing units.
Some PTA plants have been modified so they can also produce purified isophthalic acid (PIA) by substituting metaxylene for paraxylene.
Terephthalic acid
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Terephthalic acid
Uses and Outlook
Nearly all purified terephthalic acid (PTA) is consumed in polyester production including polyester fibre, polyethylene terephthalate (PET) bottle resin and polyester film. For most grades of polyester used in textiles and food and beverage containers, it is more economical to use PTA than the alternative dimethyl terephthalate (DMT) intermediate.
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