Purified terephthalic acid purification process

Meanwhile attempts to find an air oxidation route directly from p-xylene to terephthalic acid (TA) continued to founder on the relatively high resistance to oxidation of the /Moluic acid which was first formed. This hurdle was overcome by the discovery of bromide-controlled air oxidation in 1955 by the Mid-Century Corporation [42, 43] and ICI, with the same patent application date. The Mid-Century process was bought and developed by Standard Oil of Indiana (Amoco), with some input from ICI. The process adopted used acetic acid as solvent, oxygen as oxidant, a temperature of about 200 °C, and a combination of cobalt, manganese and bromide ions as catalyst. Amoco also incorporated a purification of the TA by recrystallisation, with simultaneous catalytic hydrogenation of impurities, from water at about 250 °C [44], This process allowed development of a route to polyester from purified terephthalic acid (PTA) by direct esterification, which has since become more widely used than the process using DMT. 

Purified terephthalic acid (PTA) is one of the largest-scale chemicals produced worldwide. Main fields of application for PTA include PET chips, PET films, PET fibers, and bottle resins. Two principal processes are used for the purification of raw terephthalic acid esterification distillation and ester cleavage of dialkyl terephthalates, and direct hydrogenation and crystallization [90]. The latter process was discovered by the Standard Oil Co.and later licensed worldwide, becoming the dominant process for PTA. The general process is to hydrogenate... 

The purification of terephthalic acid is complicated because it does not melt and, as it was not soluble in either water or other solvents, it could not be crystallized. On the other hand, the dimethyl ester of terephthalic acid could be easily crystallized from methanol or xylene. When the Mid Century Process was introduced by Scientific Design and Amoco in 1956, it became possible to produce and purify terephthalic acid directly. This process used air oxidation conditions similar to those for previous processes, with a ttuxed trivalent cobalt and manganese acetate catalyst in glacial acetic acid, but introduced an ammonium bromide cocatalyst in conjunction with tetrabromomethane. Cobalt or molyb-demun bromides or hydrobromic acid have also been used, and following reaction with the trivalent cobalt, provided a source of brottune free radicals. The free radicals activated the methyl groups of the /7-xylene and led to the for-... 

Purified terephthahc acid became commercially available from Amoco Chemical Co. in 1965, by which time a considerable polyester industry based on dimethyl terephthalate had already developed. The Amoco process involves purification of cmde terephthahc acid by a separate step to attain the high product purity required for polyester manufacture. The Amoco technology is the most-used worldwide, but other processes have been developed and are operating commercially. 

The principal solvolysis reactions for PET are methanolysis with dimethyl terephthalate and ethylene glycol as products, glycolysis with a mixture of polyols and BHET as products, and hydrolysis to form terephthalic acid and ethylene glycol. The preferred route is methanolysis because the DMT is easily purified by distillation for subsequent repolymerization. However, because PET bottles are copolyesters, the products of the methanolysis of postconsumer PET are often a mixture of glycols, alcohols, and phthalate derivatives. The separation and purification of the various products make methanolysis a cosdy process. In addition to the major product DMT, methanol, ethylene glycol, diethylene glycol, and 1,4-cyclohexane dimethanol have to be recovered to make the process economical.1... 

Methanolysis products are separated and purified by distillation. BHET, the monomer obtained by PET glycolysis, is normally purified by melt filtration under pressure. One of the problems encountered in neutral hydrolysis of PET is that the terephthalic acid isolated contains most of the impurities initially present in the PET waste. Hence very elaborate purification processes are required to obtain terephthalic acid of commercial purity. 

Prior to polymerization, p-xylene is first oxidized to terephthalic acid (TA) or dimethyl terephtalate (DMT). These diacid or dimethyl ester monomers are then polymerized via a condensation reaction with ethylene glycol to form the polyester. Prior to the development of a method to purify TA to make purified terephtahc acid (PTA, >99% pure) by the Mid-Century Corporation in the 1950s [10], DMT was the primary way to obtain the purified dicarboxylate. The Amoco Oil Company, now part of BP International, made several improvements to the PTA process since its inception [11]. Since the advent of the availability of PTA, it has become the monomer of choice over DMT. PTA avoids the complications of including methanol to enable purification and handling the methanol evolved during the polymerization to polyester. 

PET is prepared commercially by either ester interchange (El) or direct esterification processes (11. 12). For many years, the El process was the only one practical because of the difficulties encountered in purification of terephthalic acid (TA). The dimethyl terephthalate (DMT) was much easier to purify, and therefore much more readily available as a raw material. In the normal El batch process, ethylene glycol and DMT are mixed in a ratio ranging from... 

The cmde dimethyl terephthalate is recovered and purified by distillation in most processes. Although distillation (qv) is generally a powerful separation technique, the mode of production of the terephthaHc acid determines its impurity content, which in turn may make purification by distillation difficult. Processes resulting in the alteration of the impurities by catalytic treatment have been developed so that distillation can perform the necessary purification. 

An alternative to the foregoing process, which uses only physical methods of purification, is chemical recycling. Large amounts of PET film scrap are salvaged by a transesterification reaction. The scrap is treated with methanol in the presence of an acid catalyst to give ethylene glycol and dimethyl terephthalate, monomers that are purified by distillation or recrystallization and used as feedstocks for the production of more... 

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