Terephthalic acid process

These dimethyl terephthalate and terephthalic acid processes illustrate that it can be necessary to modify the reaction chemistry and mild (liquid phase) conditions to obtain the product in the desired yield and purity. 

Use of on-stream XRE analysis for monitoring liquid process streams has been reported for a number of applications including measurement of Fe, Cu, Co, Ni and Mo from five different points in a solution purification process of a cobalt refinery [28] analysis of Cu, As and S in copper electrolyte purification solutions [29] control of a solvent extraction process for La and Nd [30, 31] continuous monitoring of catalyst elements (Mn, Co and Br) in terephthalic acid process solutions [32] and measurement of various elements (particularly sulfur) in petroleum product and refinery streams [33, 34]. 

Chang, S.J., F.C. Chang, and H.B, Tsai, Block copolyetherester. Part 3 Preparation of block copolyetheresters by a terephthalic acid process in the presence of salts. Polymer Engineering Science, 35(2) p. 190.1995. 

Polyethylene terephthalate, commonly referred as polyester, can be made by several slightly differing routes. In the terephthalic acid process, ethylene glycol is reacted with terephthalic acid at temperatures above 200 °C, which drives the reaction forward by removal of water. An alternative process utilizes ester exchange to reach high molecular weights. 

In the terephthalic acid process, esterification of ethylene glycol and terephthalic acid is conducted directly at a moderate pressure (2.7-5.5bar) and a high temperature (220-260°C). Water is eliminated in the reaction, and is also continuously ronoved by distillation ... 

Condensation polymerization differs from addition polymerization in that the polymer is formed by reaction of monomers, each step in the process resulting in the elimination of some easily removed molecule (often water). E.g. the polyester polyethylene terephthalate (Terylene) is formed by the condensation polymerization (polycondensation) of ethylene glycol with terephthalic acid ... 

Manufacture and Processing. Terephthalic acid and dimethyl terephthalate did not become large-volume industrial chemicals until after World War II. Imperial Chemical Industries in the United Kingdom in 1949 and Du Pont in the United States in 1953 commercialized fibers made from poly(ethylene terephthalate). Dimethyl terephthalate and ethylene glycol were the comonomers used by both companies (see Fibers, polyester). 

Technical-Grade Terephthalic Acid. All technical-grade terephthahc acid is produced by catalytic, hquid-phase air oxidation of xylene. Several processes have been developed, but they all use acetic acid as a solvent and a multivalent heavy metal or metals as catalysts. Cobalt is always used. In the most popular process, cobalt and manganese are the multivalent heavy-metal catalysts and bromine is the renewable source for free radicals (51,52). 

Fig. 4. The Amoco purification process for polymer-grade terephthalic acid.

The phthalic acid and benzoic acid are reacted to form a reaction intermediate. The reaction intermediate is dissolved in sulfuric acid, which precipitates terephthalic acid (TPA). Fifty percent of the TPA is sold as a product and 50 percent is further processed at your facility into polyester fiber. The TPA Is treated with ethylene glycol to form an intermediate product, which is condensed to polyester. 

Your company manufactures terephthalic acid, a listed chemical, both for sale/distribution as a commercial product and for on-site use/processino as a feedstock in the polyester process. Because it is a reactant, it is also processed. See Figure D for how this information would be reported in Part III, Section 3 of Form R. 

Yourfacility also uses, as well as processes, sulfuric acid, a listed substance, as it sen/es as a process solvent to precipitate terephthalic acid. 

The carboxylic acid produced in the greatest fflnounts is 1,4-benzenedicarboxylic acid (terephthalic acid). About 5 X 10 Ib/year- is produced in the United States as a starting material for the preparation of polyester fibers. One important process converts p-xylene to terephthalic acid by oxidation with nitric acid ... 

The reaction occurs in a liquid-phase process at approximately 400°C using ZnO or CdO catalysts. Terephthalic acid is obtained from an acid treatment the potassium salt is recycled. ° ... 

Another approach is to use an easily oxidized substance such as acetaldehyde or methylethyl ketone, which, under the reaction conditions, forms a hydroperoxide. These will accelerate the oxidation of the second methyl group. The DMT process encompasses four major processing steps oxidation, esterification, distillation, and crystallization. Figure 10-16 shows a typical p-xylene oxidation process to produce terephthalic acid or dimethyl terephthalate. The main use of TPA and DMT is to produce polyesters for synthetic fiber and film. 

Important applications for titanium have been developed in processes involving acetic acid, malic acid, amines, urea, terephthalic acid, vinyl acetate, and ethylene dichloride. Some of these represent large scale use of the material in the form of pipework, heat exchangers, pumps, valves, and vessels of solid, loose lined, or explosion clad construction. In many of these the requirement for titanium is because of corrosion problems arising from the organic chemicals in the process, the use of seawater or polluted cooling waters, or from complex aggressive catalysts in the reaction. 

Kemkes256 assumes that the overall order relative to the esterification of terephthalic acid by 1,2-ethanediol in oligo(l,2-ethanediyl terephthalate) is two no mechanism has however been suggested. Mares257 considers that during the esterification of terephthalic acid with 1,2-ethanediol, two parallel kinetic paths take place, one corresponding to a reaction catalyzed by non-dissociated add and the other to a non-catalyzed process. In fact, Mares257 is reserved about the existence of protonic catalysis. Some other orders were found for the system terephthalic atid/l,2-ethanediol 0 (overall)318 2 (add) andO (alcohol)203 1 (add) and 1 (alcohol)181 1 (add)194 . These contradictory results could be partly due to the low solubility of terephthalic acid in 1,2-ethanediol. 

PET is the polyester of terephthalic acid and ethylene glycol. Polyesters are prepared by either direct esterification or transesterification reactions. In the direct esterification process, terephthalic acid is reacted with ethylene glycol to produce PET and water as a by-product. Transesterification involves the reaction of dimethyl terephthalate (DMT) with ethylene glycol in the presence of a catalyst (usually a metal carboxylate) to form bis(hydroxyethyl)terephthalate (BHET) and methyl alcohol as a by-product. In the second step of transesterification, BHET... 

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... 

Hydrolysis, although a simple method in theory, yields terephthalic acid (TPA), which must be purified by several recrystallizations. The TPA must be specially pretreated to blend with ethylene glycol to form premixes and slurries of the right viscosities to be handled and conveyed in modern direct polyesterification plants. Hie product of the alkaline hydrolysis of PET includes TPA salts, which must be neutralized with a mineral acid in order to collect the TPA. That results in the formation of large amounts of inorganic salts for which commercial markets must be found in order to make the process economically feasible. There is also the possibility that the TPA will be contaminated with alkali metal ions. Hydrolysis of PET is also slow compared to methanolysis and glycolysis.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. 

In the late 1950 s two groups - one at ICI (ref. 1) and the other at the Mid-Century Corporation (ref. 2) - independently discovered that p-xylene is oxidized to terephthalic acid in almost quantitative yield when soluble bromides are used together with cobalt and manganese catalysts in acetic acid solvent at temperatures > 130 °C (ref. 3). This discovery formed the basis for what became known as the Mid-Century process and later, when the Mid-Century Corporation was acquired by Amoco, as the Amoco MC process for the commercial production of terephthalic acid. A large part of the ca. 6 million tons of the latter that are manufactured annually, on a worldwide basis, are produced via this method. This makes it the most important catalytic oxidation process (ref. 4). 

Consequently, as a result of increasing environmental pressure many chlorine and nitric acid based processes for the manufacture of substituted aromatic acids are currently being replaced by cleaner, catalytic autoxidation processes. Benzoic acid is traditionally manufactured (ref. 14) via cobalt-catalyzed autoxidation of toluene in the absence of solvent (Fig. 2). The selectivity is ca. 90% at 30% toluene conversion. As noted earlier, oxidation of p-xylene under these conditions gives p-toluic acid in high yield. For further oxidation to terephthalic acid the stronger bromide/cobalt/manganese cocktail is needed. 

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