Purified terephthalic acid

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 

4-carboxybenzaldehyde, which is an impurity of terephthalic acid in the presence of an extruded Pd/C catalyst. Process conditions are typically 373 to 573 K and 200 to 1500 psi [91]. 

Although PTA catalysts have been commercialized for more than 30 years, research activities are still ongoing [92-94]. The main focus of the research has 

---

Purified terephthalic acid and dimethyl terephthalate are used as raw materials for the production of saturated polyesters. During 1993, the combined worldwide production of purified terephthafic acid plus dimethyl terephthalate exceeded 14 x 10 t (42), which is 80% of the total benzenepolycarboxyfic acid production. Terephthafic acid is also produced ia technical or cmde grades which are not pure enough for manufacture of poly(ethylene terephthalate). In almost all cases, the technical-grade material is immediately converted to purified terephthafic acid or dimethyl terephthalate, which together are the articles of commerce. 

In recent years, however, the ester exchange reaction has been decreasingly used, with manufacturers now usually preferring to use direct acid glycol condensation with purified terephthalic acid. 

This paper describes the Recopet process of chemical recycling for post-consumer PETP waste, which yields polymer-quality purified terephthalic acid. Background information is given on PETP recycling, and two routes of chemical recycling are explained. The Recopet process is described in detail, with information provided on the quality of the produets obtained. 

Terephthalic acid (p-TA or TA), a raw material for polyethylene terephthalate (PET) production, is one of the most important chemicals in petrochemical industry. Crude terephthalic acid (CTA), commonly produced by homogeneous liquid phase p-xylene oxidation, contains impurities such as 4-carboxybenzaldehyde (4-CBA, 2000-5000 ppm) and several colored polyaromatics that should be removed to obtain purified terephthalic acid (PTA). PTA is manufactured by hydropurification of CTA over carbon supported palladium catalyst (Pd/C) in current industry [1]. 

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. 

Although the superior properties of PEN have been known for many years, the unavailability of the naphthalate monomer has delayed the development of commercial markets, until relatively recently (1995) when the Amoco Chemical Company offered high purity naphthalene-2,6-dimethyl dicarboxylate (NDC) in amounts of up to 60 million pounds per year. This diester is produced by a five-step synthetic route, starting from the readily available compounds, o-xylene and 1,4-butadiene [3], Prior to this, the NDC diester was obtained by extraction of 2,6-dimethylnaphthalene (DMN) from petroleum streams, where it was present in relatively low abundance. Oxidation of DMN to crude 2,6-naphthalene dixcarboxylic (NDA) is conducted by a similar process to that used for conversion of p-xylcnc to purified terephthalic acid (TA), crude NDA is esterified with methanol, and is then distilled to yield high purity NDC. Other companies (e.g. the Mitsubishi Gas Chemical Company) followed Amoco s introduction with lesser amounts of NDC. Teijin [4] has manufactured PEN for many years for its own captive uses in films. 

Together with purified terephthalic acid, 1,3-propanediol is used to produce polytrimethylene terephthalate (PTT), a polymer with remarkable "stretch-recovery" properties. The desirable attributes of PTT have been known since the 1940s, but high production costs prevented its entrance into the polymer market (29). In the 1990s, a new fossil-based route to 1,3-propanediol was developed enabling the production of PTT for higher-value applications, and PTT polymers were introduced into the market by DuPont and Shell Chemicals (29,30). 

Purified Terephthalic Acid Production Alternative Energy Development... 

Fiber-grade terephthalic acid (7,8,9,10) Tb produce a fiber-grade quality, MTA has to undergo a further purification step. In the feed preparation system (7), MTA is dissolved in water at elevated temperatures and sent to the hydrogenation reactor (8) in which it is catalytically treated to remove the impurities. The hydrogenation catalyst is palladium based. The purified terephthalic acid is crystallized by stepwise depressurizing in a unique crystallizer train (9). 

Washington Group Internation al/lnterquisa Purified terephthalic acid Paraxylene Commercally proven, economic process for producing highest quality PTA product 1 2000... 

Mitsui Chemicals, Inc. Purified terephthalic acid (PTA) Paraxylene High quality product with simple and mild oxidation and low product cost 10 1997... 

The discovery of poly(ethylene terephthalate), PET, in the 1940s [1,2] and its commercialization initially by DuPont and by ICI in the 1950s created a large market demand for terephthalic acid and terephthalate esters of polymer purity. Because dimethyl terephthalate, DMT, is readily purified by distillation [3] (and also because the p-xylene oxidation/esteiification intermediate, methyl p-toluate, is more readily kept in solution than is p-toluic acid) the polyester fibers and films industry was initially based on terephthalate ester. With the development of improved oxidation and purification technologies, purified terephthalic acid, TPA, became available in commercial quantities by the mid 1960s. Over 75% of the worldwide PET manufacture (total world PET capacity is over six million tons/year) is currently based on TPA rather than DMT [4]. This preference for TPA results from the less complicated esterification catalysis and the absence of methanol handling when the acid is used directly. 

PTA. Abbreviation for phosphotungstic acid also for purified terephthalic acid. 

A recent interesting example of the use of alkane feedstocks to develop more sustainable processes is the direct conversion of ethane into acetic acid developed by Sabic. Acetic acid is the raw material for many key petrochemical intermediates and products, including vinyl acetate monomer (VAM), purified terephthalic acid (PTA), acetate esters, cellulose acetate, acetic anhydride, monochloroacetic acid (MCA), and so on. Acetic acid is produced commercially from several feedstocks and by several different technologies. 

Nevertheless, Amoco s growth strategy proved to be unsuccessful. So it finally spun off all but its initial chemical learning base in paraxylene intermediates. As a result, the profits from Amoco s chemical sector increased from 68 million in 1981 to 574 million in 1994, thanks in large part to its 40 percent share of the world s market for paraxylene and purified terephthalic acid, both used to make polyester, the demand for which grew dramatically, especially in Asia. By 1997, 16 percent of Amoco s sales and the same percent of its income derived from chemicals. i... 

In another example, air is added as a source of oxygen to the expansion vessel at 150 °C to oxidize 4-carboxy benzaldehyde (a coproduct of p-xylene oxidation ) to terephthalic acid. Again, no quantitative information is given about rates of reaction, conversion, purity, etc. Still other examples relate that water at 327 °C and 200 atm (subcritical not supercritical) either with or without oxygen results in the extraction and production of purified terephthalic acid. 

In the TA process, ethylene glycol and purified terephthalate acid are generally mixed in a ratio of less than 1.4 to 1 and heated in an initial phase with an antimony oxide catalyst to the boiling point of the ethylene glycol (198 °C). Because the reaction is relatively slow at this temperature, this step is often run under pressure to increase the achievable temperatures. As the reaction proceeds and the BHET is produced, the temperatures are Increased and the pressure is lowered to allow distillation of the evolved ethylene glycol. This phase resembles the PC step of the El process. 

The emission streams from purified terephthalic acid (PTA) plants commonly contain carbon monoxide, methyl bromide, and various volatile organic compounds (VOC s). Before the vent gas (offgas) is exhausted to the atmosphere, these contaminants (often regulated) must be destroyed, normally by the catalytic oxidation process. Currently, most commercially available catalysts are used at an inlet terrperature higher dian 350 C. The improvement of the catalyst activity is desired to increase the catalyst life-time and to reduce the operational cost. Additionally, the catalyst selectivity needs to be improved to minimize or eliminate the formation of polybromobenzenes (PBB s) which can cause plugging or blockage in process lines. 

Purified terephthalic acid (PTA) - Davy Process Technology, UK... 

Application GT-CAR is GTC s carboxylic acid recovery technology that combines liquid-liquid extraction technology with distillation to recover and concentrate carboxylic acids from wastewater. The GT-CAR process is economical for any aqueous stream generated in the production of dimethyl terephthalate (DMT), purified terephthalic acid (PTA), pulp/pa-per, furfural and other processes. 

---