Dimethyl terephthalate Production

Proportions and exact conditions required for optimum conversions and yields of diester are proprietary. Recovery of the dimethyl terephthalate product from unreacted starting materials, etc., is by distillation using a series of four or five columns. Polymer-grade material (m.p. 142°C b.p. 288°C) is obtained by distillation under reduced pressure from the top of the last fractionating column. 

The single largest use of methanol is in formaldehyde and dimethyl terephthalate production. Methanol is also used in the manufacture of methyl acrylate, methyl methacrylate, methyl chloride, dimethyl ether, dimethyl sulfate, and various other intermediates and dyes. Methanol is usehil in dissolving phenolic laminating resins, ethyl cellulose, cellulose nitrate, and a variety of other resins. Low-viscosity resin solutions are possible using methanol. 

Polyethylene terephthalate (PET) is a linear thermoplastic polymer, which was initially commercialized for packaging carbonated soft drinks due to its excellent gas barrier properties that allows it to retain CO. Raw materials used for production of PET are ethylene glycol and terephthalic acid (or dimethyl terephthalate). Production of PET is a two-step process in the first step trans-esterification or esterification takes place, depending on whether terephthalic acid or dimethyl terephthalate is used, and in the second step polycondensation of resulting oligomers produces PET (Massa et al.,2011). 

Ester interchange reactions are valuable, since, say, methyl esters of di-carboxylic acids are often more soluble and easier to purify than the diacid itself. The methanol by-product is easily removed by evaporation. Poly (ethylene terephthalate) is an example of a polymer prepared by double application of reaction 4 in Table 5.3. The first stage of the reaction is conducted at temperatures below 200°C and involves the interchange of dimethyl terephthalate with ethylene glycol... 

Reactions of the Methyl Groups. These reactions include oxidation, polycondensation, and ammoxidation. PX can be oxidized to both terephthahc acid and dimethyl terephthalate, which ate then condensed with ethylene glycol to form polyesters. Oxidation of OX yields phthaUc anhydride, which is used in the production of esters. These ate used as plasticizers for synthetic polymers. MX is oxidized to isophthaUc acid, which is also converted to esters and eventually used in plasticizers and resins (see Phthalic acids and otherbenzenepolycarboxylic acids). 

Other Markets. The use of methanol in the production of formaldehyde, MTBE, and acetic acid [64-19-7] accounts for approximately two-thirds of the worldwide demand for methanol. Methanol is used as feedstock for various other chemicals, such as dimethyl terephthalate (DMT)... 

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. 

Initial production of the dimethyl terephthalate started with the oxidation of -xylene to terephthaUc acid using nitric acid both companies reportedly used similar technology (43—45). Versions of the nitric acid oxidation process, which has been abandoned commercially, involved the use of air in the initial oxidation step to reduce the consumption of nitric acid (44,46,47). The terephthaUc acid was then esterified with methanol to produce dimethyl terephthalate, which could be purified by distillation to the necessary degree (48). 

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. 

Herm/es/Djnamit JS obe/Process. On a worldwide basis, the Hercules Inc./Dynamit Nobel AG process is the dorninant technology for the production of dimethyl terephthalate the chemistry was patented in the 1950s (67—69). Modifications in commercial practice have occurred over the years, with several variations being practiced commercially (70—72). The reaction to dimethyl terephthalate involves four steps, which alternate between liquid-phase oxidation and liquid-phase esterification. Two reactors are used. Eirst, -xylene is oxidized with air to -toluic acid in the oxidation reactor, and the contents are then sent to the second reactor for esterification with methanol to methyl -toluate. The toluate is isolated by distillation and returned to the first reactor where it is further oxidized to monomethyl terephthalate, which is then esterified in the second reactor to dimethyl terephthalate. 

The oxidation reactor effluent and methanol ate sent to the esterification reactor, which operates at up to 250°C and a pressure sufficient to maintain the Hquid phase. This latter is about 2500 kPa (25 atm). The oxidation products are converted to methyl -toluate and dimethyl terephthalate without a catalyst. Excess methanol is suppHed, and steam and vaporized methanol ate removed and enter a methanol recovery column. The esterification products flow to a cmde ester column, which separates the toluate from the terephthalate. The overhead stream of methyl -toluate is returned to the oxidation reactor, and the bottoms stream of dimethyl terephthalate goes to a primary distillation. The distillate is dissolved in methanol, crystallized, and sohd dimethyl terephthalate is recovered. The dimethyl terephthalate can then be either recrystallized or distilled to yield the highly pure material needed for the polyesterification reaction. 

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. 

Hydrolysis of Dimethyl Terephthalate. Hoechst Celanese and Eormosa Chemical Eibers Corp. produce a polymer-grade terephthahc acid by hydrolysis of high purity dimethyl terephthalate. Hbls-Troisdorf AG hcenses a process with this step (70). Hydrolysis occurs at 260—280°C and 4500—5500 kPa (45—55 atm) in a hydrolysis reactor without catalysis. The overhead methanol and water vapor is separated and the methanol is returned to the dimethyl terephthalate section for reuse. The reactor hquid is crystallized, cycloned, washed, and further cooled. Einahy, the slurry is centrifuged and dried. The product has less than 25 ppm of 4-formylbenzoic acid and very low levels of other impurities. There may be several hundred parts per million of monomethyl terephthalate, which is incompletely hydrolyzed dimethyl terephthalate. 

Economic Aspects. Terephthahc acid and dimethyl terephthalate are usually sold under long-term contracts. Pricing information is at times pubhshed but actual contract prices are not revealed. Price data pubhshed in 1992 were 0.60/kg for terephthahc acid and 0.57/kg for dimethyl terephthalate (42). The price is mainly influenced by the price of -xylene. The price of terephthahc acid is more than dimethyl terephthalate because a kilogram of it produces 17% more polyester. The price of dimethyl terephthalate takes this factor plus a credit for the methanol generated during polyester production into consideration. 

Terephthahc acid is pure white, and molten dimethyl terephthalate is colorless. Impurities or degradation products can be yellow or brown, so the darkness of either a solution of terephthahc acid in dimethylform amide or molten dimethyl terephthalate can be compared to APHA color standards. 

Thermoplastic copolyester elastomers are generally block copolymers produced from short-chain aUphatic diols, aromatic diacids, and polyalkjlene ether-diols. They are often called polyesterether or polyester elastomers. The most significant commercial product is the copolymer from butane-l,4-diol, dimethyl terephthalate, and polytetramethylene ether glycol [25190-06-1J, which produces a segmented block copolyesterether with the following stmcture. 

The avadabihty of PMDI also led to the development of polyurethane-modified isocyanurate (PUIR) foams by 1967. The PUIR foams have superior thermal stabiUty and combustibiUty characteristics, which extend the use temperature of insulation foams well above 150°C. The PUIR foams are used in pipe, vessel, and solar panel insulation glass-fiber-reinforced PUIR roofing panels having superior dimensional stabiUty have also been developed. More recently, inexpensive polyester polyols based on residues obtained in the production of dimethyl terephthalate (DMT) have been used in the formulation of rigid polyurethane and PUIR foams. 

Polyester Polyols. Initially polyester polyols were the preferred raw materials for polyurethanes, but in the 1990s the less expensive polyether polyols dominate the polyurethane market. Inexpensive aromatic polyester polyols have been introduced for rigid foam appHcations. These are obtained from residues of terephthaHc acid production or by transesterification of dimethyl terephthalate (DMT) or poly(ethylene terephthalate) (PET) scrap with glycols. 

Commercial thermoplastic polyesters are synthesized in a similar way by the reaction of a relatively high molecular-weight polyether glycol with butanediol and dimethyl terephthalate (14,15). The polyether chain becomes the soft segment in the final product, whereas the terephthaUc acid—butanediol copolymer forms the hard crystalline domains. 

On the basis of bulk production (10), poly(ethylene terephthalate) manufacture is the most important ester producing process. This polymer is produced by either the direct esterification of terephthaHc acid and ethylene glycol, or by the transesterification of dimethyl terephthalate with ethylene glycol. In 1990, poly(ethylene terephthalate) manufacture exceeded 3.47 x 10 t/yr (see Polyesters). Dimethyl terephthalate is produced by the direct esterification of terephthaHc acid and methanol. 

The most generally useful polyester is that made by reaction between dimethyl terephthalate (dimethyl 1,4-benzenedicarboxylate) and ethylene glycol (1,2-ethanediol). The product is used under the trade name Dacron to make clothing fiber and tire cord and under the name Mylar to make recording tape. The tensile strength of polyethylene terephthalate) film is nearly equal to that of steel. 

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 major PET manufacturers are depolymerizing scrap PET with glycols (glycolysis) or methanol (methanolysis) to form low-molecular-weight polyester diols (and BHET) and dimethyl terephthalate.3 The purified products are then used to make new products. Goodyear uses glycolysis to make REPETE, a new product which contains 10-20% recycled PET. Hoechst Celanese used methanolysis to produce DMT for repolymerization. Eastman Chemicals uses depolymerization of PET to recover used X-ray scrap. 

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... 

A two-step methanolysis-hydrolysis process37 has been developed which involves reaction of PET with superheated methanol vapors at 240-260°C and atmospheric pressure to produce dimethyl terephthalate, monomethyl terephthalate, ethylene glycol, and oligomeric products in the first step. The methanolysis products are fractionally distilled and the remaining residue (oligomers) is subjected to hydrolysis after being fed into the hydrolysis reactor operating at a temperature of ca. 270°C. The TPA precipitates from the aqueous phase while impurities are left behind in the mother liquor. Methanolysis-hydrolysis leads to decreases in the time required for the depolymerization process compared to neutral hydrolysis for example, a neutral hydrolysis process that requires 45 min to produce the monomers is reduced... 

Gamble et al.46 devised a glycolysis-methanolysis process which involves first dissolving PET in a mixture of EG, TPA, and dimethyl terephthalate oligomers (this stream actually is obtained from the molten PET formed in the polymerization process) at 230-290°C. The product stream is then reacted with superheated methanol at temperatures between 250 and 290°C to form a product mixture which is rich in dimethyl terephthalate. Most of the contaminants remain in the first reactor, which enables the process to produce high-purity dimethyl terephthalate. 

Witten A process for making dimethyl terephthalate by the concurrent oxidation and esterification of p-xylene. Similar to the Imhausen and Katzschmann processes. Developed by Chemische Werke Witten and subsequently operated by Dynamit Nobel in Germany. World production capacity for this process in 1993 was 3 million tonnes. 

Transesterification is a crucial step in several industrial processes such as (i) production of higher acrylates from methylmethacrylate (for applications in resins and paints), (ii) polyethene terephthalate (PET) production from dimethyl terephthalate (DMT) and ethene glycol (in polyester manufacturing),... 

The formation of prepolymer can also be achieved by transesterification of dimethyl terephthalate (DMT) with EG, releasing the by-product methanol. High-purity DMT is easily obtained by distillation and in the early years of PET production, all processes were based on this feedstock. During the late 1960s, highly purified TPA was produced for the first time on an industrial scale by re-crystallization. Since then, more and more processes have shifted to TPA as the feedstock and today more than 70 % of global PET production is based on TPA. The TPA-based PET production saves approximately 8 % of total capital investment and 15% of feedstock cost (Figure 2.1). 

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