Terephthalic acid, molecular formula C8H6O4, is an organic compound, white crystalline powder. It is the highest yield dicarboxylic acid. It is solid at normal temperature. Heating does not melt, sublimation above 300 ℃. If heated in a closed container, it can melt at 427 ℃. Terephthalic acid is the raw material for producing polyester, especially polyethylene terephthalate (PET). It is used to manufacture polyester resin, synthetic fiber and plasticizer.
PTA production process can be divided into oxidation unit and hydrofining unit. Raw material p-xylene is oxidized into crude terephthalic acid by air with acetic acid as solvent under the action of catalyst, and then crystallized, filtered and dried into crude product in turn; Crude terephthalic acid is hydrogenated to remove impurities, and then crystallized, centrifuged and dried into PTA products.
The purification method of crude terephthalic acid comprises the following steps: drying the crude terephthalic acid, ball milling and screening to make the particle size reach 1-5 μ m. Immerse in water at a temperature of 60 ℃ - 100 ℃, stir, clarify, skim, centrifuge and dry at 80 ℃ - 105 ℃ to obtain pure terephthalic acid. The crude terephthalic acid is the precipitate of alkali reduction wastewater after acid precipitation, and the dry basis weight content of impurities is 15% - 18%.
The main patent manufacturers of purified terephthalic acid (PTA) process are BP Amoco, DuPont ICI and Mitsui Petrochemical Co., Ltd. after years of development, the technologies of the three companies are similar, each has its own characteristics, and the level is comparable. The total production capacity of PTA units adopting BP Amoco process in the world is 7.176 million T / A, that of DuPont ICI process is 3.495 million T / A, and that of well 3 oil refining process is 1.025 million T / A., 4-C6H4(COOH)2。 Colorless crystal. Sublimation occurs when the temperature is above 300 ℃. It has very little solubility in water and is soluble in dimethyl sulfoxide, dimethylformamide and hexamethylphosphoryl triamine. It is difficult to purify because of its low solubility and high melting point.
Terephthalic acid is prepared industrially by oxidation of p-Xylene by nitric acid or by air oxidation under the catalysis of cobalt salt. Terephthalic acid can also be produced by rearrangement reaction of potassium benzoate or potassium phthalate in the presence of cadmium or zinc catalyst and carbon dioxide.
Use of terephthalic acid:
1. Terephthalic acid is one of the important bulk organic raw materials, which is widely used in various aspects of the national economy such as chemical fiber, light industry, electronics and construction. Terephthalic acid is mostly used to produce polyethylene terephthalate (polyester). It is an important raw material for synthetic resin, polyester fiber, film, plastic products, insulating paint and plasticizer, and also used in the production of medicine, dyes and other products.
2. Terephthalic acid (PTA) is a raw material for producing polyester, especially polyethylene terephthalate (PET). More than 90% of terephthalic acid (PTA) in the world is used to produce polyethylene terephthalate (PET). It requires 0.85-0.86 tons of PTA and 0.33-0.3 tons of MEG (ethylene glycol) to produce 1 ton of pet. Polyester includes fiber chips, polyester fibers, bottle chips and film chips. In the domestic market, 75% of PTA is used to produce polyester fiber; 20% is used to produce bottle grade polyester, mainly used in the packaging of various beverages, especially carbonated beverages; 5% for film grade polyester, mainly used in packaging materials, films and tapes. It can be seen that the downstream extension products of PTA are mainly polyester fibers.
3. Terephthalic acid can also be used as a chromatographic reagent. The residual alkalinity of wool was determined by GC. The fatty acid can be separated and analyzed by adding polyethylene glycol 200.
4. Terephthalic acid is widely used. More than 90% of terephthalic acid in the world is used to produce polyethylene terephthalate. Another important application of terephthalic acid is to produce plasticizers, including two types: the first type is dioctyl terephthalate (DOTP), which is the product of esterification reaction between terephthalic acid and industrial octanol (2-ethylhexanol), and is a high-quality plasticizer with high flash point and high specific resistivity, Especially suitable for the production of cable materials with high requirements for heat resistance and insulation; The second is polyester plasticizer, which is the product of esterification polycondensation reaction between terephthalic acid and Polyols (such as diethylene glycol, triethylene glycol, glycerol, propylene glycol, butanediol, etc.), and its relative molecular weight is generally between 1000 and 4000 (the relative molecular weight of polyester as plasticizer is much smaller than that of polyester used in chemical fiber and plastic packaging).