[Chemical Knowledge]：PET manufacturing method and process flow

PET(Polyethylene Terephthalate) is a polymer compound, widely used in the field of chemical production raw materials. Its manufacturing methods mainly include ester exchange method and direct esterification method. This article will introduce in detail the process, principle, advantages and disadvantages and operating points of these two manufacturing methods.

1. Transesterification Process

Transesterification is a process in which dimethyl terephthalate (DMT) is transesterified with ethylene glycol (EG) to produce bis-hydroxyethyl terephthalate (BHET), followed by a polycondensation reaction to produce PET.

1. Process

(1) Dimethyl terephthalate and ethylene glycol are mixed in a molar ratio of 1:1.05-1.10, a catalyst is added, and an ester exchange reaction is carried out at 180-220°C and 0.1-1.0MPa. The BHET produced

(2) is cooled, crystallized, separated and dried to obtain BHET with high purity.

(3) BHET is subjected to a polycondensation reaction at a high temperature under a high degree of vacuum to produce a PET resin.

2. Principle

transesterification is a reversible reaction and requires the presence of a catalyst to increase the reaction rate and product selectivity. Commonly used catalysts include acetates or oxides of metals such as zinc, manganese, and cobalt.

polycondensation reaction is an exothermic reaction, and it is necessary to control the reaction temperature and vacuum degree to avoid the occurrence of side reactions and ensure the quality and molecular weight of the product.

3. Advantages and Disadvantages
The advantages of
transesterification are that the raw materials are readily available, the reaction conditions are mild, and the product purity is high. The disadvantage is that the production process needs to consume a lot of energy and water resources, while generating more waste.

4. Key points of operation

(1) Control the temperature and pressure of the transesterification reaction to improve the reaction rate and product selectivity.

(2) In the polycondensation reaction, the reaction temperature and vacuum degree are controlled to avoid the occurrence of side reactions.

(3) In the production process, strengthen the treatment and recycling of waste, reduce production costs and environmental pollution.

2. Direct Esterification

Direct Esterification is the esterification of terephthalic acid and ethylene glycol to produce BHET, and then polycondensation reaction to produce PET. Compared with the ester exchange method, the raw materials of the direct esterification method are cheaper and easier to obtain, so it has a wider range of applications.

1. Process

(1) Terephthalic acid and ethylene glycol are mixed in a molar ratio of 1:2.5~3.5, a catalyst is added, and esterification is carried out at 220~280 ℃, normal pressure or slight positive pressure. The resulting BHET is fed to the polycondensation tank together with unreacted monomer.

(2) In a polycondensation kettle, BHET is subjected to a polycondensation reaction at a high temperature and a high degree of vacuum to produce a PET resin. At the same time, unreacted monomers are recovered for reuse.

2. Principle
The
esterification reaction is reversible and requires the use of a catalyst to increase the reaction rate and product selectivity. Commonly used catalysts include tetraethyl titanate, antimony acetate, and the like. The

polycondensation reaction is the same as the transesterification method, and the reaction temperature and vacuum degree need to be controlled to avoid the occurrence of side reactions and ensure the quality and molecular weight of the product.

3. Advantages and disadvantages
The advantages of the
direct esterification method are that the raw materials are readily available, the production cost is low, and it is suitable for large-scale production. The disadvantage is that the reaction conditions are relatively harsh, requiring high temperature and high pressure conditions, and high equipment requirements.

4. Key points of operation

(1) Control the temperature and pressure of the esterification reaction to improve the reaction rate and product selectivity. At the same time, the amount and activity of the catalyst should be ensured to avoid catalyst poisoning or failure.

(2) In the polycondensation reaction, the reaction temperature and vacuum degree are controlled to avoid the occurrence of side reactions. At the same time, it is necessary to strengthen the maintenance and maintenance of the equipment to ensure the normal operation and service life of the equipment.

In short, the production method of PET includes two kinds of ester exchange method and direct esterification method. Different manufacturing methods have different advantages and disadvantages and scope of application, so it is necessary to choose according to the specific situation in the actual production process. At the same time, during the operation, attention should be paid to controlling the reaction conditions and strengthening the waste treatment and recycling work to ensure production efficiency and product quality while reducing production costs and environmental pollution.