PET (Polyethylene Terephthalate) is a polymer compound that is widely used in the field of raw materials for chemical production. Its manufacturing methods mainly include ester exchange method and direct esterification method. This article will introduce the process, principle, advantages and disadvantages and operation points of these two manufacturing methods.
I. Ester exchange method
Ester exchange method is to carry out ester exchange reaction between dimethyl terephthalate (DMT) and ethylene glycol (EG) to generate bis(hydroxy)ethyl terephthalate (BHET), and then carry out polycondensation reaction to generate PET.
1. Process
(1) Mix dimethyl terephthalate and ethylene glycol according to the molar ratio of 1:1.05~1.10, add catalyst, and then produce PET at 180 ~220°C and 0.1~1.0MPa for transesterification reaction.
(2) The generated BHET is cooled, crystallized, separated and dried to obtain BHET with high purity.
(3) The BHET is subjected to polycondensation reaction at high temperature and high vacuum to generate PET resin.
2. Principle
The ester exchange reaction is reversible 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.
Condensation reactions are exothermic and require control of the reaction temperature and vacuum to avoid side reactions and to ensure the quality and molecular weight of the products.
3. Advantages and disadvantages
The advantages of the ester exchange method are that the raw materials are easily available, the reaction conditions are mild, and the purity of the products is high. The disadvantage is that the production process needs to consume a large amount of energy and water resources, and at the same time produces more waste.
4. Operation points
(1) Control the temperature and pressure of the ester exchange reaction to improve the reaction rate and product selectivity.
(2) In the polycondensation reaction, control the reaction temperature and vacuum to avoid the occurrence of side reactions.
(3) In the production process, strengthen the treatment and recycling of waste to reduce the production cost and environmental pollution.
Two, direct esterification method
Direct esterification method is to esterify terephthalic acid and ethylene glycol to generate bis(hydroxyethyl) terephthalate (BHET), and then polycondensation reaction to generate PET. compared with esterification method, the raw material of direct esterification method is cheaper and more readily available, and therefore has a wider range of applications.
1. Process
(1) Terephthalic acid and ethylene glycol are mixed at a molar ratio of 1:2.5~3.5, a catalyst is added, and the esterification reaction is carried out at 220~280°C, at atmospheric pressure or under slight positive pressure. The generated BHET is fed into the polycondensation kettle together with the unreacted monomer.
(2) In the polycondensation kettle, BHET is subjected to polycondensation reaction at high temperature and high vacuum to produce PET resin. Meanwhile, the unreacted monomer is recycled and reused.
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 and antimony acetate.
The polycondensation reaction is the same as the ester exchange method, which needs to control the reaction temperature and vacuum to avoid side reactions and ensure the quality and molecular weight of the products.
3. Advantages and disadvantages
The advantages of direct esterification method are that the raw materials are easy to obtain, the production cost is lower, and it is suitable for large-scale production. The disadvantage is that the reaction conditions are harsh, requiring high temperature and high pressure conditions, and the requirements for equipment are high.
4. Operation points
(1) Control the temperature and pressure of the esterification reaction to improve the reaction rate and product selectivity. At the same time, ensure the amount and activity of catalyst to avoid poisoning or failure of catalyst.
(2) In the polycondensation reaction, control the reaction temperature and vacuum to avoid the occurrence of side reactions. At the same time, the maintenance and repair of equipment should be strengthened to ensure the normal operation and service life of the equipment.
In short, the manufacturing methods of PET include two kinds of ester exchange method and direct esterification method. Different manufacturing methods have different advantages and disadvantages and scope of application, so they need to be selected according to the specific situation in the actual production process. At the same time, in the operation process, we should pay attention to the control of reaction conditions, strengthen the waste treatment and recycling and other aspects of the work, in order to ensure the production efficiency and product quality at the same time to reduce production costs and environmental pollution.
