Polyethylene can be divided into high pressure method, medium pressure method and low pressure method according to the polymerization pressure.
The high-pressure method is used to produce low-density polyethylene, this method was developed early, and the polyethylene produced by this method has accounted for about 2/3 of the total polyethylene production, but with the development of production technology and catalysts, its growth rate has lagged far behind the low-pressure method. In terms of its implementation methods, the low-pressure method includes slurry method, solution method and gas phase method. The slurry method is mainly used to produce high-density polyethylene, while the solution method and the gas phase method can not only produce high-density polyethylene, but also produce medium and low-density polyethylene, also known as linear low-density polyethylene, by adding comonomers. Various low-pressure processes are developing rapidly.
High pressure method
Method of polymerizing ethylene into low-density polyethylene with oxygen or peroxide as initiator. Ethylene enters the reactor after secondary compression, polymerizes into polyethylene under pressure 100~300 MPa, temperature 200~300 °C and initiator, and the reactants are separated by reduced pressure, so that the unreacted ethylene is recycled and recycled, and the molten polyethylene is extruded and granulated after adding plastic additives.
There are two types of polymerization reactors used: tubular reactors (tube lengths up to 2000 m) and kettle reactors. The one-way conversion rate of the tubular process is 20%~34%, and the annual production capacity of a single line is 100 kt. The one-way conversion rate of the kettle method process is 20%~25%, and the annual production capacity of a single line is 180 kt.
Low pressure method
There are three kinds of slurry method, solution method and gas phase method, except for the solution method, the polymerization pressure is below 2 MPa. The general steps include catalyst preparation, ethylene polymerization, polymer separation and granulation.
(1) Slurry method: the resulting polyethylene is insoluble in solvents and is slurry. The polymerization conditions of the slurry method are mild and easy to operate, and alkyl aluminum is commonly used as an activator, hydrogen is used as a molecular weight regulator, and a kettle reactor is mostly used. The polymer slurry from the polymerization kettle is transported to the powder dryer by the flash kettle and gas-liquid separator, and then to the granulation. The production process also includes solvent recovery, solvent refining and other steps. Using different combinations of polymerization kettles in series or parallel, products with different molecular weight distribution can be obtained.
(2) Solution method: polymerization is carried out in solvent, but ethylene and polyethylene are dissolved in solvent, and the reaction system is a homogeneous solution. The reaction temperature (≥ 140 °C) and pressure (4~5MPa) are high. It is characterized by short polymerization time, high production intensity, and can produce polyethylene of high, medium and low density, which can better control the properties of the product; However, the polymer obtained by the solution method has a low molecular weight, narrow molecular weight distribution, and low solid content.
(3) Gas phase method: ethylene is polymerized in the gaseous state, and a fluidized bed reactor is generally used. There are two kinds of catalysts, chromium and titanium, which are added to the bed by the storage tank, and the high-speed ethylene cycle is used to maintain the fluidization of the bed and exclude the heat of polymerization. The resulting polyethylene is discharged from the bottom of the reactor. The pressure of the reactor is about 2 MPa, and the temperature is 85~100 °C. The gas phase method is the most important method of production line type low density polyethylene, and the gas phase method eliminates the solvent recovery and polymer drying processes, and saves 15% of investment and 10% of operating costs than the solution method. It is 30% of the investment of the traditional high-pressure method and 1/6 of the operating cost. As a result, it has developed rapidly. However, the gas phase method needs to be further improved in terms of product quality and variety.
Medium pressure method
A chromium-based catalyst loaded on silica gel is used in a ring reactor to polymerize ethylene at medium pressure to produce high-density polyethylene.
Processing and application: can be processed by blow molding, extrusion, injection molding and other methods, and is widely used in the manufacture of films, hollow products, fibers and daily miscellaneous products. In actual production, in order to improve the stability of polyethylene to ultraviolet rays and oxidation, and improve the processing and use performance, a small amount of plastic additives need to be added. Commonly used ultraviolet absorbers are o-hydroxybenzophenone or its alkoxy derivatives, etc., carbon black is an excellent ultraviolet shield. In addition, antioxidants, lubricants, colorants, etc. are added to expand the application range of polyethylene.
