There are three types: slurry method, solution method and gas phase method. Except for solution method, the polymerization pressure is below 2 MPa. The general steps include catalyst preparation, ethylene polymerization, polymer separation and granulation.
①The polyethylene produced by the slurry method is insoluble in solvents and is in a slurry state. Slurry polymerization has mild conditions and is easy to operate. Alkyl aluminum is commonly used as activator and hydrogen is used as molecular weight regulator. Tank reactors are mostly used. The polymer slurry from the polymerization tank is passed through the flash tank and the gas-liquid separator to the powder dryer, and then to granulation. The production process also includes solvent recovery, solvent refining and other steps. By adopting different polymerizers in series or in parallel, products with different molecular weight distributions can be obtained.
Polyethylene
②Solution method The polymerization is carried out in a solvent, but both ethylene and polyethylene are dissolved in the solvent, and the reaction system is a homogeneous solution. The reaction temperature (≥140℃) and pressure (4～5MPa) are relatively high. It is characterized by short polymerization time and high production intensity. It can produce polyethylene with high, medium and low densities at the same time, which can better control the properties of the product; but the polymer obtained by the solution method has a low molecular weight, a narrow molecular weight distribution and a solid The content is low.
③Gas-phase method "Ethylene is polymerized in gaseous state, generally a fluidized bed reactor is used. There are two types of catalysts: chromium series and titanium series, which are quantitatively added into the bed from a storage tank, and high-speed ethylene is circulated to maintain the fluidization of the bed and remove the heat of polymerization. The produced 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 for the production of linear low-density polyethylene. The gas phase method eliminates the solvent recovery and polymer drying steps, and saves 15% of the investment and 10% of the operating cost compared with 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 further improvement in product quality and variety.