Modified PE Granules

Low pressure method

Polyethylene can be divided into high pressure method, medium pressure method and low pressure method according to polymerization pressure.

There are three kinds of methods: slurry method, solution method and gas-phase method, except for the solution method, the polymerization pressure is below 2 MPa. The general steps are catalyst preparation, ethylene polymerization, polymer separation and granulation.

①Slurry method: the generated polyethylene is insoluble in solvent and is in the form of slurry. Slurry method polymerization conditions are mild, easy to operate, commonly used alkyl aluminum as an activator, hydrogen as a molecular weight regulator, more kettle reactor. Polymer slurry from the polymerization kettle out of the flash kettle, gas-liquid separator to the powder dryer, and then go to granulation. The production process also includes solvent recovery, solvent refining and other steps. By using different combinations of polymerization kettles in series or parallel, products with different molecular weight distributions can be obtained.

② Solution method: polymerization is carried out in the solvent, but both ethylene and polyethylene are dissolved in the solvent, and the reaction system is a homogeneous solution. Reaction temperature (≥ 140 ℃), pressure (4 ~ 5MPa) higher. Characterized by short polymerization time, high production intensity, can produce both high, medium and low density polyethylene, and can better control the nature of the product; however, the polymer molecular weight obtained by the solution method is low, the molecular weight distribution is narrow, and the content of solids is low.

③ Gas-phase method: Ethylene is polymerized in the gas state, and a fluidized bed reactor is generally used. The catalyst has two kinds of chromium and titanium, which is added to the bed by the storage tank in quantity, and the high-speed ethylene circulation is used to maintain the fluidization of the bed and exclude the heat of polymerization reaction. The resulting polyethylene is discharged from the bottom of the reactor. The reactor pressure is about 2 MPa and the temperature is 85 to 100°C. The gas-phase method is the predominant method for production line LDPE. The gas-phase method eliminates the need for solvent recovery and polymer drying, and saves 15% in investment and 10% in operating costs over the solution method. It is 30% of the investment and 1/6 of the operation cost of the traditional high-pressure method, which has been rapidly developed. However, the gas-phase method in product quality and variety to be further improved.