Meteorological method
In the process of preparing high-density polyethylene, it is with the help of a low-pressure gas-phase fluidization reactor, chromium-based catalysts are selected, which can meet the corresponding polymerization reaction in the working environment of 85-110°C, and the main product density is 0.915-0.970g/cm3. Typical vapor polymerization processes are Unipol PE technology from Univation and Innovene G technology from INEOS. The characteristics of Unipol PE production process are that only one reactor is needed to complete the production of high-density polyethylene, and there are many types of products, complete brands, at present, the maximum production capacity of a single line reaches 600,000 t/a, the reactor used in the production process is a vertical vapor phase liquefaction bed reactor in the top expanded state, and the catalyst is injected into the reactor from the top in a solid powder state or slurry state. The Innovene G production process is not much different from the Unipol process in terms of process and operation, except that it is designed to remove powder from the circulating gas, and a loop cyclone separation device is set between the vertical liquefy bed reactor and the circulating gas cooler. The main characteristics of the vapor phase production process are low operating pressure, low cost and easy maintenance of equipment. However, the purity of raw materials is required, and all raw materials need to be refined. The biggest advantage is that the conversion rate is relatively high, the one-way conversion rate of ethylene can reach 95%, the total utilization rate can reach 98.5%, and the ethylene that has not participated in the reaction can be recycled into the refining equipment for continued use.
Slurry method
The slurry production process embodies the characteristics of ethylene and aliphatic hydrocarbon solvent mixing, and the polymerization reaction can be realized at low temperature and low pressure by using titanium or chromium catalysts. In practical applications, it can be divided into stirring kettle type and ring tube type process according to the form of different reactors. The stirred kettle reactor was invented and optimized by Hoechst and uses a dual reactor that can be used in parallel or in series. Hydrogen, ethylene, catalyst, etc. are put into the first reactor to cause a polymerization reaction, and the polymer exists in hexane in the form of slurry, so it is called the slurry preparation process. The mixing kettle type has a simple process and low requirements for the purity of raw materials, and can flexibly use series or parallel connection to produce single-peak or double-peak high-density polyethylene, which can quickly complete the switching of product grades, so this technology is widely used. A typical representative of ring tube reactors is Phillips' Phillips process, which uses chromium-based catalysts, which are activated before use, and then injected into the reactor by mixing raw ethylene with hydrogen to generate polyethylene under the catalytic reflection of the catalyst. This process is characterized by relatively low cost, relatively few processes, and the raw materials are easy to convey, but the purity requirements of the raw materials are relatively high.
Solution method
The solution method production process usually uses a stirred kettle reactor. In the process of applying the solution method, ethylene can be dissolved in a solvent at 160-200°C, and the polymerization reaction can be achieved with the help of a catalyst. The process of the production process is to dissolve ethylene in the solvent, transport it to the kettle reactor to catalyze the polymerization reaction, and the polymer generated after the reaction is completed is in a molten state, suspended in the solvent, and after separation, it can be directly extrusion and granulation. The characteristics of this process are: in the reactor, the polymerization reaction is fast, the corresponding material stays in the reactor for a short time, the time for switching between different products is short, the polymerization product can be dissolved in the solvent, there will be no dirt on the pipe wall inside the reactor, and the reactor does not need to be cleaned regularly. The disadvantages are: the temperature and pressure required for polymerization reaction are high, resulting in more equipment, in order to meet the needs of high temperature and high pressure in the reaction process, the equipment cost is high, and the process process is long, and the operating cost is high. Typical examples of solution production processes are NOVA's Sclairtech process and DOW Chemical's DOWlex process. At present, due to the long process flow, high energy consumption and high investment of the solution method, there is basically no unit in China that chooses this method for polyethylene production.
