The waste plastics are sorted, broken, melted and then enter the thermal decomposition reactor. The reactor is heated up. The waste plastics are melted into high-viscosity liquids and pyrolyzed at a certain temperature to produce relatively low molecular weight substances. The thermal decomposition temperature of various kinds of waste plastics is different, but it is basically within the range of 350~500°C. Using this technology to treat waste plastics can not only reduce environmental pollution, but also recover gasoline, diesel oil and other oil products, and the yield can reach 80. More than %, it has certain practical significance. At present, China also adopts this technology in the recovery of waste plastics, that is, it recovers fuel (oil, gas) through high-temperature pyrolysis or catalytic cracking, but it is generally smaller in scale and also in plastics. In the cracking process, due to various reasons (such as plastics producing high-viscosity melts that are difficult to transport; poor thermal conductivity of plastics, long time to reach the thermal decomposition temperature, etc.) cause high energy consumption, and it is prone to carbonization and clogging pipes, and the process is not easy to control. Disadvantages such as long processing time. Therefore, it is very necessary to research and develop a good chemical process.
Cracking waste plastics is usually divided into thermal cracking and catalytic cracking.
(1) Thermal cracking. The separation of waste plastics is relatively complex. If they are classified and then cracked, it will take some equipment investment, energy and time, and the recovery cost is higher. Thermal cracking is generally used in reactors to make those sorting impossible. Contaminated waste plastics are heated to their decomposition temperature (600~900°C) to decompose, absorb and purify, and available decomposition products are used. The difference in the thermal cracking temperature characteristics of waste plastics is mainly used to separate and recover by the use of segmented thermal cracking.
All kinds of waste plastics have their own thermal cracking temperature characteristics. Commonly used waste plastics such as polyvinyl chloride, polyethylene, polypropylene and polystyrene are usually subjected to staged pyrolysis. By controlling the pyrolysis temperature, the waste plastics mixture is divided. Segment pyrolysis. If the polystyrene is pyrolyzed during the low temperature stage, the higher value styrene monomer and light fuel oil can be recovered, and the heavy fuel oil can be recovered during the high temperature stage.
(2) Catalytic cracking. The thermal cracking reaction requires high temperature and is difficult to control. To reduce temperature, save costs and increase productivity, catalytic cracking is often used. Catalytic cracking of waste plastics to produce fuel oil has been a precedent worldwide. 3. China's Beijing, Xi'an, and Guangzhou cities have also established some small-scale waste plastic oil plants.