Recycling of paraffin and olefins from plastic bags: Many plastic bags can produce paraffin or olefin hydrocarbon gases through high-temperature decomposition, and the amount produced varies greatly depending on the type of plastic. During the thermal decomposition of plastics such as polyethylene and polystyrene, a large amount of this gas is produced, and as the reaction temperature increases, the amount of gaseous products increases while the amount of liquid products decreases.
The fresh-keeping bag decomposes oil. Low density polyethylene is thermally decomposed at 4200C for 2 hours, with 60% of its products being paraffin and 40% being olefins. If high-density polyethylene is continuously introduced into the reaction system using a melting device and thermally decomposed at 400-4500C, 94.5% of the decomposed oil and 5.5% of the decomposed gas can be obtained. The order of recovery is: propylene>ethylene, propane>methane>n-butane.
In order to improve the yield of decomposed oil, a high-pressure reactor can be used to thermally decompose polyethylene in an atmosphere of carbon dioxide, carbon monoxide, helium, and hydrogen. The reaction temperature is around 340 ℃, and the oil conversion rate can reach 87% to 93%; Adding water to control local overheating during decomposition can also improve the oil conversion rate. By using lacquer nickel or Pt-C as catalysts to hydrogenate and decompose polyethylene, the oil conversion rate can reach as high as 85% to 90%.
The polypropylene of the fresh-keeping bag undergoes thermal decomposition at around 4000C, and up to 95% of the decomposed oil is recovered. If zinc chloride is used as a catalyst for the hydrogenation decomposition of polypropylene in an autoclave, 64% of the decomposed oil with an octane number of 83 can be obtained. This type of oil is of sufficient quality to be used as gasoline; If water is added for hydrogenation decomposition, a decomposition oil with an octane number of 86 can be obtained, but the olefin content is 40%. Methane is the most abundant in decomposed gas, followed by ethane and propane.
The fresh-keeping bag decomposes gas. Thermal decomposition gasification reaction is commonly used for recovering olefin gases, which can recover ethylene from polyethylene and mainly recover propylene from polypropylene.
The fresh-keeping bag uses a melting furnace to continuously introduce polyethylene into the reaction system, which undergoes thermal decomposition at 590-8000C with a gasification rate of 75%. At around 7000C, 32% ethylene can be obtained, and the total amount of propylene and butene-l reaches 58%. If a molten salt thermal decomposition furnace is used and the decomposition temperature is increased to 850 ℃, 30% of ethylene can be obtained, 17% of propylene can be obtained, and a total of 60% of olefins can be obtained; Under the action of a catalyst, thermal decomposition is 2 to 7 times faster than without a catalyst. At 4800C, the order of catalyst activity with respect to gasification rate is (from high to low) SiO2·AL2O3、CaX、NaX、NaY、CaA、NaA。
The polypropylene of the fresh-keeping bag decomposes at 500-6000C, and the gasification rate increases with the increase of reaction temperature. At 6000C, 26% of propylene can be obtained, followed by ethylene and methane. When using a molten salt thermal decomposition furnace, the decomposition temperature reaches around 9000C, from which 20% ethylene, 20% methane, and only 10% propylene can be obtained. Similarly, solid acidic catalysts can be used to catalyze the decomposition of polypropylene, using silica and alumina as catalysts and a fluidized bed atmospheric pressure reaction device. At 4500C, methane is mainly produced, followed by propylene and ethane. The production of liquid paraffin increases compared to the absence of catalyst.