How much profit does 1 ton of plastic break? The profit of plastic crushing has the following requirements. The pyrolysis of waste plastics depends on the equipment and process conditions used. There are liquid oil-based oilification processes, gas-based gasification processes, and solid carbon. The carbonization process that is currently widely used is the pyrolysis and oilification process.
The figure shows the process of waste plastic pyrolysis and oilification, which includes pretreatment, melting, pyrolysis, production oil recovery, residue treatment, neutralization treatment, and exhaust treatment.
(1) Pre-treatment. After the waste plastic is separated from the impurities (tanks, bottles, metals) through the pre-treatment process, the waste plastic is sent to the melt and rolled and broken into particles or fragments with a diameter of about 10 mm.
(2) Thermal decomposition. The waste plastic is first heated in the melting process at 200 ~ 300 ° C and melted into a kerosene liquid state. In this process, there is a small amount of thermal decomposition, especially waste plastics containing polyvinyl chloride. First, polyvinyl chloride will decompose at 250-300 ° C to produce hydrogen chloride gas, and the hydrogen chloride is sent to a neutralization treatment process.
In the thermal decomposition step, as the temperature is increased, the decomposition reaction rate is also accelerated, but the yield of the liquid product is reduced, and an adverse carbonization phenomenon may occur. Therefore, choosing the temperature range becomes the key in process design. The liquid waste plastic is heated to 300-500 ° C to decompose. In order to obtain as much petroleum component as liquid at normal temperature, a catalyst is sometimes used. The use of a catalyst can not only improve the yield of oil, especially the yield of light oil, but also improve the quality of the oil.
(3) Recovery of generated oil. The generated oil recovery step is to cool the high-temperature thermally decomposed gas generated in the thermal decomposition step to room temperature to become a liquid oil. The quality, properties, and yield of the produced oil vary greatly depending on the type of plastic, the reaction temperature, the reaction time, and whether or not a catalyst is used.
(4) Residue disposal. The residue treatment step is a step of removing a small amount of impurities such as sand, glass, wood chips and the like generated during thermal decomposition from the furnace during the thermal decomposition step. For polyvinyl chloride plastics, hydrogen chloride gas is generated during thermal decomposition. The hydrogen chloride gas must be recovered in the neutralization process as hydrochloric acid, or neutralized with alkali such as caustic soda and slaked lime before being discharged.
(5) Exhaust treatment. The exhaust gas treatment step is a step for treating combustible gases (carbon monoxide, methane, ethane, etc.) that are difficult to aggregate in the thermal decomposition step. Open flame chimneys can be used for direct burning or as fuel for thermal decomposition. In addition, it can be reused in the system as an energy source for power steam.
Technology and Application of Pyrolysis of Waste Plastics
A General process and characteristics
According to chemical principles, pyrolysis and oilification processes can be divided into thermal cracking and catalytic cracking. The former is generally performed at a high temperature of 600-900 ° C, and the latter is performed at a lower temperature of 300-450 ° C. Figure 5-71 shows the general process of waste plastic pyrolysis and oiling.
There are four types of waste plastic pyrolysis and oilification processes: tank method (polymerization bath method and decomposition tank method), tube method (tube distillation and spiral method), fluidized bed deactivation and catalytic method (solid catalyst).
B tank pyrolysis process
There are mainly two types of tank pyrolysis processes: polymerization bath method and decomposition tank method. They have the same principle and differ only in solid waste!
The material is first melted and then decomposed, or is directly fed into the decomposition.
Thermal decomposition in the tank is very similar to the distillation process. The waste plastic added to the tank is sharply decomposed at the beginning, but the product cannot be distilled out of the tank until a certain vapor pressure is reached. Therefore, before reaching the vapor pressure of the distillable low-molecular-weight oil, it is refluxed in the tank, and after the distillation outlet is filled with steam (volatile components), it is discharged out of the tank. Thereafter, the cooling and separation processes are continued, the oil content is put into the storage tank, and the gas is used as fuel. When plastic is thermally decomposed in the tank pyrolysis furnace, the characteristics of the equipment should be analyzed by reaction engineering in order to carry out corresponding reasonable design.
The figure shows the process flow of the decomposition tank method developed by Japan's Mitsubishi Heavy Industries. The crushed waste plastic (10 mm) is fed into a melting tank with a temperature of 230-280 ° C through a rotary feeder, and the molten plastic is sent to a decomposition furnace, which is decomposed by heating at 400-500 ° C with hot air. The generated gas is recovered by cooling and liquefaction Fuel oil. HCI from molten polyvinyl chloride (PVC) is recovered in a hydrogen chloride recovery tower.
Light-weight oil is obtained by preheating and decomposing in a tank-type pyrolysis furnace. Conditions such as atmospheric pressure, low temperature, and long-term reaction must be maintained to allow simultaneous vaporization and carbonization. And deposited on the inner wall of the tank or the surface of the heating tube. In addition, when the solid waste plastic is fed, the gas generated in the dissolution tank and the separation tank is mixed with air, and it may cause an explosion after mixing.
The shape of the reaction tank is characterized by mixing and stirring during the decomposition process in the tank to make it in a fully mixed state, and external heating depends on the temperature to control the properties of the oil produced. Because it is completely mixed, this makes the part of the polymer that has just been fed in and the product that has been completely decomposed become the oil in the reflux. Therefore, the relative molecular mass distribution of the material in the tank is wide, and a long decomposition residence time is required. Because of the external heating, and the precipitation of carbon on the surface of the heating tube, this makes the heat transfer worse. Therefore, it is necessary to discharge the precipitated carbon and solid matter at regular intervals.
C tube pyrolysis process
There are two types of tube pyrolysis methods: tube distillation and spiral type. Tubular and tank pyrolysis furnaces are both externally heated and require a large amount of thermal fuel. Fig. 5-74 shows the thermal decomposition process of the tubular distillation method of Rifa Company. Distillation can be used to make waste PS into liquid monomer relatively easily, and the decomposition equipment, reaction temperature and residence time used to recover the monomer can be controlled at will.
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