In this section we discuss the process flow diagram for the pyrolysis of waste plastics. This is required to be a compact process that can be controlled in a stable and continuous way, because of movement of the high-viscosity material for each unit system in the process. This material can block the flow line and make difficult the continuous control of unit process.
In order to obtain an oil product in the pyrolysis of waste plastics, the major steps for waste plastics that are derived from the household, industry, etc. are basically shown in Figure 5.3, Waste plastics as a reactant for obtaining the valuable products are invariably contaminated with materials such as soil, iron and wood etc. and also consist of various types of plastics. This material cannot be directly used in the pyrolysis process. Therefore, in the next step separation treatments must be applied to obtain waste plastic with a homogeneous composition. If it contains a lot of various contaminative materials in waste plastics, it leads to poor economics by increasing the recycling procedure cost. Thus, the important point for the pyrolysis process is the purification of waste thermoplastics, especially excluding PVC and PET in the reactant. Here, PVC and PET in the plastic feedstock can cause unfavourables emission due to the presence of Cl as well as high char/coke yields since they are not well degraded in the general pyrolysis process, respectively. The waste material must be separated into individual components, such as thermoplastic, PVC, PET, thermosetting, iron, aluminum and paper, etc.
In the third step, the separated plastics are delivered by the feeding system such as the conveyer, hopper and extruder, etc. to the melting reactor, after cutting to a small size. The feeding system is continuously controlled with a constant reactant amount and classified as heated or nonheated case of the extruder. By preheating the plastic feedstock the melting time in the melting reactor can be shorted thus improving production rates. Moreover, the film type reactant is very bulky and voluminous, which makes it difficult to ensure continuous feeding, is easily dosed at the melting system after melting in the feeding system. In the unheated case, a hard reactant of several millimeter size is adequately controlled by a continuous feeding system. Thus, the feeding system will be determined by the profile of waste plastics that are exited from industry, agriculture and household, etc. Other important point is that if there is trouble in a continuous automatic feeding system, it can be quickly transferred to a manual system.
The next step is the melting system, where the solid plastic is changed to a low-viscosity melt. If there is sufficient time to melt the polymer in the melting reactor, the pyrolysis and/or catalytic degradation process as the next step of melting system will be well controlled without trouble in a continuous system. The residence time of plastics in the reactor depends on the plastic type and the desired viscosity extent. Thus, in order to reduce the melting time of reactant in the melting system, it needs be heated in the feeding system prior to the melting system. Moreover, as the system is scaled up to a big plant, this is a very important parameter for heating the feeding system.
In the pyrolysis and catalytic degradation of polymer at temperature 300-450°C the melted reactant is degraded into a smaller molecule and also upgraded to oil product with a high quality. Several processes of pyrolysis and catalytic degradation are available, such as pyrolysis process only, liquid-phase catalytic degradation after the melting process and catalytic degradation after the pyrolysis process, according to the characteristics of the oil produced.
For the processes of different reactor types, kiln and retort pyrolysis processes are characterized by a relatively low capital investment. However, they suffer from unfavorable economics, due to the high processing costs compared with the value of the oil product obtained. Also, the characteristics of this process are relatively long residence times of waste in the reactor, poor temperature control due to large temperature gradients across their internal dimensions, fouling walls of the reactor by carbon residue and low liquid product quality due to the production of a diverse number of pyrolysis products.
Fixed-bed pyrolysis-catalytic cracking process for oil recovery of waste plastic is in use at several commercial processes. The reactor type in the pyrolysis or/and catalytic cracking process is generally constant stirred tank reactor (CSTR) and plug flow reactor. The problem is the fact that carbon residues tend to foul the walls of the reactor and thus give poor heat transfer from the external wall to the center of the reactor. Furthermore, CSTR type can deal with a relatively high viscosity reactant, but the problem of heat transfer by a big reactor diameter can be more important, compared with that of plug flow reactor. Basically, waste plastics are melted to materials of low viscosity and then the liquefied reactant is thermally decomposed to low-molecular-weight hydrocarbons in the pyrolysis reactor. These reactants are cracked in a fixed-bed reactor using solid catalyst to yield the oil and gas products. The characteristics of this process are the quality of the oil product, very similar to that of conventional gasoline, kerosene and diesel oils, but the drawback of the catalyst is high cost and short life-cycle due to poisoning/deactivation.
The fluidized-bed process yields a uniform product and a high conversion during a short reaction time. In addition, the problem of low thermal conductivity of polymers is overcome by a fluidized system and thus heat transfer gradients are eliminated. Some advantages are high-quality product, low energy requirement supplied by combustion of a portion of the gas by-product, good temperature control, the efficient removal of impurities present in the waste plastic, application on a relatively small scale, and also a robust and relatively inexpensive process to establish. On the contrary, this process has problems with toleration in the chlorine produced, the removal of solid sludge from the fluidized bed and also its long-term durability.
In the pyrolysis process, one of the most important decision items is the degradation temperature in the reactor. The degradation temperature must be decided by the type and composition of plastics contained in mixed thermoplastics, because of their different degradation temperatures. For a reactor with a big diameter in a large-scale plant the temperature gradient must also be taken into consideration in determining the degradation temperature, because of heat transfer limitations for viscous fluids with low thermal conductivity in the large reactor. Moreover, the coke accumulated on the internal surface of reactor during a long reaction time hinders heat transfer between heat source and viscous fluid in the reactor. Thus, the heat supply of the plant is gradually increased with the progress of reaction time. Optimization process control for hindering the coke formation is an important key in a large-scale plant.
Also, the impact on the environment in the pyrolysis of waste plastics must be taken into consideration. If a PVC material is contained in the reactant, the hydrochloric acid is evolved during decomposition of PVC which causes air pollution. Thus, a system is needed in order to remove the chlorine components in gas products. The rest of the gas products consisting of light hydrocarbons can be used as fuel gas in the heating system. Also the nonvolatile material generated in the melting and pyrolysis process, which includes a small amount of volatile hydrocarbon components, is discharged to a sludge treatment system. After being sufficiently heated in the sludge system, the product obtained is used as a valuable oil, but the solid char/coke retained is landfilled or incinerated.
Finally, the product obtained is separated from the distillation tower, such as gas product, light oil product and heavy oil product. Our target is light oil product and/or heavy oil product, which is generally obtained by control of reactor temperature and distillation system such as temperature gradient, reflux ratio and reboiler temperature, etc. The distribution of the oil product must be decided by market circumstances.
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