This type of process is the preferred choice for treating plastic wastes containing a high proportion of components that may affect negatively the catalyst performance. The occurrence of a previous thermal step allows for the removal of many of these components previously to the catalytic treatment.
Figure 3.17 illustrates the flow sheet of the process developed by the company Nippon Steel Corporation, being applied in Japan on a commercial scale . The process may use as raw materials any kind of plastic wastes, including chlorine contents from PVC and limited amounts of nonplastic components, such as paper, fibres and aluminium foil. It includes a pretreatment to remove undesired materials (metals, paper, etc.). Then, the crushed plastic wastes are loaded by a screw conveyor into a mixing tank, being melted at 300°C. In this stage, PVC is thermally decomposed and more than 90% of chlorine is removed in the form of hydrogen chloride. The melted plastics are fed into the thermal cracking vessel operating at 400°C and pressures close to atmospheric. The gas stream derived from the thermal degradation is first purified from additional HCl, released in the thermal treatment, and subsequently reformed by contacting with a catalyst in a fixed bed reactor, leading to oils useful as fuel. The carbon residue precipitated in the thermal cracking reactor is discharged out of the process continuously by using a centrifugal separator and finally burnt at about 1100°C within an incinerator. Several pilot and semi-commercial plants are being setting up in Japan based on this technology.
Although the great majority of the aforementioned processes for giving added value to the product of a previous thermal cracking have been addressed towards the production of higher quality fuels, other alternatives have been proposed yielding completely different
products. In this regard, the upgrading of a polyethylene waste by its conversion into lubricating oils with high viscosity index has been patented by Chevron . The process starts with the thermal conversion of pure or waste HDPE at 600-700°C giving rise to a mixture of n-paraffins and 1-olefins (25-75 wt%). The heavy fraction in this mixture (boiling point above 650°C) is separated and then passed through a hydrogenation zone to remove the N, S and O heteroatoms that might deactivate subsequently the dewaxing catalysts. Finally, the effluent of the hydrogenation zone is isomerized using a typical dewaxing catalyst such as an intermediate pore size molecular sieve (e.g. ZSM-22, ZSM-23, SSZ-32, SAPO-11). According to this patent, the lubricating oils obtained show pour and cloud points below —9.5°C and —3.9°C respectively, the oil yields being above 50 wt%.
Other interesting products that can be obtained from waste plastics using combined thermal and catalytic processes are alkylaromatic compounds, which possess industrial applications as automatic transmission fluids (ATF), detergents (linear alkyl benzenes, LAB), and improvers of cetane number in diesel fuels . The process uses as raw material the olefins generated in a previous step of thermal and catalytic cracking, which represent a cheaper source of olefins alternative to the currently existing ones. No special details about the conditions applied for the olefin production are indicated, the emphasis being focused on the alkylation step. Alkylation catalysts comprise conventional Lewis acids (AlCl3, FeCl3, TiCl4, ZnCl2, etc.) and solid acids such as pillared clays, natural and synthetic zeolites (ZSM-5, ZSM-12, ZSM-23, USY), amorphous silica-alumina, etc.
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