Similarly to crude oil cracking the products of the waste polyolefin cracking process are hydrocarbon fractions. According to the process conditions different shares of gas, gasoline, light and vacuum gas oil fractions as well as solid residue (coke) can be obtained. The unsaturated character of these products makes the further treatment, i.e. hydrorefining or application as the feed in cracking or hydrocracking processes, necessary. Light gaseous fractions can be used as the source of C2-C4 olefins for polymerization or fuels for power generation. High-temperature processing of polyolefins, pyrolysis, similar to the petrochemical process can give gaseous olefins and liquid fractions composed of aromatics such as BTX fraction. Diverse applications of cracking products of waste plastics treatment proposed by BP are presented by Kastner and Kaminsky . A modified scheme of waste polyolefin treatment is presented in Figure 4.1. According to the proposed technological scheme the prepared waste plastics (after selecting, washing if necessary and granulating) are submitted to the cracking process and the products obtained can be used with naphtha as feed for a steam cracker or with vacuum gas oil as feed for fluid catalytic cracking. Gasoline and light gas oil fractions can be delivered to a hydrorefining unit and processed with similar refinery fractions.
Chemical composition of waste plastic cracking products depends on shares of the individual polymers (PE, PP, PS) in the feed and process parameters. This fact decides the technological application of the final products. Important products of the cracking process, both petroleum fractions and waste plastics, are coke residues. Coke residue yield increases considerably, up to 10 wt%, in cracking of municipal and industrial waste plastics since they contain various inorganic impurities and additives. It can be applied as solid fuel, like brown coal. In the fluid cracking the solid residue is continuously removed from the process by combustion in a regenerator section.
Figure 4.1 Cracking and pyrolysis of waste polyolefin processing, main products and their application . (Reproduced by permission of Hydrocarbon Processing)
Waste plastics potentially can also be processed in hydrocracking process as an additional feed stream in mixture with vacuum gas oil or crude oil residues. Careful plastic segregation is then necessary since inorganic additives and impurities of plastics can foul the hydrocracking catalyst. Noncatalytic high-temperature olefin pyrolysis (700-800°C) and coking are insensitive to fouling.
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