Waste Plastics Suitable For Cracking And Pyrolysis

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Cracking and pyrolysis are the suitable processes for the tertiary recycling of additive polymers such as polyolefins, mainly polyethylene (PE), polypropylene (PP), polybutadi-ene (PBD) and polystyrene (PS). Small admixtures of other plastics, such polyethylene terephthalate (PET), polyvinyl chloride (PVC), less than two 2%, are admissible yet not desirable. Taking into account the total share of polyolefins in plastics production (~70 wt% including PS) they are the highest environmental problem. Their chemical composition (carbon and hydrogen) close to crude oil is the reason that this type of waste plastic can be processed simply by the refinery medium-temperature methods such as thermal and catalytic cracking, visbreaking or coking (temperature generally lower than 500°C) or high-temperature noncatalytic processes (600-800°C), i. e. pyrolysis. The relatively low density of PE, PP, PS in comparison with PVC and PET is the basis of water separation.

Commingled post-consumer or municipal waste plastics selected for cracking or pyrol-ysis also contain other elements such as oxygen, nitrogen, sulphur and about ten metals such as Al, Ca, Na (>100 ppm each) and Sb, Ba, Cr, Co, Cu, Fe, Pb, Mg, P, Zn, generally below 50 ppm each. Inorganic compounds in waste plastics are contained in the different types of additives and impurities (product remains). The total content of mineral components in waste plastics (approximate analysis) attains value 0.5-1 wt% (ash) [2, 3]. In our waste cracking experiments studies some PE samples selected from municipal wastes contained ~0.6 while PS even more than 3 wt% of inorganic components [4]. Theoretical hydrogen content in PP and PE (C2H2)„ is 14.28 wt%. The reduced hydrogen content in waste plastics, e.g. 13.7 wt% [2, 3] is a result of some oxygen or nitrogen content (admixture of PET or PA), inorganic components or PS (7.69 wt% H2 content). Depending on the origin the hydrogen content in crude oil is 11-14 wt% [5]. Therefore from the cracking efficiency point of view, PE and PP are excellent feeds, better than PS since in the cracking processes they give a lower coke yield. In the course of cracking inorganic impurities and additives in waste plastics are deposited in coke (solid carbon residue). The high yield of coke in the cracking process diminishes the yield of light hydrocarbon fractions and complicates the process since it deactivates the catalyst and deposits over heat exchange surface of the reactor.

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