ZSM-5 zeolites performance assessment in catalytic pyrolysis of PVC-containing real WEEE plastic wastes

Abstract

Catalytic pyrolysis of plastic wastes is a promising way for their conversion into valuable products. By modulating the catalyst properties and operating conditions, it is possible to direct the product distribution to obtain oils that may be suitable both as fuels and as chemicals. However, the efficient and safe removal of the halogens, often contained in plastic wastes, remains as a great challenge. In this work, the catalytic behaviour of ZSM-5 zeolites in the pyrolysis of a real chlorinated plastic waste of the electric and electronic equipment sector (WEEE), consisting of PE with about 3.4% of PVC, was investigated. To that end, three zeolite samples with different acidity and accessibility were synthesized and assayed. A thermal pre-treatment was applied to the plastic waste at 350 ºC, which allowed a chlorine removal of 87% from the WEEE feedstock. The pyrolysis tests were carried out in a downdraft fixed-bed stainless steel reactor, with a catalyst/feedstock ratio of 0.2, at temperatures of 600 ºC and 450 ºC in the thermal and catalytic zones, respectively, of the reaction system. In comparison with thermal pyrolysis, that mainly produced waxes, the product distribution changed considerably by contacting the pyrolysis vapours with ZSM-5 zeolites, leading to a strong enhancement in the yield of oil and gases. The largest yield of oil (about 60 wt%), having a concentration of monoaromatics (mainly BTX) above 50 wt%, was attained over the desilicated ZSM-5 sample. Regarding chlorine distribution, about 90% was accumulated in the char fraction, probably captured by the inorganic components present in the raw WEEE waste. Coke was the second fraction in terms of Cl concentration, followed by wax and oil, whereas this halogen was almost not detected in the gases. The lowest concentration of Cl in the oil was attained with the desilicated zeolite, with a value below 90 ppm, which could facilitate the subsequent processing of this stream in refinery units.