Hydroreforming over Ni/H-beta of the thermal cracking products of LDPE, HDPE and PP for fuel production

Abstract

The thermal cracking at 400 °C of pure polyolefins—low density polyethylene (LDPE), high density polyethylene (HDPE) and polypropylene (PP) and a standard polyolefin mixture (46 % LDPE + 27 % HDPE + 28 % PP)—was studied together with the catalytic hydroreforming of the obtained oils over Ni/h-beta at 310 °C under 20 bar of hydrogen. The oils obtained after the thermal cracking of PP contain the highest amount of gasoline (58 %), while those coming from HDPE the lowest (39 %). The bromine index of the oils was very high, ranging from 54.1 (LDPE) to 83.8 (PP), indicating a high olefinic content of the oils. Additionally, the thermal cracking of the mixture indicates the occurrence of a synergestic effect among plastics, with transfer of methyl groups from PP to polyethylenes. Ni/h-beta (Si/Al = 25; Ni content = 6.2 wt%) catalyst was used in the hydroreforming since it contains a bimodal pore size distribution (0.6/3.1 nm), which improves accessibility of the oil molecules to the catalytic sites. After the hydroreforming and regardless of the plastics used, the share of lighter products (gasoline and gases) increases, reaching a remarkable 68 % of gasolines with the oils coming from PP. Regardless of the starting feed, the amount of useful fuels (gasoline + light diesel) was within 80–85 %. Additionally, the oils were successfully hydrogenated since the bromine indexes dropped below 7, indicating that more than 90 % of the starting olefins were saturated. The usage of catalysts increased the amount of aromatics in the obtained oils within 13–20 %, depending on the starting plastic. Likewise, the isoparaffin content of the gasolines was within 35–40 %, except for PP, where it was enhanced to 62 %. However, the research octane number (RON) of the gasolines from LDPE and PP and the cetane indexes of the diesel from all the plastics were promising for their application as fuels.