Production of Light Olefins from Polyethylene in a Two-Step Process: Pyrolysis in a Conical Spouted Bed and Downstream High-Temperature Thermal Cracking

Abstract

A two-step process has been used for the selective production of light olefins by the thermal cracking of high-density polyethylene. The plastic has been continuously fed into a conical spouted-bed reactor (CSBR) operating at 500 °C, which yields 93 wt % of waxes (C21+) and C12–C21 hydrocarbons. The volatile product stream has been cracked downstream in a multitubular (quartz tubes) reactor in the 800–950 °C range, with short residence times (0.016–0.032 s). A yield of 77 wt % of light olefins (C2–C4) has been obtained by operating at 900 °C in the second step. The maximum yields of ethylene, propylene, and butenes are 40.4, 19.5, and 17.5 wt %, respectively. Given the short residence time of the products in the reactor, the yield of aromatics is only 6.2 wt %. The high light olefin yield is due to the excellent performance of both steps. The CSBR allows maximizing the yield of waxes and avoiding defluidization problems. The operating conditions in the multitubular reactor (low concentration of the compou...