Upgrading vacuum residue in a supercritical CO2 environment with polypropylene and steam: Insights into products distribution and characterization of liquid products

Abstract

The pyrolysis of vacuum residue (VR) using polypropylene (PP) under supercritical carbon dioxide (scCO₂) conditions represents a novel approach to converting this abundant and difficult-to-process feedstock into lighter liquid products. In this paper, a set of experiments was performed to analyze the impact of scCO₂, PP and steam on the yields of pyrolysis products (i.e., liquid, gas and coke) and characteristics of the liquid fractions. The findings indicated notable improvements when scCO₂ was used instead of subcritical CO₂ in the pyrolysis of VR with PP. These included decreased viscosity and improved API gravity, alongside increased maltene yield and reduced coke yield. Furthermore, the inclusion of PP markedly elevated the quality of the obtained liquid, whereas steam + scCO₂ primarily affected the product distribution by raising the yield of coke and gas fractions. VR pyrolysis under scCO₂ conditions at 380 °C, 8 MPa, PP/VR ratio of 0.23 for 60 min in a batch system, without the use of catalyst, resulted in the highest liquid yield of 83.7 ± 1.9 wt%, and minimum coke yield of 9.8 ± 1.3 wt%. Under these conditions, the upgraded liquid consisted of 75.2 wt% light and middle distillates with boiling points below 350 °C. Additionally, the proportion of oxygenates in the pyrolysis oil reduced by 60.5 %, improving its heating value. Moreover, adding PP to the upgrading process resulted in a higher proportion of isoparaffins, cycloparaffins, and aromatics, coupled with a decrease in olefins, aligning the liquid specifications more closely with fuel standards.