CO₂-H₂O assisted co-pyrolysis of petroleum vacuum residue and polypropylene to improve asphaltene reduction and coke suppression: A statistical approach

Non-catalytic co-pyrolysis of petroleum vacuum residue (VR) and polypropylene (PP) was studied under CO₂ and/or steam atmospheres in a batch condition. A statistical approach was performed to analyze the effect of temperature, CO₂ initial pressure, reaction time, PP and water contents using fractional factorial design methodology. A comparison of co-pyrolysis in steam, CO₂, and steam + CO₂ atmospheres showed that CO₂ + steam is a more effective environment for reduction of asphaltene and suppressing coke formation. In detail, steam dominated the pyrolysis process, whereas CO₂ enhanced the thermal cracking reactions, thereby increasing the asphaltene conversion to maltene and decreasing coke formation. Moreover, the results indicated that the addition of PP to VR improved the yield and quality of the liquid product and diminished the coke content. The optimum conditions were the temperature of 380 °C, CO₂ initial pressure of 0.6 MPa, reaction time of 90 min with PP/ VR and water/ VR ratios of 0.23, and 0.25 (g/g), respectively. Pyrolysis of VR under such conditions resulted in the 86% maltene yield, 89.7% asphaltene reduction and 6.3% coke reduction. ¹H NMR analysis showed that paraffins occupy the major content of the liquid (81.07 vol%), followed by aromatics (12.28 vol%) and olefinic compounds (6.65 vol%). In addition, the upgraded oil had a remarkable °API gravity of 35.9 and a low viscosity of 5.27 ± 0.02 cP. Furthermore, heavy metals of Ni and V were removed from the liquid product by 56.1% and 65.6%, respectively. Pyrolysis of VR with PP in the CO₂ + steam environment would be deemed an economically viable and environmentally friendly technique of transforming the abundant low-grade VR feedstocks to valuable liquid fuels at a milder processing condition without using any catalysts.