Thermal Conversion of Athabasca Bitumen under Low- and High-Pressure Conditions: Influence on Cracking, Hydrogen Transfer, and Addition Reactions

Visbreaking is a mature thermal conversion process for heavy oils, yet the molecular effects of pressure on phase behavior and reaction chemistry of the liquid feed remain poorly understood. While thermal conversion predominantly occurs in the liquid phase, the process operates in a mixed vapor-and-liquid system. In this context, pressure does not directly influence the liquid-phase reaction kinetics but instead modulates vapor–liquid equilibrium and thereby affects the vapor phase volume fraction, influencing the broader reaction environment and overall product distribution. Experimental evidence from an earlier study shows that high pressure (2.5–4.0 MPa) suppresses coke yield (2.4 ± 0.35 wt %) at a comparable vacuum residue conversion to that at low-pressure reactions (3.9 ± 0.79 wt % at 0.5–2.0 MPa). This study aims to elucidate the influence of pressure on cracking pathways, hydrogen transfer, and addition reactions through a suite of spectroscopic analyses (UV–vis, FTIR, and 1H NMR), which were used to further characterize the structural changes in these liquid products. Spectroscopy reveals that low-pressure conditions promote aromatic condensation and the loss of alkyl constituents, as evidenced by a decreased aliphatic-to-aromatic hydrogen ratio and increased aromaticity in the products, changes that promote higher coke yields. In contrast, high initial pressure preserves more aliphatic content and favors hydrogen redistribution instead of hydrogen loss. Specifically, higher pressure led to an increased methylene-to-methyl ratio (nCH2/nCH3) and a higher methyl hydrogen content in the liquid products, which suggests enhanced methyl transfer reactions under these conditions. These findings provide valuable molecular insights into how pressure can optimize visbreaking and mitigate coke formation by retaining hydrogen-rich light species in the liquid phase.