Effect of Pressure on Visbreaking Product Composition and Properties

Visbreaking is a thermal conversion process employed to decrease the viscosity of residual oils, and cracking occurs mainly in the liquid phase. The impact of pressure on thermal cracking is expected to be minor, yet several observations in the literature indicate that pressure may affect coking, the olefin-to-paraffin ratio, and product composition. In this study, the impact of pressure on coil visbreaking at 400 °C was evaluated in a closed system at a severity sufficient to cause the onset of coking. The products from thermal conversion with initial pressures of 0.5–2.0 MPa (low pressure) differed from those with initial pressures of 2.5–4.0 MPa (high pressure). High pressure suppressed coking, and at comparable vacuum residue conversion, the coke yield was lower at high pressure than at low pressure. Pressure had a minor impact on the distillation profile of the products. Under conditions of controlled space time, pressure did not affect the olefin-to-paraffin ratio, H/C molar ratio, or microcarbon residue. Pressure had an impact on composition-dependent properties, such as the first derivative of density and refractive index with respect to temperature, viscosity, and aromatic hydrogen content. No relationship was found between initial pressure and free radicals, n-pentane insoluble content, and molar element ratios measured in the study.