Thermodynamic Model of Sediment Deposition in the LC-FINING Process



The LC-FINING ebullated bed hydrocracking process is used to hydrocrack residuum. In this process, sediment deposition in the downstream equipment sometimes affects the overall economics by limiting the operating conversion, even though the upstream reactor system is capable of achieving much higher conversion levels. The sediment is measured by SHFT value (Shell Hot Filtration Test method), which mainly measures that portion of the asphaltene that is insoluble in heavy oil at specific laboratory test conditions.
A solubility-theory-based thermodynamic model was developed to predict the SHFT value. According to this model, asphaltenes are in equilibrium (in solution) with the surrounding fluid. The model parameters are asphaltene molecular weight and its solubility parameter. Both are related. Solubility parameter is modelled as a function of the hydrogen-to-carbon (H/C) atomic ratio of the heavy oil. The molecular weights are estimated from pilot plant data by minimizing the sum of squares of the residuals between the observed SHFT value and the calculated value for all runs. Asphaltene molecular weight decreases with increasing process severity, indicating that the asphaltene becomes more aromatic by losing side chains. The simple model is able to explain the observed trends with respect to different LC-FINING catalysts, feed sources and diluent effects. Commercial data are compared with the pilot plant data and the model prediction validates the model.