Hindered Stepwise Aggregation Model for Molecular Weight Determination of Heavy Petroleum Fractions by Vapor Pressure Osmometry (VPO)

Venezuela Orinoco heavy crude oil was fractionated into diesel, vacuum gas oil (VGO), vacuum residue (VR), and asphaltene fractions, which were subjected to molecular weight (MW) measurement by vapor pressure osmometry (VPO). The VPO is known to overestimate the average molecular weight (MW) of heavy hydrocarbons, because of molecular aggregation. This paper proposes a hindered stepwise aggregation (HSA) model to simulate the molecular aggregation and used the model to estimate the true MW of heavy petroleum fractions. A data regression procedure was developed to determine the model parameters, aggregation equilibrium constant, and aggregate distribution, using a fast simulated annealing (FSA) algorithm based on the VPO data. This data analysis method is self-tuned to fit the VPO data to the HSA models of various petroleum fractions using the optimized solution of the FSA algorithm. The results showed that the VPO data of heavy petroleum fractions at various solution concentrations were in good agreement with those predicted by the HSA model. The aggregation equilibrium constant and aggregate distribution data obtained from the HSA model suggested that various degrees of molecular aggregation occur in heavy petroleum fractions. The molecules of diesel and VGO were monomers, regardless of the solution concentration. The molecules of VR formed dimer aggregates at high solution concentrations; the number of dimer aggregates exceeded that of monomers as the solution concentration increased. The molecules of asphaltenes were polymer aggregates. The size of asphaltene polymer aggregates increased significantly with the solution concentration. The MW of asphaltenes determined by the HSA model was much lower than that by the conventional linear regression method.