The phase equilibrium of alkanes and supercritical fluids

Schwarz, Cara Elsbeth (2001-12)

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2001.


Current methods for wax fractionation result in products with large polydispersity, and due to the high temperatures required, thermal degradation of the wax is often incurred. The need for an alternative process thus exists. The purpose of this project is to investigate the technical viability of supercritical fluid processing as an alternative wax fractionation technology. The main aims of this project are to select a suitable supercritical solvent, to conduct binary phase equilibrium experiments, to determine if the process is technically viable and to investigate the ability of various equations of state to correlate the phase equilibrium data. Based on limited data from the literature, propane and a propane rich LPG (Liquefied Petroleum Gas) were selected as suitable solvents. Literature data for propane and high molecular weight alkanes is scares and incomplete, thus necessitating experimental measurements. A phase equilibrium cell was designed, constructed and commissioned. The cell was designed for pressures up to 500 bar and temperatures to 200 oC, and with the aid of an endoscope, the phase transitions were detected visually. The measurements correspond well to literature values from reliable research groups. Phase equilibrium data sets for propane with nC32, nC36, nC38, nC40, nC44, nC46, nC54 and nC60 as well as LP Gas with nC36 were measured. At temperatures just above the melting point of the alkanes, the phase transition pressures can be considered to be moderate, which will positively impact the economics of the process. The phase transition pressure increases with increasing carbon number, the relationship being found to be linear when the pressure is plotted as a function of carbon number at constant mass fractions and temperature. The increase in phase transition pressure with increasing carbon number indicates that the solvent will be able to selectively fractionate the wax. At higher temperatures the gradient of the line is larger and may thus lead to improved selectivity. The higher temperatures will also lead to better mass transfer. The linear relationship indicates that limited extrapolation to higher carbon numbers may be possible. However, this needs to be verified experimentally. The inability to measure the critical point and vapour pressure curves of the higher molecular weight normal alkanes, as well as the inability of cubic equations of state to predict liquid volumes and to capture the chain specific effects such as internal rotations, results in cubic equations of state requiring large interaction parameters to fit the data. The alternative, statistical mechanical equations of state, have difficulty in predicting the critical point of the solvent correctly and thus overpredicts the mixture critical point, yet require smaller interaction parameters to fit the data. Further work is required to improve the predictability of these non-cubic equations of state. This project has proven that wax fractionation by supercritical extraction with propane is technically feasible.

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