The phase equilibrium of alkanes and supercritical fluids

[摘要] Current methods for wax fractionation result in products with large polydispersity, and dueto the high temperatures required, thermal degradation of the wax is often incurred. Theneed for an alternative process thus exists. The purpose of this project is to investigate thetechnical viability of supercritical fluid processing as an alternative wax fractionationtechnology.The main aims of this project are to select a suitable supercritical solvent, to conduct binaryphase equilibrium experiments, to determine if the process is technically viable and toinvestigate 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 (LiquefiedPetroleum Gas) were selected as suitable solvents. Literature data for propane and highmolecular weight alkanes is scares and incomplete, thus necessitating experimentalmeasurements. 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 theaid of an endoscope, the phase transitions were detected visually. The measurementscorrespond well to literature values from reliable research groups.Phase equilibrium data sets for propane with nC32, nC36, nC38, nC40, nC44, nC46, nC54and nC60 as well as LP Gas with nC36 were measured. At temperatures just above themelting point of the alkanes, the phase transition pressures can be considered to bemoderate, which will positively impact the economics of the process. The phase transitionpressure increases with increasing carbon number, the relationship being found to be linearwhen the pressure is plotted as a function of carbon number at constant mass fractions andtemperature. The increase in phase transition pressure with increasing carbon numberindicates that the solvent will be able to selectively fractionate the wax. At highertemperatures 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 relationshipindicates 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 highermolecular weight normal alkanes, as well as the inability of cubic equations of state topredict 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. Thealternative, statistical mechanical equations of state, have difficulty in predicting the criticalpoint of the solvent correctly and thus overpredicts the mixture critical point, yet requiresmaller interaction parameters to fit the data. Further work is required to improve thepredictability of these non-cubic equations of state.This project has proven that wax fractionation by supercritical extraction with propane istechnically feasible.

[发布机构] Stellenbosch University