The phase equilibrium of alkanes and supercritical fluids
Date
2001-12
Authors
Schwarz, Cara Elsbeth
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
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.
Description
Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2001.
Keywords
Supercritical fluid extraction, Waxes, Alkanes, Dissertations -- Process engineering, Theses -- Process engineering