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The Journal of Supercritical Fluids, Volume 82, 2013, Pages 63-71.
Svetlana Rudyk.
Section of Chemical Engineering, Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Campus Esbjerg, Niels Bohr vej 8, 6700 Esbjerg, Denmark.
Abstract
The current work aimed to perform a consistency test based on the Gibbs–Duhem equation in order to evaluate experimental results obtained by supercritical carbon dioxide extraction of crude oil modified with several co-solvents in pressures of 20–60 MPa at 60 °C. The collected oil represented a liquid phase fraction while the outgassing losses a vapour phase fraction obtained after extraction. The systems of crude oil and ethanol or methanol modified supercritical carbon dioxide exhibited negative deviations from the Lewis–Randall rule. The unique combinations of saturation vapour pressures that satisfied the consistency tests were determined for the systems with the co-solvents using the criterion of the even distribution of the data about zero line in the plots of area test. The obtained values can be used to determine the thermodynamic conditions where the maximum recoveries can occur and the regions where the multiphases exist.
The current study demonstrates that the consistency test is able to evaluate supercritical carbon dioxide extraction data obtained at high pressures, when using a combination mode.
Additional Information:
High diffusivity of carbon dioxide at almost no surface tension in supercritical state and strong solvent property, which could be increased by adding small volumes of co-solvents, make it an excellent medium in many extraction processes. Although extensive literature exists on solubility parameters of various substances, the choice of modifier to supercritical carbon dioxide at high pressures and elevated temperatures are usually carried out empirically. The statistical verification of the results by repeated measurements requires large consumption of energy, time and materials and do not always provide the clear discrimination of the co-solvent effects because of the data scatter at specific thermodynamic conditions or due to the difference in equipment, operational regimes or presence of porous matrix.
The consistency test based on the Gibbs–Duhem equation was suggested to compare the results of the crude oil supercritical extraction modified with various co-solvents. The combinations of minimal and maximal saturation pressures of the systems with the co-solvents were determined by trials based on even scatter of data about zero line. The consistency tests has shown that the minimal saturation pressure depends on the boiling temperature of the co-solvent while the maximal saturation pressure depends on the solvent strength of the SF modified with the specific co-solvent.
