High pressure fluid phase equilibria

[摘要] ENGLISH ABSTRACT:Supercritical extraction is being investigated as a possible alternative to the processescurrently used in the fractionation of paraffinic waxes. By removing the lighter carbonfractions from the wax, the wax hardness will be improved and its melting temperaturerange reduced, hence improving the performance of the wax product in certainapplications. In order to evaluate and operate such an extraction process optimally, it isnecessary to have a thermodynamic model that accurately represents the process system.There are, however, currently no predictive models available for these systems. In order tofit present models to the systems, accurate phase equilibrium data of the supercriticalsolvent - n-alkane systems are needed. Unfortunately, the amount of reliable publisheddata on these systems in the required operating range is very limited.A view cell was designed and developed with which these high pressure equilibria couldbe studied. The binary phase equilibria of supercritical CO2 with n-CI2, n-CI6, n-C20, n-C24,n-C28 and n-C36 and of supercritical ethane with n-CI6, n-C24 and n-C28 were measured inthe temperature range 313 - 367 K. It was found that the systems with these two solventshave very different types of phase behaviour. The n-alkane solubility is much higher inethane, but supercritical CO2 will provide a much better degree of control over theselectivity achieved in an extraction process.Of the various equations of state investigated, it was found that the Patel Teja equation ofstate provided the best fit of the CO2 - n-alkane systems and that the Soave-Redlich-Kwong equation fitted the ethane - n-alkane systems the best. The interaction parametersof both these equations of state display a functional relationship with temperature and nalkaneacentric factor, making it possible to determine parameter values for application atother operating temperatures and with other n-alkane systems.It was found that the current equations of state were not able to represent the phaseequilibria accurately over the entire range of operating conditions. The poor performanceof the equations of state can be attributed to inherent flaws in the existing equations ofstate.

[发布机构] Stellenbosch University