Supercritical fluid extraction of paraffin wax

[摘要] ENGLISH ABSTRACT:In this study the deoiling and fractionation of paraffin wax using supercriticalfluid extraction (SCFE) has been investigated. SCFE was compared withstate-of-the-art processes such as wax crystallisation, static crystallisation andshort path distillation.Ethane and carbon dioxide were investigated as supercritical solvents for thesupercritical fluid extraction of paraffin wax. Supercritical phase equilibriumdata for ethane - n-alkane and CO2 - n-alkane systems close to the mixturecritical region were obtained from the literature, and were correlated withseveral equations of state. Statistical mechanical equations of state failed tocorrelate the data close to the mixture critical region due to the neglect ofdensity fluctuations which influences phase behaviour close to critical points,or due to inadequate mixing rules. It was found that simple cubic equations ofstate such as Soave-Redlich-Kwong, Peng-Robinson and Patel-Teja couldcorrelate the data using two interaction parameters. This can be attributedmore to their flexibility as correlating tools than to their fundamental accuracy.The Patel-Teja EOS was modified by fitting it to predict low vapour pressuredata for long-chain n-alkanes. This modified Patel-Teja EOS was then fittedto the phase equilibria by adjusting two interaction parameters per binarysystem. The interaction parameters for each solvent (ethane or CO2) systemwere then fitted to generalised correlations to enable extrapolation to solvent- n-alkane systems for which no equilibrium data were available. TheSimplified Perturbed Hardchain theory (SPHC) equation of state was used tocorrelate lower-pressure solubility data used to model the extract separator.A pilot plant SCFE unit was constructed and used to obtain experimentalfractionation data of a low-molecular weight Fischer-Tropsch wax. Theexperimental results indicate that fractionation of the wax is possible and thatthe separation efficiency is enhanced by returning some of the extract to thecolumn as reflux. An equilibrium stage model was constructed and used tosimulate the extraction experiments. It was possible to obtain goodagreement between the experimental results and model predictions.Deoiling of petroleum waxes with a low n-paraffin content (which are notcurrently deoiled commercially) was investigated. Experimental SCFE andSPD results indicated that selective deoiling is not possible, since theseparation is based on differences in molecular weight (or vapour pressure).Simulations of wax crystallisation or solvent extraction and practical testsindicate that deoiling is possible, based on differences in structure andtherefore melting point of the components in the wax. Practical problemsassociated with crystallisation or solvent extraction such as filtration, the useof chlorinated solvents and low yields currently prevent the commercialdeoiling of these waxes. During crystallisation of these waxes a soft wax cakeis formed which impedes the operation of static crystallisation.A detailed study of the economics of n-paraffin wax deoiling using SCFE wasconducted. Flow sheets were proposed to minimise the energy consumptionof the SCFE process. Comparison of SPD, static crystallisation and SCFEindicates that a SPD plant will be the cheapest option for deoiling the waxfeed investigated. Fractionation of heavier waxes using SPD might not beeconomically feasible, since the distillation temperature increases dramaticallywith increasing molecular weight, which leads to higher energy cost. Formedium to long chain n-paraffin waxes SCFE should be very competitive,since the capacity of the supercritical solvent can be manipulated to extractlonger chain waxes without increasing the extraction temperature. Staticcrystallisation appears to be the more expensive deoiling option, due primarilyto the large initial capital investment cost.

[发布机构] Stellenbosch University