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Solubilities of p -hydroxybenzoic acid ( p -HBA) and mixtures of the para and ortho isomers have been determined in supercritical carbon dioxide at 318 and 328 K for pressures in the range 101−203 bar. The solubility of p -HBA in carbon dioxide ( p -HBA/CO 2 binary system) is 2 orders of magnitude lower than the binary system solubility of o -HBA. However, measurements undertaken with an equilibrium chamber of mixed para and ortho isomers demonstrate a strong enhancement in the solubility of p -HBA. Furthermore, the enhancement was found to be independent of the composition of the equilibrium chamber. The binary and ternary ( p -HBA/ o -HBA/CO 2 ) solubility data were correlated using a modified form of the Peng−Robinson equation of state. Covolume-dependent mixing rules were also used with the Peng−Robinson model in an attempt to improve the correlation of solubility data. Conventional supercritical fluid (SCF) extraction of a mixture of the isomers is capable of producing an extract which is 99% pure o -HBA in one extraction step. The potential for separation of the isomers using retrograde crystallization appears limited because of the similar crossover pressure for both isomers.
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Lucien et al. (1996) studied this question.
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