(69f) Optimisation of Energy Use in Solvent Extraction Processes Using Liquid and Compressed Gas Solvents

Solvent extraction is one of the simplest and most common processing methods for separating mixtures of different compounds. This typically involves contacting the mixture with the solvent, separation of the solvent phase from non-soluble material by density difference or filtration, and finally evaporation of the solvent to recover dissolved compounds and/or to enable re-use of the solvent. A large part of the total processing cost is in the final stage due to the energy cost of evaporating and recovering the solvent, even when efficient multi-stage vacuum evaporation is used.

Compressed gas solvents (liquefied gases and supercritical fluids) are less commonly used than liquid solvents due to the added complexity of working under pressure, but they have important and growing use in applications where lower temperature processing and low solvent residue in the product is desired, and in applications where the more favourable transport properties available are necessary or beneficial to the process. Compressed gas solvent extraction often requires a larger upfront capital expenditure, but for large scale processing the energy cost can be the most significant part of the total processing cost. The solvent compression and vapourisation cycle for compressed gas solvents can, however, be carried out very efficiently.

This paper gives a comparison of the thermodynamics, the most efficient processing methods, and the likely processing cost of processes using liquid solvents and compressed gas solvents. Liquid solvent extraction is described using ethanol and hexane extraction as examples. Compressed gas extraction is described using liquid propane and supercritical carbon dioxide as examples.