The Separation of Solvent and Dissolved Substances: Methods for Precipitation
The separation of dissolved substances from the solvent in gas extraction is a necessary operation. Any suitable separation process can be employed, but processes are pRefserred which introduce no additional substances with only small changes in condition
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The separation of dissolved substances from the solvent in gas extraction is a necessary operation. Any suitable separation process can be employed, but processes are preferred which introduce no additional substances with only small changes in conditions of state. Additional fractionation of the extract during removal of the solvent from the extract, e.g., by stepwise reducing solvent power of the solvent, is a further aspect. Substances dissolved in the supercritical solvent can be removed by reducing solvent power of the solvent or by applying a mass separating agent. In the first case, a condensed phase is formed, which is then separated from the remaining gaseous phase. Reduction of solvent power results from changes in conditions of state which result in reduced density, or from adding substances with low solvent power.
6.1 Separation by Reduced Solvent Power 6.1.1 Reducing Pressure and Increasing Temperature Solvent power of supercritical solvents depends on pressure and temperature. Therefore, different solubilities at different conditions of state can be used for separating dissolved substances from the solvent. In general, solvent power of the supercritical component increases with increasing density and vice versa. Lower density can be achieved by reducing pressure or increasing temperature. Reduced pressure leads to lower concentrations of the dissolved substances, because of lower density. The effect of pressure on the solute, the Poynting-effect, is low in case of a solid solute. In case of a liquid solute phase, pressure determines the concentration of the supercritical solvent in this liquid phase. Concentrations of the supercritical solvent can be high, and the effect on the structure of the liquid contributes to the concentration of the solute in the gaseous phase. Concentration of the supercritical solvent in the liquid phase increases with pressure and leads to a wider packing of the solute molecules, contributing to the tendency to adopt the gaseous state. Increasing temperature leads to a decrease in density (at constant pressure) and a lower solvent power of the supercritical solvent. Increasing temperature also increases vapour pressure of the solute. At low pressures, where density decreases 175 G. Brunner, Gas Extraction © Springer-Verlag Berlin Heidelberg 1994
strongly with temperature, concentrations of the solute components in the supercritical solvent decrease with temperature. At high pressures, where density decreases moderately with increasing temperature, the increase of vapour pressure dominates. The result is an increasing concentration of solute with increasing temperature (at constant pressure). Contrary to this statement, which refers to the "normal" behavior, there are conditions of state, where a reverse behavior can occur and, e.g., solubility decreases with pressure, even at high pressures. This behavior depends on the binary phase behavior and has been discussed in Chapter 3. Mter establishing thermodynamic conditions of reduced solubility, a condensed phase is formed
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