Solvent Considerations

The requisite immiscibility and viscosity of the extracting fluid have been discussed.

Several other solvent properties that are important to the extraction process are listed here.

• Selectivity, i.e. the ability to extract the material of interest in preference to other, interfering material. Solvent selectivity can be supplemented through the use of adsorbents and secondary solvents, and by other means.

• High distribution coefficient to minimize the solvent-to-feed ratio.

• Solute solubility, which is usually related to polarity differences between the two phases, leading to low solubility in the raffinate.

• Ability to recover the extracted material. Thus the formation of emulsions and other deleterious events must be minimized.

• Capacity, the ability to load a high amount of solute per unit of solvent.

• Density, as density differentials are needed for countercurrent flow. The solvent density is related to solubility in supercritical fluid extracting solvents.

• Low interfacial tension to facilitate mass transfer across the phase boundary. Interfacial tension tends to decrease with increasing solute solubility and as solute concentration increases. In liquidliquid extraction low interfacial tension allows the disruption of solvent droplets (entrained in the feed solution) with low agitation.

• Low relative toxicity.

• Nonreactive. In some instances, such as ionexchange extractions, known reactivity in the extracting fluid is used. In addition to being nonreactive with the feed, the solvent should be nonreactive with the extraction system (e.g., non-corrosive) and should be stable.

• Inexpensive. Cost considerations should emphasize the energy costs of an extraction procedure, since, for a given extraction method, capital costs are relatively constant.

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Solar Panel Basics

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