Solvent extractors mass transfer

For startup of column extractors, it generally is best to start from dilute-solute conditions to avoid unstable operation. For example, when starting a column in which the feed is the continuous phase, first fill the column with solute-lean feed liquid before starting the flow of solvent and actual feed. This way, the solvent quickly becomes dispersed and mass transfer approaches steady state from dilute conditions, promoting faster and more stable startup. 

Eor economic reasons it is best to do the extraction as fast as possible by increasing the flow rate of the supercritical gas. This is, however, limited by the pressure drop from inlet to outlet of the extractor which should not exceed 10 bar in order to avoid compaction and chaimelling. The maximal flow depends on the geometry of the extractor and on the nature and particle size of the material. Other limiting factors especially towards the end of the process are diffusion and mass transfer kinetics which require time rather than large amounts of solvent. 

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