Mass Transfer
Lewis number, which is important in problems involving simultaneous heat and mass transfer, is the ratio of

Thermal diffusivity to mass diffusivity
Momentum diffusivity to thermal diffusivity
Mass diffusivity to momentum diffusivity
None of these

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Mass Transfer
Solvent extraction is the terminology applied to the liquid-liquid extraction, which is preferred for the separation of the components of liquids, when

One of the liquid components is heat sensitive
Extracting solvent is cheaply & abundantly available
Viscosity of liquid components is very high
One of the liquid components has very high affinity towards the solvent

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