Mass Transfer
For the gas absorption, the height of a transfer unit, based on the gas phase is given by (G: superficial molar gas velocity, L: superficial molar liquid velocity, FG:mass transfer co-efficient, mols/m² , a: in-terfacial area per unit volume of tower)

F/(FG.a)
FG/(G.a)
(G.a)/FG
L/(FG.G)

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Mass Transfer
In a counter-current liquid extractor

Both liquids flow at fixed rate
Liquid's flow rate depends upon the temperature and pressure
Both liquids can have any desired flow rate
Only one of the liquids may be pumped at any desired rate

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

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Mass Transfer
A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,

Composition of vapors and that of liquid do not vary throughout the column
The top and bottom compositions are unchanged with and without total reflux
The top and bottom compositions correspond to the maximum enrichment available
Reboiler load and condenser load are minimum

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