Mass Transfer

Molecular diffusion is described by Fick’s law, flux proportional to concentration gradient; the key exam decision is whether the situation is equimolar counter-diffusion or diffusion of A through stagnant B, which adds the (1 − y_A) drift-flux correction.

Convective mass transfer uses a coefficient k_c in direct analogy to heat convection; k_c comes from Sherwood-number correlations Sh = C·Re^m·Sc^n, and the Chilton-Colburn analogy links it to the heat-transfer coefficient.

The two-film theory adds gas-film and liquid-film resistances in series (through the Henry constant) to give an overall coefficient; the interface is assumed to be at equilibrium, so no resistance sits there and the two film fluxes must be equal.

Absorber design multiplies the number of transfer units (a measure of separation difficulty) by the height of a transfer unit (a measure of packing efficiency); the operating line comes from a solute balance and its minimum slope is set by pinching against the equilibrium line.

Binary distillation is analysed with the McCabe-Thiele method: overall balances fix the products, the relative volatility gives the equilibrium curve, and stepping between the rectifying and stripping operating lines counts the theoretical stages.