Mass Transfer and Separation Processes
Mass transfer concerns diffusion and separation. Concepts: Fick's law (molecular diffusion); mass transfer coefficients (k_c) and correlations (Sherwood number); film theory and two-film theory; interfacial equilibrium; distillation (McCabe-Thiele, minimum reflux, minimum stages, feed quality q-line); absorption/stripping (Kremser equations); extraction; drying; adsorption; and membrane separations. Also know relative volatility and its role in separation difficulty.
Traps: confusing operating line with equilibrium curve; forgetting that minimum reflux corresponds to infinite stages; and misreading the q-line for subcooled, saturated, or superheated feed. Questions on absorption often test whether you know the operating line lies below the equilibrium curve for absorption (and above it for stripping).
Worked example (rule of thumb): relative volatility α = (y/x)/(y/x) evaluated against a reference; higher α means easier separation. For McCabe-Thiele, the rectifying operating line passes through (x_D, x_D) on the y = x line, and the q-line intersects the equilibrium curve at the feed point. Minimum reflux occurs when the operating line touches the equilibrium curve at the q-line intersection.