Mass Transfer and Separation Processes - Complete Guide

At its core, mass transfer describes how molecules redistribute to eliminate concentration differences—driven by random thermal motion (diffusion) or bulk fluid motion (convection). Fick’s laws govern molecular diffusion, while Sherwood, Reynolds, and Schmidt numbers link macroscopic flow to microscopic transport. Engineers use these to predict flux across boundaries, like air–water interfaces in scrubbers.

Beyond single-phase diffusion, real separations occur across phase boundaries where resistance exists in both phases and at the interface. The two-film theory models this by assigning separate resistances to gas and liquid films, with overall coefficients (K_y, K_x) derived from their series combination. Equilibrium relationships (e.g., Henry’s law, Raoult’s law) define the driving force—so accurate thermodynamic property estimation (activity coefficients, fugacity) is non-negotiable for design fidelity.

Advanced practice recognizes limitations of equilibrium-stage models: transient effects, interfacial turbulence, chemical reaction coupling (e.g., CO₂ absorption with MEA), and microscale heterogeneity (e.g., droplet size distribution in extractors) demand population balance models or CFD–mass transfer coupling. Industry increasingly uses process simulators (Aspen Plus, CHEMCAD) embedded with NRTL, UNIFAC, or electrolyte models—but these require experimental validation at pilot scale, especially for novel solvent systems or bio-based feeds.