Expert-level mass transfer covering molecular diffusion, convective mass transfer, interphase transfer, mass transfer coefficients, and design of mass transfer equipment.
Ficks first law: J = negative D times dC over dz, flux proportional to gradient. Ficks second law: dC over dt = D times d2C over dz2, transient diffusion. Binary diffusivity: Chapman-Enskog for gases, Wilke-Chang for liquids. Diffusion in solids: Fickian diffusion, concentration dependent D common.
Mass transfer coefficient: NA = kc times CA_bulk minus CA_surface. Analogy to heat transfer: Sherwood number analogous to Nusselt number. Sh = f of Re and Sc, Schmidt number replaces Prandtl number. Film theory: resistance in thin film near interface. Penetration theory: surface renewal model, short contact times.
Two-film theory: resistance in both gas and liquid films at interface. Overall coefficient: 1 over KG = 1 over kG + H over kL, Henry constant H. Gas film control: when H is large, liquid phase resistance negligible. Liquid film control: when H is small, gas phase resistance negligible.
Tray columns: stage efficiency, downcomer sizing, flooding limits. Packed columns: random or structured packing, HETP, pressure drop. Flooding: maximum vapor and liquid rates before performance degrades. Wetting rate: minimum liquid rate to ensure packing surface is wetted.
| Pitfall | Fix |
|---|---|
| Wrong film resistance assumption | Calculate both resistances and compare |
| Operating too close to flooding | Design for maximum 80% of flood velocity |
| Ignoring liquid maldistribution | Use quality distributor and redistribution every 6m of packing |
| Wrong HETP for conditions | Validate HETP with pilot column data for actual system |