How do you design an absorption column using the concept of NTU and HTU?

Column height is calculated as Z = NTU x HTU. NTU (Number of Transfer Units) represents the difficulty of separation based on driving forces and equilibrium, calculated from inlet/outlet concentrations. HTU (Height of Transfer Unit) depends on mass transfer coefficients, packing characteristics, and flow rates. For gas absorption: NTU = integral of dy/(y-y*), where y* is equilibrium concentration. HTU typically ranges from 0.3-1.0m for modern packings.