A rate-based model was developed for the design of acid gas absorbers using aqueous alkanolamine solutions. The model adopts the film theory and assumes that thermodynamic equilibrium among the reacting species exists in the bulk liquid. The diffusion-reaction equations for the reacting species in the liquid film are solved using collocation techniques. Heat effects accompanying diffusion and reaction are accounted for using appropriate heat balances on each tray. The algorithm adopts a plate-by-plate calculation starting at the bottom of the tower. Tray hydraulics was added to the algorithm to ensure proper operation of the tower. The program was developed to handle either monoethanolamine (MEA) or diethanolamine (DEA) as chemical solvents.