The unit operation that applies the concepts seen on mass transport phenomena is represented here by gas-liquid absorption columns. Referring to processes in the absence of chemical reaction and for sufficiently diluted solutions, packed absorption columns are presented and, with the method of the operating lines, the approach for determining the flow rates is illustrated. The text then delves into the aspects of the correct design of the cross-section of the absorption column, which must be such as to avoid flooding phenomena. The reference to mass transfer coefficients, liquid-gas equilibrium relations, and two-film theory, allows us to identify, through mass balances, the design equations for the height of the column, highlighting the role of the height of transfer unit (for which some literature relations are provided) and the number of transfer units (for which the analytical solutions are obtained).