ANS:C - tangential to the equilibrium curve

The minimum liquid rate to be used in an absorber corresponds to an operating line tangential to the equilibrium curve. In absorption processes, the operating line represents the relationship between the concentrations (or partial pressures) of the solute in the gas and liquid phases. The slope of the operating line is typically determined by the ratio of the liquid flow rate to the gas flow rate (liquid-to-gas ratio). For an absorber, the minimum liquid rate required corresponds to the operating line that is tangent to the equilibrium curve on a concentration (or partial pressure) vs. concentration (or partial pressure) plot. This is because at this point, any further decrease in the liquid rate would result in the operating line intersecting the equilibrium curve, leading to a condition where the gas and liquid leaving the absorber are in equilibrium with each other. Therefore, the correct statement is: Tangential to the equilibrium curve.