A carrier-gas mixture is directed through a fluid flow path in a combined heat and mass transfer device operating at a sub-atmospheric pressure. Heat and mass are transferred from or to the carrier-gas mixture via interaction with a liquid composition that includes a vaporizable component in a liquid state to substantially change the content of the vaporizable component in the carrier-gas mixture via evaporation or condensation of the vaporizable component. The mass flow rate of the carrier-gas mixture is varied by extracting or injecting the carrier-gas mixture from at least one intermediate location in the fluid flow path, and/or the mass flow rate of the liquid composition is varied by extracting or injecting the liquid composition from at least one intermediate location in the fluid flow path; and the flow of the carrier-gas mixture or the liquid composition is regulated to reduce the average local enthalpy pinch in the device.