A design of inserting in parallel an impermeable sheet or a permeable barrier to divide an open duct into two channels for double-pass operations with uniform wall concentration and external refluxes at the ends, resulting in substantially improving the mass transfer, has been proposed and investigated theoretically. The results are represented graphically and compared with those in an open duct (without an impermeable sheet or a permeable barrier inserted and, thus, with single-pass operations). Considerable improvement in mass transfer is obtainable by employing such a double-pass device with external refluxes, instead of using an open conduit with single-pass operations. The effect of impermeable-sheet or permeable-barrier location on the enhancement of mass-transfer efficiency as well as on the increment of power consumption, has been also discussed.