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