A membrane device is presented that can used for a wide range of applications from once-through filtration, crossflow filtration, molecular separation, gas/liquid absorption or reaction, gas dispersion into liquid, and degassing of liquid. The device comprises a thin flat sheet membrane that allows certain fluid or molecules go through while blocking others. The membrane sheet is fixed on a supporting structure with mini channel on two sides of the membrane for respective feed and sweep flows. The membrane sheet is sealed with gaskets with two cover plates that the membrane sheet can be replaced or cleaned. The cover plate provides connection ports to connect the feed fluid to the feed channels on one membrane surface and to connect the sweep fluid to the sweep channels on the other surface of the membrane.