Membrane dryers use the process of selective permeation of the gas components in the air. The dryer is a cylinder which houses thousands of tiny hollow polymer fibers with an inner coating. These fibers have selective permeation for the removal of water vapor. As filtered, wet compressed air enters the cylinder, the membrane coating allows water vapor to permeate the membrane wall and collect between the fibers, while the dry air continues through the fibers in the cylinder at almost the same pressure as the incoming wet air. The permeated water is vented to the atmosphere outside of the cylinder. The permeation or separation is caused by the difference in the partial pressure of a gas between the inside and the outside of the hollow fiber.
Membrane dryers are simple to operate, silent while operating, have no moving parts, low power consumption and minimal service requirements (mainly filters upstream of the dryer). Besides removing water, gas component separation can also be achieved with a membrane, depending on the characteristics of the fiber material. Separation of different gases is achieved by differences in molecular size and gas solubility in the membrane. Gases of smaller molecular size have larger diffusion and can be suitably separated by differences in mobility. As such, specific membranes can be used to make nitrogen generators, for example.