Membranes are used for separating gas mixtures where they act as a permeable barrier through which different compounds move across at different rates or not move at all. The membranes can be Nano-porous, polymer, etc. and the gas molecules infiltrate based on their magnitude, diffusivity, or solubility. The driving force used for gas separation is generally the partial pressure gradient which can be defined as the product of total pressure and mole fraction. Most membranes used for gas separation are non-porous.