Axial distribution of permeance and of ideal selectivity of a porous cylindrical tube

An innovative permselectivity test bench is used in order to determine the axial distribution of two properties of a stainless steel porous tube, which are the permeance and the gas selectivity. The effects of the operating conditions (inlet mass flowrate and inlet pressure) have been studied. A new radial form of the gas permeability equation has been developed for this work and its relationship with Darcy's permeability is established for a cylindrical configuration of the test. The pressure variation along the centre axis of the tube is determined. The effects of this pressure variation on the physical properties of gases such as density and viscosity are determined and their influence on the selectivity of gases is studied. It is found that the gas density and gas viscosity decrease along the flow. This decrease leads to an increase in the gas permeability for N2 and a decrease for CO2. Consequently, the selectivity (N2/CO2) increases. The obtained results can be useful to understand the factors affecting gas separation in case of a porous tube for continuous industrial processes.