Considerations on the performance of hollow-fiber modules with glassy polymeric membranes

Glassy polymeric membranes are widely used in the separation of gas mixture s. Typically, the permeability of these membranes has a pressure and compos ition dependence that is well described by the dual-mode transport model. N evertheless, for simplicity, most of the hollow-fiber permeator models only consider constant permeability, which can lead to inaccurate results. A co mparative theoretical study is performed on the influence of two different membrane mass transport models on gas separation hollow-fiber modules: the constant permeability and the dual-mode transport models. The comparison is performed in terms of the recovery and purity of the fast er gas, under different design and operation conditions. Simulations are pe rformed for the He/CH4 separation in a polycarbonate membrane. It is shown that the differences in performance exhibited by the two models can go up t o 10% for recovery and 5% for purity, particularly for high-permeate pressu res. Pressure drop on the retentate and permeate sides is analyzed. Two pressure drop models based on Hagen-Poiseuille equation are considered and compared : constant viscosity, evaluated for feed conditions, and composition-depend ent viscosity. It is concluded that neglecting viscosity composition depend ency can lead to errors up to 20% in pressure drop calculations, consequent ly affecting the hollow-fiber performance in terms of recovery and purity. (C) 2001 Elsevier Science B.V. All rights reserved.