MEMBRANE SEPARATION : A CASE STUDY ON PURGE GAS SYSTEM

Abstract

Membrane separation technologies are now widely used in chemical industries because of their lower costs, less maintenance, lower energy consumption and reduced pollution problems than other separation technologies. The mechanism of gas separation is based on the principle that certain gas permeates more rapidly than others. Membranes are now widely used in fertilizer plant for separation of hydrogen. The purge gas in ammonia plant contains approximately 65% of H2 and rest is N with Ar and CH 4. We can achieve hydrogen purity upto GQ% using these membrane. A mathematical model was developed considering counter current mode of flow for membrane configuration of flat Sheet, hollow fiber feed outside mode and hollow fiber feed inside mode. The assumptions of steady state, no radial concentration gradi6nt, permeability dependent of temperature only etc. were taken. The model equations were derived from mass balance equation along with Henry's law and Fick's law for molar flux relation and pressure drop relation of Hagen Poiseuille. Numerical integration was done using Fourth order Runge Kutta method. A computer program in Fortran was developed and the model was checked With reported data. The comparison of mode ref flow (co-current and counter current) and membrane configuration sheet and i'i61101■1 fiber) was done. Hollow fiber feed outside Mode with counter current flow was found having highest permeate composition and Nfas considered iii for further study with purge gas system taking three membrane , - materials (cellulose acetate, polyimide and polystyrene). The simulation study was done to see the effect of feed flow rate, pressure ratio and temperature on permeate composition for the separation of purge gas system CH2 - N2). Simulation studies showed that polyimide membrane is better than cellulose acetate and polystyrene for the separation of hydrogen from purge gas. Polystyrene was found to be most inferior for purge gas separation_ The effect of temperature was insignificant in all cases. The permeate composition was found to increase with .a increase in flow rate and a decrease in pressure ratio. It is advisable to use counter current hollow fiber food outside mode per mentor with polyimide membrane at high feed flow rate and at as high a feed pressure as permitted by the mechanical strength of membrane system for better hydrogen recovery from purge gas.