Rigorous numerical simulation of gas separation by hollow-fiber membranes
Clancy, Donald J.
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The work develops numerically-stable computer models to simulate the performance of cocurrent, and countercurrent membranes. Additionally, a previously unreported gas-flow pattern, a variance of cross flow, is discussed and simulated. These models assume solution-diffusion permeation, and account for gas non-ideality. A stable method is presented for calculating the permeate pressure drop through the hollow bore of the membrane fiber. This analysis also develops an energy balance and calculation procedures for simulating the temperature change which occurs in a hollow-fiber membrane. The cocurrent and counter-current numerical models are then validated by comparing calculated results against operating data obtained from air- and hydrogen-separation applications. The cross-flow numerical model is then correlated against data obtained from a full-scale, carbon-dioxide purification facility. It was noticed that this facility's cellulose acetate membranes exhibited a marked increase in gas permeability as the CO2 partial pressure increased. This led to an investigation and correlation of the effects of C02-induced plasticization of cellulose acetate on the increase in mixed-gas permeability. It was found that the permeability of hydrocarbons increases much more than that of CO2 as the CO2 partial pressure increases. This leads to a decrease in the amount of hydrocarbon that can be recovered from a membrane unit. This work concludes that enhanced oil recovery facilities utilizing cellulose acetate membranes operating at CO2 partial pressures in excess of approximately 150 psia could substantially benefit by avoiding CO2 plasticization effects either by operating at lower CO2 partial pressures, or lowering the degree of acetylation in the cellulose acetate membrane polymer.