Vacuum degassing using microporous hollow fiber membranes

This study evaluated the performance of vacuum degassing using microporous hollow fiber membranes for the removal of dissolved gasses from aqueous solutions. The experiments and analysis in this study were done in a batch reactor configuration. The membrane module was designed with individually sealed unconfined fibers, which fluidize in the flow providing excellent contact between the fibers and the water. The performance of the hollow fiber membrane module was monitored by measuring the change in dissolved oxygen (DO) with time in the reservoir. A preliminary model was developed to describe the degassing process. The overall oxygen mass transfer coefficient (K) in the system model was calculated assuming liquid film transfer control. The empirical correlation Sh = 0.018 Re^0.83 Sc^0.33 (Ahmed and Semmens, 1992) was used to calculate K to predict the performance of the system. Initial assumptions that C^* = 0 (i.e. pure vacuum) showed a good agreement between observed and predicted values at low Reynolds numbers. However, increasing deviation of results with increasing Reynolds number was found. A validation of the empirical correlation used and an evaluation of the effect of C^* was therefore further investigated. Mass transfer is controlled by the resistance in the liquid phase. Analysis of the total gas pressure differentials within the fibers showed C^* > 0. Partial pressures of the gases inside the fiber are dependent on the liquid phase boundary layer (C^* > 0). Below Re ∼ 2500 mass transfer across the membrane appears to be the limiting step. Above Re ∼ 2500 the performance of the system appears to be limited by the flow of the gasses within the fiber lumen. Flux of water vapor into fibers cannot be neglected in determining the total gas pressure within the fiber lumen.