Nonlinear deflection of polypropylene hollow fiber membranes in transverse flow

The mass transfer coefficient (KL) for oxygen transfer in a hollow fiber module with liquid flow normal to the fiber is much higher than for modules with flow parallel to the fibers. In the transverse flow modules studied, individually sealed fibers are mounted vertically at the bottom of a channel with water flow horizontally past the fibers. As a result, the free standing fibers deflect in the flow, like vertical cantilever beams subjected to uniform horizontal force. With this module configuration, a high mass transfer coefficient can be achieved at low liquid flow rates resulting in a more energy efficient process. Previous studies have shown that heat and mass transfer coefficients strongly depend on the orientation of the fibers with respect to the flow direction. The objective of this paper is to present a model for predicting the deflected shape of hollow fibers in a transverse flow module based on the flexural strength of the membrane material and the loading conditions. The modulus of elasticity (E) of the fiber material, measured by conducting conventional stress-strain tests, was correlated to the deflected shape of the fiber. The experimental value of E was verified by allowing the fibers to bend under their own weight. The predicted deflection of the fibers showed close agreement with the observed profile. Experiments were also performed to investigate the deflection of the fibers in the flowing water. The observed curvature of the fibers matched the predicted deflection well.