LUMEN MASS-TRANSFER IN HOLLOW-FIBER MEMBRANE PROCESSES WITH CONSTANT EXTERNAL RESISTANCES

Hollow-fiber membrane processes with a constant external resistance ha ving a constant or variable shell concentration resulting from an oper ational mode of cocurrent or countercurrent are studied. By solving nu merically the continuity mass-conservation equation with the correspon ding boundary conditions the lumen laminar mass-transfer coefficients for both cases are correlated The correlations greatly improve the cal culating accuracy of the overall mass-transfer coefficient and can be used to obtain the lumen mixed-cup concentration by an algebraic equat ion substituting the partial differential equation. A separation facto r m' is introduced to characterize the effect of the operational mode. Calculation results demonstrate that the lumen mass-transfer coeffici ent is independent of the real lumen and shell concentrations but if i s greatly influenced by m'. The countercurrent mode, compared to the c ocurrent mode, provides not only a higher mean driving force, but a hi gher lumen mass-transfer coefficient. This conclusion is novel and val id for the tube-shell heat or mass-transfer processes and is supported by the experimental data in the literature and our gas membrane separ ation experiments.