Novel designs for improving the performance of hollow fiber membrane distillation modules

Abstract

Five types of novel hollow fiber module configurations with structured-straight fibers, curly fibers, central-tubing for feeding, spacer-wrapped and spacer-knitted fibers, have been designed and constructed for the direct contact membrane distillation (DCMD) process. Their module performances were evaluated based on permeation flux experiments, fluid dynamics studies, and tracer-response tests for flow distribution as well as process heat transfer analysis. The novel designs showed flux enhancement from 53% to 92% compared to the conventional module, and the spacer-knitted module had the best performance. The fluxes of all the modified configurations, except the structured-straight module, were independent of the feed flow velocity, and the modules with undulating membrane surfaces (curly and spacer-knitted fibers) were able to achieve more than 300% flux improvement in the laminar flow regime. The improved performance was attributed to the improved fiber geometries or arrangements that can provide effective boundary layer surface renewal and more uniform flow distribution, confirmed by the sodium chloride tracer response measurements. The heat transfer analysis underscores the advantage of the module with curly fibers with the least temperature polarization effect (temperature polarization coefficient = 0.81–0.65 at T m = 303–333 K), which is favorable for enhancing permeation flux.