Concept of spacer configuration efficay (SCE) applied to optimize ladder type feed spacer filament spacing in narrow channels

Abstract

Cross-flow membrane operations are often confronted with the challenges associated with the periodic build-up of material on the membrane surfaces and lead to many operational and maintenance issues. The traditional experimental techniques while taking measurements close to the membrane walls intrude or alter the flow field and bring in complexities for analysis. Computational Fluid Dynamics (CFD) has been emerged as an effective tool to visualize accurately the flow and concentration pattern within the spacer filled membrane module without disturbing the actual flow and concentration patterns in the computational domain. ANSYS FLUENT has been used as working tool in the present study to arrive at an optimum spacer configuration for efficient cross-flow membrane separation process. In the present study, the dimensionless feed spacer filament spacing is altered in a systematic fashion to visualize/analyse the resulting impact of flow dynamics on the flow and concentration patterns generated in the vicinity of top and bottom membrane surfaces. It was interesting to note that the average mass transfer coefficient for the two membrane surfaces was not identical for all of the configurations tested in this study, especially when the dimensionless bottom filament spacing was increased to 6. Such configurations would lead to preferential build-up of salts on one membrane compared to the other and are not suitable for efficient cross-flow membrane separation processes.However the mass transfer coefficient for the two membrane surfaces were almost the same for low to moderate bottom filament spacing, despite the average top wall shear stress values being significantly higher than yielded for the bottom membrane surface and indirectly reflects the similar build-up tendency on the two walls. The concept of Spacer Configuration Efficacy (SCE), which is the ratio of Sherwood number to Power number, has been introduced in the current work to compare different spacer configurations to arrive at an optimum configuration. Spacers yielding higher SCE values reflect their tendency to enhance the mass transport of the salt from the membrane surfaces to the bulk of the solution at moderate pressure losses. Based on the present study, it was concluded that the spacer configuration having dimensionless top and bottom filament spacing equal to 4 performs better than the rest of the configurations for hydraulic Reynolds number up to 200.