Effect of feed channel spacer geometry on hydrodynamics and mass transport in membrane modules

Abstract

Among different types of membrane modules used for cross flow filtration processes, Spiral Wound Module (SWM) dominates in the area of Ultra Filtration (UF), Nano Filtration (NF) and RO (Reverse Osmosis) due to high packing density, moderate energy utilization, standardization, cost effectiveness and being readily available from different suppliers. Membrane operations are often confronted with challenges associated with periodic maintenance of membranes due to significant material build-up on the surfaces. Operational issues arising from scaling and fouling primarily include: increased membrane resistance, decreased permeate flow, increased energy requirement and decreased membrane life. These issues have been addressed by several researchers, in a limited way, by proposing better pre-treatment processes or by alternative membranes through experimental and modelling studies. However, there appears a need to change membrane secondary structures to alter the flow patterns associated with fluids within the membrane module.In spiral wound modules, net-type spacers are introduced to develop feed channel, by keeping the membrane surfaces apart. Presence of feed spacers generate secondary flow patterns within the membrane module which may lead to enhance mass transport of the solute away from the membrane to minimize concentration polarization, which is a desirable feature for efficient membrane operations. However, the undesirable features associated with their use are increased pressure drop and development of fluid stagnant zones. Therefore, the efficiency of a membrane module depends largely on the efficacy of the spacers to increase mass transport away from the membrane surface into the bulk fluid at moderate pressure loss.Literature review reveals that a number of experimental studies were conducted in past to shed light on the role of feed spacers in membrane modules. However, due to difficulty in applying flow visualization and measuring techniques in experimental studies, an in-depth understanding of the flow and concentration patterns generated within the modules was not possible. The flow visualization was made possible with the development of Computational Fluid Dynamics (CFD) techniques, but was restricted to two-dimensional analyses due to computational constraints and provided useful information regarding hydrodynamics prevailing in spacer filled narrow channels. With the ongoing developments in CFD techniques and computational resources three-dimensional studies are being conducted, which can provide in-depth analysis of concentration patterns and hydrodynamics in membrane modules.In this thesis three-dimensional modelling of flow through spacer filled narrow channels is carried out using CFD package ANSYS FLUENT to investigate the impact of feed spacer filament orientation on shear stress exerted on membrane surfaces and Power number. The impact of dimensionless filament spacing on mass transport, shear stress, pressure drop and friction factor is also investigated using a systematic approach by hooking a User Defined Function (UDF) to ANSYS FLUENT. The predicted results showed excellent agreement with the previous experimental and other numerical studies revealing that CFD predicts hydrodynamics and mass transport within feed channel of spacer obstructed membranes quite accurately. These investigations are new to membrane related studies which shed light on spacer impact on performance of RO operations.Post processing of the results revealed the complex flow patterns generated within the spacer filled narrow channels and showed that the alignment of the feed spacers with the flow direction have great influence on the generation of secondary flow patterns through the spacer filled channels. Pressure drop and Power number in spacer filled SWM appears to depend largely on the filament orientation based on current investigations. Pressure drop and power number will be higher if the filaments are inclined more towards the channel axis and vice versa.For ladder type spacers wall shear stress at the top membrane surface is always higher (approximately 3 to 8 times for the spacer arrangements considered in the study at Reh=100) than that for the bottom wall, but interestingly the mass transfer coefficient values for the two walls are not significantly different for spacer arrangement having low to moderate bottom filament spacing (L2 = 2 to 4). However, when the bottom filament spacing is further increased (L2 = 6), there is a sharp decline in the pressure drop but the area weighted mass transfer coefficient for the top membrane wall showed a sharp reduction compared to the bottom membrane wall suggesting high fouling propensity of the top membrane wall which is not a desirable feature in membrane operations.Different spacer arrangements considered in this work are compared on the basis of Spacer Configuration Efficacy (SCE), which in this thesis is defined as the ratio of Sherwood number to Power number. Spacer having higher SEC values would lead to higher mass transport of the solute away from the membrane walls to the bulk of the solution at moderate pressure losses. It has been concluded by carrying out mass transfer simulations for different spacer arrangements that the spacer arrangement having top and bottom filament dimensionless ratio equal to 4 performs better than all the other considered arrangements for hydraulic Reynolds number up to 200.The results emanated out of the current study are considered to be of practical significance and could potentially lead to the development of efficient membrane modules with optimum spacer arrangements for RO operations.