Effect of feed spacer arrangement on flow dynamics through spacer filled membranes

Abstract

Operational issues arising from scaling and fouling of membranes are addressed by pre-treatment processes and alternative membrane or membrane secondary structures. In the present work the flow patterns associated with fluids within the membrane module are investigated using Computational Fluid Dynamics (CFD) tools. The effects on flow patterns through a spacer filled Reverse Osmosis (RO) membrane with the secondary structure of the membranes (feed spacer filaments) at various angles with the inlet flow are analyzed. The presence of the feed spacers in membrane module appears to generate secondary flow patterns enhancing the prospects for self induced backwashing increasing the allowable operational time and membrane efficiency. The flow visualization in the present study is useful in understanding the complex flow patterns generated in spacer filled RO membrane modules and could possibly lead to developing a new RO membrane which is more efficient, economical and appears to be a practically viable solution to reduce costs associated with the maintenance of RO membranes.

Introduction

Reverse Osmosis 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. However, there appears a need to change membrane or membrane secondary structures to alter the flow patterns associated with fluids within the membrane module. To visualize flow through RO membranes Computational Fluid Dynamics (CFD) tools have been used extensively by various researchers. Literature review reveals that CFD tools have been used quite accurately to predict the flow behavior through RO membranes [1], [2], [3], [4].

Spiral wound membrane module (SWM) is regarded as one of the most commonly used assemblies for water treatment using membrane separation processes. Fig. 1 represents a SWM in partly unwounded state. In case of Spiral Wound Module (SWM) a number of flat membrane sheets are glued together, in pair arrangement, on three sides forming a pocket and a permeate spacer is introduced between the membranes pocket. The fourth open end of the membrane pocket is connected to a common permeate collector tube. The membrane pockets are rolled around the tube with feed spacers between each pocket [5], [6]. As a result of the design alternating feed and permeate channels are developed. Feed enters through one side of the module and is forced through the membrane. Retentate leaves the module from the opposite side of the feed inlet, whereas permeate is collected in the common permeate tube.

The net spacer in the feed channel not only keep the membrane layers apart, thus providing passage for the flow, but also significantly affects the flow and concentration patterns in the feed channel. Spacers are not only responsible for the pressure drop and limited flow zones (dead zones) creation but also promote mixing between the fluid bulk and fluid elements adjacent to the membrane surface. In other words they are intended to keep the membranes clean by enhancing mass transfer and disrupting the solute concentration boundary layer. In the past several experimental and theoretical studies were carried out to shed light on these phenomena and to optimize spacer configuration [7], [8], [9], [10], [11], [12]. So it is quite understandable that the presence of these spacers promote directional changes in the flow which reduces membrane fouling and concentration polarization. Hence the efficiency of a membrane module depends heavily on the efficacy of the spacers to increase mass transport away from the membrane surface into the bulk fluid by increasing shear rate at the membrane surface [13].

Spiral wound membranes have tightly wrapped structures which cannot be opened easily for chemical cleaning or cannot be back flushed by operating in reverse direction. For these reasons, the fouling control methods for SWM are limited to hydrodynamics, pretreatment of the feed and operational controls [15]. The fouling issues can be addressed to a large extent by varying the hydrodynamic conditions prevailing in spiral wound membrane. The feed spacers can be oriented to generate high cross flow velocities or secondary flow patterns which can develop higher scouring forces on the membrane surface to reduce fouling and concentration polarization. However, this approach will need higher pumping energy to compensate losses within the membrane module. Hence the feed spacers must be optimized to reduce the buildup on the membrane surface with moderate energy loss.

Literature review to date reveals that for the same type of spacers, spacer-filled flat channels and SWM channels show similar flow characteristics [16], [17]. Studies of Ranade and Kumar [18] in another study concluded that the transition from laminar to turbulent flow regime for most of the spacer-filled channels occurs at Reynolds numbers of 300–400 (based on hydraulic diameter) for packed beds. In the present study we have used laminar flow model as channel's Reynolds number (Re_ch) which was kept between 100 and 125 for all the cases. In the present work, an attempt has been made to study the effect on flow patterns through a spacer filled RO membrane when the secondary structures of the membranes (feed spacer filaments) are set at various angles with the inlet flow. Three cases were analyzed to investigate the effect of feed spacer orientation, with respect to the inlet flow, on wall shear stress, pressure drop and power number.