Investigation into the effectiveness of feed spacer configurations for reverse osmosis membrane modules using Computational Fluid Dynamics

Abstract

Reverse osmosis operations for water treatment are usually energy intensive and responsible for most of the product price. Several studies used flow characteristics to compare different geometries of feed spacers, but these cannot completely explain the effectiveness of feed spacers for promoting mass transfer near membranes. A few recent studies introduced a concept (Spacer Configuration Efficacy, SCE) combining mass transfer and energy consumption, but SCE has been applied only to a limited extent. The present study uses 3-dimensional steady state Computational Fluid Dynamics with mass transfer to compare four channels with feed spacer configurations (Ladder-type, Triple, Wavy and Submerged) and an empty plain channel using SCE and other performance measures. In contrast to previous studies, a saturated concentration boundary condition is employed at the membrane surface and optimised meshing of the domain is discussed. Power law correlations for SCE and other performance measures developed from the simulation results enable quick evaluation of the spacers. Results indicated that the assumed saturated solute concentration at the membrane strongly affects the mass transfer coefficient. Based on SCE, the Wavy spacer configuration showed the highest performance for Re > 120 among the obstructed geometries considered, while Ladder-type was better for Re < 120.

Influence of Spacer Design and Module Geometry on the Filtration Performance during Skim Milk Microfiltration with Flat Sheet and Spiral-Wound Membranes

“…Apart from that, the deposit layer decreases with distance from the spacer strands with membrane contact (Figure 8b). As reported by Kavianipour et al [23], the crossflow velocity varies due to the spacer strand´s influence on the flow field. Since the feed stream is influenced by the spacer strands, an area of flow separation can occur.…”

“…This can also be attributed to frictional losses. As already described by Da Costa et al [10] and Kavianipour et al [23], the feed stream changes direction as it flows above and below the spacer strands of the diamond-shaped spacer net. This causes energy dissipation, which is not present with the parallel spacer due to the linear feed flow comparable to the flow in tubular membranes or pipes.…”

“…This is due to the low crossflow velocities directly behind the spacer strand. This was also observed in computational fluid dynamics (CFD) simulations [23]. Based on the findings of Grosse-Gorgemann et al [24] and Fiebig [25] for heat transfer, Li et al [26] stated that slowly rotating transversal vortices form behind spacer strands at Reynolds numbers (based on the channel height) below 100.…”

A review of efforts to reduce membrane fouling by control of feed spacer characteristics

“…Commercially, there are two main feed spacer configurations available: woven and nonwoven [50,51]. Good spacer configurations should reduce a build-up of fouling deposits and decrease CP via keeping the solute concentration in the layer of fluid in contact with membrane surface at close to the bulk concentration [52,53]. Rejected species accumulation can be suppressed by encouraging back-mixing from the solution layer adjacent to membrane to the liquid bulk [50].…”

“…Kavianipour et al [52] used computational fluid dynamics to study four feed spacer configurations: triple, ladder-type, wavy and submerged long with spiral wound modules that are employed in reverse osmosis systems (see Figure 7). Both mass transfer phenomena and flow characteristics were investigated and the feed spacer behaviour was compared at different Reynolds numbers in the range 50≤Re≤ 200 through the laminar zone.…”