Abstract
Forward osmosis (FO) modules currently suffer from performance efficiency limitations due to concentration polarisation (CP), as well as pressure drops during operation. There are incentives to further reduce CP effects, as well as optimise spacer design for pressure drop improvements and mechanical support. In this study, the effects of applying transmembrane pressure (TMP) on FO membrane deformation and the subsequent impact on module performance was investigated by comparing experimental data to 3D computational fluid dynamics (CFD) simulations for three commercial FO modules. At a TMP of 1.5 bar the occlusion of the draw-channel induced by longitudinal pressure hydraulic drop was comparable for the Toray (16%) and HTI modules (12%); however, the hydraulic perimeter of the Profiera module was reduced by 46%. CFD simulation of the occluded channels indicated that a change in hydraulic perimeter due to a 62% increase in shear strain resulted in a 31% increase in the Reynolds number. This reduction in channel dimensions enhanced osmotic efficiency by reducing CP via improved draw-channel hydrodynamics, which significantly disrupted the external concentration polarization (ECP) layer. Furthermore, simulations indicated that the Reynolds number experienced only modest increases with applied TMP and that shear strain at the membrane surface was found to be the most important factor when predicting flux performance enhancement, which varied between the different modules. This work suggests that a numerical approach to assess the effects of draw-spacers on pressure drop and CP can optimize and reduce investment in the design and validation of FO module designs.
Highlights
An increasing global demand for potable water, as well as the efficient treatment of wastewater, has driven research into forward osmosis (FO) as an alternative to current processes [1]
The draw-channels were modelled as 3D computational fluid dynamics (CFD) geometries and validated against a range of experimental data reported previously in the literature to assess the degree of membrane deformation across module and memranes 2021, 11, x FOR PEER REVIEW Membranes 2021, 11, 161 previously in the literature to assess the degree of membrane deformation across module and membranberatynpeetsyp[1e7s,2[117].,2T1h].eTPhFe(PPFor(iPfeorraif)emrao) dmuoleduwleaswvaaslivdaaltieddatferdomfropmrepvrioeuvisoeuxs-experimental perimental dadtaat,au, suinsigngthtehemmetehtohdodfrforommoouurrpprreevvioiouusswwoorrkk[[1188]]
An assessment of efficiency and concentration polarisation (CP) showed overall efficiency increased in the SW modules at the region of membrane deformation under transmembrane pressure (TMP), but not the PF module—due to a high degree of occlusion likely detracting from the membrane area
Summary
An increasing global demand for potable water, as well as the efficient treatment of wastewater, has driven research into forward osmosis (FO) as an alternative to current processes [1]. External concentration polarisation (ECP) is found in all membrane processes and is a buildup of solutes within the boundary layer of the membrane surface, which lowers flux performance and hinders the concentration gradient that drives FO processes. ICP is reported as the main CP factor impacting flux performance in FO processes when compared to ECP, especially when the membrane active-layer faces the feed-side [10,11]. Novel flux models that account for wide-ranging temperatures or the diffusivity of draw solutes have since been developed to predict performance for a wider range of operating conditions [11,12]. The use of CFD analysis to assist in the calculation of a Reynolds number in a spacer-filled channel has not yet been applied to the calculation of mass transfer coefficients in FO CP modelling
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