Quantifying the potential of pressure retarded osmosis advanced spacers for reducing specific energy consumption in hybrid desalination

A hypothetical PRO advanced spacer that delivers a 50 % mass transfer enhancement (i.e., 50 % higher Sherwood number) is simulated for a range of feed conditions and membrane properties, to shed insights into the effect of improved PRO spacer on the overall specific energy consumption (SEC) of RO-PRO hybrid desalination. Results show that a large increase in pressure drop in the PRO module has negligible impact on power density (PD) and SEC for RO-PRO. The analysis revealed that the PRO advanced spacer has marginal impact on SEC for a typical current PRO membrane. Even so, the PRO advanced spacer has an important impact in terms of PD, which can increase by 10 %, especially under severe external concentration polarization. The sensitivity analysis demonstrates that the extent of SEC reduction or power density enhancement related to the advanced spacer is most sensitive to the structural parameter. This is because internal concentration polarization is the major cause for osmotic pressure loss in PRO, which limits the potential PRO performance improvements from advanced spacers. Nevertheless, the benefits of PRO advanced spacers can be further exploited through the continuous development of new materials for novel membranes with a reduced structural parameter

Analysis of the effect of advanced FO spacer on the specific energy consumption of hybrid RO desalination system

Research involving hybrid forward osmosis-reverse osmosis (FO-RO) desalination has gained attention recently due to its potential to reduce energy consumption compared to traditional RO. This paper aims to understand the degree of the impact of advanced spacers in the FO process on the overall specific energy consumption (SEC) of FO-RO systems. The SEC for a representative advanced spacer is simulated and analysed under standard recovery and operating conditions. The results show that advanced spacers can significantly reduce SEC by up to 9.27% under the operating conditions considered. The results also show that placing an advanced spacer on the FO membrane draw side has a greater effect in reducing SEC compared to placing it on the feed side, due to the larger extent of ECP. It was found that FO channel pressure drop has insignificant impact on the SEC. The performance of advanced spacers in SEC reduction is most effective if the contribution of external concentration polarisation (ECP) to transmembrane osmotic pressure is high, if the contribution of internal concentration polarisation (ICP) is low, and if the effective transmembrane osmotic pressure is low. Optimal mixing in FO systems is therefore crucial to reduce SEC, especially for systems with severe ECP

3D CFD study of hydrodynamics and mass transfer phenomena for spiral wound membrane submerged-type feed spacer with different node geometries and sizes

Modification of the spacer geometry is a promising approach to increase the efficiency of reverse osmosis (RO) spiral wound membrane modules. Column nodes and spherical nodes are considered in this three-dimensional computational fluid dynamic (CFD) study to evaluate the hydrodynamic and mass transfer performance of submerged spacers with different node geometries and sizes. Small-scale CFD analysis results reveal that the column node has better mass transfer performance than the spherical node geometry because column nodes divert more flow to the filaments, leading to higher local velocity at the region between the filament and wall. Furthermore, when the dimensionless node diameter ratio of the column nodes increases from 0.3 to 1.2, Sherwood number and wall shear increase by 25% and 8% respectively at the expense of higher global friction factor (44%). A sea water RO full-scale analysis revealed that column node spacers yield higher average flux than spherical nodes and conventional spacers at high feed inlet velocity (> 0.1 m/s), because the mixing effects by the spacer that improve mass transfer are more prominent

CFD as a tool for modelling membrane systems

Computational fluid dynamics (CFD) is a computer-based numerical method used to analyse systems that involve fluid flow and/or heat and mass transfer (Versteeg & Malalasekera, 2007). CFD bridges the two different approaches for solving engineering problems before the computer era, theoretical and experimental; it relies on mathematical models while being easy to adapt to almost any realistic condition (Anderson & Wendt, 1995). Another feature of CFD is its versatility, as it allows the analysis of systems for a variety of applications such as chemical reactions (Salehi et al., 2016), aerodynamics (Snel, 2003), dispersion of pollutants (Chu et al., 2005), blood flows (Byun & Rhee, 2004), among many others

CFD study of the effect of SWM feed spacer geometry on mass transfer enhancement driven by forced transient slip velocity

Recent studies have shown that the interactions between forced transient flow and eddy inducers (i.e. spacers) in spiral wound membrane modules results in significant mass transfer enhancement and reduction in concentration polarisation (CP). This paper uses CFD to investigate the effect of varying the spacer geometric parameters on the resonant frequency for an unsteady forced-slip, as well as the resulting permeate flux enhancement, for a 2D zig-zag spacer. The analysis shows that the resonant frequency is significantly affected by the interaction of the shear layer with successive downstream spacers. The effectiveness of forced-slip reaches a peak (up to 15.6% flux increase) for a spacer size in the range of 0.5<df/hch<0.6 because of the trade-off between mixing-induced forced-slip and the CP modulus. In addition, vortex shedding is suppressed for smaller spacer sizes (df/hch≤0.4), because viscous forces dominate over convective forces due to a smaller filament Reynolds number. As the distance between filaments is increased, the increase in flux due to forced-slip is greater (up to 31.5%), albeit the actual flux decreases because the boundary layer is more developed. These results also reinforce the finding that forced-slip is more efficient for spacer designs with poor mixing (i.e. high CP)

Validation and characterisation of mass transfer of 3D-CFD model for twisted feed spacer

3D-CFD simulations of a membrane channel with several variations of twisted feed spacer geometry are performed for a Reh range of 50–200 using a fine meshing approach. Although previous studies could not accurately simulate its performance, the current CFD model shows good agreement with previous experimental data. The validated model reveals that twisted spacers present higher Sherwood number (~55 %) and lower friction factor (~8 %) than conventional ladder-type spacers because the twisted features promote vortex generation and minimise the appearance of stagnant zones. Furthermore, the RR-twisted spacer outperforms the LL- and LR-twisted spacer types in terms of Sh because the concave surfaces of the spacers face towards the centre of channel, causing stronger vortices downstream of the filaments. With respect to the number of twists, Sh reaches a minimum at lm/ltwist = 3 due to relatively stagnant zones. However, Sh increases at lm/ltwist = 4 due to the formation of strong vortices in the region between the filaments. In terms of attack angle, Sh reaches a maximum at α = 45° due to the formation of stronger vortices behind the filament intersection. This paper shows that CFD modelling tools have evolved to a stage that they can be used to understand membrane phenomena with complex spacer designs

Comparison of oscillating flow and slip velocity mass transfer enhancement in spacer-filled membrane channels: CFD analysis and validation

Unsteady shear methods have the potential to generate flow perturbations near the membrane surface, which play an important role in reducing concentration polarisation and fouling tendency. In general, there are two main approaches for generating time-varying flow perturbations: 1) generating oscillations in the bulk flow; or 2) forcing a slip velocity near the membrane surface. This paper presents a detailed comparison study of both approaches by means of two-dimensional computational fluid dynamics (CFD) simulations. The results show that both approaches result in significant increases in flux and maximum wall shear at the same disturbance resonant frequency and Reynolds number. This suggests that the mechanism by which the flow perturbations are generated is not as important as the perturbation frequency, in terms of increasing wall shear and permeate flux. However, it is more important to perturb flow near the membrane surface because it reduces energy consumption compared to oscillating flow approach. In addition, this paper confirms that a white noise perturbation can be used to simplify the approach for maximising vortex-shedding-induced mass transfer enhancement, without the need to identify the peak/resonant frequency for the flow in spacer-filled membrane channels at the expense of a higher pressure loss

3D CFD study of the effect of multi-layer spacers on membrane performance under steady flow

Multi-layer feed channel spacers have shown superior mass transfer enhancement than conventional dual-layer spacers used in reverse osmosis (RO) spiral wound membrane modules. However, mass transfer indicators do not directly address the economic advantages of multi-layer spacers. To allow for a direct economic comparison of spacer designs, a simplified multi-scale techno-economic model is proposed which can provide useful cost trends. A total of 8 feed spacer geometries with different attack angles (α = 0–60°) and filament sizes (df/hch = 0.4–0.6) are first investigated using 3D-CFD. Multi-layer spacers typically increase both Sherwood number (~12%) and friction factor (~140%). The techno-economic model is then used to assess the impact of these changes on the total processing cost for RO. The latter analysis found that larger pressure drops associated with multi-layer spacers in long channels have little impact on total water processing cost for RO, with multi-layer spacers showing lower total processing costs (by 2–4%) than the dual-layer spacer for both seawater and brackish water RO. Novel spacer designs should therefore emphasise flux enhancement. The importance of including energy recovery for a more accurate economic analysis is highlighted, especially for systems with low recovery ratio

CFD study of the effect of unsteady slip velocity waveform on shear stress in membrane systems

An unsteady forced slip velocity has an important effect on the flow conditions adjacent to a membrane interface, which can help control concentration polarisation (CP) and fouling. This study explores the effect of non-sinusoidal slip velocity waveforms on mass transfer and shear stress in membrane channels. The hydrodynamics and mass transfer of unobstructed and obstructed membrane channels under the influence of slip velocity are simulated using two-dimensional computational fluid dynamics (CFD). At a Reynolds number where vortex shedding occurs, the results show that both sinusoidal and non-sinusoidal slip velocity profiles cause a similar increase in mass transfer and shear stress. However, for systems without vortex shedding, a non-sinusoidal waveform with a sudden decrease in slip velocity can significantly increase maximum shear stress (by over 20%). This effect shows a clear advantage of non-sinusoidal slip velocity profiles over sinusoidal slip velocity profiles

A CFD study on the effect of membrane permeance on permeate flux enhancement generated by unsteady slip velocity

One of the most noteworthy achievements in reverse osmosis (RO) efficiency is the improvement in membrane permeance. Although current membranes offer higher permeance (and flux) than older RO membranes, increases in permeate flux are limited by concentration polarisation (CP) and fouling. Therefore, innovation is needed to reduce CP to further increase permeate flux. An unsteady forced slip velocity can disrupt the boundary layer, thus reducing CP. This paper uses Computational Fluid Dynamics (CFD) to analyse the effect of membrane permeance on the resonant frequency for an unsteady forced slip velocity, as well as the resulting mass transfer enhancement. The results show that the resonant frequency of the unsteady forced slip velocity is not affected by the membrane permeance. Although the results show a peak in the mass transfer enhancement factor for permeance values in the range typically used for brackish water, the permeate flux can also be improved for higher membrane permeances (up to 23%) at the expense of a slightly higher pumping energy (5–7%)