Porosity of spacer-filled channels in spiral-wound membrane systems: Quantification methods and impact on hydraulic characterization

Abstract

The porosity of spacer-filled feed channels influences the hydrodynamics of spiral-wound membrane systems and impacts the overall performance of the system. Therefore, an exact measurement and a detailed understanding of the impact of the feed channel porosity is required to understand and improve the hydrodynamics of spiral-wound membrane systems applied for desalination and wastewater reuse. The objectives of this study were to assess the accuracy of porosity measurement techniques for feed spacers differing in geometry and thickness and the consequences of using an inaccurate method on hydrodynamic predictions, which may affect permeate production. Six techniques were applied to measure the porosity namely, three volumetric techniques based on spacer strand count together with a cuboidal (SC), cylindrical (VCC) and ellipsoidal volume calculation (VCE) and three independent techniques based on volume displacement (VD), weight and density (WD) and computed tomography (CT) scanning. The CT method was introduced as an alternative for the other five already existing and applied methods in practice. Six feed spacers used for the porosity measurement differed in filament thickness, angle between the filaments and mesh-size. The results of the studies showed differences between the porosities, measured by the six methods. The results of the microscopic techniques SC, VCC and VCE deviated significantly from measurements by VD, WD and CT, which showed similar porosity values for all spacer types. Depending on the maximum deviation of the porosity measurement techniques from -6% to +6%, (i) the linear velocity deviations were -5.6% and +6.4% respectively and (ii) the pressure drop deviations were -31% and +43% respectively, illustrating the importance of an accurate porosity measurement. Because of the accuracy and standard deviation, the VD and WD method should be applied for the porosity determination of spacer-filled channels, while the CT method is recommended for numerical modelling purposes. The porosity has a linear relationship with the flow velocity and a superlinear effect on the pressure drop. Accurate porosity data are essential to evaluate feed spacer performance in spiral-wound membrane systems. Porosity of spacer-filled feed channels has a strong impact on membrane performance and biofouling impact.

CFD Investigation of the Effect of the Feed Spacer on Hydrodynamics in Spiral Wound Membrane Modules

“…The channel porosity can be calculated using Equation (2). In the present work, a different element length meant a different porosity of the feed channel, and the value was higher than in other feed spacer studies, where the maximum value of the porosity measurement was 0.906 as reported by Siddiqui et al [42]. In this study, the porosity value of the elliptic cross-section was from 0.928 to 0.979, as shown in Figure 4.…”

Influence of Membrane Intrusion on Permeate‐Sided Pressure Drop During High‐Pressure Reverse Osmosis

“…It can be concluded that the porosity of spacer-filled channels does not only influence the hydrodynamics of spiral-wound membrane systems but impacts the overall performance of the system [30]. It can be concluded that the porosity of spacer-filled channels does not only influence the hydrodynamics of spiral-wound membrane systems but impacts the overall performance of the system [30].…”

“…The latter influences the porosity of the material that is a crucial parameter for any hydrodynamic predictions [30]. Permeate spacers particularly differ in height and mesh size.…”

“…Permeate spacers particularly differ in height and mesh size. The latter influences the porosity of the material that is a crucial parameter for any hydrodynamic predictions [30]. Porosity (e) is defined by the ratio of the void volume and the total volume of the spacer-filled rigid channel:…”

“…with the volume taken by the solid spacer material V Sp and the total channel volume V tot . It can be concluded that the porosity of spacer-filled channels does not only influence the hydrodynamics of spiral-wound membrane systems but impacts the overall performance of the system [30]. Porosity should be low to provide satisfactory membrane support and prevent intrusion whereas it has to be sufficiently high to offer suitable flow conditions and decrease pressure drop.…”