Abstract
In an effort to improve performances of forward osmosis (FO) systems, several innovative draw spacers have been proposed. However, the small pressure generally applied on the feed side of the process is expected to result in the membrane bending towards the draw side, and in the gradual occlusion of the channel. This phenomenon potentially presents detrimental effects on process performance, including pressure drop and external concentration polarization (ECP) in the draw channel. A flat sheet FO system with a dot-spacer draw channel geometry was characterized to determine the degree of draw channel occlusion resulting from feed pressurization, and the resulting implications on flow performance. First, tensile testing was performed on the FO membrane to derive a Young’s modulus, used to assess the membrane stretching, and the resulting draw channel characteristics under a range of moderate feed pressures. Membrane apex reached up to 67% of the membrane channel height when transmembrane pressure (TMP) of 1.4 bar was applied. The new FO channels considerations were then processed by computational fluid dynamics model (computational fluid dynamics (CFD) by ANSYS Fluent v19.1) and validated against previously obtained experimental data. Further simulations were conducted to better assess velocity profiles, Reynolds number and shear rate. Reynolds number on the membrane surface (draw side) increased by 20% and shear rate increased by 90% when occlusion changed from 0 to 70%, impacting concentration polarisation (CP) on the membrane surface and therefore FO performance. This paper shows that FO draw channel occlusion is expected to have a significant impact on fluid hydrodynamics when the membrane is not appropriately supported in the draw side.
Highlights
Forward osmosis (FO) membrane processes have undergone significant development in both the research field and the commercial production of modules
In FO processes, mass transfer is important on both sides of the membrane, and as such external concentration polarisation (ECP) occurs on the draw side in the form of dilutive ECP, which lowers the flux of the process and is further hypothesized to affect internal concentration polarisation (ICP) in the support layer [14,15]
The results show that pressures applied in typical FO membrane process operation do not exceed the elastic region of the membrane
Summary
Forward osmosis (FO) membrane processes have undergone significant development in both the research field and the commercial production of modules. Membranes 2020, 10, 111 oil and gas well-fracturing, osmotic concentration of liquid foods, and other novel applications, yet with limited current industrial implementation and largely lab-scale studies [6,7,8] Specialized applications such as utilizing fertilizer as a draw solution or brine dilution show continued industrial and commercial interest [9]. ICP is specific to FO membranes, occurring within the porous support layer of the membrane This concentration gradient causes an increase in solute concentration at the active-support interface, reducing the effective osmotic pressure difference across the membrane active layer and the driving force resulting in lower fluxes [12]. In FO processes, mass transfer is important on both sides of the membrane, and as such ECP occurs on the draw side in the form of dilutive ECP, which lowers the flux of the process and is further hypothesized to affect ICP in the support layer [14,15]
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