Influence of dynamic operation of reverse osmosis systems on fluid dynamics and mass transfer by investigation of 2-D spacer filled channels

• Published in: Desalination and water treatment 2017-04, Vol.73, p.30-45
• Main Authors: Präbst, Alexander, Kiefer, Florian, Kroiß, Alexander, Spinnler, Markus, Sattelmayer, Thomas
• Format: Article
• Language: English
• Subjects: CFD; Mass transfer enhancement; Pulsating flows; Reverse osmosis; Spacer filled channels
• ISSN: 1944-3986
• DOI: 10.5004/dwt.2017.20531

Motivation of this paper is the hypothesis of permeate flux enhancement due to the dynamic operation of Reverse Osmosis (RO) systems powered by fluctuating renewable energies or by active control. Therefore, in this paper the influence of laminar pulsating flows in spacer filled channels with two porous walls is considered and numerically analysed using the opensource Computational Fluid Dynamics (CFD) tool OpenFOAM. Three different 2-D spacer configurations typically found in literature are investigated: Cavity, Submerged and Zig-Zag. The simulations are performed using typical conditions for seawater desalination. A literature review about dynamically operated membrane systems showed that the results achieved in previous studies are contradictory. Most of them were experimentally or numerically performed in open channels at different conditions. A comparison of the results is therefore difficult. As a first approach this study discusses the dynamics of the mass transfer through the membrane using an analysis based on Biot number. For the CFD simulations, boundary conditions based on the Solution-Diffusion model are implemented and validated with literature data. It shows good agreement at steady state conditions for typical seawater and brackish water desalination conditions. A qualitative analysis of the flow patterns shows that the flow is mainly disturbed in the decelerating phase of the pulsation cycle, which increases the mixing inside the channel. Different local concentration profiles at the membrane and Sherwood number profiles showed that especially in the middle part of two cylinders the mass transfer is significantly influenced whereas at the attachment points of cylinder and membrane, the influence is only marginal. At Womersley numbers of Wo = 17.7 the concentration profiles along the membrane are highly disturbed and the patterns change significantly, also qualitatively. The influence of the Amplitude Ratio is lower than that of the influence of the Womersley number.