Hydrodynamics and concentration polarization in NF/RO spiral wound modules with ladder-type spacers: experimental and numerical work
Advancing the chemical engineering fundamentals
Membranes and Membrane Science - II (T2-8b)
Keywords: Nanofiltration, Boundary-layer disruption, CFD, Integrated modeling, pore flow modeling
The present authors have performed an intensive and consistent work, associating experimental work to computational fluid dynamics (CFD), aiming at the study of the influences of both fluid flow momentum and mass transfer phenomena on the concentration polarization in the permeation of aqueous solutions in spiral-wound modules.
It was studied the effect of momentum boundary layer development in the mass transfer rates in open channels for nanofiltration (NF) conditions (100<Re<1000, 1<DP<4 MPa, Sc=850), simulating spiral-wound modules channels [1]. The two-dimensional conditions occurring in spiral-wound module feed channels were approximated by a slit (200mmx30mmx2mm), either constituting an open channel or filled with ladder-type spacers with transverse filaments adjacent to the membrane wall. The entrance length in open channels is not influenced by permeation rates, depending only on the circulating Reynolds number, Re, in opposition to the permeation fluxes that depend also on the permeation Reynolds number, Rep. For CFD modelling, it was used an intrinsic rejection coefficient, f’, dependent on the operating parameters: the transmembrane pressure, DP, and the permeation flux, vp. The CFD code used was previously validated against experimental data and benchmark problems [2]. The fluid phase modelling was integrated with that of the membrane transport and the range of Schmidt numbers studied was extended to 570<Sc<3200 [3]. The transport modelling of the solute inside the membrane was improved through the use of hindered transport mechanisms. The solution properties were allowed to vary with the solute concentration. Numerical and experimental results were in excellent agreement and showed that the concentration polarization depends on both the circulating and the permeation Reynolds numbers and on the Schmidt number. Moreover, it was analyzed in detail the physical mechanisms governing the onset of both hydrodynamic and concentration boundary layers, the onset of the former occurring prior to that of the latter [4]. The hydrodynamic boundary layer grows by molecular diffusion of momentum as a consequence of the non-slip condition at the membrane surface, this growth being insensitive to permeation fluxes and to Schmidt numbers in the range of interest of NF. Conversely, the concentration boundary layer growth depends on those two last parameters. The increase of the circulating flux yields an increase of the laminar shear stresses near the membrane surface that act as an enhancement factor to the solute transfer, slowing down the growth of the concentration boundary layer. Further integrated modelling was developed, paying particular attention to the solute hindered transport inside the membrane, making recourse to the steric-force pore flow model to calculate the membrane intrinsic rejection coefficient as a function of the pore Peclet number, which in turn depends on the membrane pore radius [5]. The predictions were promising due to their excellent agreement with the experimental data. The presence of ladder-type spacers was also intensively studied through the analysis of the effect of the geometrical parameters, spacers’ height and distance, on the concentration boundary layer disruption. It was concluded that the spacers’ constitution must be such that concentration boundary layer disruption has to occur at both top and bottom membranes, otherwise, near the membrane without spacers adjacent to it, a continuously growing concentration boundary layer will develop [6]–[8], yielding an increasing concentration polarization.
References
[1] Geraldes V, Semiao V, de Pinho MN, 1998. Nanofiltration mass transfer at the entrance region of a slit laminar flow, Industrial & Engineering Chemistry Research, 37 (12): 4792-4800.
[2] Geraldes V, Semiao V, de Pinho MN, 2000. Numerical modelling of mass transfer in slits with semi-permeable membrane walls, Engineering Computations, 17 (2,3): 192-217.
[3] Geraldes V, Semiao V, de Pinho MN, 2001. Flow and mass transfer modelling in nanofiltration, Journal of Membrane Science, 191: 109-128.
[4] Geraldes V, Semiao V, de Pinho MN, 2002. The effect on mass transfer of momentum and concentration boundary layers at the entrance region of a slit with a nanofiltration membrane wall, Chemical Engineering Science, 57: 735-748.
[5] de Pinho MN, Semiao V, Geraldes V, 2002. Integrated modeling of transport processes in fluid/nanofiltration membrane systems, Journal of Membrane Science, 206: 189-200.
[6] Geraldes V, Semiao V, de Pinho MN, 2002. Flow management in nanofiltration spiral wound modules with ladder-type spacers, Journal of Membrane Science, 203: 87-102.
[7] Geraldes V, Semiao V, de Pinho MN, 2002. The effect of the ladder-type spacers configuration in NF spiral-wound modules on the concentration boundary layers disruption, Desalination, 146: 187-194.
[8] Geraldes V, Semiao V, de Pinho MN, 2004. Concentration polarisation and flow structure within nanofiltration spiral-wound modules with ladder-type spacers, Computers and Structures, 82 (17-19): 1561-1568.
Presented Wednesday 19, 16:20 to 16:40, in session Membranes and Membrane Science - II (T2-8b).
