Membrane distillation heat transfer enhancement by CFD analysis of internal module geometry

Abstract

Module geometry optimisation can be a crucial matter in all separation processes using selective or hydrophobic membranes, e.g. Reverse Osmosis (RO), Membrane Distillation (MD). In fact the choice of suitable channel shape and size can dramatically affect the performance of the process. With reference to the membrane distillation process, temperature polarization phenomena and pressure drops along the channels largely affect the process efficiency (i.e. the efficient use of temperature driving force for the passage of vapour through the membrane) as well as pressure distribution, module mechanical resistance and pumping costs.Several works have been presented so far in literature on the fluid flow characterization of spacer-filled channels for the case of Reverse Osmosis modules, but only few works deal with the problem of MD modules optimization.The present work aims at the CFD simulation of the fluid flow and temperature fields within spacer-filled MD module channels for a variety of spacer geometries. Both commercial and custom made geometries have been simulated in order to identify the most important parameters affecting process efficiency. The commercial CFD code ANSYS CFX11.0 has been adopted to perform simulations.Results provide valuable information to identify the main features which an optimised spacer should possess in order to minimise T-polarization and pressure drops along the channel.

Membrane Distillation: Principle, Advances, Limitations and Future Prospects in Food Industry

“…Some authors have considered the use of spacer-filled channels (Chernyshov et al, 2003;Cipollina et al, 2011;Teoh et al, 2008;Wang, 2011). The spacers can improve the flow characteristics at the membrane surface and by promoting regions of turbulence due to the formation of eddies and wakes.…”

Influence of the boundary conditions on heat and mass transfer in spacer-filled channels

“…For example, in Direct Contact Membrane Distillation r ext may represent the thermal resistance of the membrane itself in series with the condensation thermal resistance on the permeate side, and T ext will be the condensing temperature. In numerical simulations, most authors imposed at the walls either a uniform temperature / concentration [11] or a uniform heat / mass flux [14,15,16]. Tamburini et al [7] simulated their own experiments on Membrane Distillation, conducted under one-sided heat transfer (see also following Section), by imposing the mixed (3 rd type) boundary condition of Eq.…”

“…Among computational studies of membrane processes, both the one-side configuration (a) [7,10,14,23,24] and the two-side, symmetric configuration (c) [11,9,13] have been considered. For the case of a spacerless plane channel subjected to hydrodynamically and thermally fully developed flow, the difference in the Nusselt numbers for one-side and two-side heat transfer is illustrated in Figure A.1.…”

Experimental and theoretical analyses of temperature polarization effect in vacuum membrane distillation

“…Researchers has taken advantage of the availability of powerful computers along with advanced Computational Fluid Dynamic (CFD) modeling packages and started to simulate the behavior of the fluid thin film near the membrane surface [11][12][13][14][15][16][17][18][19]. The central focus of these simulation studies has been to investigate the turbulence promoter's effect on the fluid temperature, velocity and pressure profiles near the membrane surface to see how these promoters could enhance the water vapor flux.…”