Effect of feed flow pattern on the distribution of permeate fluxes in desalination by direct contact membrane distillation

Abstract

The current study aims to highlight the effect of flow pattern on the variations of permeate fluxes over the membrane surface during desalination in a direct contact membrane distillation (DCMD) flat module. To do so, a three dimensional (3D) Computational Fluid Dynamics (CFD) model with embedded pore scale calculations is implemented to predict flow, heat and mass transfer in the DCMD module. Model validation is carried out in terms of average permeate fluxes with experimental data of seawater desalination using two commercially available PTFE membranes. Average permeate fluxes agree within 6% and less with experimental values without fitting parameters. Simulation results show that the distribution of permeate fluxes and seawater salinity over the membrane surface are strongly dependent on momentum and heat transport and that temperature and concentration polarization follow closely the flow distribution. The analysis reveals a drastic effect of recirculation loops and dead zones on module performance and recommendations to improve MD flat module design are drawn consequently.

Direct contact membrane distillation module scale-up calculations: Choosing between convective and conjugate approaches

“…(25) though an iterative process. Details on the derivation of the equations and algorithm flowchart can be found in a previous investigation by Soukane et al [30].…”

“…The first one considers only the feed side of the module while the second is a conjugate approach that includes both the feed and the permeate sides. Regarding the first approach, models evolved from simple boundary conditions setting constant values for the permeate flux [26], or the heat flux [16,27], or the membrane mass transfer coefficient [28,29] to more complex formulations coupling to the module scale both heat and species transport across the membrane [30]. The second approach has often been used to circumvent the complexity of the strongly non-linear boundary condition at the membrane permeate interface, leaving to the CFD code the calculation of the temperature at the permeate side of the membrane surface.…”

“…CFD modeling of DCMD processes necessitates the use of relatively fine meshes to resolve both temperature and concentration polarization phenomena [30,33]. Resulting mesh size is generally large leading to significant computational burden.…”

“…It is demonstrated that for practical module lengths, replacing the permeate side of the DCMD module by an appropriate boundary condition leads to similar results as a conjugate approach where both feed and permeate sides are considered. The first modeling approach, denoted in the present article as "convective approach" has been used successfully to predict flow evolution in DCMD module and emphasize its effect on the distribution of permeate fluxes over the membrane surface [30]. The permeate side is replaced by an appropriate boundary condition and the steady state temperature at the permeate side of the membrane is solved through an iterative process.…”

“…The two modeling strategies use ANSYS fluent V16 to solve coupled turbulent momentum, heat and mass transfer in the module channel, although an in-house non-linear solver is called for each node subject to the convection boundary condition in the convective approach [30]. Twodimensional calculations are carried out for different module lengths and different operating conditions.…”