Analysis of effective thermal conductivity and tortuosity modeling in membrane distillation simulation

Abstract

Porous hydrophobic membranes are the main elements in MD (membrane distillation) desalination systems and need to be adequately modeled in terms of transport mechanisms to allow for theoretical assessments toward process design. The objective of this work is twofold. First, illustrate the rational choice of models based on the morphology and structure of a couple of selected membranes for which micrographs and experimental results for distillate mass flux are readily available in the literature. Second, provide a comparative analysis of well-established models for membrane effective thermal conductivity and tortuosity, so as to confirm the adequacy of the proposed model selection path. Theoretical results of the distillate flux were obtained from the numerical simulation of the heat and mass transfer phenomena in the MD process for the DCMD (direct contact membrane distillation) configuration, using a multi-region approach with OpenFOAM@. The comparative analysis is performed for PVDF membranes with different morphologies in the transversal section, for both flat and hollow fiber geometries. For the membrane with sponge-like morphology, the employment of the parallel-resistance model for the effective thermal conductivity and the fractal tortuosity model, designed for porous media characterized by randomly distributed obstructions in spherical format, led to the best agreement with respect to the experimental data for the distillate flux. On the other hand, for the membrane with macro voids in the cross-section (presence of finger-like channels), asymmetrical morphological structure, and narrow pore size distribution, it was found that the employment of the parallel-resistance model for effective thermal conductivity and the Euclidian-based tortuosity model for the random arrangement of highly interconnected pores provided the best agreement between experimental and theoretical data. The results confirm the appropriate model selection path for effective thermal conductivity and tortuosity in MD simulation by taking into consideration solely the membrane morphology.

Keywords

• Desalination modeling
• Direct contact membrane distillation (DCMD)
• Effective thermal conductivity
• Membrane tortuosity
• CFD simulation
• Model selection