Assessment of direct contact membrane distillation under different configurations, velocities and membrane properties

Abstract

Direct Contact Membrane Distillation (DCMD) is emerging as a viable solution for water desalination and purification application due to its low energy demands. In this work a validated high fidelity numerical model is developed for the DCMD system in conjugated heat and Navier–Stokes flow subject to appropriate flow conditions. The system consists of two adjacent channels representing the hot contaminated feed water and the cold fresh permeate water separated by a permeable super hydrophobic PVDF membrane. The temperature gradient across the membrane creates sufficient pressure to evaporate and drives the vapor through the pours of the membrane and condenses at the colder permeate channel. This phenomena involves a combined localized evaporation, conduction and condensation, leading to the flux of fresh water at the permeate side. The performance of the system is measured by the quantity and quality of the permeated mass flux which depends on the configuration, membrane properties and flow conditions. While permeated mass flux is considered the ultimate system metric, evaluation of heat flux, temperature polarization coefficient (TPC), and thermal efficiency has brought additional physical insight. In this work numerous DCMD parameters are considered in an attempt to quantify their influence and arriving at the optimal conditions. In view of these studies, it was evident that counter flow DCMD system built around the flat sheet PVDF membrane (130μm thickness and 85% porosity) running at higher velocity (Re=100), warmer feed (75°C) and room temperature permeate, and low membrane conduction (0.0662W/mK) can achieve the desired performance of nearly 50g/m2s mass flux, 50% average thermal efficiency and the neither mass limited nor heat limited TPC values of 0.68.