Internal feed preheating necessary for energy-efficient modular multi-effect membrane distillation

Abstract

Multi-effect membrane distillation (MEMD) systems achieve a high product recovery ratio by effectively utilizing the heat of condensation for further vaporization within the module. In this study, a novel internal feed-preheating design for improving the energy efficiency of modular MEMD systems is proposed and evaluated using a 2D CFD model. Custom user-defined functions are implemented to account for water vapor and associated energy transport across the membrane in an MEMD design with serpentine flow across various effects. Preheat channels embedded into the condensation plates help increase the temperature of feed entering the first effect by ~25 K, close to heater plate temperature. As a result, high-grade external heat input is not used for sensible heating, instead being utilized towards evaporation and pure water production, resulting in ~32 % improvement in energy efficiency. However, the added thermal resistance of the preheat channels results in lower average flux and hence recovery, which can be ameliorated by increasing the effective conductivity of the preheat channels. Increasing the thermal conductivity of preheat channels results in ~23.5 % rise in recovery ratio. Such preheat designs are necessary to reap energy efficiency improvements by increasing the total number of effects in multi-effect vacuum and gap MD systems.