An Improved Modelling Approach for the Comprehensive Study of Direct Contact Membrane Distillation

Graeme Millar,Fernanda Helfer,David V. Thiel,Abolfazl Ansari,Saman Kavousi

doi:10.3390/membranes11050308

Graeme Millar, Fernanda Helfer + Show 3 more

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https://doi.org/10.3390/membranes11050308

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Abstract

Direct Contact Membrane Distillation (DCMD) is a promising and feasible technology for water desalination. Most of the models used to simulate DCMD are one-dimensional and/or use a linear function of vapour pressure which relies on experimentally determined parameters. In this study, the model of DCMD using Nusselt correlations was improved by coupling the continuity, momentum, and energy equations to better capture the downstream alteration of flow field properties. A logarithmic function of vapour pressure, which is independent from experiments, was used. This allowed us to analyse DCMD with different membrane properties. The results of our developed model were in good agreement with the DCMD experimental results, with less than 7% deviation. System performance metrics, including water flux, temperature, and concentration polarisation coefficient and thermal efficiency, were analysed by varying inlet feed and permeate temperature, inlet velocity, inlet feed concentration, channel length. In addition, twenty-two commercial membranes were analysed to obtain a real vision on the influence of membrane characteristics on system performance metrics. The results showed that the feed temperature had the most significant effect on water flux and thermal efficiency. The increased feed temperature enhanced the water flux and thermal efficiency; however, it caused more concentration and temperature polarisation. On the other hand, the increased inlet velocity was found to provide increased water flux and reduced temperature and concertation polarisation as well. It was also found that the membrane properties, especially thickness and porosity, can affect the DCMD performance significantly. A two-fold increase of feed temperature increased the water flux and thermal efficiency, 10-fold and 27%, respectively; however, it caused an increase in temperature and concertation polarisation, at 48% and 34%, respectively. By increasing Reynolds number from 80 to 1600, the water flux, CPC, and TPC enhanced by 2.3-fold, 2%, and 21%, respectively. The increased feed concentration from 0 to 250 [g/L] caused a 26% reduction in water flux. To capture the downstream alteration of flow properties, it was shown that the ratio of inlet value to outlet value of system performance metrics decreased significantly throughout the module. Therefore, improvement over the conventional model is undeniable, as the new model can assist in achieving optimal operation conditions.

Highlights

Water scarcity is increasing globally due to population growth, climate change, and expanded industrial activities, leading to a severe global challenge [1,2,3]
Since the equations to compute membrane temperature on both sides in the semi-empirical model are dependent on water flux, and temperature on both sides in the semi-empirical model are dependent on water flux, and to include the effects of water flux on the temperature results on the numerical model, the to include the effects of water flux on the temperature results on the numerical model, the algorithm illustrated in Figure 3 has been applied to couple the temperature results of algorithm illustrated in Figure 3 has been applied to couple the temperature results of numerical simulation with the empirical model for each element
Downstream alterations of the water flux, membrane temperatures, and membrane vapour pressures along the module were compared with the results of reference [21]

Summary

Introduction

Water scarcity is increasing globally due to population growth, climate change, and expanded industrial activities, leading to a severe global challenge [1,2,3]. To clear away contaminants and salt from water in various sources, ranging from wastewater to seawater, thermal-based and membrane-based desalination processes have been widely developed [4]. Driven technologies, such as multistage flash distillation, consume high-priced energy to vaporize water. This method is increasingly being replaced by membrane-based technologies, especially reverse osmosis (RO) [5]. Membrane distillation (MD) is hydrophobic membrane-based desalination technology which is thermally driven [6]. MD has several advantages over other desalination processes

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