Theoretical and experimental study of direct contact membrane distillation

Abstract

Direct contact membrane distillation (DCMD) is characterized as a low-thermal energy process, involving evaporation and a phase change driven by the pressure difference between two fluid channels separated by a hydrophobic membrane. The temperature difference creates a driving pressure between the hot channel (feed) and the cold channel (permeate). This paper demonstrates the performance of the DCMD through high fidelity simulation and experimental observation to reveal a fundamental and qualitative understanding of the spatial distribution of the temperature, mass flux, and heat flux as well as the temperature polarization. The flow model is governed by the Navier–Stokes equations of non-isothermal fluid coupled with the energy equation for the two adjacent channel flow and the middle hydrophobic and porous membrane. The experimental study involved the development of a transparent acrylic DCMD unit operated by two peristaltic pumps where each cycles the feed and the permeate from the corresponding reservoir through the DCMD chambers that are separated by the PVDF–HFP membrane. The hot feed reservoir temperature is maintained at 40°C (4% salinity), whereas the permeate reservoir temperature is kept at 25°C (0% salinity). The system is tested using membranes of prescribed thickness, porosity, and conductivity. The model and experimental results were compared in counterflow configurations and a good agreement between the model and experimental was obtained for temperature distributions, mass flux, and temperature polarization coefficient (TPC). The system metrics were obtained for the DCMD showing a suitable TPC working range (0.3–0.55), a relatively low mass flux yield (5 kg/h m²) and a very low thermal efficiency (1.5%). These results suggest there is still a large potential in DCMD to enhance its overall yield in order to speed up their large-scale commercialization.

Keywords:

• Mass transfer
• Heat transfer
• Temperature polarization coefficient (TPC)
• Hydrophobic

Acknowledgments

The authors are grateful for the financial support from Masdar Institute of Science & Technology and for all the support received from the Waste to Energy Lab throughout this research. Special thanks also to Jennah Alonso who has helped in this work.

Notes

Presented at the 3rd International Conference on Water, Energy and Environment (ICWEE) 24–26 March 2015, Sharjah, United Arab Emirates