Date of Award
2024
Document Type
Thesis - ECU Access Only
Publisher
Edith Cowan University
Degree Name
Doctor of Philosophy
School
School of Engineering
First Supervisor
Mehdi Khiadani
Second Supervisor
Masoumeh Zargar
Third Supervisor
Abdellah Shafieian
Fourth Supervisor
Amir Razmjou
Abstract
Temperature polarisation (TP) presents a significant barrier to the global acceptance of membrane distillation (MD) technology. Temperature Polarisation Coefficient (TPC), a conventionally used dimensionless parameter, has been found ineffective for comparing different MD configurations, design conditions, and analysing TP phenomena along the membrane. This research aims to analyse TP in a direct contact membrane distillation (DCMD) system, by examining temperature profiles across and along a lengthy membrane (650 mm) under various operational (feed temperature, permeate temperature, mass flow rate and feed salinity) and design conditions (module orientation, flow channel height and flow direction), using an innovative experimental approach. For this purpose, a specialised DCMD module was constructed, equipped with miniature thermocouples for precise temperature measurements and designed to accommodate different modifications easily.
Key findings indicate that TP is not uniform along the membrane and that the temperature profile is asymmetrical across the feed and permeate sides for the majority of instances. The extent of polarisation is notably influenced by the proximity of flow inlets and outlets along the membrane. Furthermore, the study revealed while certain operational parameters, such as feed temperature, permeate temperature, and flow rates, have a significant impact on TP, other parameters, such as feed salinity, have a negligible effect on the temperature profile. Hence, while some changes in operating conditions have a direct impact on TP which influences the permeate flux, other operating conditions that don’t impact TP but affect permeate flux could be attributed to other adverse phenomena occurring in MD systems.
The research also found that substantial improvements in permeate flux can be achieved through fundamental design modifications. By altering the module orientation and flow channel height in a custom-designed DCMD system, TP was mitigated nearly to non-existence. Notably, increasing the flow channel height and altering the module orientation from the standard horizontal position with the feed side on top (FST) to a sideways orientation resulted in a 90% increase in permeate flux. Configurations with the permeate side on top (PST) and sideways orientations outperformed the FST setup, especially at higher channel heights, though improvements were minimal at lower channel heights.
Thermal convective currents, solute convective currents and secondary flows were found to play crucial roles in assisting or opposing TP and must be carefully considered in hydrodynamic analyses of DCMD systems. Additionally, the flow direction (counter-current and concurrent) was observed to influence the shape of the captured temperature profiles, while changes in flow channel height did not lead to abrupt shifts in the development trends of temperature profiles. Counter-current flow consistently resulted in slightly better permeate flux compared to concurrent flow, regardless of the flow channel height.
This comprehensive analysis underscores the limitations of TPC and highlights the importance of capturing detailed temperature profiles to understand and mitigate TP in MD systems. By incorporating strategic design modifications and considering the dynamic interactions of operational and flow conditions, the efficiency and acceptance of MD technology can be significantly enhanced.
DOI
10.25958/1z7m-2081
Access Note
Access to this thesis is embargoed until 12th March 2027
Recommended Citation
Mohamed Hijaz, H. (2024). Integrated design and operational considerations impacting temperature polarisation and performance in direct contact membrane distillation. Edith Cowan University. https://doi.org/10.25958/1z7m-2081
