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The magical water separation technology: How does membrane distillation complete separation driven by thermal energy?
With the scarcity of water resources, membrane distillation (MD) technology has gradually attracted attention and become an emerging water treatment technology. In nature, the evaporation and condensation of water are common processes, and membrane distillation is based on this natural phenomenon and uses phase change to separate water. This article will delve into the principles, technology, applications and challenges of membrane distillation, and let us unveil this magical water separation technology together.
Basic principles of membrane distillation
Membrane distillation is a naturally driven separation process in which phase change is the key. This process relies on a hydrophobic membrane that allows only water vapor to pass through. The micropores of this membrane do not allow liquid water to enter, but allow water vapor to pass through at different pressure and temperature differences. The driving force for this process comes from the partial water vapor pressure difference between different interfaces. This pressure difference occurs when there is a difference in temperature on the two sides.
The material of the membrane cannot be wetted by liquid water because the interaction between the polarity of water and the non-polarity of the membrane prevents liquid water from entering these pores.
Types of membrane distillation technology
The diversity of membrane distillation technology allows it to play a role in different scenarios. The following are several major membrane distillation technologies:
- Direct Contact Membrane Distillation (DCMD)
- Air Gap Membrane Distillation (AGMD)
- Vacuum membrane distillation (VMD)
- Scavenged Gas Membrane Distillation (SWGMD)
- Multiple Effect Membrane Distillation (V-MEMD)
- Permeable Gap Membrane Distillation (PGMD)
Direct contact membrane distillation (DCMD)
In DCMD, the two sides of the membrane are hot feed water and cooled product water. The advantage is that the membrane is the only mass transport barrier, enabling higher product water flow rates. However, its heat loss rate is relatively high, which makes DCMD limited in energy efficiency.
Air gap membrane distillation (AGMD)
In AGMD's design, an air gap is retained between the film and the cooling surface, which can effectively reduce heat conduction losses. However, this also hinders mass transfer to a certain extent, thereby reducing water production efficiency.
Scavenged gas membrane distillation (SWGMD)
SWGMD sets up an air circulation environment on the permeation side of the membrane, so that the resistance to mass transmission can be reduced through forced flow and the water production rate can be increased. However, this requires more efficient condensation equipment to accommodate larger gas flows.
Multiple-effect membrane distillation (V-MEMD)
The V-MEMD module consists of a steam riser and multiple evaporation-condensation stages. Each stage recovers heat from the previous stage, making it more energy efficient than traditional membrane distillation.
Application of membrane distillation
Membrane distillation technology has practical value in many fields. The main applications include:
- Desalination
- Fresh water treatment
- Wastewater treatment
- Water Recycling and Concentration
Solar powered membrane distillation
Membrane distillation is particularly suitable for solar-powered desalination devices due to its low energy consumption. This enables water management in remote or water-scarce areas. For example, Germany's Fraunhofer Institute for Solar Energy Systems has developed a self-financed device that uses solar energy to desalinize seawater.
Challenges and prospects
Although membrane distillation technology has many advantages, it still faces some challenges. Among them, membrane humidification is a major obstacle, which can lead to contamination of the produced water and reduced efficiency. In addition, improving energy efficiency is also a major challenge for membrane distillation technology to overcome. At present, commercial systems for membrane distillation have not been able to achieve significant improvements in energy consumption, which is a key direction for future technology development.
Faced with the water crisis, membrane distillation offers a viable solution, but can we meet this challenge and further improve the commercial viability and energy efficiency of the technology?
