11.3 Membrane distillation: principles and configurations
3 min read•august 7, 2024
is a cool water treatment method that uses temperature differences to separate water from other stuff. It's like a high-tech version of boiling water, but with a special membrane that only lets water vapor through.
This section dives into how membrane distillation works and the different ways to set it up. We'll learn about the science behind it and explore some real-world applications for cleaning water.
Principles of Membrane Distillation
Overview of Membrane Distillation
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Membrane distillation is a thermally driven separation process that uses a hydrophobic membrane to separate a into a and a
Relies on a across the membrane, which is induced by a between the feed and permeate sides
Utilizes with typically ranging from 0.1 to 1 μm, allowing water vapor to pass through while preventing liquid water from entering the pores (polypropylene, polytetrafluoroethylene)
Operates at lower temperatures compared to conventional distillation, making it suitable for using low-grade heat sources such as solar energy or waste heat
Heat and Mass Transfer in Membrane Distillation
Heat transfer occurs through the membrane by conduction and across the boundary layers by convection
Mass transfer involves the evaporation of water at the feed-membrane interface, diffusion of water vapor through the membrane pores, and condensation at the permeate-membrane interface
refers to the formation of thermal boundary layers near the membrane surface, reducing the effective temperature gradient and limiting the driving force for mass transfer
Strategies to mitigate temperature polarization include increasing , using spacers, or designing membranes with enhanced surface properties
Challenges and Considerations in Membrane Distillation
can occur when liquid water enters the membrane pores, leading to reduced separation efficiency and contamination of the permeate
Factors contributing to pore wetting include high feed pressure, presence of surfactants or organic compounds, and membrane degradation over time
Membrane is another challenge, caused by the deposition of suspended solids, scaling, or biofouling on the membrane surface
Fouling mitigation strategies involve pretreatment of the feed solution, periodic cleaning, and optimization of to minimize foulant accumulation
Membrane Distillation Configurations
Direct Contact and Air Gap Membrane Distillation
Direct contact MD (DCMD) is the simplest configuration, where the feed solution is in direct contact with the hot side of the membrane, and the permeate is in contact with the cold side
DCMD offers high due to the direct heat transfer but suffers from high conductive heat losses
Air gap MD (AGMD) introduces a stagnant air gap between the membrane and the condensing surface to reduce heat losses
AGMD has lower permeate flux compared to DCMD but improved and reduced temperature polarization
Sweeping Gas and Vacuum Membrane Distillation
Sweeping gas MD (SGMD) uses a cold inert gas to sweep the permeate side of the membrane, facilitating the removal of water vapor and maintaining a high driving force
SGMD can achieve high permeate flux and reduce the risk of pore wetting but requires an additional gas stream and may have higher energy consumption
Vacuum MD (VMD) applies a vacuum on the permeate side to create a pressure gradient and enhance the removal of water vapor
VMD can operate at lower feed temperatures and has a lower risk of pore wetting compared to other configurations but requires a vacuum pump and may have higher capital costs
Comparison and Applications of Membrane Distillation Configurations
The choice of MD configuration depends on factors such as the desired permeate quality, energy efficiency, and
DCMD is often used for and concentration of aqueous solutions, while AGMD is suitable for applications requiring higher thermal efficiency (treatment of industrial wastewater)
SGMD and VMD are promising for the treatment of volatile organic compounds and the recovery of valuable components from dilute solutions (pharmaceutical industry, food processing)
Hybrid systems combining MD with other processes (, forward osmosis) can enhance the overall performance and broaden the range of applications