11.3 Membrane distillation: principles and configurations

Membrane distillation 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 feed solution into a permeate and a concentrate
• Relies on a vapor pressure difference across the membrane, which is induced by a temperature gradient between the feed and permeate sides
• Utilizes hydrophobic membranes with pore sizes 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
• Temperature polarization 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 feed flow rate, using spacers, or designing membranes with enhanced surface properties

Challenges and Considerations in Membrane Distillation

• Pore wetting 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 fouling 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 operating conditions 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 permeate flux 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 thermal efficiency 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 scalability
• DCMD is often used for desalination 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 (reverse osmosis, forward osmosis) can enhance the overall performance and broaden the range of applications