Membrane distillation (MD) is a thermally driven separation process in which separation is driven by phase change. A hydrophobic membrane presents a barrier for the liquid phase, allowing the vapour phase (e.g. water vapour) to pass through the membrane's pores. The driving force of the process is a partial vapour pressure difference commonly triggered by a temperature difference.
Principle of membrane distillation
Most processes that use a membrane to separate materials rely on static pressure difference as the driving force between the two bounding surfaces (e.g. reverse osmosis - RO), or a difference in concentration (dialysis), or an electric field (ED). The selectivity of a membrane can be due to the relation of the pore size to the size of the substance being retained, or its diffusion coefficient, or its electrical polarity. Membranes used for membrane distillation (MD) inhibit passage of liquid water while allowing permeability for free water molecules and thus, for water vapour. These membranes are made of hydrophobic synthetic material (e.g. PTFE, PVDF or PP) and offer pores with a standard diameter between 0.1 and 0.5 μm (3.9×10−6 and 1.97×10−5 in). As water has strong dipole characteristics, whilst the membrane fabric is non-polar, the membrane material is not wetted by the liquid. Even though the pores are considerably larger than the molecules, the high water surface tension prevents the liquid phase from entering the pores. A convex meniscus develops into the pore. This effect is named capillary action. Amongst other factors, the depth of impression can depend on the external pressure load on the liquid. A dimension for the infiltration of the pores by the liquid is the contact angle Θ=90 – Θ'. As long as Θ < 90° and accordingly Θ' > 0° no wetting of the pores will take place. If the external pressure rises above the so-called liquid entry pressure, then Θ = 90°resulting in a bypass of the pore. The driving force which delivers the vapour through the membrane, in order to collect it on the permeate side as product water, is the partial water vapour pressure difference between the two bounding surfaces. This partial pressure difference is the result of a temperature difference between the two bounding surfaces. As can be seen in the image, the membrane is charged with a hot feed flow on one side and a cooled permeate flow on the other side. The temperature difference through the membrane, usually between 5 and 20 K, conveys a partial pressure difference which ensures that the vapour developing at the membrane surface follows the pressure drop, permeating through the pores and condensing on the cooler side.
Membrane distillation techniques
Many different membrane distillation techniques exist. The basic four techniques mainly differ by the arrangement of their distillate channel or the manner in which this channel is operated. The following technologies are most common:
Direct Contact MD (DCMD) Air Gap MD (AGMD) Vacuum MD (VMD) Sweeping Gas MD (SWGMD) Vacuum multi-effect membrane distillation (V-MEMD) Permeate Gap MD (PGMD)
Direct-contact MD In DCMD, both sides of the membrane are charged with liquid- hot feed water on the evaporator side and cooled permeate on the permeate side. The condensation of the vapour passing through the membrane happens directly inside the liquid phase at the membrane boundary surface. Since the membrane is the only barrier blocking the mass transport, relatively high surface related permeate flows can be achieved with DCMD. A disadvantage is the high sensible heat loss, as the insulating properties of the single membrane layer are low. However, a high heat loss between evaporator and condenser is also the result of the single membrane layer. This lost heat is not available to the distillation process, thus lowering the efficiency. Unlike other configurations of membrane distillation, in DCMD the cooling across the membrane is provided by permeate flow rather than feed preheating. Therefore, an external heat exchanger is also needed to recover heat from the permeate, and the high flow rate of the feed must be carefully optimized.
Air-gap MD
In air-gap MD, the evaporator channel resembles that in DCMD, whereas the permeate gap lies between the membrane and a cooled walling and is filled with air. The vapour passing through the membrane must additionally overcome this air gap before condensing on the cooler surface. The advantage of this method is the high thermal insulation towards the condenser channel, thus minimizing heat conduction losses. However, the disadvantage is that the air gap represents an additional barrier for mass transport, reducing the surface- related permeate output compared to DCMD. A further advantage over DCMD is that volatile substances with a low surface tension such as alcohol or other solvents can be separated from diluted solutions, due to the fact that there is no contact between the liquid permeate and the membrane with AGMD. AGMD is especially advantageous compared to alternatives at higher salinity. Variations on AGMD can include hydrophobic condensing surfaces or porous condensers for improved flux and energy efficiency. In AGMD, uniquely important design features include gap thickness, condensing surface hydrophobicity, gap spacer design, and tilt angle.
Sweeping-gas MD
Sweeping-gas MD, also known as air stripping, uses a channel configuration with an empty gap on the permeate side. This configuration is the same as in AGMD. Condensation of the vapour takes place outside the MD module in an external condenser. As with AGMD, volatile substances with a low surface tension can be distilled with this process. The advantage of SWGMD over AGMD is the significant reduction of the barrier to the mass transport through forced flow. Hereby higher surface-related productwater mass flows can be achieved than with AGMD. A disadvantage of SWGMD caused by the gas component and therefore the higher total mass flow, is the necessity of a higher condenser capacity. When using smaller gas mass flows there is a risk of the gas heating itself at the hot membrane surface, thus reducing the vapour pressure difference and therefore the driving force. One solution of this problem for SWGMD and for AGMD is the use of a cooled walling for the permeate channel, and maintaining temperature by flushing it with gas.
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![Membrane distillation: Droplet condensation regimes seen in AGMD.[11][12]](https://upload.wikimedia.org/wikipedia/commons/1/1b/Air_gap_membrane_distillation_configuration_with_different_permeate_droplet_condensation_regimes.png?utm_source=en.wikipedia.org&utm_campaign=parser&utm_content=thumbnail_unscaled)
