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Membrane distillation
Membrane distillation is a thermally driven separational programm in which separation is enabled due to phase change. A hydrophobic membrane displays a barrier for the liquid phase, allowing the vapour phase (e.g. water vapour) pass through the membrane's pores. The driving force of the process is given by a partial vapour pressure difference commonly triggered by a temperature difference.
==Principle of membrane distillation==
State of the art processes that separate mass flows by a membrane, mostly use a static pressure difference as the driving force between the two bounding surfaces (e.g. RO), a difference in concentration (dialysis) or an electric field (ED). Selectivity of a membrane is produced by, either its pore size in relation to the size of the substance to be retained,its diffusion coefficient or electrical polarity. However, the selectivity of membranes used for membrane distillation (MD) is based on the retention of liquid water with-at the same time- 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 to 0.5 µm. 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 liquid phase does not enter the pores because of the high water surface tension. 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=180 – 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 wetting 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.〔Joachim Koschikowski: Entwicklung von energieautark arbeitenden Wasserentsalzungsanlagen auf Basis der Membrandestillation Fraunhofer Verlag, 2011, 3839602602〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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