Abstract
This study presents a new concept of a simple method for the synthesis of dual layer hydrophilic/hydrophobic composite membranes for membrane distillation (MD). The membranes were prepared of poly(vinylidenefluoride) (PVDF) by phase inversion. The synthesis was realized by allowing a full or partial penetration of the polymer solution through one or two non-woven support (NWS) layers. This was achieved by proper selection of a thin NWS having high stiffness, high porosity and low surface tension, in combination with a runny polymer solution and sufficient time gap between casting and coagulation. The applied preparation method was effective in yielding dual layer composite membranes. The first layer atop the NWS was a hydrophilic or slightly hydrophobic one (contact angle (CA) of 88-92º), while the bottom layer beneath the NWS was highly hydrophobic (CA=132-140º). The difference in surface energy between the top and bottom layers originated from a difference in morphology. A smooth and dense top layer is formed as a result of an instantaneous demixing, while a porous and multi-scale network with some degrees of spherulitical structure was formed on the bottom by a delayed demixing mechanism. Direct contact MD (DCMD) results showed that the obtained flux was comparable to other composite MD membranes with high salt rejection. Membrane alignment inside the MD module is a critical element in determining the membrane performance and is shown to significantly increase flux when a top facing feed configuration is used.
Highlights
Membrane distillation (MD) is a non-isothermal separation process in which a hydrophobic membrane acts as an interface between the hot liquid and a colder permeate side
Four different membranes were prepared in this work; (M1): the polymer solution was cast on a dry non-woven support (NWS) that was placed on a flat metal plate, (M2): the polymer solution was cast on the first dry NWS which was placed atop a second dry NWS, all placed on a flat metal plate, (M3): like M2 but the second NWS was pre-wetted with DMAC, and (M4): like M2 but the second NWS was pre-wetted with ethanol, a weak non-solvent
Dual layer hydrophilic–hydrophobic flat-sheet composite PVDF membranes was prepared using a simple membrane casting process involving a proper selection of NWS material and casting parameters without changing the polymer chemistry
Summary
Membrane distillation (MD) is a non-isothermal separation process in which a hydrophobic membrane acts as an interface between the hot liquid (feed solution) and a colder permeate side. The hot water evaporates at the interface and the vapour (driven by the vapour pressure difference) is forced to cross to the other side of the membrane where it is collected as distillate. Despite its early introduction [1], large-scale application of MD is strongly limited by the lack of MD application-specific membranes that offers both high vapor flux and wetting resistance. Membrane wetting occurs when the feed/permeate penetrates through the membrane pores or when vapor condenses in the pores. An MD module design that facilitates low temperature and concentration polarization is essential [5]. Other MD membrane requirements such as low heat conductivity are desirable
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