Liquid entry pressure

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique. The latter technique is also applicable in making PVDF membranes. On the other hand, other hydrophobic MF membranes are made by the TIPS technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned to the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold pressure called liquid entry pressure of water. These membranes can therefore be used for membrane distillation. Tracketching method is applied to make MF membranes from PC. 

This is the minimum hydrostatic pressure that must be applied to the liquid feed solution before it overcomes the hydrophobic forces of the membrane and penetrates into the membrane pores. Liquid entry pressure (LEP) is a characteristic of each membrane and prevents wetting of the membrane pores when it is high. A high LEP may be achieved using a membrane material with high hydrophobicity (i.e., a large water contact angle, CA) and a small maximum pore size. However, as the maximum pore size decreases, the mean pore size of the membrane decreases and the permeability of the membrane becomes low. 

FIGURE 6.3 A schematic diagram of the experimental setup for the gas permeation and liquid entry pressure of water tests. (From Qtaishat, M., Khayet, M., and Matsuura, T., J. Memb. Sci., 341, 139-148, 2009c. With permission.)... 

Figure VI - 51. Liquid entry pressure as a function of the weight fraction of ethanol for a porous polypropylene (Accural) membrane with a pore diameter of 0.1 pm [78].

TABLE 12.1 Commercial Membranes Commonly Used in MD (Membrane Thickness, S Mean Pore Size, Porosity, e Liquid Entry Pressure of Water, LEP ,). 

The pore size range may be from several nanometers to a few micrometers. The pore size distribution (PSD) should be as narrow as possible and the feed liquid should not penetrate into the pores. The liquid entry pressure (LEP), defined as the minimum transmembrane pressure that is required for the feed solution to enter into the pores, by overcoming the hydrophobic forces, should be as high as possible. A high LEP can be achieved by the material of low surface energy (i.e., high hydrophobicity) and small maximum pore size. On the other hand, a small maximum pore size parallels... 

Feng [86] and Feng et al. [87] developed novel nanofibrous membranes for seawater desalination by air-gap MD. The PVDF nanofibrous membranes were characterized by SEM, AFM, and DSC, measurement of LEPw (liquid entry pressure of water), equilibrium contact angle, and particle separation. It was found that the pore size of the PVDF nanofibrous membrane was around 1.5 pm. The equilibrium contact angle of some nanofibrous membranes was above 120°. Feng et al. [86] attempted for the first time to use ENMs for desalination by MD. PDVF nanofibrous membrane could produce potable water (NaCl concentration <280 ppm) from a saline water of NaCl concentration 6 wt% by air-gap MD (AGMD). This new approach may eventually enable the MD process to compete with conventional... 

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