Liquid entry pressure materials

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. 

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... 

For a hydrophobic porous material with contact angle greater than 90°, the APc is >0 and depends on the liquid surface tension and the membrane pore size. As an example, considering water-air-polypropylene system, one can calculate that for a dry membrane with a pore size of 0.03 pim (30 nm) the critical entry pressure of water is more than 300 psi (>20 bar). 

To prevent imbibition of the non-wetting phase, which would destroy the continuous liquid connection needed to apply equation (21.4), the maximum imposed capillary pressure must not exceed the entry pressure for the porous-disc film holder. This pressure can be estimated by Pew = 2a/rpore, where r ore is the pore radius of the porous material used. Therefore, discs with smaller pores are required for higher capillary pressures. Standard porous discs can be found with nominal pore diameters from 50 to 1 pm. Thus, when using a porous frit with pore diameters 1 pm, a typical surfactant solution (a = 30 mN/m) gives Pcmax foam films. Due to low interfacial tensions, this value can be much lower for emulsion films. Special porous... 

EXPLOSIVES. In Ihe conventional sense, a solid, gas. or liquid material which, when triggered, will release a great amount of heal and pressure hy way of a very rapid, self-sustaining exothermic decomposition. This entry does not describe nuclear explosives. 

Of the many rentes of toxic-material entry into humans, inhalation is the most likely to occur and is the rente specified in cnrrent legislation. Toxicity data have been developed for most of the important indnstrial chemicals [19] and form the basis for threshold or allowable concentrations to which persons cam be exposed with no significant physiological effect. Evaluation of toxicity hazards is then based on the duration of exposures to concentrations above these threshold levels. The concentrations of toxic materials in air are primarily functions of vapor pressure and temperature (for toxic liquids) the rate of gas, vapor, mist, or dust release or generation and distance from the source. 

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