Porous membranes pore dimensions

Fig. 4.49a-d. AFM images of the PVDF porous membrane (pore dimension 100 nm) and those of the dense P(VDF-HFP) before (a and b) and after (c and d) the grafting process. Reprinted from [78]. Copyright 2004, with kind permission from Elsevier... 

For the porous membranes the dimension of the pore mainly determines the separation characteristics, the type of membrane material being of crucial importance for chemical, thermal and mechanical stability but not for flux and rejection. On the other hand, for nonporous membranes, the intrinsic properties of the material are mainly responsible for the separation. Some major characteristics of the three basic types are givra below ... 

Synge, RLM, Experiments on Electrical Migration of Peptides and Proteins Inside Porous Membranes Influences of Adsorption, Diffusion, and Pore Dimensions, Biochemical Journal 65, 266,1957. 

When the membrane pore size is reduced further to molecular dimensions, gas species separation can occur by molecular sieving. To separate hydrogen selectively from the other syngas components (CO, C02, CH4, and H20), porous membranes need to be able to discriminate molecules in the 0.3-0.4 nm size with 0.1 nm or less in size difference. 

Molecular sieves are porous aluminosilicates (zeolites) or carbon solids that contain pores of molecular dimensions which can exhibit seleaivity according to the size of the gas molecule. The most extensive study on carbon molecular sieve membranes is the one by Koresh and Soffer (1980,1987). Bird and Trimm (1983) also described the performance of carbon molecular sieve membranes, but they were unable to prepare a continuous membrane. Koresh and Soffer (1980) prepared hollow-fiber carbon molecular sieves, with pores dimensions between 0.3 and 2.0 run radius (see Chapter 2). 

Porous membrane - Membrane made of porous material. When it separates liquid phases its performance depends on the size of the pores and chemical properties of the material. If pore size is much larger than the molecular dimensions, the membrane exerts no influence on transport of individual components of separated liquids and only prevents mixing by convection. For smaller pores it selectively controls transport of species between the phases discriminating them by the size and/or charge. See also - membrane system, - membrane electrode. 

Polyolefins. Low density polyethylene and polypropylene have been developed as sheet and hollow fiber mlcroporous membranes, respectively, for use In plasmapheresis. Polyethylene Is made porous by stretching the annealed film ( ), while polypropylene la made porous by coextruding hollow fibers with a leachable plasticizer. Neither membrane has been prepared with small pore dimensions suitable for protein rejection. These polyolefin membranes are characterized by good chemical stability, but require special surfactant treatments to make them wettable. Their low deformation temperature precludes the use of steam sterilization. Because they are extruded without the usual antl-oxldants and stabilizers, their stability la lower than Injection molding formulations of the same polymer. 

Figure 7. Size scaling of the relative depression ATx/T of the X point of ( He) , in finite systems, according to Eqs. (38a) and (39). o ( He) clusters of radius Ro (Ref. 65) He in vicor glass, pore radius Rq = 35 A (Ref. 159) O He in porous gold, pore radius Ro = 120 A (Ref. 160) A He confined in cylindrical pores (radius do = 400 A) in polymer membrane (Ref. 159) V He in cyhndrical pores (radius do = 150A-1000A) in nucleopore filters (Ref. 192). The confining dimension is L = Ro for spherical clusters or pores, or do for cylindrical pores. The sohd fine corresponds to the size scaling with = 1.7 A and v = 2/3.

The Norton filters included an original design (tube) filter element and an improved design (lumen) filter element. Overall dimensions of both filter elements were 2.1 cm diameter by 40.2 cm length. The tube element consisted of a bundle of 28 porous tubes and the lumen element consisted of a porous cylinder with 19 tubular channels or lumens extending the length of the cylinder. The filter membrane comprised of the inner surface of the tubes or lumens and had a smaller pore diameter than the non-membrane portions of the filter. Inside diameter of both tubes and lumens was 2.8 mm resulting in a total membrane area 0.091-m2 (1 ft2) for the tube filter and 0.060-m2 (0.65 ft2)for the lumen filter. Membrane pore diameter was 0.45 (1m for both filters. 

One important, but often not clearly defined variable in the characterisation of porous membranes, is the shape of the pore or its geometry. In order to relate pore radii to physical equations, several assumptions have to be made about the geometry of the pore. For example, in the Poiseuille equation (see eq. IV 4) the pores are considered to be parallel cylinders, whereas in the Kozeny-Carman equation (eq. IV - 5) the pores are die voids between the close-packed spheres of equal diameter. These models and their corresponding pore geometries are extreme examples in most cases, because such pores do not exist in practice. However, in order to interpret the characterisation results it is often essential to make assumptions about the pore geometry. In addition, it is not the pore size which is the rate-determining factor, but the smallest constriction. Indeed some characterisation techniques determine the dimensions of the pore entrance rather than the pore size. Such techniques often provide better information about permeation related characteristics. 

By considering the flux of gas through porous membranes, a the volume V measured at a pressure jp, passing through the membrane in time t, in relation to the equation governing the flow (19,20), one can define permeability constants of various types, and having various dimensions. The equation of flow for an incompressible fluid in a pore system may be written ... 

Mass transfer of gas through a porous membrane can involve several processes depending on the pore stmcture and the solid [1]. There are four different mechanisms for the transport Poiseuille flow Knndsendiflusion partial condensation/capillaiy diffusion/selective adsorption and molecular sieving [2, 3]. The transport mechanism exhibited by most of carbon membranes is the molecular sieving mechanism as shown in Fig. 2.1. The carbon membranes contain constrictions in the carbon matrix, which approach the molecular dimensions of the absorbing species [4],... 

If the membrane happens to be porous (or microporous) and uncharged, the nature of the liquid-membrane equilibrium will be determined by the relative size of the solute molecules with respect to the pore dimensions in the absence of any specific solute-pore wall interaction. Similar considerations are also valid for liquid-porous sorbent equilibria. If the solute dimensions are at least two orders of magnitude smaller, then the solute concentration in the solution in the pore should be essentially equal to that in the external solution. However, the solute concentration in the porous membrane/porous sorbent/gel will be less than that in the external solution due to the porosity effect. Assuming that the solute exists only in the pores of the memhrane/porous sorbent with a porosity e , the value of kirn should be equal to the membrane or sorbent porosity e if the solute characteristic dimensions are at least two orders of magnitude smaller than the radius of the pore. 

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