Membrane pore size effect

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

The pore size distribution determination is illustrated in Figure 10. Again, the pore size distribution determination method does not result in the actual membrane pore size and pore size determination. It does, however, give a means of comparing different fdter media. A narrow pore size distribution is required for effective filtration and filtration validation. 

Costa, A. R., and De Pinho, M. N. (2005) Effect of membrane pore size and solution chemistry on the ultrafiltration of humic substances solutions. /. Membr. Sci. 255,49-56. 

Selective barrier structure. Transport through porous membranes is possible by viscous flow or diffusion, and the selectivity is based on size exclusion (sieving mechanism). This means that permeability and selectivity are mainly influenced by membrane pore size and the (effective) size of the components ofthe feed Molecules... 

Electrical-charge effects can be further exploited by using charged membranes (as referred to above) to increase retention of all species with like polarity. It is important to mention that it may be possible to exploit electrostatic interactions even for solutes with similar isoelectrical points, due to different charge-pH profiles for the different species present. The membrane pore-size distribution also affects selectivity by altering the solute sieving coefficients locally. Narrow pore-size distributions, especially for electrically charged membranes, will impact very positively on membrane selectivity and overall performance. 

Membrane fouling is primarily a result of membrane-solute interaction that is generally irreversible and requires chemical cleaning to restore the original separation properties of the membrane. Fouling effects can be accenmated or minimized by proper selection of membrane material properties such as hydrophobicity/hydrophUicity or surface charge, as well as membrane pore size. A discussion on the membranes that are typically used in beer processing was included in Section... 

Some authors (7, ) have used measured parameters of solute and solvent transport for calculation of membrane pore size distributions. Difficulties associated with this approach are of both experimental and theoretical nature. The experiments need to be carried out under conditions that minimize or eliminate effects of boundary phenomena (polarization) and of solute adsorption (fouling) on the measured coefficients. This is rarely done. An even more serious obstacle in this approach is the absence of quantitative and valid relations between measured transport parameters and the size parameters of a "representative pore."... 

Figure 4. Effect of membrane pore-size distribution. Rs. is calculated as a function of Am and

Pretreatment is often used to reduce fouling. Methods include heating, pH adjustment, chlorination, activated-carbon sorption, or chemical precipitation. Other factors such as membrane pore-size distribution, hydrophilicity/hydrophobicity, or surface charge can also reduce the effects of fouling. Methods which reduce concentration polarization, such as using higher axial flow velocities, lower flux membranes, or turbulence promoters, also help to reduce fouling. 

D. Freilich and G.B. Tanny, Hydrodynamic and microporous support pore size effects on the properties and structure of dynamically formed hydrous Zr(IV)-polyacrylate membranes, Desalination, 1978, 27, 233-251 A.J. van Reenen and R.D. Sanderson, Dynamically formed hydrous zirconium(IV) oxide-polyelectrolyte membranes, VI. Effect of copolymer composition on the stability of poly(acrylic acid - covinyl acetate) and poly(acrylic acid - covinyl alcohol) membranes, Desalination, 1989, 72, 329-338. 

Sethi and Wiesner (1997) predicted a most unfavourable size of 0.4 pm where the backtransport, considering all mechanisms, was at a minimum and thus the resulting flux lowest. When also considering cake permeability as a function of particle size, this minimum shifted to 0.01 - 0.1 pm. The dependence on membrane pore size and initial flux was not addressed in this study. Chellam and Wiesner (1998) pointed out the complicating effects of polydispersity on the use of such models. Most natural systems are polydisperse. 

Critical Flux or Pressure and Effect of Membrane Pore Size ... 

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