Membranes pore size distribution

By defining the optimum fractionation of case I solute and case V solute as the maximum of the value (// — /v)2, the optimum membrane pore size distribution that corresponds to the maximum fractionation can be searched for. For example, the optimum fractionations are represented in Figure 7 by a circular region located at the left-top comer of the fractionation data bank. The necessary pore size distributions for achieving such optimum fractionations were produced because membranes 2, 3, and 4 and experimental data from the membranes fell precisely into the circular region. Thus, the fractionation of case I and case V solutes can be optimized by a proper design of the pore size and pore size distribution by computer analysis and the formation of membranes that possess the calculated pore size distributions. 

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. 

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

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. 

The retention of a single solute is determined solely by the properties of the membrane (pore size distribution, charge, and adsorption characteristics) and by the operating conditions (e.g., pressure, pH and ionic strength of the solution). The retention of individual solutes from a mixture is more complicated. Fractionation may or may not be possible. 

Other major limitations are related to scaling issues due to distorted or poor inlet flow distribution nonidentical membrane pore size distribution, and uneven membrane thickness (Zhou and Tressel, 2006). 

UF membranes, pore size distribution is not sharp as alluded to earlier in the chapter. 

Atomic force microscopy can determine the key properties of synthetic membranes pore size distribution, surface morphology and surface roughness, surface electrical properties, surface adhesion. 

While the sintering parameters have a significant effect on the final membrane pore size distribution and overall porosity, the initial properties of the ceramic particles also strongly influence the properties of the final membrane. In general, the pore size of porous solids is determined mainly by the particle size and the pore shape is governed by the shape of the starting powder (Ishizaki et al., 1998) although additional relationships are summarised in Table 8.2. 

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