Pore size membrane

The membranes used and their characteristics such as salt rejection and marker test results as supplied by the manufacturer were described in Chapter 4. Since test conditions are different for the three membranes, here a characterisation is carried out to achieve comparable results. In summary, the CA-UF membrane has a cut-off of 5 kDa, whereas the TFC membranes rejected 90% of lactose. This results in pore diameters (as calculated after Worch (1993) and the Stokes Einstein equation) of 3.72 nm and 0.64 nm for the CA-UF and TFC membranes, respectively. 

The aim of the pore size measurement was not to produce an absolute value for pore size or molecular weight cut off, but rather to determine which of the four membranes is more open, A dextran 1000 standard was chosen and rejection experiments were carried out at a dextran concentration of about 50 mgL and pH 8. The feed DOC was 19.4 mgL k Dextran was chosen as it is not expected to interact strongly with membrane material (Combe et al. (1999)). 

The results in Table 7.4 show that in terms of dextran rejection, the TFC-SR membrane is the tightest membrane, followed by the TFC-S. The TFC-ULP membrane is also reasonably tight (it should be noted here that the flux of the TFC-ULP membrane is considerably lower which could cause a decreased rejection), whereas the CA-UF membrane is clearly a UF membrane with a very low dextran rejection. 

It was shown in Chapter 4, that the organics size, determined by size exclusion chromatography (SEC), was 1200, 1800, and 3000 Da for NOM, IHSS FA, and IHSS HA, respectively. The rejection of these organics would thus be expected to be greater than that of dextran 1000. The radius of a dextran 1000 molecule was estimated to be about 0.94 nm (see also Table 7.20). 

The CA-UF membrane has a verj low rejection. This membrane was classified as a NF membrane by the manufacturer, due to its high colour removal and some salt rejection. 

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If the solute size is approximately the (apparent) membrane-pore size, it interferes with the pore dimensions. The solute concentration in the permeate first increases, then decreases with time. The point of maximum interference is further characterized as a minimum flux. Figure 4 is a plot of retention and flux versus molecular weight. It shows the minimum flux at ca 60—90% retention. 

This deposit is composed of suspended particles similar to conventional filter cakes, and more importantly, a slime that forms as retained solutes exceed their solubility. The gel concentration 6 is a function of the feed composition and the membrane-pore size. The gel usually has a much lower hydrauHc permeabihty and smaller apparent pore size than the underlying membrane (27). The gel layer and the concentration gradient between the gel layer and the bulk concentration are called the gel-polarization layer. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

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. 

As RO membranes become looser their salt rejection falls (see Section 31.8.1). Eventually a point is reached at which there is no rejection of salts, but the membrane still rejects particulates, colloids and very large molecules. The membrane pore size can be tailored to a nominal molecular weight cut-off. The resulting filtering process is called ultra-filtration. 

Hyperfiltration (Reverse Osmosis) is a form of membrane distillation or desalination (desalting) operating with membrane pore sizes of perhaps 1 to 10 Angstrom units. The various individual RO component technologies have improved tremendously over the last 20 to 25 years, and resistance to fouling and permeate output rates have benefited. Nevertheless, all RO plants remain susceptible to the risk of fouling, and adequate pretreatment and operation is essential to minimize this problem. 

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. 

Matrixes Three types of paper filters (i). Filtrak, Germany, the filter Number 388 [(0.025) - soft, wide pores] filters No. 90 (0.15) and No. 90 (0.25) - dense, narrow pores) (ii). chromatographic papers FN-5, FN-11 (Germany) the kapron membrane (pore size 0.2 microns) and (iii). the membrane Vladipor MFA-MA No. 6 (Kazan industrial associations Tasma )] were applied to matrixes for biotests. The circles (6 -10 mm dia) are cut out from the matrixes materials by special instrument. The most stable results are received with use of matrixes of chromatographic papers FN-5, FN-11. 

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. 

With appropriate membrane pore size and a narrow distribution, membrane selectivity for smaller gas molecules can be high but the overall permeability is generally low due to a high flow resistance in fine pores. Several studies are being conducted to develop molecular sieve-type membranes using different inorganic materials, for example, those based on carbon (Liu, 2007), silica (Pex and van Delft, 2005), and zeolites (Lin, 2007). 

A choice of a filter with the correct membrane pore size can prevent the shift in polish rate. The pore size chosen must be significantly larger than the slurry particle sizes. For example, if the mean of the slurry particle size is 1000 A, the pore size should be a few microns, so that no normal sized particles will be filtered out. Frequent filter changes can prevent filter clogging. 

Microporous membranes - pore size in these membranes ranges from 50 to 200 A. In this case, the pores are usually only slightly larger than the solutes and this results in hindered transport through the pores. 

Carrier-impregnated PTFE type membrane Pore size 0.5 pm Diameter 47mm... 

The so-called bubble point of a membrane - a measure ofthe membrane pore size - can be determined by using standard apparatus. When determining the bubble point of small, disk-shaped membrane samples (47 mm in diameter), the membrane is supported from above by a screen. The disk is then flooded with a liquid, so that a pool of liquid is left on top. Air is then slowly introduced from below, and the pressure increased in a stepwise manner. When the first steady stream of bubbles to emerge from the membrane is observed, that pressure is termed the bubble point. ... 

The membrane pore size can be calculated from the measured bubble point Pj, by using the dimensionally consistent Equation 10.9. This is based on a simphstic model (Figure 10.6) that equates the air pressure in the cyhndrical pore to the cosine vector of the surface tension force along the pore surface [6] ... 

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. 

If the solute size is approximately the (apparent) membrane-pore size, it interferes with the pore dimensions. The solute concentration in the... 

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. 

In this book, an explanation of capacitive behaviour in similar and comparable systems is not directly possible with constant-phase elements because such a comparison is only possible if n values are equal, particularly in the study of surfaces covered with polymer coatings where a unification of the envisaged parameters is necessary. The impedances measured match with a relatively large amount of samples, of which the structure can be complex, showing many sources of non-idealities (e.g. variations in thickness of the membrane, pore size and pore density42 7). A good indication if such non-idealities occur can be found in the values of n. If they are not comparable, non-idealities occur. 

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. 

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