Concentration polarization factors affecting

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

As a result of these improvements in membrane performance, the major factors determining system performance have become concentration polarization and membrane fouling. All membrane processes are affected by these problems, so... 

Figure 4.7 Concentration polarization modulus ciolcih as a function of the Peclet number Jv8/Di for a range of values of the intrinsic enrichment factor E . Lines calculated through Equation (4.9). This figure shows that components that are enriched by the membrane (E0 > 1) are affected more by concentration polarization than components that are rejected by the membrane (E0 < 1) [13]...

This finding depends on different factors and, as previously described, was partially solved by enhancing the turbulent flow on the membrane surface. In this way, the deposition of the substrate and the catalyst was limited, avoiding the concentration polarization phenomenon that also affects the water flux across the membrane. This aspect is currently under study and some preliminary results obtained in a study on the Gemfibrozil degradation in the described PMR are reported in Table 15.4. 

Factors that affect fouling with NOM-calcium complexes include permeate flux and cross flow rate (see Chapters 3.3 and 9.4). At higher flux though the membrane, the concentration of calcium increases in the concentration polarization boundary layer at the membrane surface, as described above. Lower cross flow rates also increase the concentration of calcium in the boundary layer. The increases concentration of calcium at the membrane surface enhances the fouling of the membranes by the NOM-calcium aggregates.5... 

GPC was used as a major tool to determine molecular size distributions of asphalts and their fractions. However, one must bear in mind that data obtained by GPC do not represent actual, absolute, molecular weight values of asphalt compounds since the system has been calibrated using polystyrene standards. Besides, it is known that factors such as the adsorption of polar compounds on the gel and/or intermolecular associations of polar compounds can affect GPC results. Each factor affects the results in a different way. Adsorption on the gel would result in lower apparent molecular size values, while the association of these compounds would cause earlier elution, giving apparent molecular size values higher than the actual values. The effect of intermolecular associations in asphalts on GPC data, even at concentrations usually considered low enough to destroy all solute-solute associations, has been reported in the literature (22, 23). 

The efficiency of UF in whey processing is limited by a few factors, the most significant of which are concentration polarization and membrane fouling [6,39 1]. While both factors, which adversely affect permeate flux, may be aggravated by protein-protein and membrane-protein interactions [23,40,42-44], they may also be minimized by choosing suitable membrane material and configuration as well as the appropriate process conditions such as TMP, feed velocity or recirculation rate, temperature, and the chemical environment of whey [42,45,46]. 

Concentration polarization is another factor which affects electrode reactions. This occurs due to the difference in concentration of metal ions between the bulk solution and that at the electrode surface. At the anode the concentration of metal ions increases at high current densities because their rate of dissolution is greater than the rate of diffusion away from the metal surface into the bulk solution. Hence the process becomes more difficult and the rate of dissolution tends to decrease. At the cathode the surface concentration of the ions tends to decrease because the rate of diffusion from the bulk of the solution is lower than the rate of discharge at the surface, thus causing the deposition rate to fall. 

Several factors can affect the retention properties of the membrane and some of these will be discussed here. During ultrafiltration the transport of solute to the surface of the filter Is faster than the rate at which permeation through the membrane occurs. This is further complicated, as ultrafiltration progresses, by an increase in the concentration of retained molecules at the membrane. Both events contribute to the phenomenon called concentration polarization. This effectively introduces a second layer of membrane , and as a consequence the retention characteristics of the system are altered. The build-up of solute can be reduced by introducing some form of agitation at the filter surface. However. this procedure does not seem to be effective against the gel-type layers formed by proteins. Various procedures have been suggested to slow down this build-up of solute the solution can be diluted with an appropriate solvent the ultrafiltration process can be interrupted and the flow reversed momentarily a low operation pressure could be used. 

Consideration of Scheme 6 and of (1) leads directly to the hypothesis that the explanation of any factors affecting the stereochemistry of glycosylation reactions can be found in the manner in which these factors influence the equilibrium between the contact and solvent separated ion pairs. For example, polar solvents support charge separation better than nonpolar solvents and so are likely to shift the equilibrium toward the solvent separated ion pair and increase the proportion of a-glycoside formation. The difference in selectivity noted earlier between the use of diethyl ether and dichloromethane as solvent [14], as well as the increased p-selectivity with weaker nucleophiles in toluene solution (see Sect. 2.1) [80, 81], are thus readily understood. The importance of the concentration of the alcohol on selectivity is also apparent from (1) as is the expected influence of the concentration of the triflate counterion. To favor p-mannoside formation it is necessary to shift the contact ion pair-solvent separated ion pair equilibrium as far as possible toward the contact ion pair. However, any factors favoring the contact ion pair over the solvent separated ion pair are also likely to favor the covalent glycosyl triflate over the... 

---