Mass transfer resistance, reduction

Some deactivation processes lower the number of active sites So- Others add mass transfer resistances. In either case, they cause a reduction in the reaction rate that is reflected in a time-dependent effectiveness factor ... 

With polymeric phases containing a relatively thick silicon rubber layer inside the pores not only the reduction in pore diameter available for solute diffusion but also the mass transfer resistance in the alkyl polysi-... 

Ahn et al. have developed fibre-based composite electrode structures suitable for oxygen reduction in fuel cell cathodes (containing high electrochemically active surface areas and high void volumes) [22], The impedance data obtained at -450 mV (vs. SCE), in the linear region of the polarization curves, are shown in Figure 6.22. Ohmic, kinetic, and mass transfer resistances were determined by fitting the impedance spectra with an appropriate equivalent circuit model. 

The results also suggest that through AC impedance measurements, the performance drops caused by individual processes such as electrode kinetic resistance, membrane resistance, and mass transfer resistance can be correlated to either reduction or improvement in cell performance. If individual impedances are known, the contribution to the change in performance can be identified, which is very important in the design and optimization of high-temperature MEA catalyst layer components, structure down-selection, and MEA architecture. 

The consequences of this reduction in mass transfer may not be too serious, however, in applications such as hydroprocessing of oils. Under the usual trickle-flow conditions at which these processes are now operated, the contribution of gasAiquid mass transfer to the overall mass transfer resistance is generally negligible and considerably smaller... 

Internal Phase Droplet Diameter Drop size increases with increasing volume [34,41]. However, increasing the drop size will reduce the thickness of the membrane phase and lead to a lower mass transfer resistance. Optimization is a trade-off between interfacial area reduction and the increase in the mass transfer rate because of the reduced membrane thickness and thus small mass transfer resistance [107]. 

By contrast, when the mass transfer resistances and/or axial dispersion are considered, there is no analytical solution for an SMB operated under nonlinear isotherm conditions. A numerical solution of the applicable mathematical model must be used instead to calculate the performance of the SMB, to simulate the influence of the various design and operating parameters, and to search for the optimum flow rates and switching time that give the desired results. In this quest, the selection as a starting point of the optimum set of flow rates and switching time derived from the equilibrium theory permits a considerable reduction of the number of calculations. As discussed earlier by Ruthven and Ching [27], four... 

A novel approach to increase the overall extraction yield of a microbial product has been developed. The primary isolation steps of fermentation derived microbial products generally involve procedures including solid removal, wash and volume reduction using either solvent extraction or ion-exchange. These steps can be further simplified by the use of whole broth extraction. In the case of using solid phase adsorbents, extraction performance can be drastically affected by the size and the nature of the adsorbent used. If the adsorbent size is large, it exhibits internal diffusional resistance (pore diffusion) and the resultant mass transfer resistance reduces the overall rate of adsorption... 

In Fig. 9 three orthogonal slices through the reconstruction of the XVUSY crystal are displayed. The x-z projection shows a cylindrical mesopore that connects the interior of the crystal with the outside world . For one and the same mesopore marked with a white arrow in all three projections it is clear that no connection to the external surface via the mesopore network exists. In other words, this mesopore is a cavity in the crystal and will hardly contribute to reduction of mass transfer resistance. From independent measurements based on physisorption and mercury intrusion [29] it has been found that 30% of the mesopore volume in this material is present in cavities that are connected to the external surface only via micropores. More recently elegant proof from thermoporometry experiments for the existence of these cavities has been published [31]. 

Similar reactor configurations using flat membranes in place of hollow fibers have even been used with urease, uricase, glucose oxidase and creatinine kinase.49 An enzyme solution is, in this case, introduced into one of the membrane-separated chambers of a flat-membrane dialyzer. Operating the reactor at high feed flow rates leads to a reduction of mass transfer resistances, which can also be achieved using suitable turbulence promoters. The overall behavior of such systems appears to be similar to that of the hollow fiber enzyme reactors, except for an apparent higher efficiency.49... 

Figure 12.5 depicts schematically the gas- and aqueous-phase concentrations of A in and around a droplet. The aqueous-phase concentrations have been scaled by HART, to remove the difference in the units of the two concentrations. This scaling implies that the two concentration profiles should meet at the interface if the system satisfies at that point Henry s law. In the ideal case, described by (12.45), the concentration profile after the scaling should be constant for any r. However, in the general case the gas-phase mass transfer resistance results in a drop of the concentration from cA(oo) to cA(Rp) at the air-droplet interface. The interface resistance to mass transfer may also cause deviations from Henry s law equilibrium indicated in Figure 12.5 by a discontinuity. Finally, aqueous-phase transport limitations may result in a profile of the concentration of A in the aqueous phase from [A(/ ,)J at the droplet surface to [A(0)] at the center. All these mass transfer limitations, even if the system can reach a pseudo-steady state, result in reductions of the concentration of A inside the droplet, and slow down the aqueous-phase chemical reactions. 

Inspection of Figure 7.17 indicates that we expect a significant reduction in the effectiveness factor due to mass-transfer resistance in the first case, and little effect in the second case. Evaluating the effectiveness factors with Equation 7.65 indeed shows... 

There has been some discussion on the permeation process being controlled by surface reaction for thin membranes in the literature. In this case, the reduction of the membrane thickness will not increase the hydrogen permeation fluxes. The same phenomena will occur when the permeation process is controlled by one or more of the mass transfer resistances, including the formation of a boundary layer between the bulk gas phase and the membrane surface and flow through the support pores by Knudsen diffusion and viscous flow. There are only a few studies in the literature addressing the existence of these resistances. The reduction of these resistances is critical for the membrane to achieve its maximum permeation flux. 

The overall negative effect on membrane performances is that the permeation driving force between the membrane surfaces of upstream and downstream is lower than that between the respective bulks, causing a consequent reduction of flux. A significant presence of concentration polarization can make the advantages of preparing very thin membranes partially useless, because the mass transfer resistance tends to be located in the gas phase external to the membrane rather than in the membrane itself. 

These definitions, formally different with respect to those reported in eqn (14.10), approach each other in the case of negligible inhibition. This approach is very useful because, with this choice, CPC is directly related to the flux reduction due only to external mass transfer resistance and not to the inhibition phenomenon. Moreover, the polarization and inhibition effect are able to be separately identified and split into their own different contributions, which can thus be analyzed to provide a better understanding of the coupled influence of these two phenomena. This is done by defining another coefficient, i.e. the inhibition coefficient, IC, which is on the other hand a quantitative indicator of the inhibition phenomenon only (see next section). 

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