Transport resistance, mass

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

Rates and selectivities of soHd catalyzed reactions can also be influenced by mass transport resistance in the external fluid phase. Most reactions are not influenced by external-phase transport, but the rates of some very fast reactions, eg, ammonia oxidation, are deterrnined solely by the resistance to this transport. As the resistance to mass transport within the catalyst pores is larger than that in the external fluid phase, the effectiveness factor of a porous catalyst is expected to be less than unity whenever the external-phase mass transport resistance is significant, A practical catalyst that is used under such circumstances is the ammonia oxidation catalyst. It is a nonporous metal and consists of layers of wire woven into a mesh. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Since it is assumed that the only limiting resistance to moisture uptake is mass transport resistance, the basis for the model is contained with Eq. (39). It is assumed that the system is at steady state and that rectangular coordinates (uptake in one dimension) are appropriate. Since the system is at steady state and we are dealing with transport in one direction, the flux into a volume element must be equal to the flux out of that element. This condition is expressed as... 

By integrating Eq. (46) and applying the boundary conditions, the solution for the total moisture uptake limited by mass transport is found. In the solution shown in Eq. (47) the vapor pressures have been converted to relative humidities and it has been assumed that the partial pressure of water is much less than the total pressure. Under these conditions, Eq. (47) is the solution for mass transport resistance in spherical coordinates. As with transport in rectangular coordinates, the important variables are the partial pressures of the chamber and above the solid surface and the distance between the solid surface and chamber wall. 

Issues with mass transport resistance, especially at higher current densities, represent an important hurdle that fuel cells need to overcome to achieve the required efficiencies and power densifies that different applications require. Diffusion layers represenf one of fhe major fuel cell components that have a direct impact on these mass transport issues thus, optimization of the DLs is required through the use of differenf experimental and characterization techniques. 

After treating different fuel cells to 100 freeze-thaw cycles (from -40 to 70°C), Kim, Ahn, and Mench [261] concluded that stiffer materials used as diffusion layers improved the uniform compression with the CL, resulting in fewer issues after the freeze and thaw cycles. On the other hand, more flexible DLs failed to improve the compression the CL left open spaces for ice films to be formed, resulting in serious issues after the freeze-thaw cycles. However, even with the stiffer materials tested, such ice films were still evident and caused delamination of the DL and CL, surface damage in the CL, and breakage of the carbon fibers. This resulted in increased electrical and mass transport resistances. 

The resulting values are shown in Table 4. As expected, the diffusion coefficients of prenal and citral are smaller in water than in n-hexane. Since the mass transport coefficients in each boundary layer directly correlate with the diffusion coefficient (Eq. 3), this result confirms the assumption that the overall mass transport resistance can be predominantly referred to the aqueous catalyst phase ... 

As the pore size decreases, molecules collide more often with the pore walls than with each other. This movement, intermediated by these molecule—pore-wall interactions, is known as Knudsen diffusion. Some models have begun to take this form of diffusion into account. In this type of diffusion, the diffusion coefficient is a direct function of the pore radius. In the models, Knudsen diffusion and Stefan—Maxwell diffusion are treated as mass-transport resistances in seriesand are combined to yield... 

Under fixed experimental conditions, the rate of a gas-carbon reaction (rate of removal of carbon atoms from the surface) is dependent upon the reacting gas and the nature of the carbon. A discussion of the effect of the nature of the carbon on particular gas-carbon reactions is postponed until Sec. VII. In this section, existing data on the relative rates of gas-carbon reactions, where an investigator has reacted the same carbon, is presented. Results of primary interest are those where the reaction rates are not affected by mass transport resistance. 

The main mass transport resistance in liquid fluidized beds of relatively small particles lies in the liquid film. Thus, for ion exchange and adsorption on small particles, the mass transfer limitation provides a simple liquid-film diffusion-controlled mass transfer process (Hausmann el al., 2000 Menoud et al., 1998). The same holds for catalysis. 

Fig. 7.8 Origins of selectivity of amperometric sensors based on charge-transfer resistance and mass transport resistance...

Calibration is necessary to allow correlation between collected dialysis concentrations to external sample concentrations surrounding the microdialysis probe. Extraction efficiency (EE) is used to relate the dialysis concentration to the sample concentration. The steady-state EE equation is shown in equation (6.1), where Coutiet is the analyte concentration exiting the microdialysis probe, Ci iet is the analyte concentration entering the microdialysis probe, CtiSSue> is the analyte tissue concentration far away from the probe, Qd is the perfusion fluid flow rate and Rd, Rm, Re, and Rt are a series of mass transport resistances for the dialysate, membrane, external... 

For any implanted device that is intended for long-term use, collagen encapsulation of the device will occur due to the immune response to the device.60 Collagen deposition typically begins at roughly 5-7 days postimplantation and takes up to a month to reach completion. In terms of calibration of the microdialysis probe, this layer of material will provide additional mass transport resistance and could be denoted as a trauma layer. 

When a species is transferred from a phase to another phase by means of a membrane contactor, the mass-transport resistances involved are those offered by the two phases and that of the membrane (see Figure 20.5). The overall mass-transfer coefficient will, therefore, depend on the mass-transfer coefficient of the two phases and of the membrane. 

Figure 20.5 Mass-transport resistances involved in a membrane contactor.

To model a porous electrocatalyst we may consider a second type of mass transport (in addition to diffusion) locally within the electrode, i.e., a mass transport resistance between the electrode surface and the solution. This situation may arise, for example, when the electrode surface is covered by a thin layer of polymer electrolyte or as in a fuel cell electrode in which the electrocatalyst is also covered by a thin water layer. 

Figure 13 shows the potential and concentration distributions for different values of dimensionless potential under conditions when internal pore diffusion (s = 0.1) and local mass transport (y = 10) are a factor. As expected the concentration and relative overpotential decrease further away from the free electrolyte (or membrane) due to the combined effect of diffusion mass transport and the poor penetration of current into the electrode due to ionic conductivity limitations. The major difference in the data is with respect to the variation in reactant concentrations. In the case when an internal mass transport resistance occurs (y = 10) the fall in concentration, at a fixed value of electrode overpotential, is not as great as the case when no internal mass transport resistance occurs. This is due to the resistance causing a reduction in the consumption of reactant locally, and thereby increasing available reactant concentration the effect of which is more significant at higher electrode overpotentials. 

Figure 14 shows the distributions removing the influence of the internal mass transport resistance on the current distribution, i.e., y is very high (model Eqs. 117 and 118 apply). In this case much higher local current densities are achieved, although the problem of a non-uniform distribution in current density prevails, with as before much of the electrode not very active. 

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