Gradient of catalyst loading

In this section, it is constant however, this is not necessariiy the case. In Section 2.6 we will see that the gradient of catalyst loading is beneficial for CL performance. 

Gradient of catalyst loading along the oxygen channel... 

Schematic diagram of the catalyst loading gradient through plane (left) and in plane (right).

Cho et al. [140] examined the performance of PEM fuel cells fabricated using different catalyst loadings (20, 40, and 60 wt% on a carbon support). The best performance—742 mA/cm at a cell voltage of 0.6 V— was achieved using 40 wt% Pt/C in both anode and cathode. Antonie et al. [28] studied the effect of catalyst gradients on CL performances using both experimental and modeling approaches. Optimal catalyst utilization could also be achieved when a preferential location of Pt nanoparticles was close to the PEM side ... 

Liquid holdup is defined as the volume of liquid contained in the bed per unit bed volume. It is a function of the physical properties of the fluid phases and the bed characteristics. It is a basic parameter for reactor design, because it is related to other important parameters, namely, pressure gradient, gas-liquid interfacial area, the mean residence time of the liquid phase, catalyst loading per unit volume, axial dispersion coefficient, mass transfer characteristics, and heat transfer coefficient at the wall, etc. The optimal value of liquid holdup is desirable for better performance of TBR as a high value of liquid holdup will increase mass transfer resistance while too low a value of liquid holdup will decrease the proper utilization of the catalyst bed. Sometimes, the term total liquid saturation (j t) is used to describe the amount of liquid in the bed. It is defined as the volume of liquid present in a unit void volume of the reactor. Thus, the liquid holdup and total liquid saturation are related as ... 

A comparison is shown in Table 11-4 among various t3rpes of reactors of heat transfer, estimated heat-transfer coefficients, and calculated average temperature gradients between catalyst and cooling medium or surface. For each reactor, the heat load is based on a feed-gas flow typidal for that type of operation. For example, the early fixed-bed units employed space velocities of about 100, while the fluid- and hot-gas-recyde systems are based on a space velocity of 1,000. Conversion of 90 per cent of the gas is assumed in all cases, with a heat evolution of 70 Btu per cu ft of converted gas. 

The calculated influence of pore diffusion on the coke bum-off (Fig. 4) is also reflected by the measured carbon distribution over the particle cross section for catalyst samples, which were regenerated at different temperatures up to a defined bum-off degree of about 55%. Fig. 5 shows that a pronounced gradient of the carbon load over the particle cross section for a bum-off temperature of 530°C is developed. For a temperature of430°C no gradient is determined. The rate of the oxygen conversion is then so slow, that the diffusion in the pores has no influence and the coke is uniformly burned within the particle according to the (intrinsic) rate of the chemical reaction. Both results are consistent with the kinetic measurements und calculations (see Fig. 4). 

Selectivity, which is one of the most important characteristics of an industrial process, depends on several parameters temperature control, residence time distribution, gas and liquid hold-ups, catalyst loading, catalyst type, mass transfer rates etc... If homogeneous side reactions are awkward, fixed beds give better results.But if the desired product can react further on the catalyst, small catalyst particles have to be preferred to avoid concentration gradients in the pores and slurry reactors are the best. In this last case, the poor residence time distri-... 

A non-uniform electrode was introduced in 1989 [73]. In this electrode, the electrocatalyst concentration increased along the direction parallel to the electrode substrate. It is expected that the effect of increased catalyst loading in the direction of gas flow could balance the effect of diminishing reactants in the gas stream. In this way, the reaction rate could be substantially uniform across the electrode surface. Prasanna et al. [74] employed the catalyst-gradient method for single fuel cell fabrication, and found that it was an effective way to reduce Pt loading without... 

As expected, the influence of pore diffusion on the rate of coke combustion increases with temperature, and gradients of the oxygen concentration and carbon load occur. This effect is pronounced at a very high temperature of 650 °C (Figure 6.9.12b), although 650°C is unrealistic for a technical regeneration (T ,ax of the catalyst is about 550 °C), but is shown here as an instructive comparison. A relatively sharp reaction front within the particle is then formed, which migrates from... 

Calculations show that the effect of CL performance optimization by the gradient catalyst loading strongly depends on jo. To characterize the effect, the optimization factor kept is introduced, defined as the ratio of cell currents at optimal and uniform loadings... 

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