Catalyst grain size

Experiments at different flow rates and with difierent catalyst grain sizes confirmed that the reaction kinetics is not influenced by external or internal mass transfer. Catechol conversions (X) were always less than 0.05 allowing the reaction to be carried out in the differential kinetic region. The initial yields (Yi,o) for the monomethylated isomers were measured under steady-state conditions (after 8-10 hours of the catalyst activity stabilisation) and were used to compare the catalysts selectivities ... 

Reaction Rate as a Function of Catalyst Grain Size... 

Preliminary ains carried out at different catalyst loading, stirring rate and catalyst grain size have shown the absence of diffusional limitations. 

Table 10.4. Effect of alloying on Pt catalyst grain size and ECSA. Weight percent of the metals is calculated from reagent concentrations. Pure metal lattice parameters flo(Pt) = 3.9231 A flo(Ni) = 3.5238 A ao(Co) = 3.5447 A [144]. (Reproduced from Journal of Electroanal54ical Chemistry, 504(1), Shukla AK, Neergat M, Bera P, Jayaram V, Hegde MS. An XPS study on hinary and ternary alloys of transition metals with platinized carhon and its hearing upon oxygen electroreduction in direct methanol fuel cells, 111-9, 2001, with permission from Elsevier.)...

Inspection of Fig. 15.3 reveals that while for jo 0.1 nAcm , the effectiveness factor is expected to be close to 1, for a faster reaction with Jo 1 p,A cm , it will drop to about 0.2. This is the case of internal diffusion limitation, well known in heterogeneous catalysis, when the reagent concentration at the outer surface of the catalyst grains is equal to its volume concentration, but drops sharply inside the pores of the catalyst. In this context, it should be pointed out that when the pore size is decreased below about 50 nm, the predominant mechanism of mass transport is Knudsen diffusion [Malek and Coppens, 2003], with the diffusion coefficient being less than the Pick diffusion coefficient and dependent on the porosity and pore stmcture. Moreover, the discrete distribution of the catalytic particles in the CL may also affect the measured current owing to overlap of diffusion zones around closely positioned particles [Antoine et ah, 1998]. 

NO, N02 and 02 were used. The catalysts, with grain size 180-355 0.m, were loaded to the reactor and in situ pre-treated in a helium flow at 550°C for 30 min. Catalytic tests were carried out at 30,000 h 1 GHSV in the range 300-700°C. Feed gas composition in dry conditions 1000 ppm NO, 100 ppm N02, 1000 ppm CH4, 2.5% 02, balance He and in wet conditions equivalent to dry plus 1% H20, saturating with water balancing the He stream. 

The pellets of the commercial catalyst were crushed to grain size from 0.5 to 1 mm. A calculation on the basis of the measurements of the effective diffusion coefficient showed that the reaction proceeded in the kinetic region. Bed density of the catalyst was 1.23 g/cm3, specific surface after kinetic experiments was 36 m2/g. In the temperature range of 150-225°C reaction (342) is practically irreversible. The experiments proved (348) to be valid thus, the kinetics on low- and high-temperature catalysts is the same. 

The experiments were done at 70, 100, and 130°C and at pressures somewhat lower than atmospheric. Under these conditions reaction (368) is practically irreversible. Activated charcoal of the trademark Bayer AKT-4 ground to grain size 0.25-0.5 mm served as a catalyst. Estimation of the efficiency factor on the basis of the determination of the effective difusion coefficient of hydrogen in nitrogen or helium has shown that for this grain size the results of reaction rate measurements refer to the kinetic region. Estimation of relaxation time of the reaction rate from (67) showed the reaction to be quasi-steady at the condition of our experiments in the closed system. 

Using this model, Agrawal (1980) computed the initial demetallation rate as a function of micropore radius and microsphere radius (grain size) for the bimodal catalyst. The results are shown in Fig. 60 for a typical set of... 

Fig. 60. Initial HDM reaction rate versus micropore radius and grain size at a fixed porosity for the macroporous catalyst (Agrawal, 1980).

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

Utsunomiya et al. [11,12] reported an accumulation-type CTL-based sensor based on this mechanism. The alumina powder (y-Al2()3, 300 mesh in grain size, 30 mg) filled in a glazed ceramic pot (5 mm in diameter) including an electric heater wire was used as a sensor catalyst. First, the catalyst was heated to 500 °C to remove previous adsorbates, then was quenched to room temperature and allowed to adsorb the sample gas for a certain time. 

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