Experimental and Calculated Effectiveness Factors

When the rate is measured for a catalyst pellet and for small particles, and the diffusivity is also measured or predicted, it is possible to obtain both an experimental and a calculated result for rj. For example, for a first-order reaction Eq. (11-67) gives directly. Then the rate measured for the small particles can be used in Eq. (11-66) to obtain k. Provided is known, d) can be evaluated from Eq. (11-50) for a spherical pellet or from Eq. (11-56) for a fiat plate of.catalyst. Then 7caic is obtained from the proper curve in Fig. 11-7. Comparison of the experimental and calculated values is an overall measure of the accuracy of the rate data, effective diffusivity, and the assumption that the intrinsic rate of reaction (or catalyst activity) is the same for the pellet and the small particles. Example 11-8 illustrates the calculations and results for a flat-plate pellet of NiO catalyst, on an alumina carrier, used for the ortho-para-hydrogen conversion. 

Example Tbe pellet reactor is shown in Fig. 11-8. The reaction gases were 

Rates were measured at — 196°C and 1 atm pressure for pellets of three densities. The rate of reaction was also measured for the catalyst in the form of 60-micron (average size) particles. With this small size 1, so that / = 1.0. The rate data and pellet properties are given in Table 11-5. 

CHAPTER 11 REACTION AND DIFFUSION WITHIN POROUS CATALYSTS 

Solution Equation (11-67) is for an irreversible reaction, and so it cannot be used to calculate y p for the ortho-hydrogen reaction. The proper equation is obtained by applying Eq. (11-60) to the pellet. Solving for 77 gives 

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