Rate-Limiting Phenomena on the Industrial Scale

The rate expressions described in the preceding section were derived on the assumption that surface processes are the only steps that can limit the reaction rate. In order to obtain the intrinsic or surface rate equation, the primary requirement in kinetic measurements is the collection of experimental data under 

These conditions are not achieved at the industrial scale. Therefore, when laboratory data are translated to industrial conditions, any deviating effect and nonideal condition pertaining to the converter under consideration must be identified and quantified. In other words, the intrinsic rate constants determined in the laboratory must be combined with appropriate corrective factors for proper scale-up, design, and simulation of any industrial converter. In this section an attempt will be made to summarize the concepts and calculations that are necessary to perform this translation successfully. 

Flow Maldistribution in Catalyst Beds. Deviations from the ideal piston-flow assumption will be discussed in this section. The piston-flow assumption requires that temperature, concentrations, reaction rate, and mass flow rate are uniform over any cross section of the catalyst bed. 

Before discussing the rate-limiting phenomena occurring in the catalyst bed as a whole, it is appropriate to mention the different types of bed and flow arrangements which are commonly used in industrial converters. 

All modern ammonia converters are based on adiabatic catalyst beds with intermediate cooling. The various flow arrangements that have most frequently been used or proposed for adiabatic beds of ammonia converters are illustrated in Fig. 6.1. In each case, the catalyst volume must be accommodated within a cylinder, usually referred to as a cartridge. 

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