Individual Catalyst Pellets

The majority of the early MRI studies specific to catalysis addressed the heterogeneity in structure and transport within catalyst pellets. In-plane spatial resolution achieved in these investigations was approximately 30 pm, and the pellets themselves were of typical dimension 1-5 mm. In the majority of cases, investigations addressed the pure (usually oxide) support so that the quantitative nature of the data obtained was not lost because of the presence of metal (which introduces an unknown degree of nuclear spin relaxation time contrast into the images). 

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Maintenance of isothermal conditions requires special care. Temperature differences should be minimised and heat-transfer coefficients and surface areas maximized. Electric heaters, steam jackets, or molten salt baths are often used for such purposes. Separate heating or cooling circuits and controls are used with inlet and oudet lines to minimize end effects. Pressure or thermal transients can result in longer Hved transients in the individual catalyst pellets, because concentration and temperature gradients within catalyst pores adjust slowly. 

These experiments have revealed the possibility of the ignition of individual catalyst pellets within the bed. Such pellets can stay dry after the ignition due to the efficient progress of reaction despite the fact that they are surrounded by liquid filled pellets [Figure 5.4.4(a)], The images also reveal the presence of the beads with the characteristic concentric pattern of liquid distribution, similar to that observed earlier (Figure 5.4.2) for individual cylindrical pellets. 

The manner in which Ni and V sulfide deposits accumulate on individual catalyst pellets depends on the kinetics of the HDM reactions as influenced by catalyst properties, feed characteristics, and operating conditions. The dynamic course of deactivation of catalytic reactor beds is also determined by the kinetics of the HDM reaction. The lifetime and activity of a reactor bed are directly related to the details of the metal deposit distribution within individual pellets. This section will review deactivation behavior of reactor beds in light of our understanding of the reaction and diffusion phenomena occurring in independent catalyst pellets. Unfortunately, this is an area of research which remains mostly proprietary with too little information published. What has been published is generally lacking in detail for the same reason. 

X-ray Photoelectron Spectroscopy (XPS) and Laser Ion Induced Mass Analysis (LIMA) were used to investigate samples of catalyst A. These techniques can show the extent of potassium and lithium distribution within individual catalyst pellets. Samples that had been subjected to 50,100, and 1000 hours of steam reforming in a molten carbonate environment were analysed. A fresh sample of the catalyst was also examined for purposes of comparison. 

The one-dimensional model is by no means descriptive of everything that goes on in the reactor, because it provides calculated temperatures, concentrations, pressures, and so on only in one dimension — lengthwise, down the axis of the tube. Actually, transport processes and diffusion cause variations and gradients not only axially but also radially within tubes and within individual catalyst pellets. Furthermore, the reactor may not actually operate at steady-state, and so time might also be included as a variable. All of these factors can be described quite easily by partial differential equations in as many as four dimensions (tube length, tube radius, pellet radius, and time). 

Kinetics measured may differ per batch of catalyst as manufactured by the supplier. It is evident that these differences will never be too large, at least for the experienced manufacturers (not more than a factor of 2-3 under identical test conditions) and that these small changes can be compensated for by changing the operating conditions, such as the reaction temperature. In the same batch of catalyst the individual catalyst pellets will exhibit a stochastic distribution of their properties. Therefore, for catalyst testing and rate determinations, a catalyst sample has to be taken sufficiently large that it is representative of the average of the entire batch. This often has to be determined empirically. 

Mathematical models of tubular chemical reactor behaviour can be used to predict the dynamic variations in concentration, temperature and flow rate at various locations within the reactor. A complete tubular reactor model would however be extremely complex, involving variations in both radial and axial positions, as well as perhaps spatial variations within individual catalyst pellets. Models of such complexity are beyond the scope of this text, and variations only with respect to both time and axial position are treated here. Allowance for axial dispersion is however included, owing to its very large influence on reactor performance, and the fact that the modelling procedure using digital simulation is relatively straightforward. 

The dynamic behavior of the individual catalyst pellets can then be simulated by solving Eq. (5) for the four different layers shown in Fig. 1. The boundary conditions for Eq. (5) are... 

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