In catalyst particles

Thus, considering diffusion in pores leads to very similar results to those we obtained when describing diffusion in catalyst particles. 

The presence (or absence) of pore-diffusion resistance in catalyst particles can be readily determined by evaluation of the Thiele modulus and subsequently the effectiveness factor, if the intrinsic kinetics of the surface reaction are known. When the intrinsic rate law is not known completely, so that the Thiele modulus cannot be calculated, there are two methods available. One method is based upon measurement of the rate for differing particle sizes and does not require any knowledge of the kinetics. The other method requires only a single measurement of rate for a particle size of interest, but requires knowledge of the order of reaction. We describe these in turn. 

As already mentioned, the first step in any heterogeneous catalytic reaction is the adsorption of a gas molecule onto a solid surface. Adsorption heat measurements can provide information about the adsorption process not available using other surface analytical tools. For example, differential heat measurements can provide valuable insights into sites distribution on the catalyst surface as well as quantitative information on the changes in catalyst particle surface chemistry that result from changes in particle size or catalyst support material [148-150],... 

Of the various methods of weighted residuals, the collocation method and, in particular, the orthogonal collocation technique have proved to be quite effective in the solution of complex, nonlinear problems of the type typically encountered in chemical reactors. The basic procedure was used by Stewart and Villadsen (1969) for the prediction of multiple steady states in catalyst particles, by Ferguson and Finlayson (1970) for the study of the transient heat and mass transfer in a catalyst pellet, and by McGowin and Perlmutter (1971) for local stability analysis of a nonadiabatic tubular reactor with axial mixing. Finlayson (1971, 1972, 1974) showed the importance of the orthogonal collocation technique for packed bed reactors. 

A reactant in liquid will be converted to a product by an irreversible first-order reaction using spherical catalyst particles that are 0.4cm in diameter. The first-order reaction rate constant and the effective diffusion coefficient of the reactant in catalyst particles are 0.001 s and 1.2 X 10 ( ii s , respectively. The liquid film mass transfer resistance of the particles can be neglected. 

Although equation 3.41 is only applicable to competing first-order reactions in catalyst particles, at large values of where diffusion is rate controlling the equation is equivalent at the asymptotes to equations obtained for reaction orders other than one. 

Figure 1 shows a schematic illustration to depict the CTL emission process due to the recombination of carriers in catalyst particles. The figure shows... 

Where intraparticle diffusion appreciably affects the rate of the reaction, reduction in catalyst particle size would be necessary to increase the effectiveness factor and hence conversion. But this may not be possible due to the pressure drop limitations in conventional packed beds. In such situations, the use of Monoliths would provide the advantage of higher effectiveness factor. 

Characterization of Nonisobaric Diffusion Due to Nonequimolar Fluxes in Catalyst Particles... 

Studies with porous catalyst particles conducted during the late 1930s established that, for very rapid reactions, the activity of a catalyst per unit volume declined with increasing particle size. Mathematical analysis of this problem revealed the cause to be insufficient intraparticle mass transfer. The engineering implications of the interaction between diffusional mass transport and reaction rate were pointed out concurrently by Damkohler [4], Zeldovich [5], and Thiele [6]. Thiele, in particular, demonstrated that the fractional reduction in catalyst particle activity due to intraparticle mass transfer, r, is a function of a dimensionless parameter, 0, now known as the Thiele parameter. 

Information about transport diffusion in catalyst particles can also be deduced during the initial, unsteady state period of a permeation experiment. In this stage, the number of molecules passing the plug of catalyst per unit time will increase from zero until the rate of permeation characterizing the steady state behavior is attained. In the limit t - oo, the total amount of molecules which have permeated in the time interval 0... t is given by the relation [1,2, 12]... 

The influence of mass-transport resistance in the particles can only be excluded if the critical reaction rate is substantially lower than the mass transfer velocity. This leads to the need for good external mass transfer (i.e. to a sufficiently rapid flow rate in the packed bed), as well as to short diffusion paths in catalyst particles. 

Multiple steady states in catalyst particles have been stu-... 

R. Mann, Fluid catalytic cracking Some recent developments in catalyst particle design and unit hardware. Catalysis Today 75 509 (1993). 

Twins have been found in catalyst particles and fillings of all fcc-metals. Some particles contained two systems of twins. Double twinned Co-particle, located inside the nanotube, is shown in Fig. 3. New graphite layers... 

A series of CoMo/Alumina-Aluminum Phosphate catalysts with various pore diameters was prepared. These catalysts have a narrow pore size distribution and, therefore, are suitable for studying the effect of pore structure on the deactivation of reaction. Hydrodesulfurization of res id oils over these catalysts was carried out in a trickle bed reactor- The results show that the deactivation of reaction can be masked by pore diffusion in catalyst particle leading to erro neous measurements of deactivation rate constants from experimental data. A theoretical model is developed to calculate the intrinsic rate constant of major reaction. A method developed by Nojcik (1986) was then used to determine the intrinsic deactivation rate constant and deactivation effectiveness factor- The results indicate that the deactivation effectiveness factor is decreased with decreasing pore diameter of the catalyst, indicating that the pore diffusion plays a dominant role in deactivation of catalyst. 

An experimental study of the possibility of non-uniform sintering during oxidative regeneration of fixed bed catalytic reactors has been carried out. Both kinetic measurements and XPS results show that a different degree of sintering can be expected in catalyst particles sampled at different reactor positions. 

However, coke deposition does not usually occur uniformly in the reactor, which means that the amount of heat evolved in the combustion of the coke deposits may vary locally whithin the bed. The dynamics of propagation of the hot fronts generated also contribute to the heterogeneity of the thermal history of the catalyst particles in bed, and thus, those located near the exit region are usually exposed to comparatively higher temperatures (refs 7-9). It is therefore reasonable to expect a different activity loss by thermal degradation in catalyst particles located at different positions in the bed. In the present work, an experimental study has been carried out of the loss of catalytic activity at different positions in a fixed bed reactor, in order to assess the extent of the thermal degradation of the catalyst. 

Influence of catalyst preparation, composition, and structure on activity and selectivity. There is an extensive literature some of which has been covered incidentally already. An excellent review has been published by Ripperger and Saum. Much of the work relates to the catalysts as oxides and so will not be covered in detail in this Report. Variables that influence catalytic properties include preparation e.g., method and order of addition of active components), pretreatment (drying and calcination,pre-reduction sulphiding,composition (e.g., concentration and ratio of active components and the type of support), distribution of active components in catalyst particles,particle size, " and surface area and pore size distribution. 

Wheeler has summarized the work on internal diffusion for catalytic cracking of gas-oil. At 500°C the rate data for fixed-bed operation, with relatively large ( -in.) catalyst particles and that for fluidized-bed reactors (very small particle size) are about the same. This suggests that the effectiveness factor for the large particles is high. Confirm this by estimating rj for the -in. catalyst if the... 

Fig. 13-13 Rate of oxidation of SOy on -in. catalyst particles containing 0.2%, platinum [mr/s. s velocity 350 lbl br) ft )]...

Fouling Deposits may form in catalyst particles and constrict or clog... 

A effective diffusion coefficient of species i in catalyst particle l2t ... 

---