Catalyst pore structure

FIGURE 10.1 Diagram of bimodal catalyst pore structure. 

Illustration 6.2 indicates how void volume and surface area measurements can be combined in order to evaluate the parameters involved in the simplest model of catalyst pore structure. 

Scanning electron microscopy and other experimental methods indicate that the void spaces in a typical catalyst particle are not uniform in size, shape, or length. Moreover, they are often highly interconnected. Because of the complexities of most common pore structures, detailed mathematical descriptions of the void structure are not available. Moreover, because of other uncertainties involved in the design of catalytic reactors, the use of elaborate quantitative models of catalyst pore structures is not warranted. What is required, however, is a model that allows one to take into account the rates of diffusion of reactant and product species through the void spaces. Many of the models in common use simulate the void regions as cylindrical pores for such models a knowledge of the distribution of pore radii and the volumes associated therewith is required. 

Another disadvantage of fixed bed reactors is associated with the fact that the minimum pellet size that can be used is restricted by the permissible pressure drop through the bed. Thus if the reaction is potentially subject to diflfusional limitations within the catalyst pore structure, it may not be possible to fully utilize all the catalyst area (see Section 12.3). The smaller the pellet, the more efficiently the internal area is used, but the greater the pressure drop. 

This section is concerned with analyses of simultaneous reaction and mass transfer within porous catalysts under isothermal conditions. Several factors that influence the final equation for the catalyst effectiveness factor are discussed in the various subsections. The factors considered include different mathematical models of the catalyst pore structure, the gross catalyst geometry (i.e., its apparent shape), and the rate expression for the surface reaction. 

The reactor feed mixture was "prepared so as to contain less than 17% ethylene (remainder hydrogen) so that the change in total moles within the catalyst pore structure would be small. This reduced the variation in total pressure and its effect on the reaction rate, so as to permit comparison of experiment results with theoretical predictions [e.g., those of Weisz and Hicks (61)]. Since the numerical solutions to the nonisothermal catalyst problem also presumed first-order kinetics, they determined the Thiele modulus by forcing the observed rate to fit this form even though they recognized that a Hougen-Watson type rate expression would have been more appropriate. Hence their Thiele modulus was defined as... 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

If the two competing reactions have the same concentration dependence, then the catalyst pore structure does not influence the selectivity because at each point within the pore structure the two reactions will proceed at the same relative rate, independent of the reactant concentration. However, if the two competing reactions differ in the concentration dependence of their rate expressions, the pore structure may have a significant effect on the product distribution. For example, if V is formed by a first-order reaction and IF by a second-order reaction, the observed yield of V will increase as the catalyst effectiveness factor decreases. At low effectiveness factors there will be a significant gradient in the reactant concentration as one moves radially inward. The lower reactant concentration within the pore structure would then... 

The deactivation of FCC catalysts by feed metals is believed to be caused by a partial or total blockage of the catalyst pore structure, by the irreversible destruction of the zeolite... 

Surface area is by no means the only physical property which determines the extent of adsorption and catalytic reaction. Equally important is the catalyst pore structure which, although contributing to the total surface area, is more conveniently regarded as a separate factor. This is because the distribution of pore sizes in a given catalyst preparation may be such that some of the internal surface area is completely inaccessible to large reactant molecules and may also restrict the rate of conversion to products by impeding the diffusion of both reactants and products throughout the porous medium. 

Although there still remains some "art" in the production of high activity catalysts, surface area, pore size and other factors relevant to the accessibility of the reactant gases to the catalytic sites are clearly of primary importance. Thus any deposition of product or by-product within the catalyst pore structure is undesirable. In recent years the relationship between impure Claus feed containing hydrocarbons and catalyst lifetime has been well demonstrated (32). Carbon of hydrocarbon polymer deposition on the catalyst usually results in blocking access of the reactant gases to the internal catalytic sites. Product sulfur depos-... 

If data are available on the catalyst pore- structure, a geometrical model can be applied to calculate the effective diffusivity and the tortuosity factor. Wakao and Smith [36] applied a successful model to calculate the effective diffusivity using the concept of the random pore model. According to this, they established that ... 

In addition to catalyst pore structure, catalytic metals content can also influence the distribution of deposited metals. Vanadium radial profile comparisons of aged catalysts demonstrated that a high concentration of Co + Mo increases the reaction rate relative to diffusion, lowering the effectiveness factor and the distribution parameter (Pazos et al., 1983). While minimizing the content of Co and Mo on the catalyst is effective for increasing the effectiveness factor for HDM, it may also reduce the reaction rate for the HDS reactions. Lower space velocity or larger reactors would then be needed to attain the same desulfurization severity. 

Consideration of Catalyst Pore Structure and Asphaltenic Sulfur in the Desulfurization of Resids... 

However, before considering the fate of asphaltenic sulfur at high reactor severities, some patent aspects of catalyst pore structure would be of interest. Table IV shows a divergence of opinion as to the desirability of asphaltene exclusion. 

With this brief consideration of variations in catalyst pore structure, let us examine the pore structure of two catalysts used in this refractoriness study. One observes in Table V only slight differences between the two cobalt moly catalysts, T and R. They are typified by high surface area, small micropore mode diameters and low macropore volumes. 

The exclusion of asphaltenes is matched by the distribution of nickel inside and outside the catalyst pore structure. Vanadium distribution is inconsistent, probably influenced by coke deposition at the higher temperatures. 

Figure 11. Effect of catalyst pore structure on the NO reduction activity of a monolith catalyst for the SCR process. From Beeckman and Hegedus [32],...

Diffusion of N2 and H2 to the active Fe site within the catalyst pore structure... 

The assumption of global kinetic control is probably valid for only a handful of catalytic reaction processes. Nevertheless, some typical simulation results of the model of catalyst deactivation under kinetic control are presented here in order to emphasize some of the unique percolation-type aspects of the problem. The overall plugging time 0p, i.e., the time at which the catalyst becomes completely deactivated is shown is Figure 1, where it is plotted versus Z, the average coordination number of the network of pores, (in industrial applications, of course, the useful lifetime of the catalyst is significantly smaller than 0p). Note that as Z increases, (higher values of Z mean a more interconnected catalyst pore structure) 0p increases, i.e., the catalyst becomes more resistant to deactivation. The dependence of normalized catalytic activity (r/rQ) ([Pg.176]

Hie chemical composition is not the only factor determining the activity of catalysts. In many cases, the physical characteristics of the catalysts, such as the surface area, particle porosity, pore size, and pore size distribution influence their activity and selectivity for a specific reaction significantly. The importance of the catalyst pore structure becomes obvious when one considers the fact that it determines the transport of reactant and products from the outer catalyst surface to the catalytic surface inside the particle. 

Several experimental methods are available to characterize catalyst pore structure. Some of them, useful in quantifying mass transfer of reactant and product inside the porous particle, will be only briefly discussed here. More details concerning methods for the physical characterization of porous substances are given by various authors [5,8,9],... 

The advantages of phosphorus addition to catalyst formulations found in patents can be approximately categorized as follows (i) optimization of the catalyst pore structure by addition of phosphorus to be applied with certain types of feedstocks such as residual oil, (ii) optimization of the dispersion of Co(Ni) I Mo-containing phases by the presence of phosphorus, (iii) optimization of synergistic effects resulting from complex chemical combinations of phosphorus and other incorporated elements, (iv) optimization of catalyst preparation by use of specific phosphorus precursors, and (v) the use of phosphorus-containing catalysts under specific reaction conditions or processes as well as their use in combination with other hydrotreating catalysts. 

The alkylation of toluene with methanol over HZSM-5 proceeds at low temperatures via a protonated methanol species in the transition state [107] and weakly coadsoibed toluene as classically predicted for Friedel Crafts alkylation. The reaction rate is directly proportional to the concentration of the chemisorbed methanol (in the presence of excess toluene) as shown in Figure 6 [108]. Alkylation leads preferentially to ortho- and para- substituted products which rapidly isomerise in the zeolite pores. Specific reaction conditions and tailoring of the catalyst pore structure can be employed so that para- substituted products are preferentially... 

Computer-Aided Characterization and Design of Catalyst Pore Structure... 

A. Al-Lamy, Characterization of catalyst pore structure by image reconstruction from 3-D stochastic pore networks, Ph.D. dissertation, UMIST (1995). 

The deactivated catalyst recovered from the reactor after each run was analysed for its coke content using a LECO CS244 carbon/sulphur analyzer. The total surface area of the fresh and spent catalysts were measured using a Quantasorb Sortometer in the Catalyst Characterization Laboratories at Kuwait Institute for Scientific Research. The catalyst pore structures were also examined through a scanning electron microscope and images of the fresh and spent catalyst. 

The upper two curves in Figure 224, illustrating lower effectiveness, are perhaps the exceptions. The upper curve represents diethyl silane added to the reactor along with a non-reduced Cr(VI)/silica catalyst. As with other cocatalysts, the silanes were most effective with Cr(II)/silica. The second highest curve in Figure 224 represents Cr(II) /silica, but the cocatalyst was a polymer, poly-methylhydrosiloxane. Its diminished effectiveness can perhaps be attributed to the inability of such a large molecule to diffuse into the catalyst pore structure or the polymer/silica matrix. 

---