Kinetics catalyst porosity

Considerations based on the known physical phenomena can guide the choice of catalyst porosity and porous structure, catalyst size and shape and reactor type and size. These considerations apply both to laboratory as well as to large-scale operations. Many comprehensive reviews and good books on the problem of reactor design are available in the literature. The purpose of this book is to teach the reader the mathematical tools that are available for calculating interaction between the transport phenomena and true chemical kinetics, allowing optimization of catalyst performance. The discussed theories are elucidated with examples to provide training for application of the mathematics. 

The oxidation reactions of carbon and sulfur on hydroprocessing catalysts seem to be kinetically controlled by oxygen diffusion inside the catalyst porosity. Figure 3 shows the carbon and sulfur removal for Cat C which contains a very high amount of nickel and molybdenum, and an appreciable load of carbon. It is clear that the sulfur elimination occurs at higher temperatures than for the other catalysts and is simultaneous to carbon combustion. A tentative explanation of this phenomenon would be that the diffusion of oxygen in the microporosity is limited by coke deposit which needs to be at least partly removed to allow complete sulfur oxidation. 

The space time values are calculated by the authors based on the information presented in the source articles. Most of the studies did not report the catalyst porosity and the bulk density, therefore the space time units had to be determined in the gcat.min/cm units. These units make it difficult for using the resulting kinetic information in monoliths where it is more meaningful to use proper time units for the space time due to the open geometry used in the monoliths. The adaptation procedure will be discussed in the forthcoming section. 

Particle size affects the diffusion distance of the reactants and products. The longer distance of the diffusion, the bigger concentration gradient of reactant and products is. The effective diffusion coefficient of reactants and products is a complex coefficient. It is subject to influence by many factors, such as pore structure of catalyst, porosity of catalyst, and kinds of reactants and products. The reaction or formation rate of reactants and products is determined by reaction kinetics. Therefore, the research results of catalyst particle size, catalyst pore size, and diffiisivity of products should be discussed in detail. 

The catalysts used in this CCR commercial service must meet several stringent physical property requirements. A spherical particle is required so that the catalyst flows in a moving bed down through the process reactors and regenerator vessel. These spheres must be able to withstand the physical abuse of being educated and transferred by gas flow at high velocity. The catalyst particles must also have the proper physical properties, such as particle size, porosity, and poresize distribution, to achieve adequate coke combustion kinetics. 

For this purpose, cylindrical channels have been assumed. In randomly packed fixed beds the porosity is about 0.4, from which the relationship dp = 2.25 d is obtained. Since the focus is on heterogeneously catalyzed gas-phase reactions, it is important to not only ensure comparable conditions from a hydrodynamic point of view, but also as far as chemical reaction kinetics is concerned. Therefore, it is assumed that both reactors contain the same amount of catalyst. 

Al-Dahhan and Dudukovic, 1996 Dudukovic et al., 1999). This way, more solid-liquid contact points over which the liquid flows are created and the bed porosity is reduced, especially near the reactor wall. Following a proper procedure for packing a trickle bed with catalyst particles and fines decouples the apparent kinetics from hydrodynamics, which is highly desirable. The addition of lines is not the same as reducing the particle size of the catalyst, as in the latter case the particle effectiveness factor is smaller. 

Another important catalyst characteristic is porosity. Particularly when heavy feeds are processed, high pore volumes and pore diameters are required to reduce pore diffusion limitations. These limitations occur when the intrinsic rate of reaction is high compared with the rate of diffusion of the reactants through the catalyst particle to the active surface. The catalyst is then not used effectively, and reaction rates and selectivity become functions of particle size. If the kinetics of the reaction are known, it is possible to estimate from theory the reaction rate or threshold above which a catalyst of known size will begin to exhibit diffusion limitations. 

In the literature, higher values for the activation energy are also found [82, 83]. One reason for this could be the neglect of a pre-reduction of the platinum catalyst and also the low porosity of the sputtered catalyst. Another possibly important aspect is that here we actually measured intrinsic kinetic data compared with the diffusion-affected kinetic data in refs. [82] and [83]. 

Up to 48 ternary catalyst mixtures were prepared simultaneously in less than 1 h. Hence the sputtering procedure is much faster than the wet chemical route and in fact one of the fastest syntheses available. This advantage is gained at the expense of low layer porosity. Thus, sputtered catalysts are new artificial catalysts and not directly comparable to catalysts prepared by wet-chemical procedures. These catalysts offer the advantage of quick preparation and characterization compared with alumina-based catalysts. They can also be used for obtaining so-called intrinsic kinetics because there is no influence of diffusion. 

One approach to describe the kinetics of such systems involves the use of various resistances to reaction. If we consider an irreversible gas-phase reaction A — B that occurs in the presence of a solid catalyst pellet, we can postulate seven different steps required to accomplish the chemical transformation. First, we have to move the reactant A from the bulk gas to the surface of the catalyst particle. Solid catalyst particles are often manufactured out of aluminas or other similar materials that have large internal surface areas where the active metal sites (gold, platinum, palladium, etc.) are located. The porosity of the catalyst typically means that the interior of a pellet contains much more surface area for reaction than what is found only on the exterior of the pellet itself. Hence, the gaseous reactant A must diffuse from the surface through the pores of the catalyst pellet. At some point, the gaseous reactant reaches an active site, where it must be adsorbed onto the surface. The chemical transformation of reactant into product occurs on this active site. The product B must desorb from the active site back to the gas phase. The product B must diffuse from inside the catalyst pore back to the surface. Finally, the product molecule must be moved from the surface to the bulk gas fluid. 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

In concluding this part, three main points emerge from the summary of these results. First, the difficulty in achieving the preparation of these solids in a reproducible way can be solved only if a precision in the experimental parameters similar to that employed for physical or analytical chemistry measurements is used. This is a clear demonstration of the second point, which states that the textural parameters of the materials (porosity, specific surface area and surface composition) are under kinetic control. Temperature, solvent, catalyst, water/precursor ratio and concentration of reagents are the main parameters which, beside the nature of the organic subunit R, control the texture of the final material. The third point is the difficulty in rationalizing the effect of these parameters due to the numerous mechanisms involved in the sol-gel process and their interconnections. However, it must be kept in mind that all these parameters are also powerful tools that can be very useful for the development of further applications, because they allow one to tune the texture of the materials. 

Relating porosity, pore diffusional processes and kinetics of the catalytic reaction and of the deactivation process to one another with the aim of minimizing apparent catalyst deactivation [4],... 

This paper deals with the selective synthesis of 2-acetyl-6-methoxynaphthalene, precursor of Naproxen, over zeolite catalysts and especially over HBEA zeolites. As has been previously observed3 8, acetylation of 2-methoxynaphthalene occurs preferentially at the kinetically controlled 1-position with formation of l-acetyI-2-methoxynaphthalene (I). The desired isomer, 2-acetyl-6-methoxynaphthalene (II) and the minor isomer, l-acetyl-7-methoxynaphthalene (HI), are the other primary products. However, it will be shown that in presence of 2MN, isomerization of I can occur allowing a selective production of II, the desired product the effect of the operating conditions (solvent, temperature) and of the acidity and porosity of the zeolite catalyst will be presented. 

The general approach for modelling catalyst deactivation is schematically organised in Figure 2. The central part are the mass balances of reactants, intermediates, and metal deposits. In these mass balances, coefficients are present to describe reaction kinetics (reaction rate constant), mass transfer (diffusion coefficient), and catalyst porous texture (accessible porosity and effective transport properties). The mass balances together with the initial and boundary conditions define the catalyst deactivation model. The boundary conditions are determined by the axial position in the reactor. Simulations result in metal deposition profiles in catalyst pellets and catalyst life-time predictions. 

Physical characteristics of a support, namely porosity and specific surface area, have long been understood to play a key role in stabilizing active components of the catalysts in dispersed state. Explicitly or implicitly, they reflect topological properties of the carbon surface, namely the nature and quantity of (1) traps (potential wells for atoms and metal particles), which behave as sites for nucleation and growth of metal crystallites and (2) hindrances (potential barriers) for migration of these atoms and particles [4,5]. An increase in the specific surface area and the micropore volume results, as a rule, in a decrease in the size of supported metal particles. Formal kinetic equations of sintering of supported catalysts always take into consideration these characteristics of a support [6]. 

For the sake of comparison hydrogenation experiments with large cylindrical catalyst particles were carried out. The increase of the particle size diminished the velocity of catalytic hydrogenation. These experimental results provide a path for the process scale-up, i.e. a prediction of the hydrogenation rate on large catalyst particles starting from crushed particles. The values of the kinetic constants obtained for crushed particles were utilized and the ratio of porosity to tortuosity from the reaction-diffusion model was adjusted (0.167) to fit successfully the experimental data (Figure 10.40). 

Hydroxylation of phenol by hydrogen peroxide over solid acids exhibits an autocatalysis that has never been described in earlier works. The induction period is dependent on the acidity and is reduced by initial addition of dihydroxybenzenes or other electron-transfer agents. A new mechanism, initiated by the slow formation of dihydroxybenzenes in the induction period, should be considered. Comparison of various catalysts shows that the reaction is also dependent on the structure of the solid. Zeolites with too small a porosity are not active, according to a large space demand of the reaction. Catalysis by titanium silicalites does not show such behaviour the reactivity is low but regular. Thus, our results show that valuable comparison between catalysts cannot be deduced from tests performed by stopping the reaction at a determined time, but that kinetic studies are essential. 

The destruction of chloroform has also been studied by Lou and Lee using a Pt/AhOs alloy catalyst [23]. The authors have concentrated on the fact that many catalysts do produce undesirable products such as CCI4, C2CI4 and CO and they have considered the nature of the adsorbed species by interpreting kinetic data. The catalyst was prepared using a wash coating method to produce a catalysts with a bulk density of 0.6-0.7 g ml" with a BET surface area of 1.267 m g an active metal surface area of 65-75% and porosity of 80.03%. Catalysts were tested at atmospheric pressure and temperatures ranging from 200 to 475°C, with space velocities of 22,500 h and 57,000 h . ... 

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