Pores, of catalyst

Fig. 3.3.3 (a) Hahn echo ]H intensity during heating cycle of cyclohexane filling the pores of catalyst pellets, (b) Pore size distribution obtained from (a) in comparison with BET measurements. 

One should take into account the specific features of gas diffusion in porous solids when measuring effective diffusion coefficients in the pores of catalysts. The measurements are usually carried out with a flat membrane of the porous material. The membrane is washed on one side by one gas and on the other side by another gas, the pressure on both sides being kept... 

Benfield Aqueous Potassium Carbonate Blocks pores of catalyst by evaporation of K.2CO3 solution. 

Vetrocoke Aqueous Potassium Carbonate plus Arsenious Oxide - Blocks pores of catalyst by evaporation of KjCOj solution. - AS2O3 is also a poison -0.5% of As on the catalyst will reduce its activity by 50%. 

The dependence of SCR activity of CuHM31 on its sulfur content with respect to reaction temperatures also shows the similar behavior to HM catalyst as shown in Fig. 4B. The catalytic activity reveals an exponential decrease with the sulfur content of the catalyst at 250 °C, while no deactivation is observed at 400 °C, despite the deposition of sulfur up to 1.78 wt.% on the catalyst surface. As discussed in the previous study, it is probably due to the deposition location of the deactivating agents on the pores of catalyst structure (ref 1). 

The bulk diffusion processes within the pores of catalyst particles are usually described by the Wilke model formulation. The extended Wilke equation for diffusion in porous media reads ... 

DIFFUSION IN PORES OF CATALYSTS AND CATALYTIC REACTION RATES... 

In this problem, we explore the dimensionless mass transfer correlation between the effectiveness factor and the intrapellet Damkohler number for one-dimensional diffusion and Langmuir-Hinshelwood surface-catalyzed chemical reactions within the internal pores of flat-slab catalysts under isothermal conditions. Perform simulations for vs. A which correspond to the following chemical reaction that occurs within the internal pores of catalysts that have rectangular symmetry. 

Consider the synthesis of methanol from carbon monoxide and hydrogen within the internal pores of catalysts with cylindrical symmetry. The radius of each catalytic pellet is 1 mm, the average intrapellet pore radius is 40 A, the intrapellet porosity is 0.50, the intrapellet tortuosity factor is 2, and the gas-phase molar density of carbon monoxide in the vicinity of the external surface of the catalytic pellet is 3 x 10 g-mol/cm. A reasonable Hougen-Watson kinetic rate law is based on the fact that the slowest step in the mechanism is irreversible chemical reaction that requires five active sites on the catalytic surface, due to the postulate that both hydrogen molecules must dissociate and adsorb spontaneously (see Section 22-3.1). Do not linearize the rate law. In units of g-mol/cm -min-atm, the forward kinetic rate constant is... 

The selectivity to PO (S) was drastically varied as S=13 55 and S=5 12 % depending on the C-up and C-down operations respectively. The total gas flow rate effectively contributed to the enhancement of S as S=18 41 % with increasing the flow rate, indicating an advantage of the membrane reactor (which was characterized by a convection flow in membrane pores) rather than conventional packed bed reactors (which were characterized by molecular diffusion in the pores of catalyst particles). 

A part of reactants have been involved in reactions before they enter into the pores of catalysts from the external surface through the orifice of solid. Therefore, diffusion and reaction of reactant molecules take place simultaneously in pores. If the reaction rate of a reactant on the surface of catalyst is higher than diffusion rate, the reactant will be used up before it reaches to the deep sides of pores, indicating that only part of catalyst is utilized. In other words, the using ratio of catalyst internal surface is lower if there exists internal diffusion effect. The lower the diffusion rate and the bigger the catalyst particle, the lower the usage ratio of internal surface is. The concentration distribution of a reactant in a pore of catalyst is shown in Fig. 2.35. 

There are many factors influencing internal diffusion, such as the size of particles and pore of catalysts, molecular diffusion coefficient, temperature, pressure and other parameters in reaction kinetics etc. Among these factors, the size of catalyst particles and reaction temperature are the most important and easily adjustable parameters. The estimation and elimination of internal diffusion effect can usually use the ways as follows ... 

The internal utilization ratio of ammonia synthesis catalysts depends on particle size and radius of micro-pore of catalyst, reaction rate constant, operation temperature and pressure as well as the difference between the concentration of reaction components in gas bulk and equilibrium concentration etc., among which the most... 

The main disadvantage of these indicators is that the pores of catalyst particle have different sizes, but the data used in Eq. (5.89) is the average pore size. As a result, the utilization of the pores in the same particles may be different, and the reduction systems are various for the catalysts with complex pores. With introduction of parameter S ... 

A variety of processes for external diffusion (diffusion of gas at the border and the large pores) belong to the law of free diffusion. Therefore, the rate has nothing to do with pressure. However, increasing the total gas pressure will reduce the diffusion coefficient of water vapor, resulting in the increase of partial pressure of water vapor and residence time in micro-pore of catalyst, which may lead to the repeated redox of a-Fe crystallite, decreasing the activity of the catalyst. Therefore, the choice of pressure depends on the other synergetic conditions. ... 

The reduction of fused iron catalyst commences from the external surface of particles, and then expands inward. The reduction rate can be increased obviously by increasing the space velocity of reducing gas. The higher gas space velocity, the more favorable the reduction is, i.e., the lower the concentration of water vapor in gas, the faster the diffusion rate, the easier for the water molecules in the pore of catalyst to escape. As a result, the poisoning effect of water vapor is decreased to minimum. In addition, it is also conducive for the reduction reaction to move to the right and to raise the rate of reduction. However, when the space velocity continues to increase, the extent of increasing reduction rate will be minor. When it reaches the critical value, the space velocity of reductant gas on reduction rate has almost no impact. At the same time, in industrial production, increasing the space velocity is limited by the furnace heat supply and the temperature. 

In addition to above three influence factors, one of the main reasons for deactivation of catalyst results from carbon deposition and coking, which are caused by impurities derived from macromolecular compounds containing carbon covering on the surface of active sites or blocking pore of catalyst. 

Apparent density (pa)- Skeletal density is measm-ed by the medium of benzene, isopropanol etc, (not helium gas) and is not considered as true density but rather apparent density. Because their molecular diameters are bigger than helium and absolutely cannot enter into the iimer pores of catalyst (especially of microp-orous), the obtained skeletal volume is just an approximate value. 

When porous solid catalysts are used, diflusion processes in intra- and intercrystalline and interparticle pores often play important roles in the kinetics. Santacesaria et al. examined the effects of diffusion for both vapor-phase (393 — 427K) and liquid-phase (313 — 343K) esterification of ethanol and acetic acid catalyzed by zeolites. According to them, the vapor-phase reaction proceeded in a condensed phase formed in the pores of catalysts, and intracrystalline diffusion is always a rate-determining step in the case of mordenite and, by contrast, in the case of Y zeolite the influence of intracrystalline diffusion upon reaction rate can be neglected. 

Some very recent first-principle calculations together with kinetic Monte Carlo simulations have shown that the MBs with five or six methyl groups are not more active than those with fewer methyl groups [103], Propylene is intrinsically more favorable than ethylene when the reaction is not diffusion limited based on a side-chain hydrocarbon pool mechanism. The theoretical results are consistent with some experimental observations and can be rafionafized based on the shape selectivity of key reaction intermediates and transition states in the pore of catalyst [61,103],... 

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