Reaction parameters catalyst particle size

We have presented a general reaction-diffusion model for porous catalyst particles in stirred semibatch reactors applied to three-phase processes. The model was solved numerically for small and large catalyst particles to elucidate the role of internal and external mass transfer limitations. The case studies (citral and sugar hydrogenation) revealed that both internal and external resistances can considerably affect the rate and selectivity of the process. In order to obtain the best possible performance of industrial reactors, it is necessary to use this kind of simulation approach, which helps to optimize the process parameters, such as temperature, hydrogen pressure, catalyst particle size and the stirring conditions. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

At speeds higher than the reaction occurs with different particle sizes, and a graph of the overall rate of reaction vs. particle size is plotted. Such a graph is shown in Figure CSll.lc. For small particle sizes, the pore diffusional limitations are essentially absent (e = 1), and the overall rate of reaction is controlled by the chemical reaction. As the particle size is increased, the diffusional limitations become increasingly important, and above a certain particle size dp, the overall rate of reaction is determined by the diffusion of the reactants into the catalyst pores. The evaluation of the kinetic parameters for the reaction should be performed at impeller speeds higher than and particle sizes lower than dp. The reaction taking place on the catalyst surface itself is composed of various steps, such as (1) adsorption of the reactants on the active sites, (2) chemical reaction at the active sites, and (3) desorption of the products from the active sites. The rate of reaction can be written in terms of these varions steps (see Section 11.3). 

A recurring self-imposed task of many of the publications has been to identify the sites responsible for each type of reaction, so that structure-sensitivity has been a dominant theme. What has received much less attention is the possibility that, for example, particle size might determine the strength of hydrogen chemisorption, so that the use of constant operating conditions on a series of catalysts might produce results mainly decided by the surface concentration of hydrogen atoms. The dependence of kinetic parameters on particle size or other catalyst feature has been rarely examined. 

Internal diffusion is usually characterized by a Thiele modulus, q>, defined in Equation 4.56. It is a dimensionless parameter that represents the relative effect of the reaction on mass transfer rate. It depends on kinetic properties and on mass transfer properties, such as effective diffusion of the substrate, catalyst particle size, and geometry. 

The "holdup" model assumes that contacting is- proportional to the liquid holdup in the catalyst bed. This model, proposed by Henry and Gilbert [23], uses total holdup measurement as a basis and presupposes that each element of liquid hold-up is associated with an equivalent catalyst element and that all of these equivalences are of equal efficiency without respect to the nature of the reaction. Liquid velocity, particle size and fluid physical property affect contacting only as those parameters affect holdup. 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

The effectiveness is a measure of the utilization of the internal surface of the catalyst. It depends on the dimensions of the catalyst particle and its pores, on the diffusivity, specific rate, and heat of reaction. With a given kind of catalyst, the only control is particle size to which the effectiveness is proportional a compromise must be made between effectiveness and pressure drop. In simple cases t] can be related mathematically to its parameters, but in such important practical cases as ammonia synthesis its dependence on parameters is complex and strictly empirical. Section 17.5 deals with this topic. 

Powders possessing relatively high surface area and active sites can be intrinsically catalytically active themselves. Powders of nickel, platinum, palladium, and copper chromites find broad use in various hydrogenation reactions, whereas zeolites and metal oxide powders are used primarily for cracking and isomerization. All of the properties important for supported powdered catalysts such as particle size, resistance to attrition, pore size, and surface area are likewise important for unsupported catalysts. Since no additional catalytic species are added, it is difficult to control active site location however, intuitively it is advantageous to maximize the area of active sites within the matrix. This parameter can be influenced by preparative procedures. 

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