Catalysts catalyst, kinetic models

The kinetics of the CO+NO reactions has been studied at 300°C over a fresh and a deactivated bimetallic Pt-Rh/AhOs catalyst. Two kinetic models have been examined including competitive and non-competitive adsorptions of the reactants. The discrimination between these two assumptions has been achieved by using graphic and mathematical methods. From the comparison of kinetic and thermodynamic constants calculated from these methods with those previously obtained on RI1/AI2O3 and on Pt/AljOs, we believe that the kinetic data obtained on the fresh Pt-Rh/Al203 catalyst can be modelled by non-competitive adsorptions of the reactants assuming a preferential adsorption of NO on Rh and CO on Pt. By contrast NO and CO competitive adsorptions can only occur on the deactivated Pt-Rh/Al203 catalyst, which to assume that the active sur ce is mostly composed of Rh. 

Reaction System Catalyst Kinetic Model" Reference... 

To describe the kinetics of olefin polymerization with heterogeneous catalysts, kinetic models based on adsorption isotherm theories have been proposed [7-10], The most accepted two-step mechanism of ZN polymerization, proposed by Cossee [10-12], includes olefin coordination and migratory insertion of coordinated monomer into a metal-carbon bond of the growing polymer chain. 

Keywords Cobalt catalyst. Kinetics, Modeling, Fischer-Tropsch synthesis. Hydrocarbon Product Distribution, Anderson-Schulz-Flory. 

N, C-paUadacycle, Blackmond and coworkers propose the formation of a dimeric arylpalla-dium(II) [ArPd(/r.-X)L]2 in oxidative addition of an undefined [Pd L ] complex, generated in situ from the iV,C-palladacycle. Such a dimeric complex is supposed to be in equilibrium with the reactive ArPdXL2 see Rosner, T., Le Bars, J., Pfaltz, A. and Blackmond, D.G. (2001) Kinetic studies of Heck coupling reactions using paUadacycle catalysts and kinetic modeling of the role of dimer species. J.Am. Chem. Soc., 123,1848-55. 

Kinetic rate coefficients have been determined for the reduction of NO by CO in absence and presence of O2 via regression of transient experiments at automotive cold-start conditions over a commercial Pt/Rh/Ce02/y-Al203 catalyst. The kinetic model quantifies storage and release of O2 and NO in ceria during lean and rich half-cycles. 

Transport Criteria in PBRs In laboratory catalytic reactors, basic problems are related to scaling down in order to eliminate all diffusional gradients so that the reactor performance reflects chemical phenomena only [24, 25]. Evaluation of catalyst performance, kinetic modeling, and hence reactor scale-up depend on data that show the steady-state chemical activity and selectivity correctly. The criteria to be satisfied for achieving this goal are defined both at the reactor scale (macroscale) and at the catalyst particle scale (microscale). External and internal transport effects existing around and within catalyst particles distort intrinsic chemical data, and catalyst evaluation based on such data can mislead the decision to be made on an industrial catalyst or generate irrelevant data and felse rate equations in a kinetic study. The elimination of microscale transport effects from experiments on intrinsic kinetics is discussed in detail in Sections 2.3 and 2.4 of this chapter. 

Lox ES, Froment GF Kinetics of the Fischer-Tropsch reaction on a precipitated promoted iron catalyst. 2. Kinetic modeling, Ind Eng Chem Res 32 7 —S2, 1993. 

Ma W, Li Y, Zhao Y, Xu Y, Zhou J Kinetics of Fischer-Tropsch synthesis over Fe-Cu-K catalyst-kinetic model on the basis of mechanism (1), J Chem Ind Eng (China) 50(2) 159-166, 1999a. 

Erena, J., Arandes, J.M., Bilbao, J., Ga5mbo, A.G., and de Lasa, H. Conversion of syngas to hquid hydrocarbons over a two-component (Cr203-Zn0 and ZSM-5 zeolite) catalyst kinetic modeling and catalyst deactivation. Chem. Eng. Set 2000, 55, 1845-1855. 

Bitter, J.H., van Ddlen, A.)., Murzin, D.Yu., and de Jong, KP. (2005) Support effects in hydrogenation of drmamaldehyde over carbon nanofiber-supported platinum catalysts kinetic modeling. Chem. Eng. 

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Kelkar and McCarthy (1995) proposed another method to use the feedforward experiments to develop a kinetic model in a CSTR. An initial experimental design is augmented in a stepwise manner with additional experiments until a satisfactory model is developed. For augmenting data, experiments are selected in a way to increase the determinant of the correlation matrix. The method is demonstrated on kinetic model development for the aldol condensation of acetone over a mixed oxide catalyst. 

Parallel ketonization of acetic acid and propionic acid was one of the transformations of this type studied in our Laboratory. Ryba6ek and Setinek (94) investigated the kinetics of these reactions in the gaseous phase at 316°C using thorium oxide on activated carbon (p. 27) as the catalyst. This model system allowed the study of each reaction separately as well as of the simultaneous conversion of both acids. 

The preferred kinetic model for the metathesis of acyclic alkenes is a Langmuir type model, with a rate-determining reaction between two adsorbed (complexed) molecules. For the metathesis of cycloalkenes, the kinetic model of Calderon as depicted in Fig. 4 agrees well with the experimental results. A scheme involving carbene complexes (Fig. 5) is less likely, which is consistent with the conclusion drawn from mechanistic considerations (Section III). However, Calderon s model might also fit the experimental data in the case of acyclic alkenes. If, for instance, the concentration of the dialkene complex is independent of the concentration of free alkene, the reaction will be first order with respect to the alkene. This has in fact been observed (Section IV.C.2) but, within certain limits, a first-order relationship can also be obtained from many hyperbolic models. Moreover, it seems unreasonable to assume that one single kinetic model could represent the experimental results of all systems under consideration. Clearly, further experimental work is needed to arrive at more definite conclusions. Especially, it is necessary to investigate whether conclusions derived for a particular system are valid for all catalyst systems. 

The change of shape of the kinetic curves with monomer and inhibitor concentration at ethylene polymerization by chromium oxide catalysts may be satisfactory described 115) by the kinetic model based on reactions (8)-(14). 

Figure 9.2. Effect of catalyst potential Uwr, work function 0 and corresponding Na coverage on the rate of C2H4 oxidation on Pt/p"-Al203.1 The dashed line is from the kinetic model discussed in ref. 1. pO2=5.0 kPa, pC2H4=2-1 x 1 O 2 kPa, T=291°C, kad = 12.5 s 1. Reprinted with permission from Academic Press.

Figure 9.8. Effect of catalyst potential Uwr on the apparent activation energy and on the temperature (inset) at which the transition occurs from a high ( ) to a low (O) E value. The dashed lines and predicted asymptotic Ej, E2, E3 activation energy values are from the kinetic model discussed in ref. 11. Conditions p02=5.8 kPa, pCo=3-5 kPa.11 Reprinted with permission from Academic Press.

This section is divided into three parts. The first is a comparison between the experimental data reported by Wisseroth (].)for semibatch polymerization and the calculations of the kinetic model GASPP. The comparisons are largely graphical, with data shown as point symbols and model calculations as solid curves. The second part is a comparison between some semibatch reactor results and the calculations of the continuous model C0NGAS. Finally, the third part discusses the effects of certain important process variables on catalyst yields and production rates, based on the models. 

The semibatch model GASPP is consistent with most of the data published by Wisseroth on gas phase propylene polymerization. The data are too scattered to make quantitative statements about the model discrepancies. There are essentially three catalysts used in his tests. These BASF catalysts are characterized by the parameters listed in Table I. The high solubles for BASF are expected at 80 C and without modifiers in the recipe. The fact that the BASF catalyst parameters are so similar to those evaluated earlier in slurry systems lends credence to the kinetic model. 

Reactor Variable Study. Assuming that the kinetic models are valid, we have a means to rapidly explore the effects of making certain changes in the catalyst or in the operating conditions. Fortunately, Wisseroth published the results for two runs at 100 C and two more runs at 20 atm in his Table 3 (1 ). 

The selectivity is 100% in this simple example, but do not believe it. Many things happen at 625°C, and the actual effluent contains substantial amounts of carbon dioxide, benzene, toluene, methane, and ethylene in addition to styrene, ethylbenzene, and hydrogen. It contains small but troublesome amounts of diethyl benzene, divinyl benzene, and phenyl acetylene. The actual selectivity is about 90%. A good kinetic model would account for aU the important by-products and would even reflect the age of the catalyst. A good reactor model would, at a minimum, include the temperature change due to reaction. 

More complicated rate expressions are possible. For example, the denominator may be squared or square roots can be inserted here and there based on theoretical considerations. The denominator may include a term k/[I] to account for compounds that are nominally inert and do not appear in Equation (7.1) but that occupy active sites on the catalyst and thus retard the rate. The forward and reverse rate constants will be functions of temperature and are usually modeled using an Arrhenius form. The more complex kinetic models have enough adjustable parameters to fit a stampede of elephants. Careful analysis is needed to avoid being crushed underfoot. 

Shinnar, R. and Feng, C. A., Structure of complex catalytic reactions thermodynamic constraints in kinetic modeling and catalyst evaluation, I EC Fundam., 24, 153-170 (1985). 

Evaluation and Analysis of a Multisite Kinetic Model for Polymerization Initiated with Supported Ziegler—Natta Catalysts... 

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