Experimental Determination of Intrinsic Kinetics

The mass of the catalyst does not appear. However, physical intuition or the So terms in Equations 10.11 and 10.16 suggest that doubling the amount of catalyst should double the reaction rate. How are rate data taken on a CSTR translated to a packed-bed reactor or even to another CSTR operating with a different catalyst density  

The void fraction used in this equation and in Equation 9.1 should be the total void fraction that includes pore volume. We now distinguish it from the supeificial void fraction used in the Ergun equation and in the packed-bed correlations of Chapter 

The pore volume is accessible to gas molecules and can constitute a substantial fraction of the gas phase volume. Pore volume is included in reaction rate calculations through the use of the total void fraction. The superficial void fraction ignores the pore volume. It is the appropriate parameter for the hydrodynamic calculations because fluid velocities go to zero at the external surface of the catalyst particles. The pore volume is accessible by diffusion, not bulk flow. 

However, the intrinsic pseudohomogeneous rate used in Equation 10.39 is not identical to the rate determined from the CSTR measurements since the catalyst density will be different. The correction procedure is as follows  

Calculate (Rp, from the CSTR data using Equation 10.37. 

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For any industrial reacting system, the relevant parameters appearing in the rate expression (Eq. (5.14)) need to be obtained by carrying out experiments under controlled conditions. It is necessary to ensure that physical processes do not influence the observed rates of chemical reactions. This is especially difficult when chemical reactions are fast. It may sometimes be necessary to employ sophisticated mathematical models to extract the relevant kinetic information from the experimental data. Some references covering the aspects of experimental determination of chemical kinetics are cited in Chapter 1. It must be noted here that in the above development, the intrinsic rate of all chemical reactions is assumed to follow a power law type model. However, in many cases, different types of kinetic model need to be used (for examples of different types of kinetic model, see Levenspiel, 1972 Froment and Bischoff, 1984). It is not possible to represent all the known kinetic forms in a single format. The methods discussed here can be extended to any type of kinetie model. 

Beeckman and Hegedus [50] determined the intrinsic kinetics over two commercial vanadia on titania catalysts. A mathematical model was proposed to compute NO and SO2 conversions and the model was validated by experimental values. Slab-shaped cutouts of the monolith and powdered monolith material were used in a differential reactor. The cutouts contained nine channels with a length of 15 cm and with a channel opening and wall thickness of 0.60 and 0.13 cm, respectively. The SCR reaction over a 0.8 wt% V2O5 on titania catalyst was first-order in NO and zero-order in NH3. 

Chapter 3 concentrates on the details of kinetic modelling for the intrinsic rates of reaction. In practice it is not sufficient to choose the kinetic model with the correct functional form, but it is equally important to determine the kinetic parameters accurately. The accurate determination of the kinetic parameters depends upon the accuracy of the experimental results and the reliability of the parameter estimation routine used. The correct form of the kinetic model is thus a necessary but not sufficient condition for the accuracy and reliability of the kinetic model. Chapter 3 concentrates on the development of appropriate kinetic models, and a number of important reactions are discussed in detail. 

To assess the impact of IDR on enzyme kinetics, the value of intrinsic kinetics (V" and K) and mass transfer (Deff) parameters must be evaluated. Several strategies have been proposed to approximate the value of the intrinsic kinetic parameters. A reliable experimental procedure is the one proposed by Benaiges et al. (1986) which is basically based on comminuting the support to obtain particles so small than IDR becomes negligible (very low Osp see Eq. 4.54). Kinetic parameters can be determined then with that comminuted biocatalyst to have an estimate on the intrinsic values. Effectiveness factor can be approached then to the ratio of initial rates for the intact and comminuted biocatalyst (Kobayashi and Laidler 1973). An obvious drawback of this approach is that not always a biocatalyst particle small enough can be obtained to be free of IDR (effectiveness factor = 1). If a smooth correlation exists between effectiveness factor and particle size, extrapolation to size zero could give an approximate value and intrinsic kinetic parameters can be... 

Table 4.11.1 Experimental conditions used to determine the intrinsic kinetics of the heterogeneously catalyzed gas-phase hydrogenation of 1-hexene.

There are many types of experimental reactors that can be used for the generation of intrinsic kinetic data. Certain conditions have to be met, however, to assure that the kinetics are truly intrinsic. These conditions are given in this chapter. The reduction of kinetic data to a rate expression is a trial and error process. This process of determining kinetic parameters is treated in some detail. For complex reactions with unknown reaction paths, however, the usual methods may fail. Methods of synthesizing a consistent kinetic structure for such complex reactions are also treated in this chapter. 

The haloalkane dehalogenase DhlA mechanism takes place in two consecutive Sn2 steps. In the first, the carboxylate moiety of the aspartate Aspl24, acting as a nucleophile on the carbon atom of DCE, displaces chloride anion which leads to formation of the enzyme-substrate intermediate (Equation 11.86). That intermediate is hydrolyzed by water in the subsequent step. The experimentally determined chlorine kinetic isotope effect for 1-chlorobutane, the slow substrate, is k(35Cl)/k(37Cl) = 1.0066 0.0004 and should correspond to the intrinsic isotope effect for the dehalogenation step. While the reported experimental value for DCE hydrolysis is smaller, it becomes practically the same when corrected for the intramolecular chlorine kinetic isotope effect (a consequence of the two identical chlorine labels in DCE). 

The intrinsic kinetics describes a reaction rate that is not influenced by such transport phenomena therefore, it only depends on the factors concentration, pressure, temperature, and catalyst. For the comparison of the catalytic activity and the investigation of different catalysts, it is necessary to adjust the experimental conditions such that only the intrinsic kinetics is determined. If this is not the case, none of the obtained data are of use. The microkinetics is equivalent to the intrinsic-kinetic, with the difference that it consists of the elementary reactions. 

The evaluation of catalyst effectiveness requires a knowledge of the intrinsic chemical reaction rates at various reaction conditions and compositions. These data have to be used for catalyst improvement and for the design and operation of many reactors. The determination of the real reaction rates presents many problems because of the speed, complexity and high exo- or endothermicity of the reactions involved. The measured conversion rate may not represent the true reaction kinetics due to interface and intraparticle heat and mass transfer resistances and nonuniformities in the temperature and concentration profiles in the fluid and catalyst phases in the experimental reactor. Therefore, for the interpretation of experimental data the experiments should preferably be done under reaction conditions, where transport effects can be either eliminated or easily taken into account. In particular, the concentration and temperature distributions in the experimental reactor should preferably be described by plug flow or ideal mixing models. 

Although the deactivation of Industrial catalysts is often due to two or more different causes, the modeling of simultaneous deactivation phenomena has not been widely studied (refs. 1, 2). The occurrence of two different deactivation processes not only adds another level of complexity to the determination of the intrinsic kinetic behavior but also complicates the interpretation of the experimental results. In our previous studies regarding the thloresistance of naphtha reforming catalysts (refs. 3, 4) we have shown that the activity decay caused by the presence of sulfur compounds in the feed is often accompanied by coking. In this situation, the thioresistance cannot be obtained in a simple way from the deactivation curves. The characteristics of the sulfur poisoning have to be deduced from the overall deactivation rate. 

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