Single Event Kinetics

Schweitzer, J. M., Galtier, P., and Schweich, D., A single events kinetic model for the hydrocracking of paraffins in a three-phase reactor, Chem. Eng. Sci. 54(13-14), 2441-2452 (1999). 

N V Dewachtere, F San tael la, G F Froment. Application of a single-event kinetic model in the simulation of an industrial riser reactor for the catalytic cracking of vacuum gas oil. Chem. Eng. Sc., 54, 3653-3660, 1999. 

Despite the development of microscale modeling for reaction—diffusion in zeolite, the complex of MTO reaction mechanism impedes the application of microscale modeling to MTO process. Up to now, the reliable reaction kinetics based on element reactions in MTO process is still under development (van Speybroeck et al., 2014). However, a reduced or simplified microscale model could be applied. Basically, the diffusion effect is negligible if the crystal radius is small enough. Then mass equation, i.e., Eq. (1), could be simplified by neglecting the species ffux term. In this case, MTO processes over ZSM-5 and SAPO-34 catalyst could be simulated by use of the single-event kinetics by Alwahabi and Froment (2004a) as an input. 

Alwahabi SM, Froment GF Single event kinetic modeling of the methanol-to-olefins process on SAPO-34, Ind Eng Chem Res 43 5098—5111, 2004a. 

Such a set of equations could be integrated by means of Runge-Kutta routines, were it not that with the elementary step and Single Event Kinetics approach described in Chapter 1, it becomes extremely stiff because of the orders of magnitude difference between the concentrations of molecular and radical species. The Gear routine [1971] was developed to cope with such problems. 

Riser Reactor. Plug Flow Model with Slip with Reaction Scheme based upon Elementary Steps. Single Event Kinetics... 

Reaction scheme (bs basic site of the catalyst, R carbenium ion ethyl, R carbenium ion propyl ). Adapted from Alwahabi SM, Froment GF. Conceptual reactor design for the methanol-to-olefrns process on SAPO-34. Ind Eng Chem Res 2004 43 5112-22 Alwahabi SM, Froment GF. Single event kinetic modelingof the methanol-to-olefins process on SAPO-34. Ind Eng Chem Res 2004 43 5098- 111. 

For the kinetics of this kind of coke formation, single-event kinetic method is very useful as justified in studies for other reactions (Martens et al., 2001 Mitsios et al., 2009). Unfortunately, the chemistry of coke in DHP process is almost a virgin investigation field until now. 

Froment GF Single event kinetic modeling of complex catalytic processes, Catal Rev Sci Eng 47(1) 83-124, 2005. 

Increased computational resources allow the widespread application of fundamental kinetic models. Relumped single-event microkinetics constitute a subtle methodology matching present day s analytical techniques with the computational resources. The singleevent kinetic parameters are feedstock invariant. Current efforts are aimed at mapping catal) t properties such as acidity and shape selectivity. The use of fundamental kinetic models increases the reliability of extrapolations from laboratory or pilot plant data to industrial reactor simulation. 

The single-event microkinetic concept ensures the feedstock independence of the kinetic parameters [8]. Present challenges in microkinetic modelling are the identification of catalyst descriptors accounting for catalyst properties such as acidity [10,11] and shape selectivity [12,13]. 

A single-event microkinetic description of complex feedstock conversion allows a fundamental understanding of the occurring phenomena. The limited munber of reaction families results in a tractable number of feedstock independent kinetic parameters. The catalyst dependence of these parameters can be filtered out from these parameters using catalyst descriptors such as the total number of acid sites and the alkene standard protonation enthalpy or by accounting for the shape-selective effects. Relumped single-event microkinetics account for the full reaction network on molecular level and allow to adequately describe typical industrial hydrocracking data. 

Johnson and Fierke Hammes have presented detailed accounts of how rapid reaction techniques allow one to analyze enzymic catalysis in terms of pre-steady-state events, single-turnover kinetics, substrate channeling, internal equilibria, and kinetic partitioning. See Chemical Kinetics Stopped-Flow Techniques... 

The streak camera gives a time resolution of about 5 ps. It requires a rather complex calibration procedure since the incident light intensity appears as the thickness of trace on the screen. It is used mostly for luminescence kinetics measurements, one of its advantages being that it can record single events. 

The rate constants for isomerization steps are similar in the forward and the reverse directions. For convenience, we choose to parameterize the kinetic model in terms of the forward rate constants. In this respect, we use the concept of single-event rate coefficients developed by Froment (127). According to Froment, the rate constant for a particular step is obtained by multiplying a single-event rate coefficient by the number of single events, ne, possible for the reactant. The expression for ne is... 

For events that occur in a stochastic manner, single event measurements do not necessarily characterize the system. A single event normally represents one of several possibilities. One needs to repeat the measurement many times and analyze the data statistically. Comparison with data of ensemble measurements is one of the test of the single molecule measurements. The single-molecule data could provide insightful information regarding sequence and distribution of the function, which could not be measured otherwise. The basic equation used in kinetic analysis from ensemble studies uses molecule... 

The kinetic constant of each elementary step will therefore be the product of a single-events number ne and an intrinsic constant kreac which depends on the reaction type and on the types (m,u) of the reactant and product ions ... 

The elementary kinetic constant k is therefore the product of a term calculated from a difference of intrinsic free enthalpies 12 and a ratio of symmetry numbers, which we will call single-events number ne — 0 ob/[Pg.276]

Since the Eyring relation can be used to link a free activation enthalpy to a kinetic constant, the single-events method can be described in two different ways ... 

Either as a model composed of single-events number and intrinsic kinetic constants (see above). 

Although the single-events theory defines a kinetic model of elementary reactions occurring on the acid phase, this model must be included in a more general framework in order to model access to this acid phase. We will therefore consider the reaction path of a paraffin in Fig. 21 ... 

Remark The single-events theory itself does not make any presumptions regarding the nature of the elementary steps which could be kinetically determining for the rate(s) on the acid phase. Only the introduction of Assumption 4 leads to a certain number of restrictions, by excluding the protonation/deprotonation steps. 

Based on the previous assumptions, we can rigorously and explicitly determine an analytical expression of the apparent kinetic constants, between all the lumped families (Fx -> Fy) according to intrinsic elementary kinetic constants as specified in the Single-Events theory (Cochegrue, 2001 Schweitzer, 1998 Valery, 2002). The same applies for the denominator DEN (see next Fig. 26), in which the sum of the terms expresses the competitive chemisorption of all secondary and tertiary carbocations on the acid sites. The apparent kinetics so obtained formally return Langmuir-Hinshelwood expressions, traditional... 

This example demonstrates the benefit of using the single-events theory in the reduction of kinetic networks. 

By construction, the single-events theory introduces kinetic parameters which are not only intrinsic but, above all, independent of the feedstock. This first property offers a twofold advantage ... 

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