Fast flow reactor kinetics

Rogowski, D. F., Marshall, P., and Fontijn, A., High-temperature fast-flow reactor kinetics studies of the reactions of A1 with Cl, A1 with HCl, and AlCl with CI2 over wide temperature ranges, J. Phys. Chem. 93, 1118 (1989). 

The reactivity of the clusters can then be studied by various experimental techniques, including fast flow reactor kinetics in the postvaporization expansion region of a laser evaporation source [21, 22], ion flow tube reactor kinetics of ionic clusters [23, 24], ion cyclotron resonance [25, 26], guided-ion-beam [27], and ion-trap experiments [28-30]. Which of these techniques is applied depends on the charge state of reactants (neutral, cationic, anionic), on whether the clusters are size-selected before the reaction zone, on single or multiple collisions of the clusters with the reactants, on the pressure of a buffer gas if present, and on the temperature and collision energy of the reactant molecules. 

Several papers have been published describing the reactions of halogenomethanes with halogens the systems which have been investigated are summarized in Table 13. The kinetics and mechanism of the reaction of atomic fluorine with CF3I and CCl3Br have been investigated in a fast-flow reactor. The analysis of the results proves conclusively that both reactions... 

A complex mode mechanism was also suggested by the kinetic studies of Fontijn and co-workers, with a fast flow reactor between 300 and 1700 K [5-7]. 

Continuous reactors have traditionally been used for reactions with fast kinetics [10]. Solutions to this limitation are being developed, such as recycling or pulsed flow reactors [11]. However, an alternative solution to this limitation is simply to avoid it by re-thinking the chemical route to the target molecule. 

The results confirm that the adsorption of ammonia is very fast and that ammonia is strongly adsorbed on the catalyst surface. The data were analyzed by a dynamic isothermal plug flow reactor model and estimates of the relevant kinetic parameters were obtained by global nonlinear regression over the entire set of runs. The influences of both intra-particle and external mass transfer limitations were estimated to be negligible, on the basis of theoretical diagnostic criteria. 

Ray and Chanda [261] studied bismuth molybdates (prepared by the method of Peacock [250,251]) in an integral flow reactor. At constant W/F = 8 g h mol-1 and a feed ratio isobutene/oxygen = 1/6, a maximum selectivity of 75% was found at 400—450°C. As with propene, the reaction is first order with respect to isobutene and the rate is independent of the oxygen pressure. The reoxidation of the catalyst is very fast. Assuming that the kinetics can be described by three parallel first-order reactions for the production of methacrolein, carbon monoxide and carbon dioxide, rate coefficients were calculated (Table 18). 

In the presence of manganese and cobalt acetates the reaction becomes very fast, and the intermediate AMP cannot be detected. The kinetics of these reactions were studied in a flow reactor, and the results gave a good second-order fit (first order in peracetic acid and first order in acetaldehyde) at different catalyst concentrations. The plot of [acetaldehyde] vs. [peracetic acid] was linear with a slope of 1, indicating that equimolar quantities of the two substances are reacting. A plot of the experimental second-order rate constants (k Co) as a function of catalyst concentration gave a very good first-order fit for cobalt acetate... 

Fig. 4.14. Rapid thermal flow reactor as used to determine the kinetics of the fast first-order reaction between CO2 and OH ions COj + OH" = HCOj ...

Now we can really see why the CSTR operated at steady state is so different from the transient batch reactor. If the inlet feed flow rates and concentrations are fixed and set to be equal in sum to the outlet flow rate, then, because the volume of the reactor is constant, the concentrations at the exit are completely defined for fixed kinetic parameters. Or, in other words, if we need to evaluate kab and kd, we simply need to vary the flow rates and to collect the corresponding concentrations in order to fit the data to these equations to obtain their magnitudes. We do not need to do any integration in order to obtain the result. Significantly, we do not need to have fast analysis of the exit concentrations, even if the kinetics are very fast. We set up the reactor flows, let the system come to steady state, and then take as many measurements as we need of the steady-state concentration. Then we set up a new set of flows and repeat the process. We do this for as many points as necessary in order to obtain a statistically valid set of rate parameters. This is why the steady-state flow reactor is considered to be the best experimental reactor type to be used for gathering chemical kinetics. 

Thus, Equations 4.1-4.3, in the case of the homogeneous mixing of liquid flows, characterised by density and viscosity, provide an efficient commercial application of tubular turbulent reactors at almost any stage limited by mass exchange. The optimal operation conditions of the tubular turbulent reactors in the quasi-plug flow mode can be calculated due to the changes of the physical characteristics of the liquid flow, the kinetic parameters of the fast chemical reactions and the reaction construction parameters. 

Bench-scale flow reactor experiments are an effective way of examining the main performance features of the SCR reaction system on various catalysts. In this section, we review these features for Fe-based catalysts as a backdrop to considering more fundamental kinetics and mechanistic studies in Sect. 11.3 and transport effects in Sect. 11.4. The selective catalytic reduction of NOx by ammonia on Fe-ZSM-5 catalyst has been studied in detail by various research groups [19-22, 26, 27, 34-42]. The results from earlier studies of vanadia-based catalysts have underpinned the more recent studies of zeolite-based catalysts. For example, Koebel et al. [3, 6, 43] carried out a detailed study of the SCR chemistry on V-based catalysts. Nova et al. [5, 8, 44, 45] studied the chemistry of SCR over V-based catalyst and proposed a mechanism for the fast SCR reaction. To this end, the data here are by no means unique but are intended to highlight the important trends. 

For fast reaction times, it becomes necessary to utilize flow reactor systems, if accurate data is required for kinetic studies. In steady-state and with turbulent flow conditions, temperature and compositions, and consequently the rate of reaction, vary with axial location only, but not with time. 

While the account of kinetics in the first two chapters is sufficient for static procedures, it is insufficient if we are to consider dynamic procedures which give directly the rate of a chemical reaction. Accordingly, the rest of this chapter will cover the kinetics of tubular and continuous flow reactors, followed by a study of modern experimental methods for measuring the rate of fast reactions, completed in less than about one minute. The reader will find the description of the classical experimental procedures in the detailed texts listed at the end of the book. 

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