Flow tube reactor kinetics

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. 

Fig. 14. Flow tube reactor used to study NCl(a) formation and removal kinetics. Note that this apparatus has a two stage prereactor. The first stage is used to generate Cl atoms from F + HCl. The second stage (the first 40 cm of the main reactor tube) is used to generate N3 from the F + HN3 reaction. The inlet for ethane or H2S near the observation window is used for calibration of F or Cl atom concentrations. Reproduced with permission from Ref. 124.

Additional modes consider special reaction conditions or enviromnents. The mode TUBE is designed for a laminar flow tube reactor (LFTR), which allows reactions between products but not between products and reactants. Consequently, these reactions are ignored in the TUBE mode. Another difference to MONOMOLEC is that special reaction kinetics is not particularly considered, since a turbulent flow in the tube reactor has similar kinetics to a stirred tank reactor. 

McCullough et al heated NO/Ar mixtures to temperatures in the range 1750-2100 K in an alumina flow tube reactor and monitored the fractional decomposition of NO as a function of flow rate (residence time) in the reactor using a commercial chemiluminescent analyzer. Experiments at low temperatures were also performed but the data were excluded because of the influence of surface reactions. The high-temperature central section of the reactor was packed with small pieces of alumina to promote uniform flow, and pulsed tracer experiments were conducted to determine deviations from plug flow. A detailed kinetic and flow model was used, with some simplifications to reduce computing time, to calculate the fractional removal of NO versus flow rate. A... 

Results for the reverse rate constant have been obtained in the temperature range 1750-2100 K by McCullough et al (1977) using a flow tube reactor. The fractional decomposition of NO in two NO/Ar mixtures (1 and 5% NO) was monitored as a function of flow rate (residence time) using a chemiluminescent analyzer. A detailed kinetic and flow model was used, and a sensitivity analysis was performed. McCullough s result... 

The experiments by McCullough et al. (1977) were performed in an alumina packed-bed flow tube reactor. Dilute NO/H2/Ar mixtures were heated to temperatures in the range 1750-2040 K, and the fractional decomposition of NO was monitored as a function of flow rate using a chemiluminescent analyzer. A detailed flow and kinetic model (including surface reactions) was used to infer k, A careful error analysis, including sensitivity to other rate constants, yielded error limits of 46%. 

Figure 7.7 depicts type of plasma polymer of TFE depending on the location in a small tube reactor [7]. In the tubular reactor shown, the formation of F would occur at the upstream side of the reactor, where the monomer flow makes contact with the luminous gas phase of TFE. Then, the — CF3 could be used as a labeled species or an indicator of the change in the chemical nature of the polymer due to the kinetic pathlength of a growing species. The XPS data obtained with polymers... 

If the mass transfer is accompanied by a chemical reaction at the catalyst surface on the reactor wall, the mass transfer depends on the reaction kinetics [55]. For a zero-order reaction, the rate is independent of the concentration and the mass flow from the bulk to the wall is constant, whereas the reactant concentration at the catalytic wall varies along the reactor length. For this situation the asymptotic Sh in circular tube reactors becomes Sh. = 4.36 [55]. The same value is obtained when reaction rates are low compared to the rate of mass transfer. If the reaction rate is high (very fast reactions), the concentration at the reactor wall can be approximated to zero within the whole reactor and the asymptotic value for Sh is = 3.66. As a consequence, the Sh in the reacting system depends on the ratio of the reaction rate to the rate of mass transfer characterized by the second Damkohler number defined in Equation 6.11. 

The inlet for the neutrals may have a fixed or a variable position at the flow reactor. The sample inlet may be a controlled flow of a trace gas, a breath sample, or a headspace sample, introduced via a heated sampling line (Fig. 4.7). Variation of temperature, by external heating of the flow tube, and ion kinetic energy may be applied as well. 

To study the reaction kinetics, an experiment was carried out in a tube reactor with four different volumetric flow rates at 518 K. The ethanol and acetaldehyde concentrations were determined by chemical analysis. On the basis of the analytical data, the yield of acetaldehyde (cr/coa) and the conversion of ethanol were calculated. 

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