Fast flow systems

Fast-flow systems (FFS) consist of a flow tube typically 2- to 5-crn in diameter in which the reactants A and B are mixed in the presence of a large amount of an inert bath gas such as He or Ar. As the mixture travels down the flow tube at relatively high linear flow speeds (typically 1000 cm s l), A and B react. The decay of A along the length of the flow tube, that is, with time, is followed and Eq. (T) applied to obtain the rate constant of interest. 

The term fast flow comes from the high flow speeds. In most of these systems, discharges are used to generate A or another species that is a precursor to A hence the term fast-flow discharge system (FFDS) is also commonly applied. Since fast-flow discharge systems have been applied in many kinetic and mechanistic studies relevant to tropospheric chemistry (e.g., see Howard, 1979 Kaufman, 1984), we concentrate on them. However, all fast-flow systems rely on the same experimental and theoretical principles. 

Atomic hydrogen is formed by discharging dilute H2/He mixtures. Then these H atoms are converted to OH by the addition of excess N02, e.g., through port C. By adjusting the concentration of N02 appropriately, essentially all of the H can be converted to OH before the second reactant is added through port D. 

Under conditions where the plug flow assumption is valid, that is, concentration gradients are negligible so that the linear flow velocity of the carrier gas is the same as that of the reactants, the time (t) for A and B to travel a distance d along the flow tube is given by 

Here v is the linear flow speed, which can be calculated from the cross-sectional area of the flow tube (Ar), the total pressure (P) in the flow tube, the temperature (T), and the molar flow rates (dn / dt) of the reactants and the diluent gas  

It is possible, in some cases, to test the correlation between the probe signal and atomic concentrations by the technique of gas titration. For instance, in a calorimeter probe detection of hydrogen atoms, it was determined by comparison with H + NOCl titrations that the electrical power required to balance the probe resistance was equivalent to the full recombination energy of the hydrogen atom flow [33]. 

Impurity levels must be strictly controlled and the reaction cell walls are usually poisoned with such substances as syrupy phosphoric acid or a mixture of dimethyldichlorosilane and trimethylchlorosilane to minimize the amount of heterogeneous recombination. 

This concludes a survey of the experimental techniques that have been used to measure dissociation and recombination rate coefficients. Although it is not always the practice to include in the literature an estimation of the errors associated with a particular set of rate coefficients, it is of vital importance when extrapolations of low tempera- 

The next section includes a selection of recent investigations and the explanations that have been advanced to correlate the experimental results with theory. 

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Both processes rely on a fast flow system and the rapid quenching of product gases yields of up to 90% can be attained. It is salutory to note that US production of this highly toxic compound is 600000 tonnes pa (1992) and world production exceeds one million tonnes pa. Of this, 41% is used to manufacture adiponitrile for nylon and 28% for acrylic plastics ... 

Dewald et al. (1963) dissolved Cs in ethylenediamine (ed) producing solvated electron esin e,. When mixed with a solution of water in ed in a fast-flowing system, evidence was obtained for rapid conversion of es to ed. Dewald and Tsina (1967) also... 

Felhner and Koski3 tried to isolate reaction (1) by studying the decomposition in a fast flow system coupled to a mass spectrometer. Using the mean of Garabedian and Benson s estimate3 of At, 8.0 x 1013 sec-1, they obtain Et = 23.1+2 kcal.mole-1. Because of the experimental difficulties encountered, the significance of this result is questionable. 

Eltenton141 studied the thermal decomposition of a very dilute stream of tetramethyl lead vapour in He (total pressure = 0.4 torr) in a fast flow system (contact time 0.1-0.001 sec) over the temperature range 400-700 °C. The decomposition was essentially complete at 600 °C. A small portion of the effluent from the reaction zone passed directly into the ionization chamber of a mass spectrometer. The reaction was followed by observing the methyl radical concentration. The rate-controlling step observed under these conditions is probably the loss of the first CH3 group by the reaction... 

In this section we discuss the major experimental methods used to determine absolute rate constants for gas-phase reactions relevant to atmospheric chemistry. These include fast-flow systems (FFS), flash photolysis (FP), static reaction systems, and pulse radiolysis. The determination of relative rate constants is discussed in Section C. 

TABLE 5.4 Some Typical Detection Systems Used for Reactive Species in Fast-Flow Systems... 

An alternative method of creating atoms and radicals in excited electronic states is the use of discharge methods in fast flow systems. This topic has recently been reviewed by Kolts and Setser [70]. Metal atoms have been produced in excited states by simple d.c. discharges [71] as well as by optical pumping [72]. Electronically excited inert gas and metal atoms may be produced by electron bombardment [73, 74]. 

Until recently, two major objectives of chemical kineticists were to explain overall chemical change in terms of elementary reactions and to determine the rates of these individual steps over a wide range of temperature. Within the last twenty years, the development of experimental techniques, such as flash photolysis, shock tubes, and gaseous fast-flow systems, have made it possible to observe the rapid changes that accompany many elementary reactions. The rate of reaction is defined in terms of the appearance or disappearance of a particular chemical entity. For example, for a bimolecular atom-exchange reaction like... 

The second technique (method I of ref. 264) is to measure the relaxation. Here the infrared emission is observed from different points downstream from where the reagents are mixed in a fast-flow system. Even at the shortest times, rotational relaxation is complete, but the relaxation of the vibrational states can be followed and the distributions extrapolated back to yield a set of Rv. Pacey and Polanyi [265] have found small, but significant, differences between the Rv derived from a simple extrapolation and those determined using an analysis that allowed for the concurrent processes of reaction, diffusion, flow, radiation, and deactivation. Using a large-capacity sorption... 

Exercise 1.3. Consider a coordination compound formed from BH3 and C2H4. From the HOMO-LUMO properties of each species predict the geometric structure of the Lewis acid-base adduct. Now predict the structure of a compound formed by replacing one CO ligand of Fe(CO)5 with C2H4. Note the parallelism between the main group and transition metal examples. The second compound is a stable and isolatable compound, whereas the first is a transient intermediate in the hydro-boration of ethylene to ethyl borane and has only been characterized as a transient intermediate in a fast-flow system by modulated mass spectrometry. 

Fig. 24(b) Channel flow systems pressurized fast-flow system. 

At room temperature and below, in fast flow systems, the additional reactions are those of H atoms with HO2... 

Clyne and Thrush [404] have examined the intensity / of the chemiluminescent emission between 200 and 300 K in a fast flow system. Oxygen atoms were generated either by dissociation of pure oxygen or... 

Chemiluminescence spectra of AlF(a n) produced in crossed-beam reaction of A1 with Fj t Tandem axis LMR/resonance fluorescence/resonance absorption fast-flow system used to study N + 0H,H02 and 0 + 0H,H02... 

This reaction has recently been studied by Coxon ° in a fast flow system at total pressures near 1 torr and at 294 °K. The reactants and products were followed using spectrophotometric methods and the suggested mechanism was... 

Klein and Herron studied the reaction in a fast-flow system they produced the oxygen atoms by titration of active nitrogen with NO and measured the concentration of oxygen atoms mass spectrometrically. The rate of decay of oxygen atoms was written as... 

Fair and Thrush have recently reported a rate coefficient of (1.0 + 0.2)x 10 P.mole. sec near room temperature for the recombination of sulfur atoms in the presence of Ar. Ground-state sulfur atoms were prepared in a fast flow system by the reactions... 

The thermal decomposition of vinyl thiol, CH2=CHSH, appears to proceed by a molecular mechanism, similar to that of ethane thiol. This conclusion was derived indirectly from the thermal decomposition of ethylene sulfide at 1000 °C vide infra), in a fast flow system in which the major products are C2H4, C2H2 and H2S, the latter two compounds formed in equal quantities if the reaction products are quickly trapped out of the effluent stream, vinyl thiol can be detected. Preliminary results suggest the following steps... 

COj CO-f- i Oj CO = 1.0 1600-1200 Fast flow system must be used, otherwise back reactions complicate the issue. ... 

Carrington and Levy and Westenberg and de Haas have reviewed the early work on gas-phase reactions in this field. The apparatus involves a fast-flow system where a gas, at a relatively low pressure is passed through a microwave discharge to the resonance cavity of the spectrometer. At room temperature, it is possible to have the cavity in variable positions but at higher temperatures it is fixed °° (Figs. 62 and 63, respectively). The rate coefficients found for reactions (57)-(60)... 

The decay of fluorine atoms in the other reactions was estimated to be less than 1%.229,241 Pulse radiolysis combined with UV absorption was employed by Jodkowski et al.229 at pressures of 500 - 1000 mbar (M = SF6), while a fast-flow system with a quadruple mass spectrometer was applied at pressures in the range of 0.7 - 5.1 mbar (M = He). The influence of other homogeneous reactions on the reaction kinetics was also analyzed by computer simulation and found to be negligible. The rate constant ki = (1.3 + 0.3) x 10 10 cm3molecule"1s 1 at 298 K has been estimated at pressures of 500 - 1000 mbar of SF5.229 The theoretical analysis of the reaction kinetics was based on the approach developed by Troe et al.17 23 The fall-off curves for the reaction CH3... 

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