Chemical reaction time

F. Gas Absorption with Chemical Reaction—Time-Dependent Bulk Concentration. 346... 

Fig. 4. Variation of autocorrelation function with changes in the equilibrium constant in the fast reaction limit. A and B have the same diffusion coefficients but different optical (fluorescence) properties. A difference in the fluorescence of A and B serves to indicate the progress of the isomerization reaction the diffusion coefficients of A and B are the same. The characteristic chemical reaction time is in the range of 10 4-10-5 s, depending on the value of the chemical relaxation rate that for diffusion is 0.025 s. For this calculation parameter values are the same as those for Figure 3 except that DA = Z)B = lO"7 cm2 s-1 and QA = 0.1 and <9B = 1.0. The relation of CB/C0 to the different curves is as in Figure 3. 

Thus, typical chemical reaction times are very fast. Note, if a chemical gas phase retardant is present or if the oxygen concentration is reduced in the ambient, A would be affected and reduced. Thus the chemical time could become longer, or combustion might not be possible at all. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

The flame lift-off height, which is related to the ignition distance, was inversely affected by the excitation frequency. Since the flow time scale decreased with increasing frequency, the data were plotted as a function of the Damkohler number in Fig. 29.14, where the characteristic flow time scale was estimated by large-eddy turnover time as 1/17 and the characteristic chemical reaction time was computed using an ignition delay model [21] for ethylene jet. While the results did not show any evidence of critical Damkohler number, the range... 

Treact — superficial chemical reaction time scale = H/fciR Then after a short calculation we get the non-dimensional form of Equation (24) ... 

As discussed in detail in Sections C.3.d and C.3.e, the fastest atmospheric reactions of S02 are believed to be with H202 and perhaps with Os at higher pH values. Under extreme conditions of large fog droplets (—10 yu,m) and very high oxidant concentrations, the chemical reaction times may approach those of diffusion, particularly in the aqueous phase. In this case, mass transport may become limiting. However, it is believed that under most conditions typical of the troposphere, this will not be the case and the chemical reaction rate will be rate determining in the S(IV) aqueous-phase oxidation. 

If finite chemical reaction times are put into the columnar diffusion flame theory (76), burning rates are predicted to be linearly proportional to pressure at low pressure and independent of pressure (plateau burning) at high pressure. Based on this model, von Elbe et al. (97) proposed the simple equation ... 

Question (b) is a matter of chemical kinetics and reduces to the need to know the rate equation and the rate constants (customarily designated k) for the various steps involved in the reaction mechanism. Note that the rate equation for a particular reaction is not necessarily obtainable by inspection of the stoichiometry of the reaction, unless the mechanism is a one-step process—and this is something that usually has to be determined by experiment. Chemical reaction time scales range from fractions of a nanosecond to millions of years or more. Thus, even if the answer to question (a) is that the reaction is expected to go to essential completion, the reaction may be so slow as to be totally impractical in engineering terms. A brief review of some basic principles of chemical kinetics is given in Section 2.5. 

Some scientists currently forecast that research on the archaebacteria may lead to a better understanding of catalytic enzymes and, because of tins property, lead to catalysts that can participate at higher temperatures and thus accelerate chemical reaction time. 

For an adiabatic reaction we found two kinds of features in the curves of the dependence of the percentage consumption on the chemical reaction time. In stable regimes an increase in the time facilitates an increase in the percentage of reacted substance and an increase in the temperature. With an increase in the residence time, for sufficient initial concentration, self-ignition of the mixture is possible in contrast, extinction occurs only when the reaction time is reduced. 

We express the chemical reaction time r in terms of the normal flame propagation rate since the latter is fairly well studied experimentally as a function of various parameters (mixture composition, etc.). 

However, just as for different geometric forms, we are frequently able to introduce the concept of hydraulic diameter behind which differences in the geometric forms are hidden, we may assume that there exists an effective chemical reaction time which is identical in formulas (13)—(15), so that in this case the form of formulas (16) and (17) will not depend on the reaction kinetics. The basis for such an assumption is the fact that in a... 

Indeed, in normal (slow) combustion, which may propagate only due to heat conduction, the heat flux is a quantity of the same order as the combustion heat released in unit time. The width of the front should be of the same order as the product of the chemical reaction time and the flame propagation velocity. 

In our theory such a time is introduced through losses—the time of braking and time of heat transfer. It is precisely the interrelation of these times with the chemical reaction time that determines the relative losses. 

Theoretical analysis of gas detonation leads to the conclusion that a shock wave propagates at the detonation front, compressing and heating the gas mixture. The chemical reaction runs in the already compressed gas, and it is only after completion of the reaction that the state of the explosion products calculated in the classical theory is attained (pressure pc, velocity wc, temperature Tc). In particular, in the wave front the velocity w1 and the pressure p1 of the compressed gas are approximately twice as large as in the reaction products w1 2wc, p1 2pc. The amount of the compressed gas at the pressure px and the thickness of this layer are proportional to the chemical reaction time, r. 

Footnote to Table Here and po are the initial filler density and the density of the system, u = particle velocity, X = chemical reaction time, a = reaction time zone width, and T = polytropic index... 

The circumstellar chemistry is often subdivided into three main zones, which are determined by a comparison of the characteristic dynamic flow time, R/vx, with the chemical reaction times (Lafont et al. 1982 Omont 1987 Millar 1988). (i) In the region closest to the star (perhaps R 1014 cm), the density is sufficiently high that three-body chemical reactions occur in a time short compared to the dynamic time. In this regime, we expect the chemical abundances to approach thermodynamic equilibrium, (ii) Somewhat further away from the star (1014 cm < R < 1016 cm), there is a freeze-out of the products of the three-body reactions (McCabe et al. 1979). In this region, two-body reactions dominate the active chemistry, (iii) Finally, far from the star (R > 1016 cm), the density becomes sufficiently low that the only significant chemical processing is the photodestruction that results from absorption of ambient interstellar ultraviolet photons by the resulting molecules that flow from the central star. 

Optimal operating conditions can be maintained over a wide range of flow rates and feed compositions. The high energy density that is generated reduces the chemical reaction time. This results in a short residence time for the reactants to be converted into products. A wide range of hydrocarbons can be used for the production of synthesis gas or hydrogen, with conversion of hydrocarbons close to 100%. 

The characteristic chemical reaction time tr or characteristic time scale of the chemistry may be calculated from the reaction rate expression. For a single reaction,... 

The salt/ice bath is a modified version of the ice bath. Depending on the type of salt used, salt/ice baths are very useful for producing temperatures ranging from -55 to 0 Celsius. To prepare a salt/ice bath, simply mix the finely crushed ice with 20% of its weight in salt. Salt/ice baths can maintain their temperatures for varying amounts of time depending on the heat evolved during a particular chemical reaction, time, and/or other conditions. In some procedures the salt/ice bath will have to be replaced with a fresh batch. 

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