Fast reaction regime

There are obviously many reactions that are too fast to investigate by ordinary mixing techniques. Some important examples are proton transfers, enzymatic reactions, and noncovalent complex formation. Prior to the second half of the 20th century, these reactions were referred to as instantaneous because their kinetics could not be studied. It is now possible to measure the rates of such reactions. In Section 4.1 we will find that the fastest reactions have half-lives of the order 10 s, so the fast reaction regime encompasses a much wider range of rates than does the conventional study of kinetics. 

Figures 3 and 4 illustrate how G( r) changes with Keq in the fast reaction regime (tc -C to). For both figures, k and the equilibrium constant A e( = l

Referring to Fig. 4.3, it can be seen that with this value for ft the system will lie in the fast reaction regime, Region I, and that the packed column will be a suitable reactor. Also, fi in equation 4.13 is sufficiently large for tanh 0 to be effectively 1, so that equation 4.14 applies ... 

First of all the value of /) = VH QshDco,/ will be calculated in order to check that the fast-reaction regime will apply. 

The fast reaction regime for k rlkg 1. The rate of mass transfer completely limits the conversion rate, which follows from... 

Thus, the mass-transfer resistance is negligible. For the fast reaction regime (i.e. PA oo and Equation 4.8 simplifies to one of the following equations ... 

Figure 2-2 The concentration distribution for species A. B, and C in the fast reaction regime, based on the film theory. Reaction occurs mainly in the liquid film mass transfer and reaction are parallel processes the absorption rate of the gas is increased due to the chemical reaction...

The model described above assumes constant gas velocity and pressure in the reactor. Recently, Deckwer6 outlined a dispersion model which took into account the opposite effects of gas shrinkage and expansion caused by absorption and reduced hydrostatic head. A first-order reaction in the liquid phase was assumed. Both slow and fast reaction regimes were considered. The governing nonlinear differential equations were solved on the computer. 

Useful for studies of chemical adsorption in conditions which approach fast reaction regime. 

Like the gas holdup the gas-liquid interfacial area, a, represents an important quantity. If the reaction takes place in the fast reaction regime of diffusion-reaction theory, the interfacial area is the main design criterion. Gas holdup and interfacial area are related by... 

Figure 3. Concentration profile across a gas-liquid interface with moderately fast reaction (Regime II).

Here tD and fr are the diffusion and reaction times, respectively, and k, is the mass-transfer coefficient in the absence of reaction. For the fast reaction regime, diffusion and reaction occur in parallel in the liquid film, while for the slow reaction regime, there is no reaction in the liquid film and the mass transfer can be considered to occur independently of reaction in a consecutive manner. For the slow reaction regime, the following subregimes can be defined ... 

Here He is the Henry constant for the solute a. For the fast reaction regime, instead of the effectiveness factor adjustment for the intrinsic reaction rate, it is customary to define an enhancement factor for mass-transfer enhancement by the reaction, defined as the ratio of mass transfer in presence of reaction in the liquid, to mass transfer in absence of reaction ... 

Ha 1 slow reaction regime Ha 1 fast reaction regime... 

The broad reage of conditions between the slow reaction regime in which E - I and the instantaneous reaction regime in which is very large bat independent of reaction rate is classified generally as the "fast reaction regime." In this regime, the mass transfer coefficient kL is a function of the reaction rale. Both kL and k[ are affected also by fluid mechanics, bat, fortunately, their ratio has been found to be relatively independent of these factors. 

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