Reactant concentration, effect

Many reactions over zeolites don t seem to require strong acidity. They are probably utilizing the polarity of the crystal lattice, and taking advantage of the tremendous reactant-concentrating effect of the vast, membrane-like internal surface area of the zeolite. Perhaps we even can look at zeolites as rugged, primitive proto-enzymes. Of course, they... 

Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

In the process of establishing the kinetic scheme, the rate studies determine the effects of several possible variables, which may include the temperature, pressure, reactant concentrations, ionic strength, solvent, and surface effects. This part of the kinetic investigation constitutes the phenomenological description of the system. 

Certain free radical polymerization data gave curves when plotted according to Eq. (2-15) but straight lines accordingto Eq. (2-19). This apparent paradox was resolved by postulating that some constant portion R of reactant is unreactive and serves to diminish the effective reactant concentration, lowering it to Ca - / The appropriate form of Eq. (2-15) is then... 

These apply to a bimolecular reaction in which two reactant molecules become a single particle in the transition state. It is evident from Eqs. (6-20) and (6-21) that a change in concentration scale will result in a change in the magnitude of AG. An Arrhenius plot is, in effect, a plot of AG against 1/T. Because a change in concentration scale alters the intercept but not the slope of an Arrhenius plot, we conclude that the values of AG and A, but not of A//, depend upon the concentration scale employed for the expression of reactant concentrations. We, therefore, wish to know which concentration scale is the preferred one in the context of mechanistic interpretation, particularly of AS values. 

UV pre-irradiation [976] increased the rate of NH MnC decomposition at 351 K up to a maximum, followed by a decrease in rate with a further exposure. A similar maximum was observed for samples which had been aged for various times. These effects are ascribed to partial decomposition with the formation of products which, at low concentration, accelerate decomposition but at higher concentrations increase the stability of the reactant by effectively opposing self-heating during reaction. 

The buildup of very low concentrations of product, if it can be monitored, will provide the initial rate. During this time the concentrations of the reactants remain effectively constant. A family of such determinations provides both the form of the kinetic expression and the value of the rate constant. 

A hydrocarbon is cracked using a silica-alumina catalyst in the form of spherical pellets of mean diameter 2.0 mm. When the reactant concentration is 0.011 kmol/m3, the reaction rate is 8.2 x 10"2 kmol/(m3 catalyst) s. If the reaction is of first-order and the effective diffusivity De is 7.5 x 10 s m2/s, calculate the value of the effectiveness factor r). It may be assumed that the effect of mass transfer resistance in the. fluid external Lo the particles may be neglected. 

Fig. 5.3. Now the reaction rate is determined by AGcage and Ag age, but AGcage is almost entirely determined by simple concentration factors. Thus a comparison of Ag age and Agfat allows one to explore fundamental catalytic aspects, including real entropic effects, without preoccupation with the rather trivial effective concentration effect, associated with bringing the reactants to the same cage.

FIGURE 12.1 Effects of substrate (reactant) concentration on the rate of enzymatic reactions (a) simple Michaelis-Menten kinetics (b) substrate inhibition (c) substrate activation. 

The data are summarised in Table 9. At high concentrations of formic acid the reaction becomes less than first-order in substrate this indicates the possibility of complex-formation, but a medium effect may also be influential in the vicinity of 1 Af formic acid. Complex-formation affects the kinetics of the Tl(rrr) oxidation at all but the lowest reactant concentrations " . 

Table 1.6 Characteristic quantities to be considered for micro-reactor dimensioning and layout. Steps 1, 2, and 3 correspond to the dimensioning of the channel diameter, channel length and channel walls, respectively. Symbols appearing in these expressions not previously defined are the effective axial diffusion coefficient D, the density thermal conductivity specific heat Cp and total cross-sectional area S, of the wall material, the total process gas mass flow m, and the reactant concentration Cg [114].

If diffusion of reactants to the active sites in pores is slower than the chemical reaction, internal mass transfer is at least partly limiting and the reactant concentration decreases along the pores. This reduces the reaction rate compared to the rate at external surface conditions. A measure of the reaction rate decrease is the effectiveness factor, r, which has been defined as ... 

Catalyst effectiveness may be determined by two different methods, based on (a) an estimate of the slope of the reactant concentration, s, at the solid surface... 

The effect of reactant concentrations on reaction rate was studied using unpromoted skeletal copper catalysts initially leached at 278 K and then... 

Figure 2 The effect of different reactant concentrations on hydrogen evolution during ethanolamine dehydrogenation over unpromoted skeletal copper at standard conditions.

It is commendable experimental procedure to repeat each run in duplicate and to be satisfied if the two results agree, but this is expensive in terms of the labor costs involved. Moreover, repetition of each run is not always necessary. For example, if one is studying the effect on the reaction rate of a variable such as temperature or reactant concentration, a series of experiments in which the parameter under investigation is systematically varied may be planned. If a plot of the results versus this parameter yields a smooth curve, one generally assumes that the reproducibility of the data is satisfactory. 

In order to reduce the disparities in volume or space time requirements between an individual CSTR and a plug flow reactor, batteries or cascades of stirred tank reactors ard employed. These reactor networks consist of a number of stirred tank reactors confiected in series with the effluent from one reactor serving as the input to the next. Although the concentration is uniform within any one reactor, there is a progressive decrease in reactant concentration as ohe moves from the initial tank to the final tank in the cascade. In effect one has stepwise variations in composition as he moves from onfe CSTR to another. Figure 8.9 illustrates the stepwise variations typical of reactor cascades for different numbers of CSTR s in series. In the general nonisothermal case one will also en-... 

Figure 9.3 contains typical instantaneous yield versus reactant concentration plots and the shaded areas indicate the composition changes of the desired product that are effected by various reactor types. From the definition of the overall yield,... 

The reactant concentration C will be greater than zero throughout the entire length of the pore provided that h0 < y/2. In this case the effectiveness factor will be unity because the reaction rate is independent of concentration. For values of h0 > yfl, equation 12.3.44 would call for negative values of the reactant concentration at large values of x/L, a situation that is clearly impossible. Hence the boundary conditions on equation 12.3.43 must be changed so that both the reactant concentration and its gradient become zero at a point in the pore that we label with a coordinate xc. In this situation, the concentration profile becomes... 

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