Reaction rate agitation effect

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

For chemical reaction-rate constants greater than 10 sec-1, NT increases linearly with the total bubble surface area, i.e., linearly with the gas holdup. In other words, the agitation rate only affects the total bubble surface area and has almost no effect on the rate of absorption per unit area. This result is in accordance with the work of Calderbank and Moo-Young (C4), discussed in Section II. 

Figure 4 Effect of agitation speed on reaction rate.

If agitation fails during a semi-batch operation, the transfer of heat will essentially stop. The resulting increase in temperature depends on the concentration of the reactants at that moment, the global kinetics, and the mass transfer rate. The effect of the temperature increase is easily simulated in a reaction calorimeter both with and without addition of reactants. 

The information required to predict electrochemical reaction rates (i.e., experimentally determined by Evans diagrams, electrochemical impedance, etc.) depends upon whether the reaction is controlled by the rate of charge transfer or by mass transport. Charge transfer controlled processes are usually not affected by solution velocity or agitation. On the other hand, mass transport controlled processes are strongly influenced by the solution velocity and agitation. The influence of fluid velocity on corrosion rates and/or the rates of electrochemical reactions is complex. To understand these effects requires an understanding of mixed potential theory in combination with hydrodynamic concepts. 

In a flow system, the flow rate can be varied while the space velocity is kept constant (Figure 6). If the conversion remains constant, the influence of interphase and intrareactor effects may be assumed to be negligible. A similar test can be done in a CSTR. In that case the absence of interphase and intrareactor effects can be assumed if the reaction rate is independent of the rate of agitation. 

Increasing the pressure of the gaseous reactant not only increases the amount present in the gas phase but also increases gas/liquid transport and the solubility of the gas in the liquid phase. This, in turn, facilitates liquid/solid transport of this species. All of these factors increase the availability of the gaseous reagent to the catalyst. Fig. 5.11 shows a typical plot for the relationship between hydrogen pressure and the reaction rate at a fixed catalyst quantity and agitation rate.28 At lower values an increase in pressure promotes an increase in rate but above a given value further increases in pressure have little or no effect on the rate. In the... 

The type of agitation had a large effect on but not on specific surface. In the vortex-mixed system specific surface did increase with vortex rale, but the increase was not as large as in the stirred system. Furthermore, Ihe. specific surface in the vortex system at the lower vortex rates was the Maine as that in the stirred system at the lower stirring rates. Prolonged agi-iMlion periods are probably required to produce measurable effects on surface area and reaction rate. 

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