Reactors, agitated tank backmix

Another alternative for three-phase catalytic reactors with suspended catalyst particles is to use mechanically agitated tank reactors (Figure 6.7). In a tank reactor, the flow profile can approach complete backmixing. 

Stead et al. (S23) and Johnson and Edwards (J3) showed that homogeneity can be achieved in as short a time as 0.1 sec., with sufficient agitation in a laboratory sized stirred tank. The relation between this time and the intensity of agitation was studied by MacDonald and Piret (Ml). Eldridge and Piret (E5) then used kinetic experiments to show that a series of up to five laboratory-sized stirred tanks with sufficient agitation acted as perfect backmix reactors. 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

The stirred-tank continuous reactor may be used where a high degree of agitation is necessary, possibly where a solid is held in suspension, where the reaction requires a relatively low holding time, and where backmixing is not detrimental to the yield. 

Mechanically agitated contactors are characterized by complete backmix-ing of both liquid and gas phases thus Equation 16.24 should be applicable to these contactors. Deviation from MF is possible at low stirrer speeds. If the reaction is in regime 2, accurate values of are needed. Because predictive correlations for this are unreliable, the values should be obtained by conducting experiments in a geometrically similar laboratory reactor 20 30 cm diameter. To obtain such similarity, it is recommended that the ratio of tip speed to tank diameter should be kept constant (Juvekar, 1976). 

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