Phase deviation from plug flow liquid

Advantages of three-phase fluidized beds over trickle beds and other fixed bed systems are temperature uniformity, high heat transfer, ability to add and remove catalyst particles continuously, and limited mass transfer resistances (both external to the particles and bubbles, because of turbulence and limited bubble size, and inside the particles owing to relatively small particle diameters). Disadvantages include substantial axial dispersion (of gas, liquid, and particles), causing substantial deviations from plug flow, and lack of predictability because of the complex hydrodynamics. There are two major applications of gas-liquid-solid-fluidized beds biochemical processes and hydrocarbon processing. 

The dispersion coefficients Dq and Di are included to account for deviations from plug flow in both gas and liquid phases, as mentioned above. Equations (8-191) and (8-192) include all possibihties (or at least as many as we are willing to consider at this point), so we can now look at individual cases of interest by chipping away the particular parts that do not apply. 

Finally, it should be noted that more sophisticated models have been developed, either on a stagewise basis [62], similar to the Deans-Lapidus model for single phase fixed bed reactors, or on a stochastic respectively propabilistic basics [67,66]. Using data from laboratory and full-scale reactors, Schwarz and Roberts [44] have carried out parametric studies to evaluate the accuracy of the axial dispersion model. Their simulation showed that in case of first-order kinetics, dispersion in the liquid phase is frequently not of major importance. Deviations from plug flow become important only for short reactors and a high degree of conversion. 

The parameters in the correlations were obtained using nonlinear parameter estimation software and by minimizing the sum of the relative errors between experimental and predicted conversions for the 2.5% Pd catalyst. A comparison between predicted and experimental values of conversion for both solvents is given in Figure 2. The agreement between the two is seen to be satisfactory except for hexane at the highest conversion where deviations of the liquid-phase from plug flow may influence the result. 

In a sparged reactor, the behavior of the liquid and gas phase deviates significantly from plug flow, particularly at high gas and low liquid velocities. This deviation is generally accounted for by the axial dispersion coefficient, D. Deckwer (1992) has discussed this matter in detail outlining the various approaches used to quantify axial dispersion. Tables 10.3 and 10.4 list some of the correlations available in the hterature for estimating the liquid- and gas-phase axial dispersion coefficient, and D, respectively. 

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