Longitudinal dispersion of liquid flowing through fixed and fluidized beds

Abstract

Longitudinal liquid mixing in fluidized and fixed beds was studied using sinusoidal and pulse response techniques. The tracer used was light emissive fluorescein dye. A systematical study of liquid phase dispersion by varying particle size, fluid velocity, fraction voids, and particle density was conducted. A generalized correlation applicable to both fixed bed and fluidized bed was obtained. The application of the correlation in predicting the effect of the dispersion on reactor performance was discussed.The study of fluid mixing in a continuous flow system is of considerable importance for the design of chemical reactors. Neglecting the fluid dispersion may result in an overestimation of the conversion, the driving force, and the volume efficiency of the system. In order to estimate the dispersion coefficient in a continuous flow reactor, the dynamic response method has been generally applied. This method involves injecting a tracer into the system according to a certain function and matching the response curve with that derived from the mathematicaI model. The parameters characterizing the mixing or dispersion of the fluid in the system are then evaluated from the best fitted mathematical curve.In this work, experiments have been performed by using sinusoidal and pulse inputs of fluorescein tracer to fixed beds and fluidized beds. A systematic study of liquid phase dispersion has been conducted, particularly, the effects of particle size, fluid velocity, fraction voids, and particle density have been considered. Based on the knowledge of the ef€ect of individual factors on the dispersion coefficient and the correlations suggested by the previous investigators, a generalized correlation valid for both fixed beds and fluidized beds has been obtained. A large number of fixed bed data and fluidized bed data from literature were also incorporated into this correlation in order to confirm its validity. This work is limited to the study of liquid phase mixing in particulate systems consisting of uniform size particles. Table 1 shows a summary of the previous investigations on the longitudinal dispersion of liquid in fixed beds. Although there has been a large number of investigations on the fluid dispersion in fixed beds, only a few studies are available on the loiigitudinal dispersion of liquid in fluidized beds as shown in Table 2. The information obtained from the previous investigations on liquid mixing in fixed and fluidized beds have been fragmentary. So far no correlation that ties together the longitudinal dispersion in fixed beds with that in fluidized beds has been availabIe. The usual method of correlation of the Iongitudinal dispersion is by plotting either the Peclet number vs. the Reynolds number, or the dimensionless dispersion group, Exp/p, vs. the Reynolds, or by relating some dimensionless groups involving voidage, e. For the fixed bed, plots of the Peclet number vs. the Reynolds number has been commonly employed (10, 15, 20, 36, 37, 41, 49). However, disagreements in the magnitude of the Peclet num...

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