Mixing Characteristics of Irrigated Packed Towers
1958 Volume 22 Issue 3 Pages 144-150
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1958 Volume 22 Issue 3 Pages 144-150
Mixing characteristics of flow reactors were studied, Using irrigated packed towers to make clear dynamic characteristics of reactors, flow mcchanisms of fluid and holding time of fluid in reactors, The method employed in our study was the residence time curve method, the commonest one employed in such a case.
An equation for the residence time curve, with the boundary conditions suitable for these irri- gated packed towers, was analytically derived from the concepts of the mean velocity and the apparent diffusivity, as suggested and used by Danköler3), Gilliland5), Danckwerts2), and Yagi11), in the course of studies of flow through tubular reactors, fluibized beds, and multistage agitated tanks.
The results of the residence time curve equation thus obtained were compared with the data derived from the experiments with the irrigated packed towers whose total holdup had been measured. The region in the irrigated packed towers in which our investigations were centered, in connection with the mixing characteristics of the liquid holdup, was where the models of the mean velocity and the apparent diffusivity were reasonably applicable.
Fig. 1, Table 1 and Table 2 show the apparatus and the dimensions of packings and towers used in these experiments. With this apparatus, the total holdup and the residence time curves, from which the values of M (Table 4) were determined by means of Eq. (7), were obtained under various conditions, either with or without gas flow.
The conclusions derived from the experimental data illustrated in Figs. 2, 3, 4, 7, 8, 9 and 10 are as follows:
1) Operating characteristics of liquid hold flowing in a packed tower are specifed by the mean flow velocity, u=L/θ=L/(Qt/F), and the dimensionless group, M=uL/2E, which represents the mixing characteristics of liquid flow.
2) By Eqs. (12) and (15), the operating holdup without gas flow, Hop, and the apparent diffusivity, E, are respectively well correlated with the mean liquid velocity, u. A simplified method is proposed for estimating the static holdup, Hst, by the use of mean liquid velocity, u, instead of superficial liquid velocity; that is, by means of extraporating observed values of Ht relative to u=0; obtaining excellent results, as shown in Table 3. This is because Ht and u are linear correlation represented by a straight line, as illustrated in Fig. 3.
3) The values of M in the case of irrigated packed towers, as listed in Table 4, are very high as compared with those in other types of flow reactors, indicating that the liquid flow in a packed tower closely resembles the piston flow, the more so, by increasing the length of the packed bed and the mean velocity of liquid and decreasing the diameter of packing.
