In packed towers

The predictions of correlations based on the film model often are nearly identical to predictions based on the penetration and surface-renewal models. Thus, in view of its relative simphcity, the film model normally is preferred for purposes of discussion or calculation. It should be noted that none of these theoretical models has proved adequate for maldug a priori predictions of mass-transfer rates in packed towers, and therefore empirical correlations such as those outlined later in Table 5-28. must be employed. 

Investigators of tower packings normally report kcCi values measured at very low inlet-gas concentrations, so that yBM = 1, and at total pressures close to 100 kPa (1 atm). Thus, the correct rate coefficient For use in packed-tower designs involving the use of the driving force y — y /yBM is obtained by multiplying the reported k co values oy the value of pf employed in the actual test unit (e.g., 100 kPa) and not the total pressure of the system to be designed. 

FIG. 15-34 Flooding in packed towers, Use only customary units in the vari-ahles, [Crawford and Wilke, Chem, Eng, Prog, 47, 423 (1951), with petmis-... 

Two dimensionless variables play key roles in the analysis of single transition systems (and some multiple transition systems). These are the throughput parameter [see Eq. (16-129)] and the number of transfer units (see Table 16-13). The former is time made dimensionless so that it is equal to unity at the stoichiometric center of a breakthrough cui ve. The latter is, as in packed tower calculations, a measure of mass-transfer resistance. 

TABLE 23-8 Correlation of Kgo for Absorption of CO by Aqueous Solutions of Monoethanolamine in Packed Towers ... 

Figure 9-14. Liquid redistribution in packed towers. Used by permission of Norton Chemical Process Products Corp.

Kaiser [140] presents a correlation analysis for flooding in packed towers that is more analytical in the performance approach. It is based on single phase hydraulics. It would have been helpful for the article to present a comparison of results tvith the other more conventional techniques. 

The transfer unit concept is also applicable to distillation in packed towers. Height of the packing required is ... 

Lmin = Minimum liquid wetting rate in packed tower, ft /(hr) (ft cross-section)... 

Jesser, B. W. and J. C. Elgin, Studies of Liquid Hold-up in Packed Towers, Trans. Amer. Inst. Chem. Engr., 39, No. 3 277 (1943). 

Otake, T. and K. Okada, Liquid Hold-up in Packed Towers, Operating and Holdup Without Gas Flow, Soc. Chem. Engrs. Qapan) 17, No. 7, 176 (1953). 

Copigneaux, R, Flooding in Packed Towers, Hydro. Processing Feb. (1981) p. 99. 

The problems relating to mass transfer may be elucidated out by two clear-cut yet different methods one using the concept of equilibrium stages, and the other built on diffusional rate processes. The selection of a method depends on the type of device in which the operation is performed. Distillation (and sometimes also liquid extraction) are carried out in equipment such as mixer settler trains, diffusion batteries, or plate towers which contain a series of discrete processing units, and problems in these spheres are usually solved by equilibrium-stage calculation. Gas absorption and other operations which are performed in packed towers and similar devices are usually dealt with utilizing the concept of a diffusional process. All mass transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

Figure 4.17. Generalised correlation for flooding rates in packed towers(6l)...

Tour, R. S. and Lerman, F. Trans. Am. Inst. Chem. Eng. 35 (1939) 709-18. An improved device to demonstrate the laws of frequency distribution. With special reference to liquid flow in packed towers. 

Mass transfer coefficients and specific area in packed towers... 

Semmelbauer, R. Chem. Eng. Sci. 22 (1967) 1237. Die Berechnung der Schtitthohe bei Absorptions-vorgangen in Fullkbrperkolonnen. (Calculation of the height of packing in packed towers.)... 

The derivation of equations 13.34 and 13.35 has been carried out assuming that u0 is constant and independent of the flowrates, up to and including the flooding-point. This in turn assumes that the droplet size is constant and that no coalescence occurs as the hold-up increases. Whilst this assumption is essentially valid in properly designed spray towers, this is certainly not the case with packed towers. Equations 13.34 and 13.35 cannot therefore be used to predict the flooding-point in packed towers and a more empirical procedure must be adopted. 

Up to the present time, work has been done which allows prediction of the onset of large waves (H2), and of formation of other types of waves (VI, HI), but only on flat uniform liquid surfaces. The extent to which these results can be applied to pipe line flow is uncertain. Apparently, Gazley s papers are still the only basic reports of stratified and wave flow in horizontal pipe incidentally they also show a parallel between liquid instability in pipe flow as evidenced by wave formation, and that evidenced in packed towers by flooding. 

Maintaining Functional and Structural Efficiency in Packed Towers... 

The key properties of mixtures of air and water vapor are described in Section 9.1. Here the interactions of air and water in packed towers under steady flow conditions will be analyzed. The primary objectives of such operations may be to humidify or dehumidify the ait as needed for particular drying processes or other processes, or to cool process water used for heat transfer elsewhere in the plant. Humidification-dehumidification usually is accomplished in spray towers, whereas cooling towers almost invariably are filled with seme type of packing of open structure to improve contacting but with minimum pressure drop of air. 

In packed towers, the variation of conditions from top to bottom is continuous and not interrupted as at trays. Nevertheless, it is convenient to speak of packing heights equivalent to a theoretical tray (HETU), so that tray tower theory can be applied to the design of packed towers. 

Figure 13.15. Mechanism, nomenclature, and constructions for absorption, stripping and distillation in packed towers, (a) Two-film mechanism with equilibrium at the interface, (b) Sketch and nomenclature for countercurrent absorption or stripping in a packed tower, (c) Equilibrium and material balance lines in absorption, showing how interfacial concentrations are found, (d) Equilibrium and material balance lines in stripping, showing how interfacial concentrations are found, (e) Equilibrium and material balance lines in distillation, showing how interfacial concentrations are found.

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