Of packing towers

Sometimes the height equivalent to a theoretical plate (HETP) is employed rather than and to characterize the performance of packed towers. The number of heights equivalent to one theoretical plate required for a specified absorption job is equal to the number of theoretical plates,... 

For prehminary screening and easibility studies or for rough cost estimates, one may wish to employ a version of the isothermal method which assumes that the liquid temperatures in the tower are everywhere equal to the inlet-liquid temperature. In their analysis of packed-tower designs, von Stockar and Wilke [Ind. Eng. Chem. Fun-dam. 16, 89 (1977)] showed that the isothermal method tended to underestimate the reqmred depth of packing by a factor of as much as 1.5 to 2. Thus, for rough estimates one may wish to employ the assumption that the temperature is equal to the inlet-liquid temperature and then apply a design fac tor to the result. 

The number of variables that are known to influence the rate of extraction is exceedingly large, and includes at least the following Size, shape, and material of packing Tower diameter Packing depth... 

TABLE 15-7 Selected Sources of Packed-Tower Mass-Transfer Data... 

In a trayed absorber the amine falls from one tray to the one below in the same manner as the liquid in a condensate stabilizer (Chapter 6, Figure 6-4). It flows across the tray and over a weir before flowing into the next downcomer. The gas bubbles up through the liquid and creates a froth that must be separated from the gas before it reaches the underside of the next tray. For preliminary design, a tray spacing of 24 in. and a minimum diameter capable of separating 150 to 200 micron droplets (using the equations developed in Volume 1 for gas capacity of a vertical separator) can be assumed. The size of packed towers must be obtained from manufacturer s published literature. 

The diameter of packed towers may differ depending upon parameters developed by the packing manufacturers and random packing. Conventional packing will require appro.ximat.ely the same diameter as bubble... 

The operation of packed towers under -vacuum conditions is not well defined in the literature. However, the work of Hands and Whitt [33] specifically evaluates several systems operating from 20 to 760 mm Hg abs. Their recommended limiting -vapor velocity is... 

The height of the transfer unit has not been satisfactorily correlated for application to a wide variety of systems. If pilot plant or other acceptable data are available to represent the system, then the height of packing can be safely scaled-up to commercial units. If such data are not available, rough approximations may be made by determining Hg and Hl as for absorption and combining to obtain an Hqg (Ref. 74, pg. 330). This is only very approximate. In fact it is because of the lack of any volume of data on commercial units that many potential applications of packed towers are designed as tray towers. 

Sherwood, T. R and F. A. L. Holloway, Performance of Packed Towers—Experimental Studies of Absorption and Desorption, Trans. Amer. Inst. Chem. Eng., 36, 21 (1940). 

Carey, W. F. and Williamson, G. J. Proc. Inst. Mech. Eng. (Steam Groupi 163 (1950) 41. Gas cooling and humidification design of packed towers from small scale tests. 

FIGURE 18.10 Schematic diagram of packed tower aerator. 

The construction of packed towers is relatively straightforward. The shell of the column may be constructed from metal, ceramics, glass, or plastics material, or from metal with a... 

Rose, H. E. and Young, P. H. Proc. Inst. Mech. Eng. IB (1952) 114. Hydraulic characteristics of packed towers operating under countercurrent flow conditions. 

Davidson, J. E, Cullen, E. J., Hanson, D., and Roberts, D. Trans. Inst. Chem. Eng. 37 (1959) 122. The hold-up and liquid film coefficient of packed towers. Part I. Behaviour of a string of spheres. 

Tower diameter selection is usually made with the maximum expected gas and liquid flow rates and depends on the size and type of tower packing The portion of Fig. 3 with solid lines illustrates the pressure-drop relationship for 1.5-m. [3.81 -cm] Pull rings supplied by the manufacturer for typical superficial gas and liquid flow rates used in the chemical processing industry.1 This relationship is usually available in graphical form from any manufacturer of packed-tower packing elements. While these... 

In any type of packed tower, the liquid or internal reflux drips through the packing, and forms a thin film of liquid on the surface of the packing. Vapor percolates up through the packing, and exchanges heat and molecules with the thin film of liquid on the surface of the packing. 

To summarize, the chief disadvantages of packed towers, as compared to trays, are... 

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

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. 

Continuous changes in compositions of phases flowing in contact with each other are characteristic of packed towers, spray or wetted wall columns, and some novel equipment such as the FHGEE contactor (Fig. 13.14). The theory of mass transfer between phases and separation of mixtures under such conditions is based on a two-film theory. The concept is illustrated in Figure 13.15(a). 

The concepts NTU and HTU are defined only for binary distillations and the transfer of a single substance in absorption or stripping. Since most processes of industrial interest involve multicomponents, the HETS of packed towers is the more useful concept, and may be evaluated readily from test data and tray calculations. 

The various kinds of internals of packed towers are represented in Figure 13.35 whose individual parts may be described one-by-one ... 

The most useful measure of the separating power of packed towers is the HETP, the height equivalent to a theoretical plate or stage. It is evaluated simply as the ratio of packed height used for a certain degree of separation to the theoretical number of stages. Its relation to the fundamental quantity, HTU, or the height of a transfer unit, is... 

Gil. Grimley, S. S., Effects of liquid flow conditions on the performance of packed towers. Ph.D. thesis, Univ. London, England, 1947. 

Figure a. 16 Typical efficiency characteristics of packed towers, (a) Typical efficiency characteristics for random packings and for most corrugated-sheet structured packings (6) effect of liquid distribution on the efficiency characteristics of part o. (Part b from H. Z. Kieter, Distillation Operation. Copyright C by McGhawnMf, Me. Reprinted by permission.)... 

Figure 8.16 (Continued) Typical efficiency characteristics of packed towers, (c) Typical efficiency characteristics for wire-mesh structured packings (d) example of efficiency characteristics, measured for randem packing, that deviate from those in parts a and c. (Fart d from J. S. Eckert and L. F. Walter, Hydrocarb. Proc. Pet. Ref, February 1964. Reprinted courtesy of Hydrocarbon Processing.)...

Which method to use. Section 8.2.8 draws attention to the systematic nature of the limitations of packed-tower pressure drop correlations. Due to this systematic nature, the author warns against basing packing pressure drop calculations on any correlations whose limitations are not well known. Section 8.2.8 presents three correlations and elaborates on their limitations and application boundaries. Within their boundaries, these correlations should give reliable predictions. UBe of any other correlation is dangerous unless its limitations are explored. 

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