Packing, effective area

E] Use s BoUes Fair (Ref. 75) data base to determine new effective area to use with Onda et al. (Ref. 126) correlation. Same definitions as 5-28-D. P = total pressure, atm Mq = gas, molecular weight m = local slope of equilibrium curve Lf /Gf = slope operating line Z = height of packing in feet. 

Effective area should not be confused with wetted area. While film flow of liquid across the packing surface is a contributor, effective area includes also contribiidons from rivulets, drippings, and gas bubbles. Because of this complex physical picture, effecdve interfacial area is difficnlt to measure directly. 

The effective interfacial area is used in mass transfer studies as an undivided part of individual and overall coefficients when it is difficult to separate and determine the effective area. The work of Shulman et.al.,65 presents a well organized evaluation of other work in addition to their own. One of the difficulties in correlating tower packing performance lies in obtaining the correct values for the effective interfacial areas of the packing on which the actual absorption, desorption, chemical reaction, etc. are completed. Figures 9-47 A, B, C, D, E, F, G present a correlation for Avater flow based on the ammonia-water data of Fellinger [27] and are valid for absorption work. 

Their correlations were based on a large amount of data on gas absorption and distillation with a variety of packings, which included Pall rings and Berl saddles. Their method for estimating the effective area of packing can also be used with experimentally determined values of the mass-transfer coefficients, and values predicted using other correlations. 

FIG. 14-60 Effect of structured packing surface areas, loads, and inclination angle on packing efficiency. Efficiency expressed as number of theoretical stages per meter, the reciprocal of HETP. Sulzer data, chlorobenzene-ethylbenzene, 100 mbar, at total reflux 250-mm-diameter test column. (Reprinted courtesy of Sulzer... 

The flow channel geometry is illustrated in Fig. 8.9, and dimensions S, B, and h (whenever available) are listed in Table 8.1. The effective area of the packing, ae, is the packing surface area that is wet, i.e.,... 

Packing surface area per unit volume, ft2/ft8 Tower cross-sectionel area, ft3 ing Total effective packing surface area, ft2... 

Kato and Wen (5) found, for the case of packed beds,that there was a dependency of the Sherwood and Nusselt numbers with the ratio dp/L. They proposed that the fall of the heat and mass transfer coefficients at low Reynolds numbers is due to an overlapping of the boundary layers surrounding the particles which produces a reduction of the available effective area for transfer of mass and heat. Nelson and Galloway W proposed a new model in terms of the Frossling number, to explain the fall of the heat and mass transfer coefficients in the zone of low Reynolds numbers. 

Experimental results showed that, for physical absorption (y = 1) and absorption with instantaneous chemical reaction (y = 1), the effective areas were the same while for y 1, the interfacial area increased. The different effective areas found by Ratcliff et al. (R3), Joosten and Danckwerts (JIO), and Laurent (L2), using similar packing and similar absorbing solutions, can be explained by the different y values in the different cases. 

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