Packed effective interfacial areas

The important point to note here is that the gas-phase mass-transfer coefficient fcc depends principally upon the transport properties of the fluid (Nsc) 3nd the hydrodynamics of the particular system involved (Nrc). It also is important to recognize that specific mass-transfer correlations can be derived only in conjunction with the investigator s particular assumptions concerning the numerical values of the effective interfacial area a of the packing. 

The effective interfacial area depends on a number of factors, as discussed in a review by Charpentier [C/j m. Eng.J., 11, 161 (1976)]. Among these factors are (1) the shape and size of packing, (2) the packing material (for example, plastic generally gives smaller interfacial areas than either metal or ceramic), (3) the liquid mass velocity, and (4), for smaU-diameter towers, the column diameter. 

Rizzuti et al. [Chem. Eng. Sci, 36, 973 (1981)] examined the influence of solvent viscosity upon the effective interfacial area in packed columns and concluded that for the systems studied the effective interfacial area a was proportional to the kinematic viscosity raised to the 0.7 power. Thus, the hydrodynamic behavior of a packed absorber is strongly affected by viscosity effects. Surface-tension effects also are important, as expressed in the work of Onda et al. (see Table 5-28-D). 

Principles of Rigorous Absorber Design Danckwerts and Alper [Trans. Tn.st. Chem. Eng., 53, 34 (1975)] have shown that when adequate data are available for the Idnetic-reaciion-rate coefficients, the mass-transfer coefficients fcc and /c , the effective interfacial area per unit volume a, the physical solubility or Henry s-law constants, and the effective diffusivities of the various reactants, then the design of a packed tower can be calculated from first principles with considerable precision. 

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. 

Figure 9-47. Effective interfacial areas for random ceramic tower packings. Note for gases or vapors other than air, use abscissa, G, as G/(pgas/0.075) -°. Based on the detta of Fellinger [27]. Used by permission of the Amedcan Institute of Chemical Engineers A.I.Ch.E. Jour., Shul-man, H. L, Ulldch, C. F., Proulx, A. Z., and Zimmerman, J. O., V 1, No. 2, (1955) p. 253. All rights reserved.

This has been shown to correlate for a wide variety of tower packings, various operating conditions, and physical properties of the solute and inert gases. The k(j calculated must be used in conjunction with the effective interfacial areas determined by Shulman [65] Figure 9-47, to establish a reliable value for kGa. Figure 9-47 should be used with the abscissa as G/Vp/0.075 for inert gas other than air [67] ... 

Experimental values for several systems are given by Cornell et al. (1960), Eckert (1963), and Vital et al. (1984). A selection of values for a range of systems is given in Table 11.3. The composite mass transfer term KGa is normally used when reporting experimental mass-transfer coefficients for packing, as the effective interfacial area for mass transfer will be less than the actual surface area a of the packing. 

Clw Effective interfacial area of packing per unit volume L-1... 

Data Effective interfacial area for 25 mm packing = 280 m2/m3 Mass transfer film coefficients ... 

SAHAY, B. N. and SHARMA, M. M. Chem.Eng. Sci. 28 (1973) 41. Effective interfacial areas and liquid and gas side mass transfer coefficients in a packed column. 

The mass transfer coefficients considered so far - namely, kQ,kj, KQ,andKj - are defined with respect to known interfacial areas. However, the interfacial areas in equipment such as the packed column and bubble column are indefinite, and vary with operating conditions such as fluid velocities. It is for this reason that the volumetric coefficients defined with respect to the unit volume of the equipment are used, or more strictly, the unit packed volume in the packed column or the unit volume of liquid containing bubbles in the bubble column. Corresponding to /cg, Kq, and we define k a, k, a, K, /i, and K a, all of which have units of (kmol h m )/(kmol m ) - that is, (h ). Although the volumetric coefficients are often regarded as single coefficients, it is more reasonable to consider a separately from the Ar-terms, because the effective interfacial area per unit packed volume or unit volume of liquid-gas mixture a (m m ) varies not only with operating conditions such as fluid velocities but also with the types of operation, such as physical absorption, chemical absorption, and vaporization. 

The effective interfacial areas for absorption with a chemical reaction [6] in packed columns are the same as those for physical absorption, except that absorption is accompanied by rapid, second-order reactions. For absorption with a moderately fast first-order or pseudo first-order reaction, almost the entire interfacial area is effective, because the absorption rates are independent of as can be seen... 

Why is the effective interfacial area, a, for gas-phase controlled gas absorption much smaller than that for vaporization in packed columns (Figure 6.5)... 

Effective interfacial area of packing 280 m2/m3 Film mass transfer coefficients ... 

The reported study on gas-liquid interphase mass transfer for upward cocurrent gas-liquid flow is fairly extensive. Mashelkar and Sharma19 examined the gas-liquid mass-transfer coefficient (both gas side and liquid side) and effective interfacial area for cocurrent upflow through 6.6-, 10-, and 20-cm columns packed with a variety of packings. The absorption of carbon dioxide in a variety of electrolytic and ronelectrolytic solutions was measured. The results showed that the introduction of gas at high nozzle velocities (>20,000 cm s ) resulted in a substantial increase in the overall mass-transfer coefficient. Packed bubble-columns gave some improvement in the mass-transfer characteristics over those in an unpacked bubble-column, particularly at lower superficial gas velocities. The value of the effective interfacial area decreased very significantly when there was a substantial decrease in the superficial gas velocity as the gas traversed the column. The volumetric gas-liquid mass-transfer coefficient increased with the superficial gas velocity. 

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 contributions from rivulets, drippings, and gas bubbles. Because of this complex physical picture, effective interfacial area is difficult to measure directly. 

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