Interfacial area, effective

As noted earlier, the rate of mass transfer is proportional to the interfacial area. Effective mass transfer requires that a high specific interfacial area (interfacial area per unit volume) should be created. For a droplet dispersion such as exists in most extraction processes, the specific interfacial area is given by... 

At the present state of the art all models for the prediction of mass transfer in packed columns are insufficient to some degree. One problem is a poor knowledge of the interfacial area effective for mass transfer. Pubhshed studies differ significantly and their results are often contradieting. Frrrthermore, the existing models for predicting mass transfer coefficients are not suffieiently reliable. 

Lujuid-Pha.se Transfer. It is difficult to measure transfer coefficients separately from the effective interfacial area thus data is usually correlated in a lumped form, eg, as k a or as These parameters are measured for the Hquid film by absorption or desorption of sparingly soluble gases such as O2 or CO2 in water. The Hquid film resistance is completely controlling in such cases, and kjji may be estimated as since x (Fig. 4). This... 

Agitation of the Fluid. Agitation of the solvent increases local turbulence and the rate of transfer of material from the surface of the particles to the bulk of the solution. Agitation should prevent settling of the soHds, to enable most effective use of the interfacial area. 

The integral values of effective interfacial area can thus be obtained by measuring the reaction (extraction) rate and using physico-chemical properties of the reactants. A reaction satisfying the above conditions consists of hydrolysis of hexyl formate (11) ... 

The interfacial area in the region with agitation effect N > and Hquid height, F/, is given by... 

Model Reactions. Independent measurements of interfacial areas are difficult to obtain in Hquid—gas, Hquid—Hquid, and Hquid—soHd—gas systems. Correlations developed from studies of nonreacting systems maybe satisfactory. Comparisons of reaction rates in reactors of known small interfacial areas, such as falling-film reactors, with the reaction rates in reactors of large but undefined areas can provide an effective measure of such surface areas. Another method is substitution of a model reaction whose kinetics are well estabUshed and where the physical and chemical properties of reactants are similar and limiting mechanisms are comparable. The main advantage of employing a model reaction is the use of easily processed reactants, less severe operating conditions, and simpler equipment. 

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. 

For modest changes in temperature the influence of temperature upon the interfacial area a may be neglected. For example, in experiments on the absorption of SO9 in water, Whitney and Vivian [Chem. Eng, Pi og., 45, 323 (1949)] found no appreciable effect of temperature upon kcCi over the range from 10 to 50°C. 

It shoiild be noted that the influence of substituting solvents of widely differing viscosities upon the interfacial area a can be very large. One therefore should be cautious about extrapolating data to account for viscosity effects between different solvent systems. 

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). 

In developing correlations for the mass-transfer coefficients Icq and /cl, the various authors have assumed different but internally compatible correlations for the effective interfacial area a. It therefore would be inappropriate to mix the correlations of different authors unless it has been demonstrated that there is a valid area of overlap between them. 

Volumetric Mass-Transfer Coefficients and Kia Experimental determinations of the individual mass-transfer coefficients /cg and /cl and of the effective interfacial area a involve the use of extremely difficult techniques, and therefore such data are not plentiful. More often, column experimental data are reported in terms of overall volumetric coefficients, which normally are defined as follows ... 

Experimental values of Hqg -nd Hql for a number of distillation systems of commercial interest are also readily available. Extrapolation of the data or the correlations to conditions that differ significantly from those used for the original experiments is risky. For example, pressure has a major effect on vapor density and thus can affect the hydrodynamics significantly. Changes in flow patterns affeci both mass-transfer coefficients and interfacial area. 

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. 

Interfacial Area The effective area of contact between gas and liquid is that area which participates in the gas-liqiiid mass-exchange process. This area may be less than the actual interfacial area because of stagnant pools where liquid reaches satiiradon and no longer par-dcipates in the transfer process. 

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. 

FIG. 14-67 Effective interfacial area based on data of Fellinger. (a) 1-in Raschig rings, (h) 1-in Berl saddles. To convert square feet per cubic foot to square meters per cubic meter, multiply by, 3.28 to convert pounds per hour-square foot to kilograms per second-square meter, multiply by 0.001,356. [Shul-man. Am. Inst. Chem. Eng. J., i, 2,57 (I,9,5,5).]... 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

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