Mass transfer coefficient interfacial area effect

The physical aspects of gas-liquid, liquid-liquid, solid-liquid, and gas-liquid-solid systems are discussed in the subsequent sections of this chapter. Using the guidelines given there, it is possible to get an estimate of local and average mass-transfer coefficients, interfacial areas, and contacting patterns. This section briefly considers the effect of reactions on mass transfer, a subject treated elsewhere in this handbook and in advanced texts [29, 30]. Note that while we will refer mostly to gas-liquid systems, the same treatment would more or less apply to liquid-liquid and liquid-solid systems. In the case of gas-liquid-solid systems, it may be possible to determine the controlling resistance and simplify the analysis to a two-phase system, as far as the reaction part is concerned. 

Compared to bubble columns, airlift reactors have better liquid circulation but lower rates of mass transfer and mixing. These rates are enhanced in modified airlift reactors with perforated single or coaxial draft tubes. This enhancement is because of the breakup of gas bubbles into smaller bubbles when crossing perforated tubes. The gas-liquid interfacial area and the gas-liquid mass transfer coefficient increases. Similar effect can be achieved with the addition of packing to the riser. ... 

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. 

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. 

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. 

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

Requirements regarding laboratory liquid-liquid reactors are very similar to those for gas-liquid reactors. To interpret laboratory data properly, knowledge of the interfacial area, mass-transfer coefficients, effect of contaminants on mass-transport processes, ionic characteristics of the system, etc. is needed. Commonly used liquid-liquid reactors have been discussed by Doraiswamy and Sharma (1984). 

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. 

The use of floating bubble breakers has been used to increase the volumetric mass transfer coefficient in a three-phase fluidized bed of glass beads (Kang et al., 1991) perhaps a similar strategy would prove effective for a bed of low density beads. Static mixers have been shown to increase kxa for otherwise constant process conditions by increasing the gas holdup and, therefore, the interfacial area (Potthoff and Bohnet, 1993). 

Interfacial contact area, 10 755-756 Interfacial effects, in CA resists, 15 182 Interfacial energy, 24 157 colloids, 7 281-284 Interfacial forces, in foams, 12 4 Interfacial free energy, 24 119 Interfacial in situ polymerization, in microencapsulation, 16 442 446 Interfacial mass-transfer 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 influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

In most types of mass-transfer equipment, the interfacial area, a, that is effective for mass transfer cannot be determined accurately. For this reason, it is customary to report experimentally observed rates of transfer in terms of mass-transfer coefficients based on a unit volume of the apparatus, rather than on a unit of interfacial area. Calculation of the overall coefficients from the individual volumetric coefficients is made practically, for example, by means of the equations ... 

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

The sodium sulfite oxidation technique has its limitation in the fact that the solution cannot approximate the physical and chemical properties of a fermentation broth. An additional problem is that this technique requires high ionic concentrations (1 to 2 mol/L), the presence of which can affect the interfacial area and, in a lesser degree, the mass-transfer coefficient (Van t Riet, 1979). However, this technique is helpful in comparing the performance of fermenters and studying the effect of scale-up and operating conditions. 

G. Vazquez, M.A. Cancela, C. Riverol, E. Alvarez, J.M. Navaza, Application of the Danckwerts method in a bubble column. Effects of surfactants on mass transfer coefficient and interfacial area, Chem. Eng. J. 78 (2000) 13-19. 

In Eq. (31c), Nxn and N n are the mass transfer rates. These are calculated from multicomponent mass transfer equations. The equations used take into account the mass transfer coefficients and interfacial areas generated in the specific contactor, reaction rates, heat effects, and any interactions among the above processes. 

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