Absorption effective interfacial areas

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... 

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. 

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. 

Figure 6.5 shows values ofthe effective interfacial area thus obtained by comparing A q a values [13] for gas-phase resistance-controlled absorption and vaporization with A q values by Equation 6.54. It is seen that the effective area for absorption is considerably smaller than that for vaporization, the latter being almost equal to the wetted area. The effect of gas rates on a is negligible. 

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)... 

In many practical applications, gas-liquid mass transfer plays a significant role in the overall chemical reaction rate. It is, therefore, necessary to know the values of effective interfacial area (aL) and the volumetric or intrinsic gas-liquid mass transfer coefficients such as kLah, kL, ktaL, kg, etc. As shown in Section IX, the effective interfacial area is measured by either physical e.g., photography, light reflection, or light scattering) or chemical methods. The liquid-side or gas-side mass-transfer coefficients are also measured by either physical (e.g., absorption or desorption of gas under unsteady-state conditions) or chemical methods. A summary of some of the experimental details and the correlations for aL and kLaL reported in the literature are given by Joshi et al. (1982). In most practical situations, kgaL does not play an important role. 

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 interfacial area Cross sectional area Eractional open area Absorption factor... 

Thus assumption of the same value for interfacial area in physical and chemical absorption leads to uncertainty, especially if the mass transfer coefficient is deduced from k a measured by physical absorption or desorption and from a in chemical absorption. The effective interfacial area in the case of a fast-reaction system where the absorbing capacity is increased by a chemical reactant is substantially larger than the effective interfacial area for physical absorption or desorption, as pointed out by Joosten and Danckwerts (JIO). These authors introduced a correction factor y, the ratio between the increase in liquid absorption capacity and the increase in mass transfer due to chemical reaction ... 

An alternate correlation has been proposed by Puranik and Vogelpohl (P16) for the values of effective interfacial areaov (during vaporization), Uac (absorption with reaction), Uap (without reaction), and corresponding values of the wetted surface areauw The effective interfacial area is then divided into static and dynamic areas (Uj, + a yn) for vaporization and absorption with chemical reaction. For absorption without reaction, the effective interfacial area is only the dynamic area. Thus... 

The model showed that for larger drops the reaction front penetration was small, while for smaller drops the movement of the reaction front was very fast and most ofthe nickel is consumed in the early stage (see Fig. 4.4). Afterward there was virtually simple physical absorption of nickel and these smaller drops were no longer effective in extracting the solute, resulting in a loss of effective interfacial area for solute transport. 

A further advantage of absorption plus reaction is the increase in the mass-transfer coefficient. Some of this increase comes from a greater effective interfacial area, since absorption can now take place in the nearly stagnant regions (static holdup) as well as in the dynamic liquid holdup. For NHj absorption in H2SO4 solutions, K a was 1.5 to 2 times the value for absorption in water.Since the gas-film resistance is controlling, this effect must be due mainly to an increase in effective area. The values of K a for NH3 absorption in acid solutions were about the same as those for vaporization of water, where all the interfacial area is also expected to be effective. The factors and... 

Alper.E., Deckwer,W.-D. and P.V.Danckwerts. "Comparison of effective interfacial areas with the actual area for gas absorption in a stirred cell". Chem.Engng.Sci. 35 (1980) 1263. 

Joosten,G.E.H. and Danckwerts,P.V. "Chemical reaction and effective interfacial areas in gas absorption". Chem.Engng.Sci. 

Linek.V., Krivsky.Z. and P.Hudec. "Effective interfacial area in plastic -packed absorption columns". Chem.Engng.Sci. 

YOS 58] Yoshida F., Koyanagi T., Liquid phase mass transfer rates and effective interfacial area in packed absorption columns , Ind. and Eng. Chemistry, vol. 50, no. 3, p. 365, 1958. 

Jooesten, G.E.H., and Danckwerts, P.V., Chemical Reaction and Effective Interfacial Areas in Gas Absorption, Chem. 

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. 

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. 

Increase in interfacial area. The total surface area for diffusion is increased because the bubble diameter is smaller than for the free-bubbling case at the same gas flow rate hence there is a resultant increase in the overall absorption rate. The overall absorption rate will also increase when the diffusion is accompanied by simultaneous chemical reaction in the liquid phase, but the increase in surface area only has an appreciable effect when the chemical reaction rate is high the absorption rate for this case is then controlled by physical diffusion rather than by the chemical reaction rate (G6). 

The mechanism of transfer of solute from one phase to the second is one of molecular and eddy diffusion and the concepts of phase equilibrium, interfacial area, and surface renewal are all similar in principle to those met in distillation and absorption, even though, in liquid-liquid extraction, dispersion is effected by mechanical means including pumping and agitation, except in standard packed columns. 

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. 

Thus, the rate of absorption is independent of ki, that is of the hydrodynamic conditions. Consequently, provided that the average specific rate of absorption, , is known and C and Cg are effectively the same in all parts of the system, the specific interfacial area, a, may be determined directly from a measurement of the rate of absorption, 0. 

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