Measured interfacial area

For dilute systems it can be assumed that Gm, Em, and Pl are constant, and it normaUv is assumed that the interfacial area a of the packing is constant and is equal to the value that would exist without reaction. This last assumption needs careful consideration, since different methods for measuring a may give different results. Sharma and Danckwerts [Br Chem. Eng., 15(4), 522 (1970)] have reviewed various techniques for measuring interfacial areas. 

Westerterp et al. (W5) measured interfacial areas in mechanically agitated gas-liquid contactors. The existence of two regions was demonstrated At agitation rates below a certain minimum value, interfacial areas are unaffected by agitation and depend only on nominal gas velocity and the type of gas distributor, whereas at higher agitation rates, the interfacial areas are... 

Interfacial area measurement. Knowledge of the interfacial area is indispensable in modeling two-phase flow (Dejesus and Kawaji, 1990), which determines the interphase transfer of mass, momentum, and energy in steady and transient flow. Ultrasonic techniques are used for such measurements. Since there is no direct relationship between the measurement of ultrasonic transmission and the volumetric interfacial area in bubbly flow, some estimate of the average bubble size is necessary to permit access to the volumetric interfacial area (Delhaye, 1986). In bubbly flows with bubbles several millimeters in diameter and with high void fractions, Stravs and von Stocker (1985) were apparently the first, in 1981, to propose the use of pulsed, 1- to 10-MHz ultrasound for measuring interfacial area. Independently, Amblard et al. (1983) used the same technique but at frequencies lower than 1 MHz. The volumetric interfacial area, T, is defined by (Delhaye, 1986)... 

Earlier studies in mass transfer between the gas-liquid phase reported the volumetric mass-transfer coefficient kLa. Since kLa is the combination of two experimental parameters, mass-transfer coefficient and mterfacial area, it is difficult to identify which parameter is responsible for the change of kLa when we change the operating condition of a fermenter. Calderbank and Moo-Young (1961) separated kta by measuring interfacial area and correlated mass-transfer coefficients in gas-liquid dispersions in mixing vessels, and sieve and sintered plate column, as follows ... 

Measured interfacial area in distillation trays is consistent with this high value. 

Drop size distribution in dilute suspensions of electrical conducting liquids may be determined using the Coulter principle but the need to add what may be undesirable conductive materials limits its applicability [213-215]. The use of chemical means to measure interfacial area has been used extensively for gas-liquid dispersions. Chemical reaction methods for determining the interfacial area of liquid-liquid systems involve a reaction of a relatively unchanging dispersed-phase concentration diffusing to the continuous phase. The disadvantage of this approach is that the mass transfer can affect the interfacial tension, and hence the interfacial area [216-218]. 

The scintillation method uses short-range radioactive particles for measuring interfacial area. This technique is limited by the necessity of high immiscibility between the phases as well as the availability of suitable isotopes and target materials [228],... 

The absorption of oxygen into sulfite solutions is very often used to measure interfacial areas in pilot plant or industrial reactors, as reviewed by Keith and Beek (R9) and Laurent et al. (L4). To illustrate the strategy outlined in the previous sections, a study of this reaction by Laurent (L2) will be discussed now in relation to the experimental conditions suitable for determining interfacial area in gas-liquid reactors. The reaction is... 

Knowledge of interfacial areas, drop size distributions, and dispersed phase coalescence rates is essential for accurate description and prediction of mass transfer and chemical reaction rates in liquid-liquid dispersions. In this section, a review of the experimental methods and techniques developed for describing and measuring interfacial area, drop size distributions, and coalescence rates will be given in addition, summaries of important results and correlations are presented. 

Using 6-blade disc turbines in baffled tanks with pure liquids, i.e. coalescing systems, Calderbank measured interfacial area and hold-up and obtained an equation for the surface/volume mean bubble diameter (in SI units) ... 

Chemical methods have moreover been successfully used to measure interfacial area and true mass transfer coefficients for various equipments under different conditions (34,35). However, in many cases, there are still very few data to predict even the most essential parameters (36). 

Sharma.M.M. and P.V.Danckwerts. "Chemical methods of measuring interfacial area and mass transfer coefficients in two-phase systems." Brit.Chem.Eng. 15 (1970) 522-528. 

There is also a need for caution about the interpretation of such measurements as the precise fulfillment of the required conditions is not usually easy. For instance, chemical methods of measuring interfacial area (a) purport to measure the area which is effective under the chosen conditions. There is however good evidence that the effective interfacial area may well depend on the type of reaction proceeding in the liquid as illustrated for packed columns (49). 

Alper,E. "Chemical methods of measuring interfacial areas in two-phase systems". (Proceedings of NATO ASI on "Two-phase flow and heat transfer", Turkey, 1976 ... 

However, alkaline hydrolysis of formates conforms to the fast pseudo first order reaction regime, hence it may be employed to measure interfacial areas. 

Light transmittance methods for interfacial area have been widely employed in gas-liquid systems. Sridhar and Potter (1978) describe one of the more successful versions. The techniques are described in more detail in the liquid-liquid section. Chemical methods can be used to measure interfacial area, as described above (Section 4-7.5 Robinson and Wilke, 1974). Sampling methods are not suitable since it is impossible to withdraw a sample isokinetically. Generally, the results from chemical and physical methods do not agree. Chemical methods tend to have a bias toward the smaller bubbles of the distribution, whereas these may be missed by physical methods. 

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... 

Other correlations based partially on theoretical considerations but made to fit existing data also exist (71—75). A number of researchers have also attempted to separate from a by measuring the latter, sometimes in terms of the wetted area (76—78). Finally, a number of correlations for the mass transfer coefficient itself exist. These ate based on a mote fundamental theory of mass transfer in packed columns (79—82). Although certain predictions were verified by experimental evidence, these models often cannot serve as design basis because the equations contain the interfacial area as an independent variable. 

Direct photography of drops in done with the use of fiber optic probes using either direct or reflected light. StiU or video pictures can be obtained for detailed analysis. The light transmittance method uses three components a light source to provide a uniform collimated beam, a sensitive light detector, and an electronic circuit to measure the amplified output of the detector. The ratio of incident light intensity to transmitted intensity is related to interfacial area per unit volume. 

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) ... 

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. 

With complicated geometries, the product of the interfacial area per volume and the mass-transfer coefficient is required. Correlations of kop or of HTU are more accurate than individual correlations of k and since the measurements are simpler to determine the produc t kop or HTU. 

According to this method, it is not necessaiy to investigate the kinetics of the chemical reactions in detail, nor is it necessary to determine the solubihties or the diffusivities of the various reactants in their unreacted forms. To use the method for scaling up, it is necessaiy independently to obtain data on the values of the interfacial area per unit volume a and the physical mass-transfer coefficient /c for the commercial packed tower. Once these data have been measured and tabulated, they can be used directly for scahng up the experimental laboratory data for any new chemic ly reac ting system. 

Note that the product of the mass-transfer coefficient and the interfacial area is a volumetric coefficient and obviates the need for a value of the interfacial area. While areas for mass transfer on plates have been measured, the experimental contacting equipment cuffered significantly from that used for commercial distillation or gas absorption, and the reported areas are considered unreliable for design purposes. 

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. 

Coughlin and von Berg [Chem. Eng. Sci., 21, 3 (1966)]. Continuous heat transfer and extraction of ethylbiityric acid between kerosine and water unbaffled vessel, Pump-Mix design (Fig. 15-28). Interfacial area measured. 

Auxiliary data are the sizes of bubbles and droplets. These data and the holdups of the two phases are measured by a variety of standard techniques. Interfacial area measurements utihze techniques of transmission or reflection of light. Data on and methods for finding sohi-bihties of gases or the relation between partial pressure and concentration in hquid are also well estabhshecT... 

Gas/Liquid Interfacial Area This has been evaluated by measuring absorption rates like those of CO9 in NaOH. A correlation by Charpentier (Chem. Eng. Journal, 11, 161 [1976]) is... 

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. 

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