Measurement of Interfacial Area

To calculate the gas absorption rate qL for Eq. (9.7), we need to know the gas-liquid interfacial area, which can be measured employing several techniques such as photography, light transmission, and laser optics. 

The interfacial area per unit volume can be calculated from the Sauter-mean diameter D32 and the volume fraction of gas-phase H, as follows  

The Sauter-mean diameter, a surface-volume mean, can be calculated by measuring drop sizes directly from photographs of a dispersion according to Eq. (9.21). 

Drop size distribution can be indirectly measured by using the light-transmission technique. When a beam of light is passed through a gas-liquid dispersion, light is scattered by the gas bubbles. It was 

In theory, ml is unity and m2 is a constant independent of drop-size distribution as long as all the bubbles are approximately spherical. 

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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. 

DeJesus, J. M., and M. Kawaji, 1989, Measurement of Interfacial Area and Void Fraction in Upward, Cocurrent Gas-Liquid Flow, ANS Proc. 1989 NHTC, HTC-4, p. 137 see also 1990, Investigation of Interfacial Area and Void Fraction in Upward Cocurrent Gas-Liquid Flow, Can. J. Chem. Eng <5S 9047J12, Dec. (3)... 

C.W. Robinson, C.R. Wilke, Simultaneous measurement of interfacial area and mass transfer coefficients for a well-mixed gas dispersion in aqueous electrolyte solutions, AIChE J. 20 (1974) 285-294. 

III. Measurement of Interfacial Areas and Mass-Transfer Coefficients. 35... 

The chemical method for the measurement of interfacial area in liquid-liquid dispersions was first suggested by Nanda and Sharma (S19). They calculated the effective interfacial area a by sparingly extracting soluble esters of formic acid such as butyl formate, amyl formate, etc., into aqueous solutions of sodium hydroxide. This method has been employed by a number of workers, using esters of formic acid, chloroacetic acid, and oxalic acid, which are sparingly soluble in water (D9, DIO, FI, F2, F3, 04, P8, SI5, S20). Sankholkar and Sharma (S5) employed the extraction of diisobutylene into aqueous sulphuric acid. Sankholkar and Sharma (S6, S7) have also found that the extraction of isoamylene into aqueous solutions of sulphuric acid, and desorption of the same from the loaded acid solutions into inert hydrocarbons such as n-heptane and toluene, can be used for determining the effective interfacial area. Recently, Laddha and Sharma (L2) employed the extraction of pinenes into aqueous sulphuric acid. 

With both staged equipntent and differential contactors, availability of adequate phase-equilibrium models and rate expressions would allow application of existing correlations and simulation algorithms. For example, knowledge of metal-extraction kinetics in terms of interfacial species concentrations could be combined with correlations of film mass transfer coefficients in a particular type of equipment to obtain the interfacial flux as a function of bulk concentrations. Correlations or separate measurements of interfacial area and an estimate of dispersion characteristics would allow calculation of extraction performance as a... 

Dhanuka, V.R. and J.B. Stepanek, "Simultaneous Measurement of Interfacial Area and Mass Transfer Coefficient in Three-Phase Fluidized Bed". AIChE J. (1980) 1029-1038. 

Robinson, C.W., and Wilke, C.R., "Simultaneous Measurement of Interfacial Area and Mass Transfer Coefficients for Well-Mixed Gas Dispersion in Aqueous Electrolyte Solutions",... 

Measurements of interfacial area reported in the literature generally exceed the predictions (Figure 8), except for very high W phase saturations. When the contribution of thin films of W in contact with NW in drained cells is included, the predictions exhibit the same trend as the data. We conclude that the experimental techniques are sensitive to the thin films as well as the interfaces between macroscopic volumes of W and N W, making a direct comparison of the predictions for 3D morphologies (pendular rings at grain contacts, lenses in pore throats, interface between bulk connected phases) impossible. The predictions... 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

This mass balance concerns the liquid phase, since oxygen must be dissolved in order to be used by the cells. Due to the difficulty in measuring the interfacial area (a), especially when oxygenation is carried out by bubble aeration, it is common to use the product of kL times a (kLa), known as the volumetric oxygen transfer coefficient, as the relevant parameter. 

Some of this theoretical thinking may be utilized in reactor analysis and design. Illustrations of gas-liquid reactors are shown in Fig. 19-26. Unfortunately, some of the parameter values required to undertake a rigorous analysis often are not available. As discussed in Sec. 7, the intrinsic rate constant kc for a liquid-phase reaction without the complications of diffusional resistances may be estimated from properly designed laboratory experiments. Gas- and liquid-phase holdups may be estimated from correlations or measured. The interfacial area per unit reactor volume a may be estimated from correlations or measurements that utilize techniques of transmission or reflection of light, though these are limited to small diameters. The combined volumetric mass-transfer coefficient kLa, can be also directly measured in reactive or nonreactive systems (see, e.g., Char-pentier, Advances in Chemical Engineering, vol. 11, Academic Press, 1981, pp. 2-135). Mass-transfer coefficients, interfacial areas, and liquid holdup typical for various gas-liquid reactors are provided in Tables 19-10 and 19-11. 

Both requirements of a high specific interfacial area and a direct spectroscopic observation of the interface were attained by the CLM method shown in Fig. 2 [12]. Two-phase system containing about 100 pi volumes of organic and aqueous phases is introduced into a cylindrical glass cell with a diameter of 19mm. The cell is rotated at the speed of 7000-10,000 rpm. By this procedure, a stacked two-liquid membrane each with thickness of 50-100 pm is produced inside the cell wall, which attains the specific interfacial area over 100 cm-1. UV-Vis spectrometry was used in the original work for the measurement of the interfacial species as well as those in the bulk phases. This method can be excellently applied for the measurement of interfacial reaction rate as fast as the order of seconds. 

In this study, a and kLa are measured in a TBR in the pressure range [0.3-3.2 MPa] using fast and slow chemical absorption of carbon dioxide into diethanolamine (DEA) aqueous and organic solutions. Only the trickling regime and trickling/pulsing transition have been explored. A simple model to explain the increase of interfacial area and mass transfer... 

An example of VITTM is realized as curve 1 in Figure 6.5. The cell system investigated for the measurement was as Equation (34) in which W1 and W2 (5 ml each) containing an SE (MgS04) were separated by NB containing an SE (CV TPhB ) and an ionophore (dibenzo-18-crown-6, DB18C6). The NB solution worked as the M of interfacial area of 1 cm and thickness of 1 cm. The cell used was similar to that illustrated in Figure 6.7a. 

For the measurement of interfacial tensions no molecular models are needed. It is enough to know that the Interfacial tension is a measure of the tendency of all areas to become as small as possible. Following chapter 1.2. this contractile action can be interpreted thermodynamically or mechanically. 

This corresponds to absorption with fast pseudo-first-order kinetics. Thus the film thickness, or the value of /cl, is irrelevant and does not appear in the expression for the average rate of absorption

unit volume of reactor 4 . This important case will be the basis of a chemical method for measuring the interfacial area directly from the rate of absorption when C% Df is known. 

At the outset, we recognize that a technique that measures overall values cannot be used without the restrictions that arise from the results observed with physical methods. For example, the chemical method can hardly be used with fast-coalescing systems, since the presence of a chemical compound may well reduce the coalescence rates. In fast-coalescing systems, as observed with physical methods, the wide variation of specific contact area at different locations in the reactor negates the meaning of an average value. In fact, physical and chemical techniques should be used simultaneously to identify more fully the phenomena that occur in gas-liquid reactors. While chemical methods provide overall values of interfacial area that are immediately usable for design, we must also know the variations in the local interfacial parameters (a, dgM) within the reactor in order to deal competently with scale-up. These complementary data, measured by physical methods, should be obtained from local simultaneous measurements of two of the three interfacial parameters as discussed above. 

Sharma (S35), including the use of organic solvents (such as toluene, xylene, diethylene glycol, and polyethylene glycol) for the measurement of a and by the chemical method. In each case, the reaction between CO2 and selected amines is employed to determine a. For example, values of interfacial areas obtained in a reaction of CO2 with cyclohexylamine in xylene plus 10% isopropanol, in a 10-cm-i.d. column packed with 0.5 in. ceramic Raschig rings, are reported in Fig. 15. A comparison with the values for aqueous systems shows a 50% improvement attributable to the lower surface tension of xylene ([Pg.74]

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. 

Adsorption may be followed at fluid/fluid interfaces by measuring changes in interfacial pressure, potential, or viscosity, using spread monolayers for calibration purposes. The most accurate method for measuring rates of adsorption is by the rate of increase of interfacial area at constant interfacial pressure. If (1/A) (dA/dt) is the fractional rate of increase of interfacial area expressed in sec-1 and n is the interfacial concentration in g cm-2 found from measurements on spread monolayers, then the rate of adsorption dn/dt in g cm-2 sec-1 is given by... 

This barrier was investigated by spreading a variety of compounds at the air/water interface in order to vary the electrical potential (Mac-Ritchie and Alexander, 1963c). The rate of adsorption of protein, dissolved in the subphase, was measured at constant surface pressure by the increase of interfacial area with time. As protein adsorbs,... 

We have noted that the interfacial tension between two immiscible fluids can be modified because of tbe presence of solutes. Especially important in this regard are tbe solutes that are known as surfactants (or surface-active agents ). These are typically molecules with two distinct chemical moieties, each of which (on its own) would be soluble in one of the two bulk fluids, and more or less insoluble in the other. When one of the two fluids is water, the portion of the surfactant that prefers the water is known as hydrophilic, whereas the part that prefers the other liquid is known as hydrophobic. Hence part of the surfactant molecule would like to be in fluid A and part in fluid B. The result is that there is a strong tendency for the surfactant to accumulate at the interface between andB, where each part can be more satisfied with its chemical environment than if the surfactant were wholly in either fluid A or B. Not only do surfactants tend to accumulate at interfaces, but their presence generally results in a strong decrease in the interfacial tension relative to the value of a clean AB interface. The fact that the interfacial tension is decreased certainly makes qualitative sense if we recall the interpretation of yas the surface free energy that measures the work required to achieve an increase of interfacial area. 

A recent SECM study of electrochemical catalysis at the ITIES was based on a similar concept (23). The ITIES was used as a model system to study catalytic electrochemical reactions in microemulsions. Microemulsions, i.e., microheterogeneous mixtures of oil, water, and surfactant, appear attractive for electrochemical synthesis and other applications (63). The ITIES with a monolayer of adsorbed surfactant is of the same nature as the boundary between microphases in a microemulsion. The latter interface is not, however, directly accessible to electrochemical measurements. While interfacial area in a microemulsion can be uncertain, the ITIES is well defined. A better control of the ITIES was achieved by using the SECM to study kinetics of electrochemical catalytic reduction of //zms-l, 2-dibromo-cyclohexane (DBCH) by Co(I)L (the Co(I) form of vitamin B12) ... 

Whereas kinetic studies are quite easy to perform in homogeneous solution the extent of the interface has to be taken into consideration for biphasic systems. The most reliable way of measuring the interfacial area is by use of Fraunhofer... 

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