Specific interfacial area

In industrial equipment, however, it is usually necessary to create a dispersion of drops in order to achieve a large specific interfacial area, a, defined as the interfacial contact area per unit volume of two-phase dispersion. Thus the mass-transfer rate obtainable per unit volume is given as... 

Membrane Extraction. An extraction technique which uses a thin Hquid membrane or film has been introduced (80,81). The principal advantages of Hquid-membrane extraction are that the inventory of solvent and extractant is extremely small and the specific interfacial area can be increased without the problems which accompany fine drop dispersions (see Membrane technology). 

Static mixing of gas—Hquid systems can provide good interphase contacting for mass transfer and heat transfer. Specific interfacial area for the SMV (Koch/Sulzer) mixer is related to gas velocity and gas holdup ( ) by the following ... 

Empirical Correlations of Volumetric Mass-Transfer Coefficient Ks and Specific Interfacial Area s ... 

A summary of a number of correlations proposed for volumetric mass-transfer coefficients and specific interfacial area is presented in Table II, which includes data additional to those of Westerterp et al. (W4). It is apparent that disagreement exists as to the numerical values for the exponents. This is due, in part, to the lack of geometric similarity in the equipment used. In addition, variation in operating factors such as the purity of the system (surfactants), kind of chemical system, temperature, etc., also contribute to the discrepancies. To summarize Table II ... 

Figure 4.40 Dependence of yield on specific interfacial area between aqueous or organic phase. The four combinations AC-BD refer to the respective reactions between dl-1-phenyl-ethylamine (A), 4-amino-l-benzylpiperidine (B), 3-nitrobenzoyl chloride (C), and 3,5-dinitrobenzoyl chloride (D) [23],...

Photocyanations rely on photoinduced electron transfer [29]. This was demonstrated by monitoring cyanation yields as a function of the droplet size for oil-in-water emulsions. Hence increase in interfacial area is one driver for micro-channel processing. Typically, fluid systems with large specific interfacial areas tend to be difficult to separate and solutions for more facile separation are desired. 

The two micro bubble columns comprising the smaller micro channels reached nearly 100% conversion [5], The micro bubble column with the largest hydraulic diameter reached at best 75% conversion. The curve obtained displays the typical shape, passing through a maximum due to the antagonistic interplay between residence time and specific interfacial area. 

P 28/The Hquid feed was introduced by a pump and the gas feed using a mass-flow controller [10], The reaction was carried out using liquid flows of 20.7-51.8 ml h and gas flows of 1.7-173 mlrnin . The gas and liquid velocities amounted to 0.02-1.2 and 0.03-3.0 m s , respectively. The reaction was performed in mixed flow regimes, including bubbly, slug and annular patterns. The specific interfacial areas amoimted to about 5000-15 000 m m . The reaction was conducted at room temperature. 

Conversion normalized by residence time (Figure 5.31) increased nearly linearly with the Weber number owing to an increase in specific interfacial area [10], The normalization is needed because by increasing gas and liquid velocities both interfacial area and time are affected in an antagonistic manner. 

The reactor model is also able to describe the dependence of conversion on the specific interfacial area (Figure 5.32) which passes through a maximum owing to the antagonistic role of increasing interfacial area at the expense of reducing residence time [10]. For a liquid volume flow of 50 ml h , optimum conversion was achieved at a specific interfacial area of 12 000 m m and at a residence time of 0.093 s. 

Figure 5.32 Calculated values for conversion of butyraldehyde as a function of specific interfacial area for a micro bubble column [10],...

The sulfite oxidation is a recommended test reaction for determining the size of the specific interfacial area in gas/liquid systems, in particular as expressed by the a value [9, 10]. 

The sulfite reaction is used for the above-mentioned purpose and hence is an analytical tool for judging micro-reactor performance [5,9,10]. The sulfite oxidation as a chemical method provides complementary information to optical analysis of the specific interfacial area. 

GL 27] [R 3] ]P 29] By means of sulfite oxidation, the specific interfacial area of the fluid system nitrogen/water was determined at Weber numbers ranging from lO " to 10 [10]. In this range, the interface increases from 4000 m m to 10 000 m m . The data are - with exceptions - in accordance with optically derived analysis of the interface and predictions from calculations. At stiU larger Weber number up to 10, the specific interfacial area increases up to 17 000 m m, which was determined optically. 

Characterization of a gas/liquid microreactor, the micro bubble column Determination of specific interfacial area, in Matlosz, M., Ehreeld, W., Baselt,... 

Since the mass transfer coefficient, k, and the specific interfacial area, a, vary in a similar manner, dependent upon the hydrodynamic conditions and system physical properties, they are frequently combined and referred to as a "ka value" or more properly as a mass transfer coefficient. 

In the above equalitms, Kl is the overall mass transfer coefficient (based on phase L), a is the specific interfacial area for mass transfer related to unit column volume, X and Y are the phase solute concentrations, X is the... 

As in example BATEX, the constants Ki and K2 contain the mass transfer coefficient, the specific interfacial area and the total liquid volume. 

One of the most attractive roles of liquid liquid interfaces that we found in solvent extraction kinetics of metal ions is a catalytic effect. Shaking or stirring of the solvent extraction system generates a wide interfacial area or a large specific interfacial area defined as the interfacial area divided by a bulk phase volume. Metal extractants have a molecular structure which has both hydrophilic and hydrophobic groups. Therefore, they have a property of interfacial adsorptivity much like surfactant molecules. Adsorption of extractant at the liquid liquid interface can dramatically facilitate the interfacial com-plexation which has been exploited from our research. 

A high specific interfacial area and a direct spectroscopic observation of the interface were attained by the centrifugal liquid membrane (CLM) method shown in Fig. 2. A two-phase system of about 100/rL in each volume is introduced into a cylindrical glass cell with a diameter of 19 mm. The cell is rotated at a speed of 5000-10,000 rpm. By this procedure, a two-phase liquid membrane with a thickness of 50-100 fim. is produced inside the cell wall which attains the specific interfacial area over 100 cm. UV/VIS spectrometry, spectro-fluorometry, and other spectroscopic methods can be used for the measurement of the interfacial species and its concentration as well as those in the thin bulk phases. This is an excellent method for determining interfacial reaction rates on the order of seconds. 

Stopped flow mixing of organic and aqueous phases is an excellent way to produce dispersion within a few milliseconds. The specific interfacial area of the dispersion can become as high as 700 cm and the interfacial reaction in the dispersed system can be measured by a photodiode array spectrophotometer. A drawback of this method is the limitation of a measurable time, although it depends on the viscosity. After 200 ms, the dispersion system starts to separate, even in a rather viscous solvent like a dodecane. Therefore, rather fast interfacial reactions such as diffusion-rate-limiting reactions are preferable systems to be measured. 

Finally, Bakker and Van den Akker calculated local values for the specific mass transfer rate kfl, by estimating local Ay-values from local values of the Kolmogorov time scale /(v/e) and by deriving local values of the specific interfacial area a from local values for bubble size and bubble hold-up. 

Specific interfacial area per volume of gas-liquid emulsion m2 UIg+L... 

Under changing flow conditions it can be important to include some consideration of the hydrodynamic changes within the column (Fig. 3.48), as manifested by changes in the fractional dispersed phase holdup hn and the phase flow rates Ln and Gn which, under dynamic conditions, can vary from stage to stage. Such variations can have a considerable effect on the overall dynamic characteristics of an extraction column, since variations in hn also affect the solute transfer rate terms Qn by virtue of the corresponding variation in the specific interfacial area an. 

In these equations, a is the specific interfacial area for a significant degree of surface aeration (m2/m3), I is the agitator power per unit volume of vessel (W/m3), pL is the liquid density, o is the surface tension (N/m), us is the superficial gas velocity (m/s), u0 is the terminal bubble-rise velocity (m/s), N is the impeller speed (Hz), d, is the impeller diameter (m), dt is the tank diameter (m), pi is the liquid viscosity (Ns/m2) and d0 is the Sauter mean bubble diameter defined in Chapter 1, Section 1.2.4. 

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