Transfer rate interfacial with reaction

Depending on the value of Ha, different situations can be distinguished (see Figure 2.12) For Ha < 0.3 the reaction rate is slow compared to the mass transfer and the reaction takes place in the bulk phase. For values of the Hatta number // > 3, the reaction rate is very fast compared to the mass transfer rate and the reaction takes place only in the fluid film of the reaction phase near the interfacial area. Under these conditions, the transformation increases proportionally with the specific interfacial area between the phases (a) and the square root of the reaction rate constant (Equation 2.93) ... 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

Substituted phenols as well as phenol itself are typical constituents of (bio-)refractory waste waters and can increase a(0> 3 (Gurol and Nekouinaini, 1985). They studied the influence of these compounds in oxygen transfer measurements and attributed this effect to the hindrance of bubble coalescence in bubble swarms, which increases the interfacial area a. When evaluating the effect of these phenols on the ozone mass transfer rate, it is important to note that these substances react fast with ozone (direct reaction, kD= 1.3 103 L mol"1 s 1, pH = 6-8, T = 20 °C, Hoigne and Bader, 1983 b). 

As in Example 4.3 with an agitated tank, let the fraction of chlorine passing through the reactor unreacted be / and, because CI2 is replaced by HCI, / is also the mole fraction of chlorine in the off-gas. Since the total pressure is 1 bar, the partial pressure of the chlorine will be fu bar. Because the gas phase in the reactor is assumed to be well mixed, the equivalent interfacial chlorine concentration Ca, is fJStS, i.e. /u/0.45 = 2.22/ kmol/m3. Considering unit volume, i.e. 1 m3 of dispersion, and following equation 4.17, the rate of mass transfer across the interface is now equated to the rate of the reaction in the bulk of the liquid where the concentration of the chlorine is Qnt ... 

The acceleration of mass transfer due to chemical reactions in the interfacial region is often accounted for via the so-called enhancement factors [19, 26, 27]. These parameters are defined as a relationship between the mass transfer rate with reaction and mass transfer rate without reaction, assuming the same mass transfer driving force. 

The preceding data are consistent with hydride transfer being predominantly an interfacial reaction in sulfuric acid. This view is supported by studies of the effect of acid modifiers on the reaction in 96% H2SO4. Table I shows the effect of halogenated acetic acids, methanesulfonlc acid and water upon the hydride transfer rate. The rates were estimated in two ways and all rates are relative to that in 96 percent H2SO. ... 

Effects of Water in HF Catalyst. A number of investigators have pointed out that water has an important role in alkylation catalysts. Schmer-ling (1955) stated that the use of HF catalyst with one percent water produced a favorable result In propylene-isobutane alkylation, whereas, with a catalyst containing ten percent water, isopropyl fluoride was the principal product and no alkylate was formed. (Both reactions were at 25C.) Albright et al. (1972) found the water content of sulfuric acid to be "highly important" In affecting the quality and yield of butene-isobutane alkylate. They postulated that the water content of sulfuric acid controlled the level of ionization and hydride transfer rate In the catalyst phase. It appears that dissolved water affects HF alkylation catalyst similarly and also exerts further physical influence on the catalyst phase such as reducing viscosity. Interfacial tension, and isobutane solubility. 

The interactions with the surface and reaction kinetics have been studied in detail using various techniques, such as voltammetry, electrorefiectance measurements and surface enhanced Raman spectroscopy [139,140]. For monolayers on gold assembled from long chain thiols of the structure HS-(CH2) -C00H (with n > 9) the interfacial electron transfer rate exponentially decreases with chain length and the tunnelling parameter is in the... 

For systematic study of several gas-liquid chemical reactions using a laboratory model bubble-cap column, Sharmaet al. (S23) have shown that the presence of electrolytes, size of caps, type of slots, ionic strength, liquid viscosity, and presence of solids do not affect the mass-transfer rates. These rates chiefly depend on the gas and liquid flow rates (S23, M2). The influence of the superficial gas flow rate on ki, and Icq is indicated in Fig. 20. Interfacial area a" per unit area of plate (or per unit floor area) for plate diameters varying between 0.15 and 1.20 m have been grouped in Fig. 21, which with the following correlations (S23) can be used to scale up bubble-cap plates up to 2 or 3 m in diameter ... 

Multiphase Reactors Reactions between gas-liquid, liquid-liquid, and gas-liquid-solid phases are often tested in CSTRs. Other laboratory types are suggested by the commercial units depicted in appropriate sketches in Sec. 19 and in Fig. 7-17 [Charpentier, Mass Transfer Rates in Gas-Liquid Absorbers and Reactors, in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981]. Liquids can be reacted with gases of low solubilities in stirred vessels, with the liquid charged first and the gas fed continuously at the rate of reaction or dissolution. Some of these reactors are designed to have known interfacial areas. Most equipment for gas absorption without reaction is adaptable to absorption with reaction. The many types of equipment for liquid-liquid extraction also are adaptable to reactions of immiscible liquid phases. 

Although capsule membrane PTC is not suitable for direct scale-up to industrial level due to the inconveniences of working with capsules, the principles can be exploited in membrane reactors, with the PT catalyst immobilized on the membrane surface. This would not only enable easy recovery of both aqueous and organic phases after reaction without any problems of emulsification, but also ensure that the PT catalyst does not contaminate the product in the organic phase. Using a membrane reactor will also ensure high mass-transfer rates due to high interfacial areas per unit volume of reactor. More importantly, it will open up possibilities for continuous operation. 

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