Interfacial area chemical methods

Vazquez, G., Cancela, M.A., Riverol, C., Alvarez, E., and Navaza, J.M. (2000), Application of the Danckwerts method in a bubble column Effects of surfactants on mass transfer coefficient and interfacial area, Chemical Engineering Journal, 78(1) 13-19. 

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. 

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. 

There are a number of different types of experimental laboratory units that could be used to develop design data for chemically reacting systems. Charpentier [ACS Symp. Sen, 72, 223-261 (1978)] has summarized the state of the art with respect to methods of scaUng up lab-oratoiy data and tabulated typical values of the mass-transfer coefficients, interfacial areas, and contact times to be found in various commercial gas absorbers as well as in currently available laboratoiy units. 

Mass-transfer rates have been determined by measuring the absorption rate of a pure gas or of a component of a gas mixture as a function of the several operating variables involved. The basic requirement of the evaluation method is that the rate step for the physical absorption should be controlling, not the chemical reaction rate. The experimental method that has gained the widest acceptance involves the oxidation of sodium sulfite, although in some of the more recent work, the rate of carbon dioxide absorption in various media has been used to determine mass-transfer rates and interfacial areas. 

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. 

Two immiscible fluids, in contact with each other, share a common surface, called the interface. Operations involving transfer of matter or of heat across an interface are very common in chemical industry. In such operations a large interfacial area per unit volume is necessary if the desired transfer is to be obtained rapidly in equipment of finite size. Three common methods of providing a high ratio of interfacial area to volume are now discussed. 

The in situ wet chemical approach requires less nanocarbon modification, especially for electrodeposition, and can produce thin, uniform, multilayer films. This is the method of choice for nanocarbon-polymer hybrids as the increased interfacial area reduces problems of nanocarbon insolubility and subsequent aggregation. Gas phase deposition offers the greatest control of thin film thickness but is suitable almost exclusively to the deposition of metals and metal oxides. 

The chemical method used to estimate the interfacial area is based on the theory of the enhancement factor for gas absorption accompanied with a chemical reaction. It is clear from Equations 6.22-6.24 that, in the range where y > 5, the gas absorption rate per unit area of gas-liquid interface becomes independent of the liquid phase mass transfer coefficient /cp, and is given by Equation 6.24. Such criteria can be met in the case of absorption with... 

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. 

Maruoka and coworkers also investigated the substantial reactivity enhancement of N-spiro chiral quaternary ammonium salt and simplification of its structure, the aim being to establish a truly practical method for the asymmetric synthesis of a-amino acids and their derivatives. As ultrasonic irradiation produces homogenization (i.e., very fine emulsions), it greatly increases the reactive interfacial area, which may in turn deliver a substantial rate acceleration in the liquid-liquid phase-transfer reactions. Indeed, sonication of the reaction mixture of 2, methyl iodide and (S,S)-lc (1 mol%) in toluene-50% KOH aqueous solution at 0 °C for 1 h gave rise to the corresponding alkylation product in 63% yield with 88% ee. Hence, the reaction was speeded up markedly, and the chemical yield and enantioselectivity were comparable with those of the reaction with simple stirring (0°C for 8h 64%, 90% ee) (Scheme 5.5) [10]. 

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. 

Joshi and Sharma (1976) also evaluated interfacial area and liquid-side mass-transfer coefficients using chemical methods in columns of 0.38, 0.57, and 1.0 m diameters. The optimum liquid submergence ratio (H/dT) and impeller spacing ratio (L,/dT) were found to be in the range of 0.6-0.7 and 1.4-1.6, respectively. The following correlations were proposed ... 

The overall interfacial area for the whole reactor can be determined by chemical techniques. These techniques, however, must be used with restrictions. For example, chemical methods are difficult to use for fast-coalescing systems, since the presence of a chemical compound may reduce coalescence rates. Furthermore, in fast-coalescing systems, the specific area may depend strongly on the position in the reactor, which complicates the interpretation of an average value obtained with chemical methods. Indeed, both physical and chemical techniques should be used together to describe the phenomena that occur within gas-liquid reactors. While chemical methods allow the determination of the much-needed average interfacial area, information on the variations of the interfacial parameters, such as aL and dsv, within the reactor, which is important for scale-up, cannot be obtained by this method. 

The gas-liquid interfacial area measured form the physical techniques is generally about 35% higher than the one measured by the chemical technique. The chemical technique is generally more accurate than the physical technique. The reaction systems described in Table XXXI are reliable for the gas-liquid system. For gas-liquid-solid systems, the method is not reliable even when solids are inert, because of possible adsorptions of gas and/or liquid on the solid surface. 

The gas-liquid interfacial area has been measured by both physical and chemical methods. The accuracy of these measurements is generally very poor (15). The interfacial area has been related to the energy input per unit volume and the gas holdup by the expression (6-19)... 

Various physical and chemical methods can be applied to determine interfacial areas. Unfortunately, the different methods yield largely differing results. The methods used most often are photography and sulfite oxidation. Schumpe and Deckwer recently showed... 

Different chemical methods do not lead to equal interfacial areas either. This is demonstrated in Fig. 

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 interfacial area is known accurately only in some systems used in laboratory studies falling laminar films, laminar cylindrical jets, undisturbed gas-liquid and liquid-liquid interfaces, and solid castings of known dimensions immersed in liquids. In all reactor systems used industrially such as packed towers, spray towers, and bubble trays, the interfacial area is relatively difficult to determine. Photographic, gamma-ray, light scattering and chemical methods have been used to determine a in bubble dispersions (5, 6, 7, 8, iO, 42). For an average bubble diameter dn, a superficial gas velocity Usa and a bubble rise velocity Un,... 

In this section we consider the rate of absorption of gases into liquids that are agitated so that dissolved gas is transported from the interfacial surface to the interior by convective motion. The next section, based on this one, treats chemical methods for determining interfacial areas and mass-transfer coefficients in agitated gas-liquid reactors. 

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. 

The specific surface area of contact for mass transfer in a gas-liquid dispersion (or in any type of gas-liquid reactor) is defined as the interfacial area of all the bubbles or drops (or phase elements such as films or rivulets) within a volume element divided by the volume of the element. It is necessary to distinguish between the overall specific contact area S for the whole reactor with volume Vr and the local specific contact area 51 for a small volume element AVi- In practice AVi is directly determined by physical methods. The main difficulty in determining overall specific area from local specific areas is that Si varies strongly with the location of AVi in the reactor—a consequence of variations in local gas holdup and in the local Sauter mean diameter [Eq. (64)]. So there is a need for a direct determination of overall interfacial area, over the entire reactor, which is possible with use of the chemical technique. 

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. 

Chemical methods for determining gas-liquid interfacial areas and mass-transfer coefficients have been intensively developed for the last 10 years. The principles of these methods are deduced from the results presented in Section III,B,2 A gas A is absorbed into a liquid where it undergoes a reaction with a dissolved reactant B ... 

Our objective here is to try to answer the following questions For a proposed type of gas-liquid contactor compatible with the properties and flow rates of the phases and with the reaction type, what are the likely values of the specific interfacial area and the gas and liquid mass-transfer coefficients by which the contact performance can be predicted And what is the expected accuracy of these values Table XVIII gives typical values of these parameters in typical contactors shown in Fig. 12 for fluids with properties not very different from those of air and water (especially, liquid viscosity under 5 cP where the liquid is nonfoaming). Because this review is especially concerned with the chemical method of determining these parameters, experimental data obtained by this method will be given in subsequent tables and figures. 

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. 

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. 

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