Interfacial transfer contact area

Knowing the interfacial or contact area A (m2) between the gas and the liquid phase, the total molar flow [Pg.1520]

The rate of mass transfer (qv) depends on the interfacial contact area and on the rate of mass transfer per unit interfacial area, ie, the mass flux. The mass flux very close to the Hquid—Hquid interface is determined by molecular diffusion in accordance with Pick s first law ... 

Interfacial Contact Area and Approach to Equilibrium. Experimental extraction cells such as the original Lewis stirred cell (52) are often operated with a flat Hquid—Hquid interface the area of which can easily be measured. In the single-drop apparatus, a regular sequence of drops of known diameter is released through the continuous phase (42). These units are useful for the direct calculation of the mass flux N and hence the mass-transfer coefficient for a given system. 

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... 

Ap = Final column inside net area, ft, or in. a = Surface area of an orifice, in. a = Effective interfacial area for contacting gas and liquid phases, ft /ft. Because this is very difficult to evaluate, it is usually retained as a part of the coefficient such as Kca or Kj a a = Area of transfer surface per unit of tower volume in water cooling towers, ft /ft, or, termed contact area... 

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. 

Prepare an outline design of the reactor and carry out the chemical engineering design of the stripper, specifying the interfacial contact area which will need to be provided between the carbon dioxide stream and the product stream to enable the necessary mass transfer to take place. 

Interfacial contact area, 10 755-756 Interfacial effects, in CA resists, 15 182 Interfacial energy, 24 157 colloids, 7 281-284 Interfacial forces, in foams, 12 4 Interfacial free energy, 24 119 Interfacial in situ polymerization, in microencapsulation, 16 442 446 Interfacial mass-transfer coefficients,... 

Ultrasound is known to generate extremely fine emulsions from mixtures of immiscible liquids. Ultrasonic homogenisation has been used for many years in the food industry for the production of tomato sauce, mayormaise and other similar blended items. In chemistry such extremely fine emulsions provide enormous interfacial contact areas between immiscible liquids and thus the potential for greater reaction between the phases. This can be particularly beneficial in phase transfer catalysis. 

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. 

Choice of Dispersed Phase In general, formation of dispersed drops is preferred over formation of films or rivulets in order to maximize contact area and mass transfer. Static extractors generally are designed with the majority phase dispersed in order to maximizie interfacial area needed for mass transfer i.e., the phase with the greatest flow rate entering the column generally is dispersed. The choice of dispersed phase also depends upon the relative viscosity of the two phases. If one phase is particularly viscous, it may be necessary to disperse that phase. 

In summary, one of the weakest links in modeling reactive systems operated in bubble columns is the fluid dynamic part considering multi-phase turbulence modeling, interfacial closures, and especially the impact and descriptions of bubble size and shape distributions. For reactive systems the estimates of the contact areas and thus the interfacial mass transfer rates are likely to contain large uncertainties. 

If an appropriate relation for the contact area as a function of the internal coordinates is available, the particle growth term due to interfacial mass transfer can be modeled in accordance with the well known film theory (although still of semi-empirical nature) and the ideal gas law [68]. The modeling of the source and sink terms due to fluid particle breakage and coalescence is less familiar and still on an early stage of development. Moreover, the existing theory is rather complex and not easily available. Further research is thus needed in order to derive consistent multifluid-population balance models. 

High efficiencies require deep pools of liquid on the tray (long contact time) and relatively high gas velocities (large interfacial contact areas and mass-transfer coefficients). These conditions, however, lead to a number of difficulties. One is the mechanical entrainment of liquid droplets in the rising gas stream. At high gas velocities, when the gas disengages from the froth, small droplets of liquid are carried by the gas to the tray above. Liquid carried up the tower in this manner reduces mass transfer and consequently affects the tray efficiency adversely. 

The interfacial contact area is known, constant, and independent of the operation conditions, thus facilitating the mathematical modeling of mass transfer phenomena. 

Bubble columns are used for liquid aeration and gas-liquid reactions. Thus, finely suspended bubbles produce large interfacial areas for effective mass transfer, where the contact area per unit volume of emulsion is calculated from the expression a 6e/dg, where e is the volume fraction of injected gas. While simple to design and construct, bubble columns sustain rather large eddy dispersion coefficients, and this must be accounted for in the modeling process. For cocurrent operation, liquid of... 

In the two-phase model, the mass-transfer parameter needs to be determined using actual polymerization reactor data because the specific interfacial area is strongly dependent upon the geometry of the reactor internals. Moreover, the formation of bubbles of volatile species contributes to the total vapor-liquid contact area, and the liquid holdup on a rotating disk can change with the melt viscosity or polymer molecular weight, affecting the mass-transfer coefficient. 

The specific contact surface area necessitated for mass transfer in a gas-liquid dispersion or more generally in any type of gas-liquid reactor is defined as the interfacial contact area of all the bubbles or drops or phase element such as films or rivulets within a volume element divided by the volume of that element. ... 

In some respects the mechanism of SPE is similar to that of LLE where an extractive organic solvent is added to the aqueous sample solution and the vessel is agitated vigorously to create a temporary emulsion. The emulsion consists of very small spherical solvent droplets suspended in the aqueous phase. The interfacial contact area between the two phases must be quite large in order to promote rapid mass transfer of the analytes from the aqueous to the organic phase. To complete the desired extraction, a finite amount of time is required for the emulsion to break and the phases coalesce into separate layers. The lower layer is carefully drawn off to complete the separation. 

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