Mass transfer dynamic interfacial tension

The author and co-workers have recently started work on the effect of the dynamic interfacial tension on the interfacial behaviour in systems undergoing mass transfer and this may bring more light to the topic presented here. 

Figure 5 Dynamic interfacial tensions in mass transfer processes. (A) Schematic for variation of surfactant SDS on a droplet surface with its growth at a T-junction. The continuous phase is a Tween-20 aqueous solution and the dispersed phase is hexane in the experiment of those images. (B) Interfacial tensions at the droplet pinch-off moment with the variation of phosphoric acid concentration in the water phase. The continuous phase is a phosphoric acid aqueous solution and the dispersed phase is methyl isobutyl ketone (MIBK). (C) Interfacial tensions at the bubble pinch-off moment with the variation of CO2 concentration in the gas phase. The continuous phase is a mono ethanol amine (MEA) aqueous solution and the dispersed phase is a CO2-N2 mixture. Panel (A) This figure is adapted from Wang et al (2009) with permission of the American Chemical Society.

Gaskell and Saelim [56] reported that the rate of desulphurisation was greater than that calculated from the diffusion of S in Fe and attributed this enhanced rate to Marangoni turbulence at the interface. However, deoxidation and desulphurisation reactions will lead, sequentially, to rapid mass transfer, a low, dynamic interfacial tension, emulsification, and very rapid kinetics. However, emulsification will also lead to a higher inclusion content in the steel. 

It has been reported that there was some lowering of the dynamic interfacial tension Yms when a molten, 321-stainless steel was in contact with a mould flux [4] due to mass transfer between the metal and slag phase and this would promote entrapment. 

It is still common practice to estimate the fluid dynamic properties from empirical correlations. Such correlations are usually developed from "cold flow" measurements which are often not properly designed and evaluated. It is understood that use of empirical correlations is of limited value and their predictions may lead to serious errors. This is particularly valid for those quantities which characterize interfacial properties like mass transfer coefficients, interfacial areas and phase holdups. It is now obvious that properties like density, viscosity, and surface tension are not always sufficient to describe fluid dynamic and interfacial phenomena. 

While phase equilibria for the a-tocopherol/carbon dioxide system at high pressures have been studied by several authors [1-6], only a few measurements of dynamic viscosity [7], thermal conductivity [7] and mass transfer coefficients [3] were carried out. The present study of the interfacial tension in the a-tocopherol/carbon dioxide system at temperatures between 313 and 402 K and pressures from 10 to 37 MPa aims on the one hand at completing characterisation of this system and on the other at contributing to understanding interfacial phenomena in mass transfer processes. 

It is difficult to summarize all the phenomena discussed in this volume. However, major topics include ultralow interfacial tension, phase behavior, microstructure of surfactant systems, optimal salinity concept, middle-phase microemuIsions, interfacial rheology, flow of emulsions in porous media, wettability of rocks, rock-fluid interactions, surfactant loss mechanisms, precipitation and redissolution of surfactants, coalescence of drops in emulsions and in porous media, surfactant mass transfer across interfaces, equilibrium dynamic properties of surfactant/oil/brine systems, mechanisms of oil displacement in porous media, ion-... 

In the course of interfacial mass transfer, from molecular point of view, the process is stochastic, that means some local molecules may undergo the mass transfer in advance than the others, so that small concentration gradient (where i — x, y, z) is established at the interface. As the surface tension a is function of concentration, it follows that the surface tension gradient is also created at the interface. If is increased up to a critical point, the fluid dynamic instability will appear to induce the interfacial convection as well as the formation of orderly structure at the interface. At the same time, the rate of mass transfer may be enhanced or suppressed depending on the properties of the mass transfer system concerned such phenomena is generally regarded as interfacial effect. 

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