Emulsions mass transport

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

At any instant, pressure is uniform throughout a bubble, while in the surrounding emulsion pressure increases with depth below the surfaee. Thus, there is a pressure gradient external to the bubble which causes gas to flow from the emulsion into the bottom of the bubble, and from the top of the bubble back into the emulsion. This flow is about three times the minimum fluidization velocity across the maximum horizontal cross section of the bubble. It provides a major mass transport mechanism between bubble and emulsion and henee contributes greatly to any reactions which take place in a fluid bed. The flow out through the top of the bubble is also sufficient to maintain a stable arch and prevent solids from dumping into the bubble from above. It is thus responsible for the fact that bubbles can exist in fluid beds, even though there is no surface tension as there is in gas-liquid systems. 

The majority of RDC studies have concentrated on the measurement of solute transfer resistances, in particular, focusing on their relevance as model systems for drug transfer across skin [14,39-41]. In these studies, isopropyl myristate is commonly used as a solvent, since it is considered to serve as a model compound for skin lipids. However, it has since been reported that the true interfacial kinetics cannot be resolved with the RDC due to the severe mass transport limitations inherent in the technique [15]. The RDC has also been used to study more complicated interfacial processes such as kinetics in a microemulsion system [42], where one of the compartments contains an emulsion. 

Koizumi and Higuchi [18] evaluated the mass transport of a solute from a water-in-oil emulsion to an aqueous phase through a membrane. Under conditions where the diffusion coefficient is expected to depend on concentration, the cumulative amount transported, Q, is predicted to follow the relationship... 

Diffusion cell for heterogeneous systems (membrane method) Mass transport studies from emulsions 18... 

Weiss, J. 1999. Effect of Mass Transport Processes on Physicochemical Properties of Surfactant-Stabilized Emulsions. Department of Food Science, University of Massachusetts, Amherst. 280. 

Thorough investigation of emulsion instability caused by molecular mass transport processes. 

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

The two-phase theory of fluidization has been extensively used to describe fluidization (e.g., see Kunii and Levenspiel, Fluidization Engineering, 2d ed., Wiley, 1990). The fluidized bed is assumed to contain a bubble and an emulsion phase. The bubble phase may be modeled by a plug flow (or dispersion) model, and the emulsion phase is assumed to be well mixed and may be modeled as a CSTR. Correlations for the size of the bubbles and the heat and mass transport from the bubbles to the emulsion phase are available in Sec. 17 of this Handbook and in textbooks on the subject. Davidson and Harrison (Fluidization, 2d ed., Academic Press, 1985), Geldart (Gas Fluidization Technology, Wiley, 1986), Kunii and Levenspiel (Fluidization Engineering, Wiley, 1969), and Zenz (Fluidization and Fluid-Particle Systems, Pemm-Corp Publications, 1989) are good reference books. 

The large industrial fluid beds are normally operated with U/U exceeding 10, so that a large portion of the gas bypasses the bed in the form of bubbles. Also the diameter of the bubbles is fairly large, so that interphase mass transport is small compared to the rate of reaction. Under these conditions the extent of mixing in the emulsion phase is rather an unimportant parameter as far as the prediction of conversion is concerned. It would, however, have significant influence when non first-order reactions are involved. 

J. Weiss, Mass Transport Phenomena in OU-in-Water Emulsions, The University of Massachusetts, Amherst, Massachusetts, 1999. 

For membrane processes involving liquids the mass transport mechanisms can be more involved. This is because the nature of liquid mixtures currently separated by membranes is also significantly more complex they include emulsions, suspensions of solid particles, proteins, and microorganisms, and multi-component solutions of polymers, salts, acids or bases. The interactions between the species present in such liquid mixtures and the membrane materials could include not only adsorption phenomena but also electric, electrostatic, polarization, and Donnan effects. When an aqueous solution/suspension phase is treated by a MF or UF process it is generally accepted, for example, that convection and particle sieving phenomena are coupled with one or more of the phenomena noted previously. In nanofiltration processes, which typically utilize microporous membranes, the interactions with the membrane surfaces are more prevalent, and the importance of electrostatic and other effects is more significant. The conventional models utilized until now to describe liquid phase filtration are based on irreversible thermodynamics good reviews about such models have been reported in the technical literature [1.1, 1.3, 1.4]. 

Since Trimsol contains 11% chloride by weight, initial experiments were performed with halide-tolerant Co(III). Unfortunately, the conversion of Trimsol to CO2 by Co(III) was very inefficient Poor efficiency may have been due to insufficient oxidizing power of Co(IIl), as well as low solubility of the organic waste in acid. Since Trimsol forms stable emulsions in base, direct anodic oxidation in NaOH was attempted. Unfortunately, this process was also inefficient The best results were obtained with an electrolyte of AgN03 and HNO3, elevated temperature, and a cell current near the mass transport limit. Under these conditions, complete destruction of the Trimsol was achieved at approximately 30% coulombic efficiency. As expected, chloride liberated during the destruction of Trimsol precipitated as AgQ. Results for Ag(II), Co(in), and NaOH are compared in Figure 15. 

As a final word on this development of selectivity, and indeed the developments and illustrations provided throughout this discussion of fluidized beds, we must remember that good old Academic Reaction 1 was employed throughout. However, one should be able to insert any form of reaction kinetics desired with the expectation that the equations will become nonlinear. The concept of rate processes occurring in a series of steps is a core of these models, even though there is no strong a priori evidence to support this view. A different viewpoint, picturing mass transport from a solids-lean phase to a solids-rich, cloud-emulsion phase was reported to be superior in some respects to series models such as that of Kunii and Levenspiel [see J.J. Carberry, Trans. Inst. Chem. Eng., 59, 15 (1981) and A.A. Shaikh, Chem. Eng. TechnoL, 13, 273 (1990)]. 

In a first approach, CRP was mainly studied in miniemulsion polymerization [31-33] because the technique allows the complex nucleation and mass transport processes of an emulsion polymerization to be avoided. Consequently, the same recipes used in bulk could be employed, in particular the use of hydrophobic initiators and control agents. Many successful examples have been reported, and led to well-defined polymer chains and sometimes complex architectures in stable, submicrometric latex particles. Depending on the CRP technique, several requirements have to be considered. 

Electrodes utilizing oxidic supports very often suffer from poor porosity. In contrast to catalytic layers built up from CB (see Figure 7.8), low-surface-area oxides tend to form dense layers, which cannot easily be penetrated by reactants and products. Hence, despite their advantages in durability-limited mass transport is a significant drawback to their routine application. Since they also often possess only low electron conductivities, usually carbon material/oxide support composites are applied instead. One approach toward a controlled porous electrode structure utilizing the so-called Pickering emulsions is shown below (Figure 7.13). 

The FRRPP can also be implemented in suspension and emulsion polymerization processes. Its analysis in suspension system has turned out to be straightforward, because the suspension size scale (mm sizes) does not interfere with the reaction mechanism, even if one includes mass and thermal transport effects. In emulsion polymerization systems, the submicron size scale of emulsion particles interfere with thermal and probably mass transport effects in the system. Also, the hydrophobic portions of surfactant molecules could affect the phase equilibrium aspects of the FRRPP system. 

Gradients in surface (or interfacial) tension can accelerate the spreading of fluids, enhance the stability of surfactant-laden films of liquid, emulsions, and foams, and increase rates of mass transport across interfaces. The motion of fluid driven by a gradient in surface tension is referred to as a Marangoni flow . We have demonstrated that electrochemical reduction of IF to IF at an electrode that... 

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