Mass transfer barriers

Here we also consider sorption kinetics as the mass-transfer barrier to surfactant migration to and from the interface, and we follow the Levich framework. However, our analysis does not confine all surface-tension gradients to the constant thickness film. Rather, we treat the bubble shape and the surfactant distribution along the interface in a consistent fashion. 

Creation of a mass transfer barrier for combustible volatiles and oxygen by migration of nanoparticles toward the surface exposed to heat flux... 

Kinetic and Mass-Transfer Barriers and Mediated Oxidation... 

In a binary mixture, diffusion coefficients are equal to each other for dissimilar molecules, and Fick s law can determine the molecular mass flows in an isotropic medium at isothermal and isobaric conditions. In a multicomponent diffusion, however, various interactions among the molecules may arise. Some of these interactions are (i) diffusion flows may vanish despite the nonvanishing driving force, which is known as the mass transfer barrier, (ii) diffusion of a component in a direction opposite to that indicated by its driving force leads to a phenomenon called the reverse mass flow, and (iii) diffusion of a component in the absence of its driving force, which is called the osmotic mass flow. 

In a first approximation, the new methods correspond to the conventional solvent techniques of supported catalysts (cf Section 3.1.1.3), liquid biphasic catalysis (cf Section 3.1.1.1), and thermomorphic ( smart ) catalysts. One major difference relates to the number of reaction phases and the mass transfer between them. Owing to their miscibility with reaction gases, the use of an SCF will reduce the number of phases and potential mass transfer barriers in processes such as hydrogenation, carbonylations, oxidation, etc. For example, hydroformylation in a conventional liquid biphasic system is in fact a three-phase reaction (g/1/1), whereas it is a two-phase process (sc/1) if an SCF is used. The resulting elimination of mass transfer limitations can lead to increased reaction rates and selectiv-ities and can also facilitate continuous flow processes. Most importantly, however, the techniques summarized in Table 2 can provide entirely new solutions to catalyst immobilization which are not available with the established set of liquid solvents. 

When the solubility limit of water in the fluid is approached or exceeded, the reaction rates drop drastically [13,21]. The liquid water in the enzyme particles apparently forms a mass-transfer barrier. It has also been noted that enzyme particles tend to stick together when the optimum water concentration is exceeded [45]. Obviously this reduces the enzyme area that is accessible to substrates. 

ECM hydrogels Mimics aspects of native ECM Good cell-cell interaction Mass transfer barriers Cell retrieval issues Degradable Variable consistency... 

Alginate sponges Good cell-cell interaction Uses animal-free biomolecules Relatively easy to make Isolated spheroid cultures Co-cultures difficult Mass transfer barriers Degradable... 

To ensure that the experimental procedure adopted results in the highest-possible colloidal nanoparticle concentration, the sequence of precursor addition was reversed. A second scheme which involved mixing the metal salt powder with microemulsions already containing the stoichiometric amount of NaOH solution was tested. Both schemes succeeded in forming stabilized colloidal nanoparticles however, higher uptake was obtained when scheme 1, solubUizing the metal salt before adding NaOH, was employed. The lower uptake associated with scheme 2 was attributed to the formation of a mass transfer barrier of the metal oxide/hydroxide at the surface of the salt powder, which prevented further dissolution. 

Chapter 7 Process-induced minimization of mass transfer barriers for improved... 

Process-Induced Minimization of Mass Transfer Barriers for Improved Drying... 

Although the application of ultrasound in osmotic dehydration is, technically speaking, not a contact ultrasound application, its analysis provides an interesting insight into the principles of ultrasound-assisted drying and the different approaches to minimize mass transfer barriers applying PEF and ultrasound. It is therefore briefly discussed here. 

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