Hydrodynamics-mass transfer limitation

Mass-transfer limitations due to poor hydrodynamic conditions in the feed-side/ membrane interface are common in organophilic pervaporation (as referred above). 

Several models are available that enable us to model as precisely as possible the respective influence of hydrodynamics and the mass transfer limitation on the chromatographic behavior. 

P. K. Shukla and M. E. Orazem, "Hydrodynamics and Mass-Transfer-Limited Current Distribution for a Submerged Stationary Hemispherical Electrode under Jet Impingement," Electrochimica Acta, 49 (2004) 2901-2908. 

Nutrient uptake and release rates in reefs occur at near mass transfer limits and may show water velocity dependence, depending on the reef, and its hydrodynamic characteristics. 

The first term in Eq. (1.31) corresponds to the activation part of deposition overpotential, and the second one is due to the mass transfer limitations. If one and the same process occurs under two different hydrodynamic conditions, characterized by two different values of the limiting diffusion current densities Il i and Jl,2> the Eq. (1.31) can be rewritten in the forms ... 

The improved production efficiency of the NaOH/NaCl flood resulted from the formation of W/0, water-in-oil, emulsions under the limited shear conditions. The alkaline phase imbibes into the oleic phase as a result of the interfacial chemical reaction. The swollen oil phase, together with its altered configuration, reduces the area available for the passage of the alkaline floodwater. The produced water-oil ratio, WOR, decreases as a result of the lowered water mobility. The consistent presence of a light, emulsion phase at the very end of the oil production stage indirectly confirms the occurrence of the in situ emulsification step. The production efficiency increases with decrease in injection rate because of mass transfer limitations of the interfacial chemical reaction hydrodynamic effects would act in the opposite direction. The degree of in situ emulsification and wettability alteration, and the correspondent mobility reduction, depends on the residence time of the reactants in the core. 

The quality of the results obtained in this paper is limited by the uncertainty introduced by the phase hydrodynamics in the reaction zone and the phase equilibrium hypothesis. The authors foreseen additional studies in order to better describe phase hydrodynamics, mass transfer outside and inside the catalyst pellets and their influence on process performances. 

Shukla PK, Orazem ME (2004) Hydrodynamics and mass-transfer-limited current distribution for a submerged stationary hemispherical electrode under jet impingement. Electrochim Acta 49 2901-2908... 

In order to address these issues, researchers have incorporated convection into the process of cell seeding, suppressing some of the mass transfer limitations encountered in the static procedure. Spinner flask bioreactors (Figure 44.2b) have been implemented to create convection and, thereby, hydrodynamic forces that could help increase mass transport. Poly(glycolic acid) (PGA) scaffolds were threaded onto needles and chondrocytes suspensions with a total number of cells between 2 x 10 and 10 x 10 were used. A yield of 60% was obtained after 2 h of seeding. A more uniform distribution of the cells in the scaffold was seen (compared to the static seeding) nonetheless, the concentration of cells in the outer layer of the construct was 60 to 70% higher than that in the bulk [24]. This behavior maybe due to the poor... 

Interphase mass transfer limitation for the apparent kinetic rate is conditioned by hydrodynamics that affects catalyst wetting, interphase turbulence and interphase areas. 

In a typical experiment with semiconductor-liquid junctions, one of the most important experimental problems is the differentiation between reactions that involve chemical changes at the semiconductor electrode (corrosion with insoluble products) and chemical changes in the electrolyte that might be subject to mass transfer limitations. The technique of Rotating Ring Disc Electrode (RRDE) (17-19) provides an opportunity to differentiate between these two types of reactions under controlled hydrodynamic conditions. In its simplestform, the metallic ring is isolated... 

A number of reaction engineering models in literature have already utdized the concepts of TBC or single-bubble-class (SBC) together with the axial dispersion models to couple the hydrodynamics, mass transfer, and reaction in bubble column reactors. While some researchers claim that there is less difference in the SBC and TBC model predictions, others believe that the TBC model prediction is in better agreement with experiments. We find that the difference is actually relevant to the submodels for hydrodynamics, mass transfer, and reaction kinetic as well as gas contraction, and in particular the gas holdup model and whether the system is limited by reaction or mass transfer. Then a new reactor model is developed to replace the empirical... 

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