Mass-transport process

Walton D J, Phull S S, Chyla A, Lorimer J P, Mason T J, Burke L D, Murphy M, Compton R G, Ekiund J C and Page S D 1995 Sonovoltammetry at platinum electrodes surface phenomena and mass transport processes J. Appl. Electrochem. 25 1083... 

In the A sector (lower right), the deposition is controlled by surface-reaction kinetics as the rate-limiting step. In the B sector (upper left), the deposition is controlled by the mass-transport process and the growth rate is related linearly to the partial pressure of the silicon reactant in the carrier gas. Transition from one rate-control regime to the other is not sharp, but involves a transition zone where both are significant. The presence of a maximum in the curves in Area B would indicate the onset of gas-phase precipitation, where the substrate has become starved and the deposition rate decreased. 

The flow-dynamics and mass-transport processes can be expressed mathematically and realistic models obtained to be used in the predictions of a CVD operation and in the design of reactors. 

Carbonell, RG Sarti, GC, Coupled Deformation and Mass-Transport Processes in Solid Polymers, Industrial and Engineering Chemistry Research 29, 1194, 1990. 

Requirements regarding laboratory liquid-liquid reactors are very similar to those for gas-liquid reactors. To interpret laboratory data properly, knowledge of the interfacial area, mass-transfer coefficients, effect of contaminants on mass-transport processes, ionic characteristics of the system, etc. is needed. Commonly used liquid-liquid reactors have been discussed by Doraiswamy and Sharma (1984). 

Eqs. (40)-(45) describe the behavior of a mass transport process occurring at very large Schmidt numbers. For finite values of Sc, Chin [16] extended the 0-series expansion analysis and obtained the following asymptotic series for Sfiav on a rotating hemisphere ... 

An alternative to the stirred tank system is a column-type device which provides for constant fluid flow through a powder bed. The mass transport process was shown to be primarily determined by the length and cross-sectional area of the cylinder and the fluid flow rate [36],... 

Table 1 Summary of Experimental Methods for Evaluating Diffusion Coefficients and Investigating Mass Transport Processes of Pharmaceutical Interest... 

Mass Transport Processes within Porous Catalysts... 

MASS TRANSPORT PROCESSES WITHIN POROUS CATALYSTS... 

Whether Knudsen or bulk diffusion dominates the mass transport process depends on the relative magnitudes of the two terms in the denominator of equation 12.2.6. The ratio of the two diffusivity parameters is obviously important in establishing these magnitudes. In this regard, it is worth noting that DK is proportional to the pore diameter and independent of pressure whereas DAB is independent of pore size and inversely proportional to the pressure. Consequently, the higher the pressure and the larger the pore, the more likely it is that ordinary bulk diffusion will dominate. 

Bulk or forced flow of the Hagan-Poiseuille type does not in general contribute significantly to the mass transport process in porous catalysts. For fast reactions where there is a change in the number of moles on reaction, significant pressure differentials can arise between the interior and the exterior of the catalyst pellets. This phenomenon occurs because there is insufficient driving force for effective mass transfer by forced flow. Molecular diffusion occurs much more rapidly than forced flow in most porous catalysts. 

There are three types of mass transport processes within a microfluidic system convection, diffusion, and immigration. Much more common are mixtures of three types of mass transport. It is essential to design a well-controlled transport scheme for the microsystem. Convection can be generated by different forces, such as capillary effect, thermal difference, gravity, a pressurized air bladder, the centripetal forces in a spinning disk, mechanical and electroosmotic pumps, in the microsystem. The mechanical and electroosmotic pumps are often used for transport in a microfluidic system due to their convenience, and will be further discussed in section 11.5.2. The migration is a direct transport of molecules in response to an electric field. In most cases, the moving... 

In analysis of a mass transport process, the Peclet number, Pe, the ratio of the direct flow to the diffusion, is used and given as... 

If only linear diffusion is the operating mass transport process, it has been shown that the current (i) in a planar electrode is related to the concentration (c) gradient at the surface of the electrode (x = 0) by [332]... 

It was mentioned in Section 2.2 that even in the case of a simple electrode reaction one must take into account both heterogeneous electron transfer and mass transport processes. Let us therefore examine the mathematical relationships which govern the two processes. 

Mass transfer (continued) in monolith, 27 89 in porous catalyst, 27 60-63, 68 in tubular reactor, 27 79, 82, 87 Mass transport processes, 30 312-318 convective, 30 312-313 diffusive, 30 313-315 selectivity, 30 316... 

The thermochemical conversion of biofuels takes place in the conversion system and belongs to the science of two-phase phenomena (fluid-solid dynamics), that is, heat and mass transport processes take place inside and between a solid phase and a gas phase. This phenomenology is well illustrated by Balakrishnen and Pei [49], see Figure 40. 

Mass transport processes are involved in the overall reaction. In these processes the substances consumed or formed during the electrode reaction are transported from the bulk solution to the interphase (electrode surface) and from the interphase to the bulk... 

For the M/M electrode, steps 4 and 5 are absent since the product of the charge transfer M remains incorporated in the metal electrode. Steps 1 and 5 are mass-transport processes. 

The two major causes of uneven current distribution are diffusion and ohmic resistance. Nonuniformity due to diffusion originates from variations in the effective thickness of the diffusion layer 8 over the electrode surface as shown in Figure 10.13. It is seen that 8 is larger at recesses than at peaks. Thus, if the mass-transport process controls the rate of deposition, the current density at peaks ip is larger than that at recesses since the rate of mass transport by convective diffusion is given by... 

Richer P, Bottinga Y, Javoy M (1977) A review of H, C, N, O, S, and Q stable isotope fractionation among gaseous molecules. Ann Rev Earth Planet Sci 5 65-110 Richter FM (2007) Isotopic fingerprints of mass transport processes. Geochim Cosmochim Acta 71 A839... 

Baier, Giesen et al. have described the dynamics of step and island on Ag(lll) and Cu(lOO) [31], as well as on Au(lOO) [32] electrodes in the electrolyte. Baier and Giesen [33] have determined the activation energies of mass transport processes on Ag(lll) electrodes in the aqueous electrolyte. Haftel and Einstein [34] have studied the influence of the electrochemical potential on energy landscapes close to step-and island edges Ag(lll) and Ag(lOO). A model of the metal/solution interface involving hydrophilicity of Ag(lll) and based on the capacitance analysis, has been published by Emets et al. [35] Electron... 

It is probable that the actual mass-transport process is a combination of all three cases. However, since the percentage falloff of concentration in the rod is not much different in the three cases, the results may be used on a... 

In this study the ratio of the particle sizes was set to two based on the average value for the two samples. As a result, if the diffusion is entirely controlled by secondary pore structure (interparticle diffusion) the ratio of the effective diffusion time constants (Defl/R2) will be four. In contrast, if the mass transport process is entirely controlled by intraparticle (platelet) diffusion, the ratio will become equal to unity (diffusion independent of the composite particle size). For transient situations (in which both resistances are important) the values of the ratio will be in the one to four range. Diffusional time constants for different sorbates in the Si-MCM-41 sample were obtained from experimental ZLC response curves according to the analysis discussed in the experimental section. Experiments using different purge flow rates, as well as different purge gases... 

There are three kinds of mass transport process relevant to electrode reactions migration, convection and diffusion. The Nernst—Planck equation... 

Techniques which allow one to monitor the boundary layer as a function of time, such as total internal reflection fluorescence (TIRF) spectroscopy 4 43), permit a quantitative evaluation of interfacial mass transport processes using, for example, fluorescently-tagged macromolecules which do not adsorb, such as fluorescein-labeled dextran 40 ... 

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