Mass transport system external

In an external mass-transport system, material is added to and/or removed from the sample either during or after the exposure. In such systems the mass transport typically occurs in a separate development step subsequent to the optical exposure. Clearly, external mass-transport systems cannot be used in closed volumes, but it is possible to have a spatial frequency response that extends to zero. 

VI. EXTERNAL MASS-TRANSPORT SYSTEMS A. General Considerations... 

External mass-transport systems require either the addition or removal of material to create the desired pattern of refractive-index variation. The use of the latter method is by far the more common and we discuss two techniques for accomplishing this the use of solvents to dissolve portions of a film, and selective evaporation of a fraction of the material in the film. 

A large number of methods for improving external mass transport in membrane systems have been proposed and evaluated. Several of them may lead to a significant process improvement under defined conditions. Still, these methods - use of... 

Results in Table 5 show a worse performance of the system with recycle (always in the first reactor). All values of ethanol concentration obtained with recycle decreased when compared to the ones obtained without recycle. Therefore, the increase in the external mass transport coefficient was not sufficient to compensate for the decrease in the TRS concentration in the first reactor due to dilution. The concentration of free glucose in the effluent is still very low for both cases, with and without recycling. Only... 

A further increase of the gas-phase temperature Tg leads to a steady increase of the surface temperature % until the ignition temperature Tg ig is reached. Then the system becomes unstable as small unavoidable fluctuations of the gas temperature (or concentration etc.) may lead to a small shift of the heat removal line to slightly higher temperatures. Now the stable operation point lies on the upper part of the heat production function, and % is much higher than the fluid temperature (Tg g). Here ySAm.ex is much smaller than porekm. as the rate of the chemical reaction increases quasi-exponentially with temperature, whereas can be regarded as comparatively constant ( S Dg T ). Thus the effective rate of heat production then depends only on the rate of external mass transport, and Eq. (4.5.38) reduces to ... 

In the previous analyses of the combined effects of chemical reaction and diffusion, we have used first-order kinetics for the interfacial reaction. In this section we will examine the effect of reaction order with respect to the concentration of gaseous reactant ( , henceforth to be called simply, the reaction order ). We shall do this for the shrinking unreacted-core system without external-mass-transport resistance, and for irreversible reactions K oo). 

In the preceding discussion we assumed that external mass transport does not influence the overall rate. In laboratory scale experimental studies such conditions are readily realized by using sufficiently high gas velocities. In practical systems involving packed or fluidized beds of solid particles, however, external mass transport may be fully or partially rate controlling. 

As in the case of negligible resistance to the progress of reaction by external mass transport N oo) discussed in the previous sections, analytical solutions including the effect of N h can be obtained for systems of F = 1 [26, 34]. 

Mass Transport. An expression for the diffusive transport of the light component of a binary gas mixture in the radial direction in the gas centrifuge can be obtained directly from the general diffusion equation and an expression for the radial pressure gradient in the centrifuge. For diffusion in a binary system in the absence of temperature gradients and external forces, the general diffusion equation retains only the pressure diffusion and ordinary diffusion effects and takes the form... 

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. 

Most of the actual reactions involve a three-phase process gas, liquid, and solid catalysts are present. Internal and external mass transfer limitations in porous catalyst layers play a central role in three-phase processes. The governing phenomena are well known since the days of Thiele [43] and Frank-Kamenetskii [44], but transport phenomena coupled to chemical reactions are not frequently used for complex organic systems, but simple - often too simple - tests based on the use of first-order Thiele modulus and Biot number are used. Instead, complete numerical simulations are preferable to reveal the role of mass and heat transfer at the phase boundaries and inside the porous catalyst particles. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

Experimental measurements must be interpreted in connection with all external influences, including fluid flow, bulk viscosity, and pH of the system. Consideration must be given to whether the mass transport is occurring in one or more dimensions and whether mass transport is affected by pressure gradients and/or osmotic pressure gradients. 

Internal and external mass transfer limitations in porous catalyst layers play a central role in three-phase processes. The governing phenomena are well-known since the days of Thiele (1) and Frank-Kamenetskii (2). Transport phenomena coupled to chemical reactions is not frequently used for complex organic systems. A systematic approach to the problem is presented. 

If external potentials are applied to a system of several interconnected channels, the respective field strength in each channel will be determined by Kirchhoff s laws in analogy to an electrical network of resistors [28]. Ideally, electrokinetically driven mass transport in each of the channels will take place according to magnitude and direction of these fields. This allows for complex fluid manipulation operations in the femtoliter to nanoliter range without the need of any active control elements, such as external pumps or valves. This is of particular relevance due to the demanding limitations with respect to void volumes in the system (see Sect. 2). 

In the kinetic studies of the adsorption process, the mass transport of the analyte to the binding sites is an important parameter to account for. Several theoretical descriptions of the chromatographic process are proposed to overcome this difficulty. Many complementary experiments are now needed to ascertain the kinetic measurements. Similar problems are found in the applications of the surface plasmon resonance technology (SPR) for association rate constant measurements. In both techniques the adsorption studies are carried out in a flow system, on surfaces with immobilized ligands. The role of the external diffusion limitations in the analysis of SPR assays has often been mentioned, and the technique is yet considered as giving an estimate of the adsorption rate constant. It is thus important to correlate the SPR data with results obtained from independent experiments, such as those from chromatographic measurements. 

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