Mass-transport discrimination

Mass-transport discrimination is not limited to total exclusion, but can also occur when several species are able to enter the pore system. If the diffusivity of each species in a mixture is sufficiently varied, then the different molecules will diffuse through the zeolite at different rates, and can be separated. This is shown in Figure 3.2. 

Estimation of parameters. Model parameters in the selected model are then estimated. If available, some model parameters (e.g. thermodynamic properties, heat- and mass-transfer coefficient, etc.) are taken from literature. This is usually not possible for kinetic parameters. These should be estimated based on data obtained from laboratory expieriments, if possible carried out isothermal ly and not falsified by heat- and mass-transport phenomena. The methods for parameter estimation, also the kinetic parameters in complex organic systems, and for discrimination between models are discussed in more detail in Section 5.4.4. More information on parameter estimation the reader will find in review papers by Kittrell (1970), or Froment and Hosten (1981) or in the book by Froment and Bischoff (1990). 

The ability of varying the rate of the mass transport by agitating the solution (or the working electrode) constitutes the basis of hydrodynamic methods (hydrodynamics = liquids in motion), which are a further support to the study of electrode kinetics. Nevertheless we wish to cite them here simply to cover a drawback of cyclic voltammetry. In fact, cyclic voltammetry is unable to discriminate between oxidation and reduction processes, and vice-versa. 

In order to discriminate between reversible and non-reversible regimes, the relation between the heterogeneous rate constant and the mass transport coefficient can be defined ... 

The Hatta-number represents the ratio of maximal possible reaction and mass transfer rates and helps to specify different absorption regimes. Depending on the Hatta-number value, it is possible to discriminate between very fast, fast, average and slow chemical reactions, in respect to physical mass transport [19, 20]. 

Differences in mass transport rates provide a potential means for discriminating between different gases and vapors. It is known, for example, that transport through molecular sieves can be a sensitive function of molecular size and shape [110]. For gases and vapors that have only weak physical interactions with a porous adsorbent layer, however, transport rates are often too high to allow... 

Figure lA verifies the size discrimination of mass transport througji the hydrolyzed films. At short hydrolysis times (<40 min) only the... 

Henderson Ml, Hillman AR, Vieil E, Lopez C (1998) Combined electrochemical quartz microbalance (EQCM) and probe beam defection (PBD) validation of the technique by a study of silver ion mass transport. J Electroanal Chem 458 241-248 Henderson MJ, Hillman AR, Vieil E (1999) Ion and solvent transfer discrimination at a poly(o-toluidine) film exposed to HC104 by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD). J Phys Chem B 103 8899-8907 Henderson MJ, French H, Hillman AR, Vieil E (1999) A combined EQCM and probe beam defection study of salicylate ion transfer at a polypyrrole modified electrode. Electrochem Solid State Lett 2(12) 631-633... 

Fonnation of surface relief grating (SRG) by irradiation of amorphous films of azobenzene-fiinctionalized acrylate, methacrylate, and qroxy-based polymers with two coherent laser beams has been a topic of current inteiest/ SRG, which is formed by the mass transport induced by the photoismnerization of the azobenzene chromqrhore, rrtay find potential applications fw oasable and rewritable hologr hic memoy, polarization discriminators, and waveguide couplors. 

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

Comprehensive monographs are also available detailing the analysis of mass transfer though porous and dense membranes. Standard textbooks [e.g., Refs. 26, 27] provide the basis for discriminating between various possible transport mechanisms and the selection of models capable of describing the processes in quantitatively. 

We are often concerned with the dispersion of pollutants and other chemicals in the environment. Advection and mass flux are indiscriminate transport processes. In the water column of a lake, for example, these processes transport dissolved and particle-bound chemicals equally across the boundaries of the test volume. Settling of particles, in contrast, causes a downward flux of particle-bound chemicals while leaving dissolved chemicals in place. Similarly, surfactants or gases that join rising air bubbles are carried to the surface. These discriminate transport processes are very important in a variety of environmental situations ... 

The calculation principle on which the assessment of design for such reactors is based is a substitution of the multi-phase reaction system by a quasi-single-phase model. In two-phase systems both reactants have to get into contact at a certain place. Consequently a reaction and a transport phase are distinguished. If the mass transfer rate from the transport to the reaction phase is veiy fast compared to the actual reaction rate, the process in total is dominated by the reaction kinetics. In order to discriminate this situation from one taking the mass transfer into account, it is referred to as micro-kinetically dominated In this ease all formal kinetic laws presented for homogeneous systems may be applied directly. 

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