Measurement transport resistances

In this expression Ac describes the difference between the concentration of water vapor at the water surface and that at the desiccant surface Ac = cw - cdes. Since cdes < cw, Ac may be replaced by cw, which in turn equals pM/RT, where p and M are the vapor pressure and molecular weight, respectively, of water. With this substitution the expression for dQldt becomes A(DIAx)(pMIRT). The ratio DlAx has units length time 1, so we identify it as the reciprocal of the transport resistance. Note that r increases as the effective value of Ax increases and the effective value of D decreases. Thus l/r is the only unknown in the expression dQldt = A(Mr) pMIRT) and can be calculated from the measured rate of weight increase with different monolayers present. ... 

Subdivision into a resistance measuring the resistance to charge transfer, Rct, and an impedance that measures the difficulty of mass transport of the electroactive species, called the Warburg impedance, Zw ... 

In spite of the bulk electrolyte resistance cancelling out in Equation 6.68, small differences between the series resistance in the measurements of ZDMPC and Zanode also contribute to cathode impedance. Considering the mass transport resistance of 02 in the pores and the bulk electrolyte resistance, the cathode impedance is given by... 

The transport properties of such disordered materials (see Section II) are difficult to study, for several reasons. One is that the microscopic theory of transport is not clear even for perfectly ordered CPs, as discussed in the reviews mentioned above. Another is that a dc or low-frequency conductivity measurement on an inhomogeneous material can be viewed as measuring several resistances in series, the larger playing the major role. For instance, in a fibrillar material interfibril transport is important, in a mixed crystalline-amorphous medium the amorphous regions may limit... 

This case study clearly illustrates the usefulness of the ZLD-TEOM technique in determining intracrystalline diffusivities in zeolites, provided that effects of other transport resistances such as the surface barrier are eliminated by varying the crystal size of the zeolites. The measured steady-state diffusivity can be directly used for predicting effects of diffusion in reactions catalyzed by zeolites. More important, the TEOM makes it possible to distinguish the deactivation caused by blockage of the active sites and by increased diffusion resistance caused by blockage of cavities or channels by coke. 

If the calculated value of is equal to the measured intracrystalline lifetime, Tinira, the rate of molecular exchange between different crystals is controlled by the intracrystalline self-diffusion as the rate-limiting process. Any increase of Timn, in comparison with Tf,j L indicates the existence of transport resistances different from intracrystalline mass transport. Under the conditions of TD NMR one has A r. > Antra, thus these resistances can only be brought about by sur ce barriers. The ratio Timra/Tfn L represents, therefore, a direct measure of the influence of surface barriers on molecular transport. 

Analyzing the self-diffusion behavior of guest molecules in a microporous catalyst by the combined application of pulsed-field gradient NMR selfdiffusion techniques reveals the spatial distribution of transport resistances over the catalyst particles. In the case of coke deposits on ZSM-5, the distribution of carbonaceous residues over the crystal was found to be a function of the crystal morphology, the time onstream, and the chemical nature of the coke-producing reactant. In the case of ZSM-5 modified by H3PO4, the spatial distribution of the P compounds over the ZSM-5 crystals can be determined by self-diffusion measurements. Location of transport hindrances in a zeolite framework is based on self-diffusion measurements, in... 

The resultant values of A and E and the nature of/(C) establish the desired equation for the intrinsic rate. However, for step 3 we must know or estimate the effective diffusivity. Alternately, the global rate may be measured for small catalyst particles for which -> 1.0. Now internal (and usually external) transport resistances are negligible the bulk temperature and concentrations may be taken as equal to those at a catalyst site, and the observed rate and bulk temperature and concentration data can thus be used directly to obtain /(Q and A and E. 

In addition to the conventional application of nmr pulsed field gradient experiments to self-diffusion studies, it is also possible to determine the intracrystalline molecular life times. Referring to the corresponding classical experiment, this method has been termed nmr tracer desorption technique (7). Together with the self-diffusion measurements it provides an excellent tool for characterizing the transport properties in the intra- and intercrystalline spaces, as well as at the interface between them. So far, the nmr techniques provide the only possibility for a direct determination of the existence and of the intensity of transport resistances at this... 

Analysis of data obtained from the RDC usually involves plotting the reciprocal of the measured flux, y-1, against W l/2, and extrapolating to infinite rotation speed (W 112 —> 0), such that the diffusion layer thicknesses tend to zero and diffusion in the solution on either side of the membrane is discounted. The intercept on the j l axis then gives a measure of the flux, governed by the interfacial kinetics and the transport resistance... 

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