Rapid Adsorption-Desorption Cycles

Rapid Adsorption-Desorption Cycles For rapid cycles with particle diffusion controlling, when the cycle time is much smaller than the time constant for intraparticle transport, the LDF approximation becomes inaccurate. The generalized expression... 

For the rapid adsorption-desorption cycles such as those encountered in PSA, application of the above relation becomes dubious since, when the cycle time is much smaller than the time constant of diffusion inside the particle, change of concentration distribution during the adsorption or desorption process is very complicated. Nakao and Suzuki (1983) compared solutions of the LDF model and the rigorous partial differential equation for the case of cyclic adsorption and desorption in a spherical adsorbent particle and found that the proportionality constant, K, defined by Eq. (-11-54) becomes a function of cycle time factor defined by Eq. (11-SS). 

As soon as solvent is injected, 5h drops sharply due to desorption of the polymer. The rate of this decrease is initially rapid, as can be seen, but soon takes a much lower pace. Upon reinjection of polymer, the initially measured layer thickness was rapidly restored.The first point to note is that in contrast to F, 5h is indeed remarkably sensitive to exposure to pure solvent very small desorbed amounts give rise to considerable decreases in layer thickness. Another interesting aspect is that the experiment is entirely reversible adsorption/desorption cycles can be repeated many times without observable differences. 

In order to solve the humidity problem of most gas sensors the ICB Muenster developed and instrument called Air-Cbeck in which a short but efficient pre-sampling on Tenax material takes place. With this the sensitivity could be increased as needed, water no longer interferes and the selectivity could also be increased by analyzing the time course of the rapid thermal de.sorption step. The whole adsorption-desorption cycle can be performed automatically within less than 1 min ... 

The selection of adsorbents is critical for determining the overall separation performance of the above-described PSA processes for hydrogen purification. The separation of the impurities from hydrogen by the adsorbents used in these processes is generally based on their thermodynamic selectivities of adsorption over H2. Thus, the multicomponent adsorption equilibrium capacities and selectivities, the multi-component isosteric heats of adsorption, and the multicomponent equilibrium-controlled desorption characteristics of the feed gas impurities under the conditions of operation of the ad(de)sorption steps of the PSA processes are the key properties for the selection of the adsorbents. The adsorbents are generally chosen to have fast kinetics of adsorption. Nonetheless, the impact of improved mass transfer coefficients for adsorption cannot be ignored, especially for rapid PSA (RPSA) cycles. 

Among the bulk transition metals of group VlllB (Fe to Pt), only Ni reacts very rapidly with CO (in fact, the reaction is used in the preparation of ultrapure Ni metal). At the other extreme, no free, pure carbonyl derivatives of zerovalent Pd or Pt have been reported (but sec Section IV.B), although it is thought that they are stable (99). Cycles of adsorption at RT and desorption at 673 K of CO on very small Pd particles lead to slight disproportionation, 2CO CO2 + C 100). On Ru and Rh particles some disproportionati on occurs even upon adsorption at RT. In some cases the 002(g) is detected by NMR. 

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