Grain adsorption

The approach of Prasad and Huntress (1980a, b) might be called the chemical time dependent approach because it utilizes fixed physical conditions. However, the chemical evolution to steady-state takes so long that it may be unwise to maintain fixed physical conditions. Time scales more rapid that the 107 years needed to reach chemical steady-state include the grain adsorption time and the free-fall collapse time. Thus, in the simplest approximation, chemical steady-state can never be reached because the gas phase will be adsorbed into the grains and the cloud will have collapsed to form, presumably, a star. The observed facts that clouds do not form stars as rapidly as the free fall model and that they do possess a gas phase, both demonstrate that... 

Fig. 4.12 (a) Adsorption isotherm for (i) a solid made up of mesoporous grains (ii) a solid which is wholly microporous (iii) a granular solid with the same mesoporous system as in (i) but containing also micropores having a total micropore volume given by the plateau of isotherm (ii). The adsorption is expressed in arbitrary units, (h) t-plots corresponding to isotherms (i) and (iii). The a,-plots are similar except for the scale of... 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

P29 Adsorption - Other grains/cubic foot of air, multiply by 1766.6 to convert to parts ... 

Ceilings, P. J. and de Jongh, M. A., Grain Boundary Oxidation and the Chromium-depletion Theory of Intercrystalline Corrosion of Austenitic Stainless Steels , Corros. Sc/., 7,413 (1967) Armijo, J. S., Impurity Adsorption and Intergranular Corrosion of Austenitic Stainless Steel in Boiling HNOj-KjCrjO, Solutions , Corros. Sci., 7, 143 (1967)... 

SEM micrographs (Figure 4) show the deposition on the a-Al203 grains of small crystallites with the typical hexagonal shape of silicalite. The pore size distribution, as deduced from N2 adsorption, presents a very narrow peak centred on 0.5 nm, also in good agreement with the pore diameter of silicalite-type zeolites. 

This type of functional, which we refer to as coarse-grained, can be used to calculate both surface tension and adsorption isotherms to quite good accuracy for many fluids and interfaces. It can also be used for screening problems in the theory of electrolytes. 

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