Organic vapour, adsorption

Fig22. Multi-component organic vapour adsorption rollup effect... 

Fig. 4.1 Adsorption isotherms of some organic vapours on ammonium phosphomolybdate outgassed at 180°C. The isotherm temperatures (reading downwards) were 25°C, 25°C, 25°C, 0°C.

Activated charcoal or carbon is widely used for vapour adsorption and in the removal of organic solutes from water. These materials are used in industrial processes to purify drinking water and swimming pool water, to de-colorize sugar solutions as well as other foods, and to... 

Very recently, experiments using new techniques have been performed by Lodewyckx et al. [4], X-ray microtomography coupled with image analysis allows visualising dynamic adsorption of organic vapour and water vapour on activated carbon. Figure 17.3 in [4] shows profiles inside the bed at different times. It is remarkable that the fronts seem to be of constant pattern shape. 

The separation of micropore and mesopores plus the outer surface area was done by the combination of elution and flash thermodesorption [9]. With the latter it is possible to determine the contribution of the micropores to sorption separately as micropore desorption requires a higher activation energy because of the above mentioned effects. After injection of an organic vapour or a selection of a particular concentration of the gas mixture, adsorption took place on the sample in the column. The following carrier gas eluted the adsorbate and the peak was recorded. 

Figure 6.3. Fractal plots for the adsorption of organic vapours on the silica gel and a controlled porous glass (after Faiin et al., 1985 and Christensen and Topsoe, 1987) part of Fig. 1, Farin and Avnir, 1989.

The applicability of the Saam-Cole theory has been tested by Findenegg and his co-workers (1993,1994). Their adsorption measurements of certain organic vapours on carefully selected grades of controlled-pore glass provide semi-quantitative confirmation of the theoretical treatment adopted so far. However, it is evident that some refinement is required in the assessment of 0(rp) for materials of small pore size and that the experimental choice of the mesoporous adsorbent is important To make further progress it will be necessary to study adsorbents having narrow size and shape distributions of easily accessible mesopores. 

With the two aromatic hydrocarbons, Barrer and Kelsey (1961) found that the dm spacing increased steadily with the increase in p p°, but with the alkanes there was very little change. In the former case, it appeared that most of the uptake was in the interlamellar region. As indicated by the shape of the Type Hb isotherms, the sorption of the other organic vapours probably included an appreciable amount of multilayer adsorption on, and between, the clay platelets (cf. Figure 11.5). 

The more recent adsorption-desorption isotherms of some organic vapours of different molecular diameter are shown in Figure 12.15. These measurements were made on different samples of VPI-5, each having been outgassed for 16 hours at 673 K. Although essentially of Type I, the isotherms reveal some degree of thermodynamic irreversibility with hysteresis extending back to very low p/p°. Evidently,... 

The results presented show that the a method can be applied to adsorption isotherms of organic vapours. However, it is important to obtain reliable measurements at higher pressures, in the region 0.6-0.95p and, as the necessary precision is close to the limits of our current adsorption equipment, this implies that in future work of this nature the experimental procedures used need to be carefiilly considered. 

Hence, for example, Dollimore etal have considered thermodynamic aspects of the adsorption of organic vapours on graphites and carbon blacks. Heats of adsorption and entropies of adsorbed vapours were determined, and the authors came to the conclusion that mobile adsorption appeared to be very important in the systems. In some ways the observation that C2-C4 hydrocarbons were adsorbed flat on a graphite surface tends to support this conclusion, although Hoory and Praunitz prefer to explain their results in terms of double bond interaction with the graphite.Jonas et al. find... 

GC methods have been used to measure the adsorption of non-swelling vapours on the surface of dry (. -8.) and moist (9.-12.) cellulose. Adsorption isotherms, surface areas, isosteric heats and entropies of adsorption have been measured for a range of hydrocarbons and organic vapours. Perhaps the most useful aspect of the method is that the effect of relative humidity on surface properties may be investigated. The validity of the method as applied to surface area measurements is described in some detail below. Other applications of GC to cellulose surfaces are then summarized briefly. 

Measurements of adsorption isotherms of organic vapours on clean glass surfaces have been performed by Bottomley and co-workers. They find that, in general, monolayer coverage (based on liquid densities) is reached at approximately 75 per cent of the saturation pressure. For typical pK-apparatus they calculate that adsorption can produce errors in B as large as 30 to 100 cm mol when the adsorption occurs on a clean surface. Minute traces of grease... 

Adsorption can be enhanced in vapour mixtures. Pavlychenko reports that the adsorption of mixtures of acetone and chloroform on graphitized carbon black is substantially greater than that of either of the pure components. Absorption of organic vapours can also be significant in elastomers. O-rings... 

Adams LB, Boucher EA and Everett DH (1970), Adsorption of organic vapours by Saran-carbon fibers and powders. Carbon, 8(6), jp. 761-772. 

Forbert, R., and Schlunder, E.U., A model for the prediction of the adsorption equilibria of arbitrary organic vapours on active carbon, Chem. Eng. Process., 34(1), 49-59 (1995). 

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