Exhaustion capacity, activated carbon

Total adsorption capacity where Volume passed when C =. 75 C The exhaustion capacity of activated carbon for prophram in... 

In a recent review of technological alternatives for NOM removal, Jacangelo et al. [558] presented field data that illustrate wide variability in adsorption capacities of activated carbon in a single location (e.g., exhaustion periods between 41 and 182 days). They concluded that these results are evidence of the site-specific nature of [dissolved organic carbon] removal by [activated carbon] and that the concerns regarding reliability of treatment practices to meet the new [regulations] have a sound basis. Clearly, much fundamental work remains to be done to understand fully the complex nature of these adsorbent/adsorbate interactions and thus be able to optimize both the physical and the chemical accessibility of the carbon surface to natural organic matter. 

Most of the industrial processes of adsorption with activated carbon operate in adsorption columns where a continuous fluid stream crosses the column and an adsorbate is removed by the stationaiy fixed carbon bed. As the adsorption process proceeds, the adsorption capacity of the activated carbon diminishes due to the fact that the adsorbate molecules are filling the pores. Finally, when the adsorption capacity of the activated carbon is exhausted, the adsorbate concentration level at the outlet begins to rise until it reaches the inlet level (breakthrough plot), the carbon adsorbent becoming unsuitable for further use so that it must be replaced by fresh activated carbon. 

Activated carbon is basically hydrophobic it adsorbs preferably organic solvents. The water adsorption isotherm (Figure 22.1.14) reflects its hydrophobic character. Below a relative gas humidity of about 40% co-adsorption of water can be neglected in most applications. However, higher humidity of the waste gases may affect the adsorption capacity of the activated carbon. Figure 22.1.15 demonstrates, using toluene as an example, how the relative humidity of the exhaust air influences the activated carbon loads. 

Solvent-laden air is exhausted at the three rotogravure printing presses by several fans operating in parallel and is routed in an upward flow through four adsorbers packed with Supersorbon activated carbon. The solvent contained in the air is adsorbed on the activated carbon bed. Adsorption continues until breakthrough, when the full retentive capacity of the adsorbent for solvent vapors is used up. 

Apart from the usual adsorptive dryer station, additional adsorption steps can serve to remove trace components (e.g. sulphur compounds, alcohols, aldehydes, ketones, ester, other odorants, aromatics, etc.). All molsieves and all types of activated carbon are used as adsorbents, but also zinc and ferric oxides which, in contrast to the dryers, are usually not regenerated in situ but have to be replaced by fresh adsorbents after saturation. According to the quantity of the component(s) to be removed, adsorption occurs in single adsorbers or in series-connected twin-adsorbers. This series-connection (also called Lead/Lag configuration) is designed in such a way that the container with the unloaded adsorbent follows the active one during normal operation of the plant. When the adsorption capacity of the active adsorber is exhausted, it is taken out of operation and the vessel with the fresh adsorbent is turned into the active one. The loaded adsorbent can be replaced by a fresh one while the plant is kept in operation. 

Another interesting observation is almost identical behavior of samples exposed for SOj adsorption. Regardless the carbonization temperature the same amounts of SO, are adsorbed on the samples exhausted in the HjS breakthrough test. This suggests that after exhaustion of all active centers responsible for HjS adsorption the differences in surface chemistry, which play a role in sulfur dioxide adsorption/oxidation on fresh surfaces, seem to be somehow screened by HjS adsorption products. Nevertheless, it is interesting that still some capacity exists. 

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