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Significance and Use
Activated carbon is used extensively for removing gases and vapors from air or other gas streams. The physical and chemical characteristics of an activated carbon can strongly influence its suitability for a given application. The procedure in this guide allows the evaluation of the dynamic adsorption characteristics of an activated carbon for a particular adsorbate under conditions chosen by the user. It is necessary that the user choose test conditions that are meaningful for the application (see Section 9).
This guide can also be used to evaluate activated carbons that have been impregnated with materials to enhance their effectiveness at removing gases otherwise poorly adsorbed on activated carbon.
The procedure given in this guide is not generally applicable for evaluation of carbons used as catalysts for such purposes as decomposition of low levels of ozone or oxidation of SO2 to SO 3.
The procedure given in this guide can be applied to reactivated or regenerated activated carbons.
Fig. 1 shows the adsorbate concentration profile in an activated carbon bed at breakthrough. The bed has a zone at the inlet in which the adsorbate concentration is equal to the influent concentration. In this region the carbon is at equilibrium with adsorbate. The adsorbate concentration in the remainder of the bed drops until at the outlet it is equal to the breakthrough concentration. The shorter the length of this mass transfer zone (adsorption zone), the more effectively the carbon in the bed is utilized. A bed whose depth is less than the length of this zone will show immediate appearance of adsorbate in the effluent (breakpoint).
From the standpoint of best carbon utilization it is desirable to choose a carbon which will give as short a mass transfer zone as possible under use conditions. However, in many applications, high adsorptive capacity is more important than a short mass transfer zone. In almost every application, bed pressure drop is also a primary consideration.
In a few situations such as respiratory protection against low levels of extremely toxic gases such as radioactive methyl iodide, a short mass transfer zone (that is, high adsorption rate coefficient) is more important than ultimate capacity. In other cases such as solvent recovery, a high dynamic capacity is more important.
Although the design of adsorber beds is beyond the scope of this guide, the following points should be considered. The bed diameter should be as large as possible in order to lower the pressure drop and to maximize the amount of carbon in the bed. Subject to pressure drop constraints, the deepest possible carbon bed should be used. All else being equal, the use of smaller particle size carbon will shorten the mass transfer zone and improve bed efficiency at the expense of higher pressure drop. If pressure drop considerations are critical, some particle morphologies offer less resistance to flow than others.
The two parameters obtained by the procedure in this guide can be used as an aid in selecting an activated carbon and in sizing the adsorption bed in which this carbon will be used. The best carbon for most applications should have a high dynamic capacity for the adsorbate No coupled with a short mass transfer zone (small dc) when evaluated under the operating conditions anticipated for the adsorber.
FIG. 1 Concentration Profile of an Activated Carbon Bed at Breakthrough
1.1 This guide covers the evaluation of activated carbons for gas-phase adsorption. It presents a procedure for determining the dynamic adsorption capacity, No, and critical bed depth, d c, for an activated carbon used to remove a specific adsorbate from a gas stream under conditions chosen by the user.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific hazards statements are given in Section 8.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D2652 Terminology Relating to Activated Carbon
D2854 Test Method for Apparent Density of Activated Carbon
D2867 Test Methods for Moisture in Activated Carbon
D3467 Test Method for Carbon Tetrachloride Activity of Activated Carbon
E300 Practice for Sampling Industrial Chemicals
UNSPSC Code 11101522(Activated carbon)