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(Note: NA isalsothenumberofbedsinanintermittentsystemthatwouldbeadsorbingat any given time. The total number of beds in the system would be NA + ND) A Itcanbeshownthatthenumberofdesorbingbedsrequiredinacontinuoussystem(ND) is D related to the desorption time (D), adsorption time (), and the number of adsorbing beds, as follows: N D A D (1.12) NA D is the total time needed for bed regeneration, drying, and cooling. For instance, for an eight-hour adsorption time, in a continuously operated system of seven beds (six adsorbing, one desorbing), D would be equal to or less than 8 hours/6 beds or one hour and twenty minutes. If desorption takes longer than one hour and twenty minutes, then additional beds would be needed to provide sufficient extra capacity during desorption. 1.6.3 Estimating Carbon Requirement 1.6.3.1 Overview of Carbon Estimation Procedures Obtaining the carbon requirement (Mc or McI) is not as straightforward as determining the other adsorber design parameters. When estimating the carbon charge, the sophistication of the approach used depends on the data and tools available. For this Manual, we have adopted a rule-of-thumb procedure for estimating the carbon requirement. This procedure, while approximate in nature, appears to have the acceptance of vendors and field personnel. It is sometimes employed by adsorber vendors to make rough estimates of carbon requirement and is relatively simple and easy to use. It normally yields results incorporating a safety margin, the size of which depends on the bed depth (short beds would have less of a safety margin than deep beds), the effectiveness of regeneration, the particular adsorbate and the presence or absence of impurities in the stream being treated. 1.6.3.2 Carbon Estimation Procedure The rule-of-thumb carbon estimation procedure is based on the “working capacity” (wc, lb VOC/lb carbon). This is the difference per unit mass of carbon between the amount of VOC on the carbon at the end of the adsorption cycle and the amount remaining on the carbon at the end of the desorption cycle. It should not be confused with the “equilibrium capacity” (we,) defined above in Section 1.5. Recall that the equilibrium capacity measures the capacity of virgin activated carbon when the VOC has been in contact with it (at a constant temperature and partial pressure) long enough to reach equilibrium. In adsorber design, it would not be feasible to allow the bed to reach equilibrium. If it were, the outlet concentration would rapidly increase beyond the allowable outlet (or “breakthrough”) concentration until the outlet concentration reached the inlet concentration. During this period the adsorber would be violating the emission limit. With non-regenerable (canister) type systems, placing multiple vessels in a series can substantially decrease concerns of breakthrough. The working capacity is some fraction of the equilibrium capacity. Like the equilibrium adsorptivity, the working capacity depends upon the temperature, the VOC partial pressure, and 1-18PDF Image | Carbon Adsorbers
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