ventilation air filtration system at Tooele has six beds of charcoal.) The space in between beds allows for gas flow redistribution and gas sampling equipment, to permit analysis of the gas as it flows through the beds. (Gas analysis following the second, fourth, and final beds is typical.) Provisions to provide bed bypass or diversion of gas to alternate beds to allow bed replacement without overall system shutdown are typical in engineered systems.
The results of these design considerations are that a typical charcoal filter system for a chemical agent and munitions processing plant the size of Tooele might be 20 feet by 20 feet square (or equivalent in smaller units) by 50 feet long, containing on the order of 30 tons of charcoal.
Contaminants are quickly adsorbed in the upstream portion of the first bed, and work their way through it and subsequent beds as illustrated in Figure C-1.
Initially, the carbon becomes saturated in the upstream part of the first bed but it drops to an extremely low-level at further distances from the leading face, so that contaminants in the gas leaving the first bed are below the level of detection. At a later time, the drop-off occurs further along in subsequent beds. If the concentration was monitored downstream of the second bed, ''breakthrough'' of contaminant when detected there would indicate that it was time to replace or regenerate the carbon in the preceding beds.
The speed at which breakthrough travels through the bed increases as gas velocity and temperature are increased, and is faster for less strongly adsorbed contaminants.
The velocity of the breakthrough wave and the depth of the bed determine time to breakthrough. For low concentrations of high molecular weight contaminants, such as agents and dioxins, this time can be designed to be on the order of several months. This duration easily allows adequate time for chemical analyses between the beds to certify contaminant removal and to determine breakthrough upstream of the final bed with adequate time for bed replacement or other remedial action. The system therefore approaches the desirable features of "closed-loop" systems in that contaminants are stored on the charcoal until removed for regeneration or disposal. A charcoal bed followed by gas storage has also been suggested. The committee believes that the charcoal bed, storing the contaminants, eliminates the need for gas storage.