commercial incinerators with carbon filters. Commercial data suggest that "capacity" (the amount of dioxin adsorbed at equilibrium) should not limit the "breakthrough" time. That is, the carbon in the PFS has the capacity to adsorb dioxins from the flue gas for many years before becoming saturated. The committee did not attempt to validate the kinetic analysis through an independent analysis.
For any separable component in the gas phase, there is a maximum amount that can be adsorbed on the solid. The relation between the adsorbed component and its concentration in the gas phase is called the adsorption isotherm. This equilibrium relation varies with temperature and with the other adsorbed components (see Appendices D and E for details on adsorption phenomena).
Two methods are commonly used for affecting contact between the effluent gas and the solid carbon adsorbent: (1) continuous relatively long-term flow through a packed bed of adsorbent, and (2) injection of a small amount of finely divided solid adsorbent into the flowing gas stream, with separation of the two after a few seconds. These two approaches can result in very different amounts of adsorption:
In packed-bed filters, most of the solid comes to equilibrium with the gas at the inlet concentration of the material being adsorbed (i.e., at its maximum gas concentration). This equilibrium can be represented by an isotherm of a general shape referred to as "favorable." Adsorption of many materials on carbon yields equilibrium relations with this general shape.
In dilute-phase (flowing) contact systems (e.g., carbon injection), the solid approaches equilibrium with the gas at the outlet concentration of the material being adsorbed (i.e., at its minimum gas concentration).
Packed-bed filters are capable of adsorbing more material from the gas phase than dilute-phase contact systems. Depending on the shape of the adsorption isotherm and other operating factors, the difference could be as much as 100-fold. A drawback to packed-bed filters is that they increase the system pressure drop, which results in higher power requirements. In practice, a combination of the two approaches is often used. MWCs (municipal waste combustors) typically disperse powdered activated carbon into the flue gas and separate it with a fabric filter. Thus, some separation occurs in the dilute phase, and more is adsorbed in the thin packed bed formed by the filter cake. The packed-bed design (i.e., a fixed bed of carbon) was chosen by the Army because, in addition to its higher adsorption capacity, it has two other advantages: (1) a fixed bed minimizes the amount of spent carbon adsorbent requiting disposal, and (2) a fixed bed is always present to capture accidental releases.
There are two basic problems in determining the adsorption isotherms for a fixed carbon bed (the PFS) with the Army's baseline incineration system:
The SOPCs in the flue gas—possibly unburned agent and chlorinated dioxins/furans—are present at extremely low concentrations, frequently lower than the detection levels of today's best sampling and analytic methods. Because equilibrium adsorption isotherms have not been measured to such low levels, they must be predicted from experimental measurements at much higher concentrations or from related industrial experience at higher concentrations.
Many materials in the flue gas (SOPCs, other organics, and some vapor-phase metals) compete for adsorption sites on the activated carbon. Predictions for these interactions are not well established in theory.
Several methods have been described in the literature for determining the performance of adsorption systems. Mitretek, an Army contractor, has chosen an approach developed by Dubinin and coworkers (the Dubinin-Radushkevich [D-R] relation), a summary of which appears in Ruthven (1984) (Mitretek Systems, 1997). This approach has a reasonable theoretical justification for nonpolar materials, for which the energy of adsorption is due primarily to Van der Waals forces,