Cze

Unused zone (UZ)

Unused zone (UZ)

b) Profile of relative concentration of a pollutant through the bed.

b) Profile of relative concentration of a pollutant through the bed.

Bed length

FIG. 5.20.14 Adsorber operation.

Bed length

FIG. 5.20.14 Adsorber operation.

design engineer (kg/m3) to meet the environmental standards. The value of the integral on the right side of Equation 5.20(18) is undefined for limits of 0 and 1. However, taking limits close to these values (e.g., 0.01 and 0.99) defines the integral and solves the equation. The breakthrough time tp is then as follows:

These design equations estimate operating time before breakthrough occurs. The accuracy of the calculations depends on the estimation of K, which is determined by the combination effect of diffusion external to the adsorbent, in the pores and on the pore surface, as well as axial dispersion. Predictions of K values from existing mass-transfer correlations are available but often are unreliable. Thus, adsorber designs are generally based on laboratory data. The scale-up then involves using the same adsorbent size and superficial gas velocity.

For longer beds, the column height of the adsorption zone is a small fraction of the bed length. A longer bed leads to better utilization of the adsorbent. The longer bed also results in a lower degree of backmixing (i.e., higher mass-transfer coefficient). However, proper design is required to avoid an excessive pressure drop. The energy consumption for a blower to overcome a pressure drop is a significant part of the overall operating cost. In the absence of experimental data, the Ergun equation or

Vapor Laden Air

FIG. 5.20.15 Canister adsorption system used to remove trace contaminants from air. (Reprinted, with permission, from Balston, Inc.)

equations provided by adsorbent suppliers (e.g., UOP) can provide an estimate. The Ergun equation is as follows:

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