PAC required = -^¡^(60)(60)(24) = 15,782kg/s Ans 8.3.5 Bed Adsorption and Active Zone
In bed adsorption, the water to be treated is passed through a bed of activated carbon, in which the form of carbon is normally GAC. The method of introduction of the influent may be made similar to sand filtration. In addition, the bed may be moving countercurrent or co-current to the flow of influent. Figure 8.8 shows a cutaway view of a carbon-bed adsorption unit. By a suitable modification of valve arrangements, this unit may be operated countercurrently or co-currently, in addition to being operated with the carbon bed stationary—the mode of operation depicted in the figure.
Figure 8.9a shows a schematic of an adsorption bed of depth L. Although the influent feed shown is introduced at the top, the following analysis applies to other modes of introduction as well, including moving beds. Hence, the figure should be viewed as a relative motion of the feed and the bed. The curved lines represent the configuration of the variation of the concentration of the adsorbate at various times as the adsorbate passes through the column. Thus, the curve labeled t1 is the configuration at time t1. The lower end of the curve is indicated by a zero concentration (or any concentration that represents the limit of removal) and the upper end is indicated by the influent concentration [C0]. Thus, the volume of bed above curve t1 is an
exhausted bed. Below the curve, the bed is clean (i.e., no adsorption is taking place, because all adsorbables had already been removed by the portion of the bed at curve ii and above). The zone of bed encompassed by the curve represents a bed partially exhausted.
The curve ii then advances to form the curve t2 at time t2. The curve shows a breakthrough of concentration of some fraction of [Co]. This breakthrough concenJ tration appears in the effluent. Finally t2 advances to t3 at time t3. At this time, the bed is almost exhausted. The profile concentration represented by curve ti is called an active zone. This zone keeps on advancing until the whole bed becomes totally exhausted.
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