atmosphere. Meanwhile, a third vessel is being regenerated Steam passes through it, removing the adsorbed vocs from the Cleaned air with omly a adsorbent. The mixture of steam and vOCs coming from the top of water-cooled condenser that condenses both the VOCs and the steam. Both pass in liquid form to separator, where the VOCs, rmal dense than water and have little solubility in water, float on top and are decanted and sent to solvent recovery. COntiner a o I% hydrocarbon solvent Pig. 10-2 The typical arrangement automatic switching valves; and steam FIGURE 10.10 typical arrangement for adsorpoon of a voC from a gas stream, using three adsorbent beds automate gra separation After a suitable time period a set of automatically programmed valves changes the position of the containers in the flow sheet. (The containers do not move, their place in the piping arrangement changes )Container 1, which is most heavily loaded, goes to the regeneration position. Container 2, which is lightly loaded with VOCs, goes to the position where container I was; and container 3, which is now regenerated and very clean, goes to the position previously held by container 2, making the final cleanup on the air stream. Fig 10.2 shows the steam condensate leaving the phase separator, without specifying where it goes. As discussed above, this condensate will be saturated with dissolved VOC. The VOC concentration may be high enough to prevent its being sent back to the steam boiler, or for it to be discharged to a sewer. If there is no good way to deal with this stream, then the absorber solves a large air pollution problem but creates a small water pollution problem! Adsorbents. The most widely used adsorbent for VOCs is activated carbon. This somewhat fancier version of the charcoal used for barbecuing has an amazing amount of surface area In Example 7 15 we showed that catalyst supports typically have surface areas of 100 m-g, corresponding to internal wall thicknesses of 100 A. Adsorbents like activated carbon often have surface areas of 1000 m-g, corresponding to an internal wall thickness of 10 A,. This value is startlingly low, about four times the interatomic spacing in crystals! If adsorbents have this much surface area, then they must have internal walls only four atoms thick! Apparently they do. To make materials with this much surface area, one starts with a material, from which part can be removed on an atomic scale. In the case of activated carbon, one starts with wood (or peach pits, or coconut shells, or some other woody material) and heats it to a high enough temperature that the wood decomposes(pyrolyzes), producing a gas and leaving behind a solid carbon residue, in the form of these thin internal walls 10.3.2 Absorption(Scrubbing If we can find a liquid solvent in which the vOC is soluble and in which the remainder of the contaminated gas stream is insoluble, then we can use absorption to remove and concentrate the voC for recovery and re-use, or destruction. The standard chemical engineering method of removing any component from a gas stream--absorption and stripping--is sketched in Fig. 10.3. If we can find a liquid solvent in which the gaseous component we wish to selectively remove is much more soluble than are the other components in the gas stream, the procedure is quite traightforward. The feed gas enters the absorber, which is a vertical column in which the gas passes upward and the liquid solvent passes downward. Normally, bubble caps, sieve trays, or packing is used in the interior of the column to promote good countercurrent contact between the solvent and the gas. The stripped solvent enters the top of the column and flows countercurrent to the gas. By the time the gas has reached the top of the column, most of the component we wish to remove has been dissolved into the solvent; the cleaned gas passes on to the atmosphere or to its10－4 atmosphere. Meanwhile, a third vessel is being regenerated. Steam passes through it, removing the adsorbed VOCs from the adsorbent. The mixture of steam and VOCs coming from the top of the vessel passes to a water-cooled condenser that condenses both the VOCs and the steam. Both pass in liquid form to a separator, where the VOCs, which are normally much less dense than water and have little solubility in water, float on top and are decanted and sent to solvent recovery. After a suitable time period a set of automatically programmed valves changes the position of the containers in the flow sheet. (The containers do not move; their place in the piping arrangement changes.) Container 1, which is most heavily loaded, goes to the regeneration position. Container 2, which is lightly loaded with VOCs, goes to the position where container I was; and container 3, which is now regenerated and very clean, goes to the position previously held by container 2, making the final cleanup on the air stream. Fig 10.2 shows the steam condensate leaving the zphase separator, without specifying where it goes. As discussed above, this condensate will be saturated with dissolved VOC. The VOC concentration may be high enough to prevent its being sent back to the steam boiler, or for it to be discharged to a sewer. If there is no good way to deal with this stream, then the absorber solves a large air pollution problem but creates a small water pollution problem! Adsorbents. The most widely used adsorbent for VOCs is activated carbon. This somewhat fancier version of the charcoal used for barbecuing has an amazing amount of surface area. In Example 7.15 we showed that catalyst supports typically have surface areas of 100 m2 /g, corresponding to internal wall thicknesses of 100 A. Adsorbents like activated carbon often have surface areas of 1000 m2 /g, corresponding to an internal wall thickness of 10 A,. This value is startlingly low, about four times the interatomic spacing in crystals! If adsorbents have this much surface area, then they must have internal walls only four atoms thick! Apparently they do. To make materials with this much surface area, one starts with a material, from which part can be removed on an atomic scale. In the case of activated carbon, one starts with wood (or peach pits, or coconut shells, or some other woody material) and heats it to a high enough temperature that the wood decomposes (pyrolyzes), producing a gas and leaving behind a solid carbon residue, in the form of these thin internal walls. 10.3.2 Absorption (Scrubbing) If we can find a liquid solvent in which the VOC is soluble and in which the remainder of the contaminated gas stream is insoluble, then we can use absorption to remove and concentrate the VOC for recovery and re-use, or destruction. The standard chemical engineering method of removing any component from a gas stream--absorption and stripping--is sketched in Fig. 10.3. If we can find a liquid solvent in which the gaseous component we wish to selectively remove is much more soluble than are the other components in the gas stream, the procedure is quite straightforward. The feed gas enters the absorber, which is a vertical column in which the gas passes upward and the liquid solvent passes downward. Normally, bubble caps, sieve trays, or packing is used in the interior of the column to promote good countercurrent contact between the solvent and the gas. The stripped solvent enters the top of the column and flows countercurrent to the gas. By the time the gas has reached the top of the column, most of the component we wish to remove has been dissolved into the solvent; the cleaned gas passes on to the atmosphere or to its Fig. 10-2 The typical arrangement for adsorption of a VOC from a gas stream, using three adsorbed beds; automatic switching valves; and steam desorption, condensation, and gravity separation