The Columns Feed and its Location

In most cases, we will want to separate a feed of two components into a distillate which is largely the more volatile component and a bottoms which is mostly the less volatile component. To do so, we will normally use a distillation column fitted with both a condenser and a reboiler. Thus we commonly will be interested in more complex cases than in the previous section. There, a column was fed at the bottom with a vapor, or at the top with a liquid. 

In the more common case, we will feed the column somewhere in the middle. Such a column, which was shown schematically in Fig. 12.1-l(c), requires a more complex analysis than that for a column fed at the top or the bottom. All three key equations basic 

We begin with the case of a saturated Hquid feed. Such a feed does not change the vapor flow up the column it only alters the liquid flow down the column. In other words, G equals G and L equals L + F) where F is the feed per time per column cross-section. This means that the slope of the operating fine must change at the feed point, from LjG) to ((L + F)/G). 

However, other types of feed are obviously possible, as illustrated schematically in Fig. 12.3-2. For example, if the feed is a cold liquid that is below the saturation temperature at the composition on that stage, then the feed will necessarily condense some of the vapor rising up through the column. In this case, G will be greater than G. In the same sense, if the feed is a superheated vapor, then it will vaporize some of the liquid moving down the column, and L will be greater than L.  

To see how these new feeds alter the distillation, we define a new quantity q as equal to the moles of liquid produced per mole of feed  

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