TABLE OF CONTENTS A tower comprised of rectifying (above the feed) and stripping (below the feed) sections is capable of making a more or less sharp separation between two products or pure components of the mixture, that is, between the light and heavy key components. The light key is the most volatile component whose concentration is to be controlled in the bottom product and the heavy key is the least volatile component whose concentration is to be controlled in the overhead product. Components of intermediate volatilities whose distribution between top and bottom products is not critical are called distributed keys. When more than two sharply separated products are needed, say n top and bottom products, the number of columns required will be n - 1.
In some cases it is desirable to withdraw sidestreams of intermediate compositions from a particular column. For instance, in petroleum fractionation, such streams may be mixtures of suitable boiling ranges or which can be made of suitable boiling range by stripping in small auxiliary columns. Other cases where intermediate streams may be withdrawn are those with minor but critical impurities that develop peak concentrations at these locations in the column because of inversion of volatility as a result of concentration gradient. Thus, pentyne-1 in the presence of n-pentane in an isoprene-rich C 5 cracked mixture exhibits this kind of behavior and can be drawn off as a relative concentrate at an intermediate point. In the rectification of...
