Super fractionation stage

The concept of a super-fractionation tray is important when separating products, if either the top or bottom product consists of a multi-component mixture. For example, if we are fractionating only a mixture of propane and isobutane, the super-fractionation concept would not be significant. However, if the overhead product consisted of a mixture of ethane, propane, and isobutane (i.e., C2S, C3S, and C4S) and the overhead condenser was a partial condenser (see Fig. 49.1), then the partial condenser would serve as a super-fractionation stage. The partial condenser would contribute far more to fractionation than any of the other stages in the upper portion of the distillation tower. 

Figure 49.1 A partial condenser serves as a super-fractionation stage.

Any time heat is removed from a fractionation stage above a tower s feed point, that stage becomes a super-fractionation stage. To prove my point, note the temperatures shown in Fig. 49.4 ... 

Overflowing reflux from the hot drum into the cold drum destroyed the efficiency of the super-fractionation stage represented by the partial condenser. To restore this lost fractionation, I had the panel board operator lower the reflux drum level to 60 percent by increasing the tower top reflux rate. This time I waited a few hours before pulling my naphtha product sample. Analysis then showed there was now only 7 percent jet fuel in the naphtha product. Finally, real victory ... 

---