Overview

There are two ways to remove heat from a distillation tower: top reflux and circulating reflux. In this chapter, we call a circulating-reflux stream a pumparound.

The vast majority of fractionators have top reflux. Cold liquid from the reflux drum is pumped onto the top tray of the tower. The cold liquid flashes to a hotter vapor. For example, let s say 1500 lb/h of liquid butane at 100 F flashes to 1500 lb/h of vapor at 260 F.

The specific heat of butane is 0.6 Btu/[(lb)( F)]. The latent heat of butane is 130 Btu/lb ( latent heat means the heat needed to change a pound of liquid into a pound of vapor at the same temperature). The heat removed by the top reflux is

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15.1 The Pumparound

15.1.1 Pumparound Heat Removal

Figure 15.1 shows an alternate method, called circulating reflux or pumparound, to remove heat from a tower. Hot liquid, at 500 F, is drawn from tray 10, which is called the pumparound draw tray. The liquid pumparound is cooled to 400 F. The cooled liquid is returned to the tower at a higher elevation onto tray 9. It appears from Fig. 15.1 that the cold 400 F pumparound return liquid is entering the downcomer from tray 8. This is often good design practice. Tray 9 is called the pumparound return tray.

Figure 15.1: A pumparound or circulating reflux.

The purpose of the pumparound is to cool and partially condense the upflowing vapors. The vapors to...