Pumparounds illustration

Figure 3-6 illustrates how one combination tower was retrofitted to increase the pumparound draw temperature by 45°F. The heavy coker gasoil feed to the steam stripper was modified to be withdrawn from the tower in two streams. The 3,100 B/D of relatively light, cool gas oil was trapped out and drawn off the tower before it could fall into the pumparound section. Computer calculations predicted that segregating this lighter gas-oil cut from the pumparound section would increase both the pumparound draw and return temperatures, with only a 3% reduction in pumparound heat duty. 

A complete Type A Tower is shown in Figure 2.25. This drawing illustrates the basic process and its essential auxiliaries as well as the external heat and material balance quantities. The two pumparound heat removal systems which are shown are for a typical installation and would not necessarily be located in these particular sections of the tower, nor would every tower employ two systems. 

The following discussion assumes a pumparound system between the first and second sidestream product draw trays. The heat and material balance relationships at this section of the tower are determined by making two balances which are shown as Envelopes II and III on Figure 2.25. An expanded view of this section is illustrated by Figure 2.26 which also gives the equations which are to be used in making the calculations. The sequence of calculations is discussed as follows. 

Normally, there will be no pumparound system in the section between the highest sidestream product draw tray and the top tray. Thus, it is calculated in the normal manner for a top tray as illustrated by Figure 2.21. 

For the purposes of the illustrative design procedure, the pumparound heat removals were assumed on the following bases. 

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