Retrofitted combination tower to save energy
The hot coke drum vapors are a valuable source of high-level heat. This energy, which is partially recovered in the combination tower heavy gas-oil pumparound section, can be used for generating steam, preheating coker feed, or reboiling a gas plant. The higher the temperature level at which this heat is made available for recovery, the greater is its potential value. This is the second law of thermodynamics in action.
Figure 3-6 illustrates how one combination tower was retrofitted to increase the pumparound draw temperature by 45CF. The heavy coker gas-oil feed to the steam stripper was modified to be withdrawn from the tower in two streams.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.
After the retrofit, opening the drawoff line below tray 10 (refer to Fig. 3-6) increased the pumparound draw temperature from 575°F to 620°F without any noticeable effect on duty.
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