and reflux later. For this calculation the following rules of thumb for the ratio of
moles overflow to moles product may be used:
Overflow from the HGO drawoff 2.9
Overflow from the LGO drawoff 1.2
Overflow from the Kero drawoff 0.9–1.0
Step 5. Calculate the partial pressure on the drawoff tray as follows:
Partial pressure ¼
total draw À off product vapor moles þ total overflow moles
ð
Þ Â total pressure
total hydrocarbon vapor moles þ steam passing through tray
ð
Þ
(6)
Step 6. Using the vapor pressure curves (given in Appendix 1 of this chapter), relate
the IBP of the cut at atmospheric pressure (step 3) to the calculated partial
pressure.
Step 7. The temperature arrived at in Step 6 is the theoretical temperature of a liquid
in equilibrium with its associated vapor phase. In actual practice, because a
vapor phase has been continually bubbling through the liquid on the tray, that
12
10
8
Gas oil
Naphtha
Kero
Reduced crude
6
4
2
0
0
0.4
0.8
1.2
Pound steam per gal product
Percentage (vol.) removed by stripping
1.6
2.0
2.4
Fig. 10 The amount of stripping steam
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
145