Calculate theoretical initial boiling points
The FRL for each cut is developed from the TBP cut. Thus,
HGO 50 % TBP is 648
F. Slope of TBP is 0.6
F/%vol
Slope of FRL is 0.2
F/%. Δt50 (DRL – FRL) is 7
F
50 % FRL is 641
F. From FRL curve IBP is 626
F
LGO
This is developed in the same manner as HGO, and the IBP of the FRL in this case is
527
F
Kero
In the same way, the Kero FRL has an IBP of 395
F
Now all these temperatures are at atmospheric pressure. It is now required to
relate these temperatures to the partial pressure of the respective trays.
To calculate the approximate partial pressures and drawoff temperatures
Assumptions:
1. All vapor lighter than the product cut is considered inert.
2. Internal reflux is assumed as follows:
To HGO tray 290 mol/h
To LGO tray 250 mol/h
To Kero tray 200 mol/h
HGO tray partial pressure ¼
Moles of HGO
Total moles HC vapor þ Steam
 Total pressure
¼
1, 614:9
3, 224:9
 38:1 ¼ 19:6 psia
(11)
From vapor pressure curves, theoretical drawoff temperature is 720
F.
From Fig. 9:
Theoretical temperature–Actual temperature = 93
F.
Actual drawoff temperature is 720–93 = 627
F.
Drawoff temperatures for the LGO and Kero are calculated in the same way and
are:
LGO = 493
F.
Kero = 364
F.
To Calculate the Tower Top Temperature
Set the reflux drum temperature and pressure. In this case these will be set at 10 psig
and 100
F. Taking the pressure drop across the exchangers and piping the tower
top pressure will be 15 psig.
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
169