The vapor is usually pumped to the down distillation column located below flash zone (i.e. 
below the reign where the crude oil enters). The aim of pumping the vapor is
1- Remove the remaining gas oil from reduced crude (un-vaporized) 
2- Reduce the partial pressure leading to boiling the compounds at low temp.
The water vapor or the steam is pumped due to the following reasons:
1) Available
2) Cheap
3) Don't mix with crude oil
4) Good heat transportation
The steam quantity is calculated by using the following equation:
t
s
t
s
p
p
n
n

ns: is the no. of moles of vap.
nt
 : is the total moles
ps: is vap. press. 
pt: is the total press.
t
t n
s t
s t n
p
p p
n n
p p p
(  )

 
pn: is the press. of H.C
Ex: Estimate the no. of moles of steam required to add to the distillation column to reduce the 
boiling temperature from 400 to 350F. 
Sol.:
400 at 760mmHg
From Fig. 17, (by connection 350 with 400F) so pn=400mmHg 
Let nt =100 mol
n moles s
47.38
760
(760 400)
100 
   
Reflux and Reflux Ratio
Reflux is the ratio between the amounts of material that returned to the distillation column to 
the material that result at distillation. In order to increase the degree of separation and high 
purity, amount of liquid is returned to the column in addition to getting rid of heat making the 
column in thermal equilibrium case.