1. Distillation of Binary Mixture
Bottom plate and re-boiler
For constant molal over flow, and used to denote low rates
m-1 m
Bottom
plate
m+1 B xB
Re-boiler
xm+1 yr
Re-boiler plate xB
ChE 334: Separation Processes
Dr Saad Al-Shahrani

2. Binary Multistage Distillation
The vapor leaving the partial re-boiler is assumed to be in equilibrium with the liquid bottom product.
Equilibrium line
Operating line ym
ym+1 a b
a, b,d
bottom plate
c, d, e
Re-boiler plate
The operating line for stripping section crosses the diagonal at point (xB , xB( and its slope= yr c d xB e xB
xm+1 xm
Xm-1
Note: Re-boiler acts as an ideal plate
ChE 334: Separation Processes
Dr Saad Al-Shahrani

3. Binary Multistage Distillation
Feed plate
A feed plate is the plate over which the feed is admitted. The liquid rate over the vapor rate or both may change, depending on the thermal conditions of the feed.
Consider the 5 possible feed conditions shown in the next figures which assumes that the feed has been flashed adiabatically to feed stage pressure. F L
a. sub-cooed liquid feed:
ChE 334: Separation Processes
Dr Saad Al-Shahrani

4. Binary Multistage Distillation
As a result of this, flow of liquid in stripping section increases and flow of vapor in rectifying section decreases
b. Feed is saturated liquid (bubble point liquid feed) : L F
ChE 334: Separation Processes
Dr Saad Al-Shahrani

5. Binary Multistage Distillation
C. Partial vaporized feed : L F
The liquid portion of feed becomes liquid and the vapor portion of feed becomes vapor
= LF+VF
ChE 334: Separation Processes
Dr Saad Al-Shahrani

6. Binary Multistage Distillation
d. saturated vapor feed (dew point vapor feed) : L F
e. Superheated vapor feed F L
ChE 334: Separation Processes
Dr Saad Al-Shahrani

7. Binary Multistage Distillation
Cold feed q > 1.0
Feed at bubble point (sat. liq.), q=1.0
Feed partially vapor, 0 < q < 1.0
Feed at dew point (sat. vap.), q = 0
Feed superheated vapor, q < 0
ChE 334: Separation Processes
Dr Saad Al-Shahrani

8. McCabe Thiele Graphical Equilibrium-Stage
q for sub-cooled feed (Tb > TF) L F
ChE 334: Separation Processes
Dr Saad Al-Shahrani

9. McCabe Thiele Graphical Equilibrium-Stage
q for superheated feed (TF > Td)
Boiling liquid L F
TF= feed temperature
Tb, Td= bubble and dew point of feed respectively.
= average latent heat of vaporization
CpL, CpV = specific heat of liquid and vapor respectively.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

10. McCabe Thiele Graphical Equilibrium-Stage
Feed line
The contribution of the feed stream to the internal flow of liquid = qF
The total flow rate of liquid in the stripping section is:
and
e.g
For saturated liquid feed
For saturated vapor feed
ChE 334: Separation Processes
Dr Saad Al-Shahrani

11. McCabe Thiele Graphical Equilibrium-Stage
The contribution of the feed stream to the internal flow of vapor =F(1- q)
The total flow rate of vapor in the rectifying section is: ,
e.g
For saturated liquid feed ,
For saturated vapor feed
ChE 334: Separation Processes
Dr Saad Al-Shahrani

12. McCabe Thiele Graphical Equilibrium-Stage
For constant molal over flow
Rectifying section V yn L
xn+1 n
n+1
stripping section
m+1 ym
xm+1 m
ChE 334: Separation Processes
Dr Saad Al-Shahrani

13. McCabe Thiele Graphical Equilibrium-Stage
To locate the point where the operating lines intersect,
Let yn=ym , xn+1=xm+1 and subtract equation (1) from equation (2)
FxF
Feed line
q-line
ChE 334: Separation Processes
Dr Saad Al-Shahrani

14. McCabe Thiele Graphical Equilibrium-Stage
This equation represents a straight line, called the feed line on which all intersections of operating lines must fall.
The position of feed line depend on xF and q.
Its slope is and its intercept is
This line cross the diagonal at x= xF
ChE 334: Separation Processes
Dr Saad Al-Shahrani

15. McCabe Thiele Graphical Equilibrium-Stage
ra cold liquid q > 1 slope +
rb saturated liquid q = 1 slope 
rc (vap. + liq.) 0 < q < 1 slope -
rd saturated vapor q = 0 slope 0
re superheated q < 0 slope + XD
q =1
q >1
0 < q <1 b a c
q =0 d r y e
q <1 XB XB XF XD x
ChE 334: Separation Processes
Dr Saad Al-Shahrani

16. McCabe Thiele Graphical Equilibrium-Stage
Construction of operating lines
Locate the feed line ( ) according to the feed conditions.
Locate the rectifying line ( ). This line croces the diagonal at (xD, xD) and of intercept ( )
Draw the stripping line through point (xB, xB( an the intersection of rectifying line with the feed line.
Note: xF, xB , xD, L,D are constant
ChE 334: Separation Processes
Dr Saad Al-Shahrani

17. McCabe Thiele Graphical Equilibrium-Stage
Plate Feed Location
The feed plate is always represented by the triangle that has one corner on the rectifying line and one on the stripping line y XD XF XB a b c x
Note:
The number of plates = number of plates + re-boiler plate.
The liquid on the feed plate does not have the same composition as the feed.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

18. 1 2 3 4
Feed line 5
R.O. L 6 7 8 . 9
S.O. L 10 R xF xD xB

19. McCabe Thiele Graphical Equilibrium-Stage
Heating and cooling requirements D xD
Liq. At its bubble point temperature Va
Vap. at its dew
point temp.
Top
plate La xa T1 T2
Heat loss from a large insulated is relatively small.
For condenser
a. If the condensate is not su-bcooled (at Tbub)
 molal latent heat of vaporization of more volatile component
ChE 334: Separation Processes
Dr Saad Al-Shahrani

20. McCabe Thiele Graphical Equilibrium-Stage
b. If Su-cooled Reflux
If the reflux is cooled below the bubble point, a portion of vapor coming to the top plate (1) must condensed to heat the reflux
ΔL that is condensed inside the column is obtained from: D xD V
Top
plate L Tc T1 ΔL
Cpc=specific heat of condensation
T1= temp. of liq. On to plate bubble point of condensate
Tc= temp. of return condensate (reflux)
c= heat of vaporization of condensate for volatile component.
ChE 334: Separation Processes
Dr Saad Al-Shahrani

21. McCabe Thiele Graphical Equilibrium-Stage
The actual reflux ratio in the column is
T1 Tbc= bubble point of the condensate
ChE 334: Separation Processes
Dr Saad Al-Shahrani

22. McCabe Thiele Graphical Equilibrium-Stage
For re-boiler
Bottom
plate
condensate B xB
steam
= steam consumption
= vapor flow rate from re-boiler
s = latent heat of steam.
 = molal latent heat of mixture at the bottom
 molal latent heat of less volatile component
ChE 334: Separation Processes
Dr Saad Al-Shahrani