1. Multiple Feed and Side Stream
F1 + F2 = B + D, F1xF1 + F2xF2= BxB + DxD
There are three operating lines
a) Rectifying section n xD D xD L V
n+1 F1
xF1 q1 k
b) Intermediate line
k+1 F2
xF2 q2 m
m+1 B xB
ChE 334: Separation Processes
Dr Saad Al-Shahrani

2. Distillation of Binary Mixture
xF2
xF1
c) Stripping line
ChE 334: Separation Processes
Dr Saad Al-Shahrani

3. Binary Multistage Distillation
Feed with side stream
F = D + Ls +B
a) xs > xF xD D xD
1. Rectifying line L V Ls xs k
k+1 F xF
2. intermediate line B xB
ChE 334: Separation Processes
Dr Saad Al-Shahrani

4. Binary Multistage Distillation
2. Stripping line xF xs
Note: to obtain the locus of intersections of rectifying and intermediate lines, put yn+1=yk+1, xn=xk in equaton (2) and subtract equation (2) from equation (1).
y = yn+1=yk+1, x = xn=xk
and intersect the 45 oC line at xs,xs
ChE 334: Separation Processes
Dr Saad Al-Shahrani

5. Binary Multistage Distillation
b) xF >xs
1. Rectifying section
The feed stream and the top of column as shown in figure. L xD D xD V F xF k
2. Intermediate section
k+1 Ls xs B xB
ChE 334: Separation Processes
Dr Saad Al-Shahrani

6. Binary Multistage Distillation
3. Bottom section xS xF
ChE 334: Separation Processes
Dr Saad Al-Shahrani

7. Binary Multistage Distillation
Side stream condition L V F xF Ls xs
a) Side stream is saturated liquid Vs xs L V F xF
b) Side stream is saturated vapor
ChE 334: Separation Processes
Dr Saad Al-Shahrani

8. Binary Multistage DistillationV L
c) Side stream is (vapor + liquid) F xF
SV+SL xs
ChE 334: Separation Processes
Dr Saad Al-Shahrani

9. McCabe Thiele Graphical Equilibrium-Stage
Example: mixture of water and ethyl alcohol containing 0.16 mole fraction alcohol is continuously distilled in a plate fractionating column to give a product containing 0.77 mole fraction alcohol and a waste of 0.02 mole fraction alcohol. Its proposed to withdraw 25% of the alcohol in the entering stream as a side stream with a mole fraction of 0.5 alcohol. Determine the no. of theoretical plates required and the plate from which the side stream should be withdrawn if the feed is liquid at the boiling point and RD= 2.0
Solution
Basis = 100 k mol of feed
16 k mol of alcohol enter, and 25% (4 k mol) are to be removed in the side stream. As the side stream composition is to be 0.5, there are 8 k mol in the side stream
ChE 334: Separation Processes
Dr Saad Al-Shahrani

10. McCabe Thiele Graphical Equilibrium-Stage
Total balance: F=D+B+S
100=D+B+8
92=D+B (1) xD L
xD=0.77 D V
Sat. liq.
Component balance on alcohol
100*0.16=0.77D+0.02B+4
12=0.77D+0.02B (2)
Ls= 8
Xs=0.5 F
xF=0.16
By solving equations (1) and (2)
D= K mol
B= K mol B
xB=0.02
ChE 334: Separation Processes
Dr Saad Al-Shahrani

11. McCabe Thiele Graphical Equilibrium-Stage
a) Rectifying section
R=L/D=2.0, L=2*13.55=27.1 K mol xD L
V=L+D= = K mol
xD=0.77 D V
Sat. liq.
L/V = 27.1/40.65 = 0.67
Ls= 8
Xs=0.5
b) Intermediate section F
xF=0.16 B
xB=0.02
ChE 334: Separation Processes
Dr Saad Al-Shahrani

12. McCabe Thiele Graphical Equilibrium-Stage
b) Bottom section xD L
xD=0.77 D V
Sat. liq.
Ls= 8
Xs=0.5 F
xF=0.16 B
xB=0.02
ChE 334: Separation Processes
Dr Saad Al-Shahrani

13. McCabe Thiele Graphical Equilibrium-Stage
Mol fraction alcohol in liquid
Number of ideal plates=
8 + reboiler
Side stream plate =
4 from the top
Feed plate= 7 from the top.
ChE 334: Separation Processes
Dr Saad Al-Shahrani