1. “Chemical Engineering Equilibrium Separations”
CBE 417
“Chemical Engineering Equilibrium Separations”
Lecture: 8
24 Sep 2012

2. Overview Multicomponent Flash Flash Unit Operation (AspenPlus)
Staged systems
McCabe-Thiele

3. Distillation Column

4. Distillation Column Design (binary) Simulation (binary) Specify
Za, XD, XB (more volatile)
Reflux Ratio (Lo/D)
Optimum feed stage location
P column (condenser)
F (feed flowrate)
Feed condition
Find
N (number of stages)
Nfeed (feed stage)
D, and B (flowrates)
Heat duties
Diameter, height XD Za
Simulation (binary)
Use existing column
Simulate to see performance
Any needed modifications? XB

5. Distillation Column Overall Column Balances (SS)QC
Distillation Column P
Overall Column Balances (SS) XD hD
Material Balance (MB): Za hF
Energy Balance (EB):
heat added is (+)
heat removed (-)
adiabatic (well insulated) QR XB hB

6. McCabe-Thiele Graphical Method (binary)
Used to simplify analysis of binary distillation (ease of understanding)
Assumptions:
Pure components a, b have equal latent heats of vaporization / mole ( ) and they stay constant.
are much larger than
Sensible heat changes
Heats of mixing
Column is adiabatic (well – insulated)
Constant pressure (P) throughout the column (i.e. no P in the column)
Called Constant Molal Overflow (CMO)
Assumes for every 1 mole of light material vaporized that 1 mole of heavy material condenses from the vapor phase
Net result:
Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other)
Do not need a stage by stage energy balance
McCabe-Thiele is done with MB and thermodynamic information.

7. MB on Rectifying Section
At Steady State (SS) for light material (LK) MB: Moles In = Moles Out
For CMO
General Operating Line

8. General Operating Line: Rectifying Section
Equilibrium Stage
xN-1
yN+1 yN xN
(xD, y1)
Equilibrium Line:
Relates composition of liquid leaving stage N (i.e. xN) to the composition of vapor leaving stage N (yN)
Operating Line:
Relates composition of liquid leaving stage N (i.e. xN) to the composition of vapor entering stage N (yN+1)

9. Tie Together Equilibrium & Operating Linesy1 x1 xD y2
y1 V1 LO y3 x3 x2 y4
(xD, y1)
(x1, y1) D
(x2, y2)
(x1, y2)
(x3, y3)
(x2, y3) 1 2 3

10. MB on Stripping Section
At SS and CMO still assumed, so:
Operating Line Stripping Section
(xN, yN)
(xN-1, yN)
(xB, yB)
(xN, yB)
(xB)

11. Feed Stage F
Depending on Feed “condition” will get changes to vapor and liquid flowrates… ZF
Define q = Moles of liquid flow in Stripping section that result from one mole of feed.
Suppose:
q = 1
q = 0

12. Feed Stage Operating LineZF
rectifying section
stripping section
Feed stage & overall column MBs:
feed line eqn.

13. Plot Feed MB Line Lets put all three lines together:
subcooled liq.
Feed Condition q Slope
sat’d liq.
q>1
sat’d liquid = 1 
partial V&L
q = 1
sat’d vapor = 0 0
0<q<1
mixed V & L 0 < q < 1 neg. (-)
subcooled L > 1 pos. (+)
q = 0
sat’d vap.
superheated V < 0 pos. (+) zF
Superhtd vapor
q<0
Lets put all three lines together:
rectifying section
stripping section
feed line

14. Operating Lines (McCabe-Thiele)
q is constant
R incr.
Rectifying & stripping lines must intersect at the same point on the feed line.
Consider limits:
R = 
R where rectifying line intersects the equil. curve zF

15. Operating Lines (McCabe-Thiele)
R is constant
Rectifying & stripping lines must intersect at the same point on the feed line.
q incr. zF

16. McCabe-Thiele Graphical Method
Binary Distillation zF
“step off” equilibrium stages on the XY diagram.
Feed stage location: point where switch from rectifying operating line to the stripping operating line.

17. McCabe-Thiele Graphical Method
Binary Distillation zF
“step off” equilibrium stages on the XY diagram.
Feed stage location: point where switch from rectifying operating line to the stripping operating line.
Optimum feed stage location: switching point to obtain smallest number of stages. Switch when intersection of 3 operating lines is first crossed.

18. McCabe-Thiele Graphical Method
Binary Distillation zF
Minimum Number of Stages
Total reflux; so D = ?
and R = ??

19. McCabe-Thiele Graphical Method
Binary Distillation zF
Minimum Reflux Ratio
One or both operating lines intersect the equilibrium line.
Result: infinite number of stages.

20. McCabe-Thiele Graphical Method
Binary Distillation zF
Minimum Reflux Ratio
One or both operating lines intersect the equilibrium line.
Result: infinite number of stages.

21. McCabe-Thiele Graphical Method (binary)
Used to simplify analysis of binary distillation (ease of understanding)
Assumptions:
Pure components a, b have equal latent heats of vaporization / mole ( ) and they stay constant.
are much larger than
Sensible heat changes
Heats of mixing
Column is adiabatic (well – insulated)
Constant pressure (P) throughout the column (i.e. no P in the column)
Called Constant Molal Overflow (CMO)
Assumes for every 1 mole of light material vaporized that 1 mole of heavy material condenses from the vapor phase
Net result:
Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other)
Do not need a stage by stage energy balance
McCabe-Thiele is done with MB and thermodynamic information.

22. Problem Solving Exercise

23. Questions?