CBE 417 “Chemical Engineering Equilibrium Separations”

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

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. Rectifying Section General Operating Line: Rectifying Section xN-1 yN (xD, y1) Equilibrium Stage xN yN+1 • 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 Lines y1 V1 (x1, y1) LO D (xD, y1) (x2, y2) (x1, y2) y1 xD xD (x3, y3) (x2, y3) 1 x2 y3 x1 y2 2 x3 y4 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 Line rectifying section F stripping section ZF Feed stage & overall column MBs: feed line eqn.

13. Plot Feed MB Line subcooled liq. Feed Condition q Slope sat’d liquid = 1  q>1 sat’d liq. 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’dvap. 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 “step off” equilibrium stages on the XY diagram. Feed stage location: point where switch from rectifying operating line to the stripping operating line. zF

17. McCabe-Thiele Graphical Method Binary Distillation “step off” equilibrium stages on the XY diagram. Feed stage location: point where switch from rectifying operating line to the stripping operating line. zF 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 Minimum Number of Stages Total reflux; so D = ? and R = ?? zF

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

20. McCabe-Thiele Graphical Method Binary Distillation Minimum Reflux Ratio One or both operating lines intersect the equilibrium line. zF 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?