1. (Heat and Mass Transfer) Lecture 22: Distillation and Mass Transfer
CE 318: Transport Process 2
(Heat and Mass Transfer)
Lecture 22: Distillation and Mass Transfer
(UB CBE Spiral Initiatives)
NSC 210
5/5/2015 1

2. 3rd Midterm (To be update)
Mid-terms
1st Midterm
Average: 67/100
Median: 71
SID: 19
>=86: 10
67-86: 32
48–67: 17
< 48: 13
2nd Midterm
Average: 61/100
Median: 62
SID: 20
>=81: 10
61-81: 26
41–61: 22
< 41: 14
3rd Midterm (To be update)
Average: /100
Median:
SID:
>=81: 10
61-81: 26
41–61: 22
< 41: 14 2

3. Mid-terms Final exam: 8am – 11am; May 12
Please let me know if you have time conflict by today. 3

4. Overview of Transport Processes
Momentum
Heat
Mass
Profile
Steady state
Non-steady state 4

5. Overview of Mass Transfer
Steady State Molecular Diffusion
Fick’s Law for Molecular Diffusion
DA: gas, liquid, solid, biological materials
calculation:
Counter-diffusion;
Unimolecular diffusion;
diffusion/reaction
Convective Mass-Transfer Coefficient
Unsteady State Diffusion
Mass Transfer Equipment
Multi-components
Discontinuous interface 5

6. Convective Mass Transfer Coefficients
Schmidt number
Sherwood number 6

7. Forced Heat Convection: Semi-Empirical Method to Estimate h
Prandtl Number
Nusselt Number 7

8. Industrial Separation Processes8
Distillation Gas sorption

9. Separation Process: Drying9

10. Separation Process: Evaporation10

11. Solvent Extraction
Extraction 11

12. Lactic Acid Evaporation from water
Spiral Problem:
Lactic Acid Evaporation from water
Consider a film of liquid water having instantaneous thickness lwater running down the right side of a vertical solid wall (see Figure below). The wall has thickness lwall = 0.2 cm and thermal conductivity kwall = 0.5 J/(cm·s·oC). Its left side is maintained at 100oC. To the right of the film of liquid water is air at a pressure of 500 mm Hg, a bulk temperature T = 25oC and a bulk water vapor mole fraction ywater = 0 (completely dry); “bulk” means far from the wall (“at infinity”). Heat transfer through the air is characterized by a heat transfer coefficient h = J/(cm2·s·oC). Mass transfer of water vapor through the air is characterized by a mass transfer coefficient Fwater = mol/(cm2·s).
lwall = 0.2 cm
lwater
solid wall
P = 500 mm Hg
Air T = 25 °C in bulk
ywater = 0 in bulk
T = 100 °C
Ti = ? 12

13. Spiral Problem 2016 13

14. CBE Spiral Initiatives
CE408 Process Design: Lactic acid production (2016)
Corn
steep liquor
Lactide
Lactic acid
Fermentation of corn
Solvent extraction of lactic acid from H2O
Distillation of lactic acid and organic solvent 14

15. CBE Spiral Initiatives: Lactic Acid Production
Corn
steep liquor
CE212: Liquid extraction
CE304: VLE of lactic acid and 1-octanol
CE317: Pumping solution
CE318: Distillation
CE407: Extraction 15

16. Distillation of Lactic Acid and 1-Octanol
Spiral Problem:
Distillation of Lactic Acid and 1-Octanol
1-octanol LA
H2O
Other junk
To distillation
Cut
achieved by
distillation LA
H2O
Other junk
1-octanol
1-octanol LA
H2O
Other junk 16

17. Distillation of Lactic Acid and 1-Octanol
Spiral Problem:
Distillation of Lactic Acid and 1-Octanol
Octanol (“OCT”) is the light component (more volatile, lower boiling); its boiling point at e.g. 14 mm Hg is 94C.
Lactic acid (“LA”) is the heavy component (less volatile, higher boiling); its boiling point at e.g. 14 mm Hg is 132C. 17

18. How does distillation work?
Vapor (condense what comes off top)
More Oct, less LA
Liquid (stay behind)
More LA, less OCT 18

19. How do you make distilltion better?
Collect only some of the condensed vapor as distillate product
Send the rest of condensed vapor back down the column as reflux
Rectifying tower contains “trays”
Reflux 19

20. How does reflux work? 20 Liquid reflux from tray above
Liquid reflux to tray below
xlactic acid = x1
Rising vapor
ylactic acid = y2
Liquid reflux from tray above
xlactic acid = x0
ylactic acid = y1 L 20

21. How does reflux work? ybulk xbulk = x1
Equilibrium distribution of lactic acid at interface:
y* = xbulk g Psat / P
(Raoult’s law modified by activity coefficient; mole fractions, activity coefficient and saturated vapor pressure refer to lactic acid) 21

22. Mass Transfer across the Interface22

23. Murphree or Tray Efficiency
where A is the interfacial area per mole of vapor on the tray. The quantity hM is called the Murphree efficiency or tray efficiency.
The larger the mass transfer coefficient (ky), the larger the interfacial area (A) or the longer the residence time of the bubble spends on the tray (L / ububble), the closer hM is to unity, and the more nearly equal y1 is to y*.
A tray for which the vapor leaving the tray has a lactic acid mole fraction perfectly in equilibrium with that of the liquid leaving the tray (i.e., y1 = y*(x1)) is called an ideal or theoretical tray (hM = 1 = 100%). 23

24. Example
ybulk
xbulk = x1
Suppose the volume fraction of bubbles in the froth (bubbly liquid on tray) is f = What is A, the interfacial area per mole of vapor on the tray, for a bubble diameter of (a) 2 mm and (b) 2 cm? (The important parameter A appears in the equation for hM at the top of this page.) Assume atmospheric pressure. 24

25. Take Home Messages
Message #1: In distillation (and other staged operations), we are usually given a tray efficiency hM. When we work in terms of hM, mass transfer seems not to enter distillation calculations. However, the joy of mass transfer is actually embedded in hM.
Message #2: We learned all about distillation from Dr. Lockett in CE 407 this semester that is ending now. We will use distillation to separate lactic acid from an extraction solvent like 1-octanol in our CE 408 plant design project in spring 2016. 25