ce 318: transport process 2 (heat and mass transfer) lecture 22: distillation and mass transfer (ub...
TRANSCRIPT
CE 318: Transport Process 2(Heat and Mass Transfer)
Lecture 22: Distillation and Mass Transfer(UB CBE Spiral Initiatives)
NSC 2105/5/2015
Mid-terms
2
2nd Midterm
Average: 61/100
Median: 62
SID: 20
>=81: 10
61-81: 26
41–61: 22
< 41: 14
1st Midterm
Average: 67/100
Median: 71
SID: 19
>=86: 10
67-86: 32
48–67: 17
< 48: 13
3rd Midterm (To be update)
Average: /100
Median:
SID:
>=81: 10
61-81: 26
41–61: 22
< 41: 14
Mid-terms
3
Final exam: 8am – 11am; May 12
Please let me know if you have time conflict by today.
Overview of Transport Processes
4
Momentum Heat Mass
Profile
Steady state
Non-steady state
dy
dvxyx
dx
dTkqx
dx
dCDJ AAA
2
2
y
v
t
v xx
2
2
x
T
t
T
2
2
x
CD
t
C AA
A
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
dx
dCDJ AAA
dx
dCA
21 AAAA CCkJ )(Re,ScfkA
2
2
x
CD
t
C AA
A
Multi-componentsDiscontinuous interface
zBzAAA
ABzA NNydz
dycDN ,,,
6
Convective Mass Transfer Coefficients
ABSc DN
Schmidt number
Sherwood number
)(Re,ScfD
LkSh
ABc
21 AAcA cckN
Forced Heat Convection:Semi-Empirical Method to Estimate h
7
k
c
ckN p
pPr
Prandtl Number
Nusselt Number
Pr)(Re,fk
hDNu
TThA
qs
x
8
Industrial Separation Processes
Distillation Gas sorption
9
Separation Process: Drying
10
Separation Process: Evaporation
11
Solvent Extraction
Extraction
12
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 = 0.003 J/(cm2·s·oC). Mass transfer of water vapor through the air is characterized by a mass transfer coefficient Fwater = 0.0001 mol/(cm2·s).
lwall = 0.2 cm lwater
solid wall
P = 500 mm HgAir T = 25 °C in bulk
ywater = 0 in bulk
T = 100 °C Ti = ?
13
Spiral Problem 2016
14
CBE Spiral Initiatives
CE408 Process Design: Lactic acid production (2016)
Cornsteep liquor
Fermentation of corn Solvent extraction of lactic
acid from H2O
Distillation of lactic acid and organic solvent
Lactic acidLactide
15
CBE Spiral Initiatives: Lactic Acid Production
Cornsteep liquor
• CE212: Liquid extraction
• CE304: VLE of lactic acid and 1-octanol
• CE317: Pumping solution
• CE318: Distillation
• CE407: Extraction
16
Spiral Problem: Distillation of Lactic Acid and 1-Octanol
1-octanol
LA
H2O
Other junk
H2O
1-octanolLA
LAH2O
Other junk
1-octanol
1-octanol
LA
H2O
Other junk
To distillation
H2O
1-octanolLA
Cutachievedbydistillation
17
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.
18
How does distillation work?
Vapor (condense what comes off top)More Oct, less LA
Liquid (stay behind)More LA, less OCT
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
20
How does reflux work?
Liquid reflux to tray below
xlactic acid = x1
Rising vapor
ylactic acid = y2
Liquid reflux from
tray above
xlactic acid = x0
Rising vapor
ylactic acid = y1
L
21
How does reflux work?
ybulk xbulk = x1
Equilibrium distribution of lactic acid at interface: y* = xbulk Psat / P (Raoult’s law modified by activity coefficient; mole fractions, activity coefficient and saturated vapor pressure refer to lactic acid)
22
Mass Transfer across the Interface
23
Murphree or Tray Efficiency
https://www.youtube.com/watch?v=rk8jpKHpD2o
where A is the interfacial area per mole of vapor on the tray. The quantity M 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 M 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 (M = 1 = 100%).
24
Example
Suppose the volume fraction of bubbles in the froth (bubbly liquid on tray) is f = 0.4. 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 M at the top of this page.) Assume atmospheric pressure.
ybulk
xbulk = x1
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.