Carbon Capture Workshop
Stanford, May 26, 2011
Mass Transfer is Key
Ed Cussler
Chemical Engineering
University of Minnesota
What are these towers, anyway?
How Fat?
How Tall?
How Fat Depends On Flows
How Tall Depends on Area, Rate
concout
concin
e
Kaz
t=time
a=area/volume
K=mass transfer coefficient
Area/Volume a is Simple
How Tall Depends on Area, Rate
concout
concin
e
Kaz
t=time
a=area/volume
K=mass transfer coefficient
Rate Constant K is Complicated
Rate Constant K is Complicated
K Across a Gas Liquid Interface
1
K
1
k gas
1
kliq H
Simple Mass Transfer k as f( Diffusion D)
k
DH
l
k
D
H
t
Fast Mass Transfer k as f( Rxn Rate )
k
D
Instantaneous Mass Transfer
Not f(Rxn Rate)
k rxn
k w/o rxn
DcNaOH
1
2 DcCO2
Overall Mass Transfer f(Reaction)
Energy Efficiency vs. Amount Separated
1
efficiency,
0.75
yl = 0.05
0.5
yl = 0.1
0.25
0
0.001
yl = 0.5
0.01
amount in/out, y0 / yl
0.1
Conclusions
1. Use Largest Possible Area/Volume a.
2. Use Largest Possible H.
3. For Absorption, Use Reactive Liquid.
4. Don’t Fuss Over Diffusion.
5. Watch Energy Efficiency .
3
RO Power Consumption (kWh/m )
RO is Approaching a Similar Limit
25
20
4
3
2
15
1
0
10
2000
2004
2006
5
0
1970 1980 1990 2000 2004 2006
Year
Two Exchangers
spray tower
shell and tube
3
RO Power Consumption (kWh/m )
RO is Approaching a Similar Limit
25
20
4
3
15
2
1
10
0
2000
2004
2006
5
0
1970 1980 1990 2000 2004 2006
Year
Mass-transfer area of structured
packing Tsai et al (AIChE J, May ’11)
•
Abstract
• The mass-transfer area of nine structured
packings was measured via absorption of CO2
from air into 0.1 kmol/m3 NaOH. The masstransfer area was most strongly related to the
specific area (125–500 m2/m3), and liquid load
(2.5–75 m3/m2·h). Surface tension (30–72
mN/m) had a weaker effect. Gas velocity (0.6–
2.3 m/s), liquid viscosity (1–15 mPa·s), and flow
channel configuration had essentially no impact.