Mass Transfer is Key

Ed Cussler

Chemical Engineering

University of Minnesota

Carbon Capture WorkshopStanford, May 26, 2011

What are these towers, anyway?

How Fat? How Tall?

How Fat Depends On Flows

How Tall Depends on Area, Rate

t=timea=area/volumeK=mass transfer coefficient

Kazout

in

conce

conc

Area/Volume a is Simple

How Tall Depends on Area, Rate

t=timea=area/volumeK=mass transfer coefficient

Kazout

in

conce

conc

Rate Constant K is Complicated

Rate Constant K is Complicated

K Across a Gas Liquid Interface

1 1 1

gas liqK k k H

Simple Mass Transfer k as f( Diffusion D)

DHk

l

Dk H

t

Fast Mass Transfer k as f( Rxn Rate )

k D

Instantaneous Mass Transfer Not f(Rxn Rate)

2

rxn1

w/o rxn 2

NaOH

CO

k Dc

k Dc

Overall Mass Transfer f(Reaction)

0

0.25

0.5

0.75

1

0.001 0.01 0.1

eff

icie

ncy,

amount in/out, y0 / yl

yl = 0.5

yl = 0.1

yl = 0.05

Energy Efficiency vs. Amount Separated

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 .

1970 1980 1990 2000 2004 20060

5

10

15

20

25R

O P

ow

er

Consum

ption (

kW

h/m

3)

Year

2000 2004 20060

1

2

3

4

RO is Approaching a Similar Limit

Two Exchangers

spray towershell and tube

1970 1980 1990 2000 2004 20060

5

10

15

20

25

RO

Pow

er

Consum

ption (

kW

h/m

3)

Year

2000 2004 20060

1

2

3

4

RO is Approaching a Similar Limit

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 CO2from air into 0.1 kmol/m3 NaOH. The mass-transfer 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.