2. Outline
➢ Why study CO2 absorption?
➢ What concepts are we testing?
➢ How was the experiment conducted?
➢ What were the results?
➢ What can we conclude?
3. Background
➢ Carbon sequestration: from atmosphere or anthropogenic sources
○ Increased understanding of climate change
○ Role of greenhouse gases
○ Future needs
➢ “CO2
sequestration has the potential
to significantly reduce the level of
carbon that occurs in the atmosphere
as CO2
and to reduce the release of
CO2
to the atmosphere from major
stationary human sources, including
power plants and refineries.”1
Figure 1: Visual representation of carbon emissions and carbon
sinks. Adapted from Shrink that Footprint1
4. Background
➢ Understanding CO2 absorption of particular interest
➢ Ocean Acidification
➢ In Industry
Figure 2: Depiction of CO2 Effects on Ocean Acidification.
Adapted from National Oceanic and Atmospheric Administration.2
Figure 3: Schematic of a Carbon Capture Plant.
Adapted from Technology Center Mongstad (TCM)3
5. Objectives
Running a CO2
gas scrubber in batch mode
Because there is a finite amount of CO2 absorption, we expect a breakthrough
How do these affect absorption:
➢ Flow rate of feed gas
➢ Concentration of CO2
in feed gas
➢ Volume of solution in column
➢ pH of absorption solution
Figure 4: Different types of bubble columns used in industry.
Adapted from Types of Bubble Columns4
6. Hypotheses
➢ As CO2
concentration increases, breakthrough curve will remain at the same
point, but breakthrough will be reached more quickly.
➢ An increase in feed rate will result in the same breakthrough concentration, but
breakthrough will be reached more slowly.
➢ An increase in the NaOH concentration will increase the concentration at
which breakthrough occurs.
➢ An increase in the volume of the liquid in the absorption column will also
increase the concentration at which breakthrough occurs.
7. Theory
The absorption of carbon dioxide into water takes place through a series of
equilibrium reactions:
Utilizing Le’ Chateliers principle, it’s possible to take advantage of these equilibrium
reactions to increase the amount of CO2
the water can absorb.7
8. Theory: Modelling
Henry’s Law describes the interaction between the pressure of a gas and its
dissociation into a liquid. In this case:
➢ Henry’s Law Constant for CO2: KCO2
=2x10-3
@ 25o
C.
By using the equilibrium equations that define this process, it’s possible to find the
total CO2 concentration as a function of pH.
➢ At 25 °C, 1 atm: [CO2 (aq)]=1.2x10^-5, as given by Ion Chem6
:
9. Methodology
Materials
➢ Distilled water
➢ CO2
and N2
feeds
➢ NaOH
➢ A 1.4 meter tube + valve
➢ Beakers
➢ Micropipette
➢ Air
➢ Rotameter
➢ ExStik II pH meter
Figure 5: Schematic of Lab Apparatus
for CO2
Absorption.
10. Methodology
Procedure
➢ Calibrate CO2
sensor
➢ Calibrate pH meter
➢ Purge column of CO2
with air
➢ Make NaOH solution
➢ Evaluate the pH of the solution before starting the process
➢ Pour solution into the column
➢ CO2
sensor is inserted into the tube
➢ CO2
and N2
feeds to column are turned on
➢ Bubble CO2
through column until the percent of CO2
becomes constant.
➢ Clear/clean the tube
13. Figure 6: Breakthrough curves for the absorption of
carbon dioxide for three different flow rates
Slope estimates, BT times (sec):
487.5 mL/min : 0.00408/sec, 97
446.1 mL/min : 0.003947/sec, 132
427.8 mL/min : 0.003091/sec, 161
Lower flow rates resulted with
higher bed capacities while higher
flow rates resulted with steeper
mass transfer zones .
14. Figure 7: Breakthrough curves for absorption of
carbon dioxide using 4 different amounts of NaOH
Solutions of NaOH between 50 and
300 microLiters of resulted with
slight observable differences. The
addition of 500 microLiters resulted
with a higher breakthrough time of
168 sec. (Compare to 144 sec, 140
sec, 136 sec)
Coefficient of variations for
duplicate runs were calculated to be
between 0 and 17%
15. Figure 8: Henry’s Law for a given range of partial
pressures. Includes maximum solubility of CO2 at
room temperature. Adapted from IonChem6
Henry’s Law:
Demonstrates equilibrium
relationship between CO2 vapor
phase and liquid phase.
16. **graphs**
Figure 9: Distribution of dissolved CO2 species for a
range of pH values. Adapted from IonChem6
Distribution of dissolved CO2
species: demonstrates
disappearance of H2CO3.
Key aspect of carbonic acid
equilibrium.
17. **graphs**
Figure 10: log(Total CO2 absorbed) as a function of
the pH with varying partial pressures.
Total CO2 Concentration as a
function of pH. Supports previous
findings.
Increasing partial pressure of CO2
shifts curves upwards due to
logarithmic scaling.
Higher pressure, more dissolved
CO2.
18. **graphs**
Key model allows:
➢ Prediction of Total CO2
concentration at a given
hydroxide concentration and
partial pressure.
➢ Comparison of experimental
results to theoretical results.
➢ Demonstrates key concepts
outlined previously.
Figure 11: Total CO2 absorbed as a function of NaOH
concentration with varying partial pressures.
19. **graphs**
Determines theoretical absorption
capacity of column for various CO2
partial pressures and hydroxide
concentrations.
Figure 12: Total CO2 absorbed as a function of NaOH
concentration with varying partial pressures with
predictions.
Table 1: Run Conditions for CO2
Absorption
20. **graphs**
Figure 13: log(Total CO2 absorbed) as a function of
the pH with varying partial pressures. Tighter x-axis.
Total CO2 Concentration as a
function of pH. Supports previous
findings.
Can be used to predict Total CO2
concentration with a given pH value.
21. Conclusions and Recommendations
With an increase in the pH of the aqueous solution, we observe an increase in the
total amount of CO2
absorbance.
With a decrease in flow rate, we increased the breakthrough time, which increases
the absorbance capacity of the column. But increasing the flow rate, decreases the
amount of unused bed space.
Recommended that future studies which focus on a optimizing flow rate, where bed
capacity and breakthrough time are both maximized. (Geankoplis)5
Hunb
=(1-[tb
/ts
])HT
CO2
+2NaOH→ Na2
CO3
22. References
1
Global Carbon Emissions and Sinks Since 1750 http://shrinkthatfootprint.com/carbon-emissions-and-sinks (accessed Nov 25, 2015).
2
Pacific Marine Environmental Laboratory- NOAA. http://www.pmel.noaa.gov/pubs/outstand/feel2331/images/fig01.jpg (accessed Nov
25, 2015).
3
BBC News-Science and Environment. Whatever Happened to Carbon Capture? http://www.bbc.com/news/science-environment-
18019710 (accessed Nov 25, 2015).
4
Bubble Column Reactors . University of British Columbia http://image.slidesharecdn.com/bubblecolumn1-120629082651-
phpapp02/95/bubblecolumn1-6-728.jpg?cb=1340958467 (accessed Nov 25, 2015).
5
Geankoplis, C. J. Transport processes and separation process principles (includes unit operations), 3rd ed.; Prentice-Hall International: United
States, 2003.
6
Dissolved carbon dioxide http://ion.chem.usu.edu/~sbialkow/Classes/3650/CO2%20Solubility/DissolvedCO2.html (accessed Nov 25,
2015).
7
Carbon dioxide - Carbonic acid equilibrium http://ion.chem.usu.edu/~sbialkow/Classes/3600/Overheads/Carbonate/CO2.html
(accessed Nov 25, 2015).
