Studying factors affecting CO

2

absorption in NaOH solutionFaisal Alsaid, Vanessa Ferrero, Mathew Lee, Cindy Rivera, and

James White

Outline

➢ Why study CO2 absorption?

➢ What concepts are we testing?

➢ How was the experiment conducted?

➢ What were the results?

➢ What can we conclude?

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 atmosphereas CO

2 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

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

ObjectivesRunning a CO

2 gas scrubber in batch mode

Because there is a finite amount of CO2 absorption, we expect a breakthroughHow do these affect absorption:➢ Flow rate of feed gas➢ Concentration of CO

2 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

Hypotheses➢ As CO

2 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.

TheoryThe 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 CO

2 the water can absorb.7

Theory: ModellingHenry’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 @ 25oC.

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:

MethodologyMaterials➢ Distilled water➢ CO

2 and N

2 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.

MethodologyProcedure➢ Calibrate CO

2 sensor

➢ Calibrate pH meter➢ Purge column of CO

2 with air

➢ Make NaOH solution➢ Evaluate the pH of the solution before starting the process➢ Pour solution into the column➢ CO

2 sensor is inserted into the tube

➢ CO2

and N2

feeds to column are turned on➢ Bubble CO

2 through column until the percent of CO

2 becomes constant.

➢ Clear/clean the tube

MethodologyThe following table summarizes the various run conditions that were tested.

Table 1: Run Conditions for CO2

Absorption

Results and Discussion

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 .

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%

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.

**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.

**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.

**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.

**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

**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.

Conclusions and RecommendationsWith an increase in the pH of the aqueous solution, we observe an increase in the total amount of CO

2 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

H

unb=(1-[t

b/t

s])H

T

CO2

+2NaOH→ Na2

CO3

References1Global Carbon Emissions and Sinks Since 1750 http://shrinkthatfootprint.com/carbon-emissions-and-sinks (accessed Nov 25, 2015).2Pacific Marine Environmental Laboratory- NOAA. http://www.pmel.noaa.gov/pubs/outstand/feel2331/images/fig01.jpg (accessed Nov 25, 2015).3BBC News-Science and Environment. Whatever Happened to Carbon Capture? http://www.bbc.com/news/science-environment-18019710 (accessed Nov 25, 2015).4Bubble Column Reactors . University of British Columbia http://image.slidesharecdn.com/bubblecolumn1-120629082651-phpapp02/95/bubblecolumn1-6-728.jpg?cb=1340958467 (accessed Nov 25, 2015).5Geankoplis, C. J. Transport processes and separation process principles (includes unit operations), 3rd ed.; Prentice-Hall International: United States, 2003.6Dissolved carbon dioxide http://ion.chem.usu.edu/~sbialkow/Classes/3650/CO2%20Solubility/DissolvedCO2.html (accessed Nov 25, 2015).7Carbon dioxide - Carbonic acid equilibrium http://ion.chem.usu.edu/~sbialkow/Classes/3600/Overheads/Carbonate/CO2.html (accessed Nov 25, 2015).

Burning Questions?