Regenerating Diethanolamine Aqueous Solution for CO 2 Absorption Using Microalgae

Abstract

The high energy requirements associated with regenerating amine solution represents the main challenge for its use in carbon-dioxide (CO₂) removal processes. In this work, freshwater microalgae strains have been used to remove dissolved CO₂ from an aqueous solution of diethanolamine (DEA) and regenerate the solution with a far lower energy requirement. Using microalgae has the additional advantage of producing biomass that can be readily used to produce valuable products, such as lipids, proteins, and pigments. The ability of three strains—Chlamydomonas sp., Chlorella sp., and Pseudochlorococcum sp.—to grow in DEA solution saturated with CO₂ was tested. The effectiveness of the selected strains to utilize the dissolved CO₂ as a carbon source and reduce its concentration has also been assessed. It was found that the three tested strains grew well in 10% DEA solution saturated with CO₂, with specific growth rates of 0.365, 0.352, and 0.669/day, for Chlamydomonas sp., Chlorella sp., and Pseudochlorococcum sp., respectively. The strains were also capable of removing the dissolved CO₂ with drop rate of 0.0120, 0.135, and 0.0123 mole/mL/day. After regeneration, the amine solution regained 85% of its initial capacity. These results demonstrate a significant advantage over conventional regeneration processes of amine solutions used for CO₂ absorption.

Introduction

Carbon dioxide (CO₂) capture is an important step in many industrial applications. For example, natural gas, a mixture of combustible hydrocarbons comprised mainly of methane, contains large amounts of CO₂ that must be reduced to very low levels before the hydrocarbon mixture can be utilized as a fuel. In addition, the burning of fossil fuels for power generation is considered the largest source of CO₂ emission worldwide. This CO₂ needs to be captured. Furthermore, in hydrogen production processes such as steam reforming, the H₂ generated is usually mixed with considerable amounts of CO₂, which also needs to be separated.

To date, various technologies have been proposed for CO₂ capture, such as cryogenic separation, membrane separation, and absorption.¹ The most advanced and well-established technology is chemical absorption using an alkanolamines aqueous solution, such as diethanolamine (DEA).^2,3 DEA has been commonly used in commercialized natural gas sweetening units for CO₂ removal. But despite its high CO₂-capture efficiency, a large amount of energy is required to regenerate the solvent, which needs to be heated up to about 100–130°C.⁴ In a conventional power plant using fossil fuels, the estimated energy penalty for CO₂ capture by amine solvents is about 23–30% of the energy output.⁵

DEA reacts with CO₂ as described in Equation 1 to form bicarbonate in the presence of water: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm R}_1{ \rm R}_2{ \rm NH} + { \rm CO}_2 + { \rm H}_2{ \rm O} \leftrightarrow { \rm R}_1{ \rm R}_2{ \rm NH}_2{}^{+} + { \rm HCO}_3{}^{ -} \tag{1}\end{align*} \end{document}

In this work, microalgae has been suggested to for the removal of dissolved CO₂ and amine regeneration at room temperature—resulting in a significant decrease in energy requirement. In addition, the produced biomass can be readily used to produce lipids, proteins, and pigment. Lipids can be used for biodiesel production, while proteins and pigments have potential applications in pharmaceuticals and food.

The ability to carry out a wide array of biophysical activities makes it possible for microalgae to be applied in many biotechnological and environmental processes.⁶ The use of microalgae in a number of industrial processes has proven to be cheaper and more environmentally friendly than alternative methods. Research on the use of microalgae in various applications has experienced a tremendous increase in recent years.⁷ To the best of our knowledge, however, the use of microalgae for amine regeneration in CO₂-absorption processes has never been studied before.

The main objective of this work was to test the ability of a number of freshwater microalgae strains to grow in DEA solution saturated with CO₂ and assess their effectiveness in both reducing the concentration of dissolved CO₂ and regenerating the amine solution.

Materials and Methods

Chemicals and Strains

All chemicals were purchased from Sigma Aldrich (St. Louis, MO) and used as received, without further purification. A CO₂ (99.99%) gas cylinder was purchased from Abu Dhabi Oxygen Company (Abu Dhabi, United Arab Emirates). A freshwater strain of Chlamydomonas sp., isolated from Musffah, Abu Dhabi, was kindly provided by Professor Koroush Salihi of New York University in Abu Dhabi. The performance of this strain was compared to other freshwater strains, namely, Chlorella sp. and Pseudochlorococcum sp., which were obtained from a local marine research center in Umm Al-Quwain, United Arab Emirates.

Amine Regeneration Experiment

In 500-mL Erlenmeyer flasks, 50 mL DEA was added to 450 mL nutrients medium (free of algae) containing 8.82 mM NaNO₃, 0.17 mM CaCl₂·2H₂O, 0.3 mM MgSO₄·7H₂O, 1.29 mM KH₂PO₄, 0.43 mM K₂HPO₄, 0.43 mM NaCl, 1 mL/L of vitamin B12, and 6 mL/L of P-IV solution that consisted of 2 mM Na₂EDTA·2H₂O, 0.36 mM FeCl₃·6H₂O, 0.21 mM MnCl₂·4H₂O, 0.37 mM ZnCl₂, 0.0084 mM CoCl₂·6H₂O, and 0.017 mM Na₂MoO₄·2H₂O. Prior to adding microalgae cells, the 10% DEA solution was saturated with CO₂ by supplying pure CO₂ gas through spiral tubes, which have small holes and are placed inside the stirred solution until the pH reached a constant value (8.4). The flasks were then placed in a temperature-controlled incubator shaker (Innova 40 Benchtop, New Brunswick Scientific, Edison, NJ) at 180 r/min and 27°C, and covered with paraffin tape. On a daily bases, the CO₂ concentration was measured until equilibrium was reached and the CO₂ concentration ceased to drop. In the first couple of days, a slight drop in CO₂ was witnessed, after which the concentration remained constant. This drop was expected because the solutions were equilibrated against pure CO₂; when the system was exposed to an atmospheric environment, a new equilibrium was reached. 270 mL of this solution was then mixed with 30 mL of Chlamydomonas sp. suspension and placed in the incubator shaker at 180 r/min and 27°C. The added suspension contained 10% DEA solution in nutrient medium containing the microalgae, bringing the initial dry weight concentration of the microalgae to 0.053 mg/mL.

A control test was carried out in duplicate, using 300-mL solution of 10% DEA in medium free of microalgae (the same volume of the growth experiment) placed in the incubator shaker under the same conditions. The flasks were sealed with paraffin tape to minimize evaporation and atmospheric CO₂ fixation. The cultures were subjected to the light used to illuminate the lab. To confirm the reproducibility of the data, each experiment was repeated twice, and the average values of the results were presented. The repetition was performed using biologically separate biomass samples (not repeating the measurements of the same sample). The reproducibility of the experimental results was evaluated using standard deviations.

At the end of the experiment, the CO₂ absorption capacity of regenerated amine solution was determined by bubbling CO₂ using the same saturation system. The equilibrium concentration of CO₂ dissolved in the regenerated amine was recorded.

Determination of Dissolved CO₂ and Biomass Concentrations

A 2-mL sample was withdrawn from the growth culture on a daily basis. The sample was centrifuged at 6,000 r/min for 5 min using multispeed centrifuge (IEC CL31, Thermo Scientific, Waltham, MA) to separate the biomass. The dissolved CO₂ concentration in the supernatant was determined by the method adapted from the Chittick CO₂ analyzer apparatus (Thermo Scientific).⁸ A sample of known volume (5–25 mL, depending on the CO₂ concentration) with methyl orange indicator was placed in the reaction flask. The flask was then connected and sealed to one side of long U-Tube manometer while the other side was open to atmosphere. Hydrochloric acid (HCl, 1 M) was slowly pumped to the reaction flask from a 50-mL titration burette until the indicator color changed. The solution was continuously stirred by a magnetic stirrer to homogenize and help release CO₂ from the solution. CO₂ released from the liquid displaced the fluid in the tube. The amount of CO₂ in the amine solution was then obtained using Equation 2: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm C}_{{ \rm CO}_2} ( { \rm mol \ mL}^{ - 1} ) = [ 273 \ ( { \rm V}_{ \rm gas} - { \rm V}_{ \rm HCI} ) \ { \rm P} ]/ [ 22.4 \ { \rm T} \ { \rm V}_{ \rm sample} ] \tag{2}\end{align*} \end{document}

where, V_gas and V_HCl are the volumes of displaced solution in the manometer tube and HCl titrant (L), respectively; P is pressure (atm); T is room temperature (K); and V_sample is sample volume (mL). It should be noted that the pressure created by releasing CO₂ from liquid is negligible, and the pressure of the system is approximated as atmospheric pressure.⁹

To determine the biomass concentration, the settled cells were resuspended in distilled water, and the optical density was measured at 680 nm using a spectrophotometer (Shimadzu UV-1800 UV, Kyoto, Japan). The spectrophotometer was zeroed using distilled water. Calibration curves of biomass dry weight concentration vs optical density of the three strains were determined, as described in our previous paper, and were found to be linear.¹⁰ Therefore, the growth was determined by dividing the optical density at any time by the initial optical density, measured on day 0.

Results and Discussion

The growth of Chlamydomonas sp. in 10% DEA solution saturated with CO₂ was monitored, and the results are shown in Fig 1. The vertical dashed line shown in Fig. 1 represents the point at which the microalgae inoculum was added. As mentioned earlier, it was expected to see slight drop in CO₂ concentration in the two days prior to the addition of the microalgae because the system was saturated against pure CO₂. This period was needed to reach the new equilibrium when exposed to atmospheric environment.

Fig. 1.

Biomass growth profile of Chlamydomonas sp. in 10% DEA solution and the drop in dissolved CO₂ concentration. Vertical dashed line indicates when the microalgae inoculum was added.

The residual CO₂ was measured at different times to determine the effectiveness of the strain in dissolved CO₂ removal and compared to the control test without the microalgae (Fig 1). Values shown in the figure are average values of experiments carried in duplicates under the same conditions, and the lines are connections between the experimental data to show the trends. The reproducibility of the experimental results was evaluated using the standard deviations shown as error bars in the figures. The results clearly prove that Chlamydomonas sp. is capable of removing the dissolved CO₂, with a drop rate of 0.012 mole/mL/day. The much higher drop in CO₂ concentration in the microalgae culture compared to the control clearly indicates that the drop is due to biological activity of the microalgae. At the end of the experiment, the CO₂ absorption capacity of regenerated amine solution was determined. It was found that the amine was able to regain 85% of its initial capacity. This clearly proves that the amine was intact, and the microalgae utilized the dissolved CO₂ only. Otherwise, the equilibrium concentration of CO₂ could not exceed its solubility in water, which is 0.033 mol/L.¹¹

Microalgae concentration was also monitored, and the results are shown in Fig. 1. It was found that a lag phase of around 3 days was needed for the cells to adapt to the new substrate and growth medium. The results in Fig. 1 show a good consistency where no significant drop in CO₂ was actually witnessed until day 3. After that, the growth entered an exponential stage, with a specific growth rate, μ, of 0.365/day, determined by Equation 3: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\mu = \frac { { \rm In } ( { \rm X } _ { { \rm t } 2 } / { \rm X } _0 ) - { \rm In } ( { \rm X } _ { { \rm t } 1 } /{ \rm X } _0 ) } { { \rm t } _2 - { \rm t } _1 } \tag { 3 } \end{align*} \end{document}

where, X_t2 and X_t1 are the biomass concentrations at the end, t₂, and beginning, t₁, of the exponential growth, respectively, and X_o is the initial biomass concentration at time zero.

The performance of Chlamydomonas sp. was compared to that of Chlorella sp. and Pseudochlorococcum sp. In the first runs, the two strains were grown for four days with no significant growth or drop in CO₂ concentration. The cells were then collected by centrifugation at 6,000 r/min for 5 min using multispeed centrifuge, and used again with fresh samples of 10% DEA solution saturated with CO₂. The initial concentrations of Chlorella sp. and Pseudochlorococcum sp. were 0.088 and 0.105 mg/mL. This time, the strains showed much better growth and CO₂ removal, as shown in Fig. 2 and 3, for Chlorella sp. and Pseudochlorococcum sp., respectively. The specific growth rate and CO₂ drop rate of Chlorella sp. were found to be 0.352/day and 0.0135 mole/mL/day, whereas, the specific growth rate and CO₂ drop rate of Pseudochlorococcum sp. were found to be 0.669/day and 0.0123 mole/mL/day. The three strains showed similar performances. However, Chlorella sp. and Pseudochlorococcum sp. required more than seven and six days to start to enter the exponential growth stage, respectively. The lag phase of Chlamydomonas sp. was only three days. The results in Fig. 3 show a good consistency between CO₂ drop and microalgae growth. The results in Fig. 2, however, showed a slower growth rate compared to the drop in CO₂. It should be noted that not all substrate consumption is directed towards cell growth; cells use substrates for two functions—production of new cells and the maintenance of existing cells.

Fig. 2.

Biomass growth profile of Chlorella sp. in 10% DEA solution and the drop in dissolved CO₂ concentration.

Fig. 3.

Biomass growth profile of Pseudochlorococcum sp. in 10% DEA solution and the drop in dissolved CO₂ concentration.

To the best of the authors' knowledge, similar work has not been reported in literature. The positive results found in this work suggest that microalgae can be used to regenerate amine solution saturated with CO₂. This offers a great advantage of the conventional absorption processes of CO₂ using amine solutions.

Conclusions

Selected freshwater microalgae strains, namely, Chlamydomonas sp., Chlorella sp., and Pseudochlorococcum sp., were shown to grow in aqueous DEA solution saturated with CO₂ and to utilize the dissolved CO₂. The specific growth rates of Chlamydomonas sp. and Chlorella sp. were almost similar, whereas Pseudochlorococcum sp. showed a higher growth rate. The three strains had very close rates of reduction of dissolved CO₂. After regeneration, the amine solution regained 85% of its initial capacity.

Footnotes

Acknowledgments

The authors would to acknowledge the assistance of Mr. Zia Ur Rehman Said Rahman in repeating the control experiment.

Author Disclosure Statement

No competing financial interests exist.

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