A kinetic study of CO2 capture with potassium carbonate solutions promoted with various amino acids: Glycine, sarcosine and proline

https://doi.org/10.1016/j.ijggc.2013.10.027Get rights and content

Highlights

  • Glycine, sarcosine and proline improve the absorption of CO2 into a K2CO3 solvent.

  • Reaction kinetics between CO2 and the aforementioned amino acids are reported.

  • Glycine and MEA exhibit a comparable catalytic activity.

  • Sarcosine and proline possess a larger catalytic activity than that of DEA.

Abstract

The absorption kinetics of carbon dioxide (CO2) into amino acid promoted potassium carbonate (K2CO3) solutions has been studied using a wetted-wall column. Experiments were conducted under conditions resembling those found at industrial CO2 capture plants including concentrations up to 2.0 M and temperatures from 40 to 82 °C. Results presented here show that the addition of 1.0 M glycine, sarcosine and proline accelerates the overall rate of absorption of CO2 into a 30 wt% K2CO3 solvent by a factor of 22, 45 and 14, respectively, at 60 °C. The Arrhenius expressions for the reaction between CO2 and aforementioned amino acids are k2-Gly [M−1 s−1] = 1.22 × 1012 exp(−5434/T [K]), k2-Sar [M−1 s−1] = 6.24 × 1010 exp(−1699/T [K]) and k2-Pro [M−1 s−1] = 1.02 × 1011 exp(−2168/T [K]) where the activation energies are 45.2 kJ mol−1, 14.1 kJ mol−1 and 18.0 kJ mol−1, respectively. The reaction order with respect to glycine is found to be 1, while the reaction order with respect to sarcosine and proline is observed to be in the range of 1.3–1.6 and 1.2–1.3, respectively.

Introduction

The release of greenhouse gases and its resulting potential for global warming has raised concerns over the emission of gases such as carbon dioxide (CO2). A number of technologies for CO2 capture are currently being developed, including membranes, adsorption, cryogenics and solvent absorption (Anderson et al., 2011, Metz et al., 2005, Rao and Rubin, 2002). To date, reactive absorption of CO2 into aqueous solutions of amines is the most widely used technology for CO2 capture (Abu-Zahra et al., 2007, Rao and Rubin, 2002, Smith et al., 2013, Versteeg et al., 1996, Zhang et al., 2009). Although amine solvents such as monoethanolamine (MEA) have relatively fast absorption rates compared to alkaline solvents such as potassium carbonate (K2CO3), amines have significant challenges due to high energy requirements for CO2 regeneration and solvent losses from amine degradation and evaporation (Rochelle, 2009).

Much research has been devoted to developing alternative solvents to MEA. One of the many potential solvent candidates is potassium carbonate (K2CO3). Potassium carbonate has been used as a CO2 absorbent since the early 1900s (Cullinane, 2005). The original process has undergone improvements over the years making it a viable commercial process, often used for treating synthesis gas (Benson et al., 1954). Studies have indicated that under specific configurations, potassium carbonate is an efficient CO2 absorbent and has several advantages over MEA (Smith et al., 2009). The major benefit is the ability to run the absorption process at high temperatures resulting in a more efficient and economical regeneration process. Potassium carbonate is also associated with lower toxicity and better resistance to degradation than commonly seen with amine solvents at high temperatures and in the presence of oxygen and other minor flue gas components such as sulfur oxides (SOx) and nitrogen oxides (NOx) (Bello and Idem, 2005, Strazisar et al., 2003, Supap et al., 2005).

The major challenge associated with potassium carbonate as a solvent is its low rate of reaction resulting in poor mass transfer performance. Rate promoters such as piperazine (Bishnoi and Rochelle, 2000, Cullinane and Rochelle, 2004, Cullinane and Rochelle, 2005a, Cullinane and Rochelle, 2005b), arsenious acid (Kumar and Rao, 1989), amine derivatives (Bosch et al., 1989, Cullinane and Rochelle, 2006, Mahajani and Danckwerts, 1983), carbonic anhydrase (Collett et al., 2011, Guo et al., 2011, Lu et al., 2011), boric acid (Ahmadi et al., 2008, Endo et al., 2011, Ghosh et al., 2009, Guo et al., 2011, Thee et al., 2012a) and amino acids (Aronu et al., 2011, Holst et al., 2009, Kumar et al., 2003, Park et al., 2008, Portugal et al., 2009, Song et al., 2006) have all been suggested to enhance the reaction kinetics. Of these, amino acids have the same functional group as amines, and thus may possess a similar reactivity toward CO2 (Hook, 1997, Kumar et al., 2003). In addition, amino acids have high resistance to oxidative degradation, negligible volatility and minimal toxicity (Hook, 1997, Park et al., 2008). Amino acids have been commercially used in acidic gas removal processes, including glycine in the Giammarco–Vetrocoke process (Portugal et al., 2007, Vaidya et al., 2010), alanine and diethyl or dimethyl glycine in the BASF Alkazid solvent (Kohl and Nielsen, 1997, Vaidya et al., 2010), and precipitating amino acids in the DECAB process (Fernandez and Goetheer, 2011).

This work focuses on the use of aqueous K2CO3 promoted with various primary and secondary deprotonated amino acids (see Table 1). A previous study by van Holst et al. (2009) revealed that these amino acids exhibit a relatively high reaction rate toward CO2 and possess a rather low pKa. The rate of reaction is an important factor in reducing the size and therefore the capital costs of the absorber, while a low pKa is necessary to reduce the energy requirement for CO2 regeneration (Holst et al., 2009).

In the carbonate system, CO2 is chemically consumed by the following overall reaction (NB: in this work all species are aqueous unless otherwise stated) (Thee et al., 2012a):CO2 + CO32− + H2O  2 HCO3where the rate limiting step is:CO2 + OH  HCO3

The hydration of CO2 can also proceed via direct reaction with water, although under industrial CO2 capture conditions where the pH is greater than 9 the contribution of this reaction can be deemed negligible (Astarita et al., 1981). The rate of reaction rCO2 (d[CO2]/dt) is therefore given by:rCO2=kOH[CO2][OH]We have previously measured kOH at elevated ionic strength conditions, which are typical of those encountered in industrial CO2 capture processes, using a wetted-wall column (WWC) (Guo et al., 2011, Thee et al., 2012a). The value of kOH was determined as a function of temperature using the Arrhenius expression kOH [M−1 s−1] = 2.53 × 1011 exp(−4311/T [K]). The vapour–liquid equilibrium (VLE) relationship used to estimate the partial pressure of CO2 above the carbonate solution has also been studied previously and modeled under relevant temperatures (Endo et al., 2011).

Aqueous amino acids such as glycine, sarcosine and proline exist in three states: acidic, zwitterionic and basic or deprotonated (see Table 1). The acidic state and zwitterionic state of the amino acids are much less reactive toward CO2 than the deprotonated state (Aronu et al., 2011, Guo et al., 2013, Holst et al., 2009). In this study, deprotonation of the zwitterionic amino acids is achieved by adding an equimolar amount of a strong base such as potassium hydroxide (KOH) which dissociates completely in water (Eq. (4)).KOH (s)  K+ + OH

The deprotonation of the zwitterions can then be written as:+NH2R1R2COO + OH  NHR1R2COO + H2O

A number of researchers (Aronu et al., 2011, Holst et al., 2009, Kumar et al., 2003, Lim et al., 2012) have shown that the reaction between CO2 and the deprotonated amino acids then proceeds via a zwitterionic carbamate intermediate (Eq. (6)).CO2+NHR1R2COOk1k2OOCNH+R1R2COO

This reaction is followed by the removal of a proton from the zwitterionic carbamate by any base, B, to form a neutral carbamate as shown in Eq. (7). In our system, water (H2O), carbonate ions (CO32−), bicarbonate ions (HCO3) and the deprotonated amino acid (AA) itself can all act as bases.OOCNH+R1R2COO+BkbOOCNR1R2COO+BH+

This two-step reaction mechanism was proposed by Caplow (1968) and used previously by Danckwerts (1979) to explain the reactions of CO2 with alkanolamines. The overall reaction rate for the reaction of CO2 with potassium salts of glycine, sarcosine and proline can then be expressed as:rCO2=k2[CO2][AA]1+(k1/(kb[B]))rCO2=k2[CO2][AA]1+(k1/(kAA[AA]+kH2O[H2O]+kCOs[COs2]+kHCOs[HCOs]))

If the formation of the zwitterionic carbamate (Eq. (6)) is the rate limiting step, then 1  k−1/∑kb[B] and thus, Eq. (8) reduces to a simple second order kinetic relationship as follows:rCO2=k2[CO2][AA]

When the pH of the solution is above 10, the contribution of the CO2 + OH reaction to the overall consumption of carbon dioxide must also be taken into account (Eqs. (2), (3)). Therefore Eq. (9) becomes:rCO2=k2[CO2][AA]+kOH[OH][CO2]

However, if the proton removal from the zwitterionic carbamate is the rate limiting step (Eq. (7)), then the reverse is true (i.e. 1  k−1/∑kb[B]) and thus Eq. (8) becomes:rCO2=k2[CO2][AA](kb[B])k1kobs=k2[AA](kAA[AA]+kH2O[H2O]+kCOs[CO32]+kHCOs[HCO3])k1

In this latter case, the reaction order dependency on the amino acid concentration varies from unity (kAA[AA]kH2O[H2O]+kCO3[CO32]+kHCO3[HCO3]) to two (kAA[AA]kH2O[H2O]+kCO3[CO32]+kHCO3[HCO3]). This phenomenon is commonly observed in the absorption of CO2 into secondary alkanolamines and the aqueous potassium salt of taurine (Danckwerts, 1979, Kumar et al., 2003, Versteeg et al., 1996).

In the presence of potassium carbonate where the pH of the solution is above 10, Eq. (11a) becomes:rCO2=k2[CO2][AA](kb[B])k1+kOH[OH][CO2]

Using a wetted-wall column (WWC), we have studied the reactions between CO2 and the amino acids glycine, sarcosine and proline in their deprotonated form and measured their activation energy under temperatures, pH levels and ionic strengths relevant to industrial CO2 capture systems employing carbonate solutions. These results have implications for the operation of amino acid-promoted potassium carbonate solvent systems for carbon capture processes, and more specifically will allow more accurate design of absorber and regenerator units.

Section snippets

Materials

All chemicals employed in this study were of analytical reagent grade and used as supplied without further purification. Potassium carbonate (≥99%, Thasco Chemical Co. Ltd.) and potassium bicarbonate (≥99%, Sigma–Aldrich, Australia) were weighed to prepare a chemical equivalent of a 30 wt% K2CO3 solution with an initial CO2 loading of 0.15. This equated to a carbonate concentration of 2.4 M and bicarbonate concentration of 0.9 M. Glycine (≥99%), sarcosine (≥98%) and l-proline (NB: in this work l

NMR analysis

NMR spectroscopy was used to determine the ratio of carbamate to free amino acid in a 30 wt% potassium carbonate solution before and after CO2 absorption. Fig. 1 displays a typical 1H NMR spectrum of glycine and sarcosine in a pre-loaded 30 wt% potassium carbonate solution, while Fig. 2 displays a typical 13C NMR spectrum of proline. The chemical shift of the internal references D2O (for 1H NMR spectroscopy) and 1,4-dioxane (for 13C NMR spectrocopy) were assigned as 4.8 ppm and 66.5 ppm,

Comparison with other promoters

A comparison of the partial reaction order, activation energy (Ea), pre-exponential factor (A) and rate constant (kobs) of a range of amino acid promoters (potassium salts of glycine, sarcosine and proline and sodium salt of glycine) and amine-based promoters (MEA and DEA) at 1.0 M and 40 °C is presented in Table 3. The reactivity of all primary amine promoters under these conditions is comparable, with potassium taurate giving slightly better results and sodium glycinate a poorer result. The

Conclusions

A detailed kinetic study on the absorption of CO2 into primary and secondary amino acid promoted potassium carbonate solutions has been presented in this work under conditions similar to industrial CO2 capture plants. Results show that the addition of glycine, sarcosine and proline has significantly accelerated the apparent pseudo-first-order rate constant, and thus, the overall absorption rate of CO2 into potassium carbonate is improved. Glycine is found to exhibit a comparable, if not larger,

Acknowledgments

The authors acknowledge the financial support provided by the Australian Government through its Cooperative Research Center program for this CO2CRC research project. Infrastructure support from the Particulate Fluids Processing Center (PFPC), a Special Research Center of the Australian Research Council is also gratefully acknowledged.

References (70)

  • K. Endo et al.

    The effect of boric acid on the vapour liquid equilibrium of aqueous potassium carbonate

    Fluid Phase Equilibria

    (2011)
  • E.S. Fernandez et al.

    DECAB: process development of a phase change absorption process

    Energy Procedia

    (2011)
  • U.K. Ghosh et al.

    Absorption of carbon dioxide into aqueous potassium carbonate promoted by boric acid

    Energy Procedia

    (2009)
  • H. Hikita et al.

    The kinetics of reactions of carbon dioxide with monoethanolamine diethanolamine and triethanolamine by a rapid mixing method

    The Chemical Engineering Journal

    (1977)
  • J.v. Holst et al.

    Kinetic study of CO2 with various amino acid salts in aqueous solution

    Chemical Engineering Science

    (2009)
  • H. Knuutila et al.

    Density and N2O solubility of sodium and potassium carbonate solutions in the temperature range 25 to 80 °C

    Chemical Engineering Science

    (2010)
  • A.L. Kohl et al.

    Alkanolamines for hydrogen sulfide and carbon dioxide removal, gas purification

    (1997)
  • N. Kumar et al.

    Design of a packed column for absorption of carbon dioxide in hot K2CO3 solution promoted by arsenious acid

    Gas Separation & Purification

    (1989)
  • Y. Lu et al.

    Development of a carbonate absorption-based process for post-combustion CO2 capture: the role of biocatalyst to promote CO2 absorption rate

    Energy Procedia

    (2011)
  • V.V. Mahajani et al.

    The stripping of CO2 from amine-promoted potash solutions at 100 °C

    Chemical Engineering Science

    (1983)
  • M.A. Pacheco

    CO2 absorption into aqueous mixtures of diglycolamine® and methyldiethanolamine

    Chemical Engineering Science

    (2000)
  • S. Paul et al.

    Kinetics of absorption of carbon dioxide into aqueous potassium salt of proline

    International Journal of Greenhouse Gas Control

    (2012)
  • A.F. Portugal et al.

    Characterization of potassium glycinate for carbon dioxide absorption purposes

    Chemical Engineering Science

    (2007)
  • A.F. Portugal et al.

    Solubility of carbon dioxide in aqueous solutions of amino acid salts

    Chemical Engineering Science

    (2009)
  • T. Pröll et al.

    Acid gas absorption in trickle flow columns—modelling of the residence time distribution of a pilot plant

    Chemical Engineering and Processing: Process Intensification

    (2007)
  • K. Smith et al.

    Recent developments in solvent absorption technologies at the CO2CRC in Australia

    Energy Procedia

    (2009)
  • H.-J. Song et al.

    Solubilities of carbon dioxide in aqueous solutions of sodium glycinate

    Fluid Phase Equilibria

    (2006)
  • H. Thee et al.

    Carbon dioxide absorption into unpromoted and borate-catalyzed potassium carbonate solutions

    Chemical Engineering Journal

    (2012)
  • H. Thee et al.

    A kinetic and process modeling study of CO2 capture with MEA-promoted potassium carbonate solutions

    Chemical Engineering Journal

    (2012)
  • G.F. Versteeg et al.

    The effect of diffusivity on gas–liquid mass transfer in stirred vessels. Experiments at atmospheric and elevated pressures

    Chemical Engineering Science

    (1987)
  • U.E. Aronu et al.

    Kinetics of carbon dioxide absorption into aqueous amino acid salt: potassium salt of sarcosine solution

    Industrial & Engineering Chemistry Research

    (2011)
  • G. Astarita et al.

    Gas treating with chemical solvents

    (1983)
  • A. Bello et al.

    Pathways for the formation of products of the oxidative degradation of CO2-loaded concentrated aqueous monoethanolamine solutions during CO2 absorption from flue gases

    Industrial & Engineering Chemistry Research

    (2005)
  • H.E. Benson et al.

    CO2 absorption: employing hot potassium carbonate solutions

    Chemical Engineering Progress

    (1954)
  • M. Caplow

    Kinetics of carbamate formation and breakdown

    Journal of the American Chemical Society

    (1968)
  • Cited by (72)

    • Production of cooling water by Ti<inf>3</inf>C<inf>2</inf>T<inf>x</inf> MXene interlayered forward osmosis membranes for post-combustion CO<inf>2</inf> capture system

      2022, Journal of Membrane Science
      Citation Excerpt :

      In addition, there are only a few studies investigating the feasibility of using commercial TFC-FO membranes and single conventional CO2 absorbents (e.g. monoethanolamine, glycine, and sodium glycinate) as draw solutions [3,4,15]. Recently, amino acid/salts (AAS) such as taurine/KOH have been considered as alternative CO2 absorption liquids, because of their advantages including lower energy consumption [16–18], higher resistance to oxidative degradation, as well as the negligible volatility and minimal toxicity compared to alkanolamines [17,19–22]. To further understand the feasibility of applying FO membranes in a post-combustion CO2 capture system, it is critical to evaluate the performance of FO membranes using AAS as draw solutions.

    • CO<inf>2</inf>-selective membranes containing amino acid salts for CO<inf>2</inf>/N<inf>2</inf> separation

      2021, Journal of Membrane Science
      Citation Excerpt :

      This can be explained by the fact that sarcosinate has a methyl group attached to the amino group to become a hindered amine, which provides a greater CO2 loading capacity [8]. The membrane with PZEA-Gly as the mobile carrier [8] showed a lower permeance compared to those with PZEA-Sar or PZEA-Pro, which could be explained by the lower reactivity of glycinate compared with prolinate and sarcosinate [35]. The cation effect is also demonstrated in Table 2 by a series of AAS carriers containing glycinate but with different cations.

    View all citing articles on Scopus
    View full text