Effect of Arginine on Carbon Dioxide Capture by Potassium Carbonate

Article pubs.acs.org/EF

Effect of Arginine on Carbon Dioxide Capture by Potassium Carbonate Solution Shufeng Shen,*,†,‡ Xiaoxia Feng,†,‡ and Shaofeng Ren†,‡ †

School of Chemical and Pharmaceutical Engineering and ‡Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China ABSTRACT: Preliminary screening experiments of carbon dioxide (CO2) absorption into aqueous 35 wt % equivalent potassium carbonate (K2CO3) solution with different additives were performed in a wetted-wall column at 322 K. Arginine was found as an effective promoter to enhance the absorption rate in aqueous K2CO3 solution. The effect of arginine concentration and CO2 loading on the absorption rate into carbonate solutions is discussed. The vapor−liquid equilibrium of 35 wt % equivalent K2CO3 with 0−5 wt % arginine was obtained in the temperature range from 323 to 343 K using inert-gas stripping method. Desorption curves of the unpromoted and arginine-promoted carbonate solutions are also discussed.

1. INTRODUCTION Carbon dioxide (CO2) is one of the main greenhouse gases causing climate abnormality. CO2 emissions from fossil-fired power generation are a major contributor to climate change. It is widely accepted that postcombustion carbon capture and storage (CCS) is one of the promising routes to reduce CO2 emissions in the near-term. Over the past years, absorption with different chemical solvents has been proposed and carried out in pilot demonstration as a feasible option to a large-scale implementation.1,2 Absorption happens by contacting solvent countercurrent to gases in an absorber column to remove CO2. Then the CO2-rich solvent is subsequently fed to a regenerator to strip out CO2 for storage or utilization, and the CO2-lean solvent is sent back to the absorber for reuse. The use of aqueous potassium carbonate (K2CO3) solvent has gained widespread attention among chemical absorbents for many years due to low absorption heat, low cost, less toxicity and solvent losses, and no thermal and oxidative degradation, without formation of heat-stable salts.2−5 A key limitation associated with using K2CO3 is the slow reaction rate of absorption.2,5 Promoters are often used to improve the CO2 mass-transfer rates. Promoters, such as arsenites, piperazine, and ethanolamine, have been proved to enhance greatly the absorption rate in carbonate solutions.3−9 However, these promoters are known to be toxic or volatile, prone to oxidative and thermal degradation, and hazardous to the environment.2,10 In recent years, amino acid salts have been extensively studied for CO2 absorption.11−21 Due to having the identical functional group as that of alkanolamines, amino acid salts are expected to react with CO2 in a similar manner. Moreover, their ionic characteristics in alkaline solutions will have some advantages such as low volatility and resistance to degradation.15−17 Recently, preliminary studies have been carried out for CO2 absorption by arginine-promoted carbonate solvents in our previous work.22 Arginine is one of the basic amino acids, and it contains a primary amino group in addition to a guanidinium group in its side chain. The primary amino group in arginine can be expected to react quickly with CO2 as many primary amines (e.g., monoethanolamine (MEA)) do. The strong basic chain would also be favorable for carbamate formation and facilitate CO2 absorption. However, the effect of © 2013 American Chemical Society

arginine on vapor−liquid equilibrium in carbonate solutions and the desorption process is not clear and needs to be revealed. In this work, the absorption and desorption rates of CO2 in aqueous 35 wt % equivalent K2CO3 solution with arginine are discussed. The vapor−liquid equilibrium of 35 wt % equivalent K2CO3 with 0−5 wt % arginine was investigated from 323 to 343 K using an inert-gas stripping method. Results will be helpful for calculation of arginine rate constants from kinetic data and for design of a capture process.

2. ABSORPTION SYSTEM WITH CHEMICAL REACTION Using aqueous K2CO3 for CO2 absorption, reactions can be described as follows:4,17,22 k OH−

CO2 + OH− ←→ ⎯ HCO3− k H2O

CO2 + H 2O ←→ ⎯ HCO3− + H+

(1) (2)

In basic solutions (pH > 8), the reaction of CO2 + H2O forming HCO3− can be neglected, and the CO2 reaction with OH− is the predominant and the rate-limiting reaction. Reaction of CO2 with amino acids can be described on the basis of zwitterionic mechanism.23,24 When adding arginine into carbonate solution, apart from H2O and OH−, CO2 also reacts with the alkaline salt of arginine, KArg, forming a zwitterion as an intermediate that is subsequently deprotonated by bases, B (such as KR′NH2, the guanidinium group in the side chain of arginine, CO32−, OH−, or H2O) present in solution. CO2 + H 2N−CHR′−COO−K+ k 2,Arg −

XooooooY COO+H 2N−CHR′−COO−K+ k −2,Arg

(3)

−

COO+H 2N−CHR′−COO−K+ + B kB

→ −COOHN−CHR′−COO−K+ + BH+

(4)

Received: May 31, 2013 Revised: September 10, 2013 Published: September 10, 2013 6010

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Figure 1. Schematic diagram of the wetted wall experimental apparatus. calibration of CO2 gas analyzers with a resolution of 0.01% (H3860/ H3860B, 0−20%, Beijing Huahe Tiandi Propriety Ltd.) 3.2. Experimental Setup and Procedure. Absorption kinetic experiments were carried out in a wetted-wall column (WWC) shown in Figure 1. The WWC allows countercurrent contact between a flowing gas stream and a falling thin liquid film with a measurable surface area of column for mass transfer, and thus, a fast relative comparison of absorption rate can be obtained. The unpromoted/promoted prepared carbonate solutions were pumped through the WWC at the set temperature until a homogeneous liquid film was obtained. The solvents were operated in once-through or circulated mode. A defined gas mixture by mass flow controllers with a full-scale uncertainty of ±0.2% (D07-7B, 0−10 L/min, Sevenstar Huachuang) passed through a water saturator and then entered into the bottom of the reaction column and flowed upward, thereby allowing absorption of CO2 into the liquid film. The partial pressure of CO2 in entering the gas mixture was maintained at about 15 kPa in these experiments. CO2 concentration in the outlet gas stream was recorded by online H3860 analyzer after a two-step cooling. Physicochemical data for mass transfer were obtained from our previous work.22 Vapor−liquid equilibrium data for the 35 wt % equivalent carbonate systems with different CO2 absorption amounts were measured from 323 to 343 K and at near-atmospheric pressure using a specific vapor− liquid equilibrium apparatus (see Figure 2). Loaded solvent samples, 140 cm3 each, were placed in three successive vessels submerged in a thermally regulated water bath (±0.1 °C). N2 was first flushed through the devices to purge out the gases within the system, and then the gas phase circulated and analyzed online until steady values of CO2 concentration were recorded by the H3860 gas analyzer. Carbonate salts and arginine in the solutions are ionic; thus, their partial pressures are considered negligible. The equilibrium partial pressure of CO2, P*CO2, in the gas phase was calculated as follows:

When working with aqueous-promoted K2CO3 solution, it is necessary to take both CO2 hydration with OH− and zwitterionic reaction with arginine into account in the interpretation of the absorption rate experiments.22 The absorption rate of CO2 into a CO2−lean K2CO3 and arginine solution is described by the following equation under fast pseudo-first-order simplification:24,25 NCO2 =

EkL * )= (PCO2,b − PCO 2 HCO2

kovDCO2 HCO2

* ) (PCO2,b − PCO 2 (5)

where NCO2 is the molar flow of CO2 entering the liquid solution; kL is the physical mass-transfer coefficient;26 PCO2 is the CO2 partial pressure in the gas bulk phase, which can be represented by the log mean pressure (Pln) at the top and bottom of the wetted-wall column;6,22,24 PCO * 2 is the equilibrium partial pressure; HCO2 is the Henry constant of CO2 in solution; E is the enhancement factor, which represents the ratio between the rate of absorption in the presence of the chemical reaction and the physical rate of absorption; kov is the overall pseudo-firstorder reaction rate constant; and DCO2 is the diffusion coefficient of CO2 in solution, which can be obtained from our previous work.22

3. EXPERIMENTAL SECTION 3.1. Materials. Arginine (Arg, ≥98% purity) and histidine (His, ≥99% purity) were purchased from Aladdin Reagent, Shanghai, China. Potassium carbonate, potassium bicarbonate, and other chemicals were of analytical reagent grade and were used to prepare a chemical equivalent of 35 wt % K2CO3 solution with known CO2 loading. A standard solution of 0.05 M sodium hydroxide was purchased from Tianjin Yongda Reagent, Tianjin, China. CO2 and N2, with a given purity of 99.99%, were obtained commercially and used for the

* = (P − (PHT O − PHIRO))y IR PCO CO 2 2 2

2

(6)

where yIR CO2 is the mole fraction of CO2 measured by the gas analyzer; T P and PH2O are the total pressure and the vapor pressure of water in the equilibrium vessel at the equilibrium temperature, respectively; and 6011

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Figure 2. Vapor−liquid equilibrium apparatus. PHIR2O is the vapor pressure of water at the temperature of the buffer tank. CO2 loading in liquid phase was obtained by taking about 2 cm3 of sample from vessel 3 for analysis by titration with 0.4 M sulfuric acid. The loading of carbonate solutions with arginine was verified by the Chittick CO2 apparatus22,27 and is defined as the moles of CO2 per mole of potassium ions, [CO2/K+]. The desorption experiment apparatus is shown schematically in Figure 3. When heating and continuous bubbling by water-saturated N2,

4. RESULTS AND DISCUSSION 4.1. Absorption Performance by Promoted Carbonate Solvent. Absorption flux, NCO2, for various systems was investigated in WWC at 322 K. Two basic amino acids, arginine and histidine, were used as promoters in 35 wt % K2CO3, and the absorbent solutions were operated in a once-through mode. The pH of the absorbents was kept constant at 13.0 by KOH in order to exclude the contribution of kOH−[OH−] to kov.22 Gas and liquid flow rates were 252 and 2.88 L h−1, respectively. The CO2 concentration in the inlet gas stream was 17.2% (v/v). Under these operating conditions, the fraction of mass-transfer resistance on the gas side was lower than 3%. The results of experiments compared with unpromoted 35 wt % K2CO3 solution are shown in Figure 4. Enhancement factors (E) are also compared for these absorption systems. It can be seen that CO2 absorption performance differs greatly for the investigated systems. For the unpromoted 35 wt % K2CO3 solution (PC35), NCO2 and E are 1.306 × 10−3 mol m−2 s−1 and 193, respectively. In the promoted carbonate solutions, both arginine and histidine can greatly improve the absorption rate. With the addition of 1 wt % arginine, NCO2 can accelerate by 43.5% over the PC35 solution. Moreover, with the increase of arginine concentration, NCO2 increases markedly. For the PC35 promoted by 5 wt % arginine, NCO2 and E can reach 3.00 × 10−3 mol m−2 s−1 and 454, respectively. Histidine also shows excellent performance as a promising promoter. For the PC35 promoted by 1.2 wt % histidine, E can reach 490, which is highest in the investigated systems. The possible reason is due to the effect of their basic side chains (R groups). The two investigated amino acids both contain a primarily amino group which has been shown to have fast reaction rate with CO2.5,6 However, the guanidinium group (pKa = 12.5) in arginine and the imidazole group (pKa = 6.0) in histidine28 have different charged states in

Figure 3. Desorption experiment setup. potassium bicarbonate and carbamate in CO2-rich solvent decomposed and released CO2 with N2 discharged into the atmosphere. The liquid solvent was heated by oil bath with a temperature controlling magnetic stirrer (DF-101S, ±0.1 °C). 6012

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Figure 4. Effect of promoters on CO2 absorption performance in 35 wt % K2CO3 solutions at 322 K: PC35, 35 wt % equivalent K2CO3; PC35 + Arg1, 1 wt % arginine added; PC35 + His1.2, 1.2 wt % histidine added at 325.5 K; PC35 + Arg3, 3 wt % arginine added; PC35 + Arg5, 5 wt % arginine added.

the working solution (pH 13.0). The uncharged imidazole group in histidine acting as a secondary amine can also react with CO2 and then results in fast absorption rate. However, its solubility in water is lower than that of arginine. Arginine was considered for further investigation in this study. The effect of arginine concentration in 35% equivalent K2CO3 solution on CO2 absorption behaviors along with absorption time is shown in Figure 5. The absorbent solutions with arginine from 0 to 5 wt % were pumped from tank 1 and operated in a circulated mode at 321.5 K. With the increasing absorption time, CO2 loading increases and the pH of the solutions decreases, which results in a low absorption flux. It can be seen from Figure 5a that the absorption flux increases with the increasing arginine concentration and decreases against the absorption time under the investigated range. At the beginning of absorption, CO2 loading for different promoted absorbents is low and the arginine is active as a promoter; thus, the absorption rates differ greatly for different arginine concentration in carbonate solutions. The addition of 5 wt % arginine can promote the uptake of CO2 by a factor of 2.0−3.0. With a further increasing absorption time up to 3.0 h, the fraction of the active arginine decreases greatly and the contribution of arginine to the absorption rate is small; then, NCO2 for all solutions decreases to about 1.2 × 10−3 mol m−2 s−1. The effect of CO2 loading on NCO2 is shown in Figure 5b. When CO2 loading increases with the elapsed absorption time, NCO2 decreases greatly for all investigated carbonate solutions. These absorption behaviors could be explained from chemical reactions in section 2 and an absorption process. Both arginine and hydroxyl ion concentrations have an effect on the absorption rate. At the beginning, the high concentrations of hydroxyl ion and arginine will result in fast absorption rates. Arginine contains a primary amino group, so fast reaction with CO2 is expected. Therefore, the increase of arginine concentration will result in the marked promotion of the reaction rate. With increasing cycled operation time, the concentration of bicarbonate ion increases and the concentrations of hydroxyl ion and active arginine component decrease, so the absorption fluxes gradually drop with the increasing CO2 loading. 4.2. Effect of Arginine on Vapor−Liquid Equilibrium for 35% Equivalent K2CO3 Solutions. Equilibrium CO2 partial

Figure 5. Absorption behaviors of CO2 in 35 wt % equivalent K2CO3 with different arginine concentrations at 321.5 K: (a) absorption rate of CO2 along with absorption time and (b) CO2 loading of solutions.

pressures for the different absorbent solutions are shown in Table 1. The experimental results for a 35 wt % K2CO3 solution at 343 K from this work are presented alongside data from Tosh et al.29 and Endo et al.30 and the default ASPEN plus E-NRTL values in Figure 6. The effect of arginine and temperature on equilibrium CO2 partial pressures obtained in this study has also been presented in Figures 7 and 8, respectively. Comparison of experimental results show that the method applied in this study can reasonably replicate results from the static rig of Tosh and the dynamic rig of Endo considering the difference in concentration of K2CO3 solution, while ASPEN simulation results for 35 wt % K2CO3 solution can closely match the data at low CO2 loadings obtained in this study. It can be found from Table 1 that equilibrium CO2 partial pressures for all solutions tested are below 1.50 kPa at the temperature range investigated, which is far lower than the CO2 partial pressure in flue gases (ca. 12−15 kPa) from a postcombustion coal-fired power station. At the relatively low loadings (α < 0.15), partial pressures are not more than 0.35 kPa and then the driving force (PCO2,b − PCO * 2) has little effect on the absorption rate. The chemical reactions in liquid have a great effect on the absorption rate (eq 5). This finding can be useful for controlling the components of cycled absorbents as well as the desorption process. 6013

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Table 1. Vapor−Liquid Equilibrium Data for 35 wt % Equivalent K2CO3 with 0−5 wt % Arginine at 323−343 K P*CO2 a (kPa) absorbent solventb PC35

PC35 + 1 wt % Arg

PC35 + 3 wt % Arg

PC35 + 5 wt % Arg

CO2 loading

P (kPa)

323 K

333 K

343 K

0.10 0.15 0.20 0.25 0.30 0.35 0.10 0.15 0.20 0.25 0.30 0.35 0.10 0.15 0.20 0.25 0.30 0.35 0.1 0.15 0.20 0.25 0.30 0.35

103.33 103.33 103.53 103.53 103.43 103.33 103.43 103.33 103.33 103.33 103.53 103.23 103.23 103.43 103.43 103.33 103.33 103.33 103.43 103.43 103.33 103.43 103.43 103.43

0.074 0.188 0.181 0.428 0.559 0.748 0.128 0.172 0.247 0.410 0.563 0.697 0.077 0.128 0.213 0.430 0.630 0.887 0.051 0.099 0.144 0.379 0.537 0.825

0.109 0.267 0.434 0.653 0.786 1.068 0.175 0.223 0.311 0.608 0.815 0.930 0.084 0.132 0.360 0.604 0.894 1.199 0.072 0.115 0.209 0.497 0.758 1.133

0.147 0.327 0.603 0.803 1.077 1.384 0.124 0.268 0.391 0.765 1.043 1.186 0.110 0.176 0.465 0.755 1.118 1.456 0.074 0.143 0.283 0.617 0.968 1.379

P*CO2 = equilibrium partial pressure of CO2. bPC35: 35 wt % equivalent K2CO3; Arg: arginine, P, total pressure in the equilibrium vessel. a

Figure 7. Partial pressure of CO2 over a 35 wt % equivalent K2CO3 solution (a) at temperature from 323 to 343 K and (b) with arginine addition.

pressure, the larger is the driving force for CO2 absorption. In the investigated range, CO2 partial pressure has a little bit of decrease compared to that for unpromoted carbonate solution. This phenomenon suggests that adding arginine can increase the solubility of CO2 in carbonate solution and is favorable for CO2 capture. However, at high CO2 loading, e.g., α > 0.3, the effect of arginine on partial pressure is low; in this case large amounts of carbonate are converted to bicarbonate, and carbamate has also been formed from active promoters. The main chemical equilibria in the liquid phase are considered as above-mentioned in eqs 1−3. Addition of arginine has less effect on the absorption rate at the conditions with high loading, which is consistent with the finding from Figure 5b. As shown in Figure 8a,b, the effect of temperature on CO2 partial pressure for both 3 and 5 wt % arginine-promoted solutions is similar. At CO2 loading lower than 0.15, equilibrium CO2 partial pressure hardly changes with the variation of temperature from 323 to 343 K. However, at CO2 loading higher than 0.3, CO2 partial pressure increases along with the rising temperature. 4.3. Desorption. Desorption kinetics was carried out from 365 to 376 K using the apparatus shown in Figure 3. Unpromoted and 3 wt % arginine-promoted 35 wt % equivalent K2CO3 with the same loading (α = 0.5) were used. The flow rate of stripping gas N2 was kept constant at 16.8 L h−1.

Figure 6. Partial pressure of CO2 above a K2CO3 solution as function of loading at 343 K. PC30, PC35, and PC40 represent 30, 35, and 40 wt % equivalent K2CO3 solution, respectively.

As can be seen from Figure 7a, CO2 partial pressures increase with the increasing loading and temperature of solutions. Equilibrium temperature has little effect on CO2 partial pressure at low CO2 loading. With increasing the loading of the solution, about 0.6 kPa difference is observed when α is over 0.3. This tendency is consistent with comparable studies from Endo et al.30 It can be found that CO2 partial pressures decrease a little when arginine is added into 35 wt % equivalent K2CO3 solutions, as shown in Figure 7b. The lower the equilibrium CO2 partial 6014

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Figure 9. Desorption behaviors of unpromoted and promoted carbonate solutions against elapsed time.

5. CONCLUSION The effect of arginine on absorption and desorption kinetics and vapor−liquid equilibrium in aqueous 35 wt % equivalent K2CO3 was studied. Arginine was found as an effective rate promoter to improve the CO2 capture efficiency. The addition of 5 wt % arginine can promote the uptake of CO2 by a factor of 2.0−3.0. The CO2 partial pressures for 35 wt % equivalent K2CO3 with 0−5 wt % arginine was determined. CO2 partial pressures increase with the increasing temperature and loading of solution. The addition of arginine can lower the CO2 partial pressure, which is favorable for the CO2 absorption. Different desorption curves were observed for unpromoted and 3 wt % argininepromoted 35 wt % equivalent K2CO3. The addition of arginine can improve the desorption rate. With advantages using aqueous K2CO3 solution, the arginine/K2CO3 solvent could potentially reduce CO2 capture costs.

Figure 8. Partial pressure of CO2 over a 35 wt % equivalent K2CO3 solution with (a) 3 wt % arginine and (b) 5 wt % arginine at temperature from 323 to 343 K.

■

The temperature of the water saturator and the outlet of the condenser was kept at 303 K. The CO2 loading of the solution against desorption elapsed time has been shown in Figure 9. The time was recorded when the CO2 was found in exit gases. It can be seen from the desorption curves that different desorption behaviors were found for unpromoted and 3 wt % arginine-promoted 35 wt % equivalent K2CO3 along with different regeneration temperatures. At elevated temperature, the bicarbonate in K2CO3 solution is converted to carbonate and the CO2 is released, which results in the decreasing loading. The slope of desorption curves can represent the rate of released CO2. With the increase of the temperature from 365 to 376 K, the desorption rate is significantly improved for the unpromoted K2CO3. The addition of arginine can also improve the desorption rate. At 365 K, for 3 wt % arginine addition, the desorption rate of CO2 was significantly higher than that for unpromoted K2CO3. Its desorption behavior is similar to that of unpromoted K2CO3 at 376 K. However, small changes can be observed for promoted solution at 365−376 K. Moreover, the desorption rate of CO2 decreases markedly when the CO2 loading is lower than 0.20. The slope of the desorption curves become small. In the first 3 h, the CO2 loading decreased from 0.5 to 0.25. However, the loading decreases to about 0.15 at 10 h elapsed time. It is very helpful in designing the absorption and desorption process.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 311 88632183. Fax: +86 311 88632183. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 21206029), Hebei Provincial Natural Science Foundation of China (Grant No. B2012208022), and Hebei Provincial Scientific Research Foundation for the Returned Overseas Chinese Scholars (2013− 2015). We also gratefully thank Dr. Ujjal Ghosh from Curtin University Malaysia and Dr. Xianhui Wang from the National Institute of Clean-and-Low-Carbon Energy Beijing for support of default Aspen data.

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NOMENCLATURE D = diffusion coefficient, m2 s−1 E = enhancement factor, dimensionless H = Henry coefficient, kPa m3 kmol−1 k2,Arg = second-order rate constant, L mol−1 s−1 dx.doi.org/10.1021/ef4014289 | Energy Fuels 2013, 27, 6010−6016

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KG = overall gas-phase mass-transfer coefficient, mol m−2 s−1 Pa−1 kL = liquid-phase physical mass-transfer coefficient, m s−1 kOH− = reaction rate constant with hydroxide ion, L mol−1 s−1 kov = overall kinetic constant, s−1 NCO2 = CO2 absorption flow, mol m−2 s−1 P = total pressure in the equilibrium vessel, kPa PCO2 = CO2 partial pressure in gas bulk phase, kPa P*CO2 = equilibrium partial pressure, kPa PHIR2O = vapor pressure of water at the temperature of the buffer tank, kPa PHT 2O = vapor pressure of water in the equilibrium vessel, kPa T = temperature, K wt % = weight percentage of K2CO3 solutions yIR CO2 = mole fraction of CO2 measured by gas analyzer, dimensionless

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Greek Letters

α = CO2 loading, mol CO2 per mol of potassium ions Subscripts

CO2 = carbon dioxide H2O = water

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