Energy Procedia 51 ( 2014 ) 191 – 196

Available online at www.sciencedirect.com

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1876-6102 © 2013 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of SINTEF Energi ASdoi: 10.1016/j.egypro.2014.07.022

7th Trondheim CCS Conference, TCCS-7, June 5-6 2013, Trondheim, Norway

Solubility of carbon dioxide in aqueous solution of potassium sarcosine from 353 to 393K

Sholeh Ma'muna,b,0

* aDepartment of Chemical and Green Process Engineering, Surya University,

Jl. Boulevard Gading Serpong Blok O/1, Summarecon Serpong, Tangerang, 15810, Banten, Indonesia bSINTEF Materials and Chemistry, N-7465 Trondheim, Norway

Abstract

The equilibrium solubility of CO2 in the aqueous solution of 5m' potassium salt of sarcosine (KSar) was measured over a temperature range of 353 − 393K with CO2 partial pressures ranging from 0.003 − 41.9 kPa and CO2 loadings from 0.022 − 0.538. Two vapor-liquid-solid equilibrium (VLSE) apparatuses, LabMax and the rocking equilibrium unit, were used. The obtained VLSE data complement the previous ones, so that the CO2 solubility in the 5m' KSar solution is well represented. The VLSE data are also used to determine the solvent cyclic capacity and the heat of absorption. © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of SINTEF Energi AS.

Keywords: Post-combustion capture; carbon dioxide; equilibrium; amino acid salt; precipitating system

1. Introduction

Carbon dioxide (CO2), a major contributor for the global climate change, is produced in large quantities by fossil-fuelled power plants, industrial processes (e.g. iron and steel, non-metallic mineral, and chemical & petrochemical productions), natural gas processing, and transportation. In 2005, 26.3 Gt of CO2 was emitted globally. The global emission of CO2 has continually increased over the years until it reached 32 Gt in 2010. Many scenarios to reduce the greenhouse-gas emissions have therefore been proposed. One of the scenarios proposed by World Energy Outlook 2012 (WEO-2012) known as 450 scenario targets to maintain the atmospheric concentration of greenhouse gases at 450 ppm CO2-eq by selecting a plausible energy pathway in order to meet the goal of limiting the global increase in average temperature to 2 °C in the long term, compared to pre-industrial levels. The scenario deployed CCS much more widely in comparison to the New Policies Scenario [1].

Carbon dioxide capture using amine-based absorption is currently the lead contending technology and seems to remain competitive in the future [2]. Research on solvent development has been conducted to find new, faster, and more energy-efficient absorbents by testing alternative alkanolamines and their blends, sodium carbonate solutions, chilled ammonia, and amino acid salts [3-12].

* Corresponding author. E-mail address: sholeh.mamun@surya.ac.id

© 2013 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of SINTEF Energi AS

192 Sholeh Ma’mun / Energy Procedia 51 ( 2014 ) 191 – 196

In recent years, precipitating system has been given more attention in CO2 capture despite that precipitation is normally undesirable in industrial processes. However, if the precipitant contains CO2 (i.e., carbamate, carbonate or bicarbonate), the driving force may be maintained at increased loading, so that higher solvent loadings may be reached resulting in lower energy consumption for the solvent regeneration [13].

Amino acid salts are considered as good candidates due to their precipitation potential during CO2 capture process. Previous studies by Aronu et al. [14] and Ma'mun and Kim [15] showed that the potassium salt of sarcosine (KSar), an amino acid salt, with concentration of 5m' (mol/kg-solution) was considered as a promising candidate. This system offers better performances in comparison to those of MEA.

The CO2 solubility measurements in the 5m' KSar solution at temperatures ranging from 313 to 393K had been performed by Ma'mun and Kim [15]. However, some data points at high temperatures are still required to complete the vapor-liquid-solid equlibrium (VLSE) data for the CO2-KSar-H2O system. This present work therefore contributes to provide CO2 solubility in the 5m' KSar solution at temperature range of 353 − 393K.

2. Materials and methods

2.1. Materials

The high-grade CO2 (99.99 vol%) and N2 (99.999 vol%) gases were obtained from Yara-Praxair. Sarcosine (CH3NHCH2COOH) and potassium hydroxide (KOH) were obtained from Sigma-Aldrich and Merck Chemicals with purities of min. 98%, and min. 85%, respectively. Potassium salt of sarcosine (KSar) with concentration of 5 mol/kg-solution was prepared by neutralizing 5 moles of sarcosine with an equimolar amount of KOH in a total solution weight of 1 kg. The actual concentration of KSar was determined by titration.

2.2. VLSE measurements

The vapor pressure of 5m' KSar solution is lower than that of 30wt% MEA solution. At 393K, the vapor pressure of 5m’ KSar solution is 75 kPa, while that of 30wt% MEA is about 178 kPa. Since the vapor pressure of 5m’ KSar solution is below the atmospheric pressure at 393K, so that the VLSE measurements can be performed in LabMax® reactor (see Figure 1). The LabMax® consists of a 1−L mechanically agitated-jacketed glass reactor equipped with temperature, pressure, pH, PVM, and FBRM probes. This setup is also equipped with an external thermocouple, two RMS multimeters, a Bühler gas pump, a 2-channel Rosemount BINOS 100 IR CO2 analyzer, and a Bronkhorst® Hi-Tec N2/CO2 mass flow controller. Detail description and operating procedure of this setup can be found in ref. [15].

Figure 1. Experimental setup for VLSE measurements [15]

Flow controller

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Sholeh Ma’mun / Energy Procedia 51 ( 2014 ) 191 – 196 193

The CO2 concentration in the liquid phase was determined by analyzing a liquid sample from the reactor using a GC Apollo 9000 Combustion TOC Analyzer, whereas the total alkalinity concentration measurement was performed by an automatic titrator Metrohm 905 Titrando. The CO2 partial pressure was calculated by Eq. (1).

(1)

where

2

IRCO solution, , and oy P P denote the CO2 concentration in the gas phase through the IR analyzer, total pressure of

the system, and vapor pressure of the solution, respectively. The vapor pressures of 5m' KSar solution were measured by Ma'mun and Kim [15] and can be formulated in the Antoine equation type as follows:

(2)

In this work, one data point at 393K was measured by using a rocking equilibrium unit. The apparatus (see

Figure 2) is designed to measure VLE/VLSE experiments for high temperatures up to 423K and for high CO2 partial pressures up to 800 kPa. The setup consists of two connected autoclaves (1000 and 200 cm3) which rotate 180° with 2 rpm. The operating procedure of this setup can be found in Ma'mun et al. [16]. The CO2 concentration in the liquid phase and the total alkalinity concentration of the solvent were determined by GC and titration, whereas the CO2 partial pressure is calculated as follows:

(3)

Figure 2. Experimental set-up for VLSE measurement at 393K [16]

3. Results and discussion

3.1. Vapor-liquid-solid equilibria

Additional VLSE data for the CO2 solubility in the aqueous solution of 5m' KSar were conducted over a temperature range of 353 − 393K by using LabMax® and the rocking unit. Data points collected in this work will complement the previous VLSE data for the CO2-KSar-H2O system with the concentration of 5 mol/kg-solution. SOFT modeling approach which is an equilibrium model used for systems with limited data was also used to predict the VLSE data of both of this work and Ma'mun and Kim [15]. The model gives equilibrium partial pressure of CO2 over a solvent system based on a functional fitting towards experimental data as function of CO2 loading and temperature. In this type of model, activity coefficients are neither needed nor calculated [15]. Figure 3a shows the experimental VLSE data together with the model predictions at different temperatures. The SOFT model demonstrates a reasonable representation of the experimental data used in the regression analysis. The average absolute relative deviation (AARD) between the measured CO2 partial pressures and those calculated from the model is 18.5%. The parity plot of both the measured and calculated CO2 partial pressures shows insignificant deviation (see Figure 3b).

Oil Bath

Thermostated box

Heater + Fan

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CO2

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Autoclave

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TIC

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Data AcquisitionSystem

2 2

IRLabMax CO solution solution, IR

o oCOp y P P P

solution3457.6kPa exp 14.842K 64.53

oPT

2 Rocking unit solutiono

COp P P

194 Sholeh Ma’mun / Energy Procedia 51 ( 2014 ) 191 – 196

Figure 3a shows that the model does not predict well in the region where the precipitation starts (CO2 loading = 0.52 at 313 and 333K). The predicted values are slightly lower than the experimental data, resulting in slightly higher slope than expected. This makes the model predict higher the CO2 partial pressures in the precipitation region (CO2 loadings > 0.70). Therefore, implementation of a thermodynamic model framework could improve the result. Previous work by Aronu et al. [17] showed that the extended UNIQUAQ framework gave a very good representation of the CO2 solubility in the 3.5m' KSar solution. This model is going to be implemented for the CO2-KSar-H2O system with precipitation in which a set of equilibrium reactions for the precipitants is taken into account.

Figure 3. Equilibrium solubility of CO2 in the aqueous solution of 5m' KSar. (a) Experimental data measured in this work: ▲, 353K; +, 373K; ♦, 393K. Experimental data from ref. [15]: ○, 313K; □, 333K; ∆, 353K; ◊, 393K. Curves were calculated by the SOFT model at different temperatures. (b) Parity plot between the experimental CO2 partial pressures and the model CO2 partial pressures.

3.2. Cyclic capacity

Cyclic capacity of a CO2 capture process is defined as the total CO2 that can be absorbed from the process. This can be determined by the difference between rich and lean loadings. A method to determine the cyclic capacity can be found in Ma'mun et al. [3]. The method assumes that equilibrium is attained in both absorption and stripping steps. The cyclic capacity of 5m' KSar solution was previously discussed by Ma'mun and Kim [15] with limited experimental data. The present work provides additional VLSE data at temperatures of 353, 373, and 393K, respectively, so that the new cyclic capacity will give a better result. Figure 3a shows a complete set of VLSE data which are well distributed from 313 to 393K.

Figure 4. Determination of cyclic capacity of 5M MEA and that of 5m' KSar solutions. MEA: ■, 313K [15]; □, 313K [18]; ◊, 393K [16]; +, 393K [18]; ×, 393K [19]. KSar: ○, 313K [15]; ∆, 393K [15]; ▲, 393K (This work).

Figure 4 shows the VLE and VLSE data of the CO2-MEA-H2O and CO2-KSar-H2O systems at 313 and 393K. Comparison of the cyclic capacities for both systems can be determined by taking a coal-fired power plant case, for instance, in which the flue gas from the plant contains ~12 mol% CO2. At CO2 partial pressure of 12 kPa,

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Sholeh Ma’mun / Energy Procedia 51 ( 2014 ) 191 – 196 195

MEA gives lean and rich loadings of 0.20 and 0.53, respectively, so that its cyclic capacity is 1.62 mol CO2/kg-solution. At the same CO2 partial pressure, the lean and rich loadings of KSar are 0.36 and 0.85, respectively, and this gives the cyclic capacity of 2.45 mol CO2/kg-solution (52% higher than MEA).

Figure 3a demonstrates that the model predicts very well at temperature of 393K, thus, the lean loading of KSar was determined from the model. On the contrary, the model does not predict very well at 313K, as mentioned previously. Based on this reason, the rich loading of KSar was then determined by an extrapolating method. If the rich loading were determined by the model (0.81), the cyclic capacity would be 2.25 mol CO2/kg-solution (39% higher than MEA).

3.3. Heat of absorption

An approximate value of the heat of absorption of CO2, Habs, in an absorbent can be determined by using the Gibbs–Helmholtz equation as follows:

(4)

where the subscript 1 refers to CO2 and x1 is the mole fraction or equilibrium loading of CO2. With a complete VLSE data set at a temperature range of 313 − 393K, the heat of CO2 absorption in the 5m' KSar solution can be well represented in comparison to the previous published data by Ma'mun and Kim [15]. Constant values of the

Habs (~125 kJ/mol CO2) were observed at CO2 loadings up to 0.20 (see Figure 5). The values decrease as the loadings increase and reach a minimum value (77 kJ/mol CO2) at the CO2 loading of 0.52 where the precipitation occurs. At the precipitation region, the Habs slightly increase as the CO2 loadings increase.

Figure 5. Heat of absorption of CO2 in KSar solutions from ref. [15]. 3.5m' KSar: □, 313K; +, 353K; ×, 393K. 5m' KSar: ○, 313K. Solid line was calculated by the Gibbs-Helmholtz equation from the VLSE data of CO2-KSar-H2O system.

4. Conclusion

The equilibrium solubility of CO2 in the aqueous solution of 5m' potassium salt of sarcosine (KSar) was measured at temperatures of 353, 373, and 393K, respectively, with the CO2 partial pressures ranging from 0.003 to 41.9 kPa. The collected VLSE data of this work have complemented the previous ones for the CO2-KSar-H2O system. The complete set of the VLSE data provides a good representation of the CO2-KSar-H2O system. The cyclic capacity of this system is much higher than that of MEA (i.e. 52% higher). In addition, the estimated heat of absorption of CO2 in the KSar solution using the Gibbs-Helmholtz may provide useful information for the regeneration energy requirement.

Acknowledgements

This publication has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The author acknowledges the following partners for their contributions: Aker Solutions, ConocoPhillips, Gassco, Shell, Statoil, TOTAL, GDF SUEZ, and the Research Council of Norway (193816/S60).

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196 Sholeh Ma’mun / Energy Procedia 51 ( 2014 ) 191 – 196

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