Scholarly article on topic 'Measurement of Heat of CO2 Absorption into 2-Amino-2-methyl-1-propanol (AMP)/Piperazine (PZ) Blends Using Differential Reaction Calorimeter'

Measurement of Heat of CO2 Absorption into 2-Amino-2-methyl-1-propanol (AMP)/Piperazine (PZ) Blends Using Differential Reaction Calorimeter Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Qian Xie, Adisorn Aroonwilas, Amornvadee Veawab

Abstract In this work, a Differential Reaction Calorimeter (DRC) was used to measure differential heats of CO2 absorption into aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) + piperazine (PZ) blend. The measurement was carried out at 40, 60, and 80°C, and at CO2 loading ranging from nil to CO2 saturation. The AMP/PZ molar ratios of 2:1, 4:1, and 6:1 were tested. Effects of reaction temperature, CO2 loading, and molar ratio were analyzed according to reaction mechanism.

Academic research paper on topic "Measurement of Heat of CO2 Absorption into 2-Amino-2-methyl-1-propanol (AMP)/Piperazine (PZ) Blends Using Differential Reaction Calorimeter"

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Energy Procedia 37 (2013) 826 - 833

GHGT-11

Measurement of Heat of CO2 Absorption into 2-Amino-2-methyl-1-propanol (AMP)/Piperazine (PZ) Blends Using Differential Reaction Calorimeter

Qian Xiea, Adisorn Aroonwilasa*, Amornvadee Veawaba

aEnergy Technology Laboratory, Faculty of Engineering and Applied Science, _University of Regina, Regina, SK, Canada, S4S 0A2_

Abstract

In this work, a Differential Reaction Calorimeter (DRC) was used to measure differential heats of CO2 absorption into aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) + piperazine (PZ) blend. The measurement was carried out at 40, 60, and 80oC, and at CO2 loading ranging from nil to CO2 saturation. The AMP/PZ molar ratios of 2:1, 4:1, and 6:1 were tested. Effects of reaction temperature, CO2 loading, and molar ratio were analyzed according to reaction mechanism.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: CO2 capture; heat of absorption; AMP; piperazine; differential reaction calorimeter

1. Introduction

Gas absorption process using aqueous amine solutions is the most promising technology for capturing carbon dioxide (CO2) from fossil fuel-fired power plants [1]. However, integration of this amine-based capture process into power plants is still subject to economic and technical challenges. It is commonly known that regeneration of amine solutions and compression of CO2 product are the two energy-intensive steps that can easily bring down the output of a power plant by 20~40%. In order to reduce energy requirement of the capture process, we can either develop advanced amine solutions or optimize the process design. Both approaches rely heavily on availability of thermodynamic data such as CO2 solubility and heat of absorption (or absorption enthalpy). Heat of CO2 absorption into aqueous amine solutions is an important factor for evaluating heat balance and energy distribution for absorber and

* Corresponding author. Tel.: +1-306-337-2469; fax: +1-306-585-4855. E-mail address: aroonwia@uregina.ca.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.175

stripper, reboiler heat duty, and heat duties of other heat-transfer equipment. Therefore, it is not only a crucial property for the development of alternative amine solutions, but also a key parameter for energy assessment of the capture process. Our focus of this work is to provide accurate heat of CO2 absorption data measured for a set of alternative amine solutions.

Recently, aqueous solution of 2-amino-2-methyl-1-propanol (AMP) + piperazine (PZ) blend has been considered as a promising solvent for post-combustion CO2 removal [2-4]. Bruder et al. [5] screened aqueous AMP+PZ system with the highest concentration and cyclic capacity without forming solid precipitations at operational temperatures. The 3.0M AMP + 1.5M PZ and 4.0M AMP + 1.0M PZ were identified as two good mixing recipes. Despite a great deal of attention, experimental data on heat of CO2 absorption into AMP+PZ blend are still limited in published literatures. The heat of absorption data available today were estimated from vapor-liquid-equilibrium (VLE) data by using Gibbs-Helmholtz equation (Equation (1)).

■ ' &h" (i)

gin Pco2 9(1/r)

Here, PCo2 is C02 partial pressure, T is absolute temperature, AHdiffb> differential heat of absorption, and R is the universal gas constant. The heat data obtained from this equation, however, are usually not accurate [6, 7] because the differentiation process can magnify the error in the solubility data.

In this work, differential heat of CO2 absorption into aqueous solution of AMP+PZ blend was measured by a Differential Reaction Calorimeter (SETARAM DRC-Evolution). The measurement was carried out from zero CO2 loading up to the saturation condition and at 40, 60 and 80°C. Three mixing ratios (i.e. 3.0M AMP+0.5M PZ, 4.0M AMP+1.0M PZ and 3.0M AMP+1.5M PZ) were tested. The experimental data were compared with values derived from Gibbs-Helmholtz equation. Effects of CO2 loading, temperature and solvent composition were analyzed.

2. Experimental Section

2.1. Experimental setup and operating procedure

The schematic diagram of experimental setup is shown in Fig. 1. The key components of DRC system are two glass vessels, one serves as the reaction cell while the other is the reference cell. The two vessels were connected in parallel to a water bath, from which the thermostated water was supplied and circulated to control reaction temperature. Two storage cylinders supplying CO2 gas to the absorption cell were immersed in the water bath in order to regulate feed temperature. The reaction cell was also connected to a burette pressure regulator. For each measurement, a small amount of CO2 from the cylinders was charged into amine solution in the reaction cell. As absorption occurred, the total energy-flux released or absorbed during CO2-amine reaction was precisely measured and reported as temperature gradient versus time. The remaining CO2 that was not absorbed by amine was collected in the burette. The temperature gradient together with the number of moles of CO2 absorbed was translated into heat of absorption in a unit of kJ/mole CO2. Note that each CO2 loading span (Aa) was kept approximately 0.1 mol CO2/mol amine. Heat of absorption measured this way is semi-differential in CO2 loading (integral within each loading interval) and differential in temperature [9].

2.2. Validation of experimental system

To validate the experimental system, a few experiments were conducted and the obtained data were compared to reference values. More specifically, heat of reaction between sodium hydroxide (NaOH)

and hydrochloric acid (HCl) was measured at 25°C. The measured value of 54.3 kJ/mol NaCl was comparable to the theoretical value of 55.8 kJ/mol NaCl, thus validating the system. In addition, heat of CO2 absorption into 5.0M monoethanolamine (MEA) solution were also measured at 40oC. The obtained data for MEA were compared with measured heat data by Kim et al. [7], and with VLE derived data by Jou et al. [8] and Lee et al. [9]. As shown in Fig. 2, the heat of absorption obtained in this work shows good agreement with the literature data.

2.3. Materials

CO2 gas with a purity of more than 99.998% was obtained from Praxair. AMP (95% purity) and PZ (99% purity) were both purchased from Sigma-Aldrich. All chemicals were used without further purification. The aqueous amine solutions were prepared by mixing pure amines with deionized water to a desired concentration (molarity or mol/L denoted by M). The exact solution concentration was determined by titration with 1.0 mol/L standard HCl solution using methyl orange as the indicator.

Fig. 1. Schematic diagram of experiment setup

100 90 ^ 80 8 70

1 60 ^ 50 % 40 erf 30 " 20 10 0

o$> o is o a o o °

■ 4.0 M MEA at 40°C (this work) O 30 wt. % MEA at 40 °C ( Kim, 2007) a 30 wt. % MEA at 24-120C (Jou, 1994) 02.5 & 4.0 M MEA at 40-100C(Lee, 1974)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 o (mol CO2/ mol MEA)

1 1.1 1.2 1.3

Fig. 2. Heat of absorption of CO2 with 5.0 M MEA at 40°C compared with literature data.

3. Result and Discussion

The differential heat of absorption of CO2 with aqueous 3.0M AMP+0.5M PZ, 4.0M AMP+1.0M

AMP and 3.0M AMP+1.5 M PZ were measured at 40, 60 and 80°C. The experimental data are presented

in Table 1. Experimental heats of absorption are plotted versus CO2 loading (a) in Fig. 3 to 5.

Table 1. Heat of absorption of CO2 with AMP+PZ blends at different temperatures

a -AHjiff a -AHjiff a -AH^

(mol CO2/mol amine) (kJ/mol CO2) (mol CO2/mol amine) (kJ/mol CO2) (mol CO2/mol amine) (kJ/mol CO2) 3.0 M AMP+0.5 M PZ (Molar ratio of AMP:PZ = 6:1)

40 °C 60°C 80°C

0.101 77.77 0.099 77.89 0.100 79.78

0.200 77.26 0.200 77.71 0.200 79.09

0.301 74.86 0.297 75.47 0.301 79.26

0.401 72.52 0.398 71.89 0.402 74.36

0.501 59.54 0.497 67.63 0.501 66.04

0.601 63.95 0.598 63.32 0.595 52.32

0.703 53.87 0.699 52.65 0.615 42.35

0.802 45.71 0.791 43.95 0.627 24.79 0.894 32.61 0.830 39.34

4.0 M AMP+1.0 M PZ (Molar ratio of AMP:PZ = 4:1)

40 °C 60°C 80°C

0.050 78.48 0.101 81.41 0.101 84.19

0.100 77.78 0.201 77.34 0.201 75.74

0.150 77.06 0.302 76.48 0.301 77.80

0.250 75.53 0.402 72.39 0.399 70.17

0.351 74.47 0.502 69.57 0.499 60.22

0.451 72.89 0.602 64.46 0.581 53.72

0.548 69.99 0.701 56.84 0.618 30.24

0.568 68.83 0.770 40.61 0.641 21.42 0.676 59.49 0.812 25.26

3.0 M AMP+1.5 M PZ (Molar ratio of AMP:PZ = 2:1)

40 °C 60°C 80°C

0.099 77.76 0.098 77.95 0.101 79.87

0.201 76.58 0.200 77.74 0.201 79.15

0.302 74.73 0.301 75.16 0.300 76.37

0.401 75.24 0.401 76.26 0.399 73.28

0.502 71.46 0.502 73.72 0.498 71.47

0.601 67.04 0.601 71.12 0.599 68.32

0.684 60.26 0.700 64.14 0.695 50.22

0.779 48.31 0.800 48.96 0.732 26.83 0.826 39.81 0.830 29.74

Fig.3-5 show effect of CO2 loading on heat of reaction in two modes. At low CO2 loading interval, for all three aqueous AMP+PZ systems, heat of absorption decreases very slowly as CO2 loading increases. This behavior is different from that of MEA. As can be seen in Fig. 2, at low loading, heat of absorption of CO2 with MEA remains constant and it is independent of CO2 loading. This is due to the difference in reaction mechanism of CO2 with MEA and CO2 with AMP or PZ.

100 90 80 70 60 50 40 30 20 10 0

L 3.0 M AMP+0.5 M PZ at 40°C

i 3.0 M AMP+0.5 M PZ at 60°C •

i 3.0 M AMP+0.5 M PZ at 80°C

0.1 0.2 0.3 0.4 0.5 0.6 0.7 a (mol C02/mol amine)

0.8 0.9

Fig. 3. Heat of absorption of CO2 with 3.0 M AMP + 0.5 M PZ

Fig. 4. Heat of absorption of CO2 with 4.0 M AMP + 1.0 M PZ

100 90 80 70 60 50 40 30 20 10 0

o 3 1 11111 11111 1

: 3.0 M AMP+1.5 M PZ at 400C

: 3.0 M AMP+1.5 M PZ at 60°C

: 3.0 M AMP+1.5 M PZ at 80°C

: O 3.0 M AMP+1.5 M PZ at 40-120°C (Bruder, 2011) i i

0.1 0.2 0.3 0.4 0.5 0.6 0.7 a (mol CO2/mol amine)

0.8 0.9

Fig. 5. Heat of absorption of CO2 with 3.0 M AMP + 1.5 M PZ

MEA is a primary amine and it reacts with carbon dioxide to form stable carbamate. When CO2 loading is less than the stoichiometry maximum loading (0.5 mol CO2/mol MEA), stable carbamate is formed as the dominant products. Thus, the heat of absorption remains constant in this loading interval. Unlike MEA, AMP is a sterically hindered amine characterized by forming carbamate of low stability

[10]. Due to the low stability of AMP carbamate, part of the carbamate formed will be converted to bicarbonate through carbamate hydrolysis. Bicarbonate can also be formed through zwitterions hydrolysis

[11]. Therefore, significant amount of both carbamate and bicarbonate are formed when CO2 is added into fresh AMP+H2O systems. And the heat released when forming carbamate is usually higher than forming bicarbonate [12]. Dash et al. [13] predicted speciation profile of AMP+H2O+CO2 system from VLE data. As CO2 loading increases, free amine decreases, the tendency to form bicarbonate increases while the chance to form carbamate decreases. This leads to a slow decrease of heat of absorption. Gabrilsen et al. [14] derived Equation (2) from VLE data the heat of absorption of CO2 with aqueous AMP. Equation (2) also shows that heat of absorption (-AHdf) of CO2 with AMP decrease slightly as CO2 loading increases.

AHdiff =R\ -8161 + 47652

The reactions of PZ+H2O+CO2 system have been studied by Bishnoi and Rochelle [15] and Ermatchkov et al. [16]. PZ is a secondary diamine, and its reactions with CO2 are quite complex. The main reactions of CO2 with PZ are formation of piperazine carbamate, formation of piperazine dicarbamate and formation of protonated piperazine carbamate. The heats of reaction (-AHdiff) of the three reactions are 30.07 kJ/mol, 10.99 kJ/mol and 29.04 kJ/mol, respectively [16]. Heat of absorption of CO2 with aqueous PZ is mainly determined by these three reactions. Boshnoi and Rochelle [15] and Ermatchkov et al. [16] predicted speciation profile of PZ+H2O+CO2 system as a function of CO2 loading. In the low loading domain, piperazine carbamate is the dominant products with small fractions of piperazine diarbamate and protonated piperazine. As CO2 loading increases, the fractions of piperazine dicarbamate and protonated piperazine increase, and formation of piperazine dicarbamate and formation

of protonated piperazine carbamate play a more important role. Because they have lower heat of reaction than formation of piperazine carbamate, the heat of absorption of CO2 with PZ slowly decrease as CO2 loading increases. Liu et al. [17] measured heat of absorption of CO2 with 0.86 mol PZ. Their result shows the same trend. Because heat of absorption of CO2 with aqueous AMP or PZ both decrease slowly as CO2 loading in low CO2 loading domain, it is not surprising to see that heat of absorption of CO2 with aqueous blends of AMP and PZ decreases slowly with the loading.

At high CO2 loading domain, heat of CO2 absorption decreases rapidly with the loading. This behavior was observed for both aqueous MEA and aqueous AMP+PZ solutions as shown in Fig. 2-5. CO2 absorption by aqueous amine solutions is a combined result of chemical reaction and physical dissolution. As CO2 loading approaches chemical stoichiometry saturation loading, free amines are almost depleted, amine absorption is mainly controlled by physical dissolution at this loading domain. The heat of CO2 dissolution by physical dissolution is much lower than the heat of reaction of CO2 with amine. Therefore, at high CO2 loading domain, heat of absorption shows rapid decrease with loading.

Regarding the temperature effect, heats of absorption of 3.0M AMP+0.5M PZ at 40 and 60°C are comparable as shown in Fig. 3. At 80°C, heat of absorption for low CO2 loading interval appears to be the same as those at 40 and 60°C. However, as the CO2 loading increases beyond 0.5 mol/mol, heat of absorption at 80oC decreases rapidly and is much lower than the values at 40 and 60°C. This behavior is due to the nature of reversible reactions of CO2 with aqueous AMP+PZ solutions. When temperature reaches a certain high level, and CO2 loading is high, the reversible CO2-AMP-PZ reactions favor the CO2 desorption direction. Even there is still considerable free amine existing, it cannot absorb more CO2 chemically. This causes the decline of CO2 absorption capacity as well as heat of absorption. Therefore, temperature has influence on CO2 absorption capacity and heat of absorption at high loading domain. The same temperature effect was observed for 4.0M AMP+1.0M PZ system and 3.0M AMP+1.5M PZ system.

At a given temperature, heats of absorption with the three aqueous blended AMP+PZ solutions are rather comparable especially for the low loading interval. The effect of AMP/PZ molar ratio seems to be more pronounced at the higher CO2 loading. The higher PZ ratio results in the greater reduction in heat of absorption with the increasing loading. For comparison, the VLE derived heat of absorption data of CO2 with 3.0M AMP+1.5M PZ from Brúder et al. [5] was also included in Fig. 5. The calculated data are about 10% higher than our experimental data.

4. Conclusion

Heats of CO2 absorption into three aqueous blended solutions of AMP+PZ (3.0M AMP+0.5M PZ, 4.0M AMP+1.0M PZ and 3.0M AMP+1.5M PZ) at 40, 60 and 80°C, and at CO2 loading ranges from zero to maximum loading were measured with a reaction calorimeter DRC-Evolution. Calorimeter generally provides a good way to obtain accurate heat of CO2 absorption data. The effect of CO2 loading, temperature and solvent composition can be reflected from the experimental data. At low CO2 loading domain, heat of absorption of CO2 with blended aqueous AMP+PZ decreases slowly with the CO2 loading. As CO2 loading approaches saturation, heat of absorption shows sharp decrease. Temperature does not have obvious effect on heat of absorption at low CO2 loading region. At 80°C, CO2 absorption capacity of the three blended solutions is considerably lower than the capacity at 40 and 60°C, and the heat of absorption starts to decrease rapidly. The AMP/PZ mixing ratio has an impact on heat of absorption at high CO2 loading region.

Acknowledgements

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) are greatly appreciated.

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