Scholarly article on topic 'Development of Novel Synthetic Amine Absorbents for CO2 Capture'

Development of Novel Synthetic Amine Absorbents for CO2 Capture Academic research paper on "Materials engineering"

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{"amine absorbent" / "CO2 capture" / absorption-regeneration / "reaction rate" / "heats of reaction" / "cyclic capacity"}

Abstract of research paper on Materials engineering, author of scientific article — Firoz A. Chowdhury, Hidetaka Yamada, Yoichi Matsuzaki, Kazuya Goto, Takayuki Higashii, et al.

Abstract In the present paper, we investigated five synthetic amine based absorbents, including three formulated solvents. Aqueous solutions of the amines (mass fraction; 30% for single amine and >30% for blended solvents) were used to evaluate the performance for CO2 capture. Gas scrubbing, vapor-liquid equilibrium (VLE), and reaction calorimetry experiments were conducted in the laboratory to obtain the absorption rate, the amount of CO2 absorbed, cyclic CO2 capacity, and heat of reaction for each absorbent. The results of these absorbents were compared with the conventional absorbent monoethanolamine (MEA). Three high performing synthetic absorbents (IPAE, IPAP and IBAE) were found, and these had lower heats of reaction, higher cyclic capacities, and comparable absorption rates compared with MEA. All formulated absorbents showed excellent cyclic CO2 capacity and keeping moderately good absorption rate and lower heats of absorption. Some blended solvents were already demonstrated with real blast furnace gas at pilot test plants with capacities of 1 ton-CO2/day and 30 ton-CO2/day and showed promising results in terms of reducing absorbent regeneration energy.

Academic research paper on topic "Development of Novel Synthetic Amine Absorbents for CO2 Capture"

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Energy Procedia 63 (2014) 572 - 579

GHGT-12

Development of Novel Synthetic Amine Absorbents for CO2

Capture

1 1 2 1 Firoz A. Chowdhury *, Hidetaka Yamada , Yoichi Matsuzaki , Kazuya Goto ,

1 2 Takayuki Higashii , and Masami Onoda

1Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa-shi, Kyoto, Japan 2Advanced Technology Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi Futtsu, Chiba 293-8511, Japan

Abstract

In the present paper, we investigated five synthetic amine based absorbents, including three formulated solvents. Aqueous solutions of the amines (mass fraction; 30% for single amine and >30% for blended solvents) were used to evaluate the performance for CO2 capture. Gas scrubbing, vapor-liquid equilibrium (VLE), and reaction calorimetry experiments were conducted in the laboratory to obtain the absorption rate, the amount of CO2 absorbed, cyclic CO2 capacity, and heat of reaction for each absorbent. The results of these absorbents were compared with the conventional absorbent monoethanolamine (MEA). Three high performing synthetic absorbents (IPAE, IPAP and IBAE) were found, and these had lower heats of reaction, higher cyclic capacities, and comparable absorption rates compared with MEA. All formulated absorbents showed excellent cyclic CO2 capacity and keeping moderately good absorption rate and lower heats of absorption. Some blended solvents were already demonstrated with real blast furnace gas at pilot test plants with capacities of 1 ton-CO2/day and 30 ton-CO2/day and showed promising results in terms of reducing absorbent regeneration energy.

© 2014TheAuthors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: amine absorbent, CO2 capture, absorption-regeneration, reaction rate, heats of reaction, cyclic capacity

* Corresponding author. Tel.: +81-774-75-2360; fax: +81-774-75-2318. E-mail address: firoz@rite.or.jp

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

doi: 10.1016/j.egypro.2014.11.062

1. Introduction

Carbon dioxide (CO2) is a greenhouse gas that contributes to global warming and climate change problems. Carbon capture and storage from large point exhaust sources is promising for mitigation of these problems. Cyclic chemical absorption/regeneration process using an aqueous solution of an amine-based absorbent is the most mature and applied technology for post-combustion CO2 removal from large stationary sources that emit large amounts of CO2. To date, amine-based CO2 capture technology has been widely developed for post-combustion CO2 removal because of high capture efficiency, high selectivity, and scale-up feasibility. The main obstacle for the application of conventional amine scrubbing technology is the energy requirements for absorbent regeneration. It is estimated that more than half of the capture cost arises from absorbent regeneration. Consequently, for practical application it is essential to reduce absorbent regeneration energy consumption by improving existing absorbents and developing new ones.

Numerous studies on chemical absorption phenomena with alkanolamines such as MEA, DEA, AMP, MDEA, TEA and their related absorbents have been reported.1-4 Recently, interest in the use of mixed amine absorbents, especially blends of primary and tertiary amines (such as MEA and MDEA) or secondary and tertiary amines (such as DEA and MDEA), has increased. These mixed absorbents combine the higher equilibrium capacity of the tertiary amine with the higher reaction rate of primary or secondary amine, and have been suggested for industrial gas treatment processes.5-8 Most studies have been limited to MEA-, DEA-, MDEA-, TEA- or MDEA-based single or combined absorbents. There is a very little information available regarding rationally designed new synthetic amines or amino alcohols. Recently, new secondary and tertiary alkanolamines, including some butanol derivatives synthesized by Tontiwachwuthikul and coworkers,9,10 have been applied to CO2 capture. These absorbents provided much higher CO2 absorption and cyclic capacities than the conventional amine MEA. In our previous studies, we

w _ -m S

G S a ^ o a

A' « O

6 5 4 3 2 1

^...................,. i Preferred \ Target ..... A' MEA''

A DEA AMP A

/MI>I-.A ..--■'"' 1 i

Heats of Reaction (kJ/mol-C02)

Fig. 1. The experimental relationship between the CO2 absorption rate and heat of reaction for aqueous solutions of conventional amines (mass fraction 30 %; MEA, AMP, DEA and MDEA) at 40 °C.

designed and synthesized several secondary and tertiary amino alcohols by modification of the chemical structures, and developed screening methods for finding promising absorbents.11-15 On the basis of the screening methods, in the

present study, we examined five synthetic and three blended amine based absorbents, including the reference compound MEA, and compared their performance to MEA.

Our previous research has provided useful information on various amine absorbents for CO2 capture. The CO2 loading capacities, heats of reaction, and absorption rates of these absorbents were examined, and we found a structure-performance relationship. The relationship between the heat of reaction and CO2 absorption rate of 30 wt% aqueous conventional amines, as determined experimentally, for primary amines MEA, AMP, a secondary amine DEA and a tertiary amine MDEA are shown in Fig. 1. Figure 1 shows MEA reacts with CO2 faster than DEA, which reacts faster than AMP and MDEA. Figure 1 also shows that the heat of reaction is lower for MDEA, than DEA, AMP and MEA. This indicates that the heats of reaction and absorption rates of the alkanolamines are dependent on the substituents attached to the N-atom/alkanolamine chain. In aqueous solutions CO2 and unhindered amines form stable carbamate anions and hindered amines form bicarbonate ions. Low rates of CO2 absorption, however, make hindered amines difficult to use flue gas cleaning. In this study, firstly we tried to find high performance single-component amine from screening tests then advanced formulated amine absorbents were developed for capturing CO2 from blast furnace exhaust gases.

2. Experimental

The development of new absorbent materials that efficiently, reversibly, and economically capture CO2 is very laborious. A tremendous amount of experimental work has to be done on characterizing the new solvents with respect to different properties; for example: rapid and economic synthesis was done by one-pot synthesis with high purity yield, high performance absorbents selection were done by particular chemical structure tuning e.g. changing hydroxyl chain length ethyl to butyl and alkyl substituents e.g. isopropyl, isobutyl, and secondarybutyl around the amino group. Five such secondary amino alcohols were synthesized in our laboratory by the reaction of alkanolamine with their corresponding alkylhalides. These amines were >96% pure and their structures were established by gas chromatography (GC), liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR) spectroscopy. Conventional amines MEA, AMP, DEA, and MDEA were purchased from Wako Pure Chemical Industries (Osaka, Japan) and used as received. The details for the investigated amine absorbents are shown in Table 1. Water used in all experiments was purified with water distillation apparatus (RFD240NA, Advantec, Tokyo, Japan) and an ion-exchange apparatus (RFU424CA, Advantec) in series.

Several fundamental experiments of synthesized amines were performed in our laboratory to evaluate their CO2 capture performance. For example, 30 wt% aqueous synthesized amines were used for gas scrubbing test, vapor-liquid equilibrium test and reaction calorimetric measurements. Among these experiments solvent characteristics such as CO2 absorption capacity, absorption-regeneration rate, cyclic CO2 loading, and heats of absorption were obtained. A similar study of conventional alkanolamine (MEA) was also conducted at the same time under similar conditions for comparisons. The blended solvents (A, B and C) were formulated on the basis of solvent concentration, rich CO2 loading, lean CO2 loading and cyclic CO2 loading. Other solvent issues like foaming tendency, crystallization, density, viscosity also considered. The detailed equipment and experimental procedures of the above mentioned tests were explained in our previous work.

Table 1. Investigated amine absorbents

Synthesized and blended absorbents Conventional absorbents

1. 2-(isopropylamino)ethanol (IPAE) 9. 2-aminoethanol (MEA)

2. 3-(isopropylamino)propanol (IPAP) 10. 2-amino-2-methyl-1-propanol (AMP)

3. 4-(isopropylamino)butanol (IPAB) 11. Diethaholamine (DEA)

4. 2-(isobutylamino)ethanol (IBAE) 12. Methyldiethanolamine (MDEA)

5. 2-(sec-butylamino)ethanol (SBAE)

6. Solvent-A

7. Solvent-B

8. Solvent-C

» - -P

a . o a

& ¡¡e

Absorption at 40 °C Desorption at 70 °C

I XL'^SS^Hffiiaio

- IPAP ■IPAB

- SBAE

- IBAE

- Solvent-A

- Soivent-B

- Soivent-C

Time (mill.)

Fig. 2. Experimental CO2 absorption-regeneration profiles for 5 synthetic amines (IPAE, IPAP, IPAB, SBAE, and IBAE), and 3 blended solvents (A, B and C) compared with conventional amine MEA. (mass fraction 30wt% for single amine and >30wt% for blended solvents).

3. Results and Discussion

The experimental CO2 absorption-regeneration profiles obtained in this work for the five single synthetic amine and three blended amine based solvents are compared with the conventional absorbent MEA in Fig. 2. The amount of absorbed CO2 in the aqueous amine solution (CO2 loading) was calculated from the measured CO2 concentration in the outlet gas flow. As shown in Fig. 2, the CO2 loading increased with time at 40 °C and then decreased at 70 °C. The gradient of the curve at 50 % of the 60 min CO2 loading was defined as the absorption rate. The absorption rate is not the chemical reaction rate but the apparent CO2 transfer rate from the gas to liquid phase. This reference index was used to compare the behavior of the aqueous amine solutions. The reproducibility of the experiments was checked, and the error in all of the experimental measurements was less than 3 %. Figure 2 show that all synthetic amine had a higher desorption amount of CO2 and keeping similar absorption rate except SBAE compared with MEA. All blended solvents in Fig. 2 shows higher CO2 absorption-desorption characteristics and keeping similar absorption rate than that of MEA.

CO2 absorption involves multiple electrolyte reactions in a liquid phase, but only the overall reaction was considered. The heats of reaction (A#r) of each aqueous amine solution during CO2 absorption was acquired using a differential reaction calorimeter. AHr means the enthalpy difference between fresh aqueous amine solution and CO2-loaded solution. AHr were measured at 40 °C. Generally, AHr values depend on the CO2 loadings. Consequently, we expressed the AHr values as the differential (average) enthalpies in the range of the absorbent loading. Our results are summarized in Fig. 3. Figure 3 also shows that the heat of reaction for all investigated absorbents is much lower than the conventional MEA absorbent.

Fig. 3. Heats of reaction of this work absorbents compared with conventional absorbent (MEA).

To reduce the energy requirements of the capture process, absorbents with high absorption rates, high cyclic capacities, and low heats of reaction are needed. In Figure 4, the CO2 absorption rates for single-component amines and formulated absorbents (A, B and C) are plotted against the heats of absorption values. As mentioned for Figure 1, there was a trade-off between the heat of reaction and absorption rate for primary, secondary and tertiary amines. However, the results in Figure 1 showed that synthetic and formulated absorbents were plotted at outside of the

Fig. 4. Five synthetic and three blended solvents preliminary screening results compared with conventional amines.

trade-off relationship formed by the conventional absorbents. In this study, (synthetic and formulated) absorbents were unique in that they had low heats of reaction and maintained moderately high absorption rate. Solvents with low heat of absorption might benefit from regeneration below atmospheric pressure and at low temperatures.

Solvents that showed superior performance (higher absorption rate, and lower reaction heat) from screening tests were selected for the evaluation of their vapor-liquid equilibrium (VLE) property. Four synthetic amines IPAE, IPAP, IBAE, SBAE and three formulated solvents (A, B & C) along with the reference solvent MEA was selected for the evaluation of their VLE property. For each absorbent, measurements under conditions with two temperatures, 40, and 120 °C and partial pressures of CO2 20 and 100 kPa were conducted. The experimental results are given in Table 2-9.

Table 2. VLE solubility of CO2 in 30wt% MEA aqueous solution

120 "C

PcO2 [kPa]

[g-CO2/ L-soln.]

PcO2 [kPa]

[g-CO2/ L-soln.]

Aa (40-120) "C

[g-CO2/ L-soln.]

20 117.4 100 129.8

20 100

55.5 80.2

61.9 49.6

Table 3. VLE solubility of CO2 in 30wt% IPAE aqueous solution

PCO2 a PCO2 a

[g-CO2/ [g-CO2/

[kPa] L-soln.] [kPa] L-soln.]

20 93.9 20 10.6

100 108.3 100 27.2

' (40-120) "C

[g-CO2/ L-soln.]

Aa Difference

83.3 81.1

Table 4. VLE solubility of CO2 in 30wt% IPAP aqueous solution

Table 5. VLE solubility of CO2 in 30wt% IBAE aqueous solution

120 "C

Pco2 a Pco2 a (40-120)

[g-CO2/

[g-CO2/ [g-CO2/ T , 2

[kPa] Lion,] [kPa] Lgsoln.] L-SOln]

Aa Difference

20 102.2 20 16.7 85.5 38

100 111.1 100 42.2 68.9 39

120 "C

a (40-120) V

[g-CO2/ [kPa] L-soln.] [kPa]

[g-CO2/ L-soln.]

[g-CO2/ L-soln.]

Difference

20 70.6 20 9.4 61.2 -1

100 77.3 100 42.2 53.4 8

Table 6. VLE solubility of CO2 in 30wt% SBAE aqueous solution

a (40-120) "C

[g-CO2/ L-soln.]

[g-CO2/ L-soln.]

[g-CO2/ L-soln.]

Difference

20 89.4 20 10.0 79.4 28

100 105.0 100 24.4 80.6 63

Table 7. VLE solubility of CO2 in Solvent-A aqueous solution

120 "C

a (40-120) "C [g-CO2/

Aa Difference

[g-CO2/ [g-CO2/ (%)

2 [kPa] 2 L-soln.] (%)

L-soln.] L-soln.]

20 140.9 20 8.4 132.5 114

100 172.7 100 22.8 149.9 202

Table 8. VLE solubility of CO2 in Solvent-B aqueous solution

Pco2 a PCO2 a (40-120)

[g-CO2/

[g-CO2/ [g-CO2/ T6 , ,

[kPa] LU, [kPa] LU, L-Soln]

Aa Difference

20 158.0 20 5.0 153.0 147.0

100 221.2 100 15.0 206.2 316.0

Table 9. VLE solubility of CO2 in Solvent-C aqueous solution

120 "C

Pco2 a Pco2 a (40-120)

[g-CO2/

[g-CO2/ [g-CO2/ T , 2n

[kPa] Lg-soln./ [kPa] Lg-soln2/ ^^

Difference

20 155.8 20 6.7 149.1 141

100 198.4 100 19.4 179.0 261

The experimental results of the selected seven promising absorbents (IPAE, IPAP, IBAE, SBAE, and formulated solvents A, B, and C) compared to MEA at various conditions are presented in Table 2-9. The Aa is the absorption capacity differences between the rich and lean solvent under equilibrium condition was obtained from VLE experiment. The Aa CO2 loading, is a key index that determines the net cyclic capacity in the CO2 industrial capture

system. From the experimental results all tested absorbents have better performance than MEA and seemed to be effective solvent for CO2 capture. In Table 2-9 the absorption capacity Aa compared with MEA at various operating conditions are higher by (35~64)% for IPAE, (38~39)% for IPAP, (-1~8)% for IBAE, (28~63)% for SBAE, (114~202)% for solvent A, (147~316)% for solvent B, and (141~261)% for solvent C. This means, these seven absorbents absorb more CO2 and release more CO2 off at high temperature. Good VLE property will reduce liquid flow rate and steam required for CO2 recovery. This advantage can result in lowering solvent regeneration cost for gas treating process thereby making it more economically feasible.

Fig. 5. Four synthetic and three blended solvents CO2 absorption rates verses equilibrium cyclic CO2 capacities compared with conventional MEA. Cyclic CO2 capacity = difference between lean and rich CO2 loading at 40 °C (20kPa) and 120 °C (100kPa) under equilibrium condition obtained from VLE experiment.

Figure 5 showed the performance comparison of this work absorbent with conventional absorbents in terms of absorption rate versus cyclic CO2 capacity. All formulated absorbents (A, B and C) showed excellent cyclic CO2 capacity and keeping moderately good absorption rate compared with conventional amines (Fig. 5). Two synthetic absorbents (IPAE and IPAP) showed high cyclic capacity compared to MEA, DEA and MDEA and moderate compared to AMP. It should be noted that 30 wt% aqueous AMP contains more amine molecules (3.37 mol amine/kg solvent) than 30 wt% aqueous IPAE and IPAP (2.91 and 2.56 mol amine/kg solvent), which should be an advantage for AMP in terms of its CO2 capacity.

4. Conclusions

In the present paper we investigated five synthetic amine based absorbents with particular chemical structure tuning e.g. changing hydroxyl chain length ethyl to butyl and alkyl substituents e.g. isopropyl, isobutyl, and secondarybutyl around the amino group. Placement of functional groups within the N-atom/alkanolamine chain affects CO2 absorption-regeneration performance. Gas scrubbing, VLE and reaction calorimetry experiments were conducted to obtain the absorption rates, CO2 loadings, cyclic capacities and heats of reaction for the absorbents. The results were compared with those for the conventional absorbent absorbents. Three high performing synthetic

absorbents (IPAE, IPAP and IBAE) were found, and these had lower heats of absorption and comparable absorption rates and cyclic capacities compared with conventional amine absorbents. All formulated absorbents showed excellent cyclic CO2 capacity and keeping moderately good absorption rate and lower heats of absorption. Some blended solvents were already demonstrated with real blast furnace gas at pilot test plants with capacities of 1 ton-CO2/day and 30 ton-CO2/day and showed promising results in terms of reducing absorbent regeneration energy.

Acknowledgements

A part of this work was financially supported by the COURSE 50 project funded by the New Energy and Industrial Technology Development Organization, Japan.

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