Scholarly article on topic 'Experimental Study on CO2 Solubility in Aqueous Piperazine/Alkanolamines Solutions at Stripper Conditions'

Experimental Study on CO2 Solubility in Aqueous Piperazine/Alkanolamines Solutions at Stripper Conditions Academic research paper on "Chemical engineering"

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{"CO2 caputure" / "amine absorbent" / Piperazine / stripper / VLE}

Abstract of research paper on Chemical engineering, author of scientific article — Shota Inoue, Takuya Itakura, Takao Nakagaki, Yukio Furukawa, Hiroshi Sato, et al.

Abstract Vapor liquid equilibrium (VLE) experiments using pressurized vessel operated at wide range of temperatures from 40°C to 120°C were conducted with four binary solutions of Piperazine (PZ) and alkanolamines, which are 2- Isopropylaminoethanol (IPAE), Monoethanolamine (MEA), Diethanolamine (DEA) and Methyldiethanolamine (MDEA). Effective CO2 loadings of some PZ based binary solutions, which meant the difference between the CO2 loadings at the consistent absorber and stripper conditions, increased in comparison with the PZ only solution and especially, PZ/MDEA showed the largest effective CO2 loading. CO2 loadings at the stripper condition did not depend on additive amount of MDEA to PZ and was the same CO2 loading as PZ only solution, while those increased with increase in MDEA concentration at the absorber condition. According to the reaction products analysis for PZ/MDEA samples operated at different temperature by using 13C-NMR spectroscopy, the bicarbonate in the solution decreased significantly with increase in temperature, but the PZ carbamate dissociated CO2 much less than the carbamate even at 120°C.

Academic research paper on topic "Experimental Study on CO2 Solubility in Aqueous Piperazine/Alkanolamines Solutions at Stripper Conditions"

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Energy Procedia 37 (2013) 1751 - 1759

GHGT-11

Experimental study on CO2 solubility in aqueous Piperazine/alkanolamines solutions at stripper conditions

Shota Inoue1*, Takuya Itakura1, Takao Nakagaki1, Yukio Furukawa2, Hiroshi Sato3 and Yasuro Yamanaka4

1 Department of Modern Mechanical Engineering, Graduate School of Creative Science and Engineering, Waseda University,

3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan 2 Department of Chemistry and Biochemistry, Graduate School of Advanced Science and Engineering, Waseda University 3 Research Laboratory, IHI Corporation, 1, Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan _4 Energy Operations, IHI Corporation, 1-1, Toyosu 3-chome, Koto-ku, Tokyo 135-8710, Japan_

Abstract

Vapor liquid equilibrium (VLE) experiments using pressurized vessel operated at wide range of temperatures from 40 °C to 120 °C were conducted with four binary solutions of Piperazine (PZ) and alkanolamines, which are 2-Isopropylaminoethanol (IPAE), Monoethanolamine (MEA), Diethanolamine (DEA) and Methyldiethanolamine (MDEA). Effective CO2 loadings of some PZ based binary solutions, which meant the difference between the CO2 loadings at the consistent absorber and stripper conditions, increased in comparison with the PZ only solution and especially, PZ/MDEA showed the largest effective CO2 loading. CO2 loadings at the stripper condition did not depend on additive amount of MDEA to PZ and was the same CO2 loading as PZ only solution, while those increased with increase in MDEA concentration at the absorber condition. According to the reaction products analysis for PZ/MDEA samples operated at different temperature by using 13C-NMR spectroscopy, the bicarbonate in the solution decreased significantly with increase in temperature, but the PZ carbamate dissociated CO2 much less than the carbamate even at 120 °C.

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

Keywords: CO2 caputure, amine absorbent, Piperazine, stripper, VLE

1. Introduction

Post combustion CO2 Capture (PCC) is gaining widespread interest as one of the most practical technologies for controlling greenhouse gas emissions. In particular, PCC can be retro-fitted to existing BTG power plants and offer a flexible operation because of the downstream process. CO2 recovery from

* Corresponding author. Tel.: +81-3-5286-2497; fax: +81-3-5286-2497. E-mail address: 903-shota@ruri.waseda.jp.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi: 10.1016/j.egypro.2013.06.051

flue gas by amine absorption has been demonstrated in many projects because aqueous amine solutions have been established as a mature technology for PCC. However, those of technologies still have a problem of significant energy consumption in regenerating a large amount of CO2 absorbed solution. To address this problem, many works have focused on the development of efficient aqueous amine solutions for CO2 removal from flue gas. Regeneration energy required for CO2 recovery in stripper correlates with three basic chemical properties: the reaction heat of CO2 removal, the vapor-liquid equilibrium (VLE), and the CO2 absorption rate.

Many previous works [12] [23] reported that the reaction of CO2 with aqueous alkanolamines formed carbamate or bicarbonate. The products ratio of carbamate and bicarbonate is determined by equilibrium and rate-base reaction between CO2 and the alkanolamine. In general, the rate of CO2 absorption reaction with carbamate formation, which is typically caused by primary or secondary amines and expressed as Eq.1.1 is faster than that with bicarbonate formation, which is typically caused by tertiary amines and expressed as Eq.1.2. On the other hand, the heats of CO2 absorption reactions involving bicarbonate formation tend to be lower than that involving carbamate formation. Both features, i.e., faster CO2 absorption rate and lower heat of absorption contributes to decrease energy consumption in CO2 recovery systems because the former leads to reduce circulation flow rate of solutions and the latter is associated with the reduction of heat required in the stripper. In other words, there is a trade-off between carbamate and bicarbonate formation in terms of reducing energy consumption, and some mixed aqueous solutions of alkanolamines and other promoter are reported as candidates to solve this problem. Piperazine, which is a cyclic amine and readily absorbs CO2 as mono-carbamate (PZCOO-) and di-carbamate (PZ(COO-)2) expressed as Eq.1.3 and 1.4, respectively, is commonly used for as a promoter of alkanolamines. Figure 1 shows literature review of CO2 solubility in aqueous PZ/alkanolamines solutions. Few works have discussed the reaction mechanisms of mixed aqueous solutions of PZ and alkanolamines under practical condition of the stripper, while many works have revealed them at the absorber condition. For this reason, the experiment of the stripper condition requires some efforts in the procedure which are diluting the solution and analyzing it quickly to avoid dissociation of products and ensure accuracy and reproducibility of the experimental data in addition to extracting a high-temperature liquid solution from the pressurized vessel.

CO2 + RJR2-NH O RJR^NH+COO" (1.1)

CO2 + RjR2R3-N + H2O R!R2R3-NH+ + HCO3- (1.2)

PZ + HCO3- O PZCOO- + H2O (1.3)

PZCOO- + HCO3- o PZ(COO-)2 + H2O (1.4)

Typical absorber A

conditions ±

V 8 J Í

.AX .û. ^A A

Typical A stripper conditions

Ê - A

Plot Source

A GT Rochelle et al. [2] [3] [4] [5][13][19]

A A Samanta et al. [8] [21] [22]

▲ V Ermatchkov et al. [5] [15]

▲ PWJ Derks et al. [9] [10] [11]

F Bougie et al. [6]

P- Chung et al. [7]

A L Dubois et al. [12]

H- Liu et al. [17]

Á Pérez-Salado Kamps et al. [20]

0 20 40 60 80 100 120 140 Temperature oC

Figure 1 Literature review of CO2 solubility in aqueous PZ/alkanolamines solutions

In this study, we mainly focused on the development of PZ-based solutions which can enhance effective CO2 loading. Effective CO2 loading is obtained by the difference between two CO2 loadings at the absorber and the stripper conditions and shows a capacity index affecting the circulation flow rate of the solution. In addition, 13C-NMR spectroscopy was used to identify possible products resulting from the reaction of CO2 and the test solutions.

2. Experimental

2.1. Material

Water was distilled twice. PZ (final purity >98.0%), IPAE (final purity >99.0%), MEA (final purity>99.0%), DEA (final purity >99.0%) and MDEA (final purity >99.0%) were purchased from Tokyo Kasei Kogyo Co. Ltd. Carbon dioxide was prepared by liquefied gas cylinder and purity of feed CO2 gas was 99.0%.

2.2. Apparatus and procedure

Figure 2 shows the experimental apparatus used for vapor-liquid equilibrium tests which consists of a gas supply system, a 700-mL stainless steel cylindrical vessel (autoclave) and a CO2 analyzer (Model: VA 3000, HORIBA, Japan). The autoclave, which was designed to operate at temperatures up to 200 °C and pressures up to 2 MPa, was submerged in the silicon-oil bath and the temperature of the test solution inside was controlled to within ±0.2 °C at 40 °C and +0.4 °C at 120 °C stirring with the mechanical stirrer. Before starting experiment, the reactor vessel and outlet gas line were purged with N2sufficiently. The CO2 partial pressure was set by the inner pressure of the autoclave controlled by the back pressure regulator and the CO2 concentration of the feed gas saturated with the water vapor at the inner temperature. The outlet gas temperature was controlled by the low temperature circulator and the water-saturator controlled at the same temperature compensated the vapor loss of the outlet gas.

Reaction equilibrium was determined by stable CO2 concentration in the outlet gas detected by the CO2 analyzer, which typically took a few hours. The sample of the test solution was ejected into a pre-chilled 50-mL chamber and was quickly diluted with pure water by one hundredth exactly. The diluted sample was analyzed immediately by the Total Organic Carbon analyzer (TOC-VCH, Shimadzu) to quantify the CO2 loading in the test solution.

Table 1 Experimental condition of the validation test

Amine solution 1MMEA

CO2 partial pressure in inlet gas (kPa) 1 to 100

Solution temperature (°C) 40 to 120

Solution volume (mL) 200

Inlet gas flow rate (ccm) 1000

Mass flow controller

Inlet gas sampling line

■ ■

Back pressure regulator

Saturator Thermocouple Reactor

Oil Bath

vessek I

Gas analyzer

Outlet gas sample line

Low temp. I circulator

□ HX—►

Liquid \ sampling line Condenser

Heater

Figure 2 Experimental apparatus of the VLE test

3. Result and discussion

3.1. Validation of the experimental data

In order to validate our experimental data, a VLE test was conducted using 1M MEA as a reference amine solution which was published in many papers. Figure 3 shows VLE data of MEA/CO2/H2O at 40 °C and 120 °C plotting CO2 partial pressure vs. CO2 loading. The VLE data of this work is good agreement with the data of literature [16]. Each plot of this work is shown as each average of three measurements and the reproducibility error in this work was within 2% and 4% in the y and x axes, respectively. In this result, the effective CO2 loading of MEA is 0.18, which is indicated by the length of the arrow in Figure 3, and effective CO2 loadings in the following discussion are defined as differences between these temperatures and partial pressures.

120°C ♦ This work O Lee et al. (1976)[16]

? Stripper conditions . ■ A ' A r A : â ; A 1 0 40°C 1 Absorber ^ 1 conditions 1 ^ ' Effective i CO2Loading ,=! 1, ,

0 0.2 0.4 0.6 0.8 1

CO2Loading mol-CO2/mol-amine

Figure 3 VLE data of MEA/CO2/H2O at 40 °C and 120 °C plotting CO2 partial pressure vs. CO2

loading

3.2. CO2 loadings of PZ/alkanolamines solutions

VLE experiments with binary solutions of PZ and alkanolamines were conducted in which 0.33 M PZ was mixed with 0.66 M IPAE, MEA, DEA and MDEA, respectively. Experimental conditions are shown in Table 2 and Table 3. Figure 4 shows equilibrium CO2 loading of each solution at the absorber and stripper conditions in order of effective CO2 loading from left to right which is defined as the CO2 loading difference between the absorber and stripper conditions and is a capacity index affecting the circulation flow rate of the solution. PZ only solution indicated the largest CO2 loading at the absorber condition of all solutions. However, it also indicated the largest CO2 loading at the stripper condition and this resulted one of the smallest effective CO2 loadings of all solutions. In contrast, PZ/MDEA solution indicated the smallest CO2 loading at the absorber condition, but it also indicated the largest effective CO2 loading due to the smallest CO2 loading at the stripper condition. Many paper reported that secondary or tertiary alkanolamines produce mainly bicarbonate in CO2 absorbing reactions, while PZ mainly produce carbamate. This result implies that the difference of products affects the CO2 loading at the stripper condition.

Table 2 Experimental conditions of the VLE test for binary solutions of PZ/alkanolamines

Absorber condition Stripper condition

Temp. (°C) 40 120

Pco2 (kPa) 10 100

Table 3 Test solutions of the VLE test for binary solutions of PZ/alkanolamines

Base amine Additive alkanolamines

PZ IPAE MEA DEA MDEA

0.33 mol-L"1 0.66 mol-L"1

(D 0.8

SE ro 0.7

"n 0.6

O u 0.5

o £ 0.4

M c 0.3

ra n 0.2

□ Absorber □ Stripper «Effective

PZ only PZ/IPAE PZ/MEA PZ/DEA PZ/MDEA Figure 4 Equilibrium CO2 loading of each PZ/alkanolamine solution at the absorber and stripper

conditions

3.3. Dependency on MDEA concentration in binary solutions for CO2 loading

PZ/MDEA solution has a large effective CO2 loading and this feature attracts attention as an alternate solution for conventional MEA. Additional VLE experiments with different blend of PZ/MDEA solutions were conducted to examine the dependency on MDEA concentration for CO2 loading. Five different solutions mixed constant PZ concentration 1.16 M (10 wt%) and various MDEA concentrations ranging from 0 to 3.36 M (40 wt%). Figure 5 shows the dependency on MDEA concentration for the CO2 loading per molar PZ at the absorber and stripper conditions. As shown in Figure 5, the CO2 loadings at the stripper condition were almost constant, while the CO2 loadings at the absorber condition increased with increasing MDEA concentration. In other words, the CO2 loading at the stripper condition does not depend on MDEA concentration as an additive for PZ and this result implies that the bicarbonate derived from MDEA dissociates easily at the stripper condition compared to the dissociation of carbamates formed mainly by PZ.

Table 4 Experimental conditions of the VLE test for various blends of PZ/MDEA

Temp. (°C) Pco2 (kPa)

Absorber condition

Stripper condition

120 100

Table 5 Test solutions of the VLE test for various blends of PZ/MDEA

PZ concentration MDEA concentration

(mol-L-1) (wt%) (mol-L-1) (wt%)

0.84 10

1.16 10 1.16 13.8

1.68 20

3.36 40

MDEA concentration mol/L

Figure 5 Dependency on MDEA concentration for the CO2 loading per molar PZ at the absorber and

stripper conditions

3.4. Dependency on temperature for bicarbonate production in PZ/MDEA solutions

Analyzing products resulting from the reaction of CO2 and PZ/MDEA solutions, especially quantitative analysis of carbamates and carbonate or bicarbonate, is helpful to understand why the CO2 loadings at the stripper condition are almost constant. 13C-NMR spectroscopy analyses were performed on mixed 1.16 M PZ/1.68 M MDEA solutions saturated with CO2 at several temperatures with constant CO2 partial pressure of 100 kPa. Table 6 shows chemical reactions in the PZ/MDEA/CO2/H2O system from literature [24]. In Figure 6, the stacked bar chart shows the breakdown of reaction products associated with CO2 per PZ , which are PZ-carbamate (PZCOO-)/protonated PZ carbamate (H+PZCOO-), PZ-dicarbamate (PZ(COO-)2), MDEA carbonate (MDEACO3-) and bicarbonate (HCO3-)/carbonate(CO32-) at temperatures of 40, 80, 100 and 120 °C comparing PZ/MDEA mixture and PZ only. Figure 6 also shows analytical results computed by ASPEN PLUS process simulator V7.3 under the condition corresponding to each experimental condition. In this simulation software package, the rate-based PZ/MDEA model that consists of a single-stage equilibrium flash unit operation and four streams is provided to simulate the CO2 absorption from a gas stream of CO2 and N2 by the solution. The flash model calculates five equilibrium reversible reactions and four pairs of rate-controlled forward and backward reactions, which are shown in Table 6. In order to simulate the reactor vessel in which the reactant gas flows into the pool of PZ/MDEA solution continuously until the system achieves equilibrium, several flash blocks are serially concatenated by the gas and liquid phase streams respectively until all chemical components are constant. In experimental result at 40 °C, PZ/MDEA solution produced PZ-

dicarbamate and bicarbonate (including carbonate) more than PZ solution. MDEA behaves as a proton acceptor and this behavior avoids protonation of PZ and keeps pH at a high level. Consequently, carbamates and bicarbonate formation reactions associated with CO2 absorption, such as Eq.6, 8 and 10, are promoted effectively, which leads to the enhancement of CO2 loading. The amount of PZ carbamate including protonated PZ carbamate did not depend on temperatures below 100 °C whether MDEA was added or not, where they comparatively decreased at 120 °C. In contrast, bicarbonate including carbonate remarkably decreased with increase in temperature. Increase of CO2 loading due to MDEA addition shrank with increase in temperature and eventually there was little difference at 120 °C. These results establish that the bicarbonate derived from MDEA dissociates easily at the stripper condition compared to the dissociation of carbamates formed mainly by PZ. In comparison between experimental and analytical results in PZ/MDEA solutions, amount of PZ-di-carbamate significantly different at temperatures of 80 °C or higher, while both sums of the reaction products, i.e., the CO2 loadings of PZ/MDEA was almost comparable to each other. The results imply that the reaction rates of Eq.10 or Eq.11 [2] [4] are possibly underestimated or overestimated, respectively.

Table 6 Chemical reactions in the PZ/MDEA/CO2/H2O system [24]

Eq. Reactions

(1) Equilibrium 2H2O O H3O+ + OH-

(2) Equilibrium HCO3- + H2O O CO32- + H3O+

(3) Equilibrium PZH+ + H2O PZ + H3O+

(4) Equilibrium HPZCOO + H2O PZCOO- + H3O+

(5) Equilibrium MDEAH+ + H2O MDEA + H3O+

(6) Kinetic CO2 + OH- -»• HCO3-

(7) Kinetic HCO3- -» CO2 + OH-

(8) Kinetic PZ + CO2 + H2O PZCOO- + H3O+

(9) Kinetic PZCOO- + H3O+ - PZ + CO2 + H2O

(10) Kinetic PZCOO- + CO2 + H2O -»• PZ(COO-)2 + H3O+

(11) Kinetic PZ(COO-)2 + H3O+ -»• PZCOO- + CO2 + H2O

(12) Kinetic MDEA + CO2 + H2O MDEAH+ + HCO3-

(13) Kinetic MDEAH+ + HCO3- -»• MDEA + CO2 + H2O

-5 1.4

¿0 1.2 u

"5 1.0

O 0.8 u

Analytical

Experimental

Analytical

Experimental

□ HCO3-, CO32-

□ PZ (COO")2 Analytical I Experimental ■ PZCOO-, H+PZCOO-

■ MDEA carbonate

^ ^ ^ Analytical j Experimental

PZ/MDEA I PZ PZ/MDEAj PZ PZ/MDEA! PZ PZ/MDEA PZ 40 °C 80 °C 100 °C 120 °C

Figure 6 Temperature dependency of reaction products associated with CO2 (Solution: 1.16 M PZ/1.68 M MDEA PCO2: 100 kPa)

4. Conclusion

VLE experiments using pressurized vessel operated at wide range of temperatures from 40 °C to 120 °C were conducted with four binary solutions of PZ and alkanolamines, such as IPAE, MEA, DEA and MDEA. Effective CO2 loadings, which meant the difference between the CO2 loadings of the absorber and stripper conditions, increased in some PZ based binary solutions compared to the PZ only solution. In particular, the effective CO2 loading of PZ/MDEA solution increased significantly, indicated the largest increase among the four mixed solutions.

Effect of additive amount of MDEA on CO2 loadings was constant at the stripper condition. CO2 loadings of blended PZ/MDEA solutions are almost the same as the PZ only at the stripper condition, while those increased with increase in MDEA concentration at the absorber condition. This feature expands the effective CO2 loading of PZ/MDEA binary solution compared to that of PZ only solution.

According to the reaction products analysis for PZ/MDEA samples operated at different temperature by using 13C-NMR spectroscopy, the bicarbonate (including carbonate) in the solution decreased significantly with increase in temperature, while the PZ mono-carbamate (including protonated one) eventually dissociated at 120 °C. These results imply that bicarbonate in these solutions at the stripper condition release more CO2 than carbamates and underpin the result that CO2 loading in PZ/MDEA solutions under stripper condition was not affected by MDEA concentration. Consequently, mixed aqueous solution of PZ and alkanolamines, especially mixing MDEA that mainly form bicarbonate, could be an effective way to not only decrease heat of reaction but also expand the effective CO2 loading, which contributes to reducing the regeneration energy requirement. As far as this VLE test result of PZ/MDEA binary solutions, PZ does not contribute to expand the effective CO2 loading associated with the circulation flow rate of the solution in the PCC plant, but further discussion not only in equilibrium but also in rate-base reaction analysis is concurrently required because PZ has another aspect as an absorption promoter for MDEA.

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