Scholarly article on topic 'Synthesis, solubilities, and cyclic capacities of amino alcohols for CO2 capture from flue gas streams'

Synthesis, solubilities, and cyclic capacities of amino alcohols for CO2 capture from flue gas streams Academic research paper on "Chemical sciences"

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{"Amino alcohol" / "Carbon dioxide capture" / Solubility / "Cyclic capacity" / Synsthesis / "Flue gas"}

Abstract of research paper on Chemical sciences, author of scientific article — Kreangkrai Maneeintr, Raphael O. Idem, Paitoon Tontiwachwuthikul, Andrew G.H. Wee

Abstract Amines that have been widely used in post combustion CO2 capture processes are monoethanolamine (MEA), diethanolamine (DEA) and N-methyldiethanolamine (MDEA). If used individually, these solvents have their limitations, and efforts to resolve these have produced formulated solvents consisting of blends of amines and some chemical additives. The advantages derivable from amine blends are also limited to commercially available individual amines. It is therefore desirable to synthesize new amines or amino alcohols that could incorporate the advantages of amine blends in the same molecule or provide new materials for blending in a formulated solvent. Recently, such amino alcohols have been synthesized based on an approach of rational molecular design and synthesis. This involved a systematic modification of the structure of amino alcohols by an appropriate placement of substituent functional groups, especially the hydroxyl function, relative to the position of the amino group. Some of the resulting amino alcohols were 4-(diethylamino)-2-butanol (Reg 1); 4-(piperidino)-2-butanol (Reg 2); 4- propylamino-2-butanol (Reg 3) and 4-(ethyl-methyl-amino)-2-butanol (Reg 4). The performance of these amino alcohols in aqueous solutions in terms of solubility of CO2 and cyclic capacity were compared with those of aqueous MEA using tests conducted at temperatures of 40, 60 and 80 ∘C at CO2 partial pressures of 15 and 100 kPa. All the listed amino alcohols provided a much higher CO2 absorption capacity than MEA with Reg 3 showing the highest absorption capacities at all the temperature considered. The cyclic capacity (derived as the difference between the solubilities at 40 and 80 ∘C) of the listed solvents were also much higher than that for MEA with Reg 4 showing the highest cyclic capacity. These characteristics result in a much higher CO2 absorption and a much less energy consumption for absorbent regeneration, such as in CO2 stripping, compared to conventional amines.

Academic research paper on topic "Synthesis, solubilities, and cyclic capacities of amino alcohols for CO2 capture from flue gas streams"

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Energy Procedia 1 (2009) 1 3327-1334

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GHGT-9

Synthesis, Solubilities, and Cyclic Capacities of Amino Alcohols for CO2 Capture from Flue Gas Streams

Kreangkrai Maneeintra, Raphael O. Idema*, Paitoon Tontiwachwuthikula,

Andrew G. H. Weeb

aInternational Test Centre for Carbon Dioxide Capture (ITC), Faculty of Engineering, University of Regina, SK, Canada bDepartment of Chemistry, University of Regina, Regina, SK, Canada

Abstract. Amines that have been widely used in post combustion CO2 capture processes are monoethanolamine (MEA), diethanolamine (DEA) and N-methyldiethanolamine (MDEA). If used individually, these solvents have their limitations, and efforts to resolve these have produced formulated solvents consisting of blends of amines and some chemical additives. The advantages derivable from amine blends are also limited to commercially available individual amines. It is therefore desirable to synthesize new amines or amino alcohols that could incorporate the advantages of amine blends in the same molecule or provide new materials for blending in a formulated solvent. Recently, such amino alcohols have been synthesized based on an approach of rational molecular design and synthesis. This involved a systematic modification of the structure of amino alcohols by an appropriate placement of substituent functional groups, especially the hydroxyl function, relative to the position of the amino group. Some of the resulting amino alcohols were 4-(diethylamino)-2-butanol (Reg 1); 4-(piperidino)-2-butanol (Reg 2); 4-propylamino-2-butanol (Reg 3) and 4-(ethyl-methyl-amino)-2-butanol (Reg 4). The performance of these amino alcohols in aqueous solutions in terms of solubility of CO2 and cyclic capacity were compared with those of aqueous MEA using tests conducted at temperatures of 40, 60 and 80°C at CO2 partial pressures of 15 and 100 kPa. All the listed amino alcohols provided a much higher CO2 absorption capacity than MEA with Reg 3 showing the highest absorption capacities at all the temperature considered. The cyclic capacity (derived as the difference between the solubilities at 40 and 80oC) of the listed solvents were also much higher than that for MEA with Reg 4 showing the highest cyclic capacity. These characteristics result in a much higher CO2 absorption and a much less energy consumption for absorbent regeneration, such as in CO2 stripping, compared to conventional amines.

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Keywords: Amino alcohol; Carbon dioxide capture; Solubility; Cyclic capacity; Synsthesis; Flue gas

1. Introduction

Aqueous amines solutions are widely used in industrial processes such as petroleum refinery and fuel-fired power plant gas streams chiefly to capture acid gases such as carbon dioxide (CO2) widely blamed for global warming effects and climate change. The acid gas is absorbed into the solution at lower temperatures and desorbed from the solution by heating to higher temperatures. The amines that have been extensively used in gas processes

doi:10.1016/j.egypro.2009.01.174

are monoethanolamine (MEA), diethanolamine (DEA) and N-methyldiethanolamine (MDEA). If used individually, these solvents have their limitations, and efforts to resolve these have produced formulated solvents consisting of blends of amines and some chemical additives. The advantages deriveed from amine blends are also limited to commercially available individual amines. A new class of amines referred to as sterically hindered amines, has also been introduced as commercially attractive amines including 2-Amino-2-Methyl-1-Propanol (AMP), which requires much less energy for regeneration [1-3]. In addition, a synthetic amine which can be broadly defined as an amine that has been specifically designed to perform specific tasks, for example, selective separation of H2S from light hydrocarbons in the presence of CO2, bulk separation of CO2, etc [4-8] has been developed.

The advantage of synthetic amines over alkanolamines is that only one mole of the synthetic amine is required to react with 1 mol of CO2 instead of 2 moles in the case of some alkanolamines. Therefore, with much higher chemical solvent cyclic capacities (CC), the thermal energy consumption for CO2 regeneration is expected to be significantly less than the MEA process. Moreover, the solvent circulation rate which is one of the most important factors in the economics of gas treating with chemical solvents is much lower, with high capital cost savings [9]. Recently, synthesized amino alcohols, based on an approach of rational molecular design and synthesis, have been developed [10]. This involves a systematic modification of the structure of amino alcohols by appropriate placement of substituents, especially hydroxyl function, relative to the position of the amino group. These new amino alcohols were designed to promote CO2 capture performance and to study the effect of such placement of functional groups on the performance of the amino alcohols for CO2 capture. The names and molecular structures of these new synthesized amino alcohols are listed in Figure 1.

To evaluate the CO2 capture performance, the solubilities of the new synthesized amino alcohols have been studied for the conditions at 3 mol/L and the temperature of 40°C, 60°C, 80°C and at partial pressures around 15 and 100 kPa. Similar studies under the same conditions were also conducted using MEA because it is considered to be the most commercially used alkanolamine [11,12]. The results for the synthesized amino alcohols were then compared with those for MEA as baseline. Moreover, one of these new synthesized amino alcohols, 4-diethylamino-2-butanol (DEAB), was selected for extensive evaluation of solubility at 2 mol/L and the temperature from 25°C to 80°C and partial pressures ranging from 5 to 100 kPa. A parallel solubility study of MEA was also conducted at the same conditions for comparison of their performance in terms of absorption capacity and cyclic capacity.

2. Experimental Section

2.1 Chemicals. The solvents evaluated were MEA, (Fisher scientific with 99+ %) and the new synthetic amino alcohols which were synthesized in our laboratory [10]. Aqueous solutions of these materials were prepared using distilled water to the desired concentration of 2 or 3 mol/L of solution. Nitrogen and CO2 (Praxair Inc.) with purities of 99.9% were also used in the evaluation. All the materials were used without further purification.

2.2 Equipment and Procedure. The schematic diagram of the experimental setup used in this study is shown in Figure 2. The experimental apparatus consists of a saturation cell connected with reactor. Both the cell and the reactor were immersed in a constant temperature water bath (Cole-Parmer) maintained at the indicated temperatures by using a temperature controller (Cole-Parmer within the temperature ranging from -20 to 200°C with ± 0.01°C accuracy). The temperature in the system was measured by a J-type thermocouple ranging from - 40 to 150°C with a

4-diethylamino-2-butanol

4-isopropylamino-2-butanol

4-p ip eridino-2-butanol

4-propylamino-2-butanol

4-(ethyl-methyl-amino)-2-butanol

Figure 1. Chemical structures of synthesized amino alcohols

Figure 2. Schematic diagram of the experimental setup: (1) Flow meter (2) Saturation cell (3) Equilibrium cell (4) Pressure gauge (5) Thermocouple (6) Condenser (7) Temperature controller

resolution of ± 0.01°F/C and ± 0.03°C accuracy. Flow meters used in this system were electronic Aalborg GFM-17 gas flow meters with ± 0.15% /°C full scale accuracy which was calibrated by a digital bubble flow meter (Agilent Technologies, model HFM-650) ranging from 5mL to5 L/min with accuracy ±2% of reading. The CO2 concentration in the gas stream was measured with a portable infrared (IR) CO2 gas analyzer from Nova Analytical System Inc. (Hamilton, ON), model 302 HWP ranging from 0.0-100.0% CO2 with ±1% accuracy.

Initially, the solution was fed into the system and the gas mixture were introduced to the system at the desired partial pressure and saturated with moisture content in the saturation cell to maintain the solution concentration. The wetted gas mixture was then bubbled through the amine test solution and eventually exhausted. The gas was sent to the condenser used for recovering moisture in the gas stream before being vented to the fume hood. The process was operated under atmospheric pressure.

To ensure that equilibrium was reached, the system was kept in operation for 8-14 hours. Subsequently, the liquid sample was taken to analyze for the CO2 loading. CO2 was evaluated 3 times, for which the sample was taken every one or two hours until the CO2 loading was constant or until two consecutive readings show only a slight difference (< ± 0.05 difference). The CO2 loading for each liquid sample was determined by the titration method [13]. CO2 loading reported were the average of the three equilibrium data points. For most of the system the repeatability of the CO2 loading was generally within ±10%.

3. Results and Discussion

To verify the applicability of the experimental setup and the procedure used in this study, the solubility of CO2 in 2M MDEA aqueous solutions at 40 °C was also measured at the partial pressure of CO2 ranging from 10 to 100 kPa and compared with the previously reported data in the literature [14-16]. As shown in the Figure 3, the results are found to be in good agreement with literature over the entire pressure range considered in this study. The average and maximum percent absolute deviations between this study and previous work obtained by Jou et al.14 are 4.31 and 10.19, respectively.

Solubility of CO2 in 2.0M MDEA at 40 °C

CO2 loading (mol CO2/mol amine)

Figure 3. Comparison of this work and literature solubility of CO2 in 2.0M MDEA solution at 40°C

The experimental results of the new solvents compared to MEA at various conditions are presented in Table 1-6. From the experimental results, all new amino alcohols have better performance than MEA and seem to be effective solvents for CO2 capture.

In Table 2-6 at 40 °C, the new solvents provide a much higher CO2 absorption capacity than MEA. The absorption capacity differences compared with MEA at various operating conditions are higher by 35 to 39% for 4-diethylamino-2-butanol, 33 to 37% for 4-isopropylamino-2-butanol, 22 to 37% for 4-piperidino-2-butanol, 12 to 23% for 4-propylamino-2-butanol and 14 to 37% for 4-(ethyl-methyl-amino)-2-butanol. At 80 °C, 4-diethylamino-2-butanol and 4-piperidino-2-butanol have slightly higher CO2 absorption capacities than MEA ranging from 15 to 24% and 25 to 31% higher, respectively. The absorption capacity of 4-isopropylamino-2-butanol was 33-72% higher than MEA. In contrast, 4-propylamino-2-butanol and 4-(ethyl-methyl-amino)-2-butanol provide CO2 absorption capacity lower than MEA. These were 4%, and 40 to 70%, respectively.

For the cyclic capacity presented in Table 1-6, all of the amino alcohols present a higher cyclic capacity than MEA from 9 to 457%. Particularly, 4-diethylamino-2-butanol and 4-(ethyl-methyl-amino)-2-butanol offer excellent results in terms of higher cyclic capacity higher than MEA ranging from 70 to 122% and 152 to 457%, respectively. This indicates an advantage from the viewpoint of energy efficiency for the new solvents.

Table 1. Solubility of CO2 in 3M MEA solution.

40 °C__60 °C__80 °C CCa

P/kPa aa P/kPa aa P/kPa aa 40-80 °C

15 0.609 15 0.490 15 0.379 0.230

100 0.693 100 0.621 100 0.586 0.107

K. Maneeintr et al. /Energy Procedia 1 (2009) 1327-1334 Table 2. Solubility of CO2 in 3M 4-diethylamino-2-butanol solution.

40 °C__60 °C__80 °C CCa % CC a

P/kPa aa P/kPa aa P/kPa aa 40-80 °C Difference

15 0.826 15 0.637 15 0.437 0.389 69.13

100 0.962 100 0.840 100 0.724 0.238 122.43

a mol of CO2/mol of Amine.

Table 3. Solubility results of CO2 in 3M 4-isopropylamino-2-butanol solution.

40 °C__60 °C

P/kPa aa P/kPa aa 15 0.836 15 0.698 100 0.919 100

80 °C CCa % CC a

P/kPa aa 40-80 °C Difference

15 0.650 0.186 -19.13

100 0.781 0.138 28.97

a mol of CO2/mol of Amine.

Table 4. Solubility of CO2 in 3M 4-piperidino-2-butanol solution.

40 °C__60 °C__80 °C CCa % CC a

P/kPa aa P/kPa aa ~P/kPa aa 40-80 °C Difference

15 0.747 15 0.593 15 0.437 0.250 8.70

100 0.947 100 0.807 100 0.724 0.212 98.13

a mol of CO2/mol of Amine.

Table 5. Solubility of CO2 in 3M 4-propylamino-2-butanol solution.

40 °C__60 °C

P/kPa aa P/kPa aa 15 0.681 15 0.541 100 0.849 100 0.729

80 °C CCa % CC a

P/kPa aa 40-80 °C Difference

15 0.396 0.285 23.91

100 0.565 0.284 165.4

K. Maneeintr et al. /Energy Procedia 1 (2009) 1327-1334 Table 6. Solubility of CO2 in 3M 4-(ethyl-methyl-amino)-2-butanol solution.

40 °C__60 °C__80 °C CCa % CC a

P/kPa aa P/kPa aa P/kPa aa 40-80 °C Difference

15 0.695 15 0.323 15 0.115 0.580 152.2

100 0.948 100 0.889 100 0.352 0.596 457.0

a mol of CO2/mol of Amine. Table 7. Solubility of CO2 in 2M MEA solution.

25 °C 40 °C 60 °C 80 °C CCa 25-80 °C

P/kPa a a P/kPa a a P/kPa a a P/kPa a a

9.83 0.613 10.03 0.573 9.83 0.502 10.03 0.425 0.188

15.30 0.643 15.10 0.601 15.10 0.516 15.50 0.450 0.193

30.50 0.679 30.50 0.641 30.60 0.553 30.90 0.488 0.191

50.87 0.722 50.87 0.684 50.76 0.577 50.46 0.520 0.202

75.89 0.747 76.10 0.701 76.30 0.601 76.50 0.542 0.205

99.70 0.799 99.50 0.745 99.91 0.624 99.60 0.558 0.241

a mol of CO2/mol of Amine.

Table 8. Solubility of CO2 in 2M 4-diethylamino-2-butanol solution.

25 °C 40 °C 60 °C 80 °C CCa 25-80 °C

P/kPa a a P/kPa a a P/kPa a a P/kPa a a

9.83 0.919 10.03 0.834 9.83 0.615 9.93 0.402 0.517

15.30 0.938 14.90 0.880 15.00 0.675 15.10 0.460 0.478

30.50 0.952 30.20 0.918 30.60 0.765 30.30 0.547 0.405

50.87 0.968 50.76 0.946 51.07 0.834 50.97 0.620 0.348

75.89 0.980 76.20 0.970 76.10 0.880 76.30 0.679 0.301

99.70 1.002 99.70 0.990 99.70 0.927 99.60 0.721 0.281

a mol of CO2/mol of Amine.

One of the new amino alcohols, 4-diethylamino-2-butanol (DEAB), was selected for further evaluation. Experimental data for the solubility of CO2 in 2.0M of MEA and DEAB in terms of absorption capacity and the ease of regeneration were determined at temperatures of 25, 40, 60 and 80°C at partial pressures ranging from 9 to 100 kPa as shown in Table 7 and 8, respectively. At a low temperature, the absorption capacity of CO2 in DEAB is superior to that in MEA. However, at a high temperature, the solubility in the new solvent is higher than that in

MEA. The effects of temperature and partial pressure on the solubility of CO2 in DEAB were also studied. The solubility decreased with an increase in temperature, and increased as partial pressure increased.

At flue gas conditions, 15 kPa CO2, the absorption capacity of DEAB at 25°C is 45.9% higher compared with that of MEA. However, at 80°C, the new solvent has higher CO2 absorption capacity than MEA for 2.2%. The cyclic capacities of CO2 in 2.0M of these solvents from 25°C to 80°C are also presented in Table 7 and 8. From the Table, the cyclic capacity of CO2 of DEAB is 147.7% greater than that for MEA.

Therefore, it is obvious that DEAB and other new synthesized amino alcohols have far greater cyclic capacities than MEA. This will lead to lower circulation rates for the absorption-regeneration system, and indicates an advantage from the viewpoint of energy efficiency in the regeneration of solvent. This advantage can result in lowering the cost for gas treating process thereby making it more economically feasible. However, the solubility data is one of the most important factors in the economics of gas treating. Additional research concerning gas absorption rate needs to be evaluated. As it is well known, physical and transport properties, kinetics and mass transfer also play a substantial role in characterizing the CO2 capture system.

4. Conclusions

The new synthesized amino alcohols provide much higher CO2 absorption capacity and higher cyclic capacity than that of the conventional amine, MEA. It can be deduced that the new solvents have superior performance in terms of absorption and cyclic capacities, which should lead to a reduction in the circulation rate and energy efficiency for solvent regeneration. Finally, this can bring about a lower cost for the gas treating process for CO2 capture.

It has been demonstrated that by way of rational molecular design and placement of functional groups, novel amino alcohols for promoting CO2 capture can be developed. It has been shown that the placement of functional groups within the amino alcohols affects the performance of the amino alcohols in CO2 capture. Thus, there is a structure-performance relationship between amino alcohols and CO2 capture performance. The data obtained from the studies on the interaction of the amino alcohols with CO2 provide a basis for structural refinement and optimization of synthetic amino alcohols. It has been shown that certain amino 2-butanol compounds are highly effective reagents for capturing CO2 from gas streams. Desirable characteristics of these amino alcohols include their capacity to absorb a large amount of CO2 per unit mole and to permit the separation of CO2 and the recovery of the absorbing solution with a low amount of heat energy.

5. Acknowledgement

The financial support provided by The International Test Centre for CO2 Capture, University of Regina, The Natural Sciences and Engineering Research Council of Canada (NSERC), and Natural Resources Canada (NRCan) is gratefully acknowledged.

6. References

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