Experimental Studies on the Effect of Tertiary Amine Promoters in

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Experimental studies on the effect of tertiary amine promoters in aqueous Monoethanolamine (MEA) solutions on the absorption/ stripping performances in Post-Combustion CO2 Capture Hongxia Gao, Zeyang Wu, Helei Liu, Xiao Luo, and ZhiWu Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02390 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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Experimental studies on the effect of tertiary amine promoters in aqueous Monoethanolamine (MEA) solutions on the absorption/stripping performances in Post-Combustion CO2 Capture Hongxia Gao1, Zeyang Wu1, Helei Liu*, Xiao Luo, Zhiwu Liang* Joint International Center for CO2 Capture and Storage (iCCS), Provincial Hunan Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing CO2 Emissions, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, P.R. China

*CORRESPONDING AUTHOR: Tel.: +86-13618481627; fax: +86-731-88573033; E-mail address: [email protected] (H. Liu)[email protected] (Z. Liang). 1

These authors are first co-authors

1

ACS Paragon Plus Environment

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Abstract The absorption/desorption performance of carbon dioxide (CO2) in aqueous 5M monoethanolamine (MEA) solutions in the presence of various solution regeneration promoters were comprehensively investigated using a multiple rapid screening method. The promoters considered were N-methyldiethanolamine (MDEA), N,N-Dimethylethanolamine

N,N ‑ Diethylethanolamine

(DMEA),

1-Dimethylamino-2-propanol(1DMA2P),

(DEEA),

1-Diethylamino-2-Propanol(1DEA2P),

3-Dimethylamino-1-propanol (3DMA1P), 2-(Dimethylamino)-2-methyl-1-propanol (2DMA2M1P),

Triethanolamine(TEA),

3-(Dimethylamino)-1,2-Propanediol

(3DMA-1,2-PD) and 3-(Diethylamino)-1,2-Propanediol (3DEA-1,2-PD). The molar concentration of these promoters added into 5M MEA were set to 1 mol/L. At the atmospheric pressure, the absorption and stripping experiments were carried out at 313.15K and 353.15K, respectively. Especially, the CO2 partial pressure was controlled at constant 15kPa for absorption experiments. In addition, the equilibrium solubility of CO2 was also determined in order to evaluate the driving force of each amine system. The results showed that the highest CO2 equilibrium solubility of 0.5548 mol CO2/mol amine was obtained by MEA/1DEA2P whereas both the fastest absorption and regeneration rates were achieved for aqueous blended MEA/1DMA2P solution which made MEA/1DMA2P to exhibit the highest CO2 cyclic capacity of 1.6710 mol CO2/L. Keywords: absorption/desorption; promoters; monoethanolamine; CO2 cyclic capacity.

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1. INTRODUCTION Post-combustion CO2 capture (PCC) using aqueous alkanolamine solutions as an important technology has been extensively employed to remove CO2 from flue gases, because CO2 emissions constitute the main contributors to global warming 1. The traditional amines such as primary amine, monoethanolamine (MEA), secondary amine, diethanolamine (DEA), and tertiary amine, N-methyldiethanolamine (MDEA) have been widely studied and applied for acid gas removal

2, 3

. However, the

contradiction between high absorption heats and high CO2 absorption rates for traditional amines is currently a key problem in the application of amine for CO2 removal

4, 5

. Thus, the thermodynamic equilibrium at a low and high temperature is

rarely reached, and the ideal CO2 cyclic capacity is usually not valid at specific absorber and stripper conditions in a real amine scrubbing process. There has therefore been a significant effort to improve individual amine absorbents (i.e. amine mixers) and develop novel amine-based systems. Recently, several novel individual amine candidates such as diethylenetriamine (DETA), triethylenetetramine (TETA), piperazine (PZ), 1-methylpiperazine (1MPZ), 2-amino-2-methyl-1-propanol (AMP),

3-dimethylamino-1-propanol (3DMA1P),

1-diethylamino-2-Propanol (1DEA2P), 4-diethylamine-2-butanol (DEAB), and 2-(dimethylamino)-2-methyl-1-propanol (2DMA2M1P), have been investigated with the comparison of existing absorbents for CO2 removal by many researchers 6-21. CO2 absorbed into sterically hindered, polyamines and tertiary amines, which are reported as potential replacements for aqueous MEA systems, have been the subject of most of

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the recent studies. The sterically hindered amines such as AMP exhibit relatively both large absorption rates and CO2 cyclic capacity as well as better stripping performance 22-24

. Unfortunately, precipitation occurs at high aqueous AMP solution concentration

(i.e. over 3mol/L) resulting in the potential for clogging of pipes. Ma’mum et al.

25

focused on selecting 14 new solvents for CO2 absorption performance, showing that aqueous 2.9M 2-((2-aminoethyl)amino)-ethanol (AEEA) exhibited a somewhat higher net CO2 cyclic capacity than MEA. Also, Kim and Svendsen

26, 27

reported the

absorption performance in terms of absorption capacity of aqueous AEEA solution, which showed much higher CO2 capacity but similar absolute values of ∆Habs as compared with those of MEA under same operational conditions. Later, Puxty et al. investigated the CO2 capacity of 76 different aqueous amine solutions, and seven amines consisting of primary, secondary and tertiary amines, were identified as exhibiting outstanding absorption capacities. These were regarded as promising candidates for further investigation. Interestingly, most of these amines share common structural features including steric hindrance and a hydroxyl group within 2 or 3 carbons from the amine function. Dubois and Thomas 16 evaluated the absorption and regeneration parameters of piperidine (PIP), PZ, piperazinyl-1,2-ethylamine (PZEA) and other individual/blended amines at 298K and boiling temperature, respectively. AMP, MDEA/PZ and PIP were considered as potential and interesting solvents in their study. Subsequently, Chowdhury et al.

28, 29

compared the absorption

performance (i.e. absorption rate, amount of CO2 absorbed, CO2 cyclic capacity and reaction heat) of 26 tertiary amines to understand the relationship between chemical

4


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structure-activity and absorbents. Compared with MDEA, seven high performing absorbents including one synthetic amine and N,N-diethylethanolamine (DEEA) showed high absorption rates, high CO2 cyclic capacities and lower or comparable absorption heats, indicating advancements can be obtained from synthetic amines (novel and blends) to find energy efficient absorption solvents. In addition, a novel tertiary amine DEAB has been synthesized and developed for its excellent performances of absorption capacity, kinetics and mass transfer in capturing CO2 30, 31. Furthermore, a comprehensive set of characteristics such as physical prosperities, solubility, absorption heat, mass transfer, regeneration energy consumption and solvent stability (corrosion and degradation) for novel potential absorbents have been widely investigated by many researchers

32-35

. However, some inevitable drawbacks

have been identified for different types of amines, (i) the reaction rate of CO2 and tertiary amines is far lower than that of MEA due to their inherent characteristics; (ii) the diamines with the structure of –NH-CH2-CH2-NH- (such as DETA, tetraethylenepentamine (TEPA)36, and TETA13, 37) show weak or no resistance to thermal/oxidative degradation and corrosion; (iii) the CO2 loading operating range and narrow amine concentration limits the application of

polycyclic amines; and (iv)

their precipitation tendencies and potential for clogging of pipes leads to the investigation of sterically hindered amine as an activator and accelerator in the blends for CO2 removal. Therefore, attention is being focused on amine blends which can combine the favorable features of fast reacting amine and tertiary/sterically hindered amine and which can be viewed as economically attractive absorbents for CO2

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emission reduction. Blended amine solutions have been widely tested by many researchers and found as potential absorbents with outstanding absorption performance as well as lower regeneration energy requirements. The typical reaction between CO2 and blended MEA/MDEA solution was conducted in a laminar jet absorber and a stirred cell reactor by Gonzalez-Garza et al., Rivera-Tinoco and Bouallou, and Sema et al. 38, and they concluded that the CO2 absorption rate is enhanced by adding MEA. Also, Aroonwilas and Veawab, Setameteekul et al. and Naami et al. investigated the mass transfer performance for CO2 absorption into aqueous blended MEA/MDEA solutions in DX structured packing column at various mixing ratios. The experimental results indicated that the blending ratio presents a significant effect on the absorption performance. Based on the excellent performance of AEEA, Guo et al.

39

presented

the equilibrium solubility of CO2 in aqueous AEEA, AEEA/MDEA and AEEA/AMP solutions at CO2 partial pressure from 0.8 to 800 kPa and temperatures of 303, 313 and 323K, and the AEEA-AMP showed the highest absorption rate. The ion speciation analyses and vapor-liquid equilibrium (VLE) of the blended DEAB/MEA system were done by Shi et al., who provided guidance for predicting the amine regeneration. In addition, the CO2 capture with blends of MDEA/DETA, MDEA/PZ, AMP/MEA, and DEEA/MEA were also investigated in terms of kinetics, solubility, mass transfer and CO2 stripping. It was concluded from the studies in the literatures that the blends of MEA and tertiary amines are more attractive because it provides better performance in both absorption rate and reboiler heat duty.

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In the present study, the interaction between MEA and tertiary amines were comprehensively studied at the MEA concentration of 5 mol/L and tertiary amine concentration

of

1

mol/L.

These

tertiary

amines

included

MDEA,

N,N-dimethylacetamide(DMEA), DEEA, 1-Dimethylamino-2-propanol (1DMA2P), 1DEA2P,

3DMA1P,

(3DMA-1,2-PD),

and

2DMA2M1P,

TEA,

3-(Dimethylamino)-1,2-Propanediol

3-(Diethylamino)-1,2-Propanediol(3DEA-1,2-PD).

The

chemical structures of these tertiary amines studied in our work are shown in Figure 1. The equilibrium solubility, absorption rate, CO2 cyclic capacity and regeneration rate were measured to evaluate the performance of the 8 blends using a rapid screening method. Figure 1.

2. EXPERIMENTAL SECTION 2.1 Chemicals Reagent grade MEA, MDEA, DMEA, DEEA, 1DMA2P, 1DEA2P, 3DMA1P, 2DMA2M1P, and TEA were obtained from Shanghai Aladdin Chemical Reagent Co., Ltd, China with purities of ≥ 99%, and Reagent grade 3DMA-1,2-PD and 3DEA-1,2-PD were purchased from Adamas Reagent Co., Ltd, China with purities of ≥ 99%. The solubility experiments and the absorption/desorption screening experiments were conducted using a vapor-liquid equilibrium apparatus given in Figure 2 and a rapid screening apparatus shown in Figure 3, respectively. Both the CO2 and N2 cylinders each with purity of 99.9% were supplied by Changsha Jingxiang Gas Co. Ltd., China. All the blends were prepared by diluting the amines

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with deionized water to the desired concentration of 6 ± 0.04 mol/L. 2.2 Experimental setup and procedure 2.2.1 CO2 Solubility A glass cell reactor with a bubbling distributor was used for determining the CO2 equilibrium solubility of different amine solutions, as represented in Figure 2. The reactor was equipped with a water bath (HANUO, HX20 Shanghai Hanuo Instruments Co., Ltd., China), two gas mass flow meters (model D07, Beijing Sevenstar, China), a gas mixer, a saturation cell and a water condenser. This vapor-liquid equilibrium (VLE) apparatus was described in our previous investigations40. A series of VLE experiments were performed for different amine systems at 313.15 K and atmospheric pressure. For each experiment, the reactor was charged with 20 mL of the amine solution of the desired concentration from the liquid reservoir. The saturated cell were immersed in a water bath to keep the system temperature at 313.15K and to prevent any change in amine concentration. CO2 and N2 gas streams at the desired flow rates controlled by two gas mass flow meters were mixed in a gas mixer. Then, the mixed gas stream (i.e. simulated flue gas) with CO2 partial pressure of 15 kPa and N2 partial pressure of 85 kPa was sprayed into the saturation cell and the absorption reactor in which the CO2 was absorbed by the amine solution. In order to prevent any change in amine concentration, a condenser at top of the reactor was also applied to cool the gas/vapor stream. The experiment stopped when equilibrium was reached, indicated by a constant CO2 partial pressure of 15 kPa for the outlet gas, which can be determined by an infrared CO2 analyzer with ±1.0%

8


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F.S. accuracy (model GS10, Ennix, Germany). Figure 2. The amine concentration and the amount of CO2 of liquid samples were measured using a chittick apparatus with 1M HCl. The CO2 loading is calculated by dividing the total amount of CO2 (mol/L) by the blended amine concentration, and then the unit of CO2 loading expressed as mol CO2/mol amine, as shown in Eqs.1 and 2. Each experiment was repeated for two times to ensure repeatability. C a min eVa min e = ηC HClVHCl

a=

VCO2 22.4 × C HClVHCl

×

(1)

273.15 273.15 + t

(2)

where, Camine and Vamine are the total amine concentration and volume of blended amine solution, respectively; CHCl and VHCl are the concentration and volume of HCl solution; η is the coefficient of chemical reaction; VCO2 is the volume of CO2 released from the amine solution; and t is the room temperature, ◦C. 2.2.2 Solvent Rapid Screening The schematic diagram of the rapid absorbent screening apparatus used for the measurements of the absorption/desorption performance is presented in Figure 3. In brief, the apparatus consisted of a reactor equipped with a thermometer, a condenser, a purge tube for inlet gases, a liquid sample valve, a water bath (model DF-101S, Yuhua, China, with ±1.0% F.S. accuracy), two mass flow meters with ±1.5% F.S. accuracy and two gas cylinders. The operational temperatures of absorption and desorption experiments were controlled by the water bath, which was equipped with a magnetic stirrer to uniformly mix the solution, and to always keep sufficient 9


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interfacial contact area for gas and liquid. The water condenser at the top of the reactor was used to prevent the change in amine concentration. The mass flow meters (model D07, Beijing Sevenstar, China) used to control the actual flow rates of the CO2 and N2 were designed to determine flow rates with ±1.5% F.S. accuracy, respectively. Additionally, the CO2 partial pressure was determined by an infrared CO2 analyzer with ±1.0% F.S. accuracy (model GS10, Ennix, Germany). A detailed description of the rapid absorbent screening apparatus and its operational condition can be found in our previous investigation by Gao et al. 41 Figure 3. All absorption and desorption experiments were carried out at atmospheric pressure and at temperature equal to 313.15 K and 353.15 K, respectively. The hydrodynamic conditions, namely gas flow rates were kept constant for all the experiments while comparing only the use of the various amine blends. For each solvent screening experiment, 200 mL CO2-unloaded amine solution was used as the liquid absorbent to remove the CO2 from the mixed gas with the partial CO2 pressure of 15 kPa. Mixed gases of CO2 and N2 were employed as the feed gas for the absorption experiments, and the flow rates of CO2 and N2 were 150 and 850 mL/min, respectively. Gas and liquid samples were analyzed at 90 min in order to obtain the plots of the absorption rate and CO2 loading versus reaction time. Then, the water bath was preheated from 313.15 to 353.15K, and the reactor, loaded with rich amine solution, was placed in water bath at 80◦C. For desorption experiment, the CO2 was recovered from the rich amine solution (i.e. the rich amine solution was regenerated) by heating, stirring and

10


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nitrogen stripping with the N2 flow rate of 850mL/min for 90mins. Also, the CO2 loading of the liquid samples determined by titration using HCl and the CO2 concentration of the outlet gas measured with an infrared CO2 analyzer were collected during the desorption experiments. In addition, the liquid and gas samples were determined at an interval of 6mins during both the absorption and regeneration experiment, respectively. Also, there is no change in the CO2 cyclic capacity value with increase in number of cycles of absorption-desorption process, which has been validated in our previous work42. 2.3 Reaction Mechanism The reaction mechanisms between CO2 and amines have been extensively studied by many researchers. The zwitterion mechanism, firstly proposed by Caplow subsequently reintroduced by Danckwerts

43

and

44

, is usually used to describe the CO2

reactions with primary and secondary amines. When CO2 is absorbed into primary (i.e. MEA), a complex chemical called a zwitterion is firstly formed from the MEA and CO2 reaction, and then the carbamate is formed by the deprotonation of the zwitterion. Based on the zwitterion mechanism, the reaction between CO2 and MEA can be described as follows: Dissociation of water:

2 H 2O ↔ OH − + H 3O +

(3)

Hydrolysis of carbon dioxide:

CO2 + 2 H 2 O ↔ HCO3− + H 3O +

(4)

CO2 + OH − ↔ HCO3−

(5)

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Dissociation of bicarbonate ion: HCO − + H 2 O ↔ CO32− + H 3O +

(6)

3

Zwitterion formation: CO2 + R1 NH 2 ↔ R1 NH 2+ COO −

(7)

Deprotonation by a base:

R1 NH 2+COO− + R1 NH 2 ↔ R1 NH 3+ + R1 NHCOO−

(8)

R1 NH 2+ COO− + H 2O ↔ H 3O + + R1 NHCOO−

(9)

R1 NH 2+ COO − + OH − ↔ H 2O + R1 NHCOO −

(10)

MEA formation:

R1 NHCOO− + H 2O ↔ R1 NH 2 + HCO3−

(11)

R1 NH 3+ + H 2O ↔ H 3O + + R1 NH 2

(12)

Unlike primary and secondary amines, Donaldson and Nguyen have proposed that tertiary amines cannot directly react with CO2 but have a base catalytic effect on the hydration of CO2. This mechanism has been verified by subsequent works by using MDEA, TEA, etc45,

46

. Thus, the chemical reactions in CO2-tertiary amine-H2O

system are governed by equations (13)-(14):

R2 R3 R4 N + H + ↔ R2 R3 R4 NH +

(13)

R2 R3 R4 N + CO2 + H 2O ↔ R2 R3 R4 NH ++ HCO3−

(14)

In the case of blended amine solutions containing primary and tertiary amines the overall chemical reactions of Eqs.(3)-(14) are performed. In addition, the interaction effect between tertiary amine and zwitterion will enhance the absorption performance of blends as follows:

12


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R2 R3 R4 N + R1 NH + COO− ↔ R2 R3 R4 NH ++ HCO3−

(15)

2.4 Calculations The absorption rate and stripping rate of CO2 (rCO2) calculated using the flow rates of inlet and outlet gases are represented as follows: out χ CO 1  in  molCO2   molCO2   molN 2   2 rco2  = n − × nN 2  CO2  out    s  L ⋅ s  V [L]    1 − χ CO2  s  

(16)

in where, V is the amine solution volume; nCO and nN 2 is the molar rate of CO2 and N2 2 out of the feed gas; and χ CO is the CO2 mole fraction of the outlet gas. 2

The CO2 cyclic capacity ( Qcyc ) defined as the difference between CO2 loading after absorption (αrich) and regeneration (αlean) can be determined by:

 molCO2   molCO2   molCO2  Qcyc  = α rich  − α lean     molamine  molamine  molamine

(17)

3. RESULTS AND DISCUSSION 3.1 Equilibrium solubility The experimental determination of equilibrium solubility of CO2 in aqueous blended amine and MEA solutions were conducted at 313.15K under the CO2 partial pressure of 15 kPa for absorption experiments. The experimental results are presented in Table 1 and Figure 4. It can be seen that the CO2 equilibrium solubility of blended amine solutions was a little less than that of aqueous 5M MEA solution. However, the data of equilibrium solubility of CO2 for aqueous 3DMA-1,2-PD and 3DEA-1,2-PD solutions were not given in this study. This was due to the peculiar chemical structure of hydroxy group which leads to exhibit severe foaming when these amines react with CO2. On the basis of the literature, the experimental values showed that the CO2

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equilibrium solubility of tertiary amines are close to 0.9000 mol CO2/mol amine at the absorption condition, except for MDEA. Due to the thermodynamic limitation of aqueous MEA solution, the MEA blends with those tertiary amines give a relatively lower equilibrium solubility, which are between those of the two individual amines. However, the absorption solubility in aqueous MEA/TEA solution is the lowest among all the blends. It can be attributed to the chemical structure of TEA, which leads to low CO2 loading. In addition, the CO2 equilibrium solubility of the aqueous blended amine solutions is much higher than that of 6M MEA (0.4950 mol CO2/mol amine). The results indicate that the tertiary amine in blends has some effect which can enhance the CO2 absorption performance. The ideal CO2 cyclic capacity, defined as the difference between the CO2 equilibrium solubility for the absorption and desorption experiments, directly represents the potential driving force to overcome the mass transfer resistance for removing CO2 from mixed gases. In addition, larger CO2 cyclic capacity requires a smaller volume of amine solution for a fixed CO2 removal target, implying a higher CO2 absorption capacity at a certain molar concentration. However, the equilibrium solubility for regeneration experiments for 10 different amine systems at 353.15K were difficult to be obtained accurately. Additionally, these values of ideal CO2 capacity are hardly realistic to be reached in a real CO2 capture process using a specific absorber and stripper. Therefore, it is necessary to determine the effective CO2 cyclic capacity as well as the relative absorption and stripping rate over a specific operating condition in selecting the potential absorbents.

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Figure 4. 3.2 Absorption The plots of CO2 absorption loading (mol CO2/mol amine) versus absorption time obtained in aqueous MEA and MEA-based solutions are presented in Figure 5. It can be observed that the CO2 loading increases significantly as the absorption time increases for each amine system. The CO2 loading shows a linear function with reaction time during the first 60 min and rises gently in the last 30 min due to the smaller driving force and limiting thermodynamic equilibrium. This phenomenon is quite easy to understand because the amount of free amine molecules in the solution decreases with increasing reaction time. In addition, the CO2 absorption rates (mol CO2/(L·s)) versus CO2 loadings curves for different systems tested are presented in Figure 6. These results indicate that the addition of 1M tertiary amine dose not decrease the absorption rate of CO2 in 5M MEA solution significantly. The high absorption rate of MEA-based blends with tertiary amine additives could be explained on the basis of the increasing of the absorbent concentration as well as the interaction of the tertiary amine and MEA while in the same solution (Eq.13). In our previous work by Xiao et al.

47

, it was reported that the order of the

second-order reaction rate constant (k2) of the individual tertiary amines is as follows: TEA < MDEA < DMEA < 1DMA2P < 3DMA1P < DEEA; both the order of pKa and CO2 equilibrium solubility are TEA< MDEA < DMEA < 1DMA2P < DEEA < 3DMA1P. As expected, aqueous MEA/TEA represented the worst absorption performance as compared with all the other blends in the present work. In contrast,

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the highest absorption rate and CO2 equilibrium solubility were obtained by MEA/1DMA2P as shown in Figure 6 and 4. It can then be concluded that not only the chemical structure of the tertiary amines (i.e. hydroxyl group, chain length and side chain) but the level of interaction of the amines in the blended amine systems also have some influence on the absorption rate and CO2 loading at different reaction times for various MEA/tertiary amine systems. These effects were investigated and discussed in detail in the present work. Generally, the hydroxyl group will decrease the electric charge density of the nitrogen atom within a short interval between nitrogen atom and the hydroxyl group, implying that the inductive effect can be ignored with long intervals

48

. The

hydroxyethyl group linked to the nitrogen atom generates an inductive effect on the tertiary amines (e.g. TEA, MDEA, DMEA and DEEA), leading to a reduction of the charge on the nitrogen atom. Consequently, both the CO2 loading and absorption rate decrease as the number of hydroxyethyl group linked to the nitrogen atom increases. This phenomenon also has been validated with the order of absorption rate and CO2 loading as: MEA/TEA < MEA/MDEA < MEA/DMEA < MEA/DEEA. In addition, the effect of chain length between nitrogen atom and hydroxyl group was also investigated by comparing the MEA/DMEA and MEA/3DMA1P both with two methyl groups linked to the nitrogen atom. Clearly, MEA/DMEA exhibits relatively poor absorption rate while the MEA/3DMA1P reacts more quickly with CO2. The explanation is that the hydroxyl group in DMEA is one carbon shorter than in 3DMA1P, resulting in an increase in electric charge density at the nitrogen for DMEA.

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As well, the formation of intramolecular hydrogen in 3DMA1P is more difficult than in DMEA due to molecular size and steric hindrance. The blended MEA/3DMA1P solution thereby shows a higher absorption rate and CO2 equilibrium solubility than MEA/DMEA. Similar explanation can be applied to the comparison of chain length of alkyl substituents in tertiary amines. As shown in Figures 5 and 6, MEA/DMEA is compared with MEA/DEEA. The results show that aqueous MEA/DEEA solution has better absorption performance for a certain time than that of MEA/DMEA. This can be attributed to the substitution of methyl group in DMEA with ethyl group in DEEA, the latter of which is a bigger electron-donating group thus enhancing its activity compared with methyl group. Thus, it can be concluded that tertiary amines with hydroxyl group at γ carbon atom have better absorption performance than tertiary amines with hydroxyl group at β carbon atom49. In addition, it indicates the chain length has important effect on the absorption performance, and the longer chain length results in an increase reaction rate.50 This phenomenon was also found by evaluating the CO2 equilibrium solubility of MEA/1DMA2P and MEA/1DEA2P. However, it provides a contrary result by the comparison of the absorption rates for CO2 absorption into MEA/1DMA2P and MEA/1DEA2P. As shown in Figure 5, the MEA/1DMA2P with two methyl groups linked to nitrogen atom shows excellent absorption rate compared with that of MEA/1DEA2P. Here the high viscosity and intermolecular interaction between MEA and tertiary amine need to be taken into consideration, and may be the predominant factors in the CO2 absorption process. Thus, it can be concluded that 1DEA2P has a lower reaction rate with the zwitterion

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Energy & Fuels

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induced by its specific chemical structure feature. Furthermore, the effect of the position and number of side chain (i.e. methyl group) of MEA/1DMA2P was compared with that of MEA/DMEA. It can be concluded that the addition of methyl at β carbon atom can significantly enhance both the CO2 solubility and absorption rate49. However, both the addition of methyl at α carbon atom and the increase of methyl group number can decrease the CO2 absorption rate and

CO2 equilibrium solubility,

which

can

be

seen

by

comparing

the

MEA/2DMA2M1P and MEA/DMEA. This is due to more obvious occurrence of steric hindrance by adding the methyl group at the α carbon atom than at that of β carbon atom, leading to a decrease of the activity of nitrogen atom. In addition, it can be

concluded

from

the

experimental

results

of

MEA/1DMA2P

and

MEA/2DMA2M1P that the position of the side chain can influence the formation of the hydrogen bond as well as the activity of the tertiary amine. Therefore, aqueous 6M MEA/1DMA2P solution presented high average absorption rate of 56.0888 ×10-5 mol CO2/(L·s) and reached the highest CO2 loading of 0.5267 mol CO2/mol amine among all the blended amine systems. In conclusion, all the hydroxyethyl group, chain length, side chain, physical properties and intermolecular interaction can control the activities of the blended amines. Figure 5. Figure 6. 3.3 Desorption The true CO2 carrying capacity (CO2 cyclic capacity) for amine solutions is a

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significant indication to screen the potential absorbents. The CO2 cyclic capacity is calculated as the difference between the absorption CO2 loading and the lean CO2 loading of the desorption experiment, as mentioned. Desorption experiments at 353.15K were carried out to give an evaluation for the tested amine systems. Figures 7 and 8 represent the plots of desorption rate in mol CO2/(L·s) versus reaction time and CO2 loading in mol CO2/mol amine, respectively. It can be observed from the figure that the CO2 loading decreases rapidly in the first six minutes, then drops linearly from 6 to 60 minutes, and was thereafter constant for all solutions. The addition of a tertiary amine increases the stripping rate as represented in Figure 8. This can be explained by the fact that the tertiary amine acts a major base to catalyze the formation of MEA carbamate and is protonated species. Consequently, the heat energy consumption for stripping CO2 from the unstable MEA carbamate and bicarbonate is reduced accordingly. Interestingly, both the lowest stripping rate and cyclic CO2 capacity were obtained with 6M MEA/2DMA2M1P indicating poor CO2 capture performance. The presence of two methyl groups at the α-carbon atom and the low initial CO2 loading of DMA2M1P may be responsible. It indicates that the tertiary amine, 2DMA2M1P, has a high CO2 absorption heat and a lower absorption rate, resulting in a higher requirement for regeneration 29. Both the high reaction heat of TEA and lowest CO2 rich loading of 0.4582 mol CO2/mol amine 47 at the end of the absorption experiment of MEA/TEA enhance the difficulty to strip CO2 from aqueous MEA/TEA solution. That is why the lower stripping rate and lowest lean CO2 loading of 0.2346 mol

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Energy & Fuels

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CO2/mol amine was attained by MEA/TEA, as illustrated in Figure 4. Comparative stripping rates were found in MEA/MDEA, MEA/DMEA and MEA/DEEA while the initial CO2 loadings of these systems were 0.4953, 0.5140 and 0.5215 mol CO2/mol amine, respectively. However, the introduction of a methyl substituent rather than ethyl substituent at nitrogen atom decreases the absorption rate but increases the stripping rate and CO2 cyclic capacity. Thus, aqueous MEA/DMEA solution shows a little better stripping performance than aqueous MEA/DEEA solution. This phenomenon can also be confirmed by the comparison of the stripping rates of MEA/1DMA2P and MEA/1DEA2P. Also, the hydroxyethyl group was observed to be a suitable functional group for the enhancement of the stripping rate and cyclic CO2 capacity. Thus, MEA/MDEA represents faster stripping rate as compared to MEA/DEEA and MEA/DMEA with methyl and ethyl group, respectively. By comparing MEA/3DMA1P and MEA/DMEA, an increase in the interval (chain length) between nitrogen atom and hydroxyl group both increase the absorption rate and CO2 absorption heat, and therefore, decreases the regeneration rate. Then, the higher CO2 lean loading of 0.2882 mol CO2/mol amine was obtained by MEA/3DMA1P whereas the lean CO2 loadings for MEA/DMEA and 5M MEA were 0.2700 mol CO2/mol amine and 0.2927 mol CO2/mol amine, respectively. In conclusion, the addition of 1M 1DMA2P solution into 5M MEA solution has a relatively lower lean CO2 loading of 0.2482 mol CO2/mol amine and the highest average stripping rate, resulting in a high CO2 cyclic capacity.

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Energy & Fuels

Figure 7. Figure 8. 3.4 Cyclic capacity In the real CO2 removal process using amine solution, the CO2 cyclic capacity is a key parameter because it can directly affect the amount of the amine solution and the size of the scrubbing equipment (absorber, piping, pumps, condensers, stripper, reboiler and heat exchangers). Furthermore, the CO2 cyclic capacity is also an indication of the energy requirement for CO2 regeneration. The values of CO2 cyclic capacity in both mol CO2/mol amine and mol CO2/L for 9 amine systems were determined by the difference between rich CO2 loading

and lean CO2

loading(obtained at the end of absorption and stripping experiments, respectively), as given in Figure 9 and Table 1. To probe the influence of the type of base catalyst (i.e. tertiary amine) on the capture process, we exposed blended MEA/2DMA2M1P to lower average CO2 absorption rate as well as lowest CO2 stripping rate, resulting in the lowest CO2 cyclic capacity of 1.1118 mol CO2/L. It can be found that the CO2 cyclic capacities of blended amines with added tertiary amines were 14% higher than that of aqueous 5M MEA solution, except for MEA/2DMA2M1P. This indicates that a tertiary amine increases the stripping rate to promote the CO2 cyclic capacity. Thus, the blended amine systems have significant potential as absorbent to remove CO2. According to Table 1, the CO2 cyclic capacity followed the order: MEA/2DMA2M1P < 5M MEA < MEA/TEA < MEA/1DEA2P < MEA/3DMA1P < MEA/DEEA< MEA/DMEA

Experimental Studies on the Effect of Tertiary Amine Promoters in

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