Regeneration performance of amino acid ionic liquid (AAIL) activated MDEA solutions for CO2 capture

Chemical Engineering Journal 223 (2013) 371–378

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Regeneration performance of amino acid ionic liquid (AAIL) activated MDEA solutions for CO2 capture Zhang Feng a,b, Gao Yuan a, Wu Xian-Kun a, Ma Jing-Wen a, Wu You-Ting a,b,⇑, Zhang Zhi-Bing a,b,⇑ a b

School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China National Organic Poison Control and Resource Engineering Technology Research Center, Nanjing 210093, China

h i g h l i g h t s  Regeneration performance of [N1111][Gly] + MDEA aqueous solutions was investigated.  Regeneration efficiency and absorption rate of renewed solution were compared with the fresh solution.  The regeneration process was analyzed for IL + MDEA aqueous solutions.

a r t i c l e

i n f o

Article history: Received 19 December 2012 Received in revised form 28 February 2013 Accepted 2 March 2013 Available online 14 March 2013 Keywords: Carbon dioxide Regeneration efficiency MDEA Ionic liquids Thermal regeneration

a b s t r a c t The regeneration performance of amino acid ionic liquid (AAIL) activated N-methyldiethanolamine (MDEA) solutions was investigated to evaluate the influence of solution composition, regeneration temperature on the regeneration efficiencies and absorption rate of renewed solutions. Three high concentrated aqueous solutions: 15 wt% [N1111][Gly] (tetramethylammonium glycinate) + 30 wt% MDEA, 10 wt% [N1111][Gly] + 30 wt% MDEA and 10 wt% [N1111][Gly] + 40 wt% MDEA were saturated and then regenerated under the regeneration temperature: 363 K, 373 K and 378 K, respectively. The results reveal that most of CO2 in the liquid phase is released through the thermal regeneration before solution boiling (about 378 K). And higher regeneration temperature causes higher absorption rates of CO2 in the regeneration solutions, indicating that more [N1111][Gly] can be renewed under higher regeneration temperature. Absorption of CO2 in the regenerated absorbents over a wide range of equilibrium pressure (0– 300 kPa) shows that under the same pressure, the higher regeneration temperature leads to the high CO2 absorption capacity of the regenerated solutions, and the absorption curves of the solutions regenerated at 378 K are close to those of the fresh solutions, especially when the equilibrium partial pressure of CO2 is lower than 50 kPa. Considering the regeneration efficiency and absorption rate difference between regenerated solutions and the blank, it can be concluded that MDEA which dominates CO2 load is fully regenerated, while most IL which determines the absorption rated is renewed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Large emission of greenhouse CO2 has caused negative effect on the atmospheric environment. Most of the countries have done jointing effort to reduce the gas emission. Nowadays, CO2 scrubbing or absorption is the most developed process among all CO2 Capture options for industrial applications [1]. Aqueous solutions of alkanolamine or their mixtures, including monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP) or their mixture, have been

⇑ Corresponding authors at: School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. Tel.: +86 25 83596665; fax: +86 25 83593772. E-mail addresses: [email protected] (W. You-Ting), [email protected] (Z. ZhiBing). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.005

widely used as chemical absorbents of CO2 in industrial processes [2]. However, in industrial applications, these alkanolamine absorbents have suffered several drawbacks including volatilization, corrosion, oxidation and thermal degradation at high temperatures. Therefore, lots of approaches have been proposed in the development of CO2 absorbents [1,3–5]. Ionic liquids (ILs) are organic molten salts that form stable liquid below 373 K or even at room temperature (room temperature ionic liquids, RTILs). ILs have been considered as a kind of potential candidates for CO2 capture [1], based on their unique properties such as broad temperature range of liquid state, high thermal and chemical stability, negligible vapor pressure, easy structural modification or functionalization, and especially their strong affinity to CO2. Application of ILs in the removal of CO2 has been studied since 2001 [6–10]. However, the physical absorption of CO2 in traditional ILs (i.e., [bmim][BF4]) to significant extent usually requires

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extreme conditions like high pressure (90 bar or even higher) and low temperature. Furthermore, the absorption rate is also quite slow [11]. Taking into account the chemical interaction of amino with CO2 molecule, introduction of amino-group or other alkaline groups into IL would construct functionalized ionic liquids for the removal of CO2. Representative amino-functionalized ILs includes amino-alkylimidazolium ionic liquids [12], tetraalkylphosphonium amino acid ionic liquids [13,14], and tetraalkylammonium amino acid ionic liquids [4,15]. The absorption rate of carbon dioxide in these amino-functionalized ILs under low pressure and absorption equilibrium are reached within 60 min [13,15]. The absorption mechanism of CO2 in amino-functionalized ILs is that one CO2 molecule combines with two IL molecules [12,13,15]. However, since the pure task-specific ILs (especially amino– functionalized ILs) have higher viscosity than the ordinary ILs, the delivery of ionic liquid and the gas–liquid mass transfer in the absorption process are seriously affected. Therefore, the industrial application of amino-functionalized ILs as CO2 absorbent is still unfeasible. For instance, the viscosity of pure ILs is usually higher than 200 mPa s, much higher than that of 30 wt% MEA (2.5 mPa s). To overcome this problem, water is added into ILs to lower the viscosity and improve the absorption rate. Zhang et al. [4] studied CO2 uptake in aqueous solutions of four tetraalkylammonium amino acid ILs: [N1111][Gly](tetramethylammonium glycinate), [N2222][Gly](tetraethylammonium glycinate), [N1111][Lys](tetramethylammonium lysinate) and [N2222][Lys] (tetraethylammonium lysinate), and found that the absorption rates in 30–80 wt% ILs solutions were much higher than those in pure ILs. Then, in 2012, Jing et al. [16] further investigated the kinetics of carbon dioxide absorption into 5–30 wt% tetramethylammonium glycinate solutions. The absorption rate in 30 wt% [N1111][Gly] solution was much higher that of TEA and DEA, but a little smaller than that of MEA. It should be noted that IL aqueous solution of high concentration is not suitable for the industrial removal of carbon dioxide due to the high cost of ILs. In the light of the methods and theories of mixed amines [17], the use of ILs as active components of alkanolamine solutions becomes a possible and potential way to re-formulate the CO2 absorbents [18,19], for the purpose of obtaining green and energy-saving CO2 capture processes. In 2005, two hybrid systems, 43 wt% [bmim][BF4] + 60 wt% MDEA and 60 wt% [bmim][Ac] + 30 wt% MEA, were reported in a US patent [19] as the early trial of this idea. However, the absorption of CO2 in these solutions were limited, since [bmim][BF4] and [bmim][acetate] were not good enough activators for CO2 capture under common condition. On the other hand, the presence of a little amount water in the solutions could greatly weaken the absorption, as reported by Ahmady et al., who investigated the absorption of CO2 in aqueous mixtures of 4 mol/L MDEA with 0–2 mol/L imidazolium-based ionic liquids ([bmim][BF4], [bmim][Ac] or [bmim][DCA]) [20,21]. It was additionally found by Ahmady et al. that when the partial pressure of CO2 was between 300 and 700 kPa the absorption capacity was less than 0.2 molCO2/molamine and decreased with an increase in the IL’s concentration [21,22]. It is realized from these studies that alkanolamine aqueous solutions can be reasonably activated by conventional ILs, but more efforts are required in designing the absorbents to further improve the absorption capacity and rate. As an alternative, it was proposed in our group to use amino acid ionic liquids (AAILs), a kind of task-specific ILs, as the activators for the MDEA solutions [4,23–26]. Several amino acid ILs ([N1111][Gly], [N2222][Gly], [N1111][Lys] and [N2222][Lys]) were tried to blend with MDEA aqueous solution, and it was found that the addition of a certain amount of ILs in MDEA could greatly accelerate the absorption of CO2 under low to moderate pressures [4,27].

In addition, the regeneration experiments at 353 K and 4 kPa for 240 min showed that the aqueous solution of 15 wt% [N1111] [Gly] + 15 wt% MDEA had a higher regeneration efficiency (over 98%) than the solution of 15% potassium glycinate +15% MDEA, implying the advantage of AAILs over the solid glycinate. Practically, ILs in the aqueous solution could decrease the vapor pressure of amines, thereby reducing their loss in the generation [4]. The amino acid ionic liquid (AAIL) activated MDEA solutions can make full use of the functionalized ionic liquid’s high absorption speed and stability, and the large absorption capacity of MDEA. Thus CO2 absorbent made of amino-functionalized ionic liquids and MDEA solution would have more application prospects. Considering the economic feasibility, tetramethylammonium glycinate ([N1111][Gly]), which is of low cost and quite good absorption performance for CO2, is the suitable amino acid based ILs for industrial application. More recently, to push forward the practical application of AAILs for CO2 capture, fifteen MDEA-[N1111][Gly] solutions (30– 50 wt% MDEA + 2.5–15 wt% [N1111][Gly]) were prepared and characterized in our laboratory to explore the influence of compositions and physical properties (especially viscosity) on the absorption of CO2 [23]. It was indicated that the concentration of IL had two conflicting effects on the CO2 absorption rate. In general, an increase in the concentration of AAIL is beneficial to the absorption rate of CO2 in the high-concentrated MDEA aqueous solution. However, higher concentration of AAIL also induces the increase of solution viscosity so that the absorption rate is improved not so apparently or even negatively at high concentrations of IL [23]. This means that there exists an optimum concentration of IL to balance these two effects. The aqueous solutions with 10–15 wt% AAIL and 30–40 wt% MDEA are found to be suitable formulations for CO2 removal. Besides absorption characteristics, regeneration performance is also a very important selection criterion for designing the CO2 absorbents. There are three regeneration methods for CO2 saturated absorbents: vacuum regeneration, thermal regeneration, or a combination of vacuum and thermal regeneration. It has been revealed in our group that the regeneration efficiency of 15 wt% MDEA + 15 wt% [N1111][Gly] is over 98% under vacuum condition (<4 kPa) [4]. Noticeably, due to the loss of water under vacuum condition, some water has to be added in the regenerated absorbents to keep the concentration of the solution. As for thermal regeneration, the regeneration temperature is an important parameter that determines the regeneration efficiency. Zhao et al. found that the regeneration efficiency of sodium glycinate aqueous solution (1.0 mol/L) increased from 80.0% to 92.6% with a rise in the regeneration temperature from 363 K to 403 K, and the suitable regeneration temperature was located at 383–388 K with a regeneration efficiency of 91.8% [28]. For the aqueous blends of 2-amino-2-methyl-1-propanol and sodium glycinate, the suitable regeneration temperature was found to be 378 K, and their regeneration efficiencies were higher than those of sodium glycinate aqueous solutions [29]. It is obvious that with an increase in the regeneration temperature, the regeneration efficiency increases while regeneration time reduces [28,29]. Various efforts on the absorption and desorption of CO2 in different aqueous solutions of alkanolamines have been reported [30–32]. However, as one of the most important work in designing the IL-activated alkanolamine absorbents for CO2 capture, the regeneration performance of these IL-contained solutions is scarcely found in literature. In the present work, the regeneration characteristics of three aqueous solutions: 15wt% [N1111][Gly] + 30 wt% MDEA, 10 wt% [N1111][Gly] + 30 wt% MDEA and 10 wt% [N1111][Gly] + 40 wt% MDEA were investigated at different regeneration temperatures. The absorption rate and absorption capacity of the regenerated solutions, as well as the blank solutions, were mea-

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sured and compared to explore the influence of component concentrations and temperatures on the regeneration performance. The regeneration efficiencies at low, moderate and high pressures were also determined to analyze the suitable regeneration conditions for different absorption processes.

HCO 3 , so that CO2 can be released by reversing the forward reaction in Eq. (6). In general, HCO 3 gets proton more readily from concentrated MDEAH+ or RNHþ 3 to release CO2:

MDEAHþ þ HCO3 MDEA þ H2 O þ CO2

ð11Þ

RNHþ3 þ HCO3 RNH2 þ H2 O þ CO2

2. Theory

ð12Þ 

2.1. Absorption reaction mechanism The mechanism for the absorption reaction of CO2 in aqueous solution of MDEA was proposed by Donaldson and Nguyen [33]:

MDEA þ CO2 þ H2 O MDEAHþ þ HCO3

ð1Þ

Actually, the absorption of CO2 in MDEA aqueous solutions is slow since it is an alkali-catalyzed hydrolysis reaction. On the contrary, tetraalkylammonium amino acid ILs can be completely dissociated into hydrated cations and anions [H2N–R– COO] in the aqueous solutions. Since the amino acid anions reacts with CO2 in a way similar to the primary alkanolamines, [H2N–R– COO] is abbreviated as RNH2 in the following paragraphs. The zwitterion mechanism is generally applied to model the carbon dioxide absorption in aqueous solutions of amino acid salts or the primary alkanolamines [34]. The zwitterion is initially gained through the reaction of CO2 with the amino group,

RNH2 þ CO2 RNHþ2 COO

ð2Þ

Then, the zwitterion is deprotonated by the bases (denoted by B) in the solution, such as RNH2, H2O, OH, and MDEA,

RNHþ2 COO þ B RNHCOO þ BHþ

ð3Þ

As indicated in Eqs. (2) and (3), in the aqueous solutions of ILs, part of RNH2 is transformed into carbamate, and most of the remaining part are consumed as bases. Thus the theoretical CO2 absorption capacity of the solution ranges from 0.5 to 1.0 molCO2/ molamine, since some of the IL undergoes the 1:2 reaction mechanisms other than the 1:1 mechanism of MDEA-CO2 system. For the aqueous solution of amino acid IL + MDEA, the proton can be transferred from the zwitterion to MDEA, thus more RNH2 molecules are able to absorb CO2 in the liquid phase. Meanwhile, MDEA in the liquid can also react with CO2 through a base-catalyzed CO2 hydrolysis.

RNHþ2 COO þ MDEA RNHCOO þ MDEAHþ

ð4Þ

Therefore, according to analysis above, the equilibrium reactions in the liquid phase are suggested as follows:

RNHCOO þ H2 O RNH2 þ HCO3

ð5Þ

CO2 þ H2 O Hþ þ HCO3

ð6Þ

RNHþ3 RNH2 þ Hþ

ð7Þ

HCO3 Hþ þ CO2 3

ð8Þ

H2 O Hþ þ OH

ð9Þ

MDEAHþ MDEA þ Hþ

Moreover, the thermal decomposition of RNHCOO during the thermal generation may be involved and can be described with Eq. (5). It should be noted that during the thermal regeneration process the release rate of CO2 is mainly determined by the concentration of CO2 molecular and HCO 3 in the liquid. In fact, CO2 dissolved freely in the liquid can be liberated firstly at low temperatrues when the absorbents being heated. Then, with a futher increase  in the solution temperaure, more HCO 3 and carbamate RNHCOO are decomposed according to Eqs. (5), (11), and (12) to release CO2. The CO2 saturated solution is eventually changed back to a certain basic degree through the regeneration. 3. Experimental 3.1. Chemicals Reagent-grade MDEA (purity >99.9%), tetramethylammonium hydroxide (purity >99.9%) and glycine (purity >99.9%) were obtained from Shanghai Bangcheng Chemical Co., Ltd. The aqueous solution was prepared using distilled water that had been further degassed by boiling before use. CO2 (purity >99.95%) was bought from Nanjing Gas Supply Inc. 3.2. CO2 regeneration experiments The amino acid based IL was synthesized according to Jiang et al. [15]. Three absorbents: 15 wt% [N1111][Gly] + 30 wt% MDEA, 10 wt% [N1111][Gly] + 30 wt% MDEA and 10 wt% [N1111][Gly] + 40 wt% MDEA were prepared as CO2 absorbents. It should be noted that the solution concentration is presented as mass percent in the whole paper. Before regeneration, the absorbents of about 500 ml were saturated with CO2 at 298 K and 101.3 kPa in a falling liquid film absorber which has been described in our early paper [25]. The apparatus for the thermal regeneration of saturated absorbents at normal pressures was depicted in Fig. 1. In the thermal regeneration, Tb, the temperature of the oil bath (the regeneration temperature) was maintained at a certain value. A three-necked bottle placed in the bat was fed with 100 g saturated absorbent, and the solution was stirred with a magnetometric stirring paddle. Two condensers were placed over the bottle to minimize the evaporation loss of water. The released gas was eventually led to a suction bottle in which saturated calcium hydroxide

ð10Þ

As shown in Eqs. (5)–(10), CO2 absorbed in the liquid exists in 2  the forms of freely dissolved CO2, HCO 3 ; CO3 and RNHCOO . 2.2. Regeneration mechanism In the thermal generation process, forward reactions in Eqs. (7)– (10) play a dominant role in the release of proton to react with

Fig. 1. Experimental apparatus for the regeneration.

Z. Feng et al. / Chemical Engineering Journal 223 (2013) 371–378

3.3. Absorption of CO2 in regenerated absorbents

nCO2 ¼

P0 V S  PðV S þ V A  V L Þ þ Pv V L RT

ð13Þ

As shown in Fig. 2, the regeneration efficiency of the 30 wt% MDEA+15 wt% IL system increases with a rise in temperature. It is noted that when Tb > 360 K, the regeneration efficiency is over 80%. When Tb = 378 K, liquid begins to boil, and the regeneration efficiency is 93.6%. Therefore, 363 K, 373 K and 378 K are chosen as the three typical regeneration temperatures. In addition to the relationship between regeneration temperature (363–378 K) and regeneration efficiency, the variations of solution temperature K and the release rate N during the thermal generation are also important information for the research of regeneration. As depicted in Fig. 3, two stages are designed during the regeneration of all the three absorbents for Tb = 363 K. In the first stage, the temperature of the solution rose dramatically to 354 K and large amount of gas was released as the heating temperature was maintained 354 K. After the liquid’s temperature was kept at 354 K for 2000 s, the release rate rapidly dropped to zero. Then in the second stage, increasing the heating temperature to 363 K, the liquid’s temperature was raised to 363 K and the gas re-

8 360 6 30% MDEA + 15% IL Tb: 363K

4

350

N (release rate) T (temperaure of solution)

2

340 330

0 0

1500

3000

4500

6000

t (s) 5 360 4 40% MDEA + 10% IL Tb: 363K

3

345

N (release rate) T (temperaure of solution)

2

330

T (K)

where VS and VA denote the volumes of the storage vessel and the absorption vessel respectively. VL represents the volume of liquid and Pv the saturated vapor pressure of the absorbent. During the absorption, the temperature was maintained at a scheduled value with a water bath. The pressure was measured every 2 s by using a pressure gauge (WIDEPLUS-8). A detailed description of the experiments has been presented in our early paper [18]. With the aid of Eq. (13), the absorption capacity and the absorption rate of regenerated absorbents can be obtained and compared. As CO2 load of the regenerated absorbents were measured under a wide range of CO2 partial pressure (0–300 kPa), so that the regeneration efficiency for every absorbent at low, moderate and high pressures could be evaluated and compared. In the present work, the regeneration efficiency is defined as the ratio of the absorption loads of the regenerated solution and the blank one.

4.1. Regeneration of CO2 saturated absorbents

T (K)

After regeneration, the regenerated absorbents absorbed CO2 in a Dual-vessel balance system, and the experiment method was described in our early paper [4]. With the absorption of CO2 into the absorbent, and the total pressure P of the system decreased with the time until the absorption equilibrium was established. The amount of absorbed CO2 can be calculated using the following equation:

4. Results and discussion

N (10-5mol/s)

was applied to detect CO2. When the temperature of the liquid in the three-necked bottle approached about 313 K, bubbles of CO2 and deposits of calcium carbonate appeared in the suction bottle, indicating the release of carbon dioxide from the solutions. The flow rate of the released gas was measured using a soap-film meter. Before the soap film flowmeter, the gas temperature was measured, thus the amount of released gas could be calculated as the amount of water in the gas being excluded. The temperature values of the solution and the extruded gas, as well as the flow rate, were all recorded simultaneously every minute. The regeneration experiments were performed at three different regeneration temperatures: 363 K, 373 K and 378 K, to identify the effect of different regeneration temperature on the release rate and regeneration efficiency. Moreover, to evaluate the regeneration process, the solution was heated with two temperature stages. The whole regeneration experiment was accomplished at atmospheric pressure. In the present work, the deviations of temperature and pressure are 0.1 K and 0.1 kPa, respectively. The uncertainty of solution concentration is below 0.1%.

N (10-5mol/s)

374

1 315 0 0

2000

4000

6000

8000

t (s)

30% MDEA + 15% IL 12

90

360 9

N (10-5mol/s)

85 80

30% MDEA + 10% IL Tb: 363K

6

N (release rate) T (temperaure of solution)

345 330

3

T (K)

Regeneration efficiency (%)

95

75

315 350

355

360

365 370 Tb (K)

375

380

Fig. 2. Variations of regeneration efficiency with regeneration temperature for P0 = 97 kPa.

0 0

1500

3000

4500

6000

7500

t (s) Fig. 3. Variations of solution temperature and release rate when Tb = 363 K.

375

Z. Feng et al. / Chemical Engineering Journal 223 (2013) 371–378

N (10-5mol/s)

24

16

8

0 12.0

N (10-5mol/s)

9.0

40% MDEA + 10% IL Tb: 378K N (release rate) T (temperaure of solution)

7.5 6.0 4.5

340

3.0 320

0.0 1500

3000

340

4000

6000

8000

t (s)

20 375 30% MDEA + 15% IL Tb: 378K N (release rate) T (temperaure of solution)

12 8

380

345

0

330

0 360

N (release rate) T (temperaure of solution)

0 2000

0

4000 t (s)

6000

8000

T (K)

40% MDEA + 10% IL Tb: 373K

3

340

320

375 12 360

N (release rate) T (temperaure of solution)

4

345

T (K)

30% MDEA + 15% IL Tb: 373K

8

330

0 0

1500

3000 t (s)

360

4

9

6

7500

4500

6000

Fig. 4. Variations of solutions temperature and release rate when Tb = 373 K.

T (K)

2000

6000

16 320

0

4500

t (s)

N (10-5mol/s)

6

360

T (K)

N (10-5mol/s)

380

10.5

T (K)

N (release rate) T (temperaure of solution)

6000

1.5

0

N (10-5mol/s)

4000

360

3

N (10-5mol/s)

2000

t (s)

12 30% MDEA + 10% IL Tb: 373K

340 320

0

9

360

30% MDEA + 10% IL Tb: 378K N (release rate) T (temperaure of solution)

0

380

15

380

T (K)

lease rate rose again. In the end, the liquid’s temperature kept constant at 363 K and the release rate decrease to zero. It can be concluded that the gas gives off rapidly with the increase of the solution’s temperature; when liquid’s temperature keeps constant, the release rate drops to zero. The application of two stages with different heating temperature is to explore the process of the thermal regeneration and the relationship of solution’s temperature with the release rate of gases, since two heating temperature stages make it possible to examine and compare the release amount or release rate of CO2 at various temperature ranges. Obviously, the amount of released gas in the first stage is larger than that in the second stage. It can be seen in Fig. 3 that the cases for three absorbents regenerated at 363 K, especially the variations of liquid’s temperature, are almost similar to each other, though there is small difference in the variations of release rate for three solutions. As illustrated in Fig. 4, for Tb = 373 K, the heating temperature for the two stages is set as 363 K and 373 K, respectively. Obviously, large amount of gas is also released in the first stage. The variations of solution’s temperature and release rate for thermal regeneration under Tb = 378 K are depicted in Fig. 5. It can be seen

1500

3000 4500 t (s)

6000

7500

Fig. 5. Variations of solution temperature and release rate when Tb = 378 K.

that the case for Tb = 378 K is similar with those for Tb = 373 K or Tb = 363 K. That is, most of the gas gives off in the first stage. The characteristic temperature for the two stages is set 373 K and 378 K, respectively. Noticeably, the release amount in the second stage for Tb = 378 K is much lower than those under Tb = 373 K or Tb = 363 K. In general, Tb, the regeneration temperature decides the heating intensity and the special temperature of the second stage. As shown in Figs. 3–5, large part of gas is liberated and the maximum release rate happens for all absorbents. Obviously, as depicted in Fig. 5, the majority of the gas is released within 3000 s in the first stage when Tb = 378 K. T1 and T2 stand for the characteristic temperature of the first stage and the second stage, respectively. As

Table 1 Characteristic regeneration. Tb (K) T1 (K) T2 (K)

temperature

363 353 363

for

two

stages

373 363 373

in

thermal

378 373 378

Z. Feng et al. / Chemical Engineering Journal 223 (2013) 371–378

4.2. Absorption performance of regenerated solutions To compare the thermal regeneration performance of three solutions under different Tb, absorption of CO2 in the regenerated solutions was performed in a Dual-vessel balance system with P0 = 97 kPa. In Fig. 7, the label ‘‘blank’’ represents the blank solution and ‘‘378 K’’ is denoted to the solution regenerated at 378 K. As shown in Fig. 7, the absorption rates of CO2 in all the solutions are very high in the first 40 min, and with time going on, the absorption slows down to equilibrium. This reveals that large

5

Blank 378K 373K 363K

3

2

nCO (103 mol)

4

2

Concentration [N1111][Gly] 10 % MDEA 40 %

1 0 0

20

40 60 t (min)

80

100

5 Blank

4

2

nCO (103 mol)

shown in Table 1, there are four values for T1 and T2: 353 K, 363 K, 373 K and 378 K, and the latter three are regeneration temperature. It should be noted that in the second stage for Tb = 378 K, the liquid begins to boil and its temperaure keeps constant at 378 K which is the boiling point of water improved by MDEA and other species in the solution. It can be concluded that from Fig. 3 to Fig. 5, most of the gas gives off within 3000 s in the first stage. Moreover, for every absorbent, the value of the maximum release rate increases with a rise of heating temperature in the first stage. As mentioned in Section 2.2, during the thermal regeneration, CO2 dissolved in the liquid can be liberated first at low temperature when the absorbents being heated. Then, with a further increase of solution’s temperature, more HCO 3 will liberate CO2. That is the first stage illustrated in Figs. 3–5. The positive reactions of Eqs. (11) and (12) dominate the regeneration and RNHCOO can also be decomposed. With a further increase of solutions temperature, more CO2 can be released from the solution. It should be noted that different Tb values and the two stages are design to reveal the thermal regenaration process. And the total released CO2 in a stage could be obtained through intergrating N with time T, and the regeneration efficiency could be calculated from the release amount of CO2. However, due to the limitation of the apparatus, error in integration would be apparent. To compare the behaviors of MDEA and IL in the regeneration, thermal regeneration of IL aqueous solution when Tb = 378 K was also performed without the partition of two stages. As depicted in Fig. 6, gas gives off within 2000s when the solution is directly heated to boil, and the positive reaction of Eqs. (5) and (12) goes on with gas being released. Obviously, the maximum release rate is approached when the solution’s temperaure reaches 375 K, then with the solution temperature increasing gradually, the release rate rapidly drops. Comparison between Fig. 5 and Fig. 6 shows that for the auqeous solution of 30% MDEA + 15% IL, the thermal regerantion of MDEA seems to be dominent in the first stage, since MDEA can be regenerated more easily. It has been reported that the order of regeneration efficiency is: AMP > MDEA > Sodium Glycinate > DEA > MEA [28].

378K 373K 363K

3 2

Concentration [N1111][Gly] 10 % MDEA 30 %

1 0 0

20

40 60 t (min)

80

Blank

Concentration [N1111][Gly] 10 % MDEA 40 %

3.2 2.4

378K 373K

1.6

363K

0.8 0.0 0

10

20 30 t (min)

40

3.0

3

375

2

370

1

365

0

360 3000

T (K)

N (10-5mol/s)

380

N (release rate) T (temperaure of solution)

2.0

50

Blank

Concentration [N1111][Gly] 10 % MDEA 30 %

2.5

15% IL Tb: 378K

ln(ne-n0)/(ne-n)

4

100

Fig. 7. Absorption of CO2 in regenerated solutions when P0 = 97 kPa (‘‘blank’’ represents the blank solution and ‘‘378 K’’ is denoted to the solution regenerated at 378 K).

ln(ne-n0)/(ne-n)

376

378K 373K

1.5 363K

1.0 0.5 0.0

0

1000

t (s)

2000

Fig. 6. Thermal regeneration of IL aqueous solution when Tb = 378 K.

0

10

20

30

40

50

t (min) Fig. 8. Apparent absorption rates for regenerated solutions when P0 = 97 kPa.

377

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4.3. The regeneration efficiency under different pressure Since the absorption mechanism and CO2 load in aqueous solutions of MDEA and IL under low, moderate or high pressure are different, the regeneration efficiency of same absorbent may be different under various absorption pressures. The comparisons of CO2 loads over a wide range of pressure between the regenerated solutions and the blank solution are illustrated in Fig. 10. namine is the amount sum of IL and MDEA, and namine/nCO2 represents CO2 load in solution. Obviously, under a wide range of equilibrium pressure (0–300 kPa), CO2 loads of solutions regenerated at higher temperature are closer to that of the blank solution. Under same pressure, CO2 load increases with a rise in regeneration temperature. And with an increase in Pe, difference of CO2 load between regeneration solutions and the fresh sample is enlarged, especially for the solutions regenerated at 363 K and 373 K. For all the solu-

Regernation efficiency (%)

100

30% MDEA+15% IL 30% MDEA+10% IL 40% MDEA+10% IL

80 60

nCO / namine

2

0.0 100

0

200

300

Pe(kPa)

0.9

0.6

2

15 % IL + 30 % MDEA blank Regeneration temperature 363K 373K 387K

0.3

0.0 0

100 200 Pe (kPa)

300

0.9 0.6 10 % IL + 30 % MDEA blank Regeneration temperature 363K 373K 378K

0.3 0.0 0

100

200

300

Pe (kPa) Fig. 10. CO2 load of the regenerated solutions and the fresh solutions.

120 100

30% MDEA+10% IL 30% MDEA+15% IL 40% MDEA+10% IL

80 60 40 20 0

40

10 % IL + 40 % MDEA blank Regeneration temperature 363K 373K 378K

0.3

nCO / namine

where K is the apparent absorption rate constant and t the absorption time. n0 and ne are CO2 amount in the gas at initial time and equilibrium time, respectively. The apparent absorption rates of the regenerated solutions and the blank solutions in the first 40 min are shown in Fig. 8. As shown in Figs. 7 and 8, the absorption rates of CO2 increase with a rise in regeneration temperature. It has been revealed that the absorption capacity and the absorption rate both increase with a rise of IL’s concentration in aqueous solutions of MDEA [23]. Thus, more RNH2 in the solution would be liberated under high regeneration temperature. Obviously, even for the solution regenerated at 378 K, the absorption rates are lower than those of blank solutions, probably due to the incomplete regeneration of IL. It can be seen in Fig. 9 that for P0 = 97 kPa, the regeneration efficiencies for three solutions all increase with a rise in regeneration temperature. And regeneration efficiency of 30% MDEA + 15% IL solution is a little higher than those of the other two at regeneration temperature 363 K and 373 K. When Tb = 378 K, the regeneration efficiencies of three solutions are close to each other. It should be noted that, the regeneration efficiencies here are just under low absorption pressure (about 20 kPa). The regeneration performance under a wider range of CO2 pressure will be discussed in the next section.

0.6

2

ð14Þ

0.9

nCO / namine

lnðne  no Þ=ðne  nÞ ¼ Kt

1.2

Regeneration efficiency (%)

amount of IL which can accelerate the absorption of CO2 in MDEA aqueous solution has been renewed. According to the DampingFilm Theory, for the isothermal absorption of gas, the relationship between the partial pressure of the gas versus the time can be written in the form of n, the amount of CO2:

50 kPa

105 kPa

170 kPa

264 kPa

Pe 20 Fig. 11. Regeneration efficiencies of the solutions regenerated at 378 K.

0

363K 373K 378K Regeneration temperature

Fig. 9. Regeneration efficiencies of three solutions when P0 = 97 kPa.

tions regenerated at 378 K, their absorption load curve is more near the blank’ curve, especially when pressure < 50 kPa. Moreover, vacuum could be used to further improve the regeneration

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efficiency, and for 15% MDEA+15% IL solution, regeneration efficiency under vacuum and at 353 K is over 98% [24]. As depicted in Fig. 11, the regeneration efficiencies of the solutions regenerated at 378 K are all higher than 90%. This is the same with the results reported by Zhao [28]. For the same solution, regeneration efficiencies under different pressure have little difference. Furthermore, the regeneration efficiencies of aqueous solution with 30% MDEA + 15% IL are a little higher when Pe < 105 kPa, and when Pe > 105 kPa, the solution of 30% MDEA + 10% IL has higher regeneration efficiency. In general, desorption is just the absorption of CO2 under regeneration condition. According to the reaction equilibrium of Eqs. (5)–(10), the mass balance and the charge balance, the absorption load of CO2 in aqueous solution of MDEA and IL is mainly determined by the solution’s concentration, the absorption temperature and the equilibrium pressure of CO2. Thus, to obtain a prominent regenerated solution, high vacuum and regeneration temperature are both recommended. It is very interesting to obtain a mathematic model to calculated the absorption load of CO2 and thus to design the desorption condition. This needs many parameters, including Henry coefficient of CO2, ionicity parameters of all the species in the solution, plenty of experimental data, and so on [34]. The establishment of the model is currently on our way.

5. Conclusions In the present work, thermal regeneration of IL + MDEA aqueous solutions was performed. It was found that in the thermal regeneration, most of the gas gives off before boiling of solution and the higher regeneration temperature leads to the larger release amount of gas. A rise in regeneration temperature causes the higher absorption rates of CO2. Noticeably, even for the solution regenerated at 378 K, the absorption rates are lower than those of blank solutions, probably due to the incomplete regeneration of IL. When equilibrium pressure ranges between 0 and 300 kPa, absorption of CO2 in regenerated solutions reveals that under same pressure, the absorption load of CO2 increases with a rise in regeneration temperature. For all the solutions regenerated at 378 K, their absorption load curve is near the blank’s curve and the regeneration efficiencies are higher than 90%, especially when Pe > 50 kPa. Comparisons of the absorption rate and CO2 load between the regenerated solutions and the blank explore that MDEA which dominates CO2 load can be fully regenerated under high regeneration temperature, while regeneration of IL which determines the absorption rated is inadequate. Acknowledgements The authors are grateful for the finial support from: Natural Science Foundation of Jiangsu Province (No. BK2011633) and Environment Protection Research Project of Jiangsu Province (No. 2012017). References [1] D. Wappel, G. Gronald, R. Kalb, J. Draxler, Ionic liquids for post-combustion CO2 absorption, Int. J. Greenhouse Gas Control 4 (2010) 486–494. [2] J.M. Navaza, D. Gomez-Diaz, M.D. La Rubia, Removal process of CO2 using MDEA aqueous solutions in a bubble column reactor, Chem. Eng. J. 146 (2009) 184–188. [3] M.I. Cabaco, M. Besnard, Y. Danten, J.A.P. Coutinho, Solubility of CO2 in 1Butyl–3-methyl-imidazolium-trifluoro acetate ionic liquid studied by Raman Spectroscopy and DFT Investigations, J. Phys. Chem. B 115 (2011) 3538–3550. [4] F. Zhang, C.G. Fang, Y.T. Wu, Y.T. Wang, A.M. Li, Z.B. Zhang, Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA, Chem. Eng. J. 160 (2010) 691–697.

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