Ultrasonic Enhancement of CO2 Desorption from MDEA Solution in

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Ultrasonic enhancement of CO2 desorption from MDEA solution in microchannels Hongchen Liu, Shuainan Zhao, Feng Zhou, Chaoqun Yao, and Guangwen Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06050 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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Industrial & Engineering Chemistry Research

Ultrasonic enhancement of CO2 desorption from MDEA solution in microchannels Hongchen Liu1,2, Shuainan Zhao1,2, Feng Zhou1,2, Chaoqun Yao1, Guangwen Chen1,* 1. Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China 2. University of Chinese Academy of Sciences, Beijing 100049, China

Abstract The enhancement of CO2 desorption from N-methyldiethanolamine (MDEA) rich solution was investigated in ultrasonic microreactors. Under ultrasound irradiation, the rate of bubble growth increased significantly due to rectified diffusion and bubble coalescence. The measured CO2 desorption rate was found to be obviously enhanced by ultrasound, being almost doubled at low temperature. The effects of various parameters on the enhancing effect of ultrasound were also investigated, including desorption temperature, solution flow rate, CO2 loading, MDEA concentration, microchannel length and capillary diameter. The results indicated that ultrasound was more suitable to intensify CO2 desorption process at low temperature, by virtue of which the regeneration energy consumption and solvent loss could be efficiently reduced. Keywords: ultrasound, CO2 capture, desorption, bubble behavior, mass transfer enhancement

1. Introduction In recent years, the issue of global warming due to CO2 emission has attracted worldwide attention. The CO2 concentration has increased by about 40% since pre-industrial times and is still increasing at an unprecedented rate of approximately 0.4% per year, which mainly originates from the combustion of fossil fuels.1, 2 In order to reduce CO2 emission and mitigate global warming, CO2 capture should be adopted. There are several available technologies for CO2 capture,

*

Corresponding author. Tel.: +86-411-8437-9031, Fax.: +86-411-8469-1570

E-mail address: [email protected] (G.W. Chen). 1

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including chemical/physical absorption, adsorption, membrane separation and biological fixation.3 Among all these technologies, amine-based chemical absorption is the most mature method that has been already used for CO2 removal in a wide variety of industrial applications.4 Industrially important amine solvents mainly include monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA). The CO2 removal by conventional amine absorption technology is highly energy-intensive and costly, which would inevitably result in large increase in the cost of producing electricity.5, 6 Microreactors, as an efficient process intensification method, can offer significant advantages over conventional columns, including excellent mass and heat transfer, efficient scalability and small footprint.7-10 It holds great potential for improving separation efficiency of absorber/desorber and reducing equipment size, resulting in promoted performance at lower energy consumption. TeGrotenhuis et al.

11

studied CO2 absorption in microreactor using DEA solution. The results

showed that over 90% CO2 was removed in less than 10 seconds, suggesting the microchannel technology could miniaturize the equipment by an order of magnitude. In addition to faster absorption rate, higher absorption efficiency and lower costs were obtained in microreactors as well. Ye et al.

12

found that CO2 absorption efficiency could be improved to 99.94% by using a

rectangular microchannel under 3 MPa pressure. Yang et al.

13

further performed the economic

analysis of microchannel based CO2 capture units. The results indicated that microreactors were highly economically competitive to conventional equipment, achieving savings up to 50% at 5 MMSCFD plant capacity. Regrettably, CO2 desorption, as an important part in the amine absorption technology, received less attention in microchannels than CO2 absorption. This was mainly due to the complexity of CO2 desorption involving both mass transfer and heat transfer. Cypes et al.

14

studied toluene removal from water using a microfabricated stripping column (MFSC). The obtained mass transfer coefficient was one order of magnitude greater than that in conventional packed tower, which was attributed to the reduction in the thickness of liquid film. Nguyen et al. 15 found that rapid and complete CO2 desorption was realized when the diameter of the channel was below 300 μm, also indicating the advantages of using microchannel reactors in desorption process. Similarly, our previous work

16

showed that the mass transfer coefficients of CO2 2


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desorption process in the microchannel were one order of magnitude higher than those in conventional columns. These published results indicated that microchannel reactors have great potential for improving CO2 desorption process. Despite the fact that microreactors offer a variety of advantages in desorption process, there are still remaining challenges that need to be overcome for its wider application. For example, high energy consumption of CO2 desorption happened in the microchannel at high desorption temperature, which is caused by vaporization of large amount of water. Therefore, it is necessary to enhance the CO2 desorption at low temperature, which could reduce the regeneration energy consumption. In this regard, ultrasound has been widely used as an efficient method to accelerate gas desorption from solution. Many works have demonstrated that CO2 desorption rate could be increased by ultrasound.17-20 Ying et al.

17

studied the effect of ultrasound power on the CO2

desorption from 70 wt% MEA. The result showed that CO2 desorption rate significantly increased with the increase in the ultrasound power. Gantert et al.

19

further compared the enhancement

effect of ultrasound with varying frequencies (25 and 37.5 kHz), finding that ultrasonic frequency had no significant effect on desorption rate. Additionally, the lowest CO2 desorption energy was measured to be 2.27 GJ/t CO2, showing that ultrasound has a great potential for saving energy consumption of CO2 desorption process. The desorption acceleration under ultrasound is mainly due to ultrasound induced nucleation and bubble oscillation.18, 21 However, few studies have been done on the observation of bubble growth under ultrasound irradiation, which is due to the difficulties of observing bubble growth behaviors in the complex gas-liquid two phase flow using conventional reactors, thus resulting in a lack of understanding the intensification mechanism of ultrasound desorption process. In this work, a temperature-controlled ultrasonic microreactor was designed to study the bubble growth behavior during CO2 desorption from MDEA solution. Online observation was utilized to monitor and analyze the bubble growth behavior. In addition, the CO2 desorption rate with and without ultrasound were measured and compared to quantify the effect of ultrasound on the desorption process. Meanwhile, the effects of desorption temperature, solution flow rate, CO2 loading, MDEA concentration, microchannel length and diameter on the ultrasound enhancement were also investigated and discussed. 3



2. Experimental 2.1 Materials

MDEA (Beijing Jinlong Chemical Reagent LTD, 99.5 wt%) was used without any further purification. CO2 was used as supplied by Dalian Special Gases with a purity of 99.95 vol%. Three different concentrations (10, 20 and 30 wt%) of aqueous MDEA solutions were prepared with MDEA and deionized water. CO2 was bubbled into the MDEA solution under stirring condition to prepare different CO2 loaded MDEA solutions, ranging from 0.4-1.0 mol CO2/mol MDEA.

2.2. Apparatus The design and fabrication process of the ultrasonic microreactor were referenced from the previous work carried out in our lab.22,

23

To achieve temperature control during desorption

process, an ultrasonic-bath configuration was designed and directly coupled with the Langevin-type ultrasonic transducer (ZFHN-100-21.5, Baoding Zhengjie Electric) with a height of 90 mm. The maximum power and resonance frequency of ultrasonic transducer were 100 W and about 20 kHz, respectively. As shown in Figure 1(a), the ultrasonic-bath configuration was fabricated on an aluminum alloy plate with dimensions 80×80×10 mm, and hollowed out in the center to provide a space for circulating water to pass through. There were five fluid connectors fabricated on the side wall of aluminum alloy plate, in which one of them was used to monitor the desorption temperature by inserting a K type thermocouple, the four others were used as the inlet and outlet of circulating water and MDEA solution, respectively. The MDEA solution was pumped through a 0.8 mm PFA (Perfluoroalkoxy) tubing, which was suspended between the hollowed area by two contactor intervals. A transparent PASF (polyarylsulfone) plate was used to seal the aluminum alloy plate and observe the flow hydrodynamics. Additionally, a Teflon O-ring was placed between them to prevent leakage. The schematic of the assembly ultrasonic microreactor was shown in Figure 1(b). The combination of ultrasonic-bath configuration and ultrasonic transducer was realized by means of gluing with an ultrasonic transmission gel

4


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(THD-383, Taiheda). The resonance frequency of the manufactured system was measured to be about 20 kHz by an impedance analyzer (PV70A, Beijing Band Era, China). Another microreactor with a sandwich structure shown in Figure 2 was also fabricated and placed in an ultrasonic cleaning basin (PTA-1008, Trans-Potent Mechatronics LTD) to perform the measurement of desorption rate. It was due to that the limitation of operating temperature of our ultrasonic transducer was 60 oC, at which the CO2 desorption rate was too small to be accurately measured. In view of inaccurate temperature control of ultrasonic cleaning basin caused by large internal volume and large heat exchange with the environment, a small and closed sandwich structure microreactor was designed and shown in Figure 2. It consisted of a stainless top and bottom cover plate, a hollow PASF plate. There were also five fluid connectors fabricated on the top and bottom cover plate, and the usages were the same as those in the ultrasonic-bath configuration.

(a)

(b)

Figure 1. (a) Schematic representation of the ultrasonic-bath configuration; (b) ultrasonic microreactor assembly.

5



Figure 2. Schematic diagram of the sandwich type microreactor. 2.3 Procedures Figure 3 showed the sketch of the experimental setup for bubble behavior visualization. The ultrasonic microreactor was excited by a signal generator (AFG2112, GW InSTEK), whose signal was amplified using the power amplifier (AG 1016, T&C Power Conversion). In order to reduce the energy loss due to the reflected power from ultrasonic microreactor, an impedance matcher (T1K LF-7-070114, T&C Power Conversion) was used to match the impedance difference between the power amplifier and the ultrasonic microreactor. The bubble behavior during CO2 desorption process with and without ultrasound was monitored by a high-speed camera (Phantom M310, Vision Research) in connection with a stereo microscope (SZX16, Olympus). The CO2-loaded MDEA solution was delivered into the microchannel by a syringe pump with a flow rate of 0.25-1.50 mL/min. The desorption temperature was controlled by an external circulating water bath and set in the range from 40 to 60 oC.

Figure 3. The sketch of the experimental setup for bubble behavior visualization.

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Figure 4. The sketch of the experimental setup for the measurement of CO2 desorption rate. Figure 4 showed the experiment setup for measuring the CO2 desorption rate. The CO2-loaded MDEA solution was pumped into the PFA tubing in the sandwich type microreactor, which was placed in an ultrasonic cleaning basin. CO2 was released from MDEA solution in the microreactor, then separated from gas-liquid mixture in the separator immediately. The released CO2 was measured by a gas burette after being cooled to room temperature. During the experiment, the lab jack was adjusted to keep the water level in the leveling bottle as same as in the gas burette, and thus the change rate of liquid volume in the gas burette was equal to the CO2 desorption rate. Before carrying out each desorption test, the separator was swept by air to prevent the accumulated CO2 in the separator from affecting subsequent desorption test.

2.4 Mass transfer coefficient The reaction mechanism for the reaction of CO2 with MDEA was proposed by Donaldson and Nguyen,24 in which MDEA acts as a base and catalyzes CO2 hydration to form the bicarbonate. k

1   MDEAH + +HCO3MDEA+CO 2 +H 2 O  k-1

(1)

Based on the reaction mechanism, the reaction rate can be expressed as:

r  k1CMDEACCO2 - k- 1CMDEAH + CHCO-

3

(2)

The transfer of CO2 molecules to the gas bulk can be illustrated by the stagnant film model. With the assumption of no mass transfer resistance in the gas film 25, 26 and no pronounced change 7



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of all species concentration except CO2, the differential equation combined with desorption reaction rate for CO2 desorption was given by: DCO2

d 2CCO2 dx

2

b b   r  k- 1CMDEAH + CHCO- - k1CMDEACCO2 =k1CMDEA (CCO  CCO2 ) 2

(3)

3

where Cb MDEA and Cb CO2 are the MDEA and CO2 concentrations at the bulk liquid phase, respectively. According to the equilibrium equation of Reaction (1), Cb CO2 is calculated as follows. b CO2

C



b b CHCO -C MDEAH + 3

b K1CMDEA

=

total  2CMDEA K1 (1   )

(4)

where α is the CO2 loading in solution. The value of equilibrium constant K1 can be obtained from the work of Jamal et al.27 The boundary conditions of Equation (3) are as follows: i x  0, CCO2  C CO

(5)

2

b x   L , CCO2  C CO

(6)

2

Therefore, the CO2 desorption rate flux can be obtained by integrating Equation (3) and satisfying Equations (5) and (6). b i N CO2  k L (C CO - C CO ) 2

Ha 

2

Ha th( Ha )

(7)

1 b k1 DCO2 CMDEA kL

(8)

According to the mass balance for CO2, the CO2 desorption rate JCO2 is written as follows: total J CO2  N CO2 A  QLCMDEA ( in -  out )

(9)

In addition, a mass balance for CO2 in the bulk liquid over an elementary volume of the microchannel can be written as: -QL dCCO2  k L a (C i - CCO2 )dV

(10)

where dCCO2 represents the change in concentration of all the forms of CO2 in the bulk liquid phase (mainly existing as CO2, HCO- 3). Associated with the concentration relationship between HCO- 3 and CO2 shown in Equation (4), the average liquid side volumetric mass transfer coefficient could be derived by integrating Equation (10).

8


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kLa =

QL V

b i  out  CCO2   CCO2 Ci 2 ln( )  (  ) 1 d   C  Ci total   in CMDEA K1 (1   ) out CO2  

(11)

where Cout represents the CO2 concentration in solution at the outlet of the microchannel.

3. Results and discussion 3.1 Bubble growth during CO2 desorption process To elucidate the ultrasound enhancement mechanism on desorption process, the hydrodynamic characteristics of the MDEA-CO2 flow in microchannel was investigated. Figure 5 showed the bubble growth phenomenon during CO2 desorption process with and without ultrasound. It was obvious that the bubble growth rate was significantly increased by ultrasound exertion, which appeared to be related to two mechanisms, i.e., ultrasound induced rectified diffusion and bubble coalescence.21, 28, 29 When the ultrasound was introduced, large amounts of microbubbles were formed due to alternating positive and negative pressure waves.18 In response to ultrasound, the dissolved CO2 gas in MDEA solution would diffuse into the generated microbubble, making it grow larger until it reaches its resonance size (also termed as rectified diffusion).30-32 In addition to the rectified diffusion pathway, microbubbles can also grow by bubble-bubble coalescence.28 Due to the existence of acoustic radiation force, microbubbles tend to coalesce with each other and form larger ones, leading to accelerated growth rate of the bubble size. It should be noted that both the rectified diffusion pathway and bubble coalescence phenomenon contribute to the obviously elevated bubble growth rate with ultrasound exertion. It could also be seen in Figure 5 that a non-spherical shape of the slug cap was observed when the bubble filled the channel cross section, which was different from the previous observed case in a stainless steel microchannel.16 This phenomenon was caused by the difference in the surface wettability between PFA tubing and stainless steel microchannel.

9



Figure 5. The bubble growth during CO2 desorption process with and without ultrasound. (CMDEA= 20 wt%, α= 0.960 mol CO2/mol MDEA, QL = 0.25 mL/min, T = 40 oC) The effect of ultrasound power on the growth rate of bubble diameter with varying desorption temperature and solution flow rate were investigated and shown in Figure 6. As can be seen in Figure 6(a), the growth rate of bubble diameter significantly increased with increasing ultrasound power firstly, then the increasing tendency slowed down. It indicated that a disproportional increase occurred with the increasing ultrasound power, which was due to the reduction in the ultrasonic efficiency. The more ultrasound power input, the more bubbles generated, which led to increasing energy dissipation due to the disturbing scattering of acoustic wave on the bubble surface.19 In addition, it could be also seen that bubble growth rate increased with increasing desorption temperature due to the increase in reaction rate. Meanwhile, higher desorption temperature resulted in lower CO2 equilibrium concentration at gas-liquid interface, which led to an increase in the mass transfer driving force. The effect of solution flow rate on the growth rate of bubble diameter was shown in Figure 6(b). It was found that bubble growth rate decreased with the increase in solution flow rate. The reason lay in that pressure drop increased with increasing flow rate, leading to increased pressure in the microchannel and in turn suppressed bubble formation and growth.33 Meanwhile, the high flow rate increased liquid inertia force and promoted bubble removal from the microchannel.34 It would reduce the bubble coalescence, which appeared responsible for a sharp reduction in the growth rate of bubble diameter when flow rate increased

10


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from 0.25 to 0.50 mL/min. When the ultrasound is introduced, it promoted bubble generation and coalescence, which could offset the negative effect from the increasing flow rate. The higher ultrasound power input, the greater enhancement effect of ultrasound. Therefore, bubble growth rate increased with increasing solution flow rate from 0.50 to 1.50 mL/min when the ultrasound power input was above 10 W.

Growth rate of d B /(mm/min)

20

15

10 40 o

T [ C]

5

50 60

0 0

5

10

15

20

25

P /W

(a) 20 0W

Growth rate of d B /(mm/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5W 10W

15

20W 10

5

0 0.0

0.5

1.0

1.5

2.0

Q L /(mL/min)

(b) Figure 6. Effect of ultrasound on the growth rate of bubble diameter as a function of (a) desorption temperature at 0.50 mL/min and (b) solution flow rate at 40 oC.

11



3.2 CO2 desorption rate under ultrasound Although bubble growth rate increased significantly under ultrasound irradiation, the CO2 desorption rate was disproportionate to the growth rate. It was due to the fact that both desorption mass transfer performance and the bubble coalescence increased under ultrasound, while only the increase in the mass transfer would increase CO2 desorption rate. Therefore, it was necessary to study the enhanced mass transfer performance under ultrasound by measuring CO2 desorption rate.

3.2.1 Effects of solution flow rate and desorption temperature The effects of solution flow rate and desorption temperature on the CO2 desorption rate with and without ultrasound were investigated. Figure 7 showed that the CO2 desorption rate increased with increasing solution flow rate and desorption temperature under both circumstances with and without ultrasound, which was similar to the trend observed in our previous work.16 It was reasonable that the amount of desorbed CO2 increased at high flow rate, and the reaction rate, as well as the driving force of mass transfer, increased at high desorption temperature. In addition, it also can be seen that the CO2 desorption rate with ultrasound was higher than that without ultrasound, which confirmed the enhancing effect of ultrasound on the desorption process. In order to clearly show the effect of ultrasound on the CO2 desorption rate, the ratio of CO2 desorption rate with and without ultrasound E was calculated and shown in Figure 8. It can be seen that E decreased with increasing desorption temperature. It was caused by more gas generated at higher desorption temperature, resulting in reduction in the ultrasound time and the ultrasound power input per unit volume of the solution.34 Furthermore, more ultrasound energy would be absorbed by water in the ultrasonic cleaning basin at higher temperature due to the reduction in the water viscosity,35 which led to a lower energy efficiency of ultrasound. At low desorption temperature, E significantly increased with increasing solution flow rate at low flow rate, and the increasing tendency slowed down at high flow rate. While for high desorption temperature, E increased initially followed by a gradual decrease with the increase in flow rate. On one hand, it was attributed to an increase in the deviation from desorption equilibrium with increasing flow rate at fixed desorption temperature, which was observed in our previous work.16 12


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Meanwhile, ultrasound only provided a catalytic effect on the desorption

19

and the desorption

reaction was not beyond the equilibrium point. Thereby the enhancement of mass transfer increased with increasing flow rate. On the other hand, the intensification of mass transfer was highly related with bubble oscillation under ultrasound.21 The amount of gas generated increased with increase in flow rate, which led to an increase in the enhancement of mass transfer. However, at higher flow rate, more gas was generated and solution residence time decreased, resulting in a decrease in the enhancement of mass transfer. The combination of these effects caused an increase in E at low flow rate and desorption temperature, and a reduction at high flow rate and desorption temperature. 30 60 o

T [ C]

J CO2 /(mL/min)1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

70 80 90

10

0 0.00

0.50

1.00

1.50

Q L /(mL/min)

Figure 7. Effect of ultrasound on the CO2 desorption rate as a function of solution flow rate. (solid line: without ultrasound; dotted line: with ultrasound) CMDEA = 30 wt%, α = 0.660 mol CO2/mol MDEA, L = 50 cm, D = 0.8 mm.

13



2.5

2.0

1.5

E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

60 o

T [ C]

0.5

70 80 90

0.0 0.00

0.50

1.00

1.50

2.00

Q L/(mL/min)

Figure 8. The ratio of CO2 desorption rate with and without ultrasound versus solution flow rate. CMDEA = 30 wt%, α = 0.660 mol CO2/mol MDEA, L = 50 cm, D = 0.8 mm.

3.2.2 Effects of CO2 loading and MDEA concentration The effect of CO2 loading on ultrasound enhancement of CO2 desorption rate was studied at fixed temperature 90 oC. As can be seen in Figure 9, E decreased with the increase in CO2 loading. E was almost equal to 1 at 0.855 mol CO2/mol MDEA at each flow rate, suggesting that ultrasound had little effect on the enhancement of mass transfer. It was due to the reduced solution residence time caused by generated gas at high CO2 loading, which further lead to lower power input into the ultrasound-processed solution. Meanwhile, the reason might also be that the driving force of mass transfer significantly increased with increasing CO2 loading, which would make the enhancement effect of ultrasound less prominent. A similar effect of MDEA concentration on E was observed, as shown in Figure 10. E decreased with the increase in the MDEA concentration. The reason was the same as explained above. For fixed CO2 loading, the ultrasound time and the ultrasound power input per unit volume of the solution decreased with increasing MDEA concentration due to higher generation rate of gas phase.

14


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E

1.5

1.0 0.603

α [mol CO2/mol MDEA]

0.660 0.855

0.5 0.00

0.50

1.00

1.50

Q L /(mL/min)

Figure 9. Effect of CO2 loading on the ratio of CO2 desorption rate. CMDEA = 30 wt%, T = 90 oC, L = 50 cm, D = 0.8 mm.

2.0

1.5

E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 10%

C MDEA

0.5

20% 30%

0.0 0.00

0.50

1.00

1.50

Q L/(mL/min)

Figure 10. Effect of MDEA concentration on the ratio of CO2 desorption rate. T = 90 oC, α = 0.6 mol CO2/mol MDEA, L = 50 cm, D = 0.8 mm.

3.2.3 Effects of microchannel length and diameter The effects of microchannel length and diameter were also investigated. According to the analysis above, E was influenced by the solution residence time. In general, increasing 15



microchannel length would increase solution residence time, thus promoting the ultrasound enhancement of mass transfer. However, it was shown in Figure 11 that E had little increase at 0.25 mL/min, and was improved a bit at higher flow rate. The reason might be that the solution residence time only increased from 1 to 8 s at 1.0 mL/min and 90 oC if taking into account the large amount of CO2 generated. The ultrasound power input into the solution was still too little to have much impact on the enhancement of mass transfer. Therefore, a timely separation of generated gas was necessary to further improve the mass transfer enhancement. For 0.25 mL/min, the CO2 concentration at the outlet of microchannel was close to its equilibrium value,16 leading to less effect on E by increasing microchannel length. 1.5

E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0 0.25 0.50 0.75 1.00 1.50

QL [mL/min]

0.5 0

20

40

60

80

100

L / cm

Figure 11. Effect of microchannel length on the ratio of CO2 desorption rate. CMDEA = 30 wt%, T = 90 oC, α = 0.660 mol CO2/mol MDEA, D = 0.8 mm.

Figure 12 showed the effect of microchannel diameter on E as a function of superficial liquid velocity. It displayed that E increased with an increase in the microchannel diameter at same jL. The result agreed well with our previous work, finding that ultrasonic oscillation as well as microbubble behavior was restrained in reduced microchannel dimension.21 The microchannel wall would reduce bubble oscillation by adding additional resistance.36,

37

The smaller space

between microchannel wall and bubble surface would lead to more resistance, and thus, a weaker enhancement on the mass transfer was occurred. 16


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2.0

1.5

E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8

D

0.5

1.2

[mm]

1.8 0.0 0

2

4

6

8

10

12

j L /(mm/s)

Figure 12. Effect of microchannel diameter on the ratio of CO2 desorption rate. CMDEA = 30 wt%, T = 80 oC, α = 0.660 mol CO2/mol MDEA, L = 50 cm.

3.3 Mass transfer characteristics The mass transfer coefficients with and without sonication were calculated according to Equation (11). The effects of desorption temperature and flow rate were shown in Figure 13. It

could be seen in Figure 13(a) that kLa increased with increasing the solution flow rate and desorption temperature. For increasing the solution flow rate, it would increase the surface renewal rate and promote the recirculation inside the liquid slugs between neighboring bubbles, which led to the improvement of mass transfer. As to the effect of increasing desorption temperature, it would increase CO2 diffusivity and decrease CO2 concentration at the gas-liquid interface, thus improving the mass transfer. It could be also seen that the mass transfer coefficient with ultrasound was higher than that without ultrasound, showing that the

use of ultrasound could intensify the mass transfer performance during CO2 desorption process. The mass transfer enhancement factor ((kLa)u/kLa) was calculated and shown in Figure 13(b). It was observed that the change trend of enhancement factor with the variation of desorption temperature and flow rate was similar to that of desorption rate ratio shown in Figure 8. According to Equations (7) and (9), the CO2 desorption rate was proportional to liquid

17



side volumetric mass transfer coefficient. Therefore, the desorption rate ratio and mass transfer enhancement factor with the variation of desorption temperature and flow rate appeared similar tendency. In addition, this phenomenon was also observed under other operating conditions, such as CO2 loading, MDEA concentration, microchannel length and diameter. The liquid-side volumetric mass transfer coefficients during CO2 desorption process were in the range of 0.04-4.55 s-1 in the experiment, which were much higher than those in conventional rectors.25 The mass transfer coefficients were significantly enhanced by ultrasound at low temperature and large microchannel diameter, which were up to about 2 times higher than those without ultrasound. It indicated that ultrasound was suitable to intensify CO2 desorption process at low temperature, which would reduce the regeneration energy consumption and solvent loss.

1.5 60 o

T [ C] 1.0

70 80 90

-1

k La / (s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5

0.0 0.00

0.50

1.00

Q L / (mL/min)

(a)

18


1.50

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2.5

2.0

EF

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1.5

1.0 60 o

T [ C]

0.5

70 80 90

0.0 0.00

0.50

1.00

1.50

Q L / (mL/min)

(b) Figure 13. Effects of desorption temperature and flow rate on the (a) liquid-side volumetric mass transfer coefficient (solid line: without ultrasound; dotted line: with ultrasound) and (b) mass transfer enhancement factor. CMDEA = 30 wt%, α = 0.660 mol CO2/mol MDEA, L = 50 cm, D = 0.8 mm.

4. Conclusion To study the enhancement mechanism of ultrasound on the CO2 desorption process from MDEA solution in microchannels, the bubble behavior was investigated in a home-made ultrasonic microreactor. It showed that the growth rate of bubble during CO2 desorption was significantly increased by introducing ultrasound into the solution, which was due to ultrasound induced rectified diffusion and bubble coalescence. The bubble growth rate increased with increasing ultrasound power and desorption temperature, as well as with decreasing solution flow rate. However, high ultrasound power could offset the negative effect from the increasing flow rate, which make bubble growth rate increase with solution flow rate at high flow rate. In addition, the measurement of CO2 desorption rate with and without ultrasound was carried out to study the importance of various operating parameters on ultrasound enhancement. The results indicated that the CO2 desorption rate and mass transfer coefficient were obviously increased by ultrasound, being almost doubled at low temperature. The ultrasound enhancement 19



increased with the increasing solution flow rate, microchannel length and diameter at relatively low temperature. Meanwhile, the ultrasound enhancement decreased with the increase in the desorption temperature, CO2 loading and MDEA concentration, which was partly due to a reduction in the solution residence time. Therefore, a timely separation of generated gas from the solution was very important for further improving the ultrasound enhancement on the mass transfer.

Acknowledgments Authors would like to thank Jiansheng Chu, Hengqiang Li and Fengjun Jiao (engineers in our research group) for their help in manufacturing the microreactor and building the experimental setup. We acknowledge gratefully the financial supports for this project from the National Natural Science Foundation of China (Nos. 91634204 and U1608221), DICP (ZZBS201706), the Youth Innovation Promotion Association CAS (No.2017229), MOST innovation team in key area (No. 2016RA4053) and the STS program.

Notation a

specific surface area, m2/m3

A

interfacial area, m2

C

molar concentration, mol/L

D

microchannel diameter, m

dB

bubble diameter, m

DCO2

diffusivity of CO2 in the liquid, m2/s

E

the ratio of CO2 desorption rate with and without ultrasound

EF

the mass transfer enhancement factor

Ha

Hatta number

JCO2

CO2 desorption rate at the interface, mol/s

jL

superficial liquid velocity, m/s

k

reaction rate constants for the reaction

K

equilibrium constant

20


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Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kL

liquid-side mass transfer constants, m/s

kLa

liquid-side volumetric mass transfer coefficient, s-1

NCO2

CO2 desorption rate flux at the interface, mol/(m2·s)

L

microchannel length, m

QL

volume flow rate, L/s

r

reaction rate of CO2 desorption, mol/(m3·s)

T

temperature, oC

x

spatial variable measured from the gas-liquid interface, m

Greek symbols α

CO2 loading, mol CO2/mol MDEA

δL

laminar film thickness, m

Subscripts L

liquid phase

in

the inlet of microchannel reactor

out

the outlet of microchannel reactor

u

ultrasound

Superscripts b

liquid bulk

i

the gas-liquid interface

total

total concentration

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Ultrasonic Enhancement of CO2 Desorption from MDEA Solution in

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