Mass-Transfer Characteristics of CO2 Absorption into Aqueous

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Mass transfer characteristics of CO2 absorption into aqueous solutions of N-methyldiethanolamine + diethanolamine in a T-junction microchannel Guanyi Lin, Shan Jiang, Chunying Zhu, Taotao Fu, and Youguang Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06231 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Sustainable Chemistry & Engineering

Mass transfer characteristics of CO2 absorption into aqueous solutions of N-methyldiethanolamine + diethanolamine in a T-junction microchannel Guanyi Lin†, Shan Jiang†, Chunying Zhu†*, Taotao Fu†, Youguang Ma†* †State

Key Laboratory of Chemical Engineering, School of Chemical Engineering and

Technology, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, P. R. China *E-mail: [email protected] (C. Zhu); [email protected] (Y. Ma) Abstract: The mass transfer process of CO2 absorption into aqueous solution of N-methyldiethanolamine

(MDEA)

+

diethanolamine

(DEA)

in

T-shaped

microchannel was studied using a high speed camera. The effects of two phase flow rates and diethanolamine concentration on CO2 absorption efficiency, liquid mass transfer coefficient kL, specific surface area a and volumetric mass transfer coefficient kLa were investigated under slug flow. Results showed that the CO2 absorption efficiency increased with the increase of DEA concentration and liquid flow rate, and decreased when increasing gas flow rate. The kL increased with the increase of liquid flow rate and DEA concentration, while it was insensitive to gas flow rate. For a given liquid flow rate, both kLa and a increased rapidly with gas flow rate, and then the trend was slowing down. The increase of liquid flow rate and DEA concentration led to the decrease of the a, whereas the kLa was improved. The enhancement of the liquid flow rate on kL and kLa was more obvious under low DEA concentration. An empirical correlation for predicting the volumetric mass transfer coefficients kLa was

1

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proposed by taking the Hatta number Ha into account for characterizing the effect of chemical reaction. Keywords: Microchannel; Mixed amine; Chemical absorption; Carbon dioxide; Mass transfer INTRODUCTION The large amount of fossil energy consumption leads to the increase of CO2 concentration in the atmosphere. Carbon dioxide (CO2), as an important greenhouse gas, is the main contributor to the global warming.1,2 The absorption method is widely adopted for the removal of CO2 from gas stream using aqueous alkylamine solutions as absorbents in many industrial processes.3,4 However, many alkylamine solutions have the shortcoming that absorption and desorption cannot achieve ideal effect simultaneously. For example, monoethanolamine (MEA) and diethanolamine (DEA) could fast absorb CO2, but have slow desorption rate of CO2, high energy consumption for desorption and great corrosion to equipment. On the contrary, tertiary amines, such as N-methyldiethanolamine (MDEA) has slow absorption rate of CO2 due to the inability to directly react with CO2, while it has high desorption rate of CO2 and low consumption of regeneration.5-7 The use of mixed amine solution is receiving considerable attention because of the advantage of relatively high absorption rate of CO2 together with lower consumption of regeneration.8 The traditional industry mostly adopts packing tower to absorb CO2. Comparatively, microreactor has many advantages, such as small equipment volume, high mixing efficiency, easy scale-up and high safety, accordingly it has gained increasing attention on the improvement of CO2 absorption over the recent years.9 A large number of researchers have investigated the mass transfer process of CO2 2


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absorption by amine solution in microchannel. Li et al.10 investigated the absorption of pure CO2 gas by an aqueous solution of MEA with a mass fraction of 5 %. They found that the volumetric mass transfer coefficient increased remarkably with gas flow rate due to strong augmentation in the specific interfacial area. Ye et al.11 also studied the mass transfer process of CO2 absorbed by MEA solution in microchannel, and found that the increase of chemical reaction rate had an enhancement effect on mass transfer. Ganapathy et al.12 studied the mass transfer process between DEA solution and CO2/nitrogen (N2) mixture, and investigated the effect of CO2 concentration on volumetric mass transfer coefficient and CO2 absorption efficiency. When the concentration of DEA was low, the concentration of CO2 in the gas mixture had a more significant effect on the mass transfer. Subsequently, Ganapathy et al.13 utilized the DEA solution to absorb CO2 from the N2/CO2 mixture, and investigated the influence of the inner diameter of the channel and the diethanolamine concentration on specific surface area and mass transfer coefficient. The results showed that the decrease of the channel inner diameter could significantly improve the specific surface area and the volumetric mass transfer coefficient, and the volumetric mass transfer coefficient increased with increasing solute concentration. Up to now, most previous studies have been devoted to the CO2 absorption into single-amine solution in the microchannel, the investigation on the CO2 absorption by mixed amine solution remains still insufficient even lacking. In

this

study,

the

mass

transfer

process

of

CO2

absorption

into

N-methyldiethanolamine (MDEA) and diethanolamine (DEA) mixed amine solution 3



was investigated. The effects of gas and liquid phase flow rates and solution concentration on the CO2 absorption efficiency, mass transfer coefficient, specific surface area and volumetric mass transfer coefficient were explored. An empirical correlation, considering the enhancement of the chemical reaction on mass transfer, was proposed for predicting the volumetric mass transfer coefficients kLa. EXPERIMENTAL SECTION The gas-liquid slug ﬂow was generated in a T-junction microchannel. The channels were fabricated on a polymethyl methacrylate substrate (PMMA) and then sealed with another PMMA plate by screws. Both width d and depth w of the channel are 400 μm, and the main channel length LC is 36 mm. The schematic diagram of the experiment setup for CO2 absorption is presented in Figure 1. DEA (mass percent ≥ 99 %, Shanghai Aladdin Biochemical Technology Co., Ltd) blended with MDEA (mass percent ≥ 99 %, Shanghai Aladdin Biochemical Technology Co., Ltd) aqueous solutions and pure CO2 gas were conveyed by microsyringe pumps (Harvard PHD2000, USA) into liquid and gas inlets respectively. The mass fraction of MDEA was a constant value of 5 %, while the mass fraction of DEA was varied from 1 % to 5 %. The pressure of gas phase at inlet of channel was measured by a piezometer (ST3000, Honeywell, USA. The precision is 0.02 %), and the pressure at outlet was atmospheric pressure. The illumination was provided by a cold light source (MHAA-100 W) with the lighting area of 90 × 90 mm2. After the flow stabilized about 5 min for a new experimental condition, the gas-liquid flow process was recorded by a high-speed camera (MotionPro Y5, IDT, USA) at 2000 4


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frames per second, which is located over the microchip. Some typical pictures of gas-liquid flow are shown in Figure 2. The density ρ, viscosity μL and surface tension σ of solution were measured respectively using densimeter (DMA4500, Anton Paar, Austria), automatic ubbelohde viscometer (iVisc, LAUDA, Germany) and tensiometer (OCA15ECm Data Physics Instruments GmbH, Germany) at 293.15 K. Each sample was measured three times and average value was calculated as its experimental value. The data were given in Table 1. The experiments were carried out at 293.15 ± 1 K and atmosphere pressure, the ranges of gas flow rate QG and liquid flow rate QL were separately 20 ~ 260 mL·h-1 and 20 ~ 50 mL·h-1.

Figure 1. Schematic diagram of the experimental set-up.

Figure 2. Gas-liquid two-phase flow under different operating conditions. 5



Page 6 of 32

Table 1. Physical properties of blended amine solutions ρ/

103μL/

103σ/

104He/

(kg m-3)

(Pa s)

(N m-1)

(mol m-3 Pa-1)a

5% MDEA+1% DEA

1004.08

1.05

41.76

3.41

5% MDEA+2% DEA

1005.32

1.08

41.33

3.27

5% MDEA+3% DEA

1006.42

1.12

40.32

3.20

5% MDEA+4% DEA

1007.55

1.17

40.26

3.16

5% MDEA+5% DEA

1008.73

1.23

40.20

3.14

Aqueous solutions

a

Henry coefficients He were calculated according to Penttila’s work.14

RESULTS AND DISCUSSION CO2 absorption efficiency. The volume of a single bubble VB could be calculated by the method of Zhu et al.15

VB 

w3 6

 0.9 w3 ( LB  w)

(1)

where LB is the length of the bubble. It could be obtained through two-phase flow pictures recorded by the high-speed camera. The resolution of the image is 14 μm/pixel, the measurement error is 1 ~ 2 pixels, correspondingly, and the maximum measurement error is 28 μm for length of the bubble. The CO2 absorption efficiency X could be calculated by: X

PinVin  PoutVout PinVin

(2)

where Pin is the pressure of the bubble inlet, and the outlet pressure Pout is atmospheric pressure. Vin and Vout are the bubble volumes at the inlet and outlet of the 6


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microchannel, respectively. Figure 3 shows the effects of gas and liquid flow rates on CO2 absorption efficiency at different concentrations. It could be found that higher concentration of DEA and liquid flow rate have higher CO2 absorption efficiency, while on which the reverse effect of gas flow rate is observed. The absorption efficiency is determined by two factors: one is the residence time of the bubbles in the microchannel and the other is the mass transfer rate. The increase of the DEA concentration could accelerate chemical reaction between alkyamino and CO2 and accordingly enhance mass transfer. Meanwhile, the increase of chemical reaction rate could lead to a faster reduction of the bubble volume, in this case, the movement of the bubbles in the channel would slow down, which prolongs the residence time of the bubbles in the microchannel and promotes the CO2 absorption. Conversely, the increase of gas flow rate would greatly reduce the residence time of the bubbles and thereby result in the decrease of CO2 absorption efficiency. Differently, although the increase of liquid flow rate could shorten the bubble residence time, especially in the case of low gas flow rate, the increase of liquid flow rate could accelerate the internal circulation of liquid slug and increase the mass transfer rate between the gas and liquid phases, thereby leading to the improvement of CO2 absorption efficiency. It also could be seen from Figure 3 that the difference between CO2 absorption efficiency for different liquid flow rates decreases with the increase of DEA concentration. Particularly, the CO2 absorption efficiency in low liquid flow rate is even higher than that in high liquid flow rate for the DEA solution of 5 % mass fraction and gas flow rate of 60 mL·h-1. With the 7



increase of concentration of DEA solution, difference of volumetric mass transfer coefficient for different liquid flow rates decreases (Figure 6), accordingly, the effect of residence time on absorption efficiency of CO2 becomes notable. The competition of the two opposite effects for CO2 absorption efficiency leads to the similar absorption efficiency for two different liquid flow rates at high DEA concentration. Moreover, the increase of the liquid flow rate would markedly shorten the bubble residence time under considerably low gas flow rate (60 mL·h-1) and high DEA concentration (5% mass), thus resulting in a lower CO2 absorption efficiency. 80 QL=20mL·h-1 1% DEA QL=40mL·h-1 1% DEA QL=20mL·h-1 3% DEA

60

QL=40mL·h-1 3% DEA QL=20mL·h-1 5% DEA

X/%

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

Page 8 of 32

40

QL=40mL·h-1 5% DEA

20

0

0

50

100

150

200

250

300

350

QG/mL·h-1 Figure 3. Effects of DEA concentration and gas and liquid flow rates on CO2 absorption efficiency (X %).

Calculation of mass transfer coefficient. Mass transfer resistance is generally considered to concentrate in the liquid film for the absorption process of pure CO2 gas according to two-film theory.16 Therefore, the average mass transfer flux N from the inlet position to the outlet position in the main channel could be written as:

N  k L Ce  C 

(3)

where kL is the liquid side mass transfer coefficient, Ce is the equilibrium concentration of CO2 in the blended amine solution and is assumed to be the 8


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concentration of CO2 at the gas-liquid interface, C is the concentration of CO2 in the liquid body. The reaction between CO2 and alkylamine in this work is a rapid chemical reaction and the alkylamine is excess. Thus the concentration of CO2 in the liquid phase could be considered as C = 0. According to Henry's law, the equilibrium concentration Ce is:

Ce  PHe

(4)

where P is the mean value of the pressures at inlet and outlet of microchannel, and Henry coefficient He is given in Table 1. The molar quantity of a single bubble absorbed into the liquid in the main channel nB is:

nB 

PinVin  PoutVout RT

(5)

where R is the ideal gas constant and T is the experimental temperature which is 293.15 K. The area of a single bubble AB is calculated according to the method in the literature15 which takes into account the constant curve with radius r in the corners in square microchannel:

AB  w2  2r  4( w  2r )( LB  w) 1 40  10

rw

(6) (7)

The average mass transfer flux N in the main channel could be calculated by: N

f nB f  PinVin  PoutVout   AC AC RT

(8)

where f is the formation frequency of bubbles, the mass transfer area AC is the sum of all the bubble surface area in the microchannel at any time. In the mass transfer 9



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process, the flow of the gas-liquid is stable, and the mass transfer area AC of the overall microchannel maintains basically unchanged. The mass transfer coefficient kL could be calculated by Eq. (9): kL 

f PInVin  PoutVout  ACCe RT

(9)

Reaction Mechanism. For the absorption of CO2 into MDEA +DEA + H2O solution, two specific chemical reactions occur as follow.17

CO 2  MDEA  H 2 O  MDEAH   HCO3

(10)

CO 2 +2DEA+H 2 O  DEACOO- +DEAH +

(11)

It is assumed that reaction of CO2 with MDEA and DEA is a pseudo first order reaction.17 The overall reaction rate rov could be expressed as follow.

rov  K1  CO 2 

(12)

where K1 is the overall pseudo first order reaction rate constant of the reaction between the blended amine solution and CO2 and could be calculated by the method of Lin et al.17 The value of K1 increases with the concentration of alkanolamine in the solution. Mass transfer coefficient. Figure 4 illustrates the effects of the DEA concentration in the blended amine solution and the gas and liquid flow rates on mass transfer coefficient. It could be seen that mass transfer coefficient increases with the increase of DEA concentration as shown in Figure 4a. Carbon dioxide is absorbed into the liquid phase through the interface of gas-liquid and then reacts with DEA and MDEA rapidly. The increase of DEA concentration could promote the chemical reaction and enhance mass transfer, and accordingly brings about an increase in mass transfer 10


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coefficient.15 Moreover, the mass transfer coefficient increases with liquid flow rate, but the effect of the gas flow rate on it is not remarkable as shown in Figure 4b-d. Because the gas phase resistance is negligible, the mass transfer depends primarily on the flow state of liquid slug and liquid film. It is thought that the mass transfer between bubble and liquid mainly includes that from bubble caps to liquid slug (kcap) and from bubble body to liquid film between bubble and microchannel wall (kfilm).18 The increase of liquid flow rate would prompt the internal circulation of the liquid slug, which would improve the kcap due to mass transfer enhancement.19 Meanwhile, the increase of liquid flow rate would result in the increase of the liquid slug length and the reduction of the bubble length, which could cause high mixing efficiency between liquid slug and liquid film and the low liquid film saturation, leading to an increase in the kfilm. Consequently, the overall liquid side mass transfer coefficient kL increases. Although the increase of gas flow rate would also facilitate the internal circulation of the liquid slug and improve the value of kcap,20 it could reduce the length of the liquid slug and elongate the bubble at the same time, which would lead to a smaller kfilm because of low mixing efficiency and liquid film saturation. The combined effect of two factors would result in a slightly positive influence of gas flow rate on the mass transfer coefficient. It could be seen from Figure 4 that the increase of liquid flow rate at different concentration could cause various influences on the mass transfer coefficient. For example, for the mass fraction of 1 % diethanolamine, when liquid flow rate is increased from 20 mL·h-1 to 50 mL·h-1 at the same gas flow rate, the mass transfer 11



coefficient could doubly increase. While under the concentration of 5 % DEA, the increase of liquid flow rate improves mass transfer coefficient less than 60 %. As mentioned in the literature,19 the liquid film is easily saturated especially when DEA concentration is very low. The increase of liquid flow rate enhances the mixing between liquid slug and liquid film, and reduces significantly the saturation of liquid film, which leads to an increase of the liquid film mass transfer coefficient kfilm. However, for the higher DEA concentration, the liquid film is not easily saturated, thus the increase of liquid flow rate could not improve the liquid film mass transfer coefficient kfilm obviously. Therefore, the liquid flow rate at low DEA concentration has a more significant effect on mass transfer coefficient. 9.0

(a)

7.5

6.0

6.0

wt % DEA 1 2 3 4 5

4.5 3.0 1.5 0.0

0

50

100

150

200

QG/mL·h

9.0

250

300

kL/(×10-4m·s-1)

7.5

QL (mL·h-1) 20 30 40 50

4.5 3.0 1.5 0.0

350

(b)

0

9.0

(c)

7.5

6.0

6.0

4.5 QL (mL·h-1)

3.0 1.5 0

50

100

150

200

QG/mL·h

100

150

200

250

300

QG/mL·h-1

7.5

0.0

50

-1

kL/(×10-4m·s-1)

kL/(×10-4m·s-1)

9.0

kL/(×10-4m·s-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

Page 12 of 32

20 30 40 50 250

4.5

QL (mL·h-1) 20 30 40 50

3.0 1.5 0.0

300

(d)

0

50

100

-1

150

200

250

300

QG/mL·h-1

Figure 4. Effects of DEA concentration and gas and liquid flow rates on mass transfer coefficient kL ((a) QL = 40mL·h-1, 5 % MDEA; (b) 5 % MDEA + 1 % DEA; (c) 5 % MDEA +3 % DEA; (d) 5 % MDEA +5 % DEA)

Specific surface area. The total bubble specific surface area a in the microchannel 12


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could be calculated by the following equation: a

AC VC

(13)

where VC is the volume of the microchannel. The total bubble surface area AC increases with the gas flow rate,21 as which inclines to create longer bubbles in the microchannel as shown in Figure 2. However, when the gas flow rate rises to a certain degree, the bubbles would fill up the entire channel and the specific surface area would tend to a constant,22 as shown in Figure 5a. Furthermore, it could be seen from Figure 5b that, for the lower gas flow rate, the specific surface area a decreases with the increase of liquid flow rate and the DEA concentration. However, for the higher gas flow rate, effects of the liquid flow rate and DEA concentration on the specific surface area are insignificant. With the increase of liquid flow rate, the bubbles in channel shorten and the liquid slugs lengthen under the low gas flow rate as shown in Figure 2c, d, which would lead to a smaller specific surface area. Although the increase of DEA concentration has little effect on the formation size of bubble and liquid slug, it could raise the absorption rate, in this case, the length of the bubbles in the main channel would decrease rapidly as shown in Figure 2a, c and g, and the specific surface area would also reduce. In higher gas flow rate, the bubbles almost fill up the entire channel, and the volume of the liquid slug in the microchannel is very small as shown in Figure 2b, e, f and h. Under this situation, the DEA concentration and liquid flow rate have less effect on both the bubble volume and the specific surface area in the microchannel.

13



9.0

(a)

a/(×103 m2·m-3)

7.5 6.0

QL (mL·h-1)

4.5

20 30 40 50

3.0 1.5 0.0

0

50

100

150

QG/mL·h 9.0

200

250

300

-1

(a)

7.5

a/(×103 m2·m-3)

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

Page 14 of 32

6.0

QL (mL·h-1)

4.5

20 30 40 50

3.0 1.5 0.0

0

50

100

150

QG/mL·h

200

250

300

-1

Figure 5. Effects of DEA concentration and gas liquid flow rates on specific surface area a ((a) 5 % MDEA +3 % DEA; (b) 5% MDEA)

Volumetric mass transfer coefficient. The volumetric mass transfer coefficient could be calculated by multiplying the two values of mass transfer coefficient kL and specific surface area a. Since the influence of gas flow rate on mass transfer coefficient is unremarkable, the changing tendency of the volumetric mass transfer coefficient with gas flow rate is almost similar to the varying tendency of the specific surface area. It could be seen from Figure 6 that the volumetric mass transfer coefficient increases rapidly at first with the increase of gas flow rate, and then the rising trend slows down gradually. Although the increase of DEA concentration and 14


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liquid flow rate could reduce the specific surface area, the volumetric mass transfer coefficient still increases with the increase of DEA concentration and liquid flow rate, as the increase of mass transfer coefficient plays a leading role. However, for a given concentration of DEA, when the gas flow rate is relatively low, the difference between the volumetric mass transfer coefficients is slight or even almost disappeared at different liquid flow rates as shown in Figure 6 b-d. The situation is almost the same for a given liquid flow rate at different DEA concentrations as shown in Figure 6a. Comparatively, when the gas flow rate is low, the increase of liquid flow rate or DEA concentration could cause the decrease of the specific surface area more dramatically. Consequently, although the mass transfer coefficient could also be intensified, the volumetric mass transfer coefficient indicates only a weak increase with liquid flow rate or DEA concentration, as the decrease of the specific surface area partially offsets the enhancement in the mass transfer coefficient. As mentioned above, the increase of DEA concentration could result in a slight effect of liquid flow rate on the mass transfer coefficient, which is similarly for the volumetric mass transfer coefficient. Figure 6b-d indicate that the effect of liquid flow rate on volumetric mass transfer coefficient is more dominant for the low DEA concentration in comparison with the high DEA concentration. When the liquid flow rate increases from 20 mL·h-1 to 40 mL·h-1, the volumetric mass transfer coefficient increases about 50 % for the 1 % DEA solution, while 20 % for the 5 % DEA solution. Zhu et al.15 investigated CO2 absorption by NaOH solution in a microchannel, and observed that the volumetric mass transfer coefficient is hardly independent of the 15



liquid flow rate. This could be attributed to that the high chemical reaction rate between NaOH and CO2 results in a large reduction in the specific surface area, which brings about a set-off effect on the increase in the volumetric mass transfer coefficient. As a result, high chemical reaction rate could be obtained by increasing DEA concentration, which would cause volumetric mass transfer coefficient to be insensitive to liquid flow rate. 8

4

(b)

2.5

1 2 3 4 5

2.0

kLa/s-1

kLa/s-1

3.0

(a) wt % DEA

6

1.5 QL (mL·h-1)

1.0

20 30 40 50

2 0.5 0

0

50

100

150

200

QG/mL·h

5

250

0.0

300

0

50

100

-1

150

200

250

300

QG/mL·h-1 7

(c)

(d)

6

4

5

QL (mL·h-1)

2

20 30 40 50

1 0

kLa/s-1

3

kLa/s-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

Page 16 of 32

0

50

100

150

QG/mL·h

200

4 QL (mL·h-1)

3

20 30 40 50

2 1 250

0

300

0

50

100

-1

150

200

250

300

QG/mL·h-1

Figure 6. Effects of DEA concentration and gas-liquid flow rate on volumetric mass transfer coefficient ((a) QL=40mL·h-1, 5% MDEA; (b) 5% MDEA + 1% DEA; (c) 5% MDEA +3% DEA; (d) 5% MDEA +5% DEA)

Correlation of the volumetric mass transfer coefficient. Yue et al.21 proposed a dimensionless empirical correlation to predict the volumetric mass transfer coefficient of the CO2 absorption by water under slug flow in a microchannel as following:

Shad H  0.084 Re 0G.213 Re 0L.937 ScL0.5

(14)

The mass transfer coefficient kL is represented by dimensionless Sherwood 16


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number Sh = kLdH/DAB (dH is the hydraulic diameter, and DAB is diffusivity of CO2 in solution.), which is used for characterizing the mass transfer performance in gas-liquid absorption system in the microchannel. The gas liquid Reynolds numbers ReG = ρGdHuG/μG (μG and ρG are viscosity and density of gas, respectively.), ReL = ρLdHuL/μL (μL and ρL are viscosity and density of liquid, respectively) are used for characterizing the flow condition of the gas-liquid two phase. The Schmidt number ScL=μL/DABρL is used for characterizing the liquid phase property related to mass transfer. Akanksha et al.23 suggested that the exponential relationship between Sh and ScL is determined, regardless of the system, and the index is taken as a constant of 0.5. The empirical correlation Eq. (14) could well predict the physical absorption process of traditional reactor23 and microchannel13,24. However, for the gas-liquid two-phase mass transfer accompanying chemical reaction in the microchannel, the enhancement of chemical reaction on mass transfer should be taken into account. The mass transfer enhancement due to chemical reaction could be assessed through an enhancement factor.25 In this experiment, the reaction could be considered as a fast pseudo-first-order reaction owing to Ha > 3. Thus the enhancement factor is approximately equal to Ha.26 The dimensionless Hatta number Ha  K1 DAB / k p (where K1 is the pseudo first-order reaction kinetic constant of the reaction between the blended amine solution and CO2; kp is the mass transfer coefficient of the physical absorption process) could well characterize the chemical reaction enhancement on the mass transfer. According to the penetration theory, the mass transfer coefficient of the physical absorption process kp could be calculated using Eq. (15): 17



kp  2

DAB tC

(15)

where tC is the flowing time of gas liquid mixture in microchannel. To be more accurate, Ha number is introduced to improve the empirical dimensionless correlation, as a result, a new predicted correlation of volumetric mass transfer coefficient is achieved with enhancement factor as following:

Shad H  n1 Re Gn2 Re nL3 ScL0.5 Ha n4

(16)

where n1, n2, n3 and n4 are fitting parameters, and their values are 0.18242, 0.91322, 0.33012 and 0.82379 respectively through fitting the experimental data. The range of equation (16) is 1.659 < ReG < 21.572 and 13.281 < ReL < 33.201. The average deviation of predicting volumetric mass transfer coefficients is only 7.88 % as shown in Figure 7, indicating a satisfying agreement with experimental data. 8 +20%

wt % DEA 1 2 3 4 5

6

kLapre/s-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

Page 18 of 32

4

-20%

2

0

0

2

4

kLaexp/s-1

6

8

Figure 7. Comparison between experimental data and predicting values by Eq. (16).

CONCLUSIONS The mass transfer process of CO2 absorption by N-methyldiethanolamine (MDEA) mixed with diethanolamine (DEA) in T - junction microchannel was studied. The effects of gas and liquid flow rates and DEA concentration on CO2 absorption 18


Page 19 of 32 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


efficiency, mass transfer coefficient, specific surface area and volume mass transfer coefficient were investigated. With the increase of the concentration of diethanolamine, the rate of chemical reaction with CO2 absorbed into liquid is accelerated, leading to the increase of CO2 absorption efficiency, mass transfer coefficient and volume mass transfer coefficient, and the decrease of specific surface area due to rapid shrinkage of bubble caused by chemical absorption. The increase of gas flow rate would shorten the residence time of the bubbles in the microchannel, which could result in the reduction of CO2 absorption efficiency. While the increase of liquid flow rate could improve mass transfer rate and cause the increase of CO2 absorption efficiency. However, with the increase of the concentration of diethanolamine in the solution, the effect of liquid flow rate on the absorption efficiency of CO2 decreases. The volumetric mass transfer coefficient and the specific surface area increase with the increase of gas flow rate, however, the increasing trend becomes gradually slowing down, and the mass transfer coefficient only increases slightly. Both volumetric mass transfer coefficient and mass transfer coefficient increase with the increase of liquid flow rate, reversely, the specific surface area decreases. Considering the intensification of chemical reaction on mass transfer, the dimensionless Hatta number Ha is introduced into the correlation of volumetric mass transfer coefficient with a good prediction accuracy. ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China 19



(No. 21776200, 21576186, 91634105, 91434204), the aid of Opening Project of State Key Laboratory of Chemical Engineering of China (No. SKL-ChE-17B02, SKL-ChE-16B03). Nomenclature A

——area, m2

AC

——the sum of all the bubble surface area in the microchannel, m2

a

——specific surface area, m2·m-3

C

——CO2 concentration in liquid phase, mol·m-3

Ce

——equilibrium concentration of CO2 in liquid phase, mol·m-3

DAB

——diffusion coefficient, m2·s-1

d

——depth of microchannel, m

dH

——hydraulic diameter of microchannel, m

f

——formation frequency of the bubble, s-1

Ha

——Hatta number, Ha  K1 DAB / k p

He

——Henry coefficient, mol·m-3·Pa-1

k

——mass transfer coefficient, m·s-1

K1

——overall pseudo first-order reaction kinetic constant, s-1

ka

——volumetric mass transfer coefficient, s-1

L

——length, m

N

——mass transfer flux, mol·m-2·s-1

nB

——molar quantity absorbed into the liquid from a single bubble, mol

P

——pressure, Pa 20


Page 20 of 32

Page 21 of 32 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


P

——average pressure of at inlet and outlet of the microchannel, Pa

Q

——flow rate, m3·s-1

r

——radius of constant curve in the corners in square microchannel, m

rov

——overall reaction rate, mol·m-3·s-1

R

——molar gas constant, J·mol-1·K-1

Re

——Reynolds number, Re=dHρu/μ

Sc

——Schmidt number, Sc=μ/(DABρ)

Sh

——Sherwood number, Sh=kLdH/DAB

T

——temperature, K

tC

——flowing time of gas liquid mixture in microchannel, s

u

——superficial velocity, m·s-1

V

——volume, m3

w

——width of microchannel, m

X

——CO2 absorption efficiency

Greek letter μ

——viscosity, Pa·s

σ

——surface tension, N·m-1

ρ

——density, kg·m-3

Subscripts B

——bubble

C

——microchannel

G

——gas phase 21



L

——liquid phase

in

——inlet of microchannel

out

——outlet of microchannel

film

——liquid film

cap

——bubble cap

exp

——experimental data

pre

——prediction data

p

——physical absorption

References (1)Gao, N.; Wang, J.; Shao, L.; Chen, J. Removal of carbon dioxide by absorption in microporous tube-in-tube microchannel reactor. Ind. Eng. Chem. Res. 2011, 50(10), 6369-6374, DOI 10.1021/ie1024886. (2)Osagie, E.; Biliyok, C.; Di Lorenzo, G,; Hanak, D. P.; Manovic, V. Techno-economic evaluation of the 2-amino-2-methyl-1-propanol (AMP) process for CO2 capture from natural gas combined cycle power plant. Int. J. Greenh. Gas Control. 2018, 70, 45-56, DOI 10.1016/j.ijggc.2018.01.010. (3)Wu, Y.; Zhang, Y.; Chen, N.; Dai, S.; Jiang, H.; Wang, S. Effects of amine loading on the properties of cellulose nanofibrils aerogel and its CO2 capturing performance. Carbohydr. Polym. 2018, 194, 252-259, DOI 10.1016/j.carbpol.2018.04.017. (4)Du. Y.; Yuan. Y.; Rochelle, G. T. Capacity and absorption rate of tertiary and hindered amines blended with piperazine for CO2 capture. Chem. Eng. Sci. 2016, 155, 397-404, DOI 10.1016/j.ces.2016.08.017. 22


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(5)Danckwerts, P. V. The reaction of CO2 with ethanolamines. Chem. Eng. Sci. 1979, 34(4), 443-446, DOI 10.1016/0009-2509(79)85087-3. (6)Aroonwilas, A.; Veawab, A. Characterization and comparison of the CO2 absorption performance into single and blended alkanolamines in a packed column. Ind. Eng. Chem. Res. 2004, 43(9), 2228-2237, DOI 10.1021/ie0306067. (7)Sivanesan, D.; Youn, M.; Murnandari, A.; Kang, J.; Park, K.; Kim, H.; Jeong, S. Enhanced CO2 absorption and desorption in a tertiary amine medium with a carbonic anhydrase

mimic.

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Ind.

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Chem.

2017,

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10.1016/j.jiec.2017.03.058. (8)Kim, S.; Shi, H.; Lee, J. CO2 absorption mechanism in amine solvents and enhancement of CO2 capture capability in blended amine solvent. Int. J. Greenh. Gas Control. 2016, 45, 181-188, DOI 10.1016/j.ijggc.2015.12.024. (9)Chen, G.; Yue, J.; Yuan, Q. Gas-liquid microreaction technology: recent developments and future challenges. Chin. J. Chem. Eng. 2008, 16(5), 663-669, DOI 10.1016/S1004-9541(08)60138-X. (10)Li, C.; Zhu, C.; Ma, Y.; Liu, D.; Gao, X. Experimental study on volumetric mass transfer coefficient of CO2 absorption into MEA aqueous solution in a rectangular microchannel reactor. Int. J. Heat Mass Transf. 2014, 78, 1055-1059, DOI 10.1016/j.ijheatmasstransfer.2014.07.071. (11)Ye, C.; Chen, G.; Yuan, Q. Process characteristics of CO2 absorption by aqueous monoethanolamine in a microchannel Reactor. Chin. J. Chem. Eng. 2012, 20(1), 111-119, DOI 10.1016/S1004-9541(12)60370-X. 23



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(12)Ganapathy, H.; Shooshtari, A.; Dessiatoun, S.; Alshehhi, M.; Ohadi, M. Fluid flow and mass transfer characteristics of enhanced CO2 capture in a minichannel reactor. Appl. Energy. 2014, 119, 43-56, DOI 10.1016/j.apenergy.2013.12.047. (13)Ganapathy, H.; Shooshtari, A.; Dessiatoun, S.; Ohadi, M. M.; Alshehhi, M. Hydrodynamics and mass transfer performance of a microreactor for enhanced gas separation

processes.

Chem.

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2015,

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258-270,

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10.1016/j.cej.2014.12.028. (14)Penttila, A.; Dell’Era, C.; Uusi-Kyyny, P.; Alopaeus, V. The Henry's law constant of N2O and CO2 in aqueous binary and ternary amine solutions (MEA, DEA, DIPA, MDEA,

and

AMP).

Fluid

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Equilib.

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59-66,

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10.1016/j.fluid.2011.08.019. (15)Zhu, C.; Lu, Y.; Fu, T.; Ma, Y.; Li, H. Experimental investigation on gas-liquid mass transfer with fast chemical reaction in microchannel. Int. J. Heat Mass Transf. 2017, 114, 83-89, DOI 10.1016/j.ijheatmasstransfer.2017.06.054. (16)Whitman, W. G. The two film theory of gas absorption. Chem. Metall. Eng. 1923, 29(4), 146-148, DOI 10.1016/0017-9310(62)90032-7. (17)Lin, C. Y.; Soriano, A. N.; Li, M. H. Kinetics study of carbon dioxide absorption into aqueous solutions containing N-methyldiethanolamine + diethanolamine. J. Taiwan Inst. Chem. Eng. 2009, 40, 403-412, DOI 10.1016/j.jtice.2008.11.002. (18)van Baten, J. M.; Krishna, R. CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries. Chem. Eng. Sci. 2004, 59(12), 2535-2545, DOI 10.1016/j.ces.2004.03.010.

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(19)Yao, C.; Dong, Z.; Zhao, Y.; Chen, G. An online method to measure mass transfer of slug flow in a microchannel. Chem. Eng. Sci. 2014, 112, 15-24, DOI 10.1016/j.ces.2014.03.016. (20)Zaloha, P.; Kristal, J.; Jiricny, V.; Volkel, N.; Xuereb, C.; Aubin, J. Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels. Chem. Eng. Sci. 2012, 68(1), 640-649, DOI 10.1016/j.ces.2011.10.036. (21)Yue, J.; Chen, G.; Yuan, Q.; Luo, L.; Gonthier, Y. Hydrodynamics and mass transfer characteristics in gas-liquid flow through a rectangular microchannel. Chem. Eng. Sci. 2007, 62(7), 2096-2108, DOI 10.1016/j.ces.2006.12.057. (22)Zhu, C.; Li, C.; Gao, X.; Ma, Y.; Liu, D. Taylor flow and mass transfer of CO2 chemical absorption into MEA aqueous solutions in a T-junction microchannel. Int. J. Heat Mass Transf. 2014, 73, 492-499, DOI 10.1016/j.ijheatmasstransfer.2014.02.040. (23)Akanksha; Pant, K. K.; Srivastava, V. K. Mass transport correlation for CO2 absorption in aqueous monoethanolamine in a continuous film contactor. Chem. Eng. Process. 2008, 47(5), 920-928, DOI 10.1016/j.cep.2007.02.008. (24)Su, H.; Wang, S.; Niu, H.; Pan, L.; Wang, A.; Hu, Y. Mass transfer characteristics of H2S absorption from gaseous mixture into methyldiethanolamine solution in a T-junction microchannel. Sep. Purif. Technol. 2010, 72(3), 326-334, DOI 10.1016/j.seppur.2010.02.024. (25)Putta, K. R.; Tobiesen, F. A.; Svendson, H. F.; Knuutlla, H. K. Applicability of enhancement factor models for CO2 absorption into aqueous MEA solutions. Appl. Energy. 2017, 206, 765-783, DOI 10.1016/j.apenergy.2017.08.173. 25



(26)Wu, Z.; Zhang, Y.; Lei, W.; Yu, P.; Luo, Y. Kinetics of CO2 absorption into aqueous 1-ethyl-3-methylimidazolium glycinate solution. Chem. Eng. J. 2015, 264, 744-752, DOI 10.1016/j.cej.2014.11.133.

26


Page 26 of 32

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Synopsis.

Absorption

of

CO2

into

aqueous

monoethanolamine

+

N-methyldiethanolamine solution was investigated in microchannel, which may be a sustainable method for CO2 capture.

For Table of Contents Use Only

27



Figure 1. Schematic diagram of the experimental set-up.

Figure 2. Gas-liquid two-phase flow under different operating conditions.


Page 28 of 32

Page 29 of 32

80 QL=20mL·h-1 1% DEA QL=40mL·h-1 1% DEA QL=20mL·h-1 3% DEA

60

QL=40mL·h-1 3% DEA

X/%

QL=20mL·h-1 5% DEA QL=40mL·h-1 5% DEA

40

20

0

0

50

100

150

200

QG/mL·h

250

300

350

-1

Figure 3. Effects of DEA concentration and gas and liquid flow rates on CO2 absorption efficiency (X %).

9.0

(a)

7.5

7.5

6.0

6.0

wt % DEA 1 2 3 4 5

4.5 3.0 1.5 0.0

0

50

100

150

200

250

300

kL/(×10-4m·s-1)

kL/(×10-4m·s-1)

9.0

QL (mL·h-1) 20 30 40 50

4.5 3.0 1.5 0.0

350

(b)

0

50

QG/mL·h-1 9.0

9.0

(c)

7.5

7.5

6.0

6.0

4.5 QL (mL·h-1)

3.0 1.5 0.0

0

50

100

150

100

150

200

250

300

QG/mL·h-1

kL/(×10-4m·s-1)

kL/(×10-4m·s-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


200

20 30 40 50 250

300

(d)

4.5

QL (mL·h-1) 20 30 40 50

3.0 1.5 0.0

0

50

QG/mL·h-1

100

150

200

250

300

QG/mL·h-1

Figure 4. Effects of DEA concentration and gas and liquid flow rates on mass transfer coefficient kL ((a) QL = 40mL·h-1, 5 % MDEA; (b) 5 % MDEA + 1 % DEA; (c) 5 % MDEA +3 % DEA; (d) 5 % MDEA +5 % DEA)



9.0

(a)

a/(×103 m2·m-3)

7.5 6.0

QL (mL·h-1)

4.5

20 30 40 50

3.0 1.5 0.0

0

50

100

150

200

250

300

QG/mL·h-1 9.0

(b)

7.5

a/(×103 m2·m-3)

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

Page 30 of 32

6.0 4.5 QG= 60 mL·h-1

3.0 1.5 0.0

0

1

QG= 220 mL·h-1

QL = 20 mL·h-1

QL = 20 mL·h-1

QL = 30 mL·h-1

QL = 30 mL·h-1

QL = 40mL·h-1

QL = 40 mL·h-1

QL = 50 mL·h-1

QL = 50 mL·h-1

2

3

4

5

6

DEA concentration wt% Figure 5. Effects of DEA concentration and gas liquid flow rates on specific surface area a ((a) 5 % MDEA +3 % DEA; (b) 5% MDEA)


Page 31 of 32

8

2.0

kLa/s-1

4

(b)

2.5

1 2 3 4 5

6

kLa/s-1

3.0

(a) wt % DEA

1.5 QL (mL·h-1) 1.0

20 30 40 50

2 0.5 0

0

50

100

150

200

250

300

0.0

0

50

QG/mL·h-1 5

100

150

200

250

300

QG/mL·h-1 7

(c)

(d)

6

4

5

QL (mL·h-1)

2

20 30 40 50

1 0

kLa/s-1

kLa/s-1

3

0

50

100

150

200

4 QL (mL·h-1)

3

20 30 40 50

2 1 250

0

300

0

50

-1

100

150

200

250

300

QG/mL·h-1

QG/mL·h

Figure 6. Effects of DEA concentration and gas-liquid flow rate on volumetric mass transfer coefficient ((a) QL=40mL·h-1, 5% MDEA; (b) 5% MDEA + 1% DEA; (c) 5% MDEA +3% DEA; (d) 5% MDEA +5% DEA)

8 +20% wt % DEA 1 2 3 4 5

6

kLapre/s-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


4

-20%

2

0

0

2

4

6

8

-1

kLaexp/s

Figure 7. Comparison between experimental data and predicting values by Eq. (16).



Abstract art


Page 32 of 32

Mass-Transfer Characteristics of CO2 Absorption into Aqueous

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