[Bmim][BF4] aqueous solutions in a microchannel

Accepted Manuscript Dynamics and mass transfer characteristics of CO2 absorption into MEA/ [Bmim][BF4] aqueous solutions in a microchannel Yaran Yin, Taotao Fu, Chunying Zhu, Rongwei Guo, Youguang Ma, Huaizhi Li PII: DOI: Reference:

S1383-5866(18)30226-0 https://doi.org/10.1016/j.seppur.2018.08.045 SEPPUR 14866

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

19 January 2018 22 August 2018 24 August 2018

Please cite this article as: Y. Yin, T. Fu, C. Zhu, R. Guo, Y. Ma, H. Li, Dynamics and mass transfer characteristics of CO2 absorption into MEA/[Bmim][BF4] aqueous solutions in a microchannel, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.08.045

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Dynamics and mass transfer characteristics of CO2 absorption into MEA/[Bmim][BF4] aqueous solutions in a microchannel Yaran Yina, Taotao Fua, Chunying Zhua*, Rongwei Guoa, Youguang Maa*, Huaizhi Lib a

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300027, P. R. China

b

Laboratory of Reactions and Process Engineering, University of Lorraine, CNRS, 1, rue Grandville, BP 20451, 54001 Nancy Cedex, France

* Corresponding authors: [email protected] (Y. Ma); [email protected] (C. Zhu)

Abstract The evolution of the size and velocity of CO2 bubble absorbed into MEA/[Bmim][BF4] aqueous solutions was in-line investigated by a high-speed camera. The coupling effect between the variation of volume and the velocity of bubbles was highlighted. According to the evolution of average mass transfer coefficient, the mass transfer performance in the whole flow process was studied. The results indicated that both the length and velocity of bubbles decreased gradually in the flow process due to mass transfer. A linear correlation between the relative velocity (the ratio of average bubble velocity to two-phase superficial velocity) and the relative lost length (the ratio of the lost length to the initial length) was proposed. The increase in overall liquid-phase volumetric mass transfer 1

coefficient (kLa) at higher gas flow rate could be ascribed to the shorter residence time and larger specific surface area. While the increase of kLa at higher liquid flow rate and MEA or [Bmim][BF4] concentration was resulted from intensified mass transfer. An empirical correlation of overall liquid-phase volumetric mass transfer coefficient was proposed taking the gas-phase Reynolds number, liquid-phase Reynolds number and Damköhler number into account. Keywords: microchannel; multiphase flow; dynamics; mass transfer; CO2

1. Introduction In recent years, microfluidic technology has shown great superiority and potential for chemical process intensification and equipment miniaturization [1, 2]. In comparison with the conventional devices, microreactor shows many advantages, such as highly efficient micromixing performance, narrow residence time, small equipment volume, and safe and controllable operation process [3]. Therefore, it has been widely applied to various industrial processes, including gas separation, organic synthesis, and catalytic reaction etc. [4-9]. The gas-liquid two-phase flow, mass transfer and chemical reaction are frequently encountered in these processes. The full understanding of the dynamic evolution of bubbles is a prerequisite to study gas-liquid two-phase flow and mass transfer mechanism in microchannels. Consequently, many previous studies have been devoted to bubble formation mechanism in microfluidic systems [10-12]. However, up to date, the understanding on the hydrodynamics and mass transfer mechanism remains still insufficient during 2

the gas-liquid two-phase flow process at the microscale [13, 14]. The size and velocity of bubble would be varied due to the mass transfer during the gas-liquid flow process in the microchannel [15]. Meanwhile, chemical reaction could remarkably enhance mass transfer, which would lead to more complicated gas-liquid flow behavior and mass transfer mechanism. Generally, the gas-liquid two-phase flow patterns in the microchannel mainly include bubbly flow, Taylor flow, Taylor-annular flow, annular flow and churn flow [16-18]. Taylor flow is typically characterized by an alternating sequence of gas bubbles and liquid slugs with good stability, controllability and uniform dispersity, and accordingly receives much more attention [19, 20]. The mass transfer in the Taylor flow regime mainly includes: (1) between the bubble caps and the adjacent liquid slug, and (2) between the bubble body and the liquid film adjacent to the channel wall [21, 22]. The former is closely related to the mixing in the liquid slug due to the inner recirculation, while the latter is mainly affected by the saturation of liquid film [21]. It has been proved that a long Taylor bubble could more easily saturate the liquid film, and thereby is adverse to the mass transfer between gas and liquid two phases [23]. However, the improvement of inner recirculation in the liquid slug could enhance mass transfer under shorter slug length or higher two-phase superficial velocity [16, 24-26]. Therefore, a systematical study on the evolution of bubbles under Taylor flow is quite necessary, which would be conducive to better understanding the gas-liquid mass transfer mechanism inside the microchannel and precisely manipulating 3

microchemical process. In past decades, many empirical and semi-empirical correlations have been put forward to predict the volumetric mass transfer coefficient for physical absorption process in microreactors [22, 26-30]. However, due to neglecting the effect of chemical reaction on mass transfer, these models would create great deviation for gas-liquid chemical reaction system, which limits their applicability and versatility. As one of alcohol amine absorbents for CO2 capture, monoethanolamine (MEA) aqueous solution was used in industry at the earliest with the advantages of rapid absorption and good selectivity for CO2. However, some drawbacks limited its extensive applications such as easy degradation and oxidation, corrosive effect and high energy consumption for regeneration of solvent [31, 32]. Therefore, many hybrid absorbents containing MEA were adopted for restraining these disadvantages [33-36].

In

recent

years,

it

was

found

that

the

addition

of

1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) into MEA solution could not only overcome some defects of MEA aqueous solution as CO2 absorbent, but also enhance mass transfer between gas and liquid phases [36-38]. In this study, the dynamics and mass transfer characteristics of CO2 absorption into MEA/[Bmim][BF4] aqueous solution in a T-junction microchannel was investigated. The flow process of CO2 bubbles in the MEA/[Bmim][BF4] aqueous solutions in Taylor flow regime was monitored by a high-speed camera. The influences of the gas-liquid flow rates, MEA and [Bmim][BF4]concentrations were investigated. The 4

effect of bubble volume variation on bubble velocity was explored. According to the evolution of average mass transfer coefficient, the mass transfer performance in the whole flow process was studied. A correlation of overall volumetic mass transfer coefficient is proposed taking the effect of chemical reaction into account.

2. Experimental A square cross-section microchannel was adopted in the experiment with the depth 400 μm, width 400 μm and length 40 mm. The length of the gas inlet and liquid inlet is 5 mm respectively. The channel was fabricated in a polymethyl methacrylate (PMMA) plate by precision milling and sealed with another thin PMMA plate. A schematic view of experimental setup is sketched in Fig. 1(a). The gas phase and liquid phase from syringes were fed into the branch and main inlets by two micro syringe pumps (Harvard PHD 2000, USA) with the set flow rates QG and QL, respectively. A high-speed camera (FASTCAM SA1.1, Photron, Japan) connected with a computer used to record the flow and absorption characteristics of bubbles, and light was provided by a cold light source (MHAA-100 W). All images were recorded at 2000-4000 frames per second. The pressure of gas phase at the inlet was measured by a piezometer (ST3000, Honeywell, USA. The precision is 0.02%), and the pressure at the outlet is atmospheric pressure. Taylor flow was investigated in this work as shown in Fig. 1(b). The superficial velocity ranges of gas phase and liquid phase were 0.139
and 11.68
ωIL (%)

Density, ρL×10-3 (kg·m-3)

Viscosity, μL×103 (Pa·s)

1 3 3 3 3 5

10 0 5 10 15 10

1015.0 998.9 1014.3 1015.4 1023.9 1016.4

1.098 0.986 1.082 1.147 1.264 1.209

Surface tension, σ ×103 (N·m-1) 35.16 38.35 36.54 34.94 34.20 34.74

6

Diffusivity, D×109 (m2·s-1) 2.14 2.03 2.13 2.19 2.30 2.25

Overall reaction rate constant, kov (s-1) 475.31 1403.18 1424.82 1426.49 1438.30 2379.61

3 Results and discussion 3.1 Dissolution rate The initial bubble length (l0) is defined at the formation of regular cap and rear of new generated bubble, as shown in Fig. 1(b). The dimensionless initial bubble length (l0/w) under different experimental conditions is depicted in Fig. 2. As shown in Fig. 2(a), the initial bubble length increases with the gas flow rate, but decreases with the liquid flow rate. As shown in Fig. 2(b), the initial bubble length is almost independent of the MEA concentration, implying that the effect of chemical reaction rate on the initial length of bubbles is negligible under experimental conditions. The reduction of initial bubble length with increasing [Bmim][BF4] concentration, as shown in Fig. 2(c) could be attributed to the increase of viscosity and the decrease of surface tension due to the addition of [Bmim][BF4] (Table 1), which facilitates the bubble formation [10, 39]. The dimensionless lost length (Δl/w) of bubbles as a function of the flow time is obtained by tracing the position of cap and rear of bubbles, where Δl is defined as the difference between the initial length and the instantaneous length of bubble, and w is the width of main channel. The time-dependent Δl/w reflects the mass transfer quantity of bubble within the flow time t. The time t=0 is defined at the moment when the bubble is generated. It could be observed from Fig. 3 that the Δl/w increases rapidly at the early stage and then slows down due to the continuous

7

consumption of absorbent. The longer bubble at higher gas flow rate as shown in Fig. 1(b) and Fig. 2(a), could provide larger gas-liquid interfacial area, which would result in greater mass transfer quantity and Δl/w within flow time t in Fig. 3(a). It is noteworthy that the shrinkage of bubble is accelerated at higher gas flow rate, indicating that the loss of bubble volume caused by mass transfer in the liquid film is unneglectable. Moreover, the increment of liquid flow rate under a certain gas flow rate could improve the circulation in the liquid phase and accordingly intensify the liquid-side mass transfer [24, 40]. However, the bubble becomes shrunken in this case (as shown in Fig. 2(a)), thereby leading to a smaller gas-liquid interfacial area for mass transfer. The decrease of Δl/w with liquid flow rate in Fig. 3(b) implies that the negative contribution of interfacial area is dominant. As discussed above, the rise of Δl/w with the MEA concentration in Fig. 3(c) could be attributed to the enhanced chemical reaction [16], since the generated bubble length is hardly affected by the MEA concentration (Fig. 2(b)). Similarly, although shorter bubbles are generated at higher [Bmim][BF4] concentration (Fig. 1(b) and 2(c)), the addition of [Bmim][BF4] enhances mass transfer and accordingly leads to the increase in Δl/w, as observed in Fig. 3(d). The evolution of relative lost length (∆l/l0, the ratio of the lost length to the initial length) with time could reflect the mass transfer efficiency within the flow time t. The ∆l/l0 increases continuously with the flow time, but the increasing rate would become slowed down. The gas flow rate has negligible effect on ∆l/l0, which 8

may be due to the similar growth rate of ∆l and l0 as the gas flow rate rises. With the increase of liquid flow rate, the more significant reduction of l0 than ∆l makes ∆l/l0 increased. The improvement of ∆l/l0 with the MEA concentration could be ascribed to the increment of ∆l, since l0 is almost unaffected by the MEA concentration. When [Bmim][BF4] concentration increases, both the reduction of l0 and increment of ∆l lead to the increase of ∆l/l0.

3.2 Bubble flow velocity Logically, the mass transfer depends greatly on the multiphase flow characteristics in microchannels [13]. The cap velocity (ucap) and rear velocity (urear) of bubble were obtained by tracing their flow distance whtin the Δt, and the average bubble velocity was calculated according to ua=(ucap+urear)/2. The variation of bubble velocity with flow time is displayed in Fig. 4. The bubble velocity experiences a continuous reduction owing to strong absorption of CO2 in the liquid [13, 41]. It could be found from Fig. 4(a) that the cap velocity is always lower than the rear velocity (ucap
9

slope, resulting in an intersection in Fig. 4(c). Before the intersection, the average bubble velocity increases with the liquid flow rate, while reverse tendency occurs after this point. The existence of turning point is due to the fact that the greater volume effect, which is response to the lager ∆l/l0, would cause a severe loss of velocity. As a result, the bubble velocity shows a drastic decrease at higher liquid flow rate, and eventually is lower than that at lower liquid flow rate. As shown in Fig. 4(d) and (e), for given two-phase flow rates, the larger absorbent concentration could cause the reduction of the average bubble velocity, and the effect of MEA is more pronounced in comparison with [Bmim][BF4]. Overall, these phenomena demonstrate a strong dependence of bubble velocity on mass transfer. Apparently, the relationship between the velocity and the lost volume of bubbles in the flow process is considerably complicated [41]. In order to elucidate the influence of bubble volume variation on bubble velocity, the relative velocity (ua/UTP, the ratio of average bubble velocity to two-phase superficial velocity, UTP is the two-phase superficial velocity) was compared with the relative lost length (∆l/l0), as shown in Fig. 5. The ua/UTP presents a tendency of linear decrease with the ∆l/l0,

ua /UTP  l /l0 . It means that the larger lost degree of bubble velocity would take place at the higher ∆l/l0 (or mass transfer efficiency). The relationship between ua/UTP and ∆l/l0 is almost independent of two-phase flow rates and absorbent concentration. By fitting the experimental data, a simple equation was obtained ua/UTP=1.29(1-∆l/l0). From ∆l/l0=0 corresponding to the velocity without mass 10

transfer, ua=1.29UTP could be acquired, implying that taking the main body of bubble as a cylinder is rational [26].

3.3 Mass transfer coefficient The molar quantity of a bubble absorbed into the liquid ΔnB could be calculated by the ideal gas equation [42]: nB 

PV 0 B,0  PV t B,t RT

(1)

where P0, VB,0 are the pressure and the volume of a bubble at the time t=0, Pt, VB,t represent the pressure and the bubble volume at the flow time t, R is gas constant, and T is temperature. 2 In this study, the range of Hatta number (Ha) ( Ha  kov D / kL,p , D is the

diffusion coefficient of CO2 in the liquid phase, and kov is the overall reaction rate constant, as shown in Table 1. kL,P is the liquid-phase physical mass transfer coefficient) is 6.7~22, it indicates that the reaction could be regarded as a fast reaction [43]. The mass-transfer resistance dominates and reactions occur primarily in the film near the interface [43, 44]. The gas-film resistance is neglectable, since pure CO2 was used as gas phase [45]. Therefore, the mass transfer between a bubble and the adjacent liquid could also be calculated by [16, 25]:

nB  AB,t kL,t  C *  C  t

(2)

where kL,t is the average liquid-phase mass transfer coefficient of a bubble within the flow time t. C is the CO2 concentration in the solution. The gas-liquid reaction could

11

be considered as a pseudo first-order fast reaction [38, 43], and the excess MEA is used, therefore, the concentration of CO2 in the liquid could be approximated as zero [18, 46, 47]. C* is the equilibrium concentration of CO2 in solution and could be obtained by Henry's law [48]:

C* 

P H

(3)

where H is the Henry's constant, P is the average pressure within the flow time t. In the flow process, the length and the surface area of a bubble would change with t, hence the AB,t presents the average surface area of a bubble within the flow time t. The bubble in Taylor flow regime could be divided into three parts: the cap, the rear and the main body. The main body of bubble could be treated as a cylinder, and the cap and rear of bubble could be regarded as hemisphere [49]. Thus the surface area of a bubble could be calculated as: AB = w2   lB  w  w   lB w

(4)

where AB and lB are the surface area and length of a bubble respectively, and w is the width of main channel. Integrating Eq. (1), (2) and (3), the average liquid-phase mass transfer coefficient (kL,t) of a bubble within the flow time t could be obtained as follows:

kL,t 

 PV

0 B,0

-PV t B,t  H

RT AB,t t P

(5)

Accordingly, the average liquid-phase mass transfer coefficient for the whole flow stage could be obtained, which could represent the overall liquid-phase mass 12

transfer coefficient for the whole channel (kL):

kL 

 PV

0 B,0

-PoutVB,out  H

RT ABtout P

(6)

where tout is the time when a bubble flows out of the microchannel, Pout and VB,out represent the pressure and bubble volume at the exit, and the AB is the average surface area of a bubble in the whole flow period of tout. The overall liquid-phase volumetric mass transfer coefficient (kLa) in the flow stage is given:

kL a  k L  a

(7)

where a is the specific surface area, which is calculated by: m

a

A i 1

B,i

VC

(8)

where VC is the total volume of main channel, AB,i is the surface area of ith bubble, and m is the number of bubbles in the microchannel. For the CO2 absorption into MEA/[Bmim][BF4] aqueous solution, the gas-liquid mass transfer is accompanied with rapid chemical reaction, which greatly enhances gas-liquid mass transfer, and the enhancement effect of chemical reaction on mass transfer is positively related to reaction rate which relies on the concentration of absorbent. Obviously, the intensification of mass transfer due to chemical reaction would decline along the channel in absorption process, because the concentration of absorbent would decrease with bubble moving forwards. In this case, the average liquid-phase mass transfer coefficient indicates to be negatively 13

relative to the flow time (t) (as shown in Fig.6) and the total residence time (tR) of bubble, it actually reflects an attenuation of the enhancement effect of reaction on mass transfer along the channel due to the CO2 absorption. Therefore, during the chemical absorption process, kL,t depends primarily on the concentration of absorbent and liquid flow rate, and is almost independent of the gas flow rate. Meanwhile, MEA concentration has more obvious influence on kL,t than that of the [Bmim][BF4] concentration as shown in Fig.6. In addition, for a fixed gas flow rate, with the increase of MEA concentration, the chemical reaction rate increases, which would lead to the decrease of bubble velocity, consequently, total bubble residence time increases in microchannel (Fig. 7(b)). However, tR is insensitive to [Bmim][BF4] concentration and liquid flow rate (Fig. 7(a) and (c)). It indicates that the effect of chemical reaction on tR is dominant over the liquid viscosity and flow velocity. The shorter total residence time (tR) of bubble at higher gas flow rate could result in larger overall liquid-phase mass transfer coefficient (kL) in Fig. 7(a). For constant gas flow rate, the kL increases with the augment of liquid flow rate or absorbent concentration (Fig. 7(a), (b) and (c)). Moreover, the effect of MEA concentration on mass transfer coefficient kL is more remarkable in higher gas flow rate. The effects of different factors on kLa and specific surface area (a) for the whole flow process are illustrated in Fig. 8. It could be clearly found that kLa increases with increasing either gas/liquid flow rates or absorbent concentration. 14

Therefore, the increase of kLa with gas flow rate is stemmed from a joint contribution of kL and a. Although the a decreases with the liquid flow rate, the kLa increases because of the remarkable increase in kL [24, 40]. Moreover, although both the increase of MEA and [Bmim][BF4] concentration could cause the reduction of a, the increment of kL stemming from the intensified mass transfer is the dominant factor, which leads to the rise of kLa. It is worthy note that as shown in Fig. 8 (a) and (b), the differences of kLa under various liquid flow rates or MEA concentrations become more remarkable at higher gas flow rate. This is because the decrease of specific surface area with increasing liquid flow rate or MEA concentration is more notable at low gas flow rate, which causes an apparent negative impact on kLa. Differently, at high gas flow rate, the flow pattern would gradually evolve to the annular flow, the bubble specific surface area in this case is almost unchanged, kLa depends only on kL. In previous studies, great effort has been dedicated to the prediction model of gas-liquid mass transfer inside microchannel, many correlations have been proposed. For the physical absorption of CO2-deionized water under the Taylor flow regime in a rectangular microchannel with the hydraulic diameter of 667 μm, a correlation of kLa that accounted for operating conditions and physical properties was proposed [27]: ShL adC  0.084ReG0.213 ReL0.937 ScL0.5

(9)

where dC is the hydraulic diameter of channel, ShL is the liquid-phase Sherwood 15

number that could be expressed as ShL=kLdC/D, representing the ratio of convective mass transfer to diffusive mass transfer. ReG and ReL are the Reynolds number in the gas phase and liquid phase respectively, and could be expressed as ReG=dCUGρG/μG, ReL=dCULρL/μL. The Reynolds number Re represents the ratio of the inertial force to the viscous force. ScL is the liquid-phase Schmidt number that is defined as ScL=μL/(ρLD), and reflects the ratio of the viscous coefficient to the diffusion coefficient. Ji et al. [50] investigated the absorption performance of CO2 in distilled water, ethanol, and n-propanol in rectangular microreactors. The results showed that the liquid-phase capillary number (CaL, which is expressed as CaL=ULμL/σ, representing the ratio of viscous force to surface tension) played an important role in two-phase flow and mass transfer in the microchannels with the hydraulic diameter less than 200 μm. Therefore, the effect of CaL was taken into account: ShL adC  0.22ReG0.78 ReL0.0535 ScLCaL0.7586

(10)

Subsequently, Yao et al. [20] studied the physical absorption of CO2 bubbles into ethanol solutions under Taylor flow regime in a T-junction microchannel. The kLa was correlated in terms of the similar method. ShL adC  1.367 ReG0.421ReL0.717 ScL0.640CaL0.5

(11)

Ganapathy et al. [51] investigated the chemical absorption of CO2 into diethanolamine aqueous solution in microreactors with hydraulic diameters range from 254 to 762 μm, and the correlation of kLa was proposed: 16

ShL  1.689 10-4 ReG0.223 ReL0.829 Sc1.766 L

(12)

The comparison of kLa in this experiment with predicted values by the above models is shown in Fig. 9. It could be clearly seen that the all these models create great deviations from experiment. Obviously, the kLa in this experiment depends not only on the gas-liquid two phases flow rates, but also on the rate of chemical reaction. In previous models, the ReG and ReL were usually used to reflect the effect of gas-liquid two-phase flow rates, and ScL was used to reflect the influence of diffusion rate. However, these dimensionless numbers did not contain the effect of chemical reaction. Therefore, the Damköhler number (Da) was adopted in this paper, which could jointly reflect the influences of chemical reaction and diffusion rate and be defined as: Da =kov w2 /D

(13)

It represents the ratio of reaction rate to diffusion rate. Faster chemical reaction corresponds to the larger Damköhler number [52]. By means of ReG, ReL and Da, a new correlation of kLa is proposed: ShL adC  0.81ReG0.78 ReL0.41Da0.35

(14)

As could be seen from Fig. 10, the present correlation shows a good prediction performance with relative deviation within 20%.

4. Conclusions The evolutions of the bubble size, velocity and mass transfer coefficient were investigated experimentally for the CO2 absorption in MEA/[Bmim][BF4] aqueous 17

solutions in a microchannel. The influences of the gas-liquid flow rates and concentrations of MEA and [Bmim][BF4] on bubble dynamics and mass transfer performance were investigated systematically. The results indicated that both the velocity and length of bubble decreased due to mass transfer between gas and liquid phases. The relative velocity of bubbles (the ratio of average velocity to two-phase superficial velocity) decreased linearly with the relative lost length (the ratio of the lost length to the initial length). The increment of kLa with the gas flow rate was stemmed from the shorter bubble residence time and larger interfacial area in the microchannel, while the enhancement of kLa with the liquid flow rate or absorbents concentrations could be attributed to the intensified mass transfer by reaction. A correlation was proposed to predict kLa by taking the gas-phase Reynolds number, liquid-phase Reynolds number and Damköhler number into account.

Acknowledgments This study was supported by the National Nature Science Foundation of China (No. 21776200, 21576186, 91634105, 91434204), the aid of Opening Project of State

Key

Laboratory

of

Chemical

Engineering

(No.

SKL-ChE-13T04,

SKL-ChE-16B03) and the Programs of Introducing Talents of Discipline to Universities (Grant No. B06006).

Nomenclature a

specific area (m-1) 18

AB

surface area of a bubble (m2)

AB,i

surface area of the ith bubble in the microchannel (m2)

AB,t

average surface area of a bubble within the flow time t (m2)

AB

average surface area of a bubble within the whole flow process (m2)

C

CO2 concentration in aqueous liquid (mol·m-3)

C*

CO2 concentration in gas-liquid phase interface (mol·m-3)

Ca

capillary number (dimensionless), Ca=Uμ/D

D

diffusion coefficient (m2·s-1)

Da

Damköhler number (dimensionless), Da=kovdC2/D

G

mass flux (kg·m-2·s-1)

kL

overall liquid-phase mass transfer coefficient in the flow stage (m·s-1)

kLa

overall liquid-phase volumetic mass transfer coefficient in the flow stage

kL,P

overall liquid-phase physical mass transfer coefficient in the flow stage

(s-1)

(m·s-1) kL,t

average liquid-phase mass transfer coefficient in the flow time t (m·s-1)

kov

overall reaction rate constant (s-1)

l

length of bubble (m)

n

molar quantity (mol)

P

pressure (Pa)

P

average pressure (Pa) 19

Q

flow rate (m3·s-1)

R

gas constant (J·mol-1·K-1)

Re

Reynolds number (dimensionless), Re=dCUρ/μ

Sc

Schmidt number (dimensionless), Sc=μ/(ρD)

Sh

Sherwood number (dimensionless), Sh=kdC/D

T

temperature (K)

t

flow time of gas bubble (s)

U

superficial velocity (m·s-1)

u

bubble flow velocity (m·s-1)

V

volume (m3)

w

width (m)

Greek symbols Δ

difference

μ

dynamic viscosity (Pa·s)

ρ

density (kg·m-3)

σ

surface tension (N·m-1)

ω

mass fraction (%)

Subscrips a

average velocity

B

bubble

C

channel

20

cal.

calculated values

exp.

experimental values

L

liquid phase

R

residence

t

flow time

TP

two phases

0

flow time t=0

out

outlet of microchannel

References [1] K.F. Jensen, Microreaction engineering — is small better?, Chemical Engineering Science, 56 (2001) 293-303. [2] S. Becht, R. Franke, A. Geißelmann, H. Hahn, Micro process technology as a means of process intensification, Chemical Engineering & Technology, 30 (2007) 295-299. [3] D.I.N. Kockmann, Transport phenomena in micro process engineering, Heat & Mass Transfer, (2007). [4] K. Jähnisch, M. Baerns, V. Hessel, W. Ehrfeld, V. Haverkamp, H. Löwe, C. Wille, A. Guber, Direct fluorination of toluene using elemental fluorine in gas/liquid microreactors, Journal of Fluorine Chemistry, 105 (2000) 117-128. [5] B. Bao, J. Riordon, F. Mostowfi, D. Sinton, Microfluidic and nanofluidic phase behaviour characterization for industrial CO2, oil and gas, Lab on a chip, 17 21

(2017) 2740-2759. [6] V. Hessel, P. Angeli, A. Gavriilidis, H. Löwe, Gas−liquid and gas−liquid−solid microstructured reactors: contacting principles and applications, Industrial & Engineering Chemistry Research, 44 (2005) págs. 9750-9769. [7] R. Abdallah, V. Meille, J. Shaw, D. Wenn, B.C. De, Gas-liquid and gas-liquid-solid catalysis in a mesh microreactor, Chemical communications, 4 (2004) 372. [8] J. Kobayashi, Y. Mori, S. Kobayashi, Multiphase organic synthesis in microchannel reactors, Chemistry, an Asian journal, 1 (2006) 22-35. [9] C. Ye, G. Chen, Q. Yuan, Process characteristics of CO2 absorption by aqueous monoethanolamine in a microchannel reactor, Chinese Journal of Chemical Engineering, 20 (2012) 111-119. [10] T. Fu, Y. Ma, Bubble formation and breakup dynamics in microfluidic devices: A review, Chemical Engineering Science, 135 (2015) 343-372. [11] R.K. Shah, H.C. Shum, A.C. Rowat, D. Lee, J.J. Agresti, A.S. Utada, L.-Y. Chu, J.-W. Kim, A. Fernandez-Nieves, C.J. Martinez, D.A. Weitz, Designer emulsions using microfluidics, Materials Today, 11 (2008) 18-27. [12] J.H. Xu, S.W. Li, J. Tan, G.S. Luo, Correlations of droplet formation in T-junction microfluidic devices: from squeezing to dripping, Microfluidics and Nanofluidics, 5 (2008) 711-717. [13] R. Sun, T. Cubaud, Dissolution of carbon dioxide bubbles and microfluidic 22

multiphase flows, Lab on a chip, 11 (2011) 2924-2928. [14] T. Cubaud, M. Sauzade, R. Sun, CO2 dissolution in water using long serpentine microchannels, Biomicrofluidics, 6 (2012) 22002-220029. [15] M. Abolhasani, E. Kumacheva, A. Günther, Peclet number dependence of mass transfer in microscale segmented gas–liquid flow, Industrial & Engineering Chemistry Research, 54 (2015) 9046-9051. [16] C. Zhu, Y. Lu, T. Fu, Y. Ma, H.Z. Li, Experimental investigation on gas-liquid mass transfer with fast chemical reaction in microchannel, International Journal of Heat and Mass Transfer, 114 (2017) 83-89. [17] N. Shao, A. Gavriilidis, P. Angeli, Flow regimes for adiabatic gas–liquid flow in microchannels, Chemical Engineering Science, 64 (2009) 2749-2761. [18] L. Yang, J. Tan, K. Wang, G. Luo, Mass transfer characteristics of bubbly flow in microchannels, Chemical Engineering Science, 109 (2014) 306-314. [19] H. Ganapathy, A. Shooshtari, S. Dessiatoun, M. Alshehhi, M. Ohadi, Fluid flow and mass transfer characteristics of enhanced CO2 capture in a minichannel reactor, Applied Energy, 119 (2014) 43-56. [20] C. Yao, Z. Dong, Y. Zhao, G. Chen, An online method to measure mass transfer of slug flow in a microchannel, Chemical Engineering Science, 112 (2014) 15-24. [21] P. Sobieszuk, J. Aubin, R. Pohorecki, Hydrodynamics and mass transfer in gas-liquid flows in microreactors, Chemical Engineering & Technology, 35 23

(2012) 1346-1358. [22] J.M. van Baten, R. Krishna, CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries, Chemical Engineering Science, 59 (2004) 2535-2545. [23] N. Dietrich, K. Loubière, M. Jimenez, G. Hébrard, C. Gourdon, A new direct technique for visualizing and measuring gas–liquid mass transfer around bubbles moving in a straight millimetric square channel, Chemical Engineering Science, 100 (2013) 172-182. [24] P. Zaloha, J. Kristal, V. Jiricny, N. Völkel, C. Xuereb, J. Aubin, Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels, Chemical Engineering Science, 68 (2012) 640-649. [25] J. Tan, Y.C. Lu, J.H. Xu, G.S. Luo, Mass transfer performance of gas–liquid segmented flow in microchannels, Chemical Engineering Journal, 181-182 (2012) 229-235. [26] J. Yue, L. Luo, Y. Gonthier, G. Chen, Q. Yuan, An experimental study of air–water Taylor flow and mass transfer inside square microchannels, Chemical Engineering Science, 64 (2009) 3697-3708. [27] J. Yue, G. Chen, Q. Yuan, L. Luo, Y. Gonthier, Hydrodynamics and mass transfer characteristics in gas–liquid flow through a rectangular microchannel, Chemical Engineering Science, 62 (2007) 2096-2108. [28] G. Bercˇicˇ, A. Pintar, The role of gas bubbles and liquid slug lengths on mass 24

transport in the Taylor flow through capillaries, Chemical Engineering Science, 52 (1997) 3709-3719. [29] H. Niu, L. Pan, H. Su, S. Wang, Flow pattern, pressure drop, and mass transfer in a gas−liquid concurrent two-phase flow microchannel reactor, Industrial & Engineering Chemistry Research, 48 (2009) 1621-1628. [30] C.O. Vandu, H. Liu, R. Krishna, Mass transfer from Taylor bubbles rising in single capillaries, Chemical Engineering Science, 60 (2005) 6430-6437. [31] C.-H. Yu, A Review of CO2 Capture by Absorption and Adsorption, Aerosol and Air Quality Research, (2012). [32] A.B. Rao, E.S. Rubin, A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control, Environmental Science & Technology, 36 (2002) 4467-4475. [33] R. Krupiczka, A. Rotkegel, Z. Ziobrowski, Comparative study of CO2 absorption in packed column using imidazolium based ionic liquids and MEA solution, Separation and Purification Technology, 149 (2015) 228-236. [34] H. Liao, H. Gao, B. Xu, Z. Liang, Mass transfer performance studies of aqueous blended DEEA-MEA solution using orthogonal array design in a packed column, Separation and Purification Technology, 183 (2017) 117-126. [35] M. Saidi, Process assessment and sensitivity analysis of CO 2 capture by aqueous methyldiethanolamine + piperazine blended solutions using membrane contactor: Model development of kinetics and mass transfer rate, Separation 25

and Purification Technology, 204 (2018) 185-195. [36] M.M. Taib, T. Murugesan, Solubilities of CO2 in aqueous solutions of ionic liquids (ILs) and monoethanolamine (MEA) at pressures from 100 to 1600kPa, Chemical Engineering Journal, 181-182 (2012) 56-62. [37] B.H. Lu, J.J. Jin, L. Zhang, W. Li, Absorption of carbon dioxide into aqueous blend

of

monoethanolamine

and

1-butyl-3-methylimidazolium

tetrafluoroborate, International Journal of Greenhouse Gas Control, 11 (2012) 152-157. [38] B. Lu, X. Wang, Y. Xia, N. Liu, S. Li, W. Li, Kinetics of carbon dioxide absorption into mixed aqueous solutions of MEA + [Bmim]BF4 using a double stirred cell, Energy & Fuels, 27 (2013) 6002-6009. [39] Y. Li, K.R. Ward, M.A. Burns, Viscosity Measurements Using Microfluidic Droplet Length, Analytical chemistry, 89 (2017) 3996-4006. [40] H. Ganapathy, E. Al-Hajri, M. Ohadi, Mass transfer characteristics of gas–liquid absorption during Taylor flow in mini/microchannel reactors, Chemical Engineering Science, 101 (2013) 69-80. [41] H.W. Jia, P. Zhang, Investigation of the Taylor bubble under the effect of dissolution in microchannel, Chemical Engineering Journal, 285 (2016) 252-263. [42] C. Zhu, C. Li, X. Gao, Y. Ma, D. Liu, Taylor flow and mass transfer of CO 2 chemical absorption into MEA aqueous solutions in a T-junction microchannel, 26

International Journal of Heat and Mass Transfer, 73 (2014) 492-499. [43] W. Last, J. Stichlmair, Determination of Mass Transfer Parameters by Means of Chemical Absorption, Chemical Engineering & Technology, 25 (2002) 385-391. [44] N. Kockmann, S. Karlen, C. Girard, D.M. Roberge, Liquid–Liquid Test Reactions to Characterize Two-Phase Mixing in Microchannels, Heat Transfer Engineering, 34 (2013) 169-177. [45] W.G. Whitman, The two film theory of gas absorption, International Journal of Heat and Mass Transfer, 5 (1962) 429-433. [46] N. Shao, A. Gavriilidis, P. Angeli, Mass transfer during Taylor flow in microchannels with and without chemical reaction, Chemical Engineering Journal, 160 (2010) 873-881. [47] J.H. Xu, J. Tan, S.W. Li, G.S. Luo, Enhancement of mass transfer performance of liquid–liquid system by droplet flow in microchannels, Chemical Engineering Journal, 141 (2008) 242-249. [48] P.V. Danckwerts, Gas-liquid Reactors, McGraw Hill, New York, 1970. [49] D. A, P. S, An experimental and theoretical investigation of pure carbon dioxide absorption in aqueous sodium hydroxide in glass millichannels, Journal of CO2 Utilization, 26 (2018) 133-142. [50] X.Y. Ji, Y.G. Ma, T.T. Fu, C.Y. Zhu, D.J. Wang, Experimental investigation of the liquid volumetric mass transfer coefficient for upward gas-liquid two-phase 27

flow in rectangular microchannels, Brazilian Journal of Chemical Engineering, 27 (2010) 573-582. [51] H. Ganapathy, A. Shooshtari, S. Dessiatoun, M.M. Ohadi, M. Alshehhi, Hydrodynamics and mass transfer performance of a microreactor for enhanced gas separation processes, Chemical Engineering Journal, 266 (2015) 258-270. [52] L. Yang, M.J. Nieves-Remacha, K.F. Jensen, Simulations and analysis of multiphase transport and reaction in segmented flow microreactors, Chemical Engineering Science, (2016).

Caption of figures Fig. 1. (a) Schematic diagram of the experimental setup; (b) Examples of gas-liquid two-phase flow accompanying with mass transfer (ωMEA and ωIL are the initial mass fraction of MEA and [Bmim][BF4] respectively, l0 is the initial bubble length, gas and liquid flowed into the branch and main inlets respectively in the cross-flow way).

Fig. 2. Effects of operation conditions on the dimensionless initial bubble length: (a) gas-liquid flow rates; (b) MEA concentration; (c) [Bmim][BF4] concentration.

Fig. 3. Evolution of the dimensionless lost length ∆l/w and the relative lost length ∆l/l0: (a) effect of gas flow rate; (b) effect of liquid flow rate; (c) effect of MEA concentration; (d) effect of [Bmim][BF4] concentration. 28

Fig. 4. Evolution of bubble velocity: (a) bubble cap, rear and average velocities; (b) effect of gas flow rate on average bubble velocity; (c) effect of liquid flow rate on average bubble velocity; (d) effect of MEA concentration on average bubble velocity; (e) effect of [Bmim][BF4] concentration on average bubble velocity.

Fig. 5. Relative velocity ua/UTP of bubbles as a function of relative lost length ∆l/l0: (a) effect of gas flow rate; (b) effect of liquid flow rate; (c) effect of MEA concentration; (d) effect of MEA concentration; (e) Comparison between experimental data and predicted values.

Fig. 6. Effects of experimental conditions on the average liquid-phase mass transfer coefficient (kL,t) with the flow time: (a) gas flow rate; (b) liquid flow rate; (c) MEA concentration; (d) [Bmim][BF4] concentration.

Fig. 7. Effects of experimental conditions on kL and the residence time: (a) gas and liquid flow rates; (b) MEA concentration; (c) [Bmim][BF4] concentration.

Fig. 8. Effects on of experimental conditions on kLa and specific surface area: (a) gas and liquid flow rates; (b) MEA concentration; (c) [Bmim][BF4] concentration.

29

Fig. 9. Comparison of kLa between the experimental data and calculated values by literature correlations.

Fig. 10. Comparison of kLa between the experimental data and calculated values by the new correlation.

30

Fig. 1

31

Fig. 2

32

33

Fig. 3

34

35

Fig. 4

36

37

Fig. 5

38

39

Fig. 6

40

41

Fig. 7

42

43

Fig. 8

44

Fig. 9

45

Fig. 10

46

Highlights: 1. The time-dependent bubble size and velocity were traced. 2. The volume effect on the velocity of bubble was explored. 3. The dynamic evolution of mass transfer coefficient was studied. 4.

An empirical correlation was proposed to predict the kLa.

47