Liquid Mass Transfer Coefficient for Each

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Thermodynamics, Transport, and Fluid Mechanics

Determination of liquid/liquid mass transfer coefficient for each phase in microchannels Liantang Li, Jisong Zhang, Chencan Du, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01976 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Industrial & Engineering Chemistry Research

Determination of liquid/liquid mass transfer coefficient for each phase in microchannels Liantang Li, Jisong Zhang, Chencan Du, Guangsheng Luo∗ The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

Abstract Overall mass transfer coefficient of liquid/liquid microflows has been extensively studied, but the liquid/liquid mass transfer coefficient for each phase in microchannels has never been reported. In this work, the overall mass transfer coefficient and mass transfer coefficient for each phase (inside and outside droplet) of liquid-liquid

microflows

were

determined

using

H2SO4/cyclohexanone

oxime/n-hexane as the working system. The concentrated sulfuric acid (H2SO4) was used as the continuous phase, and the n-hexane solution containing certain concentration of cyclohexanone oxime was used as the dispersed phase. A T-junction microchannel was used to generate liquid/liquid microflows. The influences of initial concentration of cyclohexanone oxime, two phase flow rates, and phase ratio were studied. The results showed that the mass transfer coefficient for each phase increased with the increasing of continuous phase rate. New mass transfer coefficient prediction models for each phase have been established, which could predict experimental results very well.

Keywords: H2SO4 / n-hexane; mass transfer coefficient; microchannels

Corresponding author. Email: [email protected]. Tel.: +86-10-62783870. ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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. Introduction Droplet based microfluidic technology has been applied in many fields, such as biological, chemical industry, new materials, cosmetics, and foods production

1-3

. To

applicate this kind of technology better, it is significant to explore the mechanisms of droplet formation as well as the mass transfer performances in liquid/liquid microflows. A lot of works have been devoted to studying the mass transfer in microchannals, indicating that microfluidic technology could substantially enhance mass transfer process compared to conventional devices 4, 5. Microfluidics exhibits outstanding advantages in mass transfer process for the small droplet sizes, high surface-to-volume ratio, and the internal circulation within the slugs, which can effectively enhance the mass transfer process

5-15

. Xu et al.

pointed out that the mass transfer process occurred during droplet forming stage as well as moving stage, and the droplet forming stage could contribute to as high as 30% in total process

16

. The volumetric mass transfer coefficient (KLa) of water/succinic

acid/butanol system was obtained in the work of Tang et al. 17, and the values varied from 0.012 to 0.082 s-1 in the droplet flow regime. Matthew et al. also found that the mass transfer coefficients were in the range of 5 ~ 15 s

-1

under their experimental

conditions, which were nearly 100 times larger than those in common reactors

18

.

Their results also showed that the mass transfer efficiency could be as high as 95-100% within short mass transfer time in microchannels 19, 20. The effects of several factors on mass transfer in microchannels have been investigated, including flow rates and two-phase flow ratios. The results indicated that higher flow rates and flow ratio were helpful to enhance mass transfer process

21, 22

.

Besides, the mass transfer coefficient could also be influenced by channel size, concentration and residence time

23-25

. Experimental results also revealed that high

shear zones in the film leaded to much higher mass-transfer rates than that in the conventional reactor

26-28

. The thickness of boundary layer of squeezing and the wall

of microchannel also enhanced the mass transfer

29-31

. Large mass transfer area and

enhanced internal circulation both inside and outside droplets were beneficial to

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enhance the mass transfer process 32,12. Many researchers have tried to develop the empirical correlations to predict mass transfer coefficients

17, 29, 33-35

. Some of them try to find out the relationship between

overall mass transfer coefficients and energy dissipation

25, 36

. Zhao and Susanti et al

established empirical correlations, which predicts volumetric mass transfer coefficient well with deviation less than 20% using typical system (water/n-butanol) 21,37. Xu et al. established suitable models of mass transfer volume coefficient, considering droplet forming stage and moving stage

16

. Bai et al. studied the mass transfer process with

the micro-LIF technique, and obtained the mass transfer coefficient prediction model 38

. Nevertheless, all the empirical correlations only took the overall mass transfer

coefficient into consideration.

Table 1. Some mass transfer systems have been reported Mass transfer Working systems

Mass transfer Working systems

materials

materials

n-butanol/water 21, 33, 39, 40

Succinic acid

Dodecane/water 5

Phenol

n-butanol/water 40, 41

Phosphoric acid

Kerosene/water 29

Acetic acid

Kerosene/water 40

Iodine

Toluene/water 42

Acetone

Benzene/water 40

Acetone

n-heptane/water 43

Benzoic acid

CCl4/water 40

Acetone

n-butyl acetate/water 44

NaOH

n-butanol/water 24

Toluene

Water/[C4mim][PF6] 38

Rhodamine 6G

In general, almost all studies have focused on the conventional system (as shown in Table 1). To the best of our knowledge, the mass transfer coefficient inside and outside droplet size has not been obtained in these reported systems, while it is essential for the fundamental research on mass transfer process. Thus, a suitable working system should be chosen first. In our previously work

45

, we studied the

droplet forming in microchannel and found that it is a special system for the interfacial intension is much smaller than that of water system without surfactants and



the viscosity of H2SO4 is about 23 times of that of water. Cyclohexanone oxime is an important intermediate product in the process of produce caprolactam. The significant characteristic of H2SO4/alkane system with cyclohexanone oxime as mass transfer solute is that the initial concentration could affect the mass transfer resistance in each phase. In low initial concentration (0.5~1.0%) in the hexane, the mass transfer resistance was mainly from the alkane phase as a result of high partition coefficient 46. In high initial concentration (2.0%~3.0%) in the hexane, the mass transfer resistance may mainly be from the H2SO4 phase due to the changes of H2SO4 physical properties (mainly viscosity). As shown in Figure 1, the viscosity of cyclohexanone oxime/ sulfuric acid increases very fast with the increasing of cyclohexanone oxime concentration. Moreover, concentrated sulfuric acid (H2SO4) and alkane system also a common system in industry, including alkylation, rearrangement reaction, and Claisen-Schmidt reaction 46-50, few basic researches have been carried out on this kind of system. Considerable efforts should be directed towards studying the mass transfer process of H2SO4/alkane system in a T-junction microchannel, to expand the applications of this kind of system in microreactors. Here the objective of this work is to study the mass transfer characteristics using cyclohexanone oxime as mass transfer solute in H2SO4/alkane system in microchannel, investigate the mass transfer coefficient for each phase (inside and outside of droplet) by adjusting the initial concentration, and obtain the mass transfer coefficient prediction models for each phase and overall mass transfer coefficient in different initial concentrations. This work may provide much deeper understanding of liquid/liquid mass transfer in microchannels.


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Figure 1. The viscosity of H2SO4 with different content cyclohexanone oxime 2. Experiments 2.1 Chemicals Concentrated sulfuric acid (98.2 wt%,) was purchased from Beijing chemical works; n-hexane (chromatographically pure) was purchased from Tianjin Fuchen and cyclohexanone oxime was purchased from J&K. All chemicals were used without further purification.

2.2 Equipment

The experimental device used herein is shown in Figure 2, which is a T-junction microreactor with a size of 0.25 mm. H2SO4 and n-hexane were delivered through feeding pipes into the microreactor by metering pumps (Beijing Satellite Co., Ltd.). The microreactor was connected by a certain length of pipe (a length of 50 mm), which is made of a 316 stainless steel, to control the mass transfer time. The inner diameter of the pipe was 1 mm and external dimeter was 1.6 mm. A phase separator [polytetrafluoroetylene (PTFE) membrane separator with average pore size of 1 µm] was connected directly downstream to the pipe, with the aid of regulating valve to



separate the two phases of H2SO4 and n-hexane. Some hexane phase was collected from the phase separator and used to analysis. The other effluent flows though the regulating valve and as the waste liquid. The droplet sizes were obtained using GPIB-ENET/100 controller and Labview (National instrument), based on the refractivity difference of the continuous and dispersed phases in the PTFE tube. Combining the captured number of droplets in certain time and the flow rate of the dispersed phase, and the droplet sizes were calculated from the frequency of droplet generation.

Figure 2. The structure of mass transfer experimental setup

2.3 Analysis

Cyclohexanone oxime concentration in n-hexane was analyzed by Agilent Gas chromatography (GC), with a flame photometric detector. The chromatographic column was HP-INNOWAX capillary column (30 m ×0.25 mm×0.25 µm). The injector and the detector temperature were 250゜C and 280゜C, respectively. The temperature program was as follows: firstly, temperature was held at 130 oC for 1 min, then increased to 180゜C at a speed of 50゜C /min, and finally held at 180゜C for 2.5 min.


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3. Results and Discussion

3.1 Effect of cyclohexanone oxime concentration on droplet sizes of H2SO4 and n-hexane system

The phase flow ratio was kept at a constant of 1.5, while the continuous phase flow rate was ranged from 3.0 mL/min to 10.5 mL/min. The concentration of cyclohexanone oxime was varied from 0.5 wt% to 3.0wt%. The results of droplet sizes are shown in Figure 3 (a). As it reveals, an increase of droplet sizes is obviously observed upon increasing the flow rate. The droplet size is about 0.6 mm as the continuous phase flow rate is 3 mL/min, it increases to 0.72 mm as the continuous phase flow rate increases to 10.5 mL/min. The main reason for the results is the increase of dispersed flow rate. As Figure 3 (b) shows, as the dispersed flow rate is kept as a constant rate of 3 mL/min, the droplet size decreases with the increasing of the continuous flow rate, which is the same with the previous results 45. In addition, no obvious changes of the droplet sizes with the cyclohexanone oxime concentration increase from 0.5 wt% to 3.0wt% at the same two phases flow rate. Thus, the concentration of cyclohexanone oxime has little effect on the dynamic interfacial tension. The Capillary number ( Ca =

µu ) in the experimental conditions is larger γ

than 0.1, indicating they are in the dripping flow pattern

51

. Comparing the droplet

size and the width of the microchannel (de=1.0mm), the droplet size is comparable to the microchannel. The droplet flows for the system in the microrector has been realized.



Figure 3. Droplet sizes in different flow rates and initial cyclohexanone oxime concentration (a) The phase ratio is 1.5; (b) The dispersed phase flow rate is 3 mL/min.

3.2 Effect of cyclohexanone oxime concentration on mass transfer efficiency and volumetric mass transfer coefficient

The continuous phase flow rate was set to 10 mL/min, and the dispersed phase flow rate was 3 mL/min, and the cyclohexanone oxime concentration was varied from 0.5% to 3.0%. The mass transfer efficiency in different cyclohexanone oxime conditions is shown in Figure 4. The mass transfer time is only 0.18 s, the efficiency of different initial concentration could be as high as 92% to 96%, showing excellent mass transfer performance in the microchannel.

Figure 4. Mass transfer efficiency in different initial cyclohexanone oxime concentrations. (Fc=10 mL/min, Fd=3 mL/min, t=0.18 s)


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Based on the mass transfer amount, the volumetric mass transfer coefficient (Ka) could be obtained as following:

N=

∆C = Ka∆CM ∆t

(1)

while ∆t is the mass transfer time, ∆C is the concentration difference of initial and finial concentration, ∆CM ( ∆CM =

( C − C ) − ( C − C ) ) is the logarithmic mean ln ( C − C ) / ( C − C )  * i

i

i

* 0

0

* i

* 0

0

concentration difference, where Ci is the cyclohexanone oxime concentration in n-hexane, Ci* is the equilibrium concentration, C0 is the initial concentration, C0* is the initial equilibrium concentration. The volumetric mass transfer coefficient of this system is shown in Figure 5 (a). As it shows, the volumetric mass transfer coefficient varies from 3.2 s-1 to 35.3 s-1, much larger than that in traditional reactors

40

. In addition, increasing of

cyclohexanone oxime concentration (from 1.0% to 2.0%) results in a decrease of volumetric mass transfer coefficient. However, no significant decreasing of volumetric mass transfer coefficient is observed upon increasing cyclohexanone oxime concentration from 2.0% to 3.0%. The results are consistent with Figure 6, which shows different cyclohexanone oxime concentrations in n-hexane and H2SO4. As Figure 6 shows, with the cyclohexanone oxime concentration increases from 0.5% to 3.0%, cyclohexanone oxime is gradually enriched in the interfacial phase at the side of H2SO4 phase, and this phenomena also indicates that at high cyclohexanone oxime concentration the mass transfer resistance is mainly controlled by the H2SO4 phase. It reveals that the mass transfer resistance comes from different positions for the physical property of this system. In low cyclohexanone oxime concentration (0.5% and 1.0%), cyclohexanone oxime can easily transfer into the thin lay of H2SO4 phase near the liquid/liquid interface for the large partition coefficient (The value of partition coefficient ( m =

cc ) of about 35 was obtained by a sufficient contact time to cd

make sure the cyclohexanone oxime reached equilibrium in two phases.). However, as ACS Paragon Plus Environment


the concentration of cyclohexanone oxime increases, the high initial cyclohexanone oxime concentration leads to large content of cyclohexanone oxime transfer from the n-hexane phase to the interfacial phase in a quite short time, and the high viscosity of H2SO4 can make the transfer of cyclohexanone oxime from the interfacial phase to the bulk phase of H2SO4 is much hard, and the mass transfer resistance in the phase of H2SO4 will play a key role in the mass transfer process. As the initial cyclohexanone oxime concentration in n-hexane increases to 2.0% to 3.0%, the mass transfer resistance in n-hexane could be ignored.

Figure 5. The volumetric mass transfer coefficient and mass transfer coefficient of different initial cyclohexanone oxime concentrations in different flow rates.

Figure 6. Different cyclohexanone oxime concentrations in n-hexane and H2SO4.

Eq. (2) shows the relationship of volumetric mass transfer coefficient and mass transfer coefficient (K):

Ka = K ⋅ a = K ⋅

6Φ d

(2)


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while a is the specific surface area, Φ is the phase volume fraction of dispersed phase ( Φ =

Fd ), d is the droplet size. The mass transfer coefficient was obtained, Fc + Fd

and shown in Figure 5 (b). According to the relationship of overall mass transfer coefficient and mass transfer coefficients for each phase: 1 1 1 = + K k d mkc

(3)

while kc is the mass transfer coefficient for the continuous phase, kd is the mass transfer coefficient for the dispersed phase, m is the partition coefficient. For the specificity of this system, in the lower cyclohexanone oxime concentration condition, the mass transfer resistance mainly exists inside droplet, thus the overall mass transfer coefficient is mainly contributed by kd; while in the high cyclohexanone oxime concentration condition, the mass transfer resistance in the dispersed phase (inside droplet) could be ignored, and the mass transfer resistance mainly exists in the continuous phase (outside droplet), the overall mass transfer coefficient is mainly determined by m and kc. When the cyclohexanone oxime concentration is at a certain range, the mass transfer will be determined by both inside droplet and outside droplet. Thus, we can obtain the value of kd at cyclohexanone oxime concentration of 1.0%, the value of K at cyclohexanone oxime concentration of 1.5%, and the value of kc at cyclohexanone oxime concentration of 2.5%. According to Eq. (3), the value of K can be predicted using kd ,kc, and m. The comparison results of calculated values with the experimental data at cyclohexanone oxime concentration 1.5% are shown in Figure 7. As can be seen, the calculated K is in accordance with the experimental value very well, thus, the mass transfer coefficient for each phase, and the overall mass transfer coefficient could be obtained by adjust the cyclohexanone oxime concentration with the working system.



Figure 7. The comparison of calculated K with experimental K.

3.2.1 Mass transfer coefficient and volumetric mass transfer coefficient for the dispersed phase

As presented in the previous study, at low cyclohexanone oxime concentration, we can obtain the mass transfer coefficient for the dispersed phase. In the following section, we will discuss the influences of different parameters. The relationships of mass transfer coefficient and volumetric mass transfer coefficient with flow rate are shown in Figure 8. As can be seen in Figure 8 (a), the mass transfer coefficient and volumetric mass transfer coefficient increase with the increasing of the continuous phase flow rate when the phase ratio of two phases is kept at 1.5. When the continuous phase flow rate is 3 mL/min, the value of volumetric mass transfer coefficient and mass transfer coefficient are 7.5 s-1 and 0.002 m/s, respectively. While increasing the continuous phase flow rate to 9 mL/min, the value of volumetric mass transfer coefficient and mass transfer coefficient of 26.1 s-1 and 0.008 m/s are obtained, respectively. This is likely due to the circle flow could enhance the mass transfer at higher flow rate. In addition, as the dispersed phase flow rate is set as 3 mL/min in Figure 8 (b), the increasing continuous phase flow rate results in increasing mass transfer coefficient and volumetric mass transfer coefficient. The reason for this phenomenon may be that the smaller size droplets could be obtained in higher


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continuous phase flow rate, resulting in larger mass transfer area.

Figure 8. The volumetric mass transfer coefficient and mass transfer coefficient in different flow rate: (a) The phase ratio is 1.5; (b) The dispersed phase flow rate is 3 mL/min.

3.2.2 Mass transfer coefficient and volumetric mass transfer coefficient for the continuous phase

When the initial cyclohexanone oxime concentration is set to 2.5%, we can obtain the mass transfer coefficient for the continuous phase. The results in different flow conditions are shown in Figure 9. As indicated, higher mass transfer coefficient and volumetric mass transfer coefficient could be achieved at higher continuous phase flow rate. As the phase ratio was set as 1.5, the volumetric mass transfer coefficient for the continuous phase increases from 3.5 to 26.0 s-1. Mass transfer coefficient increases from 8.6×10-4 to 7.8×10-3 m/s, as the continuous phase flow rate varies from 3 to 10.5 mL/min. This may be that the increasing continuous phase flow rate enhanced the circle flow in the continuous phase, thus, the mass transfer process is enhanced. When the dispersed phase flow rate is constant as 3 mL/min, increasing the continuous phase flow rate (from 4 to 12 mL/min) leads to higher volumetric mass transfer coefficient and mass transfer coefficient. The higher continuous phase flow rate leads to the increase of the mass transfer interfacial area, due to the smaller droplet size.



Figure 9. The volumetric mass transfer coefficient and mass transfer coefficient in different flow rate: (a) The phase ratio is 1.5; (b) The dispersed phase flow rate is 3 mL/min.

3.2.3 The overall mass transfer coefficient and overall volumetric mass transfer coefficient

When the cyclohexanone oxime concentration is 1.5%, the mass transfer resistance could be determined by both inside droplet and outside droplet, and only overall mass transfer coefficient can be obtained. The values of overall mass transfer coefficient and overall volumetric mass transfer coefficient are shown in Figure 10. As shown in Figures 10 (a) and 10 (b), the overall mass transfer coefficient and overall volumetric mass transfer coefficient increase with the increasing of the continuous phase flow rate.

Figure 10. The overall mass transfer coefficient and overall volumetric mass transfer coefficient in different flow rate: (a) The phase ratio is 1.5; (b) The dispersed phase flow rate is 3 mL/min.


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3.3 Prediction model of mass transfer process in microchannel

Several models for predicting overall mass transfer coefficient of liquid-liquid microflows have been published, as listed in Table 2. We compared our experimental data with the calculated values with the three models, and the results are plotted in Figure 11. It shows that the prediction models can not provide good prediction for this working system. It is required to establish new prediction models. Table 2. Three models for different systems

Prediction model

Ka = 3.0 ×105 Ca1.0Φ0.69 52

System Octane/water

(0.2

Liquid Mass Transfer Coefficient for Each

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