Experiments of mass transfer with liquid-liquid slug flow in square microchannels

TL;DR: In this paper, a liquid-liquid mass transfer mechanism with slug flow in microreactor is investigated by means of experiments in square microchannels of 0.2 and 0.3 mm width.

About: This article is published in Chemical Engineering Science.The article was published on 2014-02-24 and is currently open access. It has received 94 citations till now. The article focuses on the topics: Mass transfer coefficient & Slug flow.

Summary

1. Introduction

• Microreactors and microchannels are non-conventional devices in chemical engineering field but have been for few years the subject of numerous research studies and programs such as the European project IMPULSE (Integrated Multiscale Process Units with Locally Structured Elements) in 2005.
• With liquid–liquid slug flow in microchannels using reactive (with instantaneous reactions) or non-reactive systems.
• Dessimoz et al. (2008) worked in rectangular microchannels of 0.4 mm equivalent diameter and carried out the instantaneous neutralisation of trichloroacetic acid by NaOH in toluene or hexane.
• The results obtained in terms of mass transfer coefficients on the droplet side are compared with the correlation suggested by the previous simulation work.

2.1. Microreactors

• The experiments are carried out in two microreactors manufactured by the LAAS (Laboratory for Analysis and Architecture of Systems, CNRS Toulouse, France), whose main microchannels have square section of 0.2170.01 and 0.3070.01 mm width (Fig. 1).
• High-aspect-ratio microchannels are fabricated in a silicon wafer by plasma etching using the deep reactive ion etching (DRIE) technique and the Bosch process (Tang et al., 2007; Laermer and Schilp, 1996).
• The structured silicon wafer is capped by a borosilicate glass wafer (Pyrexs 7740 from Corning) using anodic bonding technique.
• Liquid–liquid slug flow is produced as illustrated in Fig. 2, with droplets and continuous phase slug that regularly alternates (a droplet and a continuous phase slug constitute a “unit cell”).
• They act like filters in order to extract in a selective way the continuous phase.

2.2. Experimental bench

• The introduction of the fluids is performed by syringes with stainless steel needles placed on syringe pumps (Harvard Apparatus PHD2000 or PicoPlus).
• A high-speed camera HCC-1000 (VDS Vosskühler GmbH) coupled with a binocular Nikon SMZ-10 enables the visualization of the flow in the microchannel.
• The chip lighting is provided through optical beams.
• The others are blocked using plugged capillaries.
• The measuring system and the analytical method will be described in more details in Section 3.1.

2.3. Microfluidic connections

• Silicon and glass are tough materials but highly breakable.
• The fragility of the materials prohibits the use of connections directly screwed on the chip.
• Tight microfluidic connections were obtained using silicon septa.
• As described on Fig. 4, a PFA (perfluoroalcoxy polymer, 1/16″ OD) capillary passes through a septum placed on each inlet and outlet.
• The screwing allows the septa to be flattened against the silicium wafer.

3.1. Analytical method

• Water/acetone/toluene two-phase system was used to characterize mass transfer in square microchannels.
• In order to analyze continuous phase samples representative of this phase in the microchannel, half of its flow rate is constantly extracted.
• Therefore, to avoid the transfer of toluene and water, water saturated with toluene and toluene saturated with water have been used in the whole experiments performed at ambient temperature.
• Moreover the calibration of the system showed a linear relationship between fluid absorbance and acetone concentration up to 0.4% in weight of acetone in water.
• Viscosity is measured with the rheometer CSL2 500 (TA Instruments).

3.2. Experimental protocol

• The continuous and dispersed phases flow rates, Qc and Qd, are varied.
• For every operating conditions, the droplets length Ld, velocity Ud and frequency fd are measured.
• Its concentration Cd 0 is settled so that the concentration measurements are included in the range where concentration and absorbance of the analyzed solution are proportional.
• The monitoring of the transfer all along the microchannel is achieved by connecting the measuring cell at the different secondary channels.
• The concentration profile of acetone in the dispersed phase Cd is then obtained with the following mass balance: QdρdCd 0 ¼ QdρdCdðtÞþQ cρcCcðtÞ ð1Þ where ρ is the fluid density, and t is the residence time.

4. Results

• 1. Volumetric mass transfer coefficients identification Droplet side mass transfer coefficient kd,exp is identified from the experimental concentration profiles.
• The droplet volume Vd,exp corresponds to the ratio between the dispersed phase flow rate and the droplets frequency: Vd;exp ¼ Qd f d ð4Þ.
• Tref was chosen in the zone where the laminar flow in the microchannel is fully developed in order to avoid the entrance effects impact on kd,exp estimation (Skelland andWellek, 1964).
• By fitting Eq. (5) to the experimental profiles, coefficient (kd,expa∙ VUC/Vd) can be identified for the whole experiments.
• Table 2 presents the results of other authors: Ghaini et al. (2010) and Assmann and von Rohr (2011) considered the global mass transfer coefficient while the present work and Dessimoz et al.’s (2008) study are focusing on droplet side mass transfer coefficient.

4.2. Interfacial area modeling

• It is assumed that this parameter depends on the capillary number Cad (Kreutzer et al., 2005) defined in terms of Eq. (10): Cad ¼ μcUd s ð10Þ where μc is the viscosity of the continuous phase and s the interfacial tension of the liquid–liquid system.
• The droplet ends are assimilated to hemispheres.
• The interfacial area of the liquid–liquid system a is defined as the ratio between the surface of one droplet and the unit cell volume (given by the following): a¼ Sd LUC UdC 2 ð14Þ.
• The kd,expa values previously identified allows the estimation of the mass transfer coefficient kd,exp that ranges from 1.5e"04 to 1.4e"03 m s"1. Fig. 9 shows that, as expected, the mass transfer coefficient tends to increase with the droplets velocity.

5.1. Validation of a previous numerical study

• In a previous work, a correlation for the estimation of droplet side mass transfer coefficient has been proposed for liquid–liquid mass transfer with slug flow as a function of the flow characteristics (Di Miceli Raimondi et al., 2008).
• In the present work, capillary number is low and the droplets are highly confined resulting in flow patterns with numerous vortices as illustrated by Fig. 10.
• The maximal relative error between the model and the experiments observed is 55% and the median relative error is about 10%.

5.2.1. Mass transfer models description

• Mass transfer models proposed in literature for systems approaching liquid–liquid slug flow are described afterwards.
• These models were established under conditions where the film quickly saturates because of large bubble lengths (Bercic and Pintar, 1997) or high bubble velocities (Yue et al., 2007).
• In order to compare mass transfer mechanisms in gas–liquid flow and in liquid–liquid slug flow, three models are considered: !.
• Film saturation is not quickly achieved: this is consistent with Fourier number values that are lower than 1.

6. Conclusions

• Liquid–liquid mass transfer experiments have been carried out in square microchannels of 0.21–0.30 mmwidth.
• The microreactor is composed of a main channel where the transfer operates and secondary channels that allows the selective extraction of the continuous phase for analysis.
• In order to understand the transfer mechanism in the studied system, the experimental results are compared with models available in literature: liquid–liquid mass transfer in recirculating drops with non-oscillating interface in channel of conventional size and gas–liquid mass transfer in microchannels with Taylor flow pattern.
• Nomenclature a interfacial area (m2 m"3) C concentration in terms of mass fraction (kg of solute kg"1 of phase) Ca Capillary numberFig.
• Comparison between experimental results and models.

Figures

Fig. 6. Concentration profiles of acetone in the continuous phase (measured) and in the dispersed phase (calculated). (a) wC¼0.30 mm, Qc¼7 mL h"1, Qd¼3.5 mL h"1; (b) wC¼0.21 mm, Qc¼30 mL h"1, Qd¼20 mL h"1.

Fig. 3. Scheme of the experimental bench.

Fig. 8. Comparison between calculated (Eq. (12)) and experimental (Eq. (9)) droplet volume.

Fig. 7. Comparison between experimental and calculated (Eq. (5)) concentration profiles in the dispersed phase. (a) wC¼0.30 mm, Qc¼7 mL h"1, Qd¼3.5 mL h"1; (b) wC¼0.21 mm, Qc¼30 mL h"1, Qd¼20 mL h"1.

Fig. 1. Microreactor in silicium and glass.

Fig. 2. Illustration of the microreactor structure (main and secondary channels).

Fig. 11. Comparison between experimental and calculated mass transfer coefficients (Eq. (14)).

Fig. 10. Flow structure observed for confined droplets at low capillary number (Di Miceli Raimondi et al., 2008).

Fig. 9. Mass transfer coefficient in function of droplets velocity.

Table 1 Operating conditions of the mass transfer experiments.

Fig. 5. Measuring cell associated to the UV-spectrophotometer.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Open archive toulouse archive ouverte (oatao)" ?

The results reasonably fit with a model suggested by a previous study based on 2D direct numerical simulations. A comparison with other models available in the literature shows that in the operating conditions considered in this work, the flow pattern inside the confined droplets at microscale leads to an enhancement of mass transfer compared to droplets that are not confined. 

Q2. What are the future works mentioned in the paper "Open archive toulouse archive ouverte (oatao)" ?

Subscripts c continuous phase C channel calc calculated cap refers to the droplets caps d dispersed phase or droplets exp refers to experimental data film refers to the film L liquid phase mod estimated with a mass transfer coefficient model ref at time of reference Superscripts 0 at inlet 1 at thermodynamic equilibrium 

Q3. What is the important factor in the prediction of liquid–liquid mass transfer?

The resistance in the dispersed phase to transfer that is neglected in gas–liquid Taylor flow limits the reliability of the gas–liquid models to predict liquid–liquid mass transfer. 

Q4. How much flow rate is extracted in continuous phase samples?

In order to analyze continuous phase samples representative of this phase in the microchannel, half of its flow rate is constantly extracted. 

Q5. How is the introduction of the fluids performed?

The introduction of the fluids is performed by syringes with stainless steel needles placed on syringe pumps (Harvard Apparatus PHD2000 or PicoPlus). 

Q6. What is the coefficient of the droplet side mass transfer?

The concentration profile of solute in the dispersed phase (toluene) all along the microchannel is obtained by mass balance from which a droplet side mass transfer coefficient is identified. 

Q7. How is the manufacturing of silicon microfluidic chips achieved?

The manufacturing of silicon microfluidic chips is achieved by means of the photolithography technique (Gawron et al., 2001; Chunet al., 2006). 

Q8. What is the diffusion coefficient of acetone in toluene in high diluted?

The diffusion coefficient of acetone in toluene in highly diluted solutions at ambient temperature is estimated at 2.8e"09 m2 s"1 (Bulicka and Prochazka, 1976). 

Q9. What is the definition of the transfer between a film and a gas?

The term related to thetransfer through the film is obtained by referring to the model of mass transfer from a bubble to a laminar falling film, as a function of Fourier number (Eqs. (19)–(22)). 

Q10. What is the mass balance of acetone in the dispersed phase?

The concentration profile of acetone in the dispersed phase Cd is then obtained with the following mass balance:QdρdCd 0 ¼ QdρdCdðtÞþQ cρcCcðtÞ ð1Þwhere ρ is the fluid density, and t is the residence time. 

Q11. How is the transfer of acetone measured?

The monitoring of the transfer all along the microchannel is achieved by connecting the measuring cell at the different secondary channels. 

Q12. How can the interfacial area be estimated?

They showed that depending on the operating conditions (notably the slug velocity), the interfacial area can be estimated from the droplet surface with or without considering the surface in contact with the film between the droplet and the channel wall. 

Q13. What is the definition of a microchannel?

Microreactors and microchannels are non-conventional devices in chemical engineering field but have been for few years the subject of numerous research studies and programs such as the European project IMPULSE (Integrated Multiscale Process Units with Locally Structured Elements) in 2005. 

Q14. How can the equilibrium be written in terms of Eq. (8)?

A prior study showed that this equilibrium can be written in terms of Eq. (8) in the concentration domain considered in this work, with a constant partition coefficient m¼0.76. 

Q15. How many times did they work with lower droplets velocity?

Dessimoz et al. obtained lower values of mass transfer coefficients (around 10 times lower) but they worked with lower droplets velocity (0–0.02 m s"1). 

Q16. what is the diffusion coefficient of acetone in water?

Dc corresponds to the diffusion coefficient of acetone in water in dilute solution at ambient temperature, equal to 1.2e"09 m2 s"1 (Grossmann and Winkelmann, 2005): kLa¼ kL;capacapþkL;f ilmaf ilm ð18ÞkL;capacap ¼ 2 ffiffiffi2 pπffiffiffiffiffiffiffiffiffiffiffi DcUd wd s ! 

Q17. how long is the entrance length in a smooth channel?

The entrance length Le considering laminar flow in smooth channel can be estimated using Eq. (6) (Gao et al., 2002; Shen et al., 2006):LedCRe ¼ 0:1 ð6ÞIn the experiments carried out, the entrance length is evaluated to be less than 1.4 mm in the channel of 0.21 mmwidth, and 0.7 mm in the channel of 0.30 mm width.