HYDRODYNAMICS OF TAYLOR FLOW WITH NEAR CRITICAL CO2 IN MICROCHANNEL

A. Martina, S. Camya*, J. Aubina

a Laboratoire de Génie Chimique, Université de Toulouse, CNRS, INPT, UPS, France

Corresponding author: severine.camy@ensiacet.fr

ABSTRACT

Low viscosity, high diffusivity and the possibility to precisely control the properties of supercritical CO2 (scCO2) are advantages of the association of supercritical fluids and microchannels. Despite this, transport properties are highly sensitive to temperature, pressure and composition variations. In order to improve and better control supercritical microprocesses, fundamental knowledge on the hydrodynamics and mass transfer phenomena is critical. In this work, the flow behavior of Ethanol-CO2 (EtOH-CO2) is investigated in a T-junction microchannel at pressures from 20 to 85 bar and a temperature of 40 °C. Utilization of phase diagrams was of great importance to correctly describe the different two-phase flows at high pressure inside microchannel. In the vapor/liquid zone, shadowgraphy experiments revealed a well-organized Taylor flow of CO2 bubbles dispersed within EtOH. At constant mixture composition, the bubble length decreases with increasing pressure, whereas the bubble velocity varies little. In the large single-phase liquid domain, different flow behavior was observed depending on the state of CO2. At lower pressures (P < 65 bar), CO2 bubbles are formed in liquid EtOH at the T-junction and then progressively dissolve into the liquid. At pressures higher than the supercritical pressure of CO2, the flow is very complicated. Both components appear mixed in a single phase after the T-junction channel; further along, bubbles then start to appear and then dissolve again to form a single phase flow along the microchannel.

INTRODUCTION

A number of recent works have demonstrated the interest of supercritical processes in microchannels for organic chemistry [1], separation processes [2] and nanostructure synthesis [3]. Supercritical carbon dioxide presents the advantage of being easily accessible and represents a “green” alternative to organic solvents. The low viscosity, high diffusivity, and the possibility to precisely control the properties of supercritical CO2 (scCO2) are advantages of the association of supercritical fluids and microchannels. Despite this, transport properties are highly sensitive to temperature, pressure and composition variations. In order to improve and better control supercritical microprocesses, fundamental knowledge on the hydrodynamics and mass transfer phenomena is critical.

Investigation of the influence of operating conditions on two-phase flow behavior in microchannels at high pressure has only started to be investigated recently. In 2012, Blanch-Ojea et al. [4] observed CO2-ethanol and CO2-methanol flows in microchannels under different operating conditions where CO2 is either in the liquid, gas or supercritical state. They presented miscible single phase or vapor-liquid equilibrium with two distinct phases, depending on pressure and temperature at a constant CO2 fraction. Flow regime maps were generated for detailed operating conditions where Taylor, annular, and wavy flows are obtained. In 2013, Zhao et al. [5] discussed the influence of pressure on the formation mechanisms of gas-liquid flow in a T-junction microchannel fed with Nitrogen and de-ionized

water for pressures ranging between 1 and 50 bar. Seven typical flow patterns were observed: bubbly flow, slug flow, unstable slug flow, parallel flow, slug-annular flow, and churn flow. Luther et al. [6] quantitatively studied mass transfer in the ternary multiphase system composed of ethyl acetate, water and CO2 at 85 bar and temperatures of 23 and 37.5 °C inside a microcapillary. The change in volume and refractive index of ethyl acetate was measured. Ogden et al. [7] mapped the flow regimes of a scCO2-H2O in a Y-junction microfluidic system at 100 bar and 50 °C as a function of both total flow rate and flow rate ratio. They identified the formation of three different flow regimes (i.e. segmented, wavy and parallel) depending on the CO2 and H2O flow rates. Marre et al. [8] also studied scCO2-H2O flow regimes but in a coaxial flow geometry at 50°C and different pressures (80 bar and 180 bar). They identified different flow regimes and in particular, the dripping to jetting transition as a function of the CO2 and H2O flow rates. These authors found that the transition between dripping and jetting was influenced by pressure.

These literature studies have clearly contributed to the fundamental understanding of multiphase hydrodynamics in microchannels under high-pressure conditions. Indeed, the flow is extremely complex due to the variable nature of CO2 around the critical point. However, generic conclusions cannot yet be made since the data obtained in each study is rather specific in terms of the CO2-fluid pairs, operating conditions, equipment, and surface properties of the channel and the these high-pressure flow depend strongly on all of these parameters. It should also be pointed out that detailed information on all of the parameters which influence the flow are not always provided in these studies.

In this work, an experimental study of the two-phase flow behavior of CO2-EtOH at high pressure was conducted using in-house polymer-glass microsystems fed by high-pressure pumps. Shadowgraphy was used to characterize the bubbles in terms of length and velocity and also to investigate the characteristics of the flow as CO2 dissolves into the liquid. Thermodynamic data of two-phase flow under experimental conditions are also employed to interpret flow behavior and to calculate superficial velocity inside the microchannel. This is a novel way to explore partially miscible flow at high pressure.

MATERIALS AND METHODS

Materials

Carbon dioxide was supplied by Air Liquid (≥ purity 99.98%) and ethanol is from Sigma-Aldrich (≥ purity 96% v/v). Water is deionized before use.

Experimental set-up

Microdevices used in this study are made with a soft lithography method using a UV-sensitive polymer resin and glass slides, which is described in detail in Martin et al [9]. The microchannel has a 200 x 200 µm cross section and a total length of 12.5 cm. Connection with macro environment is ensured by glass capillaries, which are embedded in the polymer resin. The microchannels made by this fabrication protocol have been validated using high pressure CO2 and have shown to resist at least 200 bar. Figure 1 (a) and (b) show the microchannel dimensions and an example microchip.

(a)

Figure 1: Polymer-glass microchannel (a) dimensions and (b) photo.

The experimental set up is shown in Figure 2. Fluid is pumped into the microchannel using high-pressure pumps (Teledyne ISCO, models 260D and 100 DX for CO2 and ethanol respectively). To maintain the pressure in the microchannel, a 250 mL tank is used as back-pressure buffer at the outlet of the microchannel. A mass flow controller (mini CORI-FLOW ML120 Bronkhorst) was used to regulate the pump motion to maintain a constant mass flow rate at the inlet of the microchip. The flow rate of ethanol was maintained directly by the pump setting. Fluids were pre-heated before the entrance of microchip using a heated bath and the LED panel (PHLOX White led Backlight 200x200) on which the microchip is placed for flow visualization enables the system to be maintained at a constant temperature of 40 °C.

Figure 2: Experimental set-up (a) schematic diagram and (b) photo

A high speed CCD camera (MIKROTRON EoSens CL MC 1362) is used to take image flow sequences at 500 frames per second. This frame rate is necessary in order to capture the bubble formation mechanism at high flow velocities (between 20 and 60 cm/s). The shadowgraphy set up enables to CO2 bubble shapes inside channel to be visualized. The raw images were processed using the MATLAB image toolbox to obtain information on bubble size and velocity.

Phase diagram of the binary mixture CO2-EtOH

Knowledge of the phase diagram of the binary CO2-EtOH mixture is essential for understanding and interpreting the behavior of the flow at the T-junction and along the microchannel. In the phase diagram, the mixture presents a large single-phase liquid domain, a small single-phase vapor domain for low pressure and high CO2 composition and a two-phase liquid/vapor domain between these zones. Based on experimental results at 293.15 K and 303.15 K at pressures ranging from 6.8 bar to 65.2 bar from [10], the binary mixture was

(b)

(a)

(b)

calculated by Simulis Thermodynamics Software (ProSim SA).The boundaries of the phase diagram are in agreement with the experimental data when modelled by the Soave-Redlich-Kwong (SRK) equation of state using a combined PSRK mixing rule (EoS/GE approach). Excess Gibbs Energy (GE) is obtained using UNIQUAC model and binary interactions parameters are fitted on experimental data. Validation of the model equilibrium data with the experimental data obtained by [10] at different temperatures and pressures are shown in Figure 3 (a).

Figure 3: Phase diagram of CO2-EtOH (a) comparison of calculated data (lines) and experimental data (symbols) obtained by [10] at 293.15 K and 303.15 K and (b) calculated

phase diagram at 313.15 K, which is the temperature used in the process.

RESULTS

The flow patterns of the CO2-EtOH mixture were investigated for three different zones of the thermodynamic phase diagram as shown in Figure 3 (b). In zone 1 the mixture is in a liquid/vapor state and the flow is characterized by regular Taylor flow; experiments were run at xCO2 = 0.65, a temperature of 40 °C and pressures ranging from 45 to 67 bar. Zone 2 is representative of the single-phase liquid domain under sufficiently low pressure for CO2 to be in the gaseous state, i.e. well below the critical pressure; experiments were performed at 43 bar and variable composition. Zone 3 is also in the single-phase liquid domain but at a pressure higher than critical pressure of CO2. In this case, CO2 is introduced at 40 °C and 82 bar, i.e. in conditions above its critical point.

Taylor flow: bubble and slug lengths and bubble velocity

Taylor flow is characterized by the formation of regular bubbles in a continuous liquid phase; the bubbles are separated from the channel walls by a thin liquid film. The latter induces a slip velocity between the bubble and the mean flow, which generates flow recirculation in both the bubble and the liquid slug. These characteristics of Taylor flow present many advantages and allow transport processes and reactions to be greatly enhanced.

Figure 4: Shadowgraph of CO2 Taylor bubbles forming in EtOH at 60 bar and 35 °C QvEtOH = 50 µL/min and QvCO2 = 100 µL/min.

(a) (b)

Figure 4 shows the formation of CO2 Taylor bubbles in ethanol at 60 bar and 35°C. Due to the high wettability of the microchip material by ethanol, the ethanol is completely in contact with the microchannel walls allowing a thin liquid film to form around the bubbles of CO2. The CO2 bubbles are extremely regular and are separated by ethanol slugs of constant length.

The effect of inlet pressure on the bubble length (LB) and liquid slug length (LS) at 40 °C and constant mixture composition (xCO2 = 0.65) is shown in Figure 5. With increasing pressure, the bubble length decreases and the slug length increases. This is in agreement with thermodynamic equilibrium since as pressure increases the amount of CO2 that is transferred to the liquid phase also increases. The decrease in bubble length is much greater than the increase in slug length because the CO2 gas occupies a greater volume than when it is dissolved.

Figure 5: Influence of inlet pressure on (a) bubble length (LB) and (b) slug length (LS) Figure 6 (a) shows the effect of inlet pressure on the bubble velocity for two different pressure gradients (difference between microchannel inlet and pressure buffer). It can be seen that for each data set, there is little influence of pressure on the bubble.

Figure 6: Influence of pressure on bubble velocity (UB) (a) inlet pressure, and (b), difference between entrance and back pressure buffer tank.

However the bubble velocity is clearly dependent on the imposed back pressure; the upper points on the graph (blue triangles on Figure 6) were obtained at a higher pressure difference than lower points on the graph (green circles on Figure 6). Figure 6 (b) shows the bubble velocity according to pressure difference between microchannel inlet and back pressure buffer tank along the experiments. The results show that a high pressure gradient causes an increase

(a) (b)

(a) (b)

of bubble velocity but without affecting the bubble length. The reasons behind this observation are still not obvious and are still under investigation.

Dissolution of gaseous CO2 into the continuous ethanol phase

The microfluidic chips were also used to investigate the dissolution of gaseous CO2 into the continuous ethanol phase. To do this, the change in the gas-liquid hydrodynamics along the microchannel is observed for different molar fractions of CO2. Figure 7 (a-d) shows the flow regimes obtained for an increasing ratio of CO2 to EtOH, (with constant EtOH flow rate) at a constant pressure and temperature (43 bar and 40 °C, respectively). For a low CO2/EtOH ratios (Figure 7 (a, b)), it can be seen that the Taylor bubbles formed at the T-junction rapidly decrease in length along the microchannel until all of the CO2 is dissolved in the liquid. The length of microchannel required for complete dissolution increases with increasing CO2/EtOH ratio. For a higher CO2/EtOH ratio (Figure 7 (c)), it can be seen that the Taylor bubbles decrease in length along the channel at a much slower rate than that observed in Figures 7 (a,b). It is expected here that the driving force for dissolution is decreasing along the microchannel as the liquid approaches saturation in CO2. At a high CO2/EtOH ratio, as shown in Figure 7 (d), a regular Taylor flow with relatively long bubbles and short liquid slugs is obtained at the T-junction. The bubble length only decreases slightly along the first lengths of the microchannel and then remains a constant size, without ever totally dissolving. Once equilibrium is achieved, the CO2 no longer dissolves into the liquid and the bubbles stay at the same size. This is in agreement with the CO2-EtOH phase diagram, where at high CO2 composition and 40°C, the CO2-EtOH mixture is in the two-phase region.

Figure 7: Visualization of dissolution of CO2 in EtOH at 40 °C and 43 bar for (a) very low CO2 fraction and (b) low CO2 fraction (single-phase liquid region), (c) establishment of Taylor flow in nearly saturated EtOH, (d) regular Taylor flow under two-phase binary

condition.

Dissolution of supercritical CO2 into the continuous ethanol phase

The dissolution of supercritical CO2 into the continuous ethanol phase was investigated at 90 bar and 40°C. In this state, CO2 enters at conditions above its critical point which could explain a different behavior of flow at the T junction and inside the channel as shown in Figure 8.

(a) (b)

(c) (d)

Figure 8: scCO2-EtOH flow formation at 90 bar and 40 °C, xCO2= 0.63

With CO2 at supercritical condition, mixing is very fast after T-junction and fluids flow in the microchannel in a homogeneous single phase which corresponds to the liquid zone on the phase diagram at high pressure.

At the vicinity of vapor/liquid boundary, behavior of flow presents specificity due to microchannel pressure drop that decreases local pressure. Figure 9 presents a study of hydrodynamics behavior at 40 °C and 82 bar inlet pressure, 78 bar in buffer tank for different flow rate ratios (i.e. different mixture compositions).

After the T-junction of microchannel, fluids result in a cloudy single-phase mixing zone, followed by a two-phase zone where bubbles are formed. These same bubbles are then dissolved further along the microchannel. At constant EtOH flow rate, the length of the mixing zone (indicated by blue arrows in Figure 9) and the two-phase zone (red arrows in Figure 9) are dependent on CO2 flow rate. It appears that the length of the mixing zone increases with the CO2/EtOH ratio. For the intermediate two-phase zone, the corresponding length of channel also seems to increase with increasing CO2 flow rate, although this hypothesis is not clearly identified in Figures 9 (b) and (c).

Figure 9: (a) QmEtOH = 2.3 g/h and QmCO2 = 2.0 g/h (xCO2 = 0.46), (b) QmEtOH = 2.3 g/h and QmCO2 = 4.3 g/h (xCO2 = 0.64), (c) QmEtOH = 2.3 g/h and QmCO2 = 6.0 g/h (xCO2 = 0.72), (d)

QmEtOH = 2.3 g/h and QmCO2 = 8.0 g/h (xCO2 = 0.77)

(a) (b)

(c) (d)

CONCLUSION

High-speed visualization experiments of CO2-EtOH flows in transparent pressure-resistant microchannels have been performed to gain insight into the effects of pressure on two-phase flows. Regular Taylor flow has been achieved whilst in the two-phase zone of the thermodynamic phase diagram. Here, the size of the bubbles and the liquid slugs depend on the operating pressure since the composition of each phase at equilibrium is pressure dependent. Single-phase liquid zone of the phase diagram, the CO2 is fed into the microchannel in either the gaseous state or the supercritical state. The gaseous CO2 dissolved along the channel and the length of microchannel required for total dissolution increased with increasing CO2/EtOH ratio. At higher CO2 fractions, corresponding to the gas/liquid zone of the phase diagram, the total dissolution of CO2 was not observed and bubble size remained constant from a certain length of microchannel. For supercritical CO2, the mixing behavior at the T-junction is different with three identified zones: a cloudy mixing zone followed by a two-phase zone with the creation of bubbles and then a single-phase zone. The lengths of the first zones depend on CO2 flow rate.

This work presents a novel approach to describe high-pressure miscible flows using flow visualization and thermodynamic data to obtain a pressure dependent characterization.

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