Method for monitoring gas bubbles

Abstract

The present invention relates to a method for monitoring gas bubbles in a device comprising a bubble-generating portion and a tube having an inlet and an outlet. The method comprises the steps of: separately feeding a flow of gas and a flow of liquid to the bubble-generating portion; generating individual bubbles of gas in the liquid in the bubble-generating portion; flowing the generated bubbles within the tube, from the inlet to the outlet thereof; wherein the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube; and simultaneously recording images of the bubbles at a plurality of positions along the tube with one or more cameras.

Description

[0001] METHOD FOR MONITORING GAS BUBBLES

[0002] Technical field

[0003] The present invention relates to a method and a device for monitoring gas bubbles. The invention makes it possible to characterize the stability of gas bubbles in a confined environment under dynamic flow conditions.

[0004] Technical background

[0005] Foams made of gas bubbles dispersed in a continuous liquid phase find various applications in different fields, such as Enhanced Oil Recovery (EOR), Carbon Capture, Usage and Storage (CCUS) or soil decontamination. For example, Garstecki et al, Lab Chip, 2006,6, 437-446, and US patent applications US20050172476A1 , US20100172803A1 and US20140037514A1 describe methods for forming emulsion or gas bubbles in a microfluidic device comprising a T-junction.

[0006] In such applications, the foam is injected into a porous medium, and it is important to control the stability of the gas bubbles in such confined environment.

[0007] The stability of the gas bubbles (or the foam stability) refers to the ability of the gas bubbles to maintain their structure. The structure is compromised when at least two individual gas bubbles in a liquid phase merge together to form a larger single entity (which is known as bubble coalescence or bubble merging).

[0008] Bubble coalescence is highly dependent on the gas (volume) fraction in the liquid phase, and the stability of gas bubbles flowing through a porous medium differs from the stability of gas bubbles in a less-constrained medium. The physicochemical parameters controlling foam stability are poorly understood.

[0009] In order to study the stability of gas bubbles in a liquid phase and optimize foam formulation, various methods have been employed to determine a critical gas fraction at which the gas bubbles destabilize (i.e., bubble coalescence occurs).

[0010] For example, volume tests (e.g., Bikerman’s foam test) or porous media tests (e.g., injecting a foam into a 3D model core or a porous medium) are generally used. However, volume tests are often insufficiently predictive and fail to represent the dynamic flow conditions of the foam. The porous media tests are expensive and time-consuming, as each injection is carried out for a single foam fraction, requiring repetitive experiments to accurately determine the critical gas fraction as well as compare critical gas fractions obtained from different formulations.

[0011] Thus, there exists a continuous need for a fast and effective method which makes it possible to monitor gas bubbles in a liquid phase in dynamic conditions so as to characterize their stability.

[0012] Summary of the invention

[0013] It is a first object of the invention to provide a method for monitoring gas bubbles in a device comprising a bubble-generating portion and a tube having an inlet and an outlet, comprising the steps of:

[0014] - separately feeding a flow of gas and a flow of liquid to the bubblegenerating portion;

[0015] - generating individual bubbles of gas in the liquid in the bubblegenerating portion;

[0016] - flowing the generated bubbles within the tube, from the inlet to the outlet thereof; wherein the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube; and

[0017] - simultaneously recording images of the bubbles at a plurality of positions along the tube with one or more cameras.

[0018] In some embodiments, the absolute pressure at the inlet of the tube is from 20% to 10 times higher than the absolute pressure at the outlet of the tube.

[0019] In some embodiments, the absolute pressure at the inlet of the tube is at least 300 mbar higher than the absolute pressure at the outlet of the tube.

[0020] In some embodiments, the internal diameter of the tube is from 100 pm to 1 cm.

[0021] In some embodiments, the length of the tube from the inlet to the outlet is equal to or larger than 500 times the square root of the internal cross-section of the tube.

[0022] In some embodiments, the length of the tube from the inlet to the outlet is at least 1 m, preferably from 2 m to 10 m.

[0023] In some embodiments, the gas comprises or is N2, CO2, and / or air.

[0024] In some embodiments, the liquid is an aqueous solution, a non-aqueous solution, preferably oil or a mixture of oils, or an emulsion of an aqueous solution and a non-aqueous solution. In some embodiments, the liquid further comprises an additive, preferably selected from a surfactant, a co-surfactant, a polymer, an inorganic salt, solid particles (Pickering stabilizers), and combinations thereof.

[0025] In some embodiments, the outlet of the tube is open to the atmosphere, so that the absolute pressure at the outlet of the tube is the atmospheric pressure.

[0026] In some embodiments, the tube is coiled.

[0027] In some embodiments, simultaneously recording images of the bubbles at a plurality of positions along the tube is carried out by recording images with a single camera having an observation field which encompasses the plurality of positions.

[0028] In some embodiments, the flow of gas is fed at a controlled pressure, and preferably at a constant pressure; and wherein the flow of liquid is fed at a controlled flow rate, and preferably at a constant flow rate.

[0029] In some embodiments, the method further comprises measuring the flow rate of gas fed to the bubble-generating portion of the device.

[0030] In some embodiments, gas bubble coalescence occurs along the tube.

[0031] In some embodiments, the gas fraction at the inlet of the tube is at least 15 vol.%, preferably at least 20 vol.%.

[0032] In some embodiments, the method further comprises an initial phase of varying the flow rate of the fed liquid and / or varying the pressure of the fed gas, until bubble coalescence occurs along the tube.

[0033] In some embodiments, the method comprises determining the gas fraction at the plurality of positions based on the recorded images; and preferably comprises determining a critical gas fraction based on the observation of bubble coalescence on the images recorded at one or more of the plurality of positions.

[0034] In some embodiments, the method comprises a step of varying the composition of the liquid.

[0035] In some embodiments, the method is for screening various liquid compositions so as to determine a liquid composition leading to the highest critical gas fraction.

[0036] It is a second object of the invention to provide a device for monitoring gas bubbles, comprising:

[0037] - a bubble-generating portion fed by a gas flow line and a liquid flow line;

[0038] - a tube having an inlet and an outlet, the inlet of the tube being flu idical ly connected to the bubble-generating portion; and

[0039] - one or more cameras for simultaneously recording images at a plurality of positions along the tube; wherein the dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube, assuming a flow of water without gas bubbles at a temperature of 20°C and at a flow rate of 1 mL / min.

[0040] In some embodiments of the device, the bubble-generating portion is a T- junction, a coaxial bubbles generator or a microfluidic flow-focusing device.

[0041] In some embodiments of the device, the dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is from 20% to 10 times higher than the absolute pressure at the outlet of the tube.

[0042] In some embodiments of the device, the dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is at least 300 mbar higher than the absolute pressure at the outlet of the tube.

[0043] In some embodiments of the device, the internal diameter of the tube is from 100 pm to 1 cm.

[0044] In some embodiments of the device, the length of the tube from the inlet to the outlet is equal to or larger than 500 times the square root of the internal crosssection of the tube.

[0045] In some embodiments of the device, the length of the tube from the inlet to the outlet is at least 1 m, preferably from 2 m to 10 m.

[0046] In some embodiments of the device, the outlet of the tube is open to the atmosphere, so that the absolute pressure at the outlet of the tube is the atmospheric pressure.

[0047] In some embodiments of the device, the tube is coiled.

[0048] In some embodiments, the device comprises a single camera having an observation field which encompasses the plurality of positions.

[0049] In some embodiments, the device comprises a pressure controller for feeding gas at a controlled pressure, and preferably at a constant pressure; and comprises a pump for feeding liquid at a controlled flow rate, and preferably at a constant flow rate.

[0050] In some embodiments, the device comprises a flowmeter for measuring the flow rate of gas fed to the bubble-generating portion.

[0051] The present invention enables to address the need mentioned above. In particular, the invention provides a method and a system which make it possible to monitor gas bubbles in a confined environment and characterize the stability of the gas bubbles under dynamic flow conditions quickly and easily in a single experiment.

[0052] This is achieved by flowing generated bubbles within a tube having an inlet and an outlet, wherein the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube, and simultaneously recording images of the bubbles at a plurality of positions along the tube with one or more cameras.

[0053] Such a pressure gradient between the inlet and outlet within the tube (i.e., the pressure drop along the tube) causes the generated bubbles to gradually expand as they move forward (towards the outlet) due to the compressibility of gas. As a result, the gas fraction of the foam varies continuously and linearly along the tube.

[0054] By “foam" is herein meant the medium comprising the liquid and the gas bubbles. It is understood that the foam which is formed and investigated in the device of the invention is representative of a foam which would be obtained in situ in a porous medium, in an application such as EOR or carbon capture.

[0055] Thus, one advantage of the invention is that multiple gas fractions can be tested along the tube at a low cost and with high throughput, thereby allowing the evolution of the foam to be observed in situ a under dynamic flow conditions, and that, when bubble coalescence occurs within the tube, the bubble coalescence can be characterized qualitatively and quantitatively.

[0056] These different gas fractions along the tube, referred to herein as local gas fractions, can be determined based on the recorded images of the bubbles at the plurality of positions along the tube, and then the critical gas fraction can be accurately determined.

[0057] The critical gas fraction corresponds to the maximum viscosity of the foam. When the gas fraction is above or below the critical gas fraction, the viscosity of the foam is less than the viscosity of said foam at the critical gas fraction.

[0058] Thus, another advantage of the invention is that various liquids can be easily and rapidly tested using the method or the system of the invention, and by comparing the critical gas fraction obtained with each liquid, the composition of the liquid can be optimized, leading to enhanced stability of the foam.

[0059] Brief description of the drawings

[0060] Figure 1 schematically illustrates one example of a device of the invention.

[0061] Figure 2 shows one example of the image of gas bubbles along a tube recorded according to the method of the invention, using the device of Figure 1. A to E correspond to successive recording positions on the tube (distance from the tube inlet) of 1 m, 2 m, 3 m, 4 m, and 5 m, respectively. FgAand FgEcorrespond to the gas fraction at the position A and E, respectively.

[0062] Figure 3 shows the variation of the gas fraction along the tube, using the device of Figure 1. The top scheme illustrates the expansion of the gas bubbles along the tube, and the bottom graph shows corresponding gas fractions at different positions of the tube. In the graph, the distance from the tube inlet (m) can be read on the X-axis, and the gas fraction (% in decimal representation) can be read on the Y-axis. The crosses correspond to the determined gas fractions, and the line of best fit is constructed as a straight line. The dotted line corresponds to the critical gas fraction (Fg*).

[0063] Figure 4 shows the variation of the gas flow rate on the Y-axis (pL / min) over time on the X-axis (min) at a given gas pressure while varying the flow rate of liquid. The sudden change of the gas flow rate, indicated by the dotted line, corresponds to the moment when bubble coalescence occurs, which corresponds to the moment when the critical gas fraction (Fg*) is reached.

[0064] Figure 5a to 5d show bubble size distributions across different bubble size ranges. The relative bubble size can be read on the X-axis, and the distribution of the bubbles (%) falling within each size can be read on the Y-axis. Figure 5a to 5d corresponds to liquid flow rates of 1 .1 mL / min, 1 mL / min, 0.6 mL / min, and 0.4 mL / min, respectively. In each figure, a to e correspond to the recording position on the tube (distance from the tube inlet) of 1 m, 2 m, 3 m, 4 m, and 5 m, respectively.

[0065] Detailed description

[0066] The invention will now be described in more detail without limitation in the following description.

[0067] The term “gas fraction” as used herein refers to the ratio of the flow rate of gas to the total flow rate of gas and liquid (gas flow rate / (gas flow rate + liquid flow rate)), the flow rates being volume flow rates.

[0068] The term “critical gas fraction” as used herein refers to the minimum gas fraction at which bubble coalescence occurs in the device.

[0069] The term “(bubble) coalescence” as used herein refers to the merging of at least two bubbles.

[0070] Device for monitoring gas bubbles

[0071] The device for monitoring gas bubbles of the invention comprises:

[0072] - a bubble-generating portion fed by a gas flow line and a liquid flow line;

[0073] - a tube having an inlet and an outlet, the inlet of the tube being fluid ically connected to the bubble-generating portion; and

[0074] - one or more cameras for simultaneously recording images at a plurality of positions along the tube. The dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube. By absolute pressure is means the pressure measured relative to the zero pressure, which is a theoretical vacuum.

[0075] The device of the invention may be a milifluidic or microfluidic device, in other words, the device may operate with volumes in the milliliter or microliter range. In particular, the bubble-generating portion may be sized at the milimeter or micrometer scale.

[0076] In some embodiments, the bubble-generating portion may be a T-junction, a coaxial bubbles generator or a flow-focusing device.

[0077] The device may comprise a valve at the bubble-generating portion.

[0078] The dimensions of the tube may be such that, in use, the absolute pressure at the inlet of the tube is from 10% to 10 times, or 20% to 10 times higher than the absolute pressure at the outlet of the tube.

[0079] For example, the dimensions of the tube may be such that, in use, the absolute pressure at the inlet of the tube is at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, or at least 100% higher than the absolute pressure at the outlet of the tube. As other examples, the dimensions of the tube may be such that, in use, the absolute pressure at the inlet of the tube is at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, at least 8 times, at least 9 times or at least 10 times higher than the absolute pressure at the outlet of the tube. It is preferred that the absolute pressure at the inlet of the tube is less than 50 times higher than the absolute pressure at the outlet of the tube, more preferably less than 20 times higher than the absolute pressure at the outlet of the tube.

[0080] In some embodiments, the dimensions of the tube may be such that, in use, the absolute pressure at the inlet of the tube is at least 300 mbar higher than the absolute pressure at the outlet of the tube.

[0081] For example, the dimensions of the tube may be such that, in use, the absolute pressure at the inlet of the tube is at least 300 mbar, at least 400 mbar, at least 500 mbar, at least 600 mbar, at least 700 mbar, at least 800 mbar, at least 900 mbar or at least 1000 mbar higher than the absolute pressure at the outlet of the tube.

[0082] The above pressure differentials, used to define the dimensions of the tube, may be determined based on a flow of water (without gas bubbles) at a temperature of 20°C and at a flow rate of 1 mL / min.

[0083] In some embodiments, the internal diameter of the tube may be from 100 pm to 1 cm. For example, the internal diameter of the tube may be from 100 pm to 200 pm, from 200 pm to 300 pm, from 300 pm to 400 pm, from 400 pm to 500 pm, from 500 pm to 600 pm, from 600 pm to 700 pm, form 700 pm to 800 pm, from 800 pm to 900 pm, from 900 pm to 0.1 cm, from 0.1 cm to 0.2 cm, from 0.2 cm to 0.3 cm, from 0.3 cm to 0.4 cm, from 0.4 cm to 0.5 cm, from 0.5 cm to 0.6 cm, from 0.6 cm to 0.7 cm, from 0.7 cm to 0.8 cm, from 0.8 cm to 0.9 cm, or from 0.9 cm to 1 cm.

[0084] The length of the tube from the inlet to the outlet may be equal to or larger than 500 times the square root of the internal cross-section of the tube.

[0085] The length of the tube herein refers to the distance of the tube along its longitudinal axis, between the inlet and outlet of the tube. The internal crosssection of the tube herein refers to the internal surface area of the cut made perpendicular to the longitudinal axis of the tube.

[0086] The internal cross-section of the tube may take any suitable shape, for example, circular, rectangular, square, triangular, oval, elliptical, or hexagonal shape. The internal cross-section of the tube is preferably circular.

[0087] When the internal cross-section is not circular, the diameter refers to the maximum dimension of the internal cross-section.

[0088] The length of the tube from the inlet to the outlet may be at least 1 m, preferably from 2 m to 10 m.

[0089] For example, the length of the tube may be from 1 to 2 m, from 2 to 3 m, from 3 to 4 m, from 4 to 5 m, from 5 to 6 m, from 6 to 7 m, from 7 to 8 m, from 8 to 9 m, or from 9 to 10 m.

[0090] In some embodiments, the outlet of the tube may be open to the atmosphere, so that the absolute pressure at the outlet of the tube is the atmospheric pressure.

[0091] In other embodiments, the outlet of the tube is not open to the atmosphere. For example, when a supercritical gas (e.g., supercritical CO2) is used, the tube (and the bubble-generating portion) may be entirely under high pressure, while maintaining the pressure drop along the tube.

[0092] The tube is preferably wettable with the liquid used. The tube may be made of any suitable material that allows for the visualization of the bubbles along the tube, such as fluorinated ethylene propylene, polytetrafluoroethylene plastique, polyvinyl chloride, polyurethane, polyethylene, silicone, or glass.

[0093] The tube is preferably transparent, semi-transparent, or partially transparent. For example, the entire tube may be transparent, semi-transparent or partially transparent, or the tube may be transparent, semi-transparent or partially transparent only at the positions recorded by one or more cameras of the device.

[0094] The visualization of the bubbles flowing along the tube, which has not been possible by conventional porous media tests, allows for the observation of bubble in situ and in a dynamic manner.

[0095] In some embodiments, the tube may be flexible (can be curved, bended or deformed), and / or may be curved. In this case, the longitudinal axis of the tube is a curvilinear axis.

[0096] For example, the tube may be coiled. The tube may for example comprise from 1 to 20 coils, preferably from 2 to 15 coils, more preferably from 4 to 10 coils. Such a coiled tube is advantageous because a single camera can record images at the plurality of positions, as shown in Fig. 1.

[0097] In this case, the device may comprise a single camera having an observation field which encompasses the plurality of positions.

[0098] For example, the observation field may have a size of 2 cm x 2 cm.

[0099] Alternatively, several cameras having distinct observation fields, such as for example one observation field per position, can also be used. In this case, the tube is not necessarily coiled and can be substantially straight.

[0100] The one or more cameras of the device may be any suitable camera used in the domain, especially for milifluidic devices. For example, the camera may be a high-speed camera or a microscope camera.

[0101] The device may further comprise a pressure controller for feeding gas at a controlled pressure.

[0102] The pressure controller may comprise, for example, a pressure regulator or a pressure sensor.

[0103] The pressure controller preferably feeds gas at a constant pressure.

[0104] The device may further comprise a flowmeter for measuring the flow rate of gas fed to the bubble-generating portion.

[0105] When the device comprises a pressure controller and a flow meter, the flowmeter may be positioned between the pressure controller and the bubblegenerating portion.

[0106] The device may further comprise a pump for feeding liquid at a controlled flow rate.

[0107] The pump preferably feeds liquid at a constant flow rate.

[0108] The pump may be, for example, a syringe pump or a peristaltic pump.

[0109] The device may comprise two or more such pumps, especially when several liquids are used (which will be described later in detail). The device may further comprise a thermostat for controlling the temperature of the fed liquid and / or the fed gas.

[0110] One example of the device of the invention is illustrated in Fig. 1.

[0111] The device for monitoring bubbles 100 comprises a bubble-generating portion 1 fed by a gas flow line 2 and a liquid flow line 3.

[0112] The straight arrows refer to the directions of the flow of the gas, liquid or gas bubbles in a liquid phase.

[0113] In this example, the bubble-generating portion 1 is a T-junction, i.e. the gas flow line 2 and the liquid flow line 3 are connected to a single downstream line.

[0114] The device also comprises a tube 4 having an inlet 4’ and an outlet 4”. The tube is coiled in this example. The inlet 4’ is connected to the single downstream line mentioned above.

[0115] The device comprises a single camera 5 having an observation field which encompasses 5 positions A to E (for clarity, Fig. 1 illustrates an enlarged view of the dotted section of the tube).

[0116] The dimensions of the tube 4 are as described above. In this example, the outlet 4” of the tube is open to the atmosphere.

[0117] In this example, the device further comprises a pressure controller 6, a gas flowmeter 7, a pump 8, and a valve 9. The valve 9 can be adapted to close off the flow of gas, or it can be adapted to close off the flow of liquid, or it can be adapted to close off both the flow of gas and the flow of liquid.

[0118] Method for monitoring gas bubbles

[0119] The invention also relates to a method for monitoring gas bubbles in a device comprising a bubble-generating portion and a tube having an inlet and an outlet.

[0120] The device may be as described above.

[0121] As shown in Fig. 1 , the method comprises the steps of separately feeding a flow of gas and a flow of liquid to the bubble-generating portion 1 , generating individual gas bubbles in the liquid in the bubble-generating portion 1 (in Fig. 1, the individual bubbles are schematically shown as elongated shapes), flowing the generated bubbles within the tube 4, from the inlet 4’ to the outlet 4” thereof. The bubbles are preferably generated as a one-dimensional stream within the tube.

[0122] As shown in this figure, the flow of gas may be fed through the gas flow line 2 and the flow of liquid may be fed through the liquid flow line 3.

[0123] The method further comprises simultaneously recording images of the bubbles at a plurality of positions A to E along the tube 4 with one or more cameras 5. By “images” is meant static representations captured at a specific moment intime, or a sequence of frames (a video sequence).

[0124] In the method of the invention, the absolute pressure at the inlet 4’ of the tube is at least 10% higher than the absolute pressure at the outlet 4” of the tube.

[0125] For example, the absolute pressure at the inlet of the tube is at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, or at least 100% higher than the absolute pressure at the outlet of the tube. The absolute pressure at the inlet of the tube may be at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, at least 8 times, at least 9 times or at least 10 times higher than the absolute pressure at the outlet of the tube. It is preferred that the absolute pressure at the inlet of the tube is less than 50 times higher than the absolute pressure at the outlet of the tube, more preferably less than 20 times higher than the absolute pressure at the outlet of the tube.

[0126] For example, the absolute pressure at the inlet of the tube may be at least 300 mbar higher than the absolute pressure at the outlet of the tube; or at least 400 mbar, at least 500 mbar, at least 600 mbar, at least 700 mbar, at least 800 mbar, at least 900 mbar or at least 1000 mbar higher than the absolute pressure at the outlet of the tube.

[0127] The pressure values indicated above, in the context of the method, are actual pressure values in use, which depend on the nature of the foam, on the liquid flow rate, and possibly on temperature.

[0128] The gas fraction Fg at the inlet 4’ is set by the gas and liquid feeding parameters. This gas fraction changes as the pressure drops along the tube, because the generated bubbles gradually expand along the tube due to gas compressibility, as shown in Fig. 2.

[0129] In Fig. 2, the position on the left is closer to the inlet of the tube (highest absolute pressure) while the position on the right is closer to the outlet of the tube (lowest absolute pressure).

[0130] The absolute pressure at the position E is the lowest, and the absolute pressure at the position A is at least 10% higher than the absolute pressure at the position E.

[0131] As shown in Fig. 2, the bubbles gradually expand across positions A to E, resulting in each local gas fraction being different from the global gas fraction.

[0132] The gas fraction varies (increases) along the tube linearly. At the position E (FgE) is greater than the gas fraction at the position A (FgA) (FgE> FgA). For example, the gas fraction FgEmay be at least 25% greater than the gas fraction FgA. Thus, the method of the invention allows for the observation in situ of the bubble behavior in a dynamic manner.

[0133] As shown in this example of Fig. 1 , the tube may be coiled.

[0134] In this case, the step of simultaneously recording images of the bubbles at a plurality of positions along the tube may be carried out by recording images with a single camera having an observation field which encompasses the plurality of positions.

[0135] In this example, the images of the bubbles are recorded with a single camera having an observation field which encompasses the 5 positions A to E (corresponding to the positions on the tube for example at 1 m, 2 m, 3 m, 4 m, and 5 m away from the tube inlet), but in another embodiment, the images may be recorded with two or more cameras.

[0136] For example, the tube may be straight and cameras may be evenly spaced at fixed intervals along its length. Alternatively, the tube may be designed in a meandering or zigzag pattern, and cameras may be placed at specific intervals along the meander (for example, every turn in the meander). Alternatively, the tube may be a straight tube, a meandering tube or a zigzag tube, and cameras may be placed at gradually increasing intervals, or at varying intervals.

[0137] In this example, the gas bubbles at 5 positions are recorded, but any suitable number of positions can be selected for recording. For example, images of the gas bubbles may be recorded at 3, 4, 5, 6, 7, 8, 9, or 10 positions along the tube with one or more cameras.

[0138] The gas fraction at the inlet of the tube may be at least 15 vol.%. For example, the gas fraction at the inlet of the tube may be from 15 to 20 vol.%, from 20 to 30 vol.%, from 30 to 40 vol.%, from 40 to 50 vol.%, or 50 vol.%, to 60 vol.%.

[0139] The internal diameter and the length of the tube may be as described above in relation to the device for monitoring gas bubbles.

[0140] The gas preferably comprises or is N2, CO2, and / or air.

[0141] The gas may be in supercritical form.

[0142] The liquid may be an aqueous solution, preferably water, a non-aqueous solution, preferably oil or a mixture of oils, or an emulsion of an aqueous solution and a non-aqueous solution.

[0143] The liquid may further comprise an additive, preferably selected from a surfactant, a co-surfactant, a polymer, an inorganic salt, solid particles (Pickering stabilizers) and combinations thereof.

[0144] In some embodiments, the absolute pressure at the outlet of the tube may be the atmospheric pressure (the outlet of the tube may be open to the atmosphere). In other embodiments, especially in the case of a supercritical gas (e.g., supercritical CO2), the absolute pressure along the entire tube may be under high pressure, while keeping the pressure drop along the tube.

[0145] In the method, the flow of gas may be fed at a controlled pressure, and preferably at a constant pressure, and the flow of liquid may be fed at a controlled flow rate, preferably at a constant flow rate.

[0146] The pressure may be controlled using a pressure controller, such as a pressure regulator or a pressure sensor.

[0147] The flow rate of liquid may be controlled using a pump, such as syringe pump or a peristaltic pump.

[0148] The method may further comprise measuring the flow rate of gas fed to the bubble-generating portion of the device.

[0149] The flow rate of gas may be measured using a flowmeter.

[0150] In some embodiments, gas bubble coalescence may occur along the tube.

[0151] Thus, the invention makes it possible to study bubble coalescence qualitatively (for example, observation of the coalescence events in detail, the morphology of bubbles before, during, and after coalescence) and / or quantitatively (e.g., frequency of bubble coalescence or size of coalesced bubble based on bubble coalescence statistics).

[0152] The method may further comprise an initial phase of varying the flow rate of the fed liquid and / or varying the pressure of the fed gas, until bubble coalescence occurs along the tube. The occurrence of bubble coalescence may be directly determined on the recorded images or may be determined by monitoring the gas flow rate and detecting a sharp change in the gas flow rate. This is described in more detail in relation with Fig. 4 below.

[0153] The flow rate of the fed liquid may be varied at a given pressure of the fed gas.

[0154] The pressure of the fed gas may be varied at a given flow rate of liquid.

[0155] The steady state (constant value of the gas fraction at a given position along the tube) may be reached before any measurements.

[0156] In the step of separately feeding a flow of gas and a flow of liquid to the bubble-generating portion, the controlled pressure, preferably the constant pressure of the flow of gas and / or the controlled flow rate, preferably the constant flow rate of liquid may be selected such that bubble coalescence occurs along the tube.

[0157] The method may further comprise determining the gas fraction at the plurality of positions based on the recorded images. The step of determining the gas fraction at the plurality of positions based on the recorded images can be performed by image analysis from the recorded images, for example, a video sequence. For the image analysis, a conventional software program for data analysis can be used, such as MATLAB, Imaged, and Origin.

[0158] Using such data analysis, it is therefore possible to, at a given gas pressure and liquid flow rate, scan local volume fractions over a wide range, and thus precisely characterize a set of local gas fractions.

[0159] The method may further comprise determining a critical gas fraction based on the observation of bubble coalescence on the images recorded at one or more of the plurality of positions.

[0160] The bubble coalescence may be observed as a double bubble (a larger bubble resulting from two bubbles merged).

[0161] The bubble coalescence may be observed visually on the recorded images and / or may be detected based on a sharp increase in the flow rate of gas when fed gas pressure increases (or when fed liquid flow rate decreases).

[0162] The method may further comprise a step of varying the composition of the liquid.

[0163] Varying the composition of the liquid may involve changing the concentration of an additive (e.g., an inorganic salt, surfactant, co-surfactant, polymer), or presence of different additive(s).

[0164] In this case, the method may be used for screening various liquid compositions so as to determine a liquid composition leading to the highest critical gas fraction.

[0165] For example, the critical gas fraction of a liquid of a certain composition may be determined according to the method of the invention, then the device may be cleaned with the liquid, and the critical gas fraction of a liquid of a different composition may be again determined according to the method of the invention.

[0166] Alternatively, the composition of the liquid may be continuously changed, by for example switching the liquid flow among different compositions via multiple distinct pumps, and the impact of the variation of liquid composition on critical gas fraction can be directly determined.

[0167] Examples

[0168] The following examples illustrate the invention without limiting it.

[0169] Example 1 : Determination of critical gas fraction The method of the invention was implemented using the device as shown in Fig. 1.

[0170] The experimental conditions were as follows :

[0171] - Pressure of the flow of gas (pressure of the pressure controller): 400 mbar

[0172] - Flow rate of liquid: 1 mL / min

[0173] - Tube length: 5.5 m

[0174] - Tube diameter: 1.55 mm (internal diameter) and 3.18 mm (external diameter)

[0175] - Tube material: fluorinated ethylene propylene

[0176] - Camera: high-speed camera (Photron Fastcam Mini AX100)

[0177] - Recording positions: at 1 m, 2 m, 3 m, 4 m, and 5 m from the tube inlet

[0178] - Pressure gradient between the tube inlet and the tube outlet: 400 mbar

[0179] - Liquid: Aqueous solution of 1 wt.% Tween 20™ surfactant

[0180] - Gas: N2

[0181] As shown in the graph of Fig. 3, the gas fractions at the position 1 m, 2 m, 3 m, 4 m, and 5 m away from the tube inlet were determined (indicated with crosses).

[0182] The critical gas fraction was determined based on the observation of bubble coalescence on the images recorded at these positions.

[0183] In the graph of Fig. 3, the critical gas fraction is indicated by the dotted line, which was around 68%. In other words, bubble coalescence was observed at this gas fraction, about 5 m away from the inlet of the tube.

[0184] Fig. 4 confirms the occurrence of bubble coalescence. In Fig. 4, the method of the invention was implemented under the same conditions as above, except that the flow rate of liquid was varied from 5 mL / min to 0.1 mL / min.

[0185] During the experiment, as the flow liquid rate was varied over time, bubble coalescence was observed as a sudden increase of the flow rate of gas, corresponding to the moment when the critical gas fraction Fg* is reached at some position in the tube. This makes it possible to adjust the liquid and gas feeding conditions and thus the initial gas fraction so that the critical gas fraction is reached at some point along the tube.

[0186] Example 2: Bubble coalescence statistics

[0187] Using the device of Fig. 1 , the distribution of the bubble sizes (the probability of bubble coalescence) was studied, varying the flow rate of liquid.

[0188] The experimental conditions were the same as in Example 1 , except that the flow rate of liquid was changed as follows: - Flow rate of liquid: 1 .1 mL / min, 1 mL / min, 0.6 mL / min and 0.4 mL / min

[0189] The initial gas fraction (gas fraction of the injected gas at 0 m) was determined to be 60% at the liquid flow rate of 1 .1 mL / min, 70% at the liquid flow rate of 1 mL / min, 75% at the liquid flow rate of 0.6 mL / min, and 80% at the liquid flow rate of 0.4 mL / min.

[0190] Fig. 5a demonstrates that the bubbles were almost stable along the entire tube: almost 100% of bubbles at the positions 1 m, 2 m, 3 m, and 4 m (a to d) were stable, although at 4 m (d) and 5 m (e), about 10% of bubbles had a relative size of 2, indicating double bubbles (bubbles each resulting from two merged bubbles), which corresponds to the first occurrence of coalescence (at which the gas fraction is just above the critical gas fraction).

[0191] Fig. 5b demonstrates that the bubbles started to destabilize at 3 m (c), and more double bubbles were observed at downstream positions. A small quantity of triple bubbles (bubbles each resulting from three bubbles merged) was also observed at the position 5 m (e).

[0192] Likewise, Fig. 5c and Fig. 5d demonstrate that bubbles started to destabilize at 2 m (b) and 1 m (a), respectively, and more double bubbles were observed at downstream positions. A small quantity of triple bubbles and quadruple drops (bubbles each resulting from four bubbles merged) was also observed.

[0193] These figures also confirmed the occurrence of bubble coalescence when the gas fraction was greater than the critical gas fraction.

Claims

Claims1. A method for monitoring gas bubbles in a device comprising a bubble-generating portion and a tube having an inlet and an outlet, comprising the steps of:- separately feeding a flow of gas and a flow of liquid to the bubblegenerating portion;- generating individual bubbles of gas in the liquid in the bubblegenerating portion;- flowing the generated bubbles within the tube, from the inlet to the outlet thereof; wherein the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube; and- simultaneously recording images of the bubbles at a plurality of positions along the tube with one or more cameras.

2. The method according to claim 1 , wherein the absolute pressure at the inlet of the tube is from 20% to 10 times higher than the absolute pressure at the outlet of the tube.

3. The method according to claim 1 or 2, wherein the absolute pressure at the inlet of the tube is at least 300 mbar higher than the absolute pressure at the outlet of the tube.

4. The method according to any one of claims 1 to 3, wherein the internal diameter of the tube is from 100 pm to 1 cm.

5. The method according to any one of claims 1 to 4, wherein the length of the tube from the inlet to the outlet is equal to or larger than 500 times the square root of the internal cross-section of the tube.

6. The method according to any one of claims 1 to 5, wherein the length of the tube from the inlet to the outlet is at least 1 m, preferably from 2 m to 10 m.

7. The method according to any one of claims 1 to 6, wherein the gas comprises or is N2, CO2, and / or air.

8. The method according to any one of claims 1 to 7, wherein the liquid is an aqueous solution, a non-aqueous solution, preferably oil or a mixture of oils, or an emulsion of an aqueous solution and a nonaqueous solution.

9. The method according to any one of claims 1 to 8, wherein the liquid further comprises an additive, preferably selected from a surfactant, a co-surfactant, a polymer, an inorganic salt, solid particles, and combinations thereof.

10. The method according to any one of claims 1 to 9, wherein the outlet of the tube is open to the atmosphere, so that the absolute pressure at the outlet of the tube is the atmospheric pressure.

11. The method according to any one of claims 1 to 10, wherein the tube is coiled.

12. The method according to claim 11 , wherein simultaneously recording images of the bubbles at a plurality of positions along the tube is carried out by recording images with a single camera having an observation field which encompasses the plurality of positions.

13. The method according to any one of claims 1 to 12, wherein the flow of gas is fed at a controlled pressure, and preferably at a constant pressure; and wherein the flow of liquid is fed at a controlled flow rate, and preferably at a constant flow rate.

14. The method according to any one of claims 1 to 13, comprising measuring the flow rate of gas fed to the bubble-generating portion of the device.

15. The method according to any one of claims 1 to 14, wherein gas bubble coalescence occurs along the tube.

16. The method according to any one of claims 1 to 15, wherein the gas fraction at the inlet of the tube is at least 15 vol.%, preferably at least 20 vol.%.

17. The method according to any one of claims 1 to 16, further comprising an initial phase of varying the flow rate of the fed liquid and / or varying the pressure of the fed gas, until bubble coalescence occurs along the tube.

18. The method according to any one of claims 1 to 17, comprising determining the gas fraction at the plurality of positions based on the recorded images; and preferably comprising determining a critical gas fraction based on the observation of bubble coalescence on the images recorded at one or more of the plurality of positions.

19. The method according to any one of claims 1 to 18, comprising a step of varying the composition of the liquid.

20. The method according to claim 19, for screening various liquid compositions so as to determine a liquid composition leading to the highest critical gas fraction.

21. A device for monitoring gas bubbles, comprising:- a bubble-generating portion fed by a gas flow line and a liquid flow line;- a tube having an inlet and an outlet, the inlet of the tube being fluidically connected to the bubble-generating portion; and- one or more cameras for simultaneously recording images at a plurality of positions along the tube; wherein the dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is at least 10% higher than the absolute pressure at the outlet of the tube, assuming a flow of water without gas bubbles at a temperature of 20°C and at a flow rate of 1 mL / min.

22. The device according to claim 21 , wherein the bubble-generating portion is a T-junction, a coaxial bubbles generator or a microfluidic flow-focusing device.

23. The device according to claim 21 or 22, wherein the dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is from 20% to 10 times higher than the absolute pressure at the outlet of the tube.

24. The device according to any one of claims 21 to 23, wherein the dimensions of the tube are such that, in use, the absolute pressure at the inlet of the tube is at least 300 mbar higher than the absolute pressure at the outlet of the tube.

25. The device according to any one of claims 21 to 24, wherein the internal diameter of the tube is from 100 pm to 1 cm.

26. The device according to any one of claims 21 to 25, wherein the length of the tube from the inlet to the outlet is equal to or larger than 500 times the square root of the internal cross-section of the tube.

27. The device according to any one of claims 21 to 26, wherein the length of the tube from the inlet to the outlet is at least 1 m, preferably from 2 m to 10 m.

28. The device according to any one of claims 21 to 27, wherein the outlet of the tube is open to the atmosphere, so that the absolute pressure at the outlet of the tube is the atmospheric pressure.

29. The device according to any one of claims 21 to 28, wherein the tube is coiled.

30. The device according to claim 29, comprising a single camera having an observation field which encompasses the plurality of positions.

31. The device according to any one of claims 21 to 30, comprising a pressure controller for feeding gas at a controlled pressure, and preferably at a constant pressure; and comprising a pump for feeding liquid at a controlled flow rate, and preferably at a constant flow rate.

32. The device according to any one of claims 21 to 31 , comprising a flowmeter for measuring the flow rate of gas fed to the bubblegenerating portion.

Images