EP2656921A1 - Electrostatic device for collecting particles suspended in a gaseous medium - Google Patents

Abstract

The device has a collecting chamber (4) comprising a tubular shaped collecting electrode (16). A collecting device comprises a capillary tube (12), where an end (12.1) of the tube terminates upstream of the electrode in a flow direction of a gaseous flow through the chamber. Another end (12.2) is connected to a liquid tank. Polarization units perform polarization of liquid in the tube so as to make a difference in potential between the liquid at the former end of the tube and the electrode to cause corona discharge and spraying of the liquid by electro spray in the direction of the electrode. An independent claim is also included for a portable collecting system.

Description

TECHNICAL FIELD AND PRIOR ART

The present invention relates to an electrostatic device for collecting particles suspended in a gaseous medium, more particularly in air.

The detection and analysis of particles present in ambient air is a major current concern, whether for monitoring the environment with the presence in the ambient air of nanoparticles produced by human activity. health with a clear need to protect populations from airborne pathogens (legionella, influenza, etc.) and safety issues (detection of biological attacks).

Devices for electrostatic capture of airborne particles, called electrofilters, serve for example to purify the air. There are also electrostatic devices for collecting and analyzing particles. These devices show a great efficiency in the collection of submicron particles.

Among these devices, some rely on the use of an intense electric field to create a corona discharge effect; they are commonly called electrostatic precipitators or electrostatic precipitators.

An electrostatic precipitator (ESP) is an apparatus that collects particles present in a gas by applying an electric field on a trajectory of particles suspended in this gas. More precisely, this high electric field (several thousands to tens of thousands of volts per centimeter in the vicinity of the discharge electrode) is induced by two electrodes arranged close to each other: a first polarized electrode or electrode of discharge, generally in the form of wire or tip, being disposed facing a second electrode, the latter being in the form of a counter-electrode, generally of planar or cylindrical geometry. The electric field existing between the two electrodes ionizes the volume of gas located in the inter-electrode space, and in particular a sheath or ring of ionized gas located around the discharge electrode. This phenomenon is called corona discharge. The charges created, migrating towards the counter electrode, charge the particles to be separated contained in the gas. The charged particles thus created migrate to the counter electrode, where they can be collected. This counterelectrode is usually called a collection electrode. Due to the level of the required electric field, it is necessary to use a discharge electrode which has a very small radius of curvature. The discharge electrodes encountered are therefore generally either spikes or wires.

Electrostatic precipitators use high voltages to generate the corona discharge.

In addition, an electrostatic precipitator comprises means for driving environmental air through the device and means for transferring particles from a gaseous medium to an aqueous or culture medium.

Such a device is for example described in the document WO 2007/012447 . The discharge electrode is formed by a wire disposed within the cylindrical counter-electrode. A tube provides a steam supply between the discharge electrode and the counter electrode. A pump is provided to cause the air and aerosol mixture through the device. This device therefore requires an external pump that does not use high voltage, as well as a steam supply.

Devices using the electrospray method have also been described for example in the document " An electrospray-based, ozone-free air purification technology, Gary Tepper and Al, Journal of Applied Physics, 102, 113305 (2007). ). The electrospray process makes it possible to spray a liquid into fine droplets. A device implementing this method comprises two electrodes, formed for one by a capillary bringing the liquid to be sprayed and for the other by a generally flat counter-electrode. The drops thus formed are electrically charged and are dispersed in the air containing the particles to be collected, they transfer their charge to the polar particles which can then be attracted and then collected by the counter-electrode. This device comprises a fan for circulating the air through the device. This fan does not use high voltage.

These devices have a large space requirement due to the need to use a fan or a pump for the circulation of air. In addition, they require the use of high voltages either to generate the corona discharge, or to implement the electrospray process, and another source of voltage to supply the pump or the fan.

In addition, in the case of the collection of airborne particles for analysis, the particles generally collected on the surface of an electrode must be recovered. For this, a culture medium can be deposited on the surface of the collection electrode or downstream of the latter.

STATEMENT OF THE INVENTION

It is therefore an object of the present invention to provide a device for collecting particles contained in a reduced bulk gas and simplified embodiment compared to devices of the state of the art.

The previously stated purpose is achieved by a collection device, whose liquid supply is configured to generate both the corona discharge and the electrospray process.

The corona discharge charges the particles to be separated for capture on the collection electrode. It also allows the generation of an ionic wind allowing the entrainment of air through the device.

The electrospray process generates charged droplets, which enhances the capture efficiency. It also ensures the wetting of the electrode, which makes it possible to retain the particles on the surface of the collection electrode, or, beyond a certain flow rate, the particles run off along the collection electrode. for evacuation or analysis.

Thus, the collection device no longer requires a pump or fan to drive air or more generally gas through the device, it also does not require special means to ensure the transfer of the medium gaseous in the liquid medium. In addition, the means for generating the corona discharge and the means for spraying the liquid by electrospray both use high voltages.

In addition, the small size of the device makes it compatible with portable use.

Thus, the means used to generate the corona discharge and those to obtain the electrospray are merged, and the drop of polarized liquid located at the end of the capillary for electrospray spraying forms the tip of the electrode. discharge for the crown discharge.

In other words, a particle collecting device is used which, as sole motive force, uses the electric force to perform the gas entrainment, particle collection and gas phase transfer functions to the aqueous phase of the reactor. sample. For this purpose, the corona discharge and the electrospray are combined, which makes it possible to offer a simplified collection device that does not require auxiliary modules, such as a pump, a fan, etc. necessary for the operation of the collection devices of the state of the art.

Advantageously, a decontamination function of the electrode can also be performed outside the particle collection phases, for this purpose a liquid capable of decontaminating the surface of the taper-electrode is sprayed with electrospray.

• une chambre de collecte, comportant une électrode de collecte,
• des moyens d'admission du flux gazeux dans la chambre de collecte,
• au moins un tube capillaire dont une première extrémité débouche en amont de l'électrode de collecte dans le sens d'écoulement du flux gazeux à travers la chambre de collecte et une deuxième extrémité est destinée à être reliée à un réservoir de liquide,
• des moyens de polarisation dudit liquide dans le tube capillaire, de sorte à imposer une différence de potentiel entre le liquide à ladite première extrémité du tube capillaire et l'électrode de collecte, pour provoquer la décharge couronne et la pulvérisation du liquide par électrospray en direction de l'électrode de collecte.

The subject of the present invention is therefore a device for collecting particles in a gas stream comprising:

• a collection chamber, comprising a collection electrode,
• means for admitting the gas flow into the collection chamber,
• at least one capillary tube whose first end opens upstream of the collection electrode in the direction of flow of the gas stream through the collection chamber and a second end is intended to be connected to a liquid reservoir,
• means for biasing said liquid in the capillary tube, so as to impose a potential difference between the liquid at said first end capillary tube and the collection electrode, to cause the corona discharge and spraying of the liquid by electrospray towards the collection electrode.

The collection device may also include means for evacuation of said gas stream from the collection chamber, said discharge means being located downstream of said collection electrode.

In an advantageous example, the collection chamber is cylindrical, and the collection electrode has a corresponding cylindrical shape. The collection chamber may then be tubular and the collection electrode may have an annular shape whose inner diameter is substantially equal to the inner diameter of the collection chamber.

The ratio of the distance between the first end of the capillary tube and the collection electrode on the inside diameter of the collection electrode is advantageously in the range [0.5; 0.75], advantageously equal to 0.56.

For example, the collection chamber comprises a tubular side wall and two bottoms forming longitudinal ends, and the admission means are formed by orifices passing through the side wall on the side of a first longitudinal end and the evacuation means are formed in the bottom located at a second longitudinal end.

The voltage difference applied between the liquid at the first end of the capillary tube and the collection electrode is in the range [8 kV; 10 kV].

In an exemplary embodiment, the inner surface of the capillary tube is advantageously at least partly made of electrically conductive material and forms the polarization means of the liquid it contains.

In another embodiment, the capillary tube is made of electrical insulating material and the biasing means are formed by a biasing electrode located inside the capillary tube.

In another embodiment, the biasing means are located upstream of the capillary tube.

The device may advantageously comprise means for decontaminating the collection electrode formed by the capillary tube by spraying, said capillary tube being able to be connected by its second end to a reservoir of a decontamination liquid suitable for be sprayed with electrospray, for example bleach.

According to an additional characteristic, the collection electrode may be formed by a biological culture medium for the collected particles.

According to another additional feature, the collection device may comprise a plurality of parallel capillary tubes. Preferably, the collection device then comprises a deflector surrounding the ends of the capillary tubes opening into the collection chamber, said deflector being intended to guide the droplets formed by electrospray. For example, the deflector is formed by a metal ring at the same potential as the liquid.

The present invention also relates to a collection system comprising a collection device according to the invention and high voltage supply means for applying the voltage difference or differences.

This system can be advantageously portable.

The system may include an ion wind generator connected to said collection chamber for increasing the flow rate of gas flowing through the collection chamber, said ion wind generator including a discharge electrode and a counter electrode.

The present invention also relates to a collection and analysis system comprising a collection system according to the invention and means for analyzing the particles captured by the collection electrode, said analysis means being located downstream of said collection electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une représentation schématique d'un exemple de réalisation d'un dispositif de collecte,
• la figure 2A est une représentation graphique du débit Db d'air à travers le dispositif en fonction de la différence de tension V appliquée entre l'électrode de décharge et l'électrode de collecte pour une solution aqueuse comportant un tampon salin de PBS (Phosphate Buffered Saline)1X et un surfactant de type Triton X100 à 0,1 %, pulvérisée et pour de une solution aqueuse de Chlorure de Sodium pulvérisée,
• la figure 2B est une représentation graphique du courant I à travers le dispositif en fonction de la différence de tension V appliquée entre l'électrode de décharge et l'électrode de collecte pour une solution aqueuse comportant un tampon salin de PBS (Phosphate Buffered Saline)1X et un surfactant de type Triton X100 à 0,1 %, pulvérisée et pour de une solution aqueuse de Chlorure de Sodium pulvérisée,
• la figure 3A est une représentation graphique du débit Db d'air à travers le dispositif en fonction de la différence de tension V appliquée entre l'électrode de décharge et l'électrode de collecte pour différentes valeurs du rapport G/D,
• la figure 3B est une représentation graphique du courant I à travers le dispositif en fonction de la différence de tension V appliquée entre l'électrode de décharge et l'électrode de collecte pour différentes valeurs du rapport G/D,
• la figure 4 est une représentation schématique d'un exemple de réalisation d'une électrode de décharge à plusieurs capillaires,
• la figure 5 est un cliché de l'ionisation de l'air au bout de la goutte d'eau, à l'extrémité du capillaire, formant alors une électrode de décharge en forme de pointe.

The present invention will be better understood with the aid of the description which follows and the drawings in appendices in which:

• the figure 1 is a schematic representation of an exemplary embodiment of a collection device,
• the Figure 2A is a graphical representation of the flow rate Db of air through the device as a function of the voltage difference V applied between the discharge electrode and the collection electrode for an aqueous solution comprising a salt buffer of PBS (Phosphate Buffered Saline) 1X and a 0.1% Triton X100 surfactant sprayed and for an aqueous solution of Sodium Chloride spray,
• the Figure 2B is a graphical representation of the current I through the device as a function of the voltage difference V applied between the discharge electrode and the collection electrode for an aqueous solution comprising a salt buffer of PBS (Phosphate Buffered Saline) 1X and a 0.1% Triton X100 surfactant, sprayed and for an aqueous solution of Sodium Chloride,
• the figure 3A is a graphical representation of the flow rate Db of air through the device as a function of the voltage difference V applied between the discharge electrode and the collection electrode for different values of the G / D ratio,
• the figure 3B is a graphical representation of the current I through the device as a function of the voltage difference V applied between the discharge electrode and the collection electrode for different values of the G / D ratio,
• the figure 4 is a schematic representation of an exemplary embodiment of a multi-capillary discharge electrode,
• the figure 5 is a snapshot of the ionization of the air at the end of the drop of water, at the end of the capillary, then forming a tip-shaped discharge electrode.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

On the figure 1 an exemplary embodiment of a collection device according to the invention can be seen schematically.

The following description takes as an example the collection of particles contained in the air, also known as airborne particles. It will be understood that the invention applies to the collection of particles contained in any gaseous medium.

The device comprises a body 2 formed in the example represented by a tube, delimiting a collection chamber 4, admission means 6 of the air in the chamber 4 and air evacuation means 8 of the air. bedroom 4.

The tube 2 has a longitudinal axis X; and is provided with an upstream longitudinal end 2.1 and a downstream longitudinal end 2.2. The terms "upstream" and "downstream" are considered in relation to the direction of flow of the treated gas through the device which is symbolized by the arrow F, the treated gas flowing from upstream to downstream.

The device also comprises means for generating a corona effect inside the chamber 4, and means for spraying a liquid by electrospray, hereinafter referred to as "electrospray means".

In the example shown and particularly advantageously, the means 10 for generating the corona discharge and the electrospray means are merged. We will designate these means by collecting means 10.

The collection means 10 comprises a capillary tube 12 arranged, in the example shown, coaxial with the X axis and mounted through a bottom 14 of the upstream end 2.1 of the tube 2.

The capillary tube 12 has a downstream end 12.1 opening into the chamber 4 and an upstream end 12.2 intended to be connected to a liquid supply. The liquid is intended to be sprayed with electrospray. The liquid supply is for example obtained by means of a syringe pump or a pump.

The drop of liquid 20 present at the downstream end 12.1 of the capillary tube 12 forms the tip of a discharge electrode.

The capillary tube 12 may be mounted movably along the axis X so as to allow axial adjustment of the position of its downstream end 12.1 with respect to a collection electrode 16 which will be described below. The discharge electrode is located upstream of the collection electrode.

The collection means also comprise means for polarizing the liquid flowing in the capillary tube 12 for the purpose of spraying it. In the example shown, the biasing means are formed directly by the capillary tube 12 which is made of electrically conductive material and which is connected to a source of voltage. This embodiment has the advantage of further reducing the number of elements used in the invention.

Advantageously, the capillary tube is connected to ground to avoid any short circuit with external elements.

Alternatively, it can be provided that the capillary tube 12 is made of electrical insulating material and that an electrode connected to the voltage source is disposed inside the tube upstream or at the end 12.1 of the tube. The electrode is for example in the form of a wire extending along the axis of the capillary tube 12 or is fixed on the inner wall of the capillary tube. In another variant, it is possible to polarize the liquid before it enters the capillary tube 12.

The collection means also comprise a counter electrode 16, also called the collection electrode, disposed downstream of the downstream end 12.1 of the capillary tube 12. The collection electrode 16 is hollow. It extends along the direction of flow and comprises in this direction a first end and a second end, the first and second ends being located downstream of the end 12.1 of the capillary tube 12.

Counter-electrode 16 has, in the example shown, the shape of circular section cylinder mounted in the tube 2. Advantageously, the inside diameter of the counter electrode 16 is substantially equal to the inside diameter of the body 2 to reduce discontinuities in diameter in the path of the air flow. The inner surface of the chamber 4 is therefore substantially continuous. Preferably, the shape of the collection electrode corresponds to at least a portion of the inner surface of the tube.

The inner surface 16.1 of the counter-electrode 16 forms the particle collecting surface. The counter electrode is connected to a high voltage source.

The counter electrode 16 may take different forms of a cylinder of revolution. It may in particular be in the form of one or more plates, between which open capillaries. It may still have the shape of a cylinder portion, such as a half-cylinder.

The admission means 6 comprise orifices formed in the tube between the upstream end 2.1 of the tube and the downstream end of the capillary tube 12.

The evacuation means are located opposite intake means 6 with respect to the downstream end 12.1 of the capillary tube 12.

The relative position of the edge of the drop of liquid 20 located at the downstream end 16 is such that the distance G separating the downstream end 12.1 of the capillary tube and the upstream end of the counter-electrode is non-zero.

The operation of this device will now be described.

Liquid, for example water, is injected into the capillary tube 12 by means of a syringe pump, a drop of liquid 20 is then formed at the downstream end 12.1 of the capillary tube 12.

A high voltage is then applied to the counter electrode 16, while the capillary tube 12 is grounded. The drop 20 is polarized since the capillary tube is electrically conductive. The capillary 12 forms a needle, at the end of which the drop 20 forms a tip, having a strong curvature (or a small radius of curvature). A corona discharge then appears in the vicinity of the drop 20 when the electric field reaches a critical value, this discharge at the end of the drop 20 is visible on the plate of the figure 5 . In known manner, the corona discharge generates a pocket of ionized gas in the vicinity of the discharge electrode. A unipolar wind of ions and charged particles develops at the drop towards the counter-electrode 16 under the effect of the Coulomb force. The entrainment of the air is done by transfer of momentum between these charged particles and the neutral particles and molecules of the air.

Because of the position of the drop 20 at a distance and upstream of the counter-electrode 16, there is indeed a drive of the air of the admission means 6 towards the evacuation means 8, in other words of the upstream downstream. Thus, the particles are entrained towards the counter-electrode 16. There then appears a phenomenon of suction of the air towards the inside of the chamber 4 and a flow according to the arrow F. The discharge electrode and the electrode collectors 16 thus form an air flow generator.

The ions produced by the discharge, migrating towards the counter electrode, charge the particles to be separated. The charged particles thus created migrate then to the counter-electrode 16, on which they can be collected.

Simultaneously, the drop of liquid 20 at the downstream end of the capillary tube 12, which forms the tip of the discharge electrode, is subjected to electrostatic forces which tend to tear it out of the tube, thereby forming an electrospray. The drop 20 is torn off the downstream end 12.1 of the capillary tube 12 when the electrostatic forces surpass the capillary forces, the latter tending to maintain the liquid in the capillary. Thus, the electrostatic forces deform the drop to tear it from the downstream end 12.1 of the capillary 12. The drop 20 is then sprayed in droplets of micrometric or nanometric sizes towards the collection electrode 16. The droplets thus formed are electrically charged and are dispersed in the air containing the particles to be collected, they capture the particles circulating in the interelectrode space. The latter are then driven to the collection electrode 16 and then collected by it.

The droplets then impact the collection electrode 16, which has the effect of wetting the surface of the collection electrode 16 and forming a film of liquid on the electrode, which ensures the transfer of the particles of the gas phase in the liquid phase.

In addition, the fact that the collection electrode 16 is wetted improves the capture of the particles. Indeed, it prevents them from being retrained by the air flow.

Moreover, when the flow rate is sufficient, this liquid film makes it possible to recover the particles by runoff along the collection electrode, for the purpose of their analysis or their evacuation.

Finally, the liquid film can serve as a culture medium for biological particles.

For example, in the case of the use of the device for the capture of potentially pathogenic biological agents in the air (viruses, bacteria, etc.), a liquid intended to be sprayed is used, a liquid favorable to the survival of microorganisms. organisms, such as, for example, an aqueous solution comprising a 1X PBS saline buffer and a 0.1% Triton X surfactant.

The collection of the particles is therefore carried out both by corona and the droplets formed by electrospray. The production of ozone is reduced compared to a collection device using only the corona discharge.

The combination of these two effects is particularly advantageous since, in addition to the collection of particles, it makes it possible to generate a flow of air through the device, without any additional module, and to ensure the transfer of the gaseous phase to the liquid phase.

We thus obtain a fully integrated device.

In a preferred manner, the potential difference applied is between 8 kV and 10 kV.

For a potential difference of between 8 kV and 10 kV, the sprayed droplets are sufficiently deflected to reach the collection electrode and collapse on the inner surface of the collection electrode and form a liquid film on the inner surface of the collection electrode. This potential range ensures maximum particle collection.

In the case of a potential difference that is too low, for example between 3 kV and 8 kV, droplets are not sufficiently deflected, they pass through the collection electrode and collapse on the inside wall of the downstream tube. the collection electrode. These particles are not collected.

Considering D the inside diameter of the collection electrode 16. The ratio G / D is greater than or equal to 0.2, preferably greater than 0.5. Preferably, the ratio G / D is such that: 0.5 ≤ G / D ≤ 0.75, and more preferably G / D is close to 0.5, for example equal to 0.56. These G / D ratio values are favorable to the appearance of an exploitable ionic wind.

If the ratio L / R is chosen too low, for example less than or equal to 0.49, the deflection of at least a portion of the droplets may be too great, they then crash on the upstream end of the collection electrode, the liquid can accumulate. This liquid then forms an extension of the collection electrode towards the discharge electrode, which may have the effect of reducing the distance between the discharge electrode and the collection electrode, which can cause the formation electric arcs. The length of the drop is therefore taken into account in the dimensioning of the device.

In the example shown and preferably, the liquid end of the capillary tube forms both the discharge electrode for the corona discharge and ensures the electrospray effect.

Advantageously, the device according to the invention comprises means for decontaminating the collection electrode between two capture cycles, for example between two cycles of pathogen capture, thus making the device reusable.

Particularly advantageously, the capillary tube 12 and the electrospray effect are used to water the inner wall of the collection electrode 16 with a suitable fluid, for example bleach, which is a highly conductive aqueous solution.

Decontamination with bleach can be carried out very simply at the same operating point as the collection, by applying the same values of voltage difference, the same liquid flow and the same distance G. For example, a solenoid valve is located between the upstream end 12.2 of the capillary tube and controls the supply of fluid depending on the desired cycle, either to perform a collection cycle, or to perform a cycle of decontamination.

On the Figure 2A , the variation of the flow rate Db within the collection device according to the invention can be seen having a G / D ratio of 0.56 as a function of the voltage V applied to the collection electrode (the discharge electrode being the mass), the flow rate results from the flow obtained by the corona discharge and the structure according to the invention. The measurements are made in the case where the liquid is PBS 1X + 0.1% Triton X (curve I) and in the case where the liquid is salt water (curve II). Thanks to the invention, a flow rate of more than 2 l / min is obtained for a voltage at the collection electrode greater than or equal to 8kV.

On the Figure 2B it is possible to see the variation of the current I within the collection device according to the invention as a function of the voltage V applied to the electrode collection. The measurements are made in the case where the liquid is PBS 1X + 0.1% Triton X (curve I ') and in the case where the liquid is salt water (curve II').

Salt water and PBS 1X + 0.1% Triton X are two highly electrically conductive liquids. It is observed thanks to these measurements that the device is very robust to the change of liquid as soon as it is highly electrically conductive. The device can therefore be used with a large number of liquids, which makes its field of application very wide in terms of particles that can be collected.

• en polarisation du dispositif à 10 kV,
• pour un rapport G/D = 0,56,
• un débit de liquide de 5 µl/min,

on obtient un débit d'air de 4,2 l/min, une efficacité de capture moyenne de 99,99 %, et un arrosage de la paroi interne de l'électrode de collecte, pour une consommation électrique de 400 mW, ce qui est très faible.For example, for a device having the structure of the figure 1 and whose body measures 100 mm and the inner diameter is equal to 10 mm, as well as the inner diameter of the collection electrode, it has been measured that, by choosing the following operating point:

• in polarization of the device at 10 kV,
• for a ratio G / D = 0.56,
• a liquid flow rate of 5 μl / min,

an air flow rate of 4.2 l / min, an average capture efficiency of 99.99%, and a watering of the inner wall of the collection electrode, for a power consumption of 400 mW, are obtained. is very weak.

Thanks to the invention, high air entrainment, efficient capture and sufficient watering of the collection electrode can be achieved simultaneously with low power consumption while avoiding arcing.

On the Figures 3A and 3B the flow and current variations can be seen as a function of the applied voltage for different G / D ratios. More G / D is small, so the smaller G is at constant diameter, the higher the flow Db is important. On the other hand, we note that for the ratio G / D = 0.49 the electrical consumption is appreciably greater. Moreover, when G / D = 0.49, the curve of the figure 3B does not have a measuring point at 10 kV because electric arcs are formed for voltages higher than 9.5 kV because of the proximity between the end of the capillary and the collection electrode.

Nevertheless, the collection device according to the invention operates with a G / D ratio of 0.49 and a voltage difference of less than or equal to 9.5 kV.

The following Tables T1 and T2 show the sensor efficiency of the device according to the invention. For this purpose, an isokinetic sampling rod connected to a particle counter is used, the assembly being placed downstream of the collection electrode and two measurement series are carried out.

A measurement series is performed with the collection device turned off and a series of measurements is performed with the collection device in operation.

où η est l'efficacité de collecte, N_On le nombre de particules sortant du dispositif de collecte en fonctionnement et N_Off le nombre de particules dans le dispositif de collecte éteint. Il est à noter que lorsque le dispositif de collecte est éteint, l'entrainement d'air est produit uniquement par l'aspiration du compteur de particules au débit de 1,2 l/min. Des valeurs de N_On et N_Off pour quelques gammes de tailles de particules sont données dans le Tableau T1. Compte tenu du faible volume d'air contenu dans le dispositif (environ 6 cm³), on peut raisonnablement estimer que N_Off ≈ N ₀ où No est la concentration de l'aérosol dans l'air ambiant. Tableau T1 Nombre moyen de particules par litre d'air pour le dispositif de collecte éteint (N_Off) et en fonctionnement (N_On) Tension appliquée à l'électrode de collecte Etat du dispositif de collecte Nombre moyen de particules par litre de d'air collecté la canne ayant une taille comprise entre 0.25-0.28 µm 0.35-0.40 µm 0.70-0.80 µm 3.5-4 µm 10 kV N_Off 136192 625925 405650 2900 N_On 7 8 0 0 7 kV N_Off 127727 406263 351969 2670 N_On 470 247 331 0 4 kV N_Off 127727 180183 232885 827 N_On 470 394 531 1 The capture efficiency is calculated from the ratio of the number of particles passing through the collection device when it operates and when it is not operating, considering that the concentration of particles in the air is constant. $$\mathrm{\eta }=1-\frac{{\mathit{NOT}}_{\mathit{We}}}{{\mathit{NOT}}_{\mathit{off}}}$$

where η is the collection efficiency, N _is the number of particles leaving the collecting device in operation and N _off the number of particles in the collection device extinguished. It should be noted that when the collection device is switched off, the air entrainment is produced solely by the aspiration of the particle counter at the rate of 1.2 l / min. Values of N _On and N _Off for some ranges of particle sizes are given in Table T1. Given the small volume of air contained in the device (about 6 cm ³ ), it is reasonable to assume that N _Off ≈ N ₀ where No is the concentration of the aerosol in the ambient air. <b> Table T1 Average Number of Particles per Liter of Air for Collector Off <i> (N <sub> Off </ sub>) </ i> and in Operation <i> (N <sub> On </ sub>) </ i></b> Voltage applied to the collection electrode State of the collection device Average number of particles per liter of air collected cane having a size between 0.25-0.28 μm 0.35-0.40 μm 0.70-0.80 μm 3.5-4 μm 10 kV N _Off 136192 625925 405650 2900 N _On 7 8 0 0 7 kV N _Off 127727 406263 351969 2670 N _On 470 247 331 0 4 kV N _Off 127727 180183 232885 827 N _On 470 394 531 1

Table T2 summarizes the efficiencies for some ranges of particle diameter collected. Efficiency values similar to those presented in this table T2, that is to say very close to 1, are also recorded for the other measurement channels. <b> Table T2 Some Capture Efficiency Values </ b> Voltage applied to the collection electrode Measured airflow (l / min) Total air flow (L / min) 0.25-0.28 μm Efficiency (%) 0.35-0.40 μm Efficiency (%) 0.70-0.80 μm Efficiency (%) 3.5-4 μm Efficiency (%) Average efficiency (%) 10 kV 3.7 ± 0.2 4.9 ± 0.2 99.99 99.99 100 100 99.99 7kV 1.8 ± 0.2 3 ± 0.2 99.83 99.93 99,90 100 99.93 4kV 0.2 ± 0.2 1.4 ± 0.2 99.63 99.78 99.77 99.9 99.83

The results obtained show a slight dependence of capture efficiency on particle size: the smaller these are, the more difficult they are to capture even if the efficiency remains close to 100%.

The combination of an electrospray and a corona discharge achieves optimal collection efficiency.

In Table T2, for each voltage, two air flows are given.

The flow measured is that indicated by a flow meter downstream of the rod. The total flow rate is equal to the sum of the flow rate measured by the flowmeter and the suction flow rate of the particle counter (1.2 l / min). The produced airflow and the capture voltage are coupled as shown by the figure 3A so that at 4 kV, potential for which the corona discharge does not appear, the air entrainment produced by the collection device according to the invention is almost zero. These results show the effect of the collection device according to the invention on the entrainment of air through the collection chamber.

In another embodiment, the device may include a plurality of capillary tubes to increase the flow of liquid sprayed on the inner wall of the collection electrode. This allows runoff of the particles collected along the collection electrode 16.

Preferably, in the embodiment using several capillaries, the device comprises a deflector guiding the sprayed drops to the collection electrode.

On the figure 4 , we can see an example of this deflector 22. It is a metal ring disposed around the capillary tubes 12 at their downstream ends 12.1. The deflector 22 is at the same electrical potential as the tips of the capillaries.

Since the deflector 22 is polarized in the same way as the ends of the capillaries, field lines are formed between this deflector 22 and the counter electrode 18. These field lines act as an electrostatic channel and force the droplets to move towards the counter-electrode 16. In the absence of such a deflector, the drops, which have the same polarity, would tend to repel each other, which would give a divergent beam of drops. With the deflector, the field lines formed by between the deflector and the counter-electrode exert a repulsive force on the drops, so that the drops are held in the conical envelope formed by the field lines.

The collection electrode 16 is, in the example shown, formed by a metal tube. Preferably, the edges of the longitudinal ends are rounded to reduce the risk of generating electric shocks. This collection electrode may be made of any conductive material, such as a metallic material or non-metallic materials such as a gel or a conductive membrane whose electrical potential is fixed by the electrical means accompanying the device. These supports non-metallic may be for example a biological culture medium, so the electrode is directly favorable to the culture of microorganism.

The use of such supports is for example described by M. Sillanpää et al. (2007) M. Sillonpää, MD Geller, HC Phuleria, C. Sioutas, High collection efficiency electrostatic precipitator for in vitro cell exposure to concentrated ambient particulate motter (PM), Aerosol Science 39 (2007) pp. 335-347 .

The collection device according to the invention is particularly suitable for the use of collection electrode directly forming a culture medium. Indeed the electrospray spraying obtained by virtue of the invention advantageously ensures the humidification of the culture medium during the collection to avoid drying out and increases the total duration of sampling. It has been found that the drying of the culture medium limits the total duration of sampling, generally to about ten minutes.

The collection electrode may also consist of an annular support or electrically conductive cylinder and covered with a thin electrical insulator, typically less than 500 μm thick.

The liquid ejected by electrospray is for example water. Preferably, in the case of microorganism collection it is an aqueous solution favorable to survival and / or the culture of microorganisms such as for example saline, a solution of phosphate buffered saline (buffered phosphate saline or PBS), an aqueous solution containing at least one antioxidant.

In another embodiment, it is conceivable to put in series a collection device according to the invention and an annular tip-electrode ion wind generator. In the latter, the discharge electrode may have the shape of one or more points, in order to increase the flow of air entrained.

The collection device can of course be associated with means for analyzing the collected particles and being in the liquid phase on the collection electrode. The analysis means generally used in the field of analysis of airborne particles are suitable and will not be described in detail here.

Thus, thanks to the invention, there is obtained a collection device with a high compactness and extremely low power consumption, for example of the order of 400 mW. This one can thus be integrated in a portable case of the order of 10 cm ³ adapted to a nomadic use for the detection of pathogens in the most diverse contexts: hospital, industrial (biomedical, agronomy ...) or for the fight against bioterrorism.

Claims (21)

Device for collecting particles in a gas flow comprising: a collection chamber (4), comprising a collection electrode (16) of tubular shape, inlet means (6) for the gas flow in the collection chamber (4),
said collection device also comprising at least one capillary tube (12) having a first end (12.1) opening upstream of the collection electrode (16) in the direction of flow of the gas flow through the collection chamber and a second end (12.2) is intended to be connected to a liquid reservoir, means for polarizing said liquid in the capillary tube (12), so as to impose a potential difference between the liquid at said first end (12.1) of the capillary tube (12) and the collection electrode, to cause the discharge crown and spraying the liquid by electrospray towards the collection electrode. A collection device according to claim 1, wherein the first end (12.1) of the capillary tube opens at a non-zero distance (G), in the direction of gas flow, from an upstream end of the collection electrode. . Collection device according to claim 2, wherein the ratio of the distance (D) between the first end (12.1) of the capillary tube (12) and the upstream end of the collection electrode (16) on the inside diameter ( D) of the collection electrode (16) is greater than or equal to 0.2. A collection device according to claim 2, wherein the ratio of the distance (D) between the first end (12.1) of the capillary tube (12) and the upstream end of the collection electrode (16) on the inside diameter (D) of the collection electrode (16) is greater than or equal to 0.5. Collection device according to claim 2, wherein the ratio of the distance (D) between the first end (12.1) of the capillary tube (12) and the upstream end of the collection electrode (16) on the inside diameter ( D) of the collection electrode (16) is in the range [0.5; 0.75]. A collection device according to one of claims 1 to 5, wherein the collection chamber (4) is tubular and the inner diameter (D) of the collection electrode is substantially equal to the inside diameter of the collection chamber (4). ). Collection device according to one of claims 1 to 6, comprising means (8) for evacuating said gaseous flow from the collection chamber (4), said evacuation means (8) being located downstream of said electrode collection. A collection device according to claim 7, wherein the collection chamber (4) has a tubular side wall and two longitudinal end bottoms, and wherein the inlet means (6) are formed by holes passing through the side wall on the side of a first longitudinal end and the discharge means (8) are formed in the bottom located at a second longitudinal end. A collection device according to one of claims 1 to 8, wherein the voltage difference applied between the first end (12.1) of the capillary tube (12) and the collection electrode is in the range [8 kV; 10 kV]. Collection device according to one of claims 1 to 9, wherein the inner surface of the capillary tube (12) is at least partly made of electrically conductive material and forms the polarization means of the liquid contained therein. Collection device according to one of claims 1 to 9, wherein the capillary tube (12) is of electrically insulating material and the biasing means are formed by a biasing electrode located inside the capillary tube (12). Collection device according to one of claims 1 to 9, wherein the biasing means are located upstream of the capillary tube (12). Collection device according to one of claims 1 to 12, comprising means for decontaminating the collection electrode formed by the capillary tube by spraying, said capillary tube (12) being able to be connected by its second end (12.2). a reservoir of a decontamination liquid capable of being sprayed with electrospray, for example bleach. Collection device according to one of claims 1 to 13, wherein the collection electrode (16) is formed by a biological culture medium for the collected particles. Collection device according to one of claims 1 to 14, comprising a plurality of parallel capillary tubes (12). Collection device according to claim 15, comprising a deflector (22) surrounding the ends of the capillary tubes (12) opening into the collection chamber (4), said deflector (22) being intended to guide the droplets formed by electrospray. Collection device according to claim 16, wherein the deflector (22) is formed by a metal ring at the same potential as the liquid. Collection system comprising a collection device according to one of claims 1 to 17 and high voltage supply means for applying the voltage difference or differences. The collection system of claim 18, wherein the system is portable. The collection system of claim 18 or 19, comprising an ion wind generator connected to said collection chamber (4) for increasing the flow rate of gas flowing through the collection chamber, said ion wind generator including a discharge electrode and a against electrode. A collection and analysis system comprising a collection system according to one of claims 18 to 20 and means for analyzing the particles captured by the collection electrode (16), said analysis means being located downstream of said collection electrode (16).

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