Method for the selective purification of aerosols

Abstract

The invention relates to a method for the selective purification of aerosols. The invention consists in electrostatically collecting all of the particles present in an aerosol, but with separation of the mechanisms on one hand for charging particles by unipolar ion diffusion in order to charge and then collect the finest particles, and on the other hand for charging particles by means of Corona discharge in order to charge and then collect the largest particles on a substrate different from the substrate for collecting the finest particles.

Claims

1. A method for selectively purifying an aerosol comprising finest and biggest particles, the method comprising the following steps: sucking the aerosol through a conduit comprising an internal wall, from an inlet orifice to an outlet orifice; charging the finest particles, downstream of the inlet orifice, by unipolar ion diffusion in a space between a first electrode under a form of a gate surrounding a second electrode under a form of a wire, and a first conductive portion of the internal wall of the conduit; generating an electric field without a corona effect in a space between a third electrode and a second conductive portion of the internal wall of the conduit, in order to collect the finest particles of said aerosol charged by the unipolar ion diffusion by deposition onto a first collection substrate; generating by an electric current an electric field with a corona effect in a space between a fourth electrode under a form of a wire or a point and a third conductive portion of the internal wall of the conduit, in order to collect the biggest particles of said aerosol not charged by the unipolar ion diffusion by deposition onto a second collection substrate distinct from the first collection substrate; extracting purified air from the outlet orifice of the conduit.

2. The purification method as claimed in claim 1, further comprising at least one step of recycling or reusing the finest particles of said aerosol collected on the first collection substrate and/or the biggest particles of said aerosol collected on the second collection substrate.

3. The purification method as claimed in claim 1, further comprising the following steps: a/ collecting radioactive particles on the first and/or the second collection substrate during a time period t1; b/ counting pulses generated by the electric current in the space between the third electrode and the second conductive portion of the internal wall of the conduit and the space between the fourth electrode under a form of a wire or a point and the third conductive portion of the internal wall of the conduit, during a time period t2.

Description

DETAILED DESCRIPTION

(1) Further advantages and features will become more clearly apparent upon reading the detailed description, which is provided by way of a non-limiting illustration, with reference to the following figures, in which:

(2) FIGS. 1A to 1E are schematic views of different configurations of electrodes for obtaining a corona electrical discharge effect;

(3) FIG. 2 is a longitudinal cross-sectional view of a charging device or a unipolar ion diffusion charger;

(4) FIG. 3 is a schematic longitudinal cross-sectional view of a first example of a particle collection device according to the invention.

(5) Throughout the present application, the terms inlet, outlet, upstream and downstream are to be understood with reference to the direction of the suction flow through a collection device according to the invention. Therefore, the inlet orifice denotes the orifice of the device through which the aerosol containing the particles is sucked, whereas the outlet orifice denotes the orifice through which the air flow exits.

(6) FIGS. 1A to 1E and 2 have already been described in the preamble. They are not described hereafter.

(7) FIG. 3 shows an example of an electrostatic device 1 according to the invention for selectively purifying an aerosol likely to contain particles.

(8) Such a device according to the invention allows the aerosol to be purified by collecting both the finest particles, such as nanoparticles, and the biggest particles, such as micron-sized particles, whilst separating them from each other according to their size range.

(9) The purification device 1 firstly comprises a conduit 11, which is a hollow cylinder of revolution about the longitudinal axis X and which is electrically connected at zero potential.

(10) The collection device 1 comprises four distinct stages 10, 20, 30, 40, inside the conduit 11, in the upstream to downstream direction, between its inlet orifice 17 and its outlet orifice 18.

(11) The first stage is formed by a unipolar ion diffusion charger 10 and is similar to that which was previously described with reference to FIG. 2.

(12) The charger 10 thus comprises a central electrode that extends along the axis X in the form of a wire 12 connected to a power supply 13 delivering a high voltage adapted to thus create a corona discharge in the vicinity of the wire 12.

(13) It further comprises a peripheral electrode in the form of a gate 14 connected to a low-voltage power supply 16.

(14) The stage 20, downstream of the charger 10, comprises a central electrode that extends along the axis X in the form of a rod 22 connected to a power supply 23 delivering a medium voltage, adapted to create an electric collection field without a corona effect in the space 21 separating the central electrode 22 and the wall of the conduit 11. A hollow cylinder 24, conforming to the wall of the conduit and forming a first collection substrate Zn, is arranged around the rod 22 opposite thereto.

(15) The stage 30, downstream of the stage 20, comprises a central electrode that extends along the axis X in the form of a wire 32 connected to a high-voltage power supply 33, adapted to create a corona effect in the vicinity of the wire 32 and thus an intense electric field in the space 31 separating the central wire 32 from the conduit 11. A hollow cylinder 34, conforming to the wall of the conduit and forming a second collection substrate Zm, is arranged around the wire 32 opposite thereto.

(16) The stage 40 comprises a structure 41, for example, a honeycomb structure, adapted to prevent the appearance of a vortex in the conduit 11, and, downstream, a suction device 42. Depending on the configurations, the collection device according to the invention may dispense with the structure 41.

(17) The operation of the collection device previously described with reference to FIG. 3 is as follows.

(18) Air containing the particles to be collected is sucked through the inlet orifice 17 by the action of the suction device 42.

(19) The finest particles of the aerosol are electrically charged by unipolar ion diffusion in the space 15 separating the gate 14 from the conduit 11.

(20) These finest particles, with high electrical mobility, and the other bigger particles with lower electrical mobility, enter the stage 20.

(21) The electric field without a corona effect created in the space 21 between the rod 22 and the cylinder 24 ensures that the finest particles are collected on the cylinder while defining the first collection substrate Zn.

(22) The other bigger particles are not collected and are still present in the aerosol that enters the third stage 30.

(23) These biggest particles are then electrically charged under the corona discharge effect in the vicinity of the wire 32 and the intense field pervading the space 31 and are collected on the internal wall of the cylinder 34 while defining the second collection substrate Zm.

(24) The air that is purified both of the finest particles deposited on the first collection substrate Zn and of the biggest particles Zm desposited on the second collection substrate Zm is then discharged through the outlet orifice 18 of the device.

(25) Each collection cylinder 24, 34 can be extracted easily from the conduit once the intended collection has been performed.

(26) According to the desired application, each of the substrates Zn and Zm then may be analyzed using conventional physical or physico-chemical characterization techniques, such as optical or electron microscopy, surface scanner, , , spectrometry if the particles are radioactive, X-ray fluorescence (XRF) spectroscopy, micro X-ray fluorescence (-XRF), laser-induced breakdown spectroscopy (LIBS), etc. in order to determine the particle size, on the one hand, of the finest particles and, on the other hand, of the biggest particles, their concentration, their chemical composition and/or their morphology.

(27) One particular application relates to detecting the presence of radioactive particles on the collection substrates without it being necessary to extract them.

(28) Indeed, sequentially, after a collection phase over a time period t1, the device as illustrated in FIG. 3 enables the stages 20 and 30 to operate as ionization chamber for a time period t2.

(29) Via a suitable electronic device, conventional in nuclear instrumentation, the measurement of the ionization current in each stage 20 and 30 makes it possible to detect the presence of radioactive particles with a double interest: early detection of an incident in a nuclear facility by monitoring the atmospheric contamination of the air of the premises, separation of the radionuclides into two fractions of distinct sizes: the finest particles in the space 20, the biggest in the space 30.

(30) The latter point is of major importance. Specifically, it is often necessary to separate, on the one hand, the aerosol particles that it is desired to detect (such as the plutonium particles in workshops where the size of the aerosols is around 5 m (median aerodynamic diameter)) and, on the other hand, the natural aerosol bearing radon progeny (much finer aerosol) which forms undesirable background noise. This natural background noise may mask the measurement of the traces of desired radionuclides.

(31) The advantage of such a separation is clearly described in the work under reference [6], page 647 in the paragraph entitled Mitigation of Interference from Radon Progeny, and in article [7].

(32) Other variants and improvements may be implemented without however departing from the scope of the invention, especially for other applications where the advantage of separating an aerosol into two distinct particle size classes is desired, in particular for reusing the finest particles collected on the first substrate and/or the biggest particles on the second substrate.

(33) The invention is not limited to the aforementioned examples; in particular, features of the illustrated examples may be combined in variants that have not been illustrated.

CITED REFERENCES

(34) [1]: W. Hinds, Aerosol Technology, 2.sup.nd Edition, 1999. [2]: P. Intra and N. Tippayawong, Aerosol an Air Quality Research, 11: 187-209, 2011. [3]: G. W. Hewitt, The Charging of small Particles for Electrostatic Precipitation, AIEE Trans., 76: 300-306, 1957. [4]: G. Biskos, K. Reavell, N. Collings, Electrostatic Characterisation of Corona-Wire Aerosol Chargers, J. Electrostat. 63: 69-82, 2005. [5]: D. Y. H. Pui, S. Fruin, P. H. McMurry, Unipolar Diffusion Charging of Ultrafine Aerosols, Aerosol Sciences Technology 8: 173-187, 1988. [6]: P. Kulkarni, P. A. Baron, K. Willeke, Aerosol Measurement 3.sup.rd Edition 2011. [7]: C. Monsanglant-Louvet, F. Gensdarmes, N. Liatimi, S. Pontreau. Evaluation des performances des moniteurs de contamination atmospherique en conditions relles de fonctionnement [Evaluation of the performance of atmospheric contamination monitors under real operating conditions], Scientific and technical report, IRSN: 251-259, 2008.