A method and device for removing atmospheric particulates

Abstract

The present invention relates to a method (200) and a generator for instantaneous removal of atmospheric particulates by introducing charged carbon dioxide particle-ion two-phase flow (120) at the location of particulate contaminants (10); to prove mechanisms of removal of these particulate contaminants (10), tests were conducted with aerosol droplets (112) and smoke particles (114). This invention comprises three steps of adiabatically releasing (205) CO2 gas from a container / cylinder (130) containing some liquid CO2, and generating (210) carbon dioxide particles (110); electrically charging of the carbon dioxide particles (110) by contact electrification (220) at an outlet nozzle (132); and rapidly removing the atmospheric particulates (10) by electrostatic attracting (240) and aggregating (250) with the charged carbon dioxide particles (110).

Description

A Method and Device for Removing Atmospheric ParticulatesRelated Applications

[0001] The present invention claims priority to Singapore patent application no. 10202400859Y filed on 25 March 2024, the disclosure of which is incorporated in its entirety.Field of Invention

[0002] The present invention relates to the removal of atmospheric particulates, in particular in indoor environments, by using charged CO2 gas-solid two-phase ion streams.Background

[0003] Atmospheric particulates are micrometer to nanometer sized particles comprise of various components such as dust, smoke, soil particles, pathogens, liquid droplets and so on. They arc known to cause various adverse health effects such as respiratory and cardiovascular diseases. Airborne transmission through pathogen-laden liquid aerosol droplets released by human respiratory motions is a maj or transmission mechanism for many infectious diseases such as CO VID- 19, SARS, MERS and influenza. Although wearing masks could be a viable option to prevent the aerosol droplets from inhalation, it is impractical to always keep mask on even during sleep due to the discomfort during breathing. Since modem people spend most of their time staying indoors, where aerosol droplets are often trapped and surrounded nearby, there is an ever-increasing demand for active removal of indoor aerosol droplets.

[0004] There arc currently a few methods to remove indoor atmospheric aerosol droplets, such as filters, electrostatic precipitators, and ventilation.

[0005] While some of these technologies can effectively remove aerosol droplets on some occasions, none of them are capable of removing aerosol droplets instantaneously. In other words, the current technologies allow a significant portion of aerosol droplets to stay in the indoor atmosphere for minutes to hours before removal because they clear the indoor air by passively intaking surrounding air at fixed locations, which are often far from the source of the aerosol droplets. The aerosol droplets may have already reached one’s respiratory system and affect one’s health with this slow response to the removal of aerosol droplets. Therefore,a new effective method of actively removing targeted aerosol droplets in short durations is desired.

[0006] One possible mechanism to instantly remove aerosol droplets is the enhanced aggregation of aerosol droplets by the release of other substances. One technology attempting this approach is an air ionizer. Common commercial air ionizers generate large amounts of molecular ions clusters from corona discharge in air by applying kV-level high voltage via a high-voltage power source. Corona discharge by high-voltage power supplycan potentially lead to dangerous electrostatic discharge. It may also produce harmful substances such as ozone and nitrogen oxides. Moreover, the molecular ions are required to diffuse onto the surface of the aerosol droplets before aggregation, which also takes minutes to hours. Therefore, commercial air ionizers have not shown supenor capability of fast clearance of aerosol droplets.

[0007] It can thus be seen that there exists a need to provide a method and system for removing atmospheric particulates that can overcome the disadvantages of the existing prior art.Summary

[0008] The following presents a simplified summary' to provide a basic understanding of the present invention. This summary is not an extensive overview of the invention, and is not intended to identify key features of the invention. Rather, it is to present some of the inventive concepts of this invention in a generalised form as a prelude to the detailed description that is to follow.

[0009] Contact electrification is a phenomenon in which surfaces become charged when they are brought into contact and are then separated. This invention differs from corona discharge air ionizers as it generates a different type of atmospheric ions flow (i.c. by7way of carbon dioxide (CCh) gas-solid two-phase flow with an amount of solid CO 2 ions) using a fundamentally different and unreported mechanism (i.e., contact electrification) which conventional corona discharge air ionizers are incapable to generate. The flow generated by this invention removes atmospheric particulates much better and faster than the ions generated by corona discharge air ionizers.

[0010] The formation of solid CO2 particles from releasing compressed CO2 are also utilized in removing surface contamination such as in CO2 snow surface cleaning. However, the cleaning mechanism in the use of surface cleaning is completely' different from the present invention. Surface cleaning using CO2 particle jets relics on the high impact velocity' and momentum transfer of CO2 particle jets to knock off contaminants from surfaces, which iscompletely different from removing atmospheric particulates. The fundamental mechanism of this invention is found to be a combination of electrostatic interactions between charged solid CO: ions and atmospheric particulates, and the subsequent accelerated aggregation by the mass of the CO2 particles.

[0011] This invention is advantageous because it improves safety of operation by eliminating the possibility of electrostatic discharge. It does not produce unintended harmful substances such as ozone or nitrogen oxides.

[0012] This invention is fundamentally more capable of removing atmospheric particulates compared to pure gaseous ions. It is found to be drastically faster and more thorough in removing atmospheric particulates than commercial air ionizers with the same level of ion generation.

[0013] Another advantage of this invention is that it can remove atmospheric particulates at local spots within seconds, which is not possible with other technologies that rely on passively intaking surrounding air at fixed locations, such as filters.

[0014] The present invention provides a method of removing atmospheric particulates, the method comprising the steps of generating a two-phase ion stream wherein solid particles in the two-phase ion stream are charged by contact electrification and atmospheric particulates are removed by aggregating with the two-phase ion stream. Preferably, the two-phase ion stream is produced by releasing a pressurized gas from a container through a nozzle in a substantially adiabatic condition. Preferably, the two-phase ion stream is produced by releasing a gas-liquid mixture of carbon dioxide through the nozzle and aided by a pressure regulator. Preferably, the two-phase ion stream is released from the carbon dioxide container kept at an internal pressure of substantially 45 bar or more. The charged solid particle ions in the two-phase stream aggregate with the atmospheric particulates for their removal.

[0015] In another embodiment, the present invention provides a generator for clearing atmospheric particulates, wherein the generator forms carbon dioxide 10ns that can attract atmospheric particulates using the above method according to the principle of this invention.Brief Desc ription of the Drawings

[0016] This invention will be described by way of non-limiting embodiments of the present invention, with reference to the accompanying drawings, in which:

[0017] FIG. 1 A shows flow of CO2 gas from a pressure tank, electric charging solid COz-ion and removal of particulates by aggregation with the solid CO2 ions; FIGs. 1B-1C illustrate two diagrams on CO2 phase diagrams; FIG. ID illustrates charged CO2 particles-ion twophase flow at a nozzle; whilst FIG. 1 E illustrates CO2 ions normalized number concentration and mass concentration according to the present invention. FIG. IF illustrates the positive and negative ionic charge concentrations from the CO 2 particle -ion flow.

[0018] FIG. 2A illustrates charged solid COi-ion flow removing aerosol and smoke particles instantaneously; FIG. 2B illustrates experimental data of the present invention compared with data obtained with conventional ionizers and filters; FIG. 2C illustrates the positive and negative ionic charge concentrations of the present invention compared with conventional ionizers and filters; FIG. 2D illustrates experimental data of the present invention controlling aerosol of different sizes over time compared with that using compressed air, whilst FIG. 2E illustrates effect of charged CO2 particle-ion flow in clearing a smoke chamber compared with that using compressed air.

[0019] FIG. 3A illustrates a mechanism of the charge generation of the CChions; FTGs. 3B- 3C illustrate ion charge rates with unheated or heated nozzle and grounded or ungrounded nozzle; whilst FIG. 3D illustrates charge electrification at tire nozzle.

[0020] FIGs. 4A-4C illustrate experiments in which aerosol droplets are removed; whilst FIG. 4D illustrates charge electrification with solid CO2 particles.

[0021] FIGs. 5A-5C illustrate experiments in which aerosol and smoke particles are removed.

[0022] FIG. 6 illustrates particle size and concentration distributions of aerosol droplets generated by the humidifier used in the above experiments.Detailed Description

[0023] One or more specific and alternative embodiments of the present invention will now be described with reference to the attached drawings. It shall be apparent to one skilled in the art, however, that this invention may be practised without such specific details. Some of the details may not be described at length so as not to obscure the invention.

[0024] This invention relates to a new mechanism or method 200 performed to remove particulate contaminants 10 quickly, for eg. within seconds, by releasing small amounts of charged CO2 micrometer-sized solid particle ions 110 in a two-phase flow 120 into an atmosphere or enclosure for cleaning. The CO2 solid particle ions 110 are formed or generated 210 by releasing 205 pressurized CO2 gas from a CO2 gas container 130 due to its exceptional cooling effect during Joule-Thomson expansion in an adiabatic condition. The CO2 solid particle ions 110 are then charged by contact electrification 220 against walls inside a nozzle 132, which is located at an outlet of the CO2 container 130. Upon reaching the vicinity of the particulates 10 (simulated by aerosol droplets during testing), thesecharged CO2 particle ions 110 actively attract 240 the particulates 10 with their high local surface charge and provide themselves as cores for aggregation, thus enabling removal of these particulate contaminants 10. This mechanism or method 200 does not rely on diffusion of ions onto surfaces of the particulates for their removal as is the ease with known air ionizers. The CO2 particles ions that do not aggregate with the particulates will quickly sublime, leaving no extra particles in the atmosphere. Therefore, the present invention is operable to quickly removing contaminating particulates at local positions; thus, this invention has a great potential to solve the problem of air contamination when fast removal of contaminating particulates is required.

[0025] The following paragraphs describe experiments to evaluate the method 200 of the present invention against the methods of the prior art.Generation of the tw o-phase flow

[0026] The two-phase flow 120 of charged solid CO2 particles and gas generated in this invention is a jet of gas stream that contains small (i.e., nano- to micron-scale) particle ions 110. In a preferred embodiment, it is generated 210 from a cylinder 130 filled with some liquid CO2 and equipped with an outlet or nozzle 132, for eg. located at a top of the cylinder. No dip tube is installed inside the liquid CO2 cylinder to obtain the two-phase CO2 flow 120. The internal pressure in the headspace of the cylinder is typically at substantially 55 - 60 bars. A Spectron LM51-6 single stage pressure regulator 140 with a 0.25-inch NPT-F outlet port connects to the outlet of the cylinder. An experiment is initiated by first purging the system, ic. by adiabatically releasing 205 the two-phase flow 120 at an external differential pressure of substantially 1 bar for 1 minute for obtaining a consistent stream of CO2 two- phasc flow 120 before use.Measurement of size of CO2 particle ions, aerosol and smoke particles

[0027] A laser spectrometer 142 was used to measure the concentration and size distribution of the particles in the atmosphere during testing of the present inventive concept (these particles during testing include, solid CO2 particles 110, water aerosol droplets 112, and smoke particles 114). It uses laser light scattering to measure the size of each particle that passes through an air collector fluidly connected to the laser spectrometer 142. A radially symmetrical collection probe was used for all experiments. Tire air intake flow rate was set at substantially 1.2 L / min, and the interval between each measurement was 6 s. The laser spectrometer 142 was connected to a computer via a data transfer cable. The samplingparameters were controlled by the laser aerosol spectrometer software operating in the computer. After measuring the size of the particle, the size data were categorized into one of 31 different size channels from diameters ranging from 0.25 pm to 32 pm for counting the number of particles and calculating the particle concentration. The spectrometer 142 can measure up to 2 x 106particles / L.Comparing performance of two-phase flow with commercial ionizers

[0028] Controlled experiments were conducted for comparing the performance of the different types of gas flows for removing aerosol droplets in the atmosphere. The experiments were conducted inside a closed chamber 150 (formed with Polymethyl methacrylate - PMMA) with dimensions of substantially 50 cm x 50 cm x 100 cm. For generating the small nano- to micron-sized aerosol droplets 112 in the closed chamber, an ultrasonic humidifier 144 filled with tap water was used; it was placed at the center of the bottom (50 cm x 100 cm) of tire chamber 150. Tire rate of generation of the aerosol by the humidifier was substantially 35 mL / hr. The size and concentration distributions of the aerosol droplets 112 indicated by lines L and M are shown in FIG. 6. Several types of gas flows were used for removing the aerosol droplets generated by the humidifier. The CO > two-phase flow 120 was generated as described in the above description. Flow of air 20 was generated by releasing air from a gas cylinder filled with compressed air. A single stage pressure regulator with a 0.25-inch NPT-F outlet port was connected to the outlet of the cylinder for regulating the flow of air. Flows of ionized air 24 from three different commercial ionizers 26, 26a, 26b, 26c were used. The ionizers 26, 26a, 26b, 26c had fans blowing that created strong flows of air w ith the ions generated out of their outlets. The gas flows produced by the different methods were introduced into the chamber 150 via an inlet hole 152 with a diameter of substantially 1.27 cm (0.5 inch) on one of the walls (i.e., with dimensions of 50 cm x 100 cm) of the chamber 150. ft was located 5 cm away from both the nearest walls (a 50 cm x 100 cm wall and a 50 cm x 50 cm wall). When the flow was generated from a cylinder (i.e., for the CO2 two-phase flow 120 and air flow 20), a stainless- steel tube with a length of substantially 5 cm and a diameter of substantially 1.27 cm (0.5 inch) was used to connect the pressure regulator of the cylinder to the inlet hole 152 of the chamber. For the commercial ionizers 26, 26a, 26b, 26c, they were operated in their most powerful modes based on the options available. The ionizers were found to require 5 to 30 seconds before a steady flow of ions blown out by the fans was produced. Hence, all experiments and measurements were conducted 1 minute after switching on tire commercialionizers. The concentration of ion produced by the ionizers was not uniform spatially throughout the outlets of the ionizers. Therefore, only the specific region of the outlet that had the highest concentration of ion from each ionizer was directed into the inlet hole 152 of the chamber 150. Both compressed air flow 20 and CO2 two-phase flow 120 were released at an external differential pressure of less than 0.1 bar according to the smallest graduation of a pressure gauge connected to the pressure regulator.

[0029] The experiment involved first switching on the humidifier 144 for introducing the aerosol droplets 112 into the closed chamber 150. The humidifier remained turned on throughout the experiment. 10 minutes after switching on the humidifier 144, the probe of the laser spectrometer 142 was inserted 5 cm into the closed chamber 150 through the wall with the dimensions of 50 cm x 50 cm. The probe was located at 5 cm above the bottom of the chamber and 1 cm horizontally away from the side wall of the chamber closest to it. The concentration of the aerosol droplets 112 was recorded by the laser spectrometer 142 for 1 minute. Subsequently, one type of gas flow for removing the aerosol droplets was separately introduced into the chamber through the inlet hole. The flow was directed from the inlet hole to the tip of the probe that was 15 cm away. All measurements were conducted at least three times.

[0030] The concentration of ions produced by the different methods was measured by an ion counter 146. The inlet of the ion counter was placed 15 cm away from and both horizontally and vertically in line with the inlet hole 152, at the same location that the probe of laser spectrometer was placed. Both positive and negative ions were measured at least three times.Clearing smoke particles

[0031] The experiment was conducted with 5Lthrcc-ncck round-bottom flasks 50. Each flask 50 was first washed with a brush and detergent to remove any stains (e.g., smoke particles) on the inner wall of the flask. It was then rinsed by tap water, deionized water and ethanol. The flask was dried overnight in an oven operated at 90 °C. After taking it out of the oven, it was allowed to cool down to room temperature for 1 hr before use. The flask rested on a cork ring with the opening of the three necks pointing outward horizontally. The top and bottom necks were each fitted with a glass stopper. Two incense sticks 60 that were used to generate smoke particles 114 were fixed into a rubber stopper and ignited. The rubber stopper was then fitted into the center neck of the flask. The incense sticks 60 burned inside the flask for 7 minutes to generate the smoke particles 114. After generating the smoke, the rubber stopper with the incense sticks at the center neck and glass stopper at tire bottom neck wereremoved. These two necks were each covered with a 0 2 pm pore gas filter that was fixed in place via a glass fitting. For introducing the CO2 two-phase flow 120 or air flow 20, the glass stopper on the top neck was replaced with a stainless-steel tube (length of 5 cm and diameter of 0.5 inch) that was connected to the pressure regulator of the cylinder. The gaps between the outer surface of the stainless-steel tube and the inner wall of the top neck were filled to prevent any gas leakage. The gas flow for removing the smoke particles was then introduced through the stainless-steel tube into the flask for 2 minutes at an external differential pressure of less than 0.1 bar with the aid of a pressure gauge connected to the pressure regulator.Determining the significance of solid-solid contact electrification by varying amount of solid CO2 particles

[0032] The number of particles produced by the CO2 two-phase flow 120 was varied for determining whether solid-solid contact electrification 220 between the solid CO2 particles 110 and tire surfaces of the pressure regulator / nozzle enabled tire charging of the CO2 particles 110. A hair dry er 70 was used to provide heat onto the pressure regulator / nozzle 132 for reducing the amount of CO2 particles produced. The hair dryer could be operated with three different modes that corresponded to different amounts of heat produced. A 2-m perfluoroalkoxy alkane (PFA) tube with a diameter of 0.635 cm (0.25 inch) was connected to the outlet of the pressure regulator 140 of the cylinder 130 filled with some liquid CO2.Charge measurement of CO2 particle-ion flow and gas cylinder at different grounding conditions

[0033] A stainless-steel tube with a 5 cm length and a 1.27 cm (0.5-inch) diameter was connected to the nozzlc / outlct of the pressure regulator of the CO2 cylindcr / containcr. The CO2 gas cylinder 130 was electrically insulated by wrapping PTFE sheets around the body and the bottom of the cylinder, as well as the pressure regulator 140. The pressure regulator 140 was connected electrically to resistors with various resistance connected in series (or no resistor in the “grounded” case), and then to ground. The electric wire was removed in the ungrounded case. Before the start of the measurement, the pressure regulator 140 connected to the CO2 gas cylinder 130 was first cooled down by allowing CO2 gas to first release from the gas cylinder outside the testing chamber at an external differential pressure of 1 bar for 1 minute. Then, the CO2 particle-ion two-phase flow 120 was blown for another 1 minute into a Faraday cup 80 connected to an electrometer 82 to measure the charge of the flow. Another electrometer 82a was used to measure the potential difference across one of theresistors connected between the pressure regulator and ground. In the cases in which the total series resistance was 2.2 Mil or 1 GO, the potential difference across the single 2.2 MQ or 1 GQ resistor was measured. In the case in which the total series resistance was 31 GQ. the potential difference across the 1 GQ resistor nearest to the ground was measured. The electrometer 82,82a was connected to a computer that allowed automatic data acquisition to be done automatically via a program. The current was then calculated with the potential difference and the resistance of the measured resistor by Ohm’s Law.Reducing the ion concentration of CO2 particle-ion flow with the same particle concentration

[0034] To reduce the ion concentration of the CO2 particle-ion two-phase flow 120 without decreasing the particle concentration, the CO2 gas cylinder 130 was first electrically insulated by wrapping PTFE sheets around the body and the bottom of the cylinder, as well as the pressure regulator 140. CO2 flow was then allowed to be adiabatically released 205 at an external differential pressure of 1 bar for 5 minutes before the experiments. The CO2 flow after the 5-minute pre-release maintained both the positive and negative ion concentration of the CO2 flow between 2 x 104lon / cin ’ to 5 x 104ion / cm3. A longer time of pre-release did not further lower the ion concentration of the CO2 flow.Reducing the particle concentration of CO2 particle -ion flow with tire same ion concentration

[0035] A lower concentration of the solid particles in the CO2 flow was obtained by releasing CO2 from the cylinders with an internal pressure at 45 to 50 bar, which is lower than the internal pressure of the CO2 cylinders in all other experiments (i.c., 57 bar). This lower internal pressure was achieved when almost all liquid CO2 inside the cylinder was used up. Below substantially 45 bar, no more liquid CO2 was in the cylinder to maintain the internal pressure by evaporation. This CO2 particle-ion two-phase flow 120 had both positive and negative ion concentrations at around 5 x 104ion / cm3.Electrostatic attraction of water aerosol by two-phase flow according to this invention

[0036] This experiment involved two streams that flow ed in parallel and separated by a short distance of 5 cm away from each other horizontally. Tire first stream was the flow of aerosol droplets 112 generated by the ultrasonic humidifier 144 at a rate of substantially 35 mL / hr. The flow of aerosol droplets 112 was directed vertically upward . The second stream was the CO2 two-phase flow 120. A 50-cm stainless-steel tube with a diameter of 0.635 cm (0.25 inch) was connected to the outlet of the pressure regulator 140 of the cylinder / container 130filled with liquid CO2. The outlet of the tube was placed at the same height and 5 cm horizontally away from the outlet of the humidifier 144. The humidifier was first switched on. After the stream of aerosol droplets 112 stabilized, the CO2 two-phase flow 120 was released 205 at an external differential pressure of substantially 0.2 bar for 10 to 15 s parallel to the stream of aerosol. Videos of the gas flow were taken by a smart phone. The experiment was repeated for air flow 20. A 50-cm perfluoroalkoxy alkane (PFA) tube with a diameter of 0.635 cm (0.25 inch) was connected to the nozzle of the pressure regulator 40 of the cylinder 22 containing air.Measuring the change of charge when CO2 pellets interacted with water aerosol

[0037] An ultrasonic humidifier 144 was placed at the center of the bottom (50 cm * 100 cm) of the closed (PMMA) chamber 150 (i.e., with dimensions of 50 cm x 50 cm * 100 cm). A Faraday cup 80 connected to an electrometer 82 was placed 20 cm horizontally away from the humidifier 144 along the long side of the chamber 150 A ceramic bowl 81 was placed inside the Faraday cup 80. The Faraday cup was covered by a lid. The electrometer 82 was connected to a computer that allowed continuous data acquisition to be done automatically via a program. After covering the Faraday cup 80 with a lid, the humidifier 144 was turned on to introduce the aerosol droplets 112 into the chamber 150 for 10 minutes. Two portions of 50 g food-grade CO2 solid pellets 90,90a (with diameter of 3 mm and length of ~10 mm) were used. One portion of 50 g of pellets 90 was used as-is. The other portion 90a of 50 g was charged by contact electrification by crushing the pellets with a mortar and pestle wrapped with an aluminum foil for 30 s. At the start of the measurement, the lid of the Faraday cup was removed, and one portion of the CO2 pellets was transferred into the ceramic bowl 81 located inside the Faraday cup 80. The initial charge of the CO2 pellets was measured at this point. After that, the Faraday cup was left uncovered (e g., by the lid). The top opening of the Faraday cup thus allowed aerosol droplets 112 in the atmosphere of the chamber 150 to enter freely into the Faraday cup and interact with the CO2 pellets. The change of charge due to the interaction with the aerosol droplets 112 was measured for 100 s. A control experiment without aerosol in the closed chamber 150 was also conducted. All experiments were performed three times.Formation of water droplets and smoke stain on a plate

[0038] The CO2 two-phase flow 120, air flow 20, and flow 24 of ionized air were used in this experiment. Tire flow of gas generated from each method was first passed through an inlethole 152 on awall ofthe closed chamber 150 (i.e., with dimensions of 50 cm x 50 cm x 100 cm). The diameter of the inlet hole was 1.27 cm (0.5 inch) for the formation of the water droplets 112 and 0.635 cm (0.25 inch) for the formation of the smoke stain. The inlet hole 152 was located at the horizontal center of the wall with dimensions of 50 cm x 50 cm and 10 cm above the bottom of the chamber (i.e., with dimensions of 50 cm x 100 cm). A stainless-steel tube that had a length of 5 cm and a diameter that matched the size of the hole (i.e., 0.5 inch or 0.25 inch) was used to connect the outlet of the pressure regulator (i.e., when cylinders were involved for the cases of the CO2 two-phase flow 120 and air flow 20) to the inlet hole 152. A target plate 95 was placed 15 cm horizontally away from the inlet hole 152. The center of the taiget plate 95 was horizontally and vertically in line with the inlet hole. The target plate 95 was a polypropylene corrugated sheet with dimensions of 120 mm x 120 mm for the formation of water stain, and a polystyrene (PS) Petri dish with dimensions of 120 mm x 120 mm for the formation of smoke stain.

[0039] For the experiment that involved the formation of the aerosol droplets 112, the ultrasonic humidifier 144 was placed at the center ofthe bottom (i.e., with dimensions of 50 cm x 100 cm) of the closed chamber 150. ft was switched on to introduce the aerosol droplets 112 into the chamber 150 for 10 minutes at a rate of substantially 35 mL / hr. For the experiment that involved the formation of the smoke stain, eight incense sticks 60 were placed at the four comers of the bottom of the chamber 150. They were burned to introduc e smoke particles 114 into the chamber. The flow of gas was then introduced into the chamber through the inlet hole that was in line with the target plate for 15 s. Both CO2 two-phasc flow 120 and air flow 20 entered the chamber at an external differential pressure of 0.1 bar for the experiment that involved the formation water droplets 112, and 2 bars for the experiment that involved the formation of the smoke stain. For the ionizer 26, 26a, 26b, 26c, it was switched on to the most powerful mode and was allowed to operate for 1 minute first before using it in the experiment.

[0040] Materials used for the experiment described above include:• Carbon dioxide (CO2) cylinders 130 and air cylinders 22• Single stage pressure regulators 140,40• Food-grade CO2 pellets 90,90a (~3 mm in diameter and ~10 mm in length)• Ultrasonic humidifier 144• Ion Generator• lonisator Desktop• Air purifier / ionizer 26, 26a, 26b, 26c• Incense sticks 60• Polypropylene corrugated sheets 95 with dimensions of substantially 120 x 120 x 3 mm• Polystyrene (PS) Petri dishes (Greiner) with dimensions of substantially 120 x 120 x 17 mm• Poly (methyl methacrylate) (PMMA) chamber 150 with dimensions of substantially 50 cm x 50 cm x 100 cm• Ethanol (analytical grade)Characterization of the unique COz aas-solid two-phase flow

[0041] FIGs. 1A-1F characterise the CO2 particle-ion two-phase flow 120 of the present invention. FIG. 1A shows the mechanism or method 200 of removing atmospheric particulates, which includes generating 210 charged CO2 particles 110 by adiabatically releasing 205 of pressurized CO2 gas and the subsequent contact electrification 220 of the CO2 particles 110 against walls inside the nozzle 132. The CO2 particle-ion two-phasc flow 120 is then directed to the aerosol droplets 112 and to remove the aerosol droplets by electrostatic attraction 240 and active aggregation 250. FIGs. 1B-1C are pressuretemperature and pressure-enthalpy phase diagrams of CO2. The generation 210 of CO2 solid particles 110 by adiabatically releasing 205 pressurized CO2 gas is thermodynamically possible. FIG. ID shows the CO2 particle-ion 110 flow released from the gas cylinder 130. FIG. IE shows the normalized number concentration (dN / dlog(Dp)) and mass concentration (dMZdlog(Dp)) plots for the CO2 particles 110. FIG. IF shows the ion concentration of the CO2 particle-ion two-phase flow 120 compared to a compressed air flow 20.

[0042] Referring back, FIG. 1A shows the adiabatic release 205 of pressurized CO2 gas into the atmosphere under room temperature can create a gas-solid two-phase flow 120 with CO2 solid particles 110. Temperature changes when real gases experience adiabatic expansion from high pressure to low pressure through a nozzle. This is known as the Joule-Thomson effect. At room temperature, most gases, including CO2, decrease their temperature during this process. The value of the temperature change, dT, with respect to pressure change, dP. under constant enthalpv H is defined as the Joule-Thomson coefficient, [IJT = (— ) . During gas expansion, dP is always negative, so when UJT is positive, the temperature of the gasdecreases by expansion. CO2 has exceptionally large Joule-Thomson coefficients at around1.1 K / bar at room temperature in the gas phase and increase with the decrease in temperature during the adiabatic gas expansion, which exceed 2 K / bar at 240 K or below.

[0043] From the prcssurc-tcmpcraturc phase diagram in FIG. IB, the maximum pressure of CO2 gas at room temperature (i.e., 293 K) is about 57 bar, above which CO2 will be liquefied. All common CO2 cylinders, include CO2 fire extinguishers, store liquefied CO2 or a CO2 liquid-gas mixture. Hence, the CO2 gas releasing 205 from any new CO2 cylinder 130 has an initial internal pressure of about 57 bar under room temperature. The CO2 flow temperature is thus expected to at least decrease from 293 K to lower than 200 K by the gas expansion from a CO2 gas cylinder 130 (i.e., from 57 bar to atmospheric pressure). Based on the estimation above using the Joule-Thomson coefficients of CO2, CO2 gas can be cooled down to lower than its triple point (216.6 K, 5.18 bar) during the adiabatic expansion of CO2 liquid-gas mixture at room temperature to one atmospheric pressure, leading to the formation of solid CO2 particles 110. CO2 liquid-gas mixture at high pressure and room temperature thus can convert into gas-solid two-phase during adiabatic gas expansion to atmospheric pressure, as shown in the pressure-enthalpy phase diagram in FIG. 1C.

[0044] In FIG. 1C, generation 210 of CO2 solid particles 110 in the CO2 flow released by gas expansion from a CO2 liquid-gas mixture stored in a common gas cylinder / container 130 was observed As shown in FIG. ID, white solid CO2 particles 110 were ejected along the gaseous CO2 flow from the nozzle 132 when the pressure regulator 140 was only slightly turned on. The CO2 particles 110 moved in the same direction as the CO2 gas The size distribution of micromctcr-sizcd CO2 particles 110 was also measured by the laser spectrometer 142 To collect the CO2 microparticles, the CO2 gas-solid two-phase flow 120 was blown directly to the sample collection probe at a minimal differential pressure (i.e., < 0.1 bar). It was found most of the CO2 microparticles 110 had diameters of less than 1 pm from the number concentration versus particle size normalized to the bin width of the channel (dN / dlog(Dp)) from the laser spectrometer 142 (line J in FIG. IE). Normalizing the concentration of the particles to the bin width allows the size distribution to be properly presented by removing the influence from the difference in bin width across different channels of the equipment. On the other hand, more CO2 molecules tend to form larger particles as shown by line K in the normalized mass concentration plot (dM / dlog(Dp)) as seen in FIG. IE.

[0045] Due to the existence of CO2 solid particles 110 from the pressure release 205 of CO2 liquid-gas mixture, ions may be generated by contact electrification 220 of the solid CO2particles 110 in the flow. Thus, the positive and negative ion concentration from the CO2 two-phase flow 120 was measured by an ion counter 146. The CO2 two-phase flow 120 and compressed air flow 20 first passed through a stainless-steel tube with a 5 cm length and a 1.27 cm (0.5-inch) diameter after leaving the nozzle to focus the flow. Then, the flows were directed to the inlet of the ion counter 146 placed 15 cm away from the outlet of the stainless- steel tube at the trajectory of the flows. It was found that the CO2 two-phase flow 120 can generate both negative and positive ions at a concentration of 2 x 106ions / cm3and 106ions / cm3respectively, with more negative ions than positive ones, as seen in FIG. IF. In contrast, the pressure release of compressed air flow 20 did not produce any significant number of ions higher than the background ion concentration, which is at the order of 103ions / cm3.Superior aerosol droplets removing ability from the CO2 particle-ion mixture flow

[0046] FIGs. 2A-2E illustrate how the CO2 solid particle-ion two-phase flow 120 removes aerosol droplets 112 instantaneously. FIG. 2A shows the expenmental setup for testing the capability' of the instant removal of water aerosol droplets 112 at a local position on the path of the gas flows. FIGs. 2B-2C illustrate PM10, PM2.5 and PM1 concentrations and ion concentrations at the local position after introducing different gas flows. FIG. 2D shows 5- minute monitoring of PM2.5 and PM1 at the local position with CO2 tw o-phase flow 120 or air flow 20. FIG. 2E are images showing removing smoke particles 114 inside a 5L three- neck round bottom flask 50 with CO2 particle-ion two-phase flow 120 (right) versus compressed air flow' 20 (left).

[0047] A number of previous studies have found that aerosol droplets can be removed when a large number of ions were introduced into the atmosphere. The introduction of solid particles which can be quickly sublime may also boost the chance of aggregation of aerosol droplets without leaving more aerosol droplets in the atmosphere after this treatment. The combination of substantial number of ions and transient particles in the CO2 two-phase flow 120 of the present invention is able to remove aerosol droplets 112 better than the release of pure gaseous ions in ionized air 24. To test this capability of removing undesirable aerosol droplets 112 instantly by the CO2 solid particle-ion two-phase flow' 120 at specific desired locations, the instant decrease of aerosol droplet concentration in the path of the CO2 solid particle-ion two-phase flow was measured in an environment with high aerosol droplet concentration (FIG. 2A). The capability of instantly removing aerosol droplets of tire CO2solid particle-ion 2 -phase flow 120 was compared to those of three commercial ionizers26, 26a, 26b, 26c for indoor household and vehicle use.

[0048] The aerosol removal test was conducted inside the closed PMMA chamber 150 with dimensions of 50 cm * 50 cm * 100 cm, as seen in FIG. 2A. An ultrasound humidifier 144 produced aerosol droplets 112 inside the PMMA chamber at a rate of 35 mL / hr to maintain the high aerosol droplet concentration. Water aerosol with high particle concentrations was used to mimic the high local aerosol concentration surrounding a person who just sneezes or coughs. The spectrometer 142 was used to measure the change in the particle concentration and calculate the values of important particulate matter (PM) (i.e., PM1, PM2.5, PM10) during the first 6 seconds of the introduction of flows, as 6 seconds is the shortest measurement interval available from the spectrometer. The flows were required to pass through the inlet hole 152 with a diameter of 1.27 cm (0.5 inch) on the wall of the PMMA chamber 150 to enter the chamber. The CO two-phase flow 120 and compressed air flow 20 first passed through a stainless-steel tube with a 5 cm length and a 1.27 cm (0.5-inch) diameter after leaving the nozzle to focus the flow before reaching the inlet hole 152. The collection probe of the spectrometer 142 was placed 15 cm away from the inlet hole 152 on the path of the flow in order to minimize the detection of the particles from the COz solid particle-ion flow, which was not the part of the aerosol to be removed.

[0049] The results showed that CO2 two-phase flow 120 can almost eliminate all aerosol droplets 112 in all sizes, including PM1, PM2.5 and PM10, in an instant, while commercial ionizers 26, 26a, 26b, 26c failed to do so completely (see FIG 2D) despite the same level of ions generated by the CO2 two-phase flow and the commercial ionizers. No removal of aerosol droplets was found by the ions generated by commercial ionizers within 6 seconds, which was the same as the performance from compressed air flows. The PM1, PM2.5 and PM10 values remained at the initial levels at around 1400, 35000 and 80000 pg / m3respectively in these cases. The ion concentrations generated from the commercial ionizers were quantified by the ion counter 146 using the same method of measuring the ion concentration of CO2 two-phase and compressed air flows. In order to directly reflect the ion generating capability under the aerosol removal experiment, the flow from the commercial ionizers also pass through the inlet hole 152 with the same diameter (i.e., 1.27 cm or 0.5 inch) before reaching the ion counter 146.

[0050] Commercial ionizers generate gaseous ions by the ionization of gas molecules using high-voltage supply. This direct method of ion generation by commercial ionizers 26, 26a, 26b, 26c should be expected to generate much more ions than the contactelectrification 220 by the random motion of the solid particles 110 within the CO2 particleion 2-phase flow 120. Surprisingly, this invention CO2 particle-ion two-phase flow 120 provided a comparable number of negative ions to the most powerful commercial ionizer in this study, both at around 2 * 106ions / cm3(sec FIG. 2B). The other two ionizers generated substantially fewer ions than the invention CO2 particle-ion two-phase flow 120. Only about 1.25 x 10' positive ions / cm3and 4.65 x 105negative ions / cm3were generated by the commercial ionizers 26, 26a, 26b, 26c. In addition, it was found that all three commercial ionizers only generated unipolar ions, as opposed to CO2 particle-ion two-phase flow' 120 (see FIG. 2C). The large differences in the ion-generating capability of the three commercial ionizers alone did not lead to any difference in the performance of instant removal of aerosol droplets 112. Therefore, the results indicate that the solid particles 110 generated by the CO2 two-phase flow' 120 are at least part of the factor to the excellent performance of instant removal of aerosol droplets.

[0051] Besides the instant removal of aerosol droplets, the long-term suppression of aerosol droplets is also important, since aerosol droplets can remain in the atmosphere for minutes or even hours. Therefore, the duration of measurement of aerosol removal in the path of the CO2 particle-ion two-phase flow 120 was extended to 5 minutes. At this longer time scale, only the smaller aerosol droplets 112 (i.e., PM1 and PM2.5) are important as particles with a diameter at the order of 10 pm will either precipitate by gravity or evaporate into smaller aerosol droplets within tens of seconds.

[0052] As shown in FIG 2D, while compressed air 20 showed a slow' reduction of larger aerosol droplets 112 (i.e., PM2.5) to around 60% of the no-treatment control after 5 minutes, the CO2 particle-ion tw o-phase flow 120 wras capable to maintain the local PM2.5 at below' 1% of the original concentration during the full duration. The difference in the persistent removal of small aerosol droplets (i.e., PM1) was even more striking. Compressed air 20 was incapable of removing PM1 particles at all even after 5 minutes, w'hile the CO2 particleion two-phase flow 120 can suppress the PM1 particles at the same low level throughout the experiment.

[0053] To showthe universality ofthe CO2 particle-ion 2-phase flow technology, the removal of another common source of particulate air pollutants, ie. smoke particles 114, was tested. Smoke particle is also a better type of particle than w ater aerosol droplets for visualizing the removal of particles, as w'ater aerosol may become adsorbed onto the wnll even without any treatment. In this experiment, smoke particles 114 produced by burning incense sticks 60 were first introduced for 7 minutes in a 5L three-neck round bottom flask 50 before theparticulate removal experiment. A thick layer of smoke was observed inside the flask 50 after the introduction of smoke particles 114 (as illustrated in the “Before” images in FIG. 2E). After that, one of the three necks was connected to the nozzle of the COz cylinder 130 or compressed air cylinder 22 through a stainless-steel tube with a 5 cm length and a 1.27 cm (0.5-inch) diameter. Because gas was blown into the flask, the other two necks served as the outlets for releasing the pressure build-up inside the flask. Gas filters were installed at those two necks to ensure no smoke particles were removed by the air flow alone. The necks of the flasks were oriented horizontally so that large particles precipitated at the bottom of the flasks prior to the introduction of the flow would not be blown up again during the experiment. As shown in FIG. 2E, no visible smoke could be observed after blowing in COz particle-ion 2-phase flow 120 for two minutes. In contrast, compressed air 20 cannot remove the smoke particles 114 from the atmosphere after two minutes. This experiment demonstrated that the CO2 particle-ion 2-phasc flow 120 can also effectively remove other particulate pollutants such as smoke particles 114, significantly better than gas flows without ions or particles.Fundamental mechanism of the generation of ions in the COz particle-ion 2-phase flow

[0054] The drastic difference in the generation of ions between gas-solid two-phase flow 120 (i.e., the CO2 particle-ion flow) and pure gas flow 20,24 (i.e., the compressed air flow) indicated that the CO2 solid particles 110 are a major contributor to the ions in the CO2 particle-ion 2-phase flow 120. However, it is still not fully clearifthe gaseous CO2 may also contribute to a substantial part of the generated ions in the particle-ion flow . Therefore, the net charges in the CO2 particle-ion two-phase flow 120 with different reduced levels of solid particles 110 were measured. The reduction of CO2 solid particles in the flow was achieved by heating up the nozzle with a hair dryer 70. Since the generation 210 of CO2 solid particles 110 in the flow is due to the cooling effect when the CO2 gas flow passes adiabatically through the nozzle 132, heating up the nozzle part is an effective method to suppress the generation of CO2 solid particles without altering the flow rate of the CO2 flow. A 2-m heat- insulated PFA tube was connected to the nozzle 132 for releasing the CO2 flow in order to block the CO2 flow from being disturbed by the air flow from tire hair dryer and to prevent further heat up of the CO2 flow after leaving the nozzle. With the high heat mode of the hair dryer 70, a CO2 flow without any visible particle or white mist was obtained. Reduced number of particles relative to the CO2 flow without heating up the nozzle were also observed when low and medium heat were used A Faraday cup 80 disposed at 15 cm away from theoutlet of the PFA tube was connected to an electrometer 82 for measuring the net charge of the flow.

[0055] FIGs. 3A-3D illustrate the fundamental mechanism or method 200 of the charge generation on the COi particles 110 according to the present invention. FIG. 3B shows the net charge of the COz particle-ion flow 120 with different levels of CO2 solid particles 110. The CO2 solid particles 110 were found to be a major contributor of the charge within the particle-ion two-phase flow 120. FIG. 3C illustrates the net charge of the CO2 particle-ion flow 120 with grounded versus ungrounded nozzle, and FIG. 3D illustrates the current collected from the nozzle 132 under different grounding resistance. FIG. 3A illustrates the fundamental mechanism or method 200 of the charge separation 230 by contact electrification 220 between CO2 particles 110 and the surface of the nozzle 132.

[0056] As shown in FIG 3B, about -0.39 nC net charge was obtained from the CO2 two- phase flow 120 released in 1 second without reducing the number of solid particles with heat, hi contrast, the CO2 two-phase flow 120 under high heat (i.e., w ithout any CO2 solid particles 110) showed negligible (i.e., lower than -104nC / s) net charge. The net charges from the CO2 flow each second were also decreased to -0.27 nC and -0.05 nC with reduced number of CO2 solid particles 110 in the flow, as seen in FIG. 3B. These results clearly show that the solid particle 110 component in the CO2 tw o-phase flow 120 is the major contributor of the high number of ions in the CO2 particle-ion 2-phase flow 120.

[0057] Since the net charge of the whole CO2 flow was not zero, it implied that charge separation 230 should occur between the CO2 solid particles 110 and some materials that were not part of the flow. As the CO2 solid particles 110 were always blown out from the nozzle 132 and not in contact with any solid surface before reaching the ion counter 146 or Faraday cup 80 for ion and charge measurements, it was likely that the CO2 solid particles 110 obtained their net charge by the contact electrification 220 against the nozzle 132. In contact electrification 220, the two contacting surfaces always carry the opposite polarity’ of charge at the moment when the two surfaces are separated 230. Since a net negative charge was obtained by the CO2 flow 120, the nozzle 132 was expected to charge positively. Also, the accumulated charge on a surface (e.g., the positive charge on the nozzle surface) energetically disfavours the further accumulation of charge with the same polarity (e.g., positive charge) during charge separation 230. Because of that, less charge will be obtained by the other surface (e.g., the surfaces of solid CO2 particles) as well. Therefore, it should also be expected to see a decrease in the net charge of the CO2 flow 120 if the routes ofcharge dissipation on the surface of the nozzle 132 was cut off bv electrical insulation if the hypothesis was correct.

[0058] To test the above hypothesis, the CO2 flow 120 first passed through a stainless-steel tube with a 5 cm length and a 1.27 cm (0.5-inch) diameter connected to the nozzle 132 to focus the flow. The nozzle 132 was usually electrically connected to the ground through an electric wire and a load (i.e., a series of resistors), unless the nozzle 132 and CO2 cylinder 130 were required to be electrically insulated (i.e., the “ungrounded nozzle” case shown in FIG. 3C). The CO2 cylinder 130 was fully wrapped with PTFE sheets to electrically insulate from any other physical grounding route. A Faraday cup 80 disposed at 15 cm away from the outlet of the stainless-steel tube was connected to an electrometer 82 for measuring the net charge of the flow. Another electrometer 82a measured the potential difference across one of the resistors connected between the pressure regulator and ground. This potential difference was converted into current by Ohm’s law to calculate the surface charge on the nozzle side. Electrically insulating the CO2 cylinder 130 also ensured tire source of current collected from the nozzle 132 was the charge obtained by the contact electrification 220 with the CO2 two-phase flow 120.

[0059] As shown in FIG. 3C, after a one-minute release, the net charge in a one-second portion of the CO2 flow 120 dropped from -3 nC to -1 nC when the nozzle 132 was electrically insulated. Furthermore, the surface charge on the nozzle 132 also decreased from about 13 nC / s to 8 nC / s when the resistance of charge dissipation increased from 2.2 M£1 to 31 GQ. as seen in FIG. 3D. Both results thus supported the hypothesis that the contact electrification 220 between the CO2 particles 110 and the nozzle 132 was the fundamental mechanism of both the contact electrification 220 and tire charge separation 230 process in the CO2 particlc-ion 2-phasc flow 120.The effect of electrostatic interaction on removing aerosol droplets

[0060] The invention CO2 particle-ion 2-phase flow 120 can generate as many ions as the commercial ionizers. The electrostatic attraction 240 between the charged CO2 solid particles 110 and the aerosol droplets 112 facilitates the removal of the aerosol droplets. The CO2 solid particles 110 are first released into the atmosphere and move towards the aerosol droplets 112 to be removed. When a CO2 solid particle 110 moves near to an opposite- charged aerosol droplet 112, the aerosol droplet may be electrostatically attracted 240 by the CO2 solid particle 110 and then adsorb onto it. As a result, large aggregates of CO2 solidparticles with adsorbed aerosol droplets are formed. These large aggregates are then quickly removed from the atmosphere by gravity.

[0061] Therefore, in order to investigate the effect of electrostatic attraction on the removal of aerosol droplets, the instant decreases of aerosol concentration of water on the path of the CO2 particle-ion flows with three different ion concentrations (i.e., 2 x 106ion / cm3, 5 x 104ion / cm3, and 2 x 104ion / cm3) but the same particle concentration were also measured under the same conditions as the experiment depicted in FIG. 2B. Water aerosol was chosen because water tends to charge positively against most materials, while the CO2 particle-ion two-phase flow 120 had more negative ions than positive ones. The low er ion concentrations with same particle concentrations from the CO2 two-phase flow 120 were achieved by a prerelease of CO2 flow from an electrically insulated cylinder for 5 minutes. With this prerelease, the nozzle 132 was charged highly positively and thus ineffective to charge the CO2 solid particles 110 negatively by contact electrification.

[0062] FIGs. 4A-4D show the electrostatic charge from tire CO2 particle-ion two-phase flow 120 is a factor of the instant removal of aerosol droplets 112. In FIG. 4A, PM10, PM2.5 and PM1 concentrations at the local position on the path of the gas flows after introducing CO2 particle-ion flows with different ion concentrations and the same particle concentration. The setup for this experiment was the same as FIG. 2B. FIG. 4B are images of the trajectory of a stream of water aerosol releasing in parallel with the CO2 particle-ion flow 120 (left) and the compressed air flow 20 (right). FIG. 4C illustrates the change of charge and the initial charge of CO2 particles in an environment fdled with water aerosol droplets. Adsorption of positively charged water aerosol was found with negatively charged CO2 particles 110 but not uncharged CO2 pellets 90a. A clear decreasing trend in the ability to remove aerosol droplets was found with decreasing ion concentrations (sec FIG. 4A). Compared to the complete removal of aerosol droplets 112 by the CO2 two-phase flow 120 with the high ion concentration of 2 x 106ion / cm3, the CO2 two-phase flow 120 with the ion concentration of 5 x 104ion / cmJcould only remove around 80% to 90% of the aerosol droplets 112. Moreover, the CO2 two-phase flow 120 with only 2 x 104ion / cm3totally lost the capability' of removing aerosol droplets 112 instantaneously even with the same amount of CO2 solid particles 110. This clearly indicated that the charged particles 110 in the CO2 two-phase flow 120 is necessary for the instant removal of aerosol droplets 112.

[0063] Releasing the CO2 flow in parallel to the stream of water aerosol (in FIG. 4B) was carried out to visualize the electrostatic attraction between a stream of water aerosol and the CO2 particle-ion flow 120 The stream of water aerosol was attracted by the CO2 flow 120and moved towards the direction of the CO2 flow. Tn contrast, there was no change in the direction of the aerosol stream when a flow of compressed air 20 was used to replace the CO2 flow at the same position and with the same external differential pressure.

[0064] Despite the negatively charged CO2 particlc-ion flow could change the trajectory of the stream of positively charged water aerosol, it was still not fully clearly whether the electrostatic attraction 240 between the CO2 solid charged by contact electrification 220 and water aerosol was actually contributing to gathering the aerosol droplets and removing them from the atmosphere. If the aerosol droplets were adsorbed on the CO2 solid surface due to electrostatic attraction, the net charge on the CO2 solid surface should become more positive with time. Therefore, tire real-time change of charge on the contact-charged CO2 solid particles 110 under an environment full of water aerosol droplets was measured. The CO2 solid particles were made by crushing CO2 pellets 90 with metal-coated mortar and pestle, and the total initial charge of the particles after crushed were around -100 nC (sec FIG. 4C). The amount of charge per mass of the particles after crushed were at a similar order of magnitude as the CO2 particle-ion flow, which also indicated that the charge in the CO2 two- phase flow 120 should be obtained from the contact electrification 220 between the CO2 particles 110 and the metal nozzle 132. The particles were then placed inside a ceramic bowl in a Faraday cup 80 connected to an electrometer 82. The charge of the particles was monitored for 100 seconds. It was found that positive charge was adsorbed onto the CO2 particles steadily and a total of +1.7 nC was added to the CO2 particle after 100 seconds under the water aerosol-rich environment, compared to only +0.25 nC under the ambient environment without the water aerosol droplets. Control experiments with uncharged CO2 pellets 90a (i.e , initial charge less than 1 nC as shown in FIG, 4C) with the same mass as the charged CO2 particles were also conducted to verify whether the water aerosol droplets were indeed attracted into the Faraday cup 80 by the CO2 particles. Only less than +0.05 nC of water aerosol was adsorbed onto the uncharged CO2 pellets 90a under the water aerosolrich environment. These results proved that the electrostatic attraction 240 between the CO2 solid particles 110 in the particle-ion tw o-phase flow' 120 and aerosol droplets 112 is a part of the mechanism or method 200 of removing the aerosol droplets 112 by the CO2 particleion 2-phase flow 120.The effect of solid CO2 particles on removing aerosol droplets

[0065] However, the electrostatic effect between the CO2 particlc-ion 2-phasc flow' and the aerosol droplets cannot explain the massive difference in removing aerosol droplets by CO2two-phase flow and commercial ionizers with comparable amounts of generated ions (see FIG. 2B). It is speculated that the solid particles 110 in the CO2 two-phase flow 120, a nonexistent component in the gaseous ion flow 24 from the commercial ionizers, played a vital role in the excellent performance of substantially instant removal of aerosol droplets by the CO2 two-phase flow 120.

[0066] To investigate the effect of the CO2 solid particles on the removal of aerosol, comparison of the instant decreases of aerosol concentration of water on the path of the CO2 particle-ion flows with two different CO2 solid particle concentrations but the same ion concentration (i.e., 5 x 104ion / cm3) under the same conditions as the experiment conducted in FIG. 2B was carried out. The CO2 flow with a lower particle concentration was achieved by releasing CO2 from a CO2 cylinder 130 with a lower internal pressure of CO2. According to the Joule-Thomson effect and the phase diagrams of CO2, a smaller pressure drop from the gas expansion decreases the temperature drop of the flow, thus resulting in a smaller extent of CO2 particle generation 210. Tire ion concentration of the CO2 flow was controlled at 5 x 104ion / cm3because (1) the high ion concentration at 2 x 106ion / cm3was not achievable with the CO2 flow with fewer solid particles 110, and (2) the CO2 flow with normal particle level and the ion concentration of 5 x 104ion / cm3can still remove a big portion of the water aerosol droplets (see FIG. 4A).

[0067] FIGs. 5A-5C illustrate that the solid particles 110 in the CO2 particle-ion two-phase flow 120 are crucial to the substantially instant removal of aerosol droplets 112. FIG. 5A shows the PM 10, PM2.5 and PM 1 concentrations at the local position on the path of the gas flows after introducing CO2 particle-ion flows with different particle concentrations and the same ion concentration. Tire setup for this experiment w as the same as that in FIG 2A. FIG 5B arc images of the formation of aggregates of watcr / acrosol droplets 112 (top) and smoke particles 114 (bottom) by blowing gases (CO2 two-phase flow 120, compressed air flow 20, and ion flow 24 by a commercial ionizer 26, 26a, 26b, 26c) on a target plate 95 within the aerosol-filled environment. FIG. 5C illustrates the fundamental mechanism of the instant removal of aerosol droplets 112 by the synergistic effect of electrostatic charge attraction 240 and solid particles 110 aggregation with the aerosol droplets 112 within the CO2 particle ion two-phase flow 120.

[0068] As shown in FIG. 5A, it was found that the CO2 with fewer particles cannot instantaneously remove the aerosol droplets anymore even with the same level of ions, regardless of the sizes of the aerosol droplets. This result indicated that the existence of sufficient CO2 solid particles 110 is the reason for the fast removal of aerosol droplets 112.

[0069] While the importance of the CO2 solid particles in the fast removal of the aerosol droplets had been confirmed by the expenments above, it was still unclear how the CO2 particles 110 help in removing the aerosol droplets. Since it was proposed in the above experiments that the aerosol droplets were removed by forming aggregates, it is suspected the CO2 particles 110 may also enhance the formation of larger aggregate of aerosol droplets with their larger size and surface charge compared to gaseous ions. The large size of the CO2 particles can serve as the platform for the aerosol droplets to aggregate 250, and the large surface charge can strengthen the electrostatic attraction 240. The momentum of the CO2 particles 110 from the flow may also accelerate the removal of the large aerosol aggregates from the atmosphere by quickly bringing them to adhere on other surfaces instead of waiting for gravity to work. An experiment was set up to visualize both the formation of large aggregates and the adhesion 250 of the aggregates carried by the CO2 solid particles 110 to a nearby surface. First, the PMMA chamber 150 used in the instant aerosol removal experiment was filled with water aerosol particles 112 or smoke particles 114. A target plate 95 was placed on the path of the CO2 particle-ion flow 120 inside the aerosol-filled chamber 150. By blowing the CO2 flow 120 with a high differential pressure (i.e., 1 bar) into the chamber 150 with water aerosol for 15 seconds, water droplets 112 with diameters of a few millimetres were found all over the target plate 95 (FIG. 5B). Similarly, large patches of smoke stains were formed on the target plate 95 when the chamber 150 was filled with smoke particles 114. On the contrary , only negligible water droplets or smoke particles were found on the target plate 95 when a high-pressure compressed air flow 20 or the flow 24 from a commercial ionizer was used. This result demonstrated that the CO2 particles 110 facilitate tire formation of large aggregates of aerosol droplets 112 or smoke particles 114 and transport the aggregates to nearby surfaces to achieve rapid removal of the aerosol particles 112 or smoke particles 114.

[0070] Based on the experimental findings, a new three-step mechanism or method 200 of the substantially instantaneous removal of aerosol droplets by the CO2 particle-ion 2-phase flow is proposed, as shown in FIG. 5C. This mechanism or method 200 is completely different from the mechanism of removing aerosol droplets using gaseous ions flow 24, where the gaseous ions first require charging the surface of the aerosol droplets by diffusion. In the first step, the charged CO2 particles 110 quickly approach the targeted aerosol droplets 1 12 by the adiabatic release of the CO2 two-phase flow 120. When the charged CO2 particles 110 arc near the aerosol droplets 112 along the particle-ion flow, the aerosol droplets without and with the opposite polarity of charge as tire CO2 particle 110 are electrostatically attracted240 by the CO2 particles 1 10 and adsorbed onto the surface of the CO2 particles 110, forming CCh-aerosol aggregates. Since there are both positively charged and negatively charged CO2 particles 110, aerosol droplets 112 with any polarity of charge can be removed. After that, the C Ch -aero so I aggregates continue to move away from the target plate 95 by the momentum of the original CO2 particle until they are permanently removed from the atmosphere by adhering to surfaces (e.g., a wall or the floor). This mechanism or method 200 is unique to CO2 particle-ion 2-phase flow 120 because it is only achievable with the three decisive properties of the CO2 particle-ion two-phase flow 120 that the conventional gaseous ion does not possess. (1) The highly charged CO2 particles 110 can attract multiple aerosol droplets 112 or smoke particles 114 at the same time. (2) The large surface area of the CO2 particles 110 allows multiple aerosol droplets / smoke particles to adsorb simultaneously. (3) Relatively large mass and momentum of the CO2 particles 110 allows the aggregates to continue moving along the onginal trajectory of the CO 2 particle-ion flow, further accelerating the removal of the aerosol droplets / smoke particles.

[0071] This invention relates to a facile and effective technology’ for substantially instantaneous removal of aerosol droplets / smoke particles by’ introducing CO2 particle-ion 2-phase flow 120 at the location of the aerosol droplets / smoke particles. This technology involves three spontaneous steps: (1) adiabatically releasing 205 of high-pressure CO2 gas and generating 210 of CO2 particles 110, (2) contact electrifying or charging 220 of tire CO2 particles 110 with inside surfaces of the nozzle 132 connected at the outlet of the CO2 cylinder 130, and (3) electrostatically attracting 240 contaminating particles onto the charged CO2 particles 110, and aggregating 250 the contaminating particles with rapid removal with the charged CO2 particles 110, as shown by the aerosol droplets / smoke particles.

[0072] The CO2 particle-ion 2-phasc flow 120 can be generated by releasing pressurized CO2 gas into the atmosphere. The formation and the sizes of the CO2 particles 110 are characterised. The charge of the CO2 particle-ion flow 120 under various conditions and verified that the large amount of charge of the CO2 two-phase flow 120 is due to the contact electrification 220 between the CO2 particles 110 and the nozzle 132 of the gas outlet are investigated. The effectiveness and generality of the CO2 particle-ion two-phase flow 120 to remove multiple types of aerosol droplets at different time scales are demonstrated. It is established that the effective removal of aerosol droplets by this invention is based on the novel fundamental mechanism of active aggregation 250 of aerosol droplets / smoke particles that utilizes both the surface charge and the physical existence of the charged CO2 particles 110. This mechanism or method 200 eliminates the need for slow diffusion of ions onto thesurface of aerosol droplets / smoke particles during the molecular ion-induced aerosol aggregation by commercial ionizers 26, 26a, 26b, 26c. Hence, the aggregation of contaminating particles with the charged CO: particles 110 according to this invention is greatly accelerated, and the aerosol droplcts / smokc particles can then be removed rapidly, such as, within seconds.

[0073] The present mechanism or method 200 is also advantageous in that it improves safety of operation by eliminating the possibility of electrostatic discharge. It also does not produce unintended harmful substances, such as ozone or nitrogen oxides.

[0074] The rapid clearance of air pollutants in indoor environments is important to human health, yet it has been surprisingly challenging to achieve — current technologies either does not the take the pin-point approach of removing atmospheric particulates and / or are too slow to respond to the release of aerosol droplets / contaminant particles into the atmosphere. The technology of releasing charged CO particle ions 110 provided by the invention provides a solution to quickly remove aerosol droplets or particulate contaminants and pave the way for future development of air cleaning products based on this novel mechanism or method 200, for eg. of the accelerated aggregation 250 by the synergistic effect of electrostatic attraction 240 of the charged CO2 solid particles 110 with the particulate contaminants 10.

[0075] While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations and combinations of variations disclosed in the text description and drawings thereof could be made to the present invention without departing from the scope of the present invention. For example, this invention can be implemented with a generator for generating the charged CO2 two-phase flow 120 and a fan or blower to recirculate air in an environment to mix with the gas-solid two-phase flow 120 containing charged CO2 solid particles 110 to accelerate particulate contamination removal present in the environment.

Claims

CLAIMS1. A method of removing atmospheric particulates, the method comprising: generating a two-phase ion stream, wherein solid particles in the two-phase ion stream arc charged by contact electrification and atmospheric particulates arc removed by aggregating with the charged solid particles in the two-phase ion stream.

2. The method according to claim 1, wherein generating the two-phase ion stream is produced by releasing a pressurized gas from a container through a nozzle in a substantially adiabatic condition.

3. The method according to claim 2, wherein generating the two-phase ion stream is produced by releasing a gas-liquid mixture of carbon dioxide through the nozzle.

4. The method according to claim 3, wherein the two-phase ion stream is released from the carbon dioxide gas container kept at an internal pressure of substantially 45 bar or more.

5. A generator for removing atmospheric particulates, wherein the generator generates charged carbon dioxide particles that attract atmospheric particulates using the method according to any one of claims 1-4.

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