DESCRIPTION

FIELD OF THE INVENTION

The present invention lies in the technical field of air filtration devices based on multilayer filtration systems for indoor environments.

Specifically, the present invention relates to a filtration device based on a multi-layer filtration system for filtering particulate pollutants made of inorganic, organic and/or biological material having a size (diameter) greater than 4 nm.

The present invention relates further to a method of decontaminating and regenerating the aforementioned filtration device.

BACKGROUND ART

In the last decades, the interest for air quality in indoor environments has drawn increasing attention from international scientific community, political institutions and environmental governances, in order to improve the comfort, health and well-being of residents.

Qualitative and quantitative variations in the air quality in indoor environments over the years have, indeed, been shown by several studies on this subject pointing out an increase of the pollutants. Since people are estimated to spend about 90% of their time in both private and public indoor environments, such as households, fitness centers, schools, workplaces, and so on, it results that air quality in indoor environments impacts significantly on health and quality of life in general.

For many people, the risks from exposure to indoor air pollution may be even greater than those related to outdoor pollution.

Indoor environments represent a mix of outdoor pollutants, mainly associated to vehicular traffic and industrial activities, which may enter through infiltrations and/or natural and/or mechanical ventilation systems, as well as indoor contaminants originating from inside the building, from combustion sources, emissions from materials and furnishings, centralized heating and cooling plants, humidification systems, electronic equipment, cleaning products, house pets, and from the occupants’ behaviour (smoking, painting), etc.

Controlling the concentrations of respiratory aerosols in indoor environments, in order to reduce the transmission of infectious agents, is essential and may be achieved by controlling the source (face mask use, physical distancing) and by technical controls (ventilation and filtration).

Recently, the detection of SARS-CoV-2 in indoor environments aroused increasing concern. Indeed, SARS-CoV-2 may be transmitted via aerosols at a distance of more than two meters between people.

Apart from SARS-CoV-2, other airborne respiratory viruses can spread similarly to SARS-CoV-2 via main transmission routes, namely direct contact, respiratory droplets, and airborne transmission.

However, whereas infection control measures (e.g., hand-washing and mask-wearing) can reduce the first two transmission modes, the third route, that is the airborne route, is difficult to prevent, since respiratory viruses are ubiquitous in the environment, with virus particles constantly circulating in the air. Airborne transmission of other respiratory viruses, such as influenza or rhinovirus, has already been studied in hospitals, since aerosol transmission has been demonstrated as the most probable mode of transmission for adults.

In this respect, a series of treating systems for removing contaminants has been developed.

US11105522 B2 discloses an air treating system, characterized by an air motor forcing air through the system itself, a pre-treating stage with a particulate filter for removing larger contaminants from the air, and an antimicrobial (e.g., copper and silver) filter for killing or damaging microorganisms, a UV chamber including an ultraviolet lamp and a catalytic (e.g., TiCh-coated) device, and a reflective (e.g., mirror-finish anodized aluminum) coating amplifying the UV radiation, so as to kill microorganisms, a post- UV step including a Volatile Organic Compounds (VOCs) reducing filter for removing odours and VOCs from the air. US20210010692 Al discloses a system designed to withstand all chemical and biological contaminants in the air. Specifically, this system has been designed for killing the anthrax spore, the most difficult biological to kill. The system exists since more than 4 years, protecting and supporting the growth of the human embryo, the most sensitive cell. The aim is to provide a transformational air purification technology (in-duct or in-room), in order to remove infectious pathogens. The system envisages the purification, including filtering of the air through oxidizing and VOC adsorbing pre-filtration, secondary filtering of the air through UV filtration inside the system and final filtering of the air through the final particulate filtration downstream of the UV filtration. The disclosure provides a method further comprising the stage of filtering the air through the particulate pre-filtration inside the housing, upstream of the VOC pre-filtration. The system comprises further one or more filters containing carbon, KMnO4 and combinations thereof.

WO2019194890 A9 discloses a biological air purifier based on electrically filtering and removing contaminants by bio-oxidation. The filtration concept is electrically based and does not have filtering barriers.

CN204478340U discloses a regenerative air purification system, applying to air- conditioning systems and ventilating systems. The purification process and the regenerative process may be accomplished simultaneously, comprising: pre-filtration filters, a gas adsorbing filtration device, photocatalyst material (a mix of nano-titanium or titanium dioxide and active carbon absorbing material) for gases, a heating system for gases. The filtration concept is based on filtering barriers and, subsequently, on gas traps.

W02010008336 Al discloses a method for capturing airborne agents or products of agents, such as microorganisms, including viruses and microbial antigens, toxins and allergens, comprising forming at least one trap for electrically charged aerosol (in emulsion, suspension form).

EP2466085 A2 refers to a centrifugal, wet-type air cleaner, with a centrifugal fan for generating a vortex flow, whereby pollutants may be eliminated centrifugally by creating a depressurized room on the basis of by simultaneous water atomization. The document aims to provide the purification of indoor/outdoor air in different fields, using a centrifugal filtration concept with water atomization without filtering barriers.

US6872241 B2 refers to an anti-pathogenic air filtration medium comprising a fibrous substrate, the fibers thereof having featuring a coating comprising a polymer. The coating provides a destructive environment for airborne pathogens. In particular, the filter medium may be used in an indoor air treating system capable of both filtering the air and eliminating pathogens. The filter medium may also be used to create a new bio-protective gas mask not only offering protection against chemical warfare agents, but also providing protection against biological pathogens. The fibrous substrate is any natural or synthetic porous material made of a plurality of intermingled fibers. The polymer is any polymer capable of withstanding acidic, basic, oxidizing or strongly solubilizing substances without decomposing.

Problem o f the known art

The above-described air decontamination systems known from the state of the art are generally based on chemical or biological (e.g., bio-oxidation) and not thermal systems, or on the simple contaminants collection.

Furthermore, the air filtration and decontamination systems known from the state of the art sometimes do not show biocidal action on all microorganisms, in particular on all future SARS-CoV-2 variants.

Lastly, the air filtration and decontamination systems known from the state of the art sometimes do not envisage self-regenerating functionalities.

SUMMARY OF THE INVENTION

In such a context, the technical task underlying the present invention is to provide a filtration device for filtering particulate pollutants made of inorganic, organic and/or biological material, which is free from the aforementioned drawbacks.

From this perspective, it is an object of present invention an indoor filtration device for filtering particulate pollutants made of inorganic, organic and/or biological material with a size (diameter) greater than 4 nm, having a filtration efficiency of 99.999 ± 0.001%, and comprising: A) a three-layer filter pack assembly, the outer layers whereof comprise a silicon carbide (SiC) ceramic filter and the intermediate layer comprises a quartz fiber filter (QFF), and

B) a final quartz fiber filter applied on non-woven fabric (NWF) and steel mesh, located downstream of the pack assembly A).

In particular, the aforementioned filtration device is of regenerating type and also has a SARS-CoV-2 inactivating action through a decontamination and regeneration method comprising a first stage of heating the aforementioned pack assembly, followed by SARS-CoV-2 inactivation, and/or a second stage of combustion of the collected particulate material, resulting in a biocidal action.

Advantageously, the inventive filtration system may be used for HVAC (“Heating, Ventilation and Air Conditioning”) applications, according to the size of the space to be treated, from applications in buildings/edifices (hospital, university/school, museum, cinema/theatre settings, etc.), down to smaller sizes, such as a single room (e.g., in food serving settings), to portable applications in household settings.

Still advantageously, compared to the known air purification systems in indoor environments and/or resulting in a biocidal action, the filtration device of the invention is not based on induced filtration systems, based in turn on combinations of UV-A light and TiCh activated silicon carbide filters (photochemical mechanism), electrostatic deposition on honeycomb cells, biodegrading action.

Yet advantageously, the filtration device of the invention is responsible for an abrupt decrease of organic, inorganic and/or biological particulate matter concentrations, having a filtration efficiency of 99.999 ± 0.001%.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the operation diagram of a preferred embodiment of the filtration system of the invention, in terms of air intake and purification and heat treatment stages (100 °C and/or 600 °C, for SARS-CoV-2 inactivation and regeneration of the filtration system, respectively).

DETAILED DESCRIPTION OF THE INVENTION For the purposes of the present invention, the terms “ comprising / containing" one or more components does not exclude the presence of other components apart from the one/ones explicitly listed.

For the purposes of the present invention, the phrase according to which an item “ consists of or is made of or composed of one or more components means that the presence in the item of any additional component apart from the one/ones explicitly listed is excluded.

For the purposes of the present invention, by “pack assembly” is meant a multi-layer packing made of a plurality of overlapping filters.

According to a preferred embodiment, each ceramic filter of the outer layers of said pack assembly A) has a porosity ranging from 25 to 70 ppi, preferably ranging from 30 to 65 ppi, more preferably equal to 60 ppi, and a thickness ranging from 0.8 to 1.2 cm, still more preferably equal to 1.0 cm.

For the purposes of the present invention, by “nominal diameter” is meant the one measured by the so-called particle retention liquid.

The principle underlying such a measuring technique is based on the premise that, in a filtration process, the particle retention efficiency of a depth filter is often expressed in terms of particle size (in pm) at which a certain retention efficiency level (e.g., 98%) is obtained. The filtration efficiency is, in turn, determined by analysis of particle removal from a fluid, using particle counters: the original fluid, with a known suspended particle concentration, and the filtrate (fluid passing through the filter) are measured with an in-liquid particle counter.

Preferably, the quartz fiber filter included in the intermediate layer of said pack assembly A) has a nominal pore diameter ranging from 1.0 to 2.5 pm, preferably equal to 2.0 pm, and a thickness ranging from 0.35 mm to 0.40 mm, preferably equal to 0.38 mm.

Still preferably, the quartz fiber filter included in the intermediate layer of said pack assembly A) is heat resistant to a temperature of 900 °C and chemically resistant to SO ₂ , HC1, SO ₃ , SO ₄ ² ', NO, and NO ₃ ‘. According to a preferred embodiment, the final filter B) has a nominal pore diameter ranging from 1.0 to 2.5 pm, preferably equal to 2.0 pm, and a thickness ranging from 0.35 nm to 0.40 nm, preferably equal to 0.38 nm.

Preferably, the final filter B) is heat resistant to a temperature of 900 °C and chemically resistant to SO2, HC1, SO3, SC ² ', NO, and NO?’.

Still preferably, each ceramic filter of the outer layers of the pack assembly A) is configured to mechanically support and transfer heat to the quartz fiber filter (QFF) of the intermediate layer of the pack assembly A).

The pack assembly A may be possibly contained in a steel casing, depending on the application context.

According to a preferred embodiment, the outer layers of the pack assembly A) contain means, preferably electric resistors, capable of heating the entire pack assembly A).

A further object of the present invention is the method of decontaminating and regenerating the filtration device 1 comprising the following steps: b-1) heating the pack assembly A) to 100 °C to inactivate the SARS-CoV-2 trapped in the intermediate layer of the assembly A) comprising the quartz fiber filter; and/or b-2) heating said quartz fiber filter contained in the assembly to 600 °C for combustion of all the collected particulate material resulting in a completely biocidal and regenerating action.

Preferably, the above-mentioned method provides performing step bl), alternatively step bl) may be repeatedly performed at regular time intervals ti, preferably ranging from 12 to 24 hours, followed by subsequent step b2), alternatively step b2) may be performed repeatedly at the aforementioned regular time intervals ti or at regular time intervals t2, preferably ranging from 24 hours to 30 days, depending on the treated air flow and on the environmental contamination level, wherein ti < t2.

A preferred embodiment of the filtration device of the invention is schematically illustrated in FIG. 1, where the operation method of such a device in the filtrating stage indicated by a) in the Figure and in the bacterial/viral particle inactivating and regenerating stage (stage b) is also depicted. The device is provided with a housing 2 having an inlet 3 suitable for receiving the pack assembly A), through which the air to be filtered in the environment enters said housing, and has at least one outlet 4, preferably two outlets, suitable for receiving the final filter B) made of quartz fiber on non-woven fabric supported on a steel mesh, through which the filtered air passes exiting from the housing 2.

Optionally, when the filtration device is used for indoor applications, it further comprises at least one active carbon odour filter to be located upstream of each outlet 4 and downstream of the inlet system 3.

Inside the housing 2, the filtration device is provided with a fan 5, preferably located between the aforementioned inlet 3 and said at least one outlet 4, associated to the inlet 3, to facilitate the inflow of air to be filtered and the outflow of filtered air.

In step a) of actually filtering, polluted air enters the device 1 via the inlet 3 passing through the pack assembly A). By means of the fan 5, the filtered air is forced out from the at least one outlet 4.

In stage b) of inactivating/regenerating, the device is closed such that only the air inside the device circulates and the device is heated by resistors located inside the outer filters of the pack assembly A) until reaching the target inactivation (100 °C)/regeneration (600 °C) temperature.

The inactivation/regeneration of the filtration device 1 is facilitated, apart from the electric heating, by the air forced by the fan 5 to circulate and pass several times through the assembly A).

In the following experimental part, merely illustrative reference is made to the filtration efficiency of the inventive filtration device.

EXPERIMENTAL PART

The developed system is based on a base laboratory prototype consisting of a three- layer pack assembly consisting of: a silicon carbide (SiC) ceramic filter having a porosity equal to 60 ppi, thickness equal to 1 cm; a quartz fiber filter (QFF, SiCh, thickness of 0.38 mm, 99.998% retention of particles having a diameter of 0.3 pm), heat resistant to a temperature of 900 °C and chemically resistant to SO2, HC1, SO3, SC ² ', NO, NO?’, except for HF; a silicon carbide (SiC) ceramic filter having a porosity equal to 60 ppi, thickness equal to 1 cm; a final QFF filter applied on non-woven fabric (NWF) and steel mesh, located downstream of the three-layer pack assembly, defining a final security barrier for air purification and avoiding the release of fibrous material from the system.

The operation diagram of the system, in terms of air intake and purification and heat treatment stages (100 °C and 600 °C, for SARS-CoV-2 inactivation and regeneration of the filtration system, respectively) is shown in FIG. 1.

Measurement method

The filtration matrices and the filtration test to which they were subjected are listed below: silicon carbide (SiC) ceramic filter having a porosity equal to 30 ppi, thickness equal to 1 cm; silicon carbide (SiC) ceramic filter having a porosity equal to 60 ppi, thickness equal to 1 cm; quartz fiber filter (QFF, SiCh, 85 g/m ² , thickness of 0.38 mm, 99.998% retention of 0.3 pm particles), heat resistant to a temperature of 900 °C and chemically resistant to SO2, HC1, SO3, SC ² ', NO, NOs’, except for HF.

From the filtration matrices, 47 mm diameter filters were punched, on each of which the filtration test was performed. In order to perform such tests, a basalt fiber seal was disposed around the silicon carbide filters. Each material was accommodated inside a sealed, metal biconical sample-holder. As for the seal, the size which is actually exposed has a diameter equal to 34 mm. The filtration matrices were subjected to filtration test by determining the numerical concentration of particles upstream (outdoor air) and downstream (filtered air) of the same, using a total particle counter (particle diameter > 4 nm), having a measurement frequency equal to 1 Hz, and an optical particle counter (0.3 pm < particle diameter < 10 pm), having a flow rate equal to 2.7 cm/s.

The air subjected to the filtration treatment was the air from the urban atmosphere of Milan, taken in outdoor environment, which was representative of the most significant environmental and biological contaminations.

QFF samples were tested as such (VIRGIN), following sampling and collection thereon of the particulate matter from the atmosphere of Milan (SAMPLED: mass collected on the filter equal to 3.185 ± 0.979 mg), and after complete combustion and regeneration in a muffle (REGENERATED).

QFF filter mass was determined with a microanalytical balance having a sensitivity equal to 1 pg.

Next, final tests were performed, in which the QFF filter was packed between two silicon carbide filters (one having a porosity of 30 ppi and one having a porosity of 60 ppi), in order to form a three-layer filtration membrane (Packed test 1), and in which, besides the three-layer filtration membrane, the final QFF filter was also inserted (Packed test 2).

Experimental results

The filtration efficiency data (mean ± 95% probability confidence interval) are as follows: silicon carbide (SiC) ceramic filter having a porosity equal to 30 ppi, thickness equal to 1 cm: filtration efficiency = 10.566 ± 4.273% silicon carbide (SiC) ceramic filter having a porosity equal to 60 ppi, thickness equal to 1 cm: filtration efficiency = 35.197 ± 8.287% VIRGIN quartz fiber filter (QFF, SiCh, 85 g/m ² , thickness 0.38 mm, 99.998% retention at 0.3 pm), heat resistant to a temperature of 900 °C and chemically resistant to SO2, HC1, SO3, SCU ² ', NO, NO?’, except for HF: filtration efficiency = 99.998 ± 0.002%

SAMPLED quartz fiber filter (QFF, SiO2, 85 g/m ² , thickness 0.38 mm, 99.998% retention at 0.3 pm), heat resistant to a temperature of 900 °C and chemically resistant to SO2, HC1, SO3, SO4 ² ', NO, NOs’, except for HF: filtration efficiency = 99.999 ± 0.001%

REGENERATED quartz fiber filter (QFF, SiO2, 85 g/m ² , thickness 0.38 mm, 99,998% retention at 0.3 pm), heat resistant to a temperature of 900 °C and chemically resistant to SO2, HC1, SO3, SO4 ² ', NO, NOs’, except for HF: filtration efficiency = 99.999 ± 0.001%

- Packed test 1 : filtration efficiency = 94.736 ± 1.581%

- Packed test 2: filtration efficiency = 99.999 ± 0.001%

As data show, the silicon carbide (SiC) filter does not perform any significant filtering function, however it proves to be essential in providing the QFF filter with mechanical stability and as thermal conductor for regenerating the same at high temperatures.

Whether it be virgin, sampled or regenerated, the QFF filter shows a filtration efficiency > 99.99%.

The three-layer pack system (Packed test 1, SiC-QFF-SiC) shows an apparent filtration efficiency drop (94.736 ± 1.581) due to the SiC particles released from the ceramic filters. To solve it, a final backup QFF filter laid out on a steel mesh grid, downstream of the three-layer pack system (SiC-QFF-SiC) forms the final four-layer system (Packed test 2, SiC-QFF-SiC-QFF), ensuring the same performance in terms of filtration efficiency of the QFF filter and the biocidal and regenerating action.