2023/09/20 @ Osaka International Convention Center

International Symposiumof the Graduate School of Engineering,Osaka Metropolitan University

World-Leading Engineering Research

Conceptual proposal for a compact
droplet remover using photocatalyst
Masafumi Akiyoshi *, Satoru Watano (Osaka Metropolitan Univ.)
Tsuyoshi Ochiai(Kanagawa Institute of Industrial Science and Technology, KISTEC )Web site: http://anticovid19.starfree.jp/* akiyoshi-masafumi@omu.ac.jp（In Japanese）

Size distribution of particles
released from the oral cavity
2μm

150μm
Talking

Coughing

In medical studies, particles released fromoral cavity are distinguished between
aerosols and droplets at a diameter of 5μm.
The actual size distribution of particles
released from the oral cavity is two-peaked,
with 150μm droplets falling in about twoseconds and reaching only about two meters,
while 2μm aerosols drift in the air for a longtime. In some cases, the droplets isdehydrated to form aerosol-sized droplet
nuclei.

G.R. Jhonson et al., Modality of human expired aerosol size distributions,
J. Aerosol Science, 42(2011)839-851.

Fall down speed (m/s)

Droplets changes to droplet
nuclei with dehydration in air

Aerosol ←SARS-CoV-2
VirusDiameter(μm)→ Droplet

5μm

Nobuyuki Takegawa, Aerosol, Droplet Transmission, andAirborne Transmission, Earozoru Kenkyu, 36 (2021) 65-74.

A specialized device that removes droplets
flying between person-and-person
Droplets that would
usually reach thefacing person

Droplets released
from the oral cavity

1) Suction
Silent type fan
Contaminated
air contains
droplets

Droplets captured on the filter

The filter with visiblelight-responsible
photocatalyst2) Capture
Purified air

Visible LED modules

3) Decompose

Within 2m(Typical width of meeting table
is about 80cm)

Ultra-low-cost droplet removal system "Hikari Cleaner"using visible light responsive photocatalyst
NOT commercial productShading leaked light with Japanese paper

Luminary light between
person-and-person

The size is 12 cm square and5 cm high. Fan noise is only
19 dB. Power consumption isless than 5 W, and can be
powered by a mobile battery

Using visible light-responsive photocatalyst, it does notneed to be completely shielded from leaking light and canbe made with a simple structure. It is made by combining
commercial PC parts, therefore it cost only 1200 yen perunit. The photocatalyst filter can be manufactured with asimple non-woven fabric filter and Toshiba's "Renecat"
spray, which is commercially available. The suction
performance can be improved by using a more powerful fan.

Light (Photon)
Particle of
Photocatalyst

Reaction in photocatalyst

Reduction

O2Band Gap

Conduction band

Excitedby light

Valence band・O2-H2Oh+need UV-light・OH

Oxidation

WO3 2.5eV
Excited byvisible light(～495nm)

superoxideanions
Oxidizing power
higher than ozonee- Electronhole
TiO2(anatase) 3.2eV

Visible light（380nm～, 3.1eV～）

Chemicallydecompose a varietyof organic materials
by oxidation
Finally organic materials are breaking down
completely into water and CO2

OH radicals
Because reactive oxygen
species disappear in about
one millionth of a second,
they can only move to a few
microns from the surface.

Visualization of droplet suction by
special imaging system
Special video recording wasconducted to visualize droplets inthe air. Within a range of about
50cm, we can see that droplets
emitted by speech from the oralcavity with a "booming" sound areinhaled and stopped by the filterin the same way as a mask.

Transmittance rate of droplets to a filter
A duct was set up in a cleanbooth using HEPA filter unit, andthe rate at which ultrasonicsprayer mist, simulating dropletsfrom the oral cavity, was captured
by a non-woven fabric filter wasevaluated. As a result, it wasconfirmed that droplets of 5 μm
or larger could be almost
completely captured.
Catching andslowly decomposing

The non-woven fabric filter used in this studyCollection rate of droplets flying in space
Air flow
0.3 m/s

300mm

500mm
PCXNebulizer:
nozzle height
400mm

Small Droplet
remover: puton the deskParticle counter:nozzle height
400mm

Large clean booth: 1.5×1.5×2.4m

Density of Particles / m-3

109

Performance of the small droplet remover inopen space was estimated using large cleanbooth. Particle counters were placed offcenter axis of nebulizer and the remover. The
nozzle of the nebulizer was set horizontallyand the mist flied almost straight with afollowing wind of 0.3 m/s from air purifier units.

108107106105

Particle Counter Height 400 mm
Particle size 1-5μm, Cleaner OFF
Particle size 1-5μm, Cleaner ON
Particle size 5-25μm, Cleaner OFF
Particle size 5-25μm, Cleaner ON104
103

0100200

PCX: position of Particle Counter / mm

Large droplets with diameters of 5.0 to 25μm were reduced to about 1/10 in all
position. Aerosols of 1.0 to 5.0 μm, which
are close to the peak diameter of the aerosol
emitted from the actual oral cavity, were also
reduced to about 1/10 from center to 15cm,but at 20 and 25cm almost no reduction wasobserved.

5
k=8.84×10-4/min

Infectivity / PFUHCHO density / mg・m-3

Decompose performance for organic gas and virus in aerosol-2

k1=1.81×10 /min
k2=1.41×10-3/min

k1=2.35×10-2/min
k2=7.15×10-5/min

10.50

k=1.76×10-2/min

k=3.81×10-2/min

100106
Blank
With small droplets remover

k=6.70×10-3/min

Blank
Standard nonwoven fabric filter
Nonwoven fabric filter + 10wt% WO 3 slurry
Ceramic fiber paper + 10wt% WO 3 slurry
Ceramic fiber paper after 500℃ baking + 10wt% WO 3 slurry
Glass fiber mesh + 10wt% WO 3 slurry

200Time / min300

105010Time / min

20400

Using a 38L size acrylic desiccator,changes in the concentration offormaldehyde (HCHO), a kind of organic
gas, were measured using a formaldehyde
meter htV-m. The non-woven fabric filter
that was initially used was organic, so
there was a limit to the improvement inperformance even if the amount ofphotocatalyst loaded was increased. A
filter made of inorganic material was used,and by applying appropriate pretreatment,a significant improvement in performance
was achieved.

A solution containing bacteriophage Qβ
was sprayed with a nebulizer in a 370 L
glove box to make an aerosols. At themeasurement time, 10 L air was sampledthrough a gelatin filter and the infectivity
was evaluated by the plaque method.

Although aerosols suspended in the
air for long time cannot be caught
by the rough filter, it was suggested
that the small droplet remover usinga photocatalyst can inactivate
viruses contained in aerosols.