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Influence of oxygen concentration, feed gas flow rate and air humidity on the output of ozone produced by corona discharge: Frailty and Life Satisfaction in Elderly

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Ozone (trioxygen) shows a lack of stability and quickly breaks down to dioxygen, meaning that ozone in any way can not be stored. It is therefore necessary to produce ozone on site where it is used. This leads to the idea of having an ozone generator with high efficiency. Ozone generator is an electrical device to convert oxygen as feed gas into ozone. Besides electrical parameters, feed gas plays an important role for ozone output. Feed gas for an ozone generator may be ambient air, dry air or oxygen rich gas. The influence of air humidity, oxygen concentration, flow rates of feed gas and their effects on ozone output of the corona discharge ozone generator have been investigated. For small‐scale ozone production (i.e. one hundred grams per hour or less), using oxygen produced by pressure swing adsorption method (PSA) is considered to be a reasonable solution. The authors intend to connect the studies with the practical reality of using low‐powered ozone generators in tropical area with high humidity condition.
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Cite this paper: Vietnam J. Chem., 2019, 57(5) 604-608 Article
DOI: vjch.201900095
604 Wiley Online Library © 2019 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Influence of oxygen concentration, feed gas flow rate and air humidity
on the output of ozone produced by corona discharge
Le Cao Cuong1, Nguyen Hoang Nghi1*, Tran Vinh Dieu1,
Doan Thi Yen Oanh2*, Dang Duc Vuong1
1Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung, Hanoi 100000, Viet Nam
2Publishing House for Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang
Quoc Viet, Cau Giay, Hanoi 100000, Viet Nam
Received July 23, 2019; Accepted for publication July 26, 2019
Abstract
Ozone (trioxygen) shows a lack of stability and quickly breaks down to dioxygen, meaning that ozone in any way
can not be stored. It is therefore necessary to produce ozone on site where it is used. This leads to the idea of having an
ozone generator with high efficiency. Ozone generator is an electrical device to convert oxygen as feed gas into ozone.
Besides electrical parameters, feed gas plays an important role for ozone output. Feed gas for an ozone generator may
be ambient air, dry air or oxygen rich gas. The influence of air humidity, oxygen concentration, flow rates of feed gas
and their effects on ozone output of the corona discharge ozone generator have been investigated. For small-scale ozone
production (i.e. one hundred grams per hour or less), using oxygen produced by pressure swing adsorption method
(PSA) is considered to be a reasonable solution. The authors intend to connect the studies with the practical reality of
using low-powered ozone generators in tropical area with high humidity condition.
Keywords. Ozone, generator, ozone output, oxygen concentration, humidity, flow rate.
1. INTRODUCTION
Ozone as a powerful oxidant is widely used for
water and air disinfection because it can destruct
varied compounds especially organic ones such as
bacteria and fungi which comes in contact with
ozone molecules, or more accurately speaking, with
the extra oxygen radicals disassociated from
ozone.[1] Ozone is commonly known as chemically
not stable and spontaneously breaks down into
oxygen after a few hours. This is the reason why
ozone cannot be stored in high pressured cylinder,
hence ozone must be produced on site where it is
used. For this reason, ozone application is always
coupled with ozone generators. Quality of ozone
generators depends on electrical parameters such as
voltage, frequencies and, also, it strongly depends on
the quality of the feed gas (oxygen-rich gas). Feed
gas for ozone generators must be dry and oxygen
rich. Flow rate of feed gas also plays a significant
role. Three requirements for professional ozone
production include (i) high ozone output (ozone
production power), (ii) high ozone concentration and
(iii) low concentration of nitrogen oxides. The study
in this paper mainly focuses on the quality of feed
gas and its effect on ozone output and concentration.
The influence of air humidity, oxygen concentration
and flow rate on the ozone output and concentration
will be shown and discussed in details. It should be
noted that, ozone generators of low power (i.e. a few
grams per hour or less) usually use ambient air as
feed gas. Therefore, investigation of the influence of
ambient air humidity on work of low power
generator has practical meanings for use in the area
with high humidity such as North Vietnam. Besides,
oxygen generators based on PSA technique have
been designed and manufactured in BKIDT
Company (Hanoi). Oxygen as feed gas produced by
PSA is considered to be a suitable solution for small
and high efficiency ozone generators in comparison
with those by fractional distillation of liquefied air.
2. MATERIALS AND METHODS
The output P of ozone in the corona discharge
generator is investigated. The ozone output value is
shown in a mass over time value g/hr, i.e. the mass
of ozone produced by ozone generator in a given
period of time. This value can be calculated through
the flow rate R of feed gas (m3/hr) and ozone
concentration C (g/m3) by:
P = R x C
Vietnam Journal of Chemistry Nguyen Hoang Nghi et al.
© 2019 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.vjc.wiley-vch.de 605
It is commonly known that flow rate is measured
in LPM (liter per minute) and ozone concentration in
g/m3; in this case the output is calculated as:
P (g/hr)= [R x (LPM x 60) x 0.001] x C(g/m3)
The above conversion formula shows the
relation between ozone output (g/h), the flow rate
(L/min) of feed gas and ozone concentration (g/m3).
Feed gas plays a very important role in corona
discharge ozone production. There exist three types
of feed gases used for ozone generation. They are
ambient air, dry air and concentrated oxygen. The
dry air refers to air which has moisture removed so
the dew point is 60 oC or lower. (At 60 oC, mass
of water in 1 m3 of air is near zero (0.008 g/m3), at
+20 oC, it is 17.5 g/m3, i.e. 2,180 times greater).
Concentrated oxygen made by the fractional
distillation of liquefied and stored in pressured
cylinders (> 96 %) or in the form of liquid oxygen
issued in large-scale ozone production, for instance
P~100 kg ozone per hour. Oxygen produced by PSA
is used on site and for small-scale ozone production
(i.e. hundreds grams of ozone per hour).
For small output ozone generator (a few up to
~100 gr. per hour), ambient air and oxygen-rich gas
produced by PSA equipment are used. Our research
focuses on these feed gases for small generators.
Ambient air with high relative humidity up to 90 %
or higher is usually observed in the tropical area.
The experiments are performed in a room where the
air is controlled by a humidifier to change the
relative humidity from 25 % to 96 % at the
temperature of around 24 oC. Ozone output vs.
humidity is measured while air flow rates are kept
unchanged (5 L/min). The influence of oxygen
concentration on ozone output is investigated by
using the 98 % oxygen from high pressure cylinder
and its mix with air.
To realize the above mentioned tasks, the
following devices are used: an ozone corona
discharge generator and oxygen PSA generator
manufactured in BKIDT company (Hanoi), an ozone
and oxygen concentration meters (in gr/m3 and in
%), a gas rotor flow meter and a 98 % oxygen from
a pressure cylinder. Ozone production discharge is
the dielectric-barrier discharge (DBD) that is the
electrical discharge between two electrodes
separated by a quartz dielectric barrier. Thanks to a
dielectric barrier, discharge becomes silent
(inaudible) and soft. This kind of discharge is called
corona discharge that differs from the spark or arc
discharge. Corona (DBD) discharge is also called
volume discharge, i.e. it is pervasive to all space
between electrodes. When oxygen flows to the cell
filled with corona media, the bombardment between
radicals (high energy electrons) and oxygen
molecules takes place with high probability.
Oxygen generator PSA used in this work is
based on the ability of porous zeolite (adsorbent) to
selectively adsorb gas molecules. Nitrogen molecule
from air is kept in zeolite layers but oxygen crosses
it and goes to user.
The scheme for ozone measurement is shown in
figure 1. Inlet gas comes to the ozone generator
(ozone cell) from three sources: (i) high pressure
oxygen cylinder, (ii) oxygen from PSA generator
and (iii) humidified ambient air. The oxygen
concentration, humidity and feed gas flow rate are
measured and controlled. An ozone meter is used to
measure the ozone concentration.
Figure 1: Scheme for ozone measurement
(Ozone output vs oxygen concentration, feed gas flow rate and air humidity)
Vietnam Journal of Chemistry Influence of oxygen concentration, feed…
© 2019 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.vjc.wiley-vch.de 606
3. RESULTS AND DISCUSSION
3.1. Influence of oxygen concentration on the
ozone output
There are two values characterizing the quality of an
ozone generator: the ozone output (or ozone power
production) P (g/hour) and the ozone concentration
C (g/m3). Both are important for application. The
first refers to the power of the generator that needs
to know for different purposes of application. As
commonly known, the greater ozone concentration is
higher soluble in water.
Figure 2 shows the increase of ozone output
from near 3 g/hr to ~24 g/hr vs. oxygen
concentration increase from 21 % to 98 %. Ozone
output of a generator using oxygen is usually twice
more than using dry air (2X). In this work, this ratio
is near 8 (8X), (24 g/hr vs. 2.85 g/hr). Very high
productivity of oxygen compared with ambient air as
feed gas is reached because ambient air was not dry
(air humidity very high, > 80 %, at 24 oC),
consequently, output is low. The feed gas of 35 %
oxygen increases ozone output twice in comparison
with the humid ambient air (5.94 vs. 2.85 g/hr).
The high ozone productivity of oxygen (98 %)
feed gas was achieved because high oxygen
concentration increases the probability of
bombardment between oxygen molecules and
radicals (electrons and ions) created by corona
discharge. Ozone concentration vs. oxygen
concentration shows a similar result: the higher
oxygen concentration in feed gas is, the higher
ozone concentration (21 % oxygen produced ~9
g/m3 ozone, 98 % oxygen gives 80 g/m3 ozone).
Figure 2: Ozone output (g/hr) and ozone concentration (g/cubic meter) vs. oxygen concentration (%) of inlet
gas (ambient temperature 24 oC, flow rate 5 L/min, electric current of ozone generator ~AC 2.5 A, 220 V
The use of oxygen from pressure cylinder or
liquid oxygen usually takes place in industrial
application of ozone (for example where the ozone
output reaches hundreds of kilograms ozone per
hour). For small-scale ozone generator (50-200g per
hour), feed gas is oxygen produced by PSA oxygen
generator using zeolites as adsorbent. Oxygen
concentration produced by PSA equipment made in
BKIDT Company (Hanoi) is high enough (near 96-
98 %). Ozone output using this oxygen reaches 24
g/hr, which equals to that using oxygen produced by
fractional distillation of liquefied air in Hanoi.
Besides, concentrated oxygen does not cause
formation of nitrogen dioxides in ozone cell that
prevents formation of nitric acid as a factor
damaging metallic details of ozone generator.
3.2. Influence of feed gas humidity on ozone
output
There are a lot of studies on effect of humidity in
ozone production including numerical simulation.[2]
In this paper, the investigation of effects of moisture
in feed gas was carried out in a narrow range of
humidity from 25 to 96 % that is usually observed in
the ambient air in tropical area. The moisture causes
a reduction of ozone output in corona discharge
generator by some mechanisms. Radicals (high
energy electrons) in corona interact with the water
molecules and it results in the formation of radical
OH*. In turn, the OH* radicals react with ozone
molecules and destroys the last.
Naturally, in water vapor there always exists
anion hydroxide OH. Ozone reacts with OH to
Vietnam Journal of Chemistry Nguyen Hoang Nghi et al.
© 2019 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.vjc.wiley-vch.de 607
create radicals O2
∗−2 , HO2*, OH* + O2* as
follows:
O3 + (OH) 𝐎𝟐
∗−𝟐 + HO2*
or O3 + OH → HO2 + O2
and next: 2 O3 + HO2 → OH* + O2*+ O2
These reactions consume an amount of ozone so
it results in ozone output decrease.[3] According to
Salvermoser and Kogelschatz, high energy electrons
in corona discharge causes formation of radical OH*
in water vapor that sets up a catalytic reaction cycle
resulting in enhanced ozone destruction.
There is another way to explain the decrease in
ozone output by moisture in feed gas. Moisture stuck
in dielectric surface changes its conductivity that
decreases the power of corona discharge.
As mentioned above, moisture in ambient air
causes damage to the generator by forming nitrogen
oxides and nitric acid inside ozone cell.
It is not easy to get dry air (i.e. air with very low
dew point, -60 to -20 oC). It is believed that for small
ozone generators (a few grams per hour), inlet air
with humidity of lower than 50-60 % should be used
because relative humidity in the range of 20-50 %
does not significantly decrease ozone output (see
Fig. 3).
Relative humidity, % (at 24oC)
Figure 3: Ozone output (g/hr) vs. relative humidity of air as inlet gas (ambient temperature ~ 24 oC, flow rate
~ 5 L/min, electric current of ozone generator ~ AC 2.8 A, 220 V). Ozone output decreases 1.4 times while
humidity increases from 21 to 96 %
3.3. Influence of feed gas flow rates on ozone
output and ozone concentration
Concentration change: Flow rate of feed gas
controls the mass of oxygen molecules that interacts
with plasma in the ozone cell to produce ozone. Low
flow rate causes an increase of the contact time of
oxygen molecules with the radicals (high energy
electrons) in corona and on the other hand, yielded
ozone slowly is released from ozone cell. In total, it
results in keeping high ozone concentration in ozone
cell. The increase of flow rate leads to the decrease
of ozone concentration (88 g/m3 at 3 L/min down to
29 g/m3 at 20 L/min) (figure 4). From the application
point of the view, high ozone concentration
enhances the solubility of ozone in water. For
instance, ozone dissolved in water increases 5 times
(from 10 to 50 mg/L), while ozone concentration
increases from 25 to 125 g/m3. Therefore, for water
disinfection high ozone concentration is desired.
Output change: Flow rate also significantly
exerts influence on ozone output of an ozone
generator.[4] Low flow rate produces low ozone.
Low flow rate inhibits the oxygen volume going to
and taking part in reaction in ozone cell. Let us
imagine an ozone cell having a given, no-changing
volume of oxygen (flow rate is very small). The
reaction to produce ozone consumes an amount of
oxygen inside the cell. Consequently, an oxygen
lack in the cell leads to a limit of ozone yield.
Pumping feed gas into ozone cell (rising the flow
rate) increases the amount of oxygen in the cell,
which results in an enhancement of ozone output. In
addition increasing flow rates, ozone output will get
saturated, or even slightly decreases, when flow rate
gets to a certain high value. In the ozone cell, the
molecules being interacted by the electrons can be
considered as immobile, because its thermal and
flow velocity is negligible compared to the speed of
the electron passing through it (~10-15 s). Therefore,
this motion does not effect on the ozone output.
According to Bernoulli principle, the speed of the
fluid goes up and its pressure goes down (ozone cell
is made in form of a tubes of small radius like to the
venturi tube). As a result, density of the gas
(oxygen) inside the ozone cell becomes lower, so
Vietnam Journal of Chemistry Influence of oxygen concentration, feed…
608
probability of bombardment between oxygen
molecules and radicals in corona media becomes
smaller, leading to a decrease of ozone output. In
sum, ozone output, first, will increase with the
increase of flow rates, then, at a certain high flow
rate output becomes saturated or even slightly
decrease (figure 4).
From the application point of view, it is
necessary to have both high output and high ozone
concentration. High ozone concentration is
important particularly for water disinfection because
ozone solubility in water is higher using ozone of
higher concentration.
Figure 4: Ozone output (g/hr) and ozone concentration (g/cubic meter) vs flow rate of oxygen as feed gas.
Concentration of oxygen ~ 98 %, ambient temperature ~ 24 oC
4. CONCLUSIONS
1. Using oxygen feed gas increases ozone output up
to 8 times in comparison with using high humid
ambient air.
2. Using oxygen produced by Pressure Swing
Adsorption oxygen generator (PSA) designed and
made in BKIDT Company (Hanoi) yields the same
ozone output as using oxygen from pressure
cylinder.
3. The ambient air with humidity from 25 % up
to 50 % (at 24 oC) almost does not change ozone
output. But humidity higher (70-80 %) significantly
reduces ozone output.
4. Increasing the flow rate would first causes an
increase of output, and then at a certain high flow
rate, the output reaches saturation or even slightly
decreases. There is a need to choose optimal flow
rate for every sort of ozone generator.
Acknowledgments. The experiments have been
carried out at the Lab of BKIDT Company (Hanoi).
The authors were thankful to Miss L. D. Lam, L. T.
Lam, N. V. Thin and V. X. Huong for performing
some measurements.
REFERENCES
1. Nguyen Hoang Nghi, Le Cao Cuong, Tran Vinh Dieu
and Doan Thi Yen Oanh. Study of water disinfection
by analyzing the ozone decomposition process,
Vietnam J. Chem., 2018, 56(5), 590-594.
2. R. Peyrous. The Effect of Relative humidity on ozone
production by corona discharge in oxygen or air A
numerical simulation - Part II: Air, Ozone Science
and Engineering, 2008, 12(1), 41-64.
3. Nguyen Hoang Nghi et al. Book: Water in nature and
the principles of water filtration and disfection,
Chapter 8, Publishing House for Science &
Technique, 2013 (in Vietnamese).
4. F. Tanaka, T. Iwaishi, T. Sakugawa, H. Akiyama.
Influence of gas flow rate and pressure in reactors on
ozone production using a compact pulsed power
generator, IEEE Pulsed Power Conference, 2011.
Corresponding authors: Nguyen Hoang Nghi
Hanoi University of Science and Technology
1, Dai Co Viet, Hai Ba Trung, Hanoi 100000, Viet Nam
E-mail: nghi.bachkhoa@gmail.com
Doan Thi Yen Oanh
Publishing House for Science and Technology
Vietnam Academy of Science and Technology,
18, Hoang Quoc Viet, Cau Giay, Hanoi 100000, Viet Nam
Email: doanyenoanh@vjs.ac.vn
... Eventually, the flow rate would first cause an increase of output, and then at a certain high flow rate, the output reaches saturation or even slightly decreases. So, there is a need to choose the optimal flow rate for every sort of ozone generator 99 . ...
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