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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Generation of Reactive Species in Surface Micro Discharge Tube with Mist Flow
for Water Treatment
Tomohiro Shibata1 and Hideya Nishiyama2
1
Graduate School of Engineering, Tohoku University, Sendai, Miyagi, Japan
2
Institute of Fluid Science, Tohoku University, Sendai, Miyagi, Japan
Abstract: Non-thermal plasmas with water are developed for lots of applications such as water treatment and chemical synthesis. In this study, the water treatment method using surface
micro discharge tube with mist flow is developed. The dissolution characteristics of ozone,
hydrogen peroxide are experimentally clarified. Furthermore, the effect of humidity and liquid
pH for ozone and hydrogen peroxide dissolution is investigated by numerical simulation.
Keywords: Plasma, Mist flow, Numerical simulation, Water treatment
2. Experiments
0.7
H2O2
O3
0.6
ROS
0.5
10
0.4
5
0.3
0.2
0.1
0
2
4
6
8
10
12
Energy yield [g/kWh]
Fig.1 Schematic illustration of experimental setup.
Concentration [mg/l]
1. Introduction
Water pollution is serious problem not only for humans
but also for the entire ecological system. Recently, the
conventional waste water treatment, e.g., biological and
chemical methods, has been replaced by the plasma
treatment. In the plasma treatment, organic compounds
are generally decomposed by only part of the ozone generated by plasma. This is because ozone has high oxidation potential and is effective for decomposition of organic compounds in water [1]. However, many radicals, such
as H2O2, O･, OH･, O3*, N2*, e-, etc., generated by plasma
cannot be used for water treatment because the discharge
area is separated from water in general plasma treatment
such as ozone treatment. If the plasma is generated near
the solution, the radicals can be utilized for water purification. The water treatment systems utilizing the discharge of bubbles [2,3], above water [4] and with mist
flow [5] have been developed. It has been reported that
the method of spraying waste water into reactive plasma
has the highest relative energy efficiency[6]. Therefore,
the authors have reported the water treatment method
using a surface micro discharge (SMD) tube [7]. However,
the chemical reaction between plasma and liquid is complicated and the understanding is not sufficient. For understanding the detailed chemical reaction, the numerical
simulation is useful method.
In this study, the dissolution characteristics of hydrogen
peroxide (H2O2), ozone (O3) and reactive oxygen species
(ROS) are measured as indicator species for chemical
reactions using the SMD tube. Particularly, H2O2 is one of
the most important species for water treatment by plasma.
Because H2O2 is an useful indicator for hydroxyl (OH)
radical which has high oxidation potential. Furthermore,
the zero-dimensional simulation of chemical reaction in
atomized liquid introduced into the SMD tube are conducted and compared with the experimental results for
further understandings of chemical reaction between
plasma and liquid.
0
pH
Fig.2 Concentrations and Energy yields of dissolved
chemical species versus pH.
2.1 Experimental apparatus
Figure 1 shows a schematic of the experimental setup,which mainly consists of electric power supply, ultrasonic atomizer units, a SMD tube, mist separators and an
air pump. Two ultrasonic atomizer units are used to gen-
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
erate sufficient amount of mist. Air or Ar is used as carrier
gases. The SMD tube is made of Teflon with a thickness
of 0.5 mm and has an inner mesh electrode made of
stainless and an outer grounded electrode made of copper.
The inner diameter and the length of discharge area is 22
mm and 50 mm respectively. The sinusoidal voltage (10
kVpp, 1000 Hz) is applied to the inner mesh electrode.
Reactive plasma is generated on the inner wall of SMD
tube by dielectric barrier discharge (DBD). The power
consumption is about 1 W (300 W/m2).
The amount of dissolved H2O2 is measured by a
coulometric titration. The amount of O3 and ROS are
measured by a water quality meter (MultiDirect, AQUA
LYTIC) with N,N-diethyl-p-phenylenediaminesulfate
(DPD).
These thicknesses are decided to equalize the volume
ratio in model with those of SMD tube when the plasma
thickness is assumed about 0.1 mm. Γpg,i is the diffusion
flux between plasma and gas phase simulated as follows
equation (5).
2.2 Experimental results
As shown in Fig. 2, the amounts of the dissolved species are influenced by the solution pH. This pH dependence results from ozone self-decomposition. Ozone
self-decomposition starts with reaction (1) and (2). O2generated in (2) reacts with O3 and the radical chain reaction starts.
O3 + OH- → ･O2- + HO2･
(1)
HO2･ ↔ H+ + ･O2(2)
An increase in the pH results in a slight increase of the
H2O2 concentration and pass through the maximum at pH
10. This is because the ozone decomposition is enhanced
and generated H2O2 as a final product in alkaline solution.
However, the H2O2 concentration is decrease in the pH
region above 10, because H2O2 acts as acid in this pH
region. On the other hand, ROS is dissolved in acid solution effectively, because the HO2 radicals are generated by
(2) in H+ rich solution. In addition, the O3 concentration is
almost the same at any pH.
Fig. 3 Simulation model.
3. Numerical simulation
3.1 Simulation model
Figure 3 shows our simulation model. Our model has
two step simulation and three phases (plasma, gas and
liquid) are included.
The first step simulation considers plasma and gas
phase based on the model of Sakiyama’s paper [8]. 53 and
21 species are contained in plasma and gas phases respectively, as shown in table 1. The diffusions between plasma
and gas are simulated about 21 common specie. The
number densities of species i in plasma and gas phase (np,i
and ng,i) are simulated as following equation (3) and (4)
respectively
(3)
(4)
where t is the time. Gi and Li are the generation and loss
terms for species i, respectively. dp (=0.1 mm) and dg
(=5.4 mm) are the thickness of plasma and gas phase.
Table 1 Considered species for each phase in 1st and 2nd
step simulations.
(5)
where Dg,i is the diffusion coefficient in gas phase for
species i. The background gas is humid air and the assumed gas temperature is 300 K. The H2O concentration
is varied 0 % to 3 %. The Gaussian-like pulse electric
field is applied on the plasma phase with 1 kHz. The peak
value of electric field is recalculated each cycle for fix the
power consumption at 300 W/m2. The simulation time is
100 ms that is the residence time of a droplet in our experiment.
The second step simulation considers gas and liquid
phases [9-12]. Gas and liquid phases contain 10 and 21
species respectively, as shown in table 1. The concentrations of species in gas and liquid phases (Cg,i and Cl,i) are
simulated as following equation (7) and (8) respectively
(7)
(8)
where Wl (= 100 ppm) is the mist concentration. Γgl,i is the
diffusion flux for species i between gas and liquid phase
simulated as following equation (9)
(9)
where kmt,i is a combined rate coefficient of species i for
gas phase plus interfacial mass transport,
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
(10)
Hi is the Henry’s coefficient for species i, R (= 0.0821 atm
ℓ K-1 mol-1) is the ideal gas constant and Tg is the gas
temperature. dl (= 2μm) is the droplet diameter. The effective Henry’s law constant H’i for a species i which undergo
1022
time of droplet in our experiment). After diffusion simulation, only liquid phase is simulated for 10 s as after treated solution.
3.2 Numerical results
Figure 4 shows the time evolution of the gas phase O3
number density in 1st step simulation for any humidity
with experimental result. Ozone can be diffused sufficiently into gas phase from plasma layer within treatment
time (0.1 s).
1.5
1020 Initial H 2O concentration
1019
1018
1%
2%
3%
10-4
10-3
10-2
time [s]
10-1
Dissolved O3 [mg/l]
O3 density [/m3]
Experimental result
1021
Experimental result
Numerical simulations
1% initial H2O
2% initial H2O
1
3% initial H2O
only 1000 ppm ozone (assumed)
0.5
0
Fig. 4 Time evolution of O3 number density in gas phase.
2
4
6
8
10
12
Fig. 6 O3 concentrations as a function of solution pH.
10
1018
Initial H 2O concentration
1%
2%
3%
30
Dissolved H2O2 [mg/l]
19
1017
1016
1015
10-4
10-3
10-2
time [s]
10-1
Fig. 5 Time evolution of H2O2 number density in gas
phase.
ionic dissociation such as HNO3 and H2O2 is deferent
from H. H’ is simulated as following equation (11),
(11)
where Ki is the ionic dissociation constant for species i
and [H+] is the H+ concentration. As shown in equation
(7), the gas phase species are simulated only diffusion in
second step. The diffusion is simulated 0.1 s (residence
Experimental result
Numerical simulations
1% initial H2O
2% initial H2O
20
3% initial H2O
only 1000 ppm ozone
(assumed)
0.02
0.01
10
0
2
4
6
8
10
Dissolved H2O2 [mg/l]
H2O2 density [/m3]
1020
0
12
pH
Fig. 7 H2O2 concentrations as a function of solution pH.
The simulated number density of O3 increase as time and
approaches to 1.7×1021 m-3 of the experimental result.
This experimental O3 number density is measured after
one pass through the SMD tube. The simulated O3 number density reaches about 2 × 1021 m-3 and is independent
of background gas humidity.
Figure 5 shows the time evolution of H2O2 number
density in gas phase for any humidity. H2O2 is also dif-
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
fused into gas phase and the number density reaches
about 2.7×1018 m-3 to 2×1019 m-3. The final number density of H2O2, which varied between 2.7×1018 m-3 and
2×1019 m-3, depends on the background gas humidity.
Because the source of H2O2 is H2O.
The final number density of O3, H2O2 and other species
in 1st step are used for background gas in 2nd step simulation.
Figure 6 shows the concentration of ozone after 10 s
simulation for any initial solution pH with experimental
results.
The simulated ozone concentrations in liquid phase is at
most 0.02 mg/ℓ with the background gas which has been
simulated in 1st step. These concentrations are considerably lower than experimental result. The simulated ozone
concentration in liquid phase with 1000 ppm ozone as
background gas shows almost same concentration with
experimental result. Because the simulated gas phase O3
number density in 1st step is lower than that of real experiment. The working gas is returned back to tank in our
experiment. Ozone with long life time is not decomposed
after passing through the SMD tube and reentry to the
SMD tube. Therefore, the O3 number density increase as
operation time increase.
Figure 7 shows the concentration of H2O2 after 10 s
simulation for any initial solution pH with experimental
result. The simulated concentration with the background
gas which has been simulated in 1st step with 1 % of initial H2O number density agrees with experimental results.
Although the decrease in H2O2 concentration above pH 10
is shown in simulation, the pH dependence does not agree
with that of experiment, namely that the H2O2 concentration slightly increase as pH increase and pass through the
maximum at pH 10. However, the simulated H2O2 concentration with 1000 ppm ozone as a background gas
shows maximum value at pH 10. This background gas
condition showed the same order of simulated ozone
concentration as that of experiment in Fig. 6. These simulations shown in Fig. 7 indicate that the pH dependence of
dissolved H2O2 concentration mainly result from the dissolution of ozone and the ozone self decomposition.
4. Conclusions
In this study, the generations of reactive species, such
as O3 and H2O2, in mist flow by SMD tube are investigated for water treatment applications. Furthermore, the
zero dimensional simulation containing the reaction of
plasma, gas and liquid phase are conducted. The obtained
results are summarized as follows.
(1) The reactive species such as H2O2, O3, and ROS
are dissolved into the droplets effectively. The
dissolved H2O2 concentration is influenced by
solution pH. An increase in the pH results in a
slight increase of the H2O2 concentration and
shows the maximum at pH 10. On the other hand,
(2)
(3)
(4)
(5)
the dissolved ozone concentration is not varied
with solution pH.
The ozone concentration in gas phase is simulated by zero dimensional simulation. The ozone
concentration increase with time and close to
64.7 ppm of experimental result. The O3 number
density in gas phase is not varied with initial H2O
concentration in background gas.
The simulated H2O2 concentration increase with
time. The last concentrations increase from 0.16
to 0.72 ppm with increase of background gas
humidity.
The simulated O3 concentration in liquid phase
with background gas simulated in 1st step is lower than that of experiment. When assuming 1000
ppm ozone as a background gas, the simulated
O3 concentration is nearly the same with that of
experiment.
The simulated H2O2 concentration in liquid
phase with 1 % H2O background gas agrees with
that of experiment. The pH dependence of concentration of H2O2 in liquid phase is indicated in
the simulation with 1000 ppm ozone as the
background gas. The pH dependence of H2O2
dissolution result from the dissolution of ozone
and the ozone self decomposition.
Acknowledgments
This study was partly supported by Grant-in-Aid for
Challenging Exploratory Research (24656117) in JSPS
and a Grant-in-Aid for JSPS Fellows (24･9008). The authors would like to thank Assoc. Prof. H. Takana for valuable discussion, Mr. T. Nakajima and Mr. K. Katagiri for
technical supports with IFS, Tohoku University.
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21 International Symposium on Plasma Chemistry (ISPC 21)
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Cairns Convention Centre, Queensland, Australia
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