Download Gas-liquid two-phase chemical reaction model of reactive plasma inside a bubble for water treatment

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Gas-liquid two-phase chemical reaction model of reactive plasma inside a
bubble for water treatment
T. Hayashi1, S. Uehara2, H. Takana2 and H. Nishiyama2
1
Graduate School of Engineering, Tohoku University, Sendai, Miyagi, Japan
2
Institute of Fluid Science, Tohoku University, Sendai, Miyagi, Japan
Abstract: Numerical simulation of chemical reactions inside a bubble is conducted. The
model is developed based on the experiment. The numerical model is a zero-dimensional
model in order to conduct simulations of a repetitive discharge for relatively long duration.
OH produced in a gas phase is transported into liquid phase by a diffusion trough the
bubble interface. The concentrations of chemical species in gas- liquid phases are obtained.
Keywords: chemical reaction model, water purification, discharge inside a bubble
1. Introduction
Water pollution is urgent issue to be solved recently.
Ozone has been widely utilized as for water purification
systems since ozone has high oxidation potential and long
lifetime.
However, the oxidation potential of ozone is not strong
enough to decompose persistent organic substances such
as dioxin and acetic acid completely.
Recent years, it has been paid attention to advanced
oxidation processes (AOPs) using non-thermal plasma
[1-2]. Hydroxyl (OH) and oxygen (O) radicals generated
by non-thermal plasma have stronger oxidation potentials
than that of ozone. Due to the short lifetime of these
radicals, initiating plasma in water or near water surface is
effective method to decompose the pollutant by radicals.
One of the effective methods is discharge inside
bubbles injected in water [3]. The advantage of this
method is that it requires less energy to initiate the
discharge than the direct discharge in water since the
discharges can occur easily in a gas phase. Another
positive aspect is that the method does not release
by-products to the environment.
Decomposition of acetic acid by OH radical generated
by nano-second pulse discharge inside bubbles is reported
in the previous experiments [4].
For further improvements of the decomposition
performance, it is essential to clarify the radical
generation process by the discharge inside bubbles and
obtain the physicochemical factors which increase the OH
radical generation in water.
In this paper, a gas-liquid two-phase chemical reaction
model of numerical simulation for discharged bubble was
originally developed to provide the fundamental data for
the improvement of the water treatment process. This
model includes the chemical reactions in gas and liquid
phases and the diffusion through the bubble interface. The
numerical simulations were conducted under various
conditions based on the experiments [4].
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2. Gas-liquid two-phase numerical model of chemical
reaction inside a bubble
The schematic image of developed zero-dimensional
two phase (gas and liquid) model of radical generation is
shown in Fig. 1.
Fig. 1. Schematic of the chemical reaction model in a Ar
gas bubble.
In the model, a domain where a streamer progressing
along a bubble interface is assumed to be a domain as gas
phase [5].
The reaction of O 2 is assumed to be able to neglect
since the operating gas is Argon gas. After second pulse
is applied, oxygen is generated by the chemical reaction
which originate from H 2 O and OH. However, in order to
operate the calculation longer to clarify the
physicochemical characteristics of OH when the pulse is
applied repetitively, chemical reaction of O 2 is not
considered.
Chemical model is developed based on the one
proposed by Matsui et al. [6].
Chemical species are diffused into liquid phase through
the interface of bubble based on the Henry’s law with
Henry’s constant H i for each species. The governing
equation is as follows:
1
dC gas ,i
dt
dC liq ,i
dt
= −k mtWL C gas ,i + k mtWL
= k mt C gas ,i − k mt
1
C liq ,i + G gas ,i − L gas ,i
H i RTg
1
C liq ,i + Gliq ,i − Lliq ,i
H i RTg
(1)
(2)
where, C i , and R, T g denote a concentration of species i,
gas constant and a temperature, respectively. k mt is the
mass transport coefficient which is determined by the
bubble form. G and L denote the term which related to a
generation and a loss of species, respectively.
The temperature (295 K) and pressure (1 atm) is
constant both in liquid and gas phases. W L , the ratio of
liquid volume to the bubble volume is taking into account
for the effect of the volume difference. Applied electric
potential is obtained by the numerical iterative calculation
using the value from the previous experiment [4].
Firstly, the simulations for a single nano-pulse
discharge were conducted. The influences of vapor
concentrations and applied voltages on the time evolution
and the diffusion fluxes of chemical species were
evaluated. Secondly, the simulations for repetitive nanopulse discharges were conducted. OH radical in liquid
phase which transported from a gas phase by a diffusion
trough the bubble interface was investigated.
3. Results and discussion
3.1. Single discharge inside a bubble
Fig. 2 shows the production rates of OH in a gas phase.
OH is generated rapidly by H 2 O dissociation through the
electron impact during the discharge in a gas phase. The
OH concentration decays rapidly with the generation of
H 2 O 2 and HO 2 .
Fig. 3. Time evolution of OH, H 2 O 2 , HO 2 and O 2 in
liquid phase when applying single pulse.
3.2. Repetitive discharges inside a bubble
Fig. 4 shows the time evolution of OH, H 2 O 2 , HO 2 and
O 2 in gas phase at V = 5 kV, f = 2000 Hz until 18 μs when
the bubble is dispatched [4]. The electron increases from
10-9 mol/l to 10-5 mol/l and changes periodically under
discharge. OH, H 2 O 2 , HO 2 change periodically with
production and decay corresponding to the electron
behaviour. However, O 2 increases gradually due to the
neglect of chemical reaction with electron.
O2
H2O2
OH
HO2
electron
Fig. 4. Time evolution of electron OH, H 2 O 2 , HO 2 and
O 2 in gas phase when applying repetitive pulse.
Fig. 5 shows the time evolution of same chemical
species in liquid phase. The concentration of OH in
liquid phase decreases 10-4 down compared with that in
gas phase. All chemical species increases gradually with
time.
Fig. 2. Time evolution of production rates of OH in a gas
phase when applying single pulse.
Fig. 3 shows the concentrations of chemical species in
liquid phase. OH increases gradually with respect to time
by the diffusion from gas phase trough the bubble
interface. However, the final concentration of OH in
liquid phase is 10-4 times lower than the one produced in a
gas phase during the discharge.
O2
H2O2
OH
HO2
Fig. 5. Time evolution of OH, H 2 O 2 , HO 2 and O 2 in
liquid phase when applying repetitive pulse.
2
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4. Conclusions
Numerical simulation of chemical reaction inside a
bubble in water is conducted with using the value
obtained in experiment.
5. Acknowledgements
This work was supported by JSPS KAKENHI Grant
Number 26249015.
6. References
[1] T. Shibata, et al. J. Phys. D: Appl. Phys., 47,
105203 (2014)
[2] T. Shibata, et al. Plasma Chem. Plasma Process.,
34, 1331-1343 (2014)
[3] H. Nishiyama, et al. J. Fluid Sci. Technol., 8, 65-74
(2013)
[4] H. Nishiyama, et al. Plasma Chem. Plasma
Process., online (2015)
[5] W. Tian, et al. J. Phys. D: Appl. Phys., 47, 055202
(2014)
[6] Y. Matsui, et al. Plasma Sources Sci. Technol., 20,
034015 (201)
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