Conversion of BTX by Rotating Arc Plasma at Atmospheric Pressure
Hyun-Woo Park1), Sooseok Choi2), Dong-Wha Park1)
1) Department of Chemical Engineering and RIC-ETTP(Regional Innovation Center for
Environmental Technology of Thermal Plasma), Inha University, 253 Yonghyun-dong, Nam-gu,
Incheon, 402-751, Republic of Korea
2) Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama, 226-8502 Japan
Abstracts: The purpose of this study is to investigate the decomposition of
volatile organic compounds (VOCs) gases of BTX (Benzene, Toluene and
m-Xylene) in air by rotating AC arc plasma. The highest energy efficiency in
the conversion process was m-xylene, followed by toluene and benzene. The
maximum conversion of benzene, toluene and m-xylene was 79%, 100% and
100% at the specific input energy (SIE) of 1,617 J/L. Main by-products of
the conversion process were CO2, CO, NO2 and NO.
Keyword: Rotating arc plasma, VOCs, BTX, Input energy, By-product
Introduction
Volatile organic compounds (VOCs) generated
from various industrial plants are serious
environmental problem, because they spread easily
in air and are hazardous substances on human beings
by causing many diseases such as allergic reactions;
headache; eye, nose or throat irradiation; dry cough;
dizziness and nausea; tiredness and even cancer [1].
Therefore, many technologies have been developed
for the conversion of VOCs into an environmentally
benign material. Among those technologies, plasma
processes including arc discharge, silent discharge
[2], ferroelectric pellet-packed reactor [3], pulsed
corona discharge [4], non-thermal plasma combined
with catalysts at room temperature [1,5-7], and
dielectric barrier discharge [8] have been introduced
as attractive methods for the decomposition of
VOCs. It is because that a conversion process based
on plasma has high treatment efficiency.
Nevertheless, there are two important issues to be
solved those are reducing power consumption and
controlling by-products emission from the
conversion process [9].
In order to evaluate a rotating arc plasma system
for an efficient conversion of VOCs, it was used to
decompose Benzene, Toluene, and m-Xylene (BTX)
in the present experimental work. The economic
Fig. 1. Schematic diagram of rotating arc plasma system
feasibility of the rotating arc plasma system was
analyzed by investigating effects of specific input
energy (SIE) on the BTX conversion efficiency and
yields of by-products such as CO2, CO, NO2, and
NO were also measured according to various
operating conditions.
Experimental
Figure1 shows the schematic diagram of the
rotating arc plasma system for the BTX
decomposition. BTX gases were added to air at the
concentration of 600 ppm using a syringe pump, and
then the mixed gas was directly used as plasma
forming gas which flow rate was controlled by a
mass flow controller (MFC). A high voltage AC
power supply was employed for the rotating arc
system, and its maximum frequency was 40 kHz. In
order to evaluate the conversion efficiencies of BTX,
its concentrations were analyzed by the method of
Fourier Transform Infrared spectrometry (FT-IR).
Waveforms of arc voltage and current were
measured by using an oscilloscope to examine a
dynamic behavior of the arc discharge and the
electric input power at the same time. Thermal
efficiency of the torch and SIE were calculated as
follow equations:
Table 1. Experimental conditions
Flow Rate
Pin
Torch
Efficiency
Pnet
SIE
(L/min)
(W)
(%)
(W)
(J/L)
20
630
85.6
539
1,617
40
642
85.8
551
827
60
631
85.6
540
540
80
666
86.4
575
431
100
710
87.2
619
371
Results and discussion
Thermal efficiency of the torch (%)
Pin − [m& C p ,cw (T0 − Ti )]
=
× 100 (1)
Pin
SIE (J/L) =
60 Pnet
V&
(2)
where, Pin (W) is the input power; m& (g/s) is the
mass flow rate of the cooling water; C p ,cw (J/g·ºC)
is its heat capacity; T0 and Ti (ºC) are coolant
temperature at inlet and outlet points, respectively.
Pnet (W) was calculated through the input power
and the thermal efficiency of the torch. V& (L/min)
is the total flow rate of the plasma forming gas
composed of air and BTX. Since the SIE is supplied
energy for unit volume of a liter, 60 is multiplied in
the equation (2).
The conversion of BTX and the yield of byproducts were measured by the FT-IR as follows:
η=
cin − cout
× 100
cout
Figure 2 presents the measured voltage waveform
at gas flow rate of 40 L/min and applied frequency
of 40 kHz, respectively. In the general arc discharge,
its length is increased as the voltage increased. It is
clearly appeared that the re-strike phenomenon
occurred in the present AC discharge. The period of
a cycle of the re-strike was 9.6 ms at 40 L/min and
40 kHz. RMS voltage was 750 V and average RMS
current was 870 mA in the case of Fig. 2.
Fig.3 is the motion pictures of the arc rotation
according to gas flow rate. As the flow rate was
decreased, the plasma volume was increased. Also,
as the flow rate was increased, it was increased that
the number of arc rotation per an unit time.
Fig. 4 shows the conversion of BTX as a function of
SIE. As SIE increases, the conversion also increases
linearly. The high conversion efficiency at a fixed
SIE is placed in order of m-xylene, toluene, and
benzene. The chemical bond strength and the
molecule stability are the main factors for the
decomposition of these VOCs [10].
(3)
1.5
1.0
cout , By
∑ niηcin
(4)
where η is the conversion efficiency of BTX (%);
cin and cout are concentrations (ppm) of BTX at
inlet and outlet, respectively; and YBy is the yield
of by-products; cout , By is the outlet concentration of
by-products (ppm); ni is the number of carbon
atoms of BTX. According to above equations,
experimental conditions used in this study are listed
in Table 1.
Voltage (kV)
YBy =
0.5
0.0
-0.5
-1.0
-1.5
-0.04
-0.02
0.00
0.02
0.04
Time (sec)
Fig. 2. Voltage waveform of the rotating arc plasma (Flow
rate=40L/min, f=40 kHz)
100
benzene
toluene
m-xylene
Conversion of BTX ((%))
80
60
40
20
600
800
1
1000
1200
140
00
1600
1800
Fig.. 4. Conversion of BTX as a sppecific input en
nergy (f=40 kHz)
Fig.
F 5 presennts the yieldds of CO2 and
a CO as a
fun
nction of SIE
E. The yield oof CO2 and CO
C increasess
in proportion
p
t the increaase of SIE. As the SIE
to
E
incrreased, CO2 yield increaased rapidly, on the other
han
nd, the increaase of CO yiield was not so high. Thee
totaal yield of CO2 and CO aat 1,617 J/L ranged from
m
63%
% to 93% acccording to B
BTX speciess. The rest of
thatt ranged from
m about 7 to 20% was ex
xpected as thee
hyd
drocarbon annd solid carrbon becausee the reactor
walll was deposiited with sooot after the ex
xperiment.
70
benzene
b
to
oluene
m
m-xylene
60
Yield of CO2 (%)
The bond energy of thhe carbon-caarbon in benzzene
K
at 298
2 K and thhat of the meethyl
ring is 144 Kcal/mol
group in m--xylene and toluene is 100
1 Kcal/mool at
298 K [11]]. It appearrs that mosst reactions are
initiated by the inelasticc collisions of
o electrons and
m
T
The
VOCs with the low
activated molecules.
ionization potentials
p
usuually have higher
h
oxidaation
efficiencies in the gas phase [1]. Because
B
of this
reason, the conversion
c
effficiencies off toluene andd mxylene weree higher thaan that of benzene.
b
At the
same SIE off 1,617 J/L, the
t maximum
m conversionns of
benzene, tolluene, and m-xylene
m
weere 79%, 1000%,
and 100%, respectively.
r
Together with boond strengtth, converrsion
a
by-prodducts yield rate in BTX
B
efficiency and
conversion process seeems to be influenced by
several factoors. First, thhe reaction of
o BTX withh the
active moleecules generated by thhe rotating arc
plasma is one factoor. Second, the therrmal
decomposition by the arcc plasma woould be occurrred.
Finally, thee oxidation reaction wiith O2 and O
might decom
mpose the VOCs.
V
To stuudy these facctors
more, the following experimennts were also
performed. First,
F
the inttermediate chemical
c
speecies
produced during
d
the degradation
d
of BTX were
w
assumed andd analyzed byy using a QM
MS, because it is
difficult to analyze directly the acctive molecuules.
directly the active molecules. Secoond, heat efffect
depended onn the net poower was estimated throough
calculating on the coolant temperaature differeence
between the inlet and the outlet. Thiird, it seems that
the BTX deegradation inn air depends greatly onn the
process of thhe oxidation.. In order to analyze that,, the
generation of
o ozone andd carbon dioxxide, and carrbon
monoxide inn the plasma discharge was analyzed.
400
Specific Input Energy (J/L)
50
40
30
20
10
200
400
600
800
100
00
1200
1400
1600
1800
1400
1600
1800
Specific Input Energy (J/L)
25
2
be
enzene
toluene
m-xylene
20
2
Yield of CO (%)
Fig. 3. Motionn pictures of rottating arc plasm
ma according too gas
flow raate: (a) 20L/miin, (b) 40L/minn, (c) 60L/minn, (d)
80L/miin, and (e) 100L
L/min
0
200
15
10
5
200
400
6
600
800
100
00
1200
Specific Input Energy (J/L)
Fig.. 5. Effect of sppecific input ennergy on the yieelds of CO2 andd
CO
The maximum total yields of CO2 and CO were
63%, 80%, and 93% for benzene, toluene, and mxylene, respectively. Therefore, it is thought that a
considerable oxidation promotes the conversion
process during the decomposition of the BTX. Fig.6.
shows the FT-IR spectra of benzene at 40 L/min.
Main by-products of this system were CO2, CO,
NO2, and NO. In Fig. 6, it was confirmed that the
peak of benzene was reduced by the plasma
discharge treatment. In addition, a significant
increase in CO2 and CO peaks can be observed.
There are reasons for the generation of NOx. First,
the heat generated from the arc plasma produced the
thermal NOx. Generally the thermal NOx is
generated at greater than 1,000 °C. Second, the
ionization potentials are different according to gas
species. The ionization potentials of N2, NO,
benzene, toluene, and m-xylene are 4.8, 3.0, 9.3, 8.8,
and 8.7 eV, respectively [11]. Therefore, N2 and NO
have relatively higher oxidation efficiency. Because
of NOx generation, it is revealed that additional
equipment for the treatment of NOx is needed for an
environmentally benign process.
Conclusions
BTX gases diluted in air were successfully
decomposed by using the rotating AC arc plasma in
the present work. At the maximum SIE of 1,617 J/L,
the conversion efficiencies of benzene, toluene and
m-xylene were 79%, 100% and 100%, respectively.
As SIE was increased, the conversion of BTX was
also increased. Main reasons of the BTX
degradation in air were the oxidation and the partial
oxidation. The yield of CO2 and CO increased in
Absorbance intensity
CO2
inlet
outlet
CO2
NO2
benzene
NO
CO2
H2O
CO
1000
2000
3000
4000
-1
Wavenumber (cm )
Fig. 6. FT-IR spectra of benzene (flow rate at 40 L/min)
proportion to the increase of SIE. The maximum
yields of CO2 and CO were 63%, 80%, and 93%,
respectively, at the highest SIE. Main by-products of
this system were CO2, CO, NO2, and NO.
Acknowledgments
This work was supported by the Regional
Innovation Center for Environmental Technology of
Thermal Plasma (ETTP) at Inha University
designated by MKE (2011).
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