ETHYLENE EPOXIDATION IN A CYLINDRICAL DIELECTRIC BARRIER
DISCHARGE SYSTEM: INFLUENCE OF FEED POSITION
T. Suttikula, T. Sreethawonga,b, S. Chavadeja,b, H. Sekiguchic
a
The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok10330, Thailand
b
Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University,
Bangkok 10330, Thailand
c
Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku,Tokyo 1528552, Japan
Abstract:
The main objective of this work was to investigate the ethylene
epoxidation in a cylindrical dielectric barrier discharge (DBD) system focusing
on the effect of C2H4 feed position which was varied in terms of the fraction of
electrode length from 0 (at the inlet of outer electrode) to 1 (at the outlet of
outer electrode). The EO selectivity increased with increasing C2H4 feed
position up to 0.5, while the H2 and C2H2 selectivities decreased. The C2H4 feed
position of 0.25 was considered to be an optimum position because it provided
comparatively high EO selectivity and yield with reasonably low power
consumption per EO molecule produced. Afterwards, the operational
parameters, such as O2/C2H4 feed molar ratio, applied voltage, input frequency,
and total feed flow rate were investigated to achieve the best conditions. The
EO selectivity and yield increased obviously from 12.7 to 30.6 and from 2.3 to
8.0, respectively, when using the optimum ethylene feed position of 0.25, an
O2/C2H4 feed molar ratio of 0.25, an applied voltage of 13 kV, an input
frequency of 550 Hz, and a total feed flow rate of 75 cm3/min.
Keywords: Epoxidation; Ethylene oxide; Dielectric barrier discharge; Feed
position
1. Introduction
Ethylene epoxidation has been of great
interest because ethylene oxide (EO) is used
primarily as a chemical feedstock for producing
various useful chemicals such as ethylene glycol,
antifreeze, solvent, adhesives, and detergents.
Ag/Al2O3-based catalysts are normally used
industrially for producing ethylene oxide. However,
these catalytic processes have some limitations, such
as high operating temperature, high energy
consumption, coke formation, and the deactivation
of catalysts. Non-thermal plasma which can be
operated under ambient pressure and temperature,
leading to low energy consumption is a potential
technique to overcome these drawbacks. In this
work, the ethylene epoxidation in a cylindrical
dielectric barrier discharge (DBD) system was
investigated focusing on the effect of C2H4 feed
position. Various operational parameters, i.e.
O2/C2H4 feed molar ratio, applied voltage, input
frequency, and total feed flow rate were
subsequently investigated in order to maximize the
EO production efficiency.
2. Experimental Setup and Reaction
Activity Experiments
The experimental study of ethylene
epoxidation was investigated in a cylindrical DBD
reactor, which was operated at ambient temperature
and atmospheric pressure. A schematic of the
cylindrical DBD system in this work is shown in
Figure 1. The reactor consisted of two concentric
stainless steel electrode tubes with a 1.9 cm ID, and
a 16 cm height for the outer electrode and a 0.8 cm
OD, and a 14 cm height for the inner electrode. A
dielectric glass tube with a 1 mm thickness was
placed on the outer surface of the inner electrode to
provide a fixed gap distance of 0.5 cm. For
adjustment of C2H4 feed position, stainless steel
tubes with a 0.3 cm OD were connected to the
outside of the outer electrode at various fractions of
electrode length in the range of 0 to 1, as shown in
Figure 2.
The power used to generate plasma was
domestic alternating current power, 200 V and 50
Hz, which was transmitted to a high voltage current
via a power supply unit. The output voltage was
adjusted by a function generator, whereas sinusoidal
wave signal was controlled and monitored by an
oscilloscope. Reactant gases composed of ethylene,
oxygen, and helium were fed into the cylindrical
DBD plasma system and were controlled by
electronic mass flow controllers. The studied DBD
plasma system was initially operated under the base
conditions [4], which were an O2/C2H4 feed molar
ratio of 1/4, an electrode gap distance of 0.5 cm, an
applied voltage of 15 kV, and an input frequency of
500 Hz. The product gas was trapped by a water trap
filter before being fed to an on-line gas
chromatograph. The compositions of feed and
product gases were analyzed by the gas
chromatograph (Perkin-Elmer, AutoSystem GC)
equipped with both a thermal conductivity detector
(TCD) and a flame ionization detector (FID). For the
TCD channel, the packed column (Carboxen 1000)
was used for separating the product gases, which
were hydrogen (H2), oxygen (O2), carbon monoxide
(CO) and carbon dioxide (CO2). For the FID
channel, the capillary column (OV-Plot U) was used
for ethylene oxide (EO) and by-product analysis.
3. Results and discussion
3.1 Effect of C2H4 feed position
The effect of C2H4 feed position in the
cylindrical DBD system was initially investigated
for ethylene epoxidation reaction. The C2H4 feed
position was varied from 0 (at the inlet of outer
electrode) to 1 (at the outlet of outer electrode) while
an O2/C2H4 feed molar ratio of 1/4, an applied
voltage of 15 kV, an input frequency of 500 Hz, a
total feed flow rate of 75 cm3/min, and an electrode
gap distance of 0.5 cm were used to operate the
cylindrical DBD system [4].
Flow meter
Mixing
chamber
O2
Power
supply
He
DBD reactor
C2H4
Mass flow
controller
Vent
GC
Figure 1. Schematic diagram setup of cylindrical DBD system
for ethylene epoxidation reaction.
Outlet gas
1
Outer electrode (1.9 cm ID)
0.75
Inner electrode (0.8 cm OD)
Dielectric glass tube (1mm thickness)
0.5
C2H4 feed
position
16 cm
0.25
0
Inlet gas (O2 and He feed)
Figure 2. The configuration of the cylindrical DBD reactor.
As shown in Figure 3, the EO selectivity
gradually increased and reached a maximum at a
C2H4 feed position of 0.5. Whereas the EO yield
rapidly decreased with increasing C2H4 feed
position; however, EO was no longer produced
beyond the C2H4 feed position of 0.75. From the
results, the C2H4 feed position of 0.25 was
considered to be an optimum for ethylene
epoxidation because it provided comparatively high
EO selectivity and EO yield and was used for further
investigation. This is because this optimum ethylene
feed position of 0.25 could minimize the ethylene
cracking reactions, leading to lowering the
undesirable by-products including CO, H2, CH4, C2
products and higher hydrocarbons.
3.0
18
16
2.5
14
EO yield (%)
10
8
1.0
6
2
0.0
0
Mixed feed
0
0.25
0.5
0.75
1
2.2
2.1
16
2.0
After the study of the effects of C2H4 feed
position and O2/C2H4 feed molar ratio, the applied
voltage was investigated in the range of 12-17 kV.
From Figure 5, the EO selectivity and yield have the
same trend that gradually increased when the applied
voltage is increased to 13 kV, and then decreases
with further increasing applied voltage up to 17 kV.
EO selectivity
EO yield
1.7
12
10
1.4
1/5
1/3.33
1/4
1/2
1/2.5
1/0.67
8
Feed O2/C2H4 molar ratio
Figure 4. EO yield and selectivity as a function of O2/C2H4 feed
molar ratio (an C2H4 feed position of 0.25, an applied voltage of
15 kV, an input frequency of 500 Hz, and a total feed flow rate
of 75 cm3/min).
9
35
8
30
EO yield (%)
7
6
25
5
EO yield
20
4
EO selectivity
3
EO selectivity (%)
3.3 Effect of Applied Voltage
14
1.8
1.5
Figure 3. EO yield and selectivity as a function of C2H4 feed
position (an O2/C2H4 feed molar ratio of 1/4, an applied voltage
of 15 kV, an input frequency of 500 Hz, and a total feed flow
rate of 75 cm3/min).
The O2/C2H4 feed molar ratio was varied in
the range of 1/5 to 1/0.66 to determine its effect on
the ethylene epoxidation performance of the
cylindrical DBD system. From the results as shown
in Figure 4, the EO selectivity gradually decreased
with increasing O2/C2H4 feed molar ratio and
reached a minimum when the O2/C2H4 feed molar
ratio was higher than 1/3.33. The EO yield rapidly
increased when O2/C2H4 feed molar ratio increased
from 1/5 to 1/4, and then adversely decreased with
further increasing O2/C2H4 feed molar ratio beyond
1/4. This is because oxygen active species was not
sufficient at the lowest O2/C2H4 feed molar ratio of
1/5 while the O2/C2H4 feed molar ratio over 1/4
provided too much oxygen active species, leading to
promoting hydrocarbon combustion. Hence, the
O2/C2H4 feed molar ratio of 1/4 was considered to be
an optimum ratio because it provided performance,
exhibiting the highest EO yield, comparatively high
EO selectivity.
1.9
1.6
C2H4 Feed position (fraction of electrode length)
3.2 Effect of O2/C2H4 feed molar ratio
18
EO selectivity (%)
4
0 .5
EO Selectivity (%)
12
EO yield
EO yield (%)
EO selectivity
2.0
1.5
Therefore, the applied voltage of 13 kV was
considered to be an optimum value because it
provided a maximum ethylene epoxidation
performance in terms of the highest EO selectivity
and yield.
15
2
1
12
13
14
15
17
10
Applied voltage (kV)
Figure 5. EO yield and selectivity as a function of applied
voltage (an C2H4 feed position of 0.25, an O2/C2H4 feed molar
ratio of 1/4, an input frequency of 500 Hz, and a total feed flow
rate of 75 cm3/min).
3.4 Effect of Input Frequency
The input frequency, which also directly
affects the field strength in the plasma zone, was
next investigated in the cylindrical DBD system by
varying in the input frequency range of 450-600 Hz.
The EO selectivity significantly increased with an
increase in the input frequency up to 550 Hz, and
adversely decreased with further increasing input
frequency over 550 Hz (Figure 6). The EO yield had
a similar trend as the EO selectivity that it rapidly
increased with increasing input frequency from 450
to 500 Hz, then remained almost unchanged in the
35
10
30
8
25
6
20
4
EO yield
15
2
0
10
450
500
550
600
Frequency (Hz)
Figure 6. EO yield and selectivity as a function of input
frequency (an C2H4 feed position of 0.25, an O2/C2H4 feed molar
ratio of 1/4, a, an applied voltage of 13 kV, and total feed flow
rate of 75 cm3/min).
3.5 Effect of Total Feed Flow Rate
The effect of total feed flow rate was further
investigated in order to achieve the maximum
ethylene epoxidation performance. Under the
obtained optimum conditions from the previous
parts (a C2H4 feed position of 0.25, an O2/C2H4 feed
molar ratio of 1/4, an applied voltage of 13 kV, and
an input frequency of 550 Hz), different total feed
flow rates of 60, 75, 100, and 125 cm3/min,
corresponding to residence times of 2.2, 18, 1.3, and
1.1 min, respectively, were studied. From Figure 7,
the EO selectivity and yield initially increased to
reach a maximum at the total feed flow rate of 75
cm3/min, and then gradually decreased with further
increasing total feed flow rate beyond 75 cm3/min.
Conclusions
The effects of C2H4 feed position and
various operational parameters on ethylene
epoxidation reaction were investigated in a
cylindrical DBD system. From the results, the C2H4
feed position of 0.25 significantly enhanced the EO
selectivity. Afterwards, the operational parameters,
including O2/C2H4 feed molar ratio, applied voltage,
1.8
1.3
1.1
10
35
8
30
6
25
EO selectivity
4
EO yield
2
EO selectivity (%)
EO yield (%)
EO selectivity
Residence time (min)
2.2
EO yield (%)
12
input frequency, and total feed flow rate, were
optimized in order to achieve the best conditions.
0
60
20
EO selectivity (%)
input frequency range of 500-550 Hz, and finally
decreased with further increasing input frequency to
600 Hz. Therefore, it can be concluded that the
highest ethylene epoxidation performance was
obtained when the input frequency of 550 Hz was
used because the maximum EO selectivity and
comparatively high EO yield were achieved.
15
75
100
125
10
3
Flow rate (cm /min)
Figure 7. EO yield and selectivity as a function of total feed
rate (an C2H4 feed position of 0.25, an O2/C2H4
feed molar ratio of 1/4, an applied voltage of 13 kV,
and an input frequency of 550 Hz).
flow
From the overall results, the highest
ethylene epoxidation performance was achieved
under the following optimum conditions: a C2H4
feed position of 0.25, an O2/C2H4 feed molar ratio of
1/4, an applied voltage of 13 kV, an input frequency
of 550 Hz, and total feed flow rate of 75 cm3/min,
exhibiting the maximum EO selectivity and yield.
Acknowledgements
The authors would like to gratefully
acknowledge the Dudsadeepipat Scholarship,
Chulalongkorn University for providing a Ph.D.
scholarship to the first author and Center for
Petroleum, Petrochemicals, and Advanced Materials,
Chulalongkorn University, Bangkok, Thailand for
providing a partially financial support.
References
[1]Oyama ST (2008) Mechanism in homogeneous
and heterogeneous epoxidation catalysis. Elsevier,
Oxford
[2]S. Chavadej, A. Tansuwan, and T. Sreethawong
(2008) Plasma Chem Plasma Process 28: 643
[3]Sreethawong T, Suwannabart T, Chavadej S
(2008) Plasma Chem Plasma Process 28:629
[4]Sreethawong T, Permsin N, Suttikul T, Chavadej
S (2010) Plasma Chem Plasma Process 30:503