The application of low current non-thermal plasma-catalysis in Fischer-Tropsch synthesis at very high pressure: the effect of pressure on hydrocarbon product yields

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
The application of low current non-thermal plasma-catalysis in Fischer-Tropsch
synthesis at very high pressure: the effect of pressure on hydrocarbon product
yields
B.B. Govender, S.A. Iwarere and D. Ramjugernath
Thermodynamics Research Unit, University of KwaZulu-Natal, Durban, South Africa
Abstract: The effect of varying pressure (0.5 to 10 MPa) in a Fischer-Tropsch synthesis
(FTS) was investigated using a combination of a low-current DC arc discharge and a
mullite coated 6wt%-Co/5wt%-Al 2 O 3 catalyst. The C 1 -C 3 hydrocarbon product yields
generally increased with increasing pressure. The influence of the catalyst was observed in
that the product yields were higher for plasma-catalysis than that compared to pure plasma
FTS.
Keywords: Very high pressure, low current, non-thermal plasma, arc discharge, catalysis
1. Introduction
Non-thermal plasmas generated at low current (I < 1A)
have been limited to ignition at low to atmospheric
pressure, until recently, when several studies have shown
that a low current (I < 1A) arc discharge can be ignited
and sustained under non-reactive and reactive conditions
at very high pressures (P > 1MPa) owing to the
technological developments that allow the generation and
sustaining of an arc discharge at these conditions.
Preliminary experiments using the inert gases; pure
argon [1], argon/H 2 mixture [2] and pure helium [3] were
undertaken to investigate the sustainability of an arc
discharge at various operating conditions, namely: gas
pressure, inter-electrode gap, ignition current, and
discharge time. Izquierdo et al. (2008) showed that an
argon arc discharge was stable under low currents (0.1 to
500 mA) and very high pressures up to 10 MPa. Fulcheri
et al. (2010) showed that a helium arc discharge was
stable at very high pressures between 5 and 7 MPa. Nonreactive studies provided critical information regarding
discharge stability and operating conditions that enabled
the application of the arc discharge in reactive systems
such as hydrocarbon synthesis [4, 5], dry reforming of
methane [6] and fluorocarbon synthesis [7]. The
production of hydrocarbons from syngas (H 2 + CO),
yielding products reminiscent to that of FTS, was
explored by Rohani et al. (2011) and Iwarere et al. (2014).
Their work demonstrated that hydrocarbon (FT) synthesis
is possible using a low current discharge at very high
pressure up to 15 MPa without the presence of a
conventional FTS catalyst. Iwarere et al. (2014) also
observed that the concentration of C 1 -C 3 hydrocarbons
increased with increasing pressure and decreasing current.
In this study a low current high pressure arc discharge
is combined with an industrially representative Co/Al 2 O 3
catalyst (i.e. plasma-catalysis) in order to increase the
product yields and energy efficiency achieved by the pure
plasma FTS. Plasma-catalysis is promising as its
application in dry reforming of methane revealed higher
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syngas conversion and product yields than pure plasma
assisted reforming [8, 9].
2. Experimental Section
2.1 Catalyst preparation
A ceramic 6wt%-Co/5wt%-Al 2 O 3 catalyst was
prepared by a two-step coating process of a pre-formed
mullite ceramic (supplied by Ceradvance Engineering
Ceramics. South Africa). Firstly, 5 wt.% γ-Al 2 O 3 was
washcoated onto a pre-formed mullite substrate according
to the experimental method developed by Villegas et al.
(2007). Secondly, the alumina washcoating of the mullite
was followed by; wet impregnation of Co(NO 3 ) 2 .6H 2 O,
calcination at 450oC and ex-situ hydrogen reduction at
350oC. After reduction, the catalyst was immediately
inserted into the reactor in a configuration which enabled
the electrodes to generate the arc discharge in the annulus
of the catalyst, as shown in Figure 1.
Fig. 1. Configuration of the two electrodes within the
annulus of the 6wt%-Co/5wt%-Al 2 O 3 mullite coated
catalyst (cross sectional view).
2.2 FTS experimental procedure
Syngas with a H 2 /CO ratio of 2.2 was transferred at the
required operating pressure from a mixing cylinder to the
1
tip-to-tip plasma reactor, which is described elsewhere [34]. The mobile electrode was moved towards the fixed
electrode using an axial positioning system until contact
of the electrodes was obtained. The high voltage DC
power supply (set at a fixed ignition current of 350 mA
and a driving voltage of 8 kV for this work) was switched
on. The arc was ignited as soon as the mobile electrode
was retracted. When an inter-electrode gap width of 1 mm
was reached, the reaction was allowed to proceed for a
pre-determined time (60s and 10s in this work), after
which period the power supply was switched off and the
arc was extinguished. The reaction products were
withdrawn from the reactor at a sample point and were
analysed off-line by a Shimadzu 2010 plus GC fitted with
a TCD and FID detector. The errors for the C 1 -C 3
hydrocarbon concentrations, attributed to the sample
analysis repeatability, were estimated as: ±6% for C 1 -C 3
hydrocarbon concentrations for plasma-catalysis at 10 s,
±10% for C 1 -C 3 hydrocarbon concentrations for plasmacatalysis at 60 s and ±10% for C 1 -C 3 hydrocarbon
concentrations for pure plasma at 60 s.
Figure 2. Schematic of the low current very high pressure
tip-to-tip arc discharge reactor [4].
Table 1. Plasma-catalysis operating conditions
Operating conditions
Discharge time = 10s
Ignition current (mA)
350
Ignition voltage (kV)
8
Inter-electrode gap (mm)
1
Pressure (MPa)
H 2 /CO ratio
Catalyst
0.5, 1, 2, 3, 4, 5, 6,
7, 8, 9, 10
2.2
6wt%-Co/5wt%-Al 2 O 3
mullite coated
Discharge time = 60s
Ignition current (mA)
350
Ignition voltage (kV)
8
Inter-electrode gap (mm)
1
Pressure (MPa)
H 2 /CO ratio
Catalyst
2
0.5, 1, 2, 3, 4, 5, 6
2.2
6wt%-Co/5wt%-Al 2 O 3
mullite coated
3. Results and Discussion
The effect of varying operating pressure on the
production of light hydrocarbons (presented in Figures 3
to 7) was investigated for plasma-catalytic FTS for
different discharge durations of 60 s and 10 s under the
operating conditions listed in Table 1. The general trend
for plasma-catalysis, as observed for pure plasma, was
that higher pressure favours hydrocarbon chain growth in
accordance with Le Chatelier’s principle. The major
hydrocarbons produced in order of yield for the plasmacatalytic process were: C 1 (methane) >> C 2 (ethane +
ethylene) > C 3 (propane + propylene). Trace quantities of
hydrocarbons were synthesised in the reactor due to the
fact that the discharge (reactive) volume was 10-5 times
smaller than the total volume of the plasma reactor (i.e.
the C 1 -C 3 hydrocarbons produced within the discharge
region were mixed with the large volume of unreacted
syngas resulting in a mixture containing <1%
hydrocarbons and > 99% syngas).
3.1 Plasma-catalysis at 60s
A discharge duration of 60 s was investigated for the
pressure range of 0.5 to 6 MPa. Beyond 6 MPa, the arc
was highly unstable, which was potentially due to the
production of high yields of water. The larger quantity of
water, verified using a GCMS, which is usually in the
gaseous state at lower pressures, may have condensed at
higher pressures leading to the extinguishing of the arc
before completion of the 60s discharge period.
3.2 Plasma-catalysis at 10s
At a discharge duration of 10 s, the arc was stable
between 0.5 and 10 MPa due to lower water yields as a
result of the reduced discharge time. Methane, propane
and propylene concentrations increased significantly from
8 to 10 MPa. However, the ethylene concentration
decreased considerably as pressure increased from 1 MPa
to 10 MPa. This behaviour of ethylene formation differs
from that of pure plasma where the ethylene yield
increases with increasing pressure. The reduction of
ethylene at higher pressures for plasma-catalysis may
have been due to its readsorption onto the catalyst surface
where secondary reactions occur [11] or the reinsertion of
ethylene into growing chains to form heavier
hydrocarbons [11, 12]. Other factors leading to low
ethylene yields, suggested for classical FTS using
catalyst, is ethylene’s higher surface mobility and lower
activation energy barrier [12].
The methane, ethane, propane and propylene
concentrations for plasma-catalysis at 10 s and 10 MPa
were 7, 3, 4 and 4 times greater, respectively, than that
compared to the pure plasma process at 60s and 10 MPa,
demonstrating the influence of the catalyst on the
hydrocarbon yields.
Furthermore, plasma-catalysis leads to a lower pressure
requirement for the production of propane and propylene,
as compared to pure plasma FTS. For pure plasma FTS,
propane is formed from 4 MPa onwards, while for
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6000
Pure plasma (60s)
Plasma catalysis (10s)
4000
12
Plasma catalysis (10s)
10
8
6
4
2
0
0
2000
1000
Fig. 6. The
concentration.
0
Fig. 3. The
concentration.
2
4
influence
6
8
P / MPa
of pressure on
10
Plasma catalysis (60s)
140
Pure plasma (60s)
120
Plasma catalysis (10s)
100
80
4
6
8
P / MPa
influence of pressure
10
on
propane
Pure plasma (60s)
5
Plasma catalysis (10s)
4
3
2
1
0
60
0
40
Fig. 7. The
concentration.
20
0
0
6
8
10
P / MPa
Fig. 4. The influence of pressure on ethane concentration.
2
4
70
60
Plasma catalysis (60s)
Pure plasma (60s)
50
Plasma catalysis (10s)
40
30
20
10
0
0
Fig. 5. The
concentration.
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2
6
methane
160
Ethane conc. / ppm
Pure plasma (60s)
3000
0
Ethylene conc. / ppm
Plasma catalysis(60s)
14
Propylene conc. / ppm
Methane conc. / ppm
Plasma catalysis (60s)
5000
16
Propane conc. / ppm
plasma-catalysis, propane is formed from a lower pressure
of 0.5 MPa. Similarly, propylene is formed at a lower
pressure of 4 MPa for plasma-catalysis as compared to 10
MPa for pure plasma FTS.
2
4
6
8
10
P / MPa
influence of pressure on ethylene
2
4
6
8
P / MPa
influence of pressure
10
on
propane
4. Conclusions
The addition of a catalyst to plasma-assisted FTS
increased all hydrocarbon product yields significantly
beyond that of pure plasma FTS. For plasma-catalysis, the
maximum concentrations of methane, propane, and
propylene were obtained at 10 MPa for the discharge
duration of 10s, while the maximum concentrations of
ethane and ethylene were obtained at 2 MPa for a
discharge duration of 60s. Furthermore, in plasmacatalysis, a lower pressure is required for the formation of
propane and propylene as compared to pure plasma FTS.
5. Acknowledgements
This research was funded by the Department of Science
and Technology (DST) and the National Research Fund
(NRF). The resources used were based at the
Thermodynamics Research Unit (TRU) located at the
University of KwaZulu-Natal (South Africa).
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6. References
[1] E. Izquierdo, J. Gonzalez-Aguilar & L. Fulcheri,
Plasma Science, ICOPS 2008. IEEE 35th International
Conference on. IEEE (2008).
[2] E. Izquierdo, J. Gonzalez-Aguilar & L. Fulcheri, High
Temperature Material Processes, 13, p. 71-76 (2009).
[3] L. Fulcheri, V. Rohani, F. Fabry, N. Traisnel, Plasma
Sources Science and Technology, 19(4), 045010 (2010).
[4] V. Rohani, S. Iwarere, F. Fabry, D. Mourard, E.
Izquierdo, D. Ramjugernath, L. Fulcheri, Plasma
Chemistry and Plasma Processing 31, p. 663–679 (2011).
[5] S. Iwarere, V. Rohani, D. Ramjugernath, F. Fabry, L.
Fulcheri, Chemical Engineering Journal., 241, p. 1–8
(2014).
[6] S. A. Iwarere, V.-J. Rohani, D. Ramjugernath, L.
Fulcheri, International Journal of Hydrogen Energy
(2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.005.
[7] S. A. Iwarere, A. Lebouvier, L. Fulcheri, D.
Ramjugernath, Journal of Fluorine Chemistry, 166, p. 96–
103 (2014).
[8] M. Kraus, B. Eliasson, U. Kogelschatz, A. Wokaun,
Physical Chemistry Chemical Physics, 3, p. 294-300
(2001).
[9] M.-W. Li, C.-P. Liu, Y.-L. Tian, G.-H. Xu, F.-C.
Zhang and Y.-Q. Wang. Energy & Fuels, 20, p. 10331038 (2006).
[10] L. Villegas, F. Masset & N. Guilhaume, Applied
Catalysis A: General, 320, p. 43–55 (2007).
[11] G. P. van der Laan & A. A. C. M. Beenackers,
Catalysis Reviews - Science and Engineering, 41(3 &n4),
p. 255–318 (1999).
[12] T. Bhatelia, C. Li, Y. Sun, P. Hazewinkel, N. Burke,
V. Sage, Fuel Processing Technology, 125, p. 277–289
(2014).
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