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 current on hydrocarbon product
yields and energy efficiency
B.B. Govender, S.A. Iwarere and D. Ramjugernath
Thermodynamics Research Unit, University of KwaZulu-Natal, Durban, South Africa
Abstract: The effect of ignition current (200 to 450 mA) in a Fischer-Tropsch synthesis
(FTS) was investigated at 2 MPa using a combination of a low-current DC arc discharge
and a mullite coated 6wt%-Co/5wt%-Al 2 O 3 catalyst. The plasma-catalytic FTS yielded
greater C 1 -C 3 hydrocarbon concentrations and consumed significantly less electrical energy
than the pure plasma FTS for the current range investigated.
Keywords: very high pressure, low current, non-thermal plasma, arc discharge, catalysis
1. Introduction
In the past decade researchers have shown that nonthermal plasmas (NTP) generated by an arc discharge can
be ignited and sustained under low current (I < 1A) and
very high pressure (P > 10 MPa) conditions, where
previously low current discharges have been limited to
ignition at low to atmospheric pressure.
The sustainability of a low current and very high pressure
arc discharge was initially investigated using the nonreactive gases: pure argon [1], argon/H 2 mixture [2] and
pure helium [3]. Izquierdo et al. [1] reported that an argon
arc discharge was sustainable under low currents (0.1 to
500 mA) and very high pressures up to 10 MPa and
Fulcheri et al. [3] subsequently reported that a helium arc
discharge was stable up to 7 MPa.
Non-reactive NTP studies at very high pressure were
followed by the reactive systems: hydrocarbon synthesis
[4, 5], dry reforming of methane [6] and fluorocarbon
synthesis [7]. The conversion of syngas (H 2 + CO) to
light hydrocarbons, resembling that of FTS products, was
explored by Rohani et al. [4] and Iwarere et al. [5]. Their
work showed that an arc discharge can be used to
synthesize FTS products at low current and very high
pressures, up to 15 MPa, without the presence of a
conventional FTS catalyst. Iwarere et al. also reported that
the concentration of C 1 -C 3 hydrocarbons increased with
decreasing ignition current [5].
This study will investigate the FTS process using a
combination of a low current very high pressure arc
discharge and an industrially representative Co/Al 2 O 3
catalyst (i.e. plasma-catalysis). The effect of the ignition
current on the product yields and energy efficiency will
be assessed. Plasma-catalysis has shown advantages over
pure plasma processes such as dry reforming of methane,
where plasma-catalysis lead to greater reactant
conversions and product yields than that compared to pure
plasma-assisted reforming [8, 9].
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2. Experimental Section
2.1 Catalyst preparation
A Co-based catalyst was prepared by the washcoating
of alumina and wet impregnation of cobalt onto a ceramic
substrate. A pre-formed mullite ceramic (procured by
Ceradvance Engineering Ceramics, South Africa) was
initially washcoated with 5 wt.% γ-Al 2 O 3 according to
the experimental method described by Villegas et al. [10].
The alumina provided a greater surface area for cobalt
dispersion, which was deposited by the wet impregnation
of Co(NO 3 ) 2 .6H 2 O, followed by calcination at 450oC to
produce cobalt oxide and finally ex-situ hydrogen
reduction at 350oC to produce metallic cobalt. The
reduced catalyst was immediately inserted into the plasma
reactor in a configuration which allowed 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).
1
2.2 FTS experimental procedure
A syngas (H 2 /CO) ratio of 2.2 was transferred at an
operating pressure from a mixing cylinder to the tip-to-tip
plasma (batch) reactor, which is described elsewhere [34]. A fixed syngas pressure of 2 MPa was selected for this
work as the relatively lower pressure ensured
sustainability of the arc over the entire range of ignition
currents investigated. After loading of the reactor, 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 driving voltage of 8 kV and ignition
currents between 200 and 450 mA for this work, was
engaged. Ignition of the arc was achieved as soon as the
mobile electrode was retracted from the fixed electrode.
When an inter-electrode gap of 1 mm was obtained, the
reaction was allowed to proceed for a pre-determined time
of 60 s, after which period the power supply was switched
off and the arc was extinguished. The reactor was then
discharged in order to remove residual current that may
have remained in the system, which was followed by the
withdrawing of a sample from the reactor at a sampling
point. The sample was 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: ±7% for C 1 -C 3 hydrocarbon concentrations
for plasma-catalysis at 60 seconds and ±7% 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].
3. Results and Discussion
The influence of the ignition current (200 to 450 mA) on
the FTS hydrocarbon product concentrations (presented in
Figures 3 to 5) were investigated using plasma-catalysis
under the fixed operating conditions presented in Table 1.
Methane was produced in the greatest quantity, followed
by the C 2 hydrocarbons (ethane and ethylene) and the C 3
hydrocarbons (propane and propylene). The total product
concentration for plasma-catalysis did not exceed 2% due
to the fact that the discharge (reactive) volume was 10-5
times smaller than the total volume of the plasma reactor
i.e. a high degree of dilution of the reaction products with
the unreacted syngas.
2
Table 1: Plasma-catalysis operating conditions
Operating conditions
Ignition current (mA)
Current variation study
200, 250, 300, 350,
400, 450
Ignition voltage (kV)
8
Inter-electrode gap (mm)
1
Discharge time (s)
60
Pressure (MPa)
2
H 2 /CO ratio
Catalyst
2.2
6wt%-Co/5wt%-Al 2 O 3
mullite coated
3.1 Pure plasma
In pure plasma FTS, an arc discharge was generated
throughout the current range of 250 to 450 mA for a
discharge time of 60 s. However, the arc discharge was
highly unstable after 10 s at an ignition current of 250 mA
and the arc finally self-extinguished at 250 mA after 20 s
(i.e. before the 60 s discharge duration). The methane
concentration shown in Figure 3 increased with
decreasing current i.e. production increased with an
increase in the non-thermal nature of the plasma. The
ethylene, propane and propylene concentrations for the
pure plasma process, not shown in this paper, were below
1 ppm for the currents between 300 and 450 mA.
3.2 Plasma-catalysis
In plasma-catalysis, the observed arc discharge at 200,
250 and 300 mA appears to be glow-like discharge, while
it follows the normal arc discharge at 350, 400 and 450
mA. In the arc regime, the C 1 and C 2 concentrations
increased with decreasing current as was observed in the
pure plasma study. This trend is more prominent for
methane, ethane and ethylene. In the glow-like regime,
the selectivity of methane, ethane, ethylene, propane and
propylene were 7, 2, 2, 15 and 4 times greater,
respectively, than the selectivity in the arc regime at 350
mA, owing to the volumetric nature (i.e. larger treatment
volume) by the glow-like discharge. Furthermore, the
maximum methane and ethane concentrations, obtained at
250 mA, were 100 and 40 times greater, respectively, than
the pure plasma concentrations obtained at the same
current.
The introduction of a catalyst in the plasma FTS
process, resulted in several phenomena that were not
observed for pure plasma FTS, which were: improved
discharge stability throughout the current range
investigated; higher product yields; a higher specific input
energy (SIE) per mole of syngas, presented in Figure 7;
and finally a lower specific energy required (SRE) per
mole of methane produced, presented in Figure 8. Several
authors have shown that these enhancements may be due
to the modification of the electric field by the catalyst as
well as micro-discharge formation in catalyst pores [13].
In addition, they suggested that the non-thermal plasmas
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300
may modify the physicochemical properties of the
catalyst leading to changes in the reaction pathways [13].
Plasma-catalysis
Pure plasma
Plasma-catalysis
Pure plasma
250
Voltage / V
Methane conc. / ppm
20000
2000
200
200
150
150
350
450
I / mA
Fig. 6: Rms voltage versus supplying current
20
150
250
350
450
I / mA
Fig. 3: The influence of current on the methane
concentration.
250
3000
Plasma-catalysis
Plasma-catalysis
Pure plasma
100
10
Pure plasma
SIE / (kJ/molsyngas)
Ethane conc. / ppm
1000
2500
2000
1500
150
1
150
250
350
450
I / mA
Fig. 4: The influence of current on the ethane
concentration.
250
350
450
I / mA
Fig. 7: Specific input energy versus supplying current
100000
Plasma-catalysis
C3 conc. / ppm
80
propylene (Plasma-catalysis)
60
40
SRE / (MJ/molMethane)
Pure plasma
propane (Plasma-catalysis)
10000
1000
100
20
150
0
150
350
450
I / mA
Fig. 5: The influence of current on the C 3 concentrations.
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250
250
350
450
I / mA
Fig. 8: Specific required energy versus supplying current.
3
4. Conclusions
The combination of a Co-based catalyst and an arc
discharge improved all hydrocarbon product yields
beyond that of pure plasma FTS, in the current range
investigated. The hydrocarbon concentrations increased
with decreasing current for plasma-catalytic and pure
plasma FTS. A glow discharge formed between 200 and
300 mA and an arc discharge formed between 350 and
450 mA for the plasma-catalytic process. In plasmacatalysis, the maximum C 1 -C 3 hydrocarbon yields were
obtained in the glow region (between 200 and 350 mA),
with methane exceeding a concentration of 1%.
Furthermore, plasma-catalysis yielded much greater C 1 hydrocarbon concentrations and consumed
C3
significantly less electrical energy than the pure plasma
FTS.
[13] E.C. Neyts & A. Bogaerts, Journal of Physics D:
Applied Physics, 47, 224010 (18pp) (2014).
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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