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 O-19-5 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 O-19-5 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. O-19-5 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). 3 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. 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