1

Hydrocarbons Synthesis from Syngas by Very High Pressure Plasma

Samuel Iwarere1, Vandad Rohani

2*, Deresh Ramjugernath

1, Frédéric Fabry

2, Laurent Fulcheri

2

1 Thermodynamic Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College

Campus, Durban 4041, South Africa

2 MINES ParisTech, PERSEE Center, 06904 Sophia-Antipolis, France

*Corresponding author: vandad-julien.rohani@mines-paristech.fr Tel: + 33 (0) 4.97.15.70.52

Abstract

Experimental studies related to chemical processes based on the use of very high pressure plasma

are scarce. In the domain of chemical synthesis enhanced by plasma, the influence of pressure has

not been yet reported in literature for conversion of syngas into hydrocarbons. This may be

attributed to the difficulties of ignition and sustaining a stable electric discharge at pressures higher

than 1 MPa. Yet, the use of high pressure plasma could constitute a new route to synthesize

hydrocarbons in gaseous phase. Thus, this paper presents experimental results of hydrocarbons

synthesis from a syngas having a H2:CO ratio of 2.2:1, i.e. close to the one commonly met in the

classical Fischer-Tropsch process, in a tip-tip non-thermal arc discharge reactor at very high

pressure (in the range 0.5 - 15 MPa) supplied by a high-voltage DC power supply without any

catalyst. The molecular synthesis is here studied through the influence of two process setting

parameters: operating pressure and supplying current. The results show that C1 to C3 molecules can

be synthesized by this way and their concentration significantly varies with the two studied

parameters. The best results in terms of products concentration after 60 s treatment were obtained:

(i) for 12 MPa pressure at 0.35 A current, with concentration of C1 and C2+C3 around 6000 ppm

and 90 ppm respectively, (ii) for 0.2 A current at 0.5 MPa pressure, with concentration of C1 and

C2+C3 around 4500 ppm and 5000 ppm respectively. In addition, the energy efficiency of the

process was evaluated, based on the determination of specific energies as they are defined in the

classical Fischer-Tropsch process. The results demonstrate that the optima in terms of chemical

conversion and energy efficiency coincide together.

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Keywords: Hydrocarbons synthesis; Syngas; Non-thermal plasma; Very high pressure; Fischer-

Tropsch; Specific required energy

1. Introduction

Hydrocarbons synthesis in plasma phase is a recent subject of scientific research. The main

motivation of that is to find a way to reduce or ideally avoid the use of solid complex catalysts in

thermo-chemical synthesis. The Fischer-Tropsch process, in its most advanced versions, is one of

the best examples of wide complex catalysts user. The use of complex catalysts (multi-functional) is

required in order to reduce the cost of the refining post-process by increasing the selectivity of the

synthesis step and/or to drive the process to a higher flexibility [1,2]. But more a solid catalyst is

complex and so selective, higher is its cost. Moreover, the poisoning of the solid catalyst imposes to

regularly replace or retreat it for a continuous hydrocarbons production. Some research works have

been done over the years on plasma-based alternative technologies to reduce or replace the use of

solid catalysts for organic compounds synthesis [2-9].

Plasma is a particular state of matter having some benefic properties for chemical processing and,

nowadays, chemistry in plasma phase is more and more studied for many applications : reforming

[10-22], nanoparticle synthesis [23,24], coating [25,26], etching [27], etc. Most of the existing

plasma technologies operate at sub-atmospheric pressures, i.e. P 0.1 MPa. Except in the lighting

field, few works have been dedicated to the creation and using very high pressure plasmas (P 1

MPa). This lack can be attributed to the difficulties of ignition and sustaining stable electric

discharges in gases under pressures higher than 1 MPa.

Yet, hydrocarbons synthesis from syngas needs for high pressures because they promote the

molecular chain growth according to the Le Chatelier’s principle. So, to explore the hydrocarbons

synthesis in plasma phase, it is firstly necessary to find a way of stable discharge ignition under

high pressures. For that, an original reactor in which an electric discharge can be ignited and

maintained under high and very pressures (up to 15 MPa) was previously developed and studied by

Fulcheri et al. [3]. Stability and electrical behavior of non-thermal discharges produced in helium at

high pressures in this reactor were reported in [3], while the results of preliminary tests of

hydrocarbons synthesis by generation of non-thermal discharges in syngas at 2.2 MPa in the same

reactor was reported in [2]. In this second work, it was proved that molecular organic compounds

can be synthesized from syngas in a high pressure plasma phase. Thus, C1 and C2 molecules were

produced by this way, following the chemical equation taking place in the arc column:

OHHCHCCHHCOd

2624242 15.015.04.055.2 (1)

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Moreover, C3 molecules were also produced but at higher pressures (> 2.2 MPa), underlining on the

way the central role of the pressure in the chemical reactions leading to the molecular chains

growth.

So, the aim of this paper is to go farther into the exploration of hydrocarbons synthesis from syngas

in plasma phase with the reactor previously tested in [2,3], by providing information regarding the

influence of operating pressure, and additionally of supplying current, on the molecular chains

formation. It presents synthesis results obtained for a fixed initial H2:CO mixture ratio equal to

2.2:1 (i.e. close to the one commonly met in the classical Fischer-Tropsch process [28]) and a

variable pressure going from 0.5 MPa to 15 MPa. Although first valuable works were published in

recent years on the plasma enhanced hydrocarbons synthesis [29-31], the study presented here is

novel in its exploring pressure range because this range has never been studied in the framework of

plasma processes for chemical synthesis.

2. Material and Methods

2.1. Gas

Contrary to the previous work in which the experiments were done in a He/H2/CO mixture [2], all

investigations in this study are conducted with a gaseous mixture exclusively composed of

hydrogen and carbon monoxide. The gases are supplied by Air Liquide Company: N18 for CO and

Alpha Gas 2 for H2. Table 1 presents the gas specifications as obtained from the specification card

on each of the gas cylinders. The H2 cylinder is observed to be in accordance with the specification

cards, but GC analyses showed that the CO cylinder supplied contained more CH4 than specified.

Therefore, the initial CH4 concentration of the H2 and CO gas mixture in the reactor as determined

by GC analysis before every experiment contains 200 ppm (± 0.5%) of CH4.

Table 1.

2.2. Plasma reactor

The high-pressure plasma reactor used in this study is fully described by Fulcheri et al. in [3].

A schematic diagram of the experimental setup is presented in Figure 1 and a description of the

reactor setup is detailed by Rohani et al. in [2]. The volume of the reactor chamber is equal to 2.56

cm3. The reactor was constructed from 316 L stainless steel and is cylindrical in design. Two face-

to-face borosilicate Pyrex™ sight windows are integrated into the outer wall of the reactor chamber

and is used to view the discharge ignited in the chamber, as well as provide information in the case

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where spectroscopic studies of the electric discharge is required. The discharge chamber is leak-

tight with O-rings to prevent leakage of air into the system, as well as the introduction of impurities

into the discharge chamber. The cathode and the anode electrodes are tungsten rods of 150 and 19

mm in length respectively with both having a diameter of 4 mm while the cathode has a cone-like

tip at an angle of 60o. The fixed electrode in this setup is the cathode, while the mobile electrode is

the anode. The cathode is connected to the negative polarity of the high-voltage, and the anode is

connected to the neutral point of the direct current (DC) power supply. A hand-wheel with a pitch

of 0.25 mm per revolution is employed to adjust the inter-electrode gap by moving the mobile

electrode from the point of contact with the fixed electrode to the desired gap.

Figure 1.

A high-voltage DC power supply is used to generate the discharge. The high-voltage DC power

supply is a Technix-SR-15-R-10000 model with maximum output voltage and current of 15 kV and

660 mA respectively. This power source is based on a double resonance technology. The power

supply unit consists of an inductor-capacitor (LC) resonant circuit, which acts as a filter and thus

makes it possible for smooth control of the current flowing to the load. The electrical power flowing

through the high voltage transformer is regulated by the filter impedance, which directly depends on

the signal frequency. The high-voltage DC is connected to two resistors 1k each which are in

series, they help to protect the power supply by limiting the current. Therefore, the general

operating principle of the high-voltage DC power supply starts from the primary rectifier, which

provides continuous voltage at a regulated current to the inverter. The inverter is composed of an

insulated gate bipolar transistor (IGBT) bridge. The inverter produces a square waveform voltage at

high output frequency up to 50 kHz. This high frequency voltage waveform is controlled by the LC

resonant circuit with its primary function been the control of electrical power into the high voltage

transformer. The alternate current high voltage from the transformer is rectified and the final output

current is made to fit within the set current in the display panel of the power supply by the mean

current controller system. Consequently, the current is imposed by the operator but the voltage is

self-adjusted by the system. Applied current and voltage varying lightly in time at a frequency of

some tens kHz around an averaged value, their values are given in terms of rms values [3]. The

schematic diagram of this high-voltage DC power source is presented with detailed description by

Fulcheri et al. [3,32].

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2.3 Experimental procedure

Experiments were carried out using carbon monoxide and hydrogen gas mixtures with the

specifications listed in Table 1.

The experimental procedure is an adaptation of that one presented by Rohani et al. [2], the reactor

chamber is purged twice using an inert gas (helium) in order to evacuate gas impurities that may be

in the system from the previous experiments. After purging with the helium, CO is first supplied

through an input valve into a stainless steel mixing cylinder which has a capacity of 300 cm3. The

gas cylinder is filled to a desired pressure measured as the partial pressure of the gas. Then, H2 is

supplied as the second reactant gas into the same mixing cylinder and filled to the desired pressure

based on the reaction stoichiometry molar ratio of the gas mixture. For all the experiments

conducted in this work, a syngas having a H2:CO ratio of 2.2:1 is used.

Samples are taken in triplicate in order to analytically verify the composition ratio of the gas

mixtures using a PR2100 GC. After verification was deemed satisfactory, the gas mixtures are

introduced into the reactor chamber via a two-way valve and the chamber is pressurized up to the

desired operating pressure. The pressure in the reactor is monitored using a pressure transducer and

a pressure gauge.

Before igniting the electric arc, a hand-wheel is used to bring the mobile electrode into contact with

the fixed one. When both electrodes are in contact, the power supply is switched on, and the mobile

electrode is gradually adjusted up to the desired distance in such a way as to allow creating and

stretching an electric arc between the two electrodes under these pressure conditions. The electric

discharge is sustained by the power source during the experiments duration. After each experiment,

a gaseous sample is taken for GC analyses.

The gas mixtures in the reactor are sampled before and after discharge using a 60 ml syringe fitted

with a manual valve. The gas collected is then injected into a PR2100 gas chromatograph and the

respective peaks obtained are integrated for qualitative purposes while the compositions are

analyzed quantitatively to determine the concentration of each species based on calibrated gas

standards for the C1, C2, C3, and C4 species. One flame ionization detector (FID) and two thermal

conductivity detectors (TCD) using helium and nitrogen as carrier gas operate concurrently on the

PR2100 gas chromatograph. A FID/methanizer is also employed in order to improve the detection

of the CO and CO2 gases.

A PR2100 GC is used for the purpose of quantitative and qualitative analyses of the outlet gaseous

phase products while a QP2010S GCMS with a Supel-Q™ PLOT column and helium as carrier gas

is employed for the reconfirmation of some species and identification of unknown synthesis species.

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The PR2100 GC used is composed of two parallel analysis circuits: one functioning with nitrogen

as carrier gas and the other with helium. It contains two 13X-type molecular sieves and one Porapak

Q column.

For the purpose of electrical analysis, a high voltage probe Elditest GE3830 with bandwidth

between 0 and 100 kHz and a Chauvin Arnoux E3N Hall-effect current probe with bandwidth up to

100 kHz are connected to an Agilent DSO 1004A digital oscilloscope with a bandwidth of 60 MHz.

Due to the technology of power supply, voltage and current are not fully constant in time but faintly

oscillating around a root mean square value. The root mean square voltage (Urms) and current (Irms)

are directly measured by oscilloscope, while the supplying energy (E in Joule) in calculated namely:

tIUE rmsrms .. (2)

In this study, the treatment duration (t) is fixed to 60 s in order to allow a significant quantity of

syngas to pass through the arc discharge according to the previous work presented in [2].

Gas pressure and temperature inside the reactor are measured with a Keller-PA type pressure

transducer and a K-type thermocouple inserted into the discharge chamber. The pressure transducer

and temperature sensor have uncertainties of ±1 % over the entire range from 0.1 to 20 MPa and

±0.1 K respectively, while the electrical probes have an own uncertainty equal to ±5 %.

In order to study the reactor performance with respect to syngas conversion, the formed products

are identified by comparing their chromatograms to the calibrated standards for saturated and

unsaturated C1, C2, C3, C4 hydrocarbons. To obtain the actual amount of each individual molecule

produced by the treatment process, the initial concentrations of gases in the reactor chamber before

each treatment procedure are taken into account and subtracted from the final concentrations

measured after treatment. The partial volume of each produced molecule is calculated with the

formula given in [2], thus the amount of organic molecules produced in the reactor can be

calculated as:

)(. i

k

k

f

k

M

reacprod CC

V

Vn (3)

Where, Vreac is the reactor internal volume (2.56 x 10-6

m3) and VM is the molar volume following

the ideal isothermal gas law at 298 K, their ratio giving the initial amount of syngas (nsyng). Ck and

Cki are the final and the initial concentrations of the produced hydrocarbon k, respectively. As we

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are still far from the optimum conditions for conversion in this exploratory work, the pressure does

not vary significantly between before and after the treatment (less than 5%) due to the low

conversion rate of syngas into organic compounds. So the pressure drop for each experiment is

neglected.

The energy performance of the reactor is evaluated through the calculation of the specific input

energy (SIE in J/molsyng) which corresponds to the energy input per mole of syngas, and the specific

required energy (SRE in J/molprod) which is the energy required to produce one mole of product-gas,

the product-gas being here considered as a gaseous mixture only composed of all produced

hydrocarbons measured.

k

i

k

f

kprodreac

M

CC

SIE

n

ESREand

V

EVSIE

)(

. (4)

3. Results and Discussion

The experimental conditions under which all experiments were carried out are presented in table 2.

Table 2.

For each quantitative chromatography measurement of concentration, the errors are estimated

thanks to the repeatability of the measured sample (RSM) and the accuracy of the calibrated gas

standards (ACGS) while taking into account the background noise (BN) in both cases. Thus, the

sample measurement error (SME) and the calibration error (CE) for individual analysis are given

by:

)ppm05.0(BN%)3(R)ppm(SME SM (5)

0.05ppm)BN(5%)(ACE(ppm) CGS (6)

These errors bars are assigned to each experimental data as indicated in the graphical results.

The two parameters studied in this paper are the operating pressure and the supplying current. Thus,

the results are presented and discussed under the form of a parametric study through two

subsections. The first subsection (3.1) is dedicated to the study of pressure variation effect on the

hydrocarbons synthesis while the second one (3.2) is dedicated to the study of current variation

effect on it. Note that for all experiments done, the gas inside the reactor is not significantly heated

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during the 60 s treatment; the initial temperature being equal to 25°C and the final temperature

never exceed 50°C.

3.1 Effect of operating pressure on the synthesis

Firstly, it is important to note that the only molecules produced in all following experiments of this

subsection are C1, C2 and C3. C4 molecules are not significantly produced in these operating

conditions. The concentration curves of the obtained molecules (C1, C2 and C3) versus operating

pressure are presented in Figures 2 and 3. These curves show that the molecule mainly produced for

all pressure values is CH4. Its concentration varies from 100 ppm up to 5800 ppm, but the

concentration curve is not monotonic: a minimum is observed for a pressure equal to 2 MPa, while

a maximum is reached for 12 MPa. Nevertheless, a significant quantity of C2H6, much lower than

CH4 all the same, is obtained. C2H6 concentration goes from 12 ppm to 78 ppm by following a trend

similar to the one of CH4 with a shift of the minimum to 8 MPa.

Figure 2.

Figure 2 shows that the concentration of produced CH4 decreases with the increase of pressure

between 0.5 and 2 MPa. However, a gradual rise in the concentration of produced CH4 is observed

from 2 MPa until a maximum located at 12 MPa. This indicates that CH4 production rate can be

optimized by increasing the operating pressure up to 12 MPa in reference to the investigated

operating conditions.

Figure 3.

In Figure 3, one can see that the concentrations of C2 and C3 follow the same global trend as the

CH4 concentration. However, the minimum of concentration for these molecules is shifted towards

the high pressures. The positive contribution of the pressure increase is significant between 4 and 10

MPa considering that there is a rise of approximately 82%, 76%, 98% and 95% for C2H6, C2H4,

C3H8, and C3H6 respectively between 4 MPa and 12 MPa. This significant gain confirms again the

strong influence of operating pressure on the hydrocarbons synthesis by this process. Moreover, it is

important to underline that the concentrations of hydrocarbons produced here, from a pure syngas

having an initial H2:CO ratio of 2.2:1, are largely higher than ones presented in the previous paper

[2] in which the synthesis experiments were done from a helium/syngas mixture, with an initial

H2:CO ratio of 4:1. Thus, the optimal initial ratio must be located around the stoichiometry value of

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the reactants in reaction (1) which is close to the reference ratio of the classical Fischer-Tropsch

process (2:1). Moreover, the addition of helium in the mixture seems to degrade the synthesis, so it

is recommended to operate with a pure syngas.

At this stage, in order to evaluate the energy performance of the process in terms of specific input

and required energies, an accurate calculation of the supplying energy was done thanks to equation

(2) and precise measurements of rms voltage self-adjusted at the terminals of reactor by the DC

power supply for a fixed imposed current value. Figure 4 presents the plot of rms voltage measured

versus operating pressure for generating an electric discharge at 0.35 A. Note that the supplying

power (not traced) has a similar curve due to its mathematical proportional link with the rms voltage

at a fixed current. The voltage curve shows that in the investigated pressure range, the rms voltage

is minimum for a pressure close to 5 MPa. The shape of this curve is similar to the one obtained by

Fulcheri et al. in the study of rms voltage applied to the same reactor in pure helium versus

pressure, at fixed imposed current [3]. So, the rms voltage follows here the law defined as the

pressure-extended Ayrton’s law in this cited paper.

Figure 4.

From these voltage measurements, the specific input energy (SIE) per mole of syngas can be

calculated; the plot is shown in Figure 5. On this one, the SIE monoteneously decreases with the

increase of pressure. A high SIE is an indication that the generated arc goes near a thermal arc in

which a part of energy serves as gas heating. At 0.5 MPa, the calculated SIE is equal to 104 kJ/mol

while from 4.5 MPa, the SIE becomes lower than 103 kJ/mol. On the same figure, the plot of

specific required energy (SRE) per mole of product-gas is presented. The curve shows that we need

for more energy to produce one mole of product-gas by syngas conversion at lower pressure. The

SRE reaches a maximum for a pressure equal to 3 MPa, while it has a minimum value at 12 MPa.

Lower is the SRE, better is the process energy efficiency. Moreover, one can remark the

correspondance between the minimum of SRE and the maximum of C1, C2, C3 hydrocarbons

absolute concetrations which have been all obtained at the same pressure value of 12 MPa. This

observation would prove the existence of an optimum operating pressure value which maximizes at

once the chemical conversion rate and the energy efficiency of the process in our investigated

conditions. In addition, one can note that high SIE values do not lead to an increase of hydrocarbons

production because the results seems to demonstrate here the exact contrary, meaning that the

plasma effect is better when the energy density of plasma is low, i.e. when the plasma is far from

equilibrium. Nevertheless, it is important to maintain uncoupled the direct effect of pressure and the

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effect coming from the distance to the equilibrium because there is not any reason to have a strong

correspondance between them at the chemical level.

Figure 5.

3.2 Effect of operating current on the synthesis

In parallel of the pressure influence study, the influence of arc current was investigated. To do that,

the 0.2 – 0.4 A current range was explored at a fixed pressure value of 0.5 MPa in the same

experimental conditions as previously used in 3.1 subsection in terms of inter-electrode gap and

discharge duration. As in the previous subsection, are firstly presented here the curves of produced

hydrocarbons concentrations measured by CGS versus the studied parameter which is now the

imposed supplying current instead of operating pressure (Figure 6). Figure 6 shows the production

of saturated and unsaturated C1, C2, C3 hydrocarbons for five different values of supplying current.

As well as in the pressure influence study, methane remains the most produced molecule for all

current values at 0.5 MPa. However, C2 molecules have a concentration in the same order of

magnitude as methane at 0.2 A, ethane being even at the same production rate as methane at this

supplying current. C3 concentrations have at least two orders of magnitude less than the one of

methane for all tested currents. It is important to note that additional products such as propyne

(C3H4), 1,3-butadiyne (C4H2), 1,2-butadiene (C4H6), 1-butene (C4H8), 1-buten-3-yne (C4H4), and

benzene (C6H6) were identified by GCMS then quantified by GC for the experiments done at 0.2 A,

but their concentrations are in minority, i.e. estimated between 1 and 2 ppm. For higher currents, the

concentration of these molecules falls under 1 ppm, preventing at the same time to draw any

accurate variation trend because of the measurement uncertainty. The respective concentration of

the hydrocarbons produced in majority (i.e. C1, C2 and C3) decreases gradually with the rise of

supplying current. These results tends to demonstrate that lower is the current, better is the

production of longer hydrocarbons in gaseous phase. Indeed, just for the comparison between C1

and C2, one can observe that slops of C2 concentrations curves around the point at 0.2 A are higher

than the one of C1, meaning that for a supplying current lower than 0.2 A, C2 concentrations could

become higher than methane concentration. The verification is currently in progress thanks to new

experiments and the results will be presented in a next paper.

Figure 6.

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To analyze the influence of the supplying current value, the self-adjusted rms voltage was measured

in order to determine the supplying energy for each investigated current value. Figure 7 presents the

plot of rms voltage versus current. This curve, having a continuous negative slope, seems to well

correspond to the characteristic curve of a non-thermal arc discharge as described in [33].

Moreover, the tangent slope of the curve almost reaches zero at 0.4 A, meaning that this operating

point is not far from the point of transition to thermal arc state [3]. Note that, between 0.2 A and 0.3

A, the rms voltage highly decreases with the current increase. Thus below 0.3 A, by way of voltage,

the reduced field applied to syngas, which is a very important parameter because directly linked to

the electronic temperature of the non-thermal plasma, has a strong deviation. So, according to that

and to discharges physics theory [33], one can conclude that by moving the electrical operating

point towards the low currents on this curve (having as consequence an increase of the self-adjusted

voltage), the discharge is taken away from the equilibrium state. By coming back in Figure 6, this

deviation is translated into an abrupt drop of products concentration of two orders of magnitude on

a current variation of 0.1 A only. Thus, these results tend to demonstrate that more the discharge is

far from the equilibrium state, more it is favorable to the production of longer hydrocarbons in

gaseous phase. In other words, to improve the conversion process at a given pressure, remoteness

from the equilibrium seems to be necessary.

Figure 7.

As done for the pressure influence study, the influence of current on the specific input and required

energies has been also established. The SIE and SRE curves versus supplying current are presented

in Figure 8. These ones point out that we input more energy per mole of syngas at lower current

(0.2 A) than at higher current (0.4 A) due to the drop of the rms voltage even though the amount of

syngas is constant. The SIE has a minimum at 0.35 A, indicating that this operating point is the

lowest one in terms of absolute energy input into the system. On the other hand, the curve of SRE

shows that we need for less energy to produce one mole of product-gas at lower current than at

higher current, so tending to put in evidence the benefits of the non-thermal effects of the discharge

on the enhanced chemistry. So, the results prove once again that the remoteness of the discharge

from the equilibrium state is benefic for the plasma enhanced syngas-to-hydrocarbons conversion.

In spite of the strong reproducibility of observation, an explanation cannot be given at this stage of

research because the phenomena are not fully understood. Next studies will focus on this

understanding by using different non-thermal plasma analysis technics and diagnostics.

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Figure 8.

Conclusions

The aim of the work was to demonstrate the strong influence of high pressure on hydrocarbons

synthesis from syngas in a tip-tip arc discharge reactor. For that, new experimental results have

been obtained regarding the quantitative influence of operating pressure, and additionally of

supplying current, on hydrocarbons synthesis from syngas having a H2:CO ratio of 2.2:1 (close to

the one commonly met in the classical Fischer-Tropsch process) for pressures ranging from 0.5 to

15 MPa and electrical current going from 0.20 to 0.40 A, for which, experimental data are very

scarce in literature. The results coming from this preliminary study have indicated that the main

molecules synthesized are in order of importance: CH4, C2H6, C2H4, C3H6, and C3H8, for all

investigated experimental conditions. The parametric analysis has shown that it is possible to

optimize at the same time the absolute concentration of produced hydrocarbons and the specific

energy required to produce them, by increasing the operating pressure and/or decreasing the

supplying current. From these results, two main conclusions can be drawn: (i) the optimum value of

operating pressure for hydrocarbons production from syngas is 12 MPa in the investigated

conditions; (ii) more the discharge generated in the syngas is far from equilibrium, higher is the

efficiency of the syngas-to-hydrocarbons conversion. For this last point, it is possible to play on the

supplying current in order to take away the electric discharge from the equilibrium.

To summarize, the best results in terms of products concentration after 60 s treatment were

obtained: (i) for 12 MPa pressure at 0.35 A current, with concentration of C1 and C2+C3 around

6000 ppm and 90 ppm respectively, (ii) for 0.2 A current at 0.5 MPa pressure, with concentration of

C1 and C2+C3 around 4500 ppm and 5000 ppm respectively. The best results in terms of specific

input energy (SIE) and specific required energy (SRE) were obtained for 12 MPa pressure at 0.35 A

current, with values of 600 kJ/molsyng and 80 MJ/molprod respectively.

Given that this technology is at a new stage and considering the volume ratio between the discharge

volume and the reactor internal volume, which shows the discharge volume to be 10-5

times smaller

than the total reactor volume [2], it is normal to obtain relatively small concentrations of products

for the moment. Future work will be focused on reaction kinetics and mechanism of molecular

organic compounds synthesis in gaseous phase inside the discharge volume and outside it at very

high pressure in order to improve the understanding of the arc effect in the process, essential step

for going towards its optimization.

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Acknowledgements

This work was supported under the PROTEA projectN° 25887NA between the Plasma group at

CEP, MINES ParisTech, 06904 Sophia-Antipolis, France and the Thermodynamic Research Unit

under the School of Engineering at the University of KwaZulu-Natal, Durban 4041, South-Africa.

Noël Traisnel is acknowledged for their assistance in technical and other related matters.

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16

List of Figures and Tables Captions

Figure 1. Schematic diagram of the experimental setup including the plasma reactor supplied by the

high-voltage DC power supply [2].

Figure 2. Concentration of CH4 produced versus pressure from an initial mixture having H2:CO a

ratio of 2.2:1 submitted during 60 s to a discharge of 0.35 A. The inter-electrode gap is equal to 1

mm.

Figure 3. Concentration of C2 and C3 hydrocarbons, under their alkane (solid line) and alkene

(dotted line) forms, produced versus pressure from an initial mixture having a H2:CO ratio of 2.2:1

submitted during 60 s to a discharge of 0.35 A (inter-electrode gap = 1 mm).

Figure 4. Rms voltage measured at terminals of reactor versus operating pressure for a fixed

imposed current of 0.35, a H2:CO ratio of 2.2:1, an inter-electrode gap of 1 mm and a discharge

duration of 60 s.

Figure 5. Specific input energy (black diamond) and specific required energy (blue circle) versus

operating pressure for a fixed imposed current of 0.35 A, a H2:CO ratio of 2.2:1, an inter-electrode

gap of 1 mm and a discharge duration of 60 s. The ordinate axes are in logarithm scale.

Figure 6. Concentration of C1, C2 and C3 hydrocarbons, under their alkane (solid line) and alkene

(dotted line) forms, produced versus supplying current from an initial mixture having a H2:CO ratio

of 2.2:1 submitted during 60 s to a discharge at 0.5 MPa (inter-electrode gap = 1 mm). The ordinate

axes are in logarithm scale.

Figure 7. Rms voltage measured at terminals of reactor versus supplying current for a fixed

pressure of 0.5 MPa, a H2:CO ratio of 2.2:1, an inter-electrode gap of 1 mm and a discharge

duration of 60 s.

Figure 8. Specific input energy (black diamond) and specific required energy (blue circle) versus

supplying current for a fixed pressure of 0.5 MPa, a H2:CO ratio of 2.2:1, an inter-electrode gap of 1

mm and a discharge duration of 60 s.

17

Table 1. Specifications of the gases used in the experiments with the maximum concentrations of

impurities.

Table 2. Operating conditions chosen in the experiments of hydrocarbons synthesis from syngas

with the high-voltage DC power supply.

18

Table 1.

19

Table 2.

20

Figure 1.

21

Figure 2.

22

Figure 3.

23

Figure 4.

24

Figure 5.

25

Figure 6.

26

Figure 7.

27

Figure 8.