Main Gaseous Products of Microwave Discharge in Various Liquid Hydrocarbons

Article abstract of DOI:10.1134/S0018143919030032

Abstract: Main gaseous products (H₂, C₂H₂, C₂H₄, CH₄) formed by microwave discharge in a number of liquid alkanes, cycloalkanes, and aromatic hydrocarbons have been studied using gas chromatography. It has been shown that the products of the discharge in these cycloalkanes and aromatic compounds bearing no side groups almost do not contain methane or ethylene, unlike the case of alkanes.

Full text of DOI:10.1134/S0018143919030032

ISSN 0018-1439, High Energy Chemistry, 2019, Vol. 53, No. 4, pp. 331–335. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Khimiya Vysokikh Energii, 2019, Vol. 53, No. 4, pp. 325–329.
PLASMA CHEMISTRY
Main Gaseous Products of Microwave Discharge
in Various Liquid Hydrocarbons
K. A. Averina, Yu. A. Lebedeva, *, and A. V. Tatarinova
aTopchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences (TIPS RAS), Moscow, 119991 Russia
*e-mail: lebedev@ips.ac.ru
Received December 12, 2018; revised December 21, 2018; accepted December 25, 2018
Abstract—Main gaseous products (H2, C2H2, C2H4, CH4) formed by microwave discharge in a number of
liquid alkanes, cycloalkanes, and aromatic hydrocarbons have been studied using gas chromatography. It has
been shown that the products of the discharge in these cycloalkanes and aromatic compounds bearing no side
groups almost do not contain methane or ethylene, unlike the case of alkanes.
Keywords: microwave discharge, plasma, microwave discharge in liquid, hydrocarbons, chromatography
DOI: 10.1134/S0018143919030032
INTRODUCTION
a directional coupler, a spectrum analyzer, and an
oscilloscope. The attenuator males it possible to get a
smoothly varying forward power in the range from
100 W to 2.5 kW. The discharge section is a waveguide-
to-coaxial adapter, the center conductor of which
serves as an antenna for introducing microwave energy
into the discharge section. A movable short plunger
was used for matching. The central electrode of the
coaxial line is made of copper tubing with an outer
diameter of 1.5 mm. Reducing the diameter of the
antenna made it possible to organize a discharge with
a forward power on the order of 100 W. The discharge
was excited at the end of the antenna in a quartz cell
(of a 55 mm diameter) placed in a protective screen.
The cylindrical quartz cell was partially filled with
a hydrocarbon, and it was blown with argon above the
surface of the liquid for 2 min. By applying microwave
power, the antenna end heats up, the hydrocarbon
evaporates, and a discharge is initiated in its vapor in
the liquid volume. No additional energy source
required for initiation.
Electric discharges in liquids attract the attention
of researchers and are one of the priorities in the study
of the physics of gas discharge and low-temperature
plasma [1–5]. This is primarily due to the promising
application of such discharges in solving environmen-
tal problems. In addition, they can be used to produce
various gas and solid products.
Various types of discharge are currently being used
to generate plasma in liquids, but microwave dis-
charges are the least studied object. Publications on
this issue appeared in the early 2000s and make several
dozen papers (the number of papers concerning liquid
organic compounds is even smaller), in contrast to
hundreds of publications on other types of discharge.
An introduction to the current state of research in this
area is given in recent reviews [6, 7].
This paper presents the results of studying the main
gas products of microwave discharge in various hydro-
carbons (alkanes, cyclanes, and aromatics): n-hep-
tane, octane, isooctane, decane, pentadecane, hexa-
decane, cyclohexane, benzene, toluene, ortho-xylene,
and the petroleum solvent Nefras S2 80/120.
In the literature there are some data on the main
gaseous products of microwave discharge in liquid n-
dodecane [8] or alcohol solutions [9, 10] and on the
minor products in liquid n-hexane, n-heptane, chlo-
roform, and their mixtures [11].
The discharge was initiated in the region of the
maximum microwave field at the end of the central
conductor of the coaxial line (Fig. 2) in various hydro-
carbons differing in structure and boiling point (see
Table 1): n-heptane, octane, isooctane, decane, pen-
tadecane, hexadecane, cyclohexane, benzene, tolu-
ene, ortho-xylene, and Nefras S2 80/120. The liquids
used were not deaerated prior to the experiments.
The volume of the liquid in the cell was about
EXPERIMENTAL
40 mL, ensuring that the end of the internal electrode
The experiments were carried out on the setup of the coaxial line (antenna) would be below the liquid
detailed in [12, 13] and sketched in Fig. 1. It includes a surface. No auxiliary gases were supplied through the
microwave generator, a circulator, a water attenuator, channel in the central electrode. The pressure above
331

332
AVERIN et al.
10
9
8
7
Ar
6
5
4
12
11
3
2
Microwave
1
Fig. 1. Schematic of the experimental setup: (1) coaxial-
to-waveguide adapter, (2) short-circuiting piston,
(3) dielectric, (4) antenna, (5) liquid, (6) discharge area,
(7) quartz reactor, (8) metal mesh screen, (9) valve,
(10) sampling, (11) gas chromatograph, and (12) analog-
to-digital converter.
the surface of the liquid was equal to atmospheric
pressure.
The composition of the main gaseous products
(H2, C2H2, CH4, C2H4) of the discharge in liquid
hydrocarbons was studied using an LKhM-8 chro-
matograph with a thermal conductivity detector and a
HaySep S-packed chromatographic column. The
instrument was equipped with a digital data acquisi-
tion and processing system (ADC and the software
suite Phoenix). Nitrogen was used as the carrier gas.
The absolute calibration of the instrument was carried
out using individual components and ready-made cal-
ibration gas mixtures.
Fig. 2. Photograph of the discharge in liquid n-heptane
(chamber diameter 55 mm, antenna diameter 6 mm).
a sufficiently high microwave field strength is required
at the antenna to ensure a high ionization rate. This, in
turn, requires an increase in the power input to the
reactor. Despite the fact that the loss tangent of non-
polar liquid hydrocarbons is small (about 10−4), at a
high forward power, some of it is spent on heating.
An Agilent syringe was used for sampling in both
cases.
The reduction of the required power while ensuring
the necessary microwave field strength can be
achieved in two ways. One is to decrease the antenna
diameter (extension of the inner conductor of the
coaxial line), and the other is to select the antenna
length. In order to optimize the antenna assembly, we
simulated the reactor electrodynamics using the soft-
ware package Comsol 3.5. The computational domain
corresponded to the real geometry of the reactor with
cylindrical symmetry (Fig. 3). The reactor was sup-
posed to be filled with a liquid hydrocarbon. Most
nonpolar hydrocarbons have a dielectric constant of
~2, and calculation results are valid for a wide class of
hydrocarbons. The calculation results are shown in
Fig. 4. In the calculations, the length (L) of the pro-
truding part of the central conductor of the coaxial
line was varied. A drop of field strength over a distance
of 40 mm occurs at the liquid/air interface due to the
difference between the dielectric constants.
RESULTS
One of the objectives in conducting experiments
with liquid hydrocarbons is to reduce the proportion
of the liquid evaporated during the experiment. Evap-
oration leads to the appearance of vapor of the sub-
strate hydrocarbon in the gas sample (detailed chro-
matographic examination of the sample showed that
the volumetric vapor content can be commensurate
with the amount of the main products). It is impossi-
ble to avoid this altogether, since a high-temperature
plasma entity is created in the bulk of the liquid (tem-
perature in the antenna region is about 2000 K) and
heat is removed through the liquid, leading to its evap-
oration. The same process is the main source of
hydrocarbon molecules conveyed to the discharge
region. But the direct heating of the liquid by micro-
From Fig. 4 it follows that the optimal length of the
wave radiation can be reduced. For plasma generation, protruding part of the antenna is 2.2 mm. It is this
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MAIN GASEOUS PRODUCTS OF MICROWAVE DISCHARGE
333
Table 1. Main gaseous products of microwave discharge in various liquid hydrocarbons
H2, vol %
CH4, vol % C2H4, vol % C2H2, vol %
Substance, boiling point and structure
n-Heptane С7Н16 (Tboil = 98.42°C)
71.2
3
7.8
7.5
6.3
6.1
5.5
18
n-Octane С8H18 (Tboil = 125.52°C)
Decane С10Н22 ((Tboil = 174.1°C)
Pentadecane С15Н32 (Tboil = 270.6°C)
Hexadecane С16Н34 (Tboil = 286.79°C)
Isooctane С8Н18 (Tboil = 99.3°C)
72
2.2
1.7
1.3
0.4
18.3
20.3
25
71.7
67.6
65.6
28.5
71
6.2
0
4.1
9.1
18.7
17.5
Cyclohexane С6Н12 (Tboil = 80.74°C)
Benzene С6Н6 (Tboil = 80.1°C)
73.4
88.8
86.1
0
0
0
11.2
12.1
Toluene С6Н5–СН3 (Tboil = 110.6°C)
CH3
1.8
ortho-Xylene С6Н5–(СН3)2 (Tboil = 144°C)
CH3
CH3
74.6
66.5
3.6
6
5.6
8.6
14.4
19.9
Petroleum solvent Nefras S2 80/120 (mixture of light
hydrocarbons with boiling range of 33–205°C)
length that was used in the experiments and made it noted that the above consideration is valid for the pure
possible to reduce the power to 100 W required for the hydrocarbon and corresponds to the initial period of
exciting the discharge with the end diameter of the time. During operation of the discharge, carbon-con-
inner conductor of the coaxial line of 2 mm. The taining particles are formed in it, which are transferred
geometry of the protruding part of the antenna is to the liquid and are distributed by convective currents
shown in the lower right corner of Fig. 3. It should be throughout its volume. These particles absorb well
HIGH ENERGY CHEMISTRY Vol. 53 No. 4 2019

334
AVERIN et al.
Metal
|E|, V/сm
Curve top points,
from left to right
2000
d42
d46
z, m E, V/сm
0.018 1936
0.020 1998
0.023 2026
0.027 1873
0.033 1491
0.036 1302
1500
1000
500
Air
d55
80
Quartz
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Liquid
z, m
L
Fig. 4. Distribution of microwave field strength along the
axis of the discharge chamber at different lengths of the
protruding part of the antenna (calculation with a forward
power of 100 W).
0
d25
d5
Fluoroplastic
60
CONCLUSIONS
Using gas chromatography, the composition of the
main gaseous products of microwave discharge in liq-
uid hydrocarbons (n-heptane, octane, isooctane, dec-
ane, pentadecane, hexadecane, cyclohexane, ben-
zene, toluene, ortho-xylene, and Nefras S2 80/120)
was investigated. It has been shown that:
—the yield of acetylene increases and the yield of
hydrogen decreases with an increase in the molecular
weight of alkanes (in the C7–C16 series, including
cycloalkanes);
11 mm
7 mm
∅ 2 mm
∅ 3 mm
∅ 5 mm
MW
Fig. 3. Geometry of the computational domain.
—hydrogen and acetylene are predominantly
formed in aromatic compounds; and
microwave radiation, thereby leading to a sharp
increase in the loss tangent and energy absorption in
the liquid. Ultimately, this process leads to a decrease
in the field strength at the end of the antenna and leads
to the extinction of the discharge. Thus, the selection
of the electrodynamic characteristics of the antenna is
essential at the initial stage of discharge and facilitates
its initiation process.
—the discharge-generated products of the unsub-
stituted cycloalkanes and aromatic compounds stud-
ied contain almost no methane or ethylene. As the
number of side groups increases, the composition
approaches the composition of the discharge products
in alkanes.
FUNDING
The work was performed within the state program of the
Topchiev Institute of Petrochemical Synthesis, Russian
Academy of Sciences.
A previous detailed study of the composition of
gaseous products of microwave discharge in liquid
hydrocarbons showed that the main products are
hydrogen, acetylene, ethylene, and methane. It is
these gases whose yield depending on the parent
hydrocarbon was investigated in the present work. The
results are given in Table 1.
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Translated by S. Zatonsky
HIGH ENERGY CHEMISTRY Vol. 53 No. 4 2019