Conversion of a Waste Refrigerant Mixture containing CFC-12, HCFC-22 and
HFC-134a to Non-Crosslinked Fluoropolymer in a Non-Thermal Plasma
Sazal K. Kundu,1 Eric M. Kennedy,1 John C. Mackie,1 Clovia I. Holdsworth,2 Thomas S. Molloy,1 Vaibhav V.
Gaikwad,1 and Bogdan Z. Dlugogorski1
1
Process Safety and Environment Protection Research Group, School of Engineering, The University of Newcastle,
Callaghan, NSW 2308, Australia
2
Discipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW
2308, Australia
Abstract: The reaction of a waste refrigerant mixture of CFC-12, HCFC-22 and HFC-134a
(in an argon/methane bath gas) in a dielectric barrier discharge non-thermal plasma reactor
has been studied. The conversion of the major refrigerant, CFC-12, ranged from 59–76 %
for an input energy density range of 3–13 kJ/L. The reaction results in the formation of a
range of gaseous products as well as non-crosslinked polymers of two distinct fractions.
Keywords: Waste refrigerant, dielectric barrier discharge, non-crosslinked polymer
1. Introduction
The manufacture and use of fluorine-containing
refrigerants are controlled by international agreements [1,
2] because of the deleterious effects these chemicals have
on the earth’s atmosphere. Refrigerants CFC-12 and
HCFC-22 are harmful to the stratospheric ozone layer.
These chemicals, along with others such as HFC-134a,
also have high global warming potentials (GWPs) [3]. In
2003, new legislation was introduced in Australia,
mandating the requirement that the refrigeration and air
conditioning industries recover, return and safely dispose
of ozone depleting and synthetic greenhouse gas
refrigerants. The Ozone Protection and Synthetic
Greenhouse Gas Management Act is national legislation
[4] and these laws require industry to handle HFCs, such
as R134a and R404A, in the same way it handles CFCs
like R12 and HCFCs like R22.a
There are new obligations for industry under this
legislation, and most importantly technicians handling
these materials must now possess an Australia-wide
competency-based licence in order to operate. In addition,
companies buying or selling refrigerants must be
authorised to do so. Under the legislation, the recovery of
ozone depleting and synthetic greenhouse gas refrigerants
is compulsory and avoidable venting of fluorocarbon
refrigerants into the atmosphere is an offence under the
act.
Industry participants do not have to cover all the costs
of recovery, and a portion of the cost incurred recovering
refrigerant may be charged to customers. Refrigerant
Reclaim Australia (RRA) was established to manage a gas
recovery program and is obligated to ensure that all
government (and international) obligations are complied
with for wholesalers and distributors. RRA is also tasked
with identifying and working with companies wanting to
develop reprocessing and destruction technologies for
recovered refrigerant. RRA provides a rebate for the
return of recovered refrigerants, and organizes the
destruction of material for which re-purification of the
recovered refrigerant mixture is not viable.
According to data available between 2004 and 2008 [5,
6], growth in the total volume of refrigerants reclaimed
and destroyed in Australia increased significantly (237
tonnes in 2004 vs. 411 tonnes in 2008). In 2008,
approximately 51 tonnes of CFCs, 260 tonnes of HCFCs
and 144 tonnes of HFCs were destroyed by RRA, where
the compositions of the total destroyed refrigerants
indicate that R12, R22 and R134a were major
components (R12–5.04 %, R22–55.3 %, R134a–14.97 %)
while the other components included R11 (3.85 %), R125
(7.44 %), R152a (5.56 %), R32 (4.90 %) with some minor
components (e.g., R115, R124, R143a, hydrocarbons
etc.).b Detailed information on composition of refrigerants
recovered in Australia can be found elsewhere [6].
The methods available for commercial-scale disposal of
the stockpiled refrigerants include thermal pyrolysis or
the technique which has been used in Australia, known as
PLASCON, a process technology developed by CSIRO
(Australia’s Commonwealth Scientific and Industrial
Research Organisation) and SRL Plasma Ltd., Australia
[7]. As the PLASCON process is focused on the
destruction of refrigerants, there is a scope to develop an
alternative or complementing process in Australia,
especially where the cost of the treatment process can be
offset by the production of useful materials. In addition to
established disposal methods, non-thermal plasma based
technologies have also been investigated for the
destruction of these refrigerants [8, 9]. Apart from
destructive technologies, researchers also have attempted
to convert the waste refrigerants into environmentfriendly and useful products. This research pathway
includes the application of conventional thermal reactors
(catalytic and non-catalytic) [6, 7] as well as plasma
reactors [10, 11].
a
CFC-12 or R12: CCl2F2, HCFC-22 or R22: CHClF2, HFC-134a or
R134a: CF3CH2F, R404A: mixture of CF3CHF2, CF3CH3 and CF3CH2F.
b
R11: CCl3F, R125: CF3CHF2, R152a: CH3CHF2, R32: CH2F2, R115:
CF3CClF2, R124: CF3CHClF, R143a: CF3CH2F.
In a recent research note, we reported that
fluorocarbons including chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons
(HCFCs)
and
hydrofluorocarbons (HFCs) can be converted into noncrosslinked polymers in a dielectric barrier discharge nonthermal plasma reactor [12]. In the present article, an
investigation is presented on the conversion of a waste
refrigerant mixture (includes CFC-12, HCFC-22 and
HFC-134a) in an argon bath gas and in complete absence
of oxygen and nitrogen applying the same reactor and
where methane is included in the feed stream. The article
includes discussion on the conversion of the components
of the waste refrigerants, characterisation of electrical
discharge, polymer characterisation and a simplified
reaction mechanism representing the key reactions steps
occurring in the plasma.
3. Result and Discussion
3.1 Products Generated During Reaction
Reaction of the waste mixture of CCl2F2, CF3CH2F and
CHClF2 with CH4 in argon bath gas resulted in the
formation of a number of gaseous products including
CHF3, CH3Cl, CH2ClF, C2HClF4, CH2Cl2, CHCl2F,
C2H2Cl2F2, H2, HF, HCl and solid polymer. The
polymeric products deposited on the dielectric surfaces
have been identified as fluorine-containing polymers. A
detailed mass balance is presented in Table 1,
highlighting that more than 95 % of the mass was
recovered and accounted for.
3.2 Plasma Chemistry and Reaction Pathways
Metastable argon atoms (Ar*) form via collision of
neutral argon atoms with kinetic plasma electrons:
e- + Ar → e- + Ar*
(R01)
Reactant molecules become fragmented via the
collisions with kinetic plasma electrons as well as with
metastable argon atoms:
Ar*/e- + CCl2F2 → CClF2 + Cl + Ar/e(R02)
Ar*/e- + CClF2 → CF2 + Cl + Ar/e(R03)
Ar*/e- + CF3CH2F → CF3CHF + H + Ar*/e- (R04)
Ar*/e- + CH4 → CH3 + H + Ar/e(R05)
Ar*/e- + CH3 → CH2 + H + Ar/e(R06)
Carbon-carbon bonds of CF3CH2F and its fragmented
species can be cleaved via reaction with kinetic plasma
electrons:
e- + CF3CH2F → CF3 + CH2F + e(R07)
e- + CF3CHF → CF3 + CHF + e(R08)
Radicals, formed via plasma activity, then participate in
abstraction reactions. Some examples are given below:
H + CHClF2 → CHF2 + HCl
(R09)
F + CH4 → CH3 + HF
(R10)
Radical combination reactions can form different
molecules and radical species. Formation of CF 3 radical
from CF2 radical and formation CHF3 can be explained
by:
CF2 + F + (M) → CF3 + (M)
(R11)
CF3 + H + (M) → CHF3 + (M)
(R12)
where M implies a third-body molecule.
Ionic species as well as radicals, formed in plasma,
participate in polymerisation reactions. Ionic species are
formed via collisions of neutral species with kinetic
plasma electrons. For example:
e- + CF3 → CF3+ + 2e(R13)
2. Experimental Section
A detailed description of the experimental setup can be
found elsewhere [12]. In brief, a dielectric barrier
discharge reactor of two concentric alumina (99.8 %
purity) tubes and 4.5 mm discharge gap has been used for
this investigation. The power supply delivered a
sinusoidal output of up to 20 kV (rms) at 21.5 kHz.
Lissajous figures were used to calculate the power input
to the reactor, which was then used to estimate the input
energy density. The feed gases include argon (99.999 %,
Coregas), methane (99.95 %, Linde) and a refrigerant
mixture (85 % CFC-12, 13.5 % HFC-134a and 1.5 %
HCFC-22, recovered in Australia by Refrigerant Reclaim
Australia). In all experiments, the total flow feed rate was
100 cm3/min of which the concentration of waste
refrigerant was 1.25 % and the concentration of methane
was 1.25 %. Carbon containing gas phase products were
identified by GCMS and quantified by micro-GC. H2 and
acid gases were quantified by a gas chromatograph
dedicated for H2 analysis and a FTIR spectrometer,
equipped with a Teflon gas cell, respectively. In order to
collect
polymeric
fractions
for
subsequent
characterisation, each experiment was conducted for 90
minutes. Polymers were deposited on the surfaces of the
dielectrics and were collected using tetrahydrofuran as
solvent. A Waters (GPCV 2000) Gel Permeation
Chromatograph (GPC) and a Brüker (Avance 600 MHz or
Avance 400 MHz, depending on availability) NMR
spectrometer were used for the characterisation of the
polymeric material.
Table 1. Overall mass balance at 9 kJ/L for 90 minute experimental duration
HCl
Others
Polymers
1.12
1.28
0.93
8.52
1.74
3.27
3.85
5.78
25.7
118
25.5
225
% Balance
HF
4.92
H2
33.3
C2H2Cl2F2
145
CHCl2F
21.9
CH2Cl2
5.94
C2HClF4
66.6
CH2ClF
CHClF2
503
CH3Cl
CF3CH2F
78.2
CHF3
CCl2F2
Feed
(mg)
Products
(mg)
CH4
Gas
95.6
3.3 Conversion of Refrigerant Components
The reactions were examined over an input energy
density range of 3–13 kJ/L for which the applied voltage
range was 14.5–15.9 kV (peak-peak). The conversion of
CCl2F2, CF3CH2F and CH4 increases with increasing
input energy density, while the conversion of CHClF2
initially increases with input energy density; however, it
drops slightly above 9 kJ/L (Fig. 1).
80
75
♦ CCl2F2
◊ CH4
▲CF3CH2F
60
● CHClF2
60
45
50
30
40
15
30
% Conversion
% Conversion
70
0
0
3
6
9
12
15
Input energy density (kJ/L)
Temperature of outer electrode (°C)
Fig. 1: Percent conversion of reactants as a function of
input energy density (kJ/L).
The drop of conversion of CHClF2 above 9 kJ/L can be
explained by its formation from the fragmented products
of CCl2F2:
H + CClF2 + (M) → CHClF2 + (M)
(R14)
120
100
80
60
40
20
0
3
6
9
12
15
Input energy density (kJ/L)
Fig. 2: Temperature of outer electrode as a function of
input energy density (kJ/L).
Fig. 2 plots the temperature profile of outer electrode
which indirectly indicates the temperature of the bulk gas.
As the gas environment is highly corrosive, an indirect
measurement was applied. At 9 kJ/L input energy density,
the temperature of outer electrode reaches 74 °C where
the conversions of CH4, CCl2F2, CF3CH2F and CHClF2
were 73 %, 73 %, 52 % and 17 % respectively. The
conversion level of CH4, CCl2F2, CF3CH2F and CHClF2
reached 79 %, 76 %, 58 % and 16 % respectively at 13
kJ/L where the temperature of outer electrode remained at
95 °C. This phenomenon shows the non-thermal
characteristics of the system where the bulk gas remains
at low temperature compared to the electron temperature
while providing a significant level of reactant
conversions.
3.4 Discharge Characterisation
Fig. 3 shows instantaneous current and voltage
waveforms related to CH4, CCl2F2, CF3CH2F and CHClF2
reactants in argon bath gas at 9 kJ/L. It was previously
reported that at the same frequency and at similar applied
voltage, the discharge in pure argon is filamentary[13].
The ionisation energies of Ar, CH4, CCl2F2, CF3CH2F and
CHClF2 are 15.76 eV [13], 12.62 eV [13], 12.30 eV [14],
12.64 eV [15] and 12.20 eV [16] respectively.
Fig. 3: V-I trace at 9 kJ/L.
The possibility of Penning ionisation is very low as the
lowest energy of metastable argon atoms (11.55 eV [13])
is not sufficient to ionise reactant molecules. Instead, a
modified argon discharge is formed as the reactant
molecules preferentially ionise compared to argon
molecules. Therefore, the discharge developed can be
described as shifting slightly from filamentary to
homogenous glow discharge under the conditions studied.
3.5 Polymer Characterisation
The polymeric materials formed under the investigated
input energy density range (3–13 kJ/L) are readily soluble
in organic solvents like tetrahydrofuran (THF) and
chloroform, indicating that they are non-crosslinked.
According to gel permeation chromatographic analysis
(GPC) there are two distinct distributions of polymeric
products. Based on polystyrene standards, the apparent
number average molecular weight (Mn) of the higher
molecular weight fraction polymers is 80 000 g mol-1,
with a polydispersity index (PDI) of 1.4. The Mn value for
low molecular weight fraction polymers is 500 g mol-1
(PDI–1.4).
The functional groups present in the polymers and their
connectivities were identified by 13C, 19F, DEPT 135 and
DEPTQ 135 NMR spectroscopic techniques. The DEPT
135 experiments were used to distinguish CH3 and CH
from CH2 units. In contrast, the DEPTQ 135 experiments
were used to distinguish CH3 and CH from CH2 and
quaternary carbons signals. The NMR spectra, presented
in figures 4 to 7, display sharp and broad peaks which are
representative of low and high molecular weight fractions
of the polymers respectively. The characterisation
presented in the current manuscript will focus on the high
molecular weight fraction polymer as its analysis is also
applicable to the low molecular weight fraction polymers.
Fig. 4: 19F NMR spectrum for the polymeric mixture
synthesised at 9 kJ/L.
The peaks in the region of 150 to 210 ppm, 90 to
150 ppm and 40 to 90 ppm in 19F NMR spectrum
(Fig. 4) represent CF, CF2 and CF3, respectively. CF3 is
either an end group or a branch group. The CF2 group
found in the 19F NMR spectrum can be either CF2 or
CHF2 groups where CF2 can be present in the main chain
while CHF2 can either be an end or a branch group. CF, in
contrast, can be representative of either CHF or CHClF
groups.
13
C NMR spectrum shows CH2, CHCl, CHF, CHClF,
CHF2 at 42, 58, 88, 98 and 112 ppm shifts (Fig. 5). The
peak at 125 ppm includes overlapping signals from CF 2,
CF3 and CH groups. This peak is resolved in the DEPTQ
135 spectrum (Fig. 6) at 120 ppm, disclosing a quaternary
carbon group and at 130 ppm a CH group. The quaternary
carbon groups, combined with CF2 and CF3 groups, are
absent in the DEPT 135 spectrum (Fig. 7). The CH peak
at 130 ppm of DEPTQ spectrum may be described as a
double bond between two equivalent CH groups attached
to a CH2 group (-CH2-CH=CH-CH2-). DEPT 135 and
DEPTQ 135 spectra show one peak at 53 ppm which can
be assigned as CH peak and this peak overlaps with the
CHCl peak (at 58 ppm) in the 13C NMR spectrum.
Fig. 5. 13C NMR spectrum for the polymeric mixture
synthesised at 9 kJ/L.
Fig. 6. DEPTQ 135 NMR spectrum for the polymeric
mixture synthesised at 9 kJ/L (QC: quaternary carbon).
Fig. 7. DEPT 135 NMR spectrum for the polymeric
mixture synthesised at 9 kJ/L.
These spectral assignments indicate that the polymers
formed are random in nature, where CH2, CF2, CHCl,
CHF and –CH=CH– are all present in the main polymer
chain. The CH group at 53 ppm in the 13C NMR spectrum
is present in the main polymer chain with CHF2, CHClF
or CF3 branches. The low molecular weight fraction
polymers may have all or some of the groups identified
for high molecular fraction polymer; however, they are
also random copolymers with a relatively short chain
length.
4. Conclusion
The reaction of a waste refrigerant mixture of CCl2F2,
CF3CH2F and CHClF2 has been examined (with CH4
additive) in argon bath gas in a dielectric barrier discharge
reactor. The products include a spectrum of gas phase
species as well as non-crosslinked polymers which have
been characterised as random copolymers. The findings
show this may be an alternative pathway for treating
waste refrigerants and producing useful materials.
5. Acknowledgement
The authors would like to thank the Australian
Research Council for financial support of this project.
Sazal K. Kundu and Vaibhav V. Gaikwad are indebted to
the Australian Government and the University of
Newcastle, Australia for postgraduate scholarships. We
thank Dr. Monica Rossignoli and Ms Azrinawati Mohd
Zin at School of Environmental and Life Sciences, The
University of Newcastle, Australia for their assistance
with NMR and GPC analyses respectively.
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