- Title
- Conversion of fluorine-containing ozone-depleting and greenhouse gases to fluoropolymers applying a dielectric barrier discharge non-equilibrium plasma
- Creator
- Kundu, Sazal
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis describes research in the conversion of fluorine-containing ozone-depleting and greenhouse gases (abbreviated as “fluorocarbons”) into environmentally benign substances using a non-equilibrium plasma process. Conversion of a variety of fluorocarbons, including CFC-12 (CCl₂F₂), CFC-11 (CCl₃F), HCFC-22 (CHClF₂), HFC-134a (CF₃CH₂F) and a mixture of CFC-12, HCFC-22 and HFC-134a were investigated in detail. Among these chemicals, CFC-11, CFC-12 and HCFC-22 are ozone depleting substances while all are potent greenhouse gases. Conversion of HFC-134a was examined with and without the addition of methane, while methane was used as a reactant in all other experiments. Oxygen and nitrogen were excluded from all experiments and under these conditions the targeted fluorocarbons convert to gaseous as well as polymeric products. A dielectric barrier discharge(DBD) reactor, constructed from two concentric dielectric barriers, a 2.0–2.4 cm plasma zone and a 4.5–4.7 mm discharge gap, was used to generate the non-equilibrium plasma. The reaction of the fluorocarbon results in the formation of corrosive hydrofluoric acid (HF) gas, restricting reactor materials selection. In order to overcome this process constraint, the double barrier DBD reactor configuration with coaxial electrodes was selected to ensure that the electrodes were protected from exposure to HF. In terms of dielectric strength, quartz is highly suitable for the DBD reactor, however it is not suitable in a gas stream containing HF. In contrast, alumina is an HF-resistant material and has been used in studying the high temperature conversion of CFCs and halons although its dielectric strength is lower than that of quartz. With the aim of developing reactors designed for handling HF, a detailed preliminary study was undertaken using these two dielectric materials, quartz and alumina, examining the conversion of methane in an argon bath gas. It was found that the higher relative permittivity of alumina compared to quartz results in higher power dissipation in the alumina reactor compared to a similar quartz reactor, for a given applied voltage, due to increased capacitance of the system.Unlike thermal conversion methods, excitation and discharge quenching in the DBD system is too rapid to allow the development of any significant thermal equilibrium between electrons and ions and temperature rise of the process stream is relatively small. Measurements of the temperature in the reaction zone (using temperature of the outer electrode as a proxy) reveal that the reactants convert at a relatively low bulk gas temperature compared to conventional thermal or thermal plasma reactors. Carbon-containing gas phase products were identified by gas chromatography/mass spectroscopy (GC/MS) and quantified using a micro gas chromatograph (μGC). Hydrogen, produced during reactions, was quantified by a dedicated GC employing argon as carrier gas. A specially designed teflon Fourier transform infrared (FTIR) gas cell (11.7 mm path length) was used to quantify acid gases (HCl and HF). The data acquired from this analytical train were used to estimate mass balances, where overall balances in excess of 95 % were achieved for all experiments. Polymeric products, generated during the reaction of the fluorocarbons, were deposited in the plasma zone. The polymeric materials were collected by rinsing the reactor tubes with tetrahydrofuran solvent and characterised by gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy. The polymers were primarily non-crosslinked in nature, and are readily soluble in chloroform and tetrahydrofuran solvents. This property of the polymers enabled their analysis by solution state ¹³C and ¹⁹F NMR spectroscopies. Based on analyses of ¹³C NMR, spectra, the composition of polymeric materials changed according to their source material, viz, from one fluorocarbon to other. The conversion of HFC-134a in an argon bath gas (feed condition: 1.25 % HFC-134a in 100 cm³ min⁻¹) was 52 % at 2 kJ L⁻¹ (the outer electrode temperature of 36°C) and 82% (where the outer electrode temperature was 101°C). This temperature profile demonstrates the non-equilibrium nature of the system, where the electron temperature is relatively high compared to bulk gas temperature. The applied voltage range was 13.4 – 15 kV (peak-peak) for this conversion range of HFC-134a, and corresponds to an input energy density range of 2 – 12 kJ L⁻¹. The addition of methane with the HFC-134a in the argon bath gas reduces the level of conversion of HFC-134a compared to reactions in the absence of methane. For example, at an input energy density of 6 kJ L⁻¹, the conversion level of HFC-134a (feed condition: 1.25 % HFC-134a in 100 cm³ min⁻¹) was 75 % while the addition of an equimolar concentration of methane reduced the conversion level of HFC-134a to 47 %. The reaction of CFC-12 was examined, with methane included as an additive in argon bath gas and a conversion range of 44 to 71 % for CFC-12 (feed condition: 1.25 % CFC-12, 1.25 % CH4 in 100 cm³ min⁻¹) was observed over an input energy density range of 3 – 13 kJ L⁻¹. CFC-11 and HCFC-22 were also examined (with methane as additive) under similar feed conditions, where the conversion ranged from 37 – 63 % and 53 – 79 % respectively over an input energy density range of 3 – 13 kJ L⁻¹. A fluorocarbon mixture (85 % CFC-12, 1.5 % HCFC-22 and 13.5 % HFC-134a) with methane (similar feed conditions) was studied and the conversion of the major fluorocarbon, CFC-12 was found to be 59 – 76 % for the input energy density of 3 – 13 kJ L⁻¹. According to mass balance analysis, the products from the reaction of CFC-12 with methane under the feed condition described earlier and an input energy density of 5 kJ L⁻¹ contain 48 % gas phase substances and 52 % polymer (measured in wt %). The elemental composition of C, H, Cl and F (in wt %) for this polymer is 27.51 %, 1.82 %, 44.92 % and 25.88 % respectively. HF and HCl (if chlorine is present in the fluorocarbon) are major gas phase products while H2 is an additional major gas phase product where methane is employed as an additive. A reaction mechanism describing the formation of gas phase and polymeric products is presented. As the reactions were conducted in an argon bath gas, the reactions of metastable argon atoms and direct electron impact reactions contribute to the resulting product distribution. As the discharge is primarily an ionisation phenomena, any interactions between ionised plasma species and the external electric field must be considered and factored into the elucidation of the prevailing reaction mechanisms particularly the formation of polymers. While the formation of polymers in plasma is described in many ways including the ionic mechanism, radical mechanism and atomicpolymerisation, this thesis considers the ionic mechanism as a basis with additional hypotheses to explain the product spectrum. Accordingly, Coulombic interactions between charged species and the electric field due to the applied voltage are important when considering the polymer's formation mechanism and a polymerisation mechanism is described in this thesis which includes the contributions of cations on polymer formation.
- Subject
- non-equilibrium plasma; dielectric barrier discharge; alumina dielectric; greenhouse gas; non-crosslinked polymer; plasma polymerisation
- Identifier
- http://hdl.handle.net/1959.13/1296450
- Identifier
- uon:19263
- Rights
- Copyright 2015 Sazal Kundu
- Language
- eng
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