Plasma-Assisted Non-Oxidative Methane Coupling to Olefins

Methane conversion to olefins, and specifically to ethylene, by means of non-thermal plasma technologies has attracted attention over the past years; electric energy is used to produce energetic electrons for molecule bonds breaking, instead of gas heating, thereby overcoming the disadvantages of high operating temperatures. The numerous works on non-thermal plasma-assisted methane coupling, employing different kinds of non-thermal plasma technologies evidence the high research interest. Recently, nanosecond pulsed discharges have been employed due to high conversion of electrical into chemical energy. However, ethylene is never produced as major product whereas the technical and economic feasibility of plasma-driven processes have never been discussed. Further, general conclusions about the interplay of applied conditions/plasma reactor characteristics and plasma reactor performance are rather limited. The aim of the current doctoral thesis is to address the abovementioned points by applying a holistic approach: novel plasma reactor systems are first developed, investigated and optimized at lab scale, followed by techno-economic evaluation and sustainability assessment. Plantwide process models that simulate the whole plasma-assisted process and life cycle assessment are employed to facilitate the evaluation. The evaluation outcome serves, then, as feedback for further reactor improvements and advances towards i) lower ethylene energy cost that allows for positive economic potential and ii) large scale operation, tackling current limitations such as short operating periods, reactor clogging due to carbon formation and very broad operating windows. Low energy cost methane conversion to ethylene in a hybrid plasma-catalytic reactor system is first investigated. Methane conversion to acetylene by a nanosecond pulsed discharge (NPD) and subsequently, acetylene hydrogenation to ethylene using a Pd-based catalyst placed in the post-plasma zone is possible. Ethylene is formed as major product at 25.7% yield per pass, demanding 1642 kJ/molC2H4 in a single reactor, when operating at applied voltage ~17 kV, pulse repetition frequency 3 kHz, discharge gap 2.4 mm and total feed flow rate 200 sccm of H2:CH4 = 1:1 and utilizing 0.5 g of catalyst. The hybrid reactor is self-sustained since both heat and H2 required for the hydrogenation reaction are provided by methane cracking in the plasma zone itself, while operating periods of ~1 h are achieved. Higher ethylene yields and thus lower energy costs can be attained via hydrogenation catalyst activity enhancement using NPD treatment. The impact of treatment energy via pulse voltage, pulse frequency and treatment time individual variation on acetylene-to-ethylene hydrogenation reaction performance is investigated. Up to 11.2% increase in C2H2 conversion is attained after plasma treatment at specific energy input of 5340 J/gcat whereas C2H4 selectivity remains constant. The catalyst characterization reveals that C2H2 conversion increase is attributed to the higher active metal surface attained after the plasma treatment while no surface distractions (i.e. phase change and active metal sputtering) occur. Finally, it is concluded that the plasma-treatment impact on catalyst performance is driven by the specific energy input, irrespective of the operating conditions (pulse voltage, pulse frequency and treatment time). Direct plasma-assisted non-oxidative methane coupling to ethylene is also possible. Ethylene can be formed as major product at ~20 % yield per pass without utilizing any hydrogenation catalyst downstream the plasma zone, demanding 2020 kJ/molC2H4. This is attained by employing an NPD reactor, featuring rapid product quenching rates, (recyclable) hydrogen, co-feeding (CH4:H2=1:1) and operating at elevated pressures (5 bar). A study on the reaction pathways involved in non-oxidative methane coupling in NPD reactor via isotopic analysis is conducted to explain the product selectivity shift. Plasma-assisted reactions with isotopes, serving as tracers, are performed in two ratios (CH4:D2=1:1 and CH4:D2=1:3) and elevated pressures (up to 5 bar) while acetylene hydrogenation reactions are also performed in a tubular reactor under conventional furnace heating to simulate post-plasma zone conditions. It is concluded that C2H2 is mainly formed via CH3 radicals coupling to ethane followed by stepwise dehydrogenation (C2H6 → C2H5 → C2H4 → C2H3 → C2H2) when operating at ambient pressure. However, two additional reactions, not important at ambient pressure, are enabled when operating at elevated pressures: i) direct CH3 radicals coupling to ethylene and ii) acetylene hydrogenation to ethylene catalyzed by the copper-based electrode. The combined effect of the two reactions result in C2H4 formation as major product at pressures higher than 3 bar. Plantwide simulations of the hybrid plasma-catalytic and direct ethylene formation processes are performed. After performing sensitivity analyses on the key process parameters (reflux ratio, operating pressure and column stages) and heat integration, the economically preferable operating window is defined and the economic potential of plasma-assisted processes for ethylene production is evaluated. It is concluded that plasma-assisted processes are not economically viable, given that break-even electricity prices of 23 and 35 USD/MWh are calculated for the one-step and two-step process, respectively, while the current electricity prices for some energy intensive industries in certain countries can be as low as 50 USD/MWh. Further, life cycle assessment is conducted to estimate the carbon footprint of the plasma-assisted process and benchmark it against the carbon footprint of other green-considered ethylene production processes. Coupling of plasma-assisted process with wind turbines is necessitated to attain significantly low carbon footprint (1.3 kg CO2-eq./kgC2H4). Finally, the ethylene energy cost that allows for positive economic potential is set as new targeted performance. To meet the targeted performance, the plasma reactor is redesigned and reoptimized; a plate-to-plate configuration is tested. Discharge characterization is also applied to link the operating parameter (pulse repetition frequency, discharge gap and pressure) values with the discharge regime and CH4 fragmentation pattern, which drives the plasma chemistry. Acetylene yield of 33.5% using 870 kJ/molC2H2 is attained. Moreover, stable performance and long operating periods that are required at large scale operation are tackled and general guidelines (high load-impedance matching) for optimum plasma reactor performance are also established.status: publishe