- Population growth driven sustainability issues, including the food resource issue, are pressing problems in need of attention. The goal of the present thesis is to examine the ability of an electric non-thermal plasma based microreactor to destroy ethylene, a known plant hormone responsible for losses in the range of 20% to 50% of fruits and vegetables in cold storage.
The research hypotheses investigated were that an electric glow discharge (non-thermal plasma) will efficiently destroy ethylene present at the ppm level in ethylene/air mixtures, and that a carbon nanotube based emitter electrode will enable lower voltage ignition of the glow discharge.
For the purpose, a microreactor device was constructed and equipped with a carbon nanotube based emitter electrode to produce a positive corona discharge in which the corona discharge is created between the surface of the carbon nanotube supported emitter electrode and a stainless steel grounded electrode. The performance of the system towards the oxidation of ethylene in air was investigated by a preliminary screening experiment, followed by systematic parameter sweeps of the identified important factors. Measurements of the reactor performance were based on gas chromatograms employing an on-line instrument equipped with both Thermal Conductivity and Helium Ionization detectors, and used conversion, defined as X%=([ethylene]in-[ethylene]out)/[ethylene]in*100%, as the measured variable for reactor performance.
Early experiments with the CNT based reactor indicated that while the CNTs may enable instantaneous spark discharges within the reactor at lower voltages, the CNT electrodes seemed to be destroyed in the process. For this reason work employed reactors from which the CNT structures had been removed.
The screening experiments showed that the most important factors in the destruction of ethylene in the gas mixtures was the thickness of the reactor, with a thicker reactor resulting in higher conversion, the initial concentration of ethylene, with lower concentrations of ethylene resulting in a higher fraction of the ethylene present destroyed, and the residence time of the gas flow within the reactor, with longer residence times (i.e., lower flowrates) resulting in a larger fraction of the ethylene being destroyed. Surprisingly, different applied reactor voltage was of little effect, nor was the interaction between residence time and reactor thickness.
The systematic examination of the microreactor performance focused on the conversion of ethylene as the residence time increases for inlet concentrations of ethylene between 200 ppm and 600 ppm, reactor thickness of 250 μm and 750 μm, and applied reactor current of 5 mA and 6.5 mA. Results showed ethylene conversion between X=35% and X=75%, again with very significant dependence on the reactor thickness, residence time, and inlet ethylene concentration. It should be noted that while higher conversion was observed for lower inlet concentrations of ethylene, that the absolute number of molecules of ethylene destroyed is higher for higher concentrations of ethylene, and this is reflected in the energetic efficiency of the system, with about 3x10⁻¹⁰ moles ethylene destroyed per Joule of electric power (3x10⁻¹⁰ mol /J) applied to the reactor at low inlet concentrations, growing to about (16x10⁻¹⁰ mol/J )at high inlet concentrations of ethylene.
The observed conversion of ethylene showed a sigmoid shaped dependence on residence time where the data could be fitted using a logistic function. This is possibly due to a localized corona discharge within the reactor that grows laterally within the reactor to fill the available reactor volume, even as the reacting volume is advocated by the moving fluid. This model results in little growth of the reaction volume at low residence times (high flow rates) and yielding low conversion, and maximal growth of the reacting volume at high residence times resulting in higher ethylene conversion. Confirming this model requires further work in future.