Type of Document Dissertation Author Shih, Kai-Yuan Author's Email Address email@example.com URN etd-04092010-102917 Title Analysis of External Pressure and Solution Temperature and Conductivity on Pulsed Electrical Discharge in Aqueous Solution or Bubbles Degree Doctor of Philosophy Department Chemical Engineering, Department of Advisory Committee
Advisor Name Title Bruce R. Locke Committee Chair Milen Kostov Committee Member Ravindran Chella Committee Member Igor Alabugin University Representative Keywords
- Electrical Discharge
- Hydrogen Peroxide
- Energy Yield Efficiency
Date of Defense 2010-04-02 Availability unrestricted AbstractElectrical discharge induced non-thermal plasma is considered a “green” treatment method for gas or water pollution control because of its capability to generate reactive radical and reactive molecular species, high efficiency, low cost, non-harmful residuals or by-products at the end of degradation and the capability to operate in ambient conditions. It has been intensively studied for applications in chemistry and chemical, materials, environmental, and biomedical engineering.
In the present study, we focus on the fundamental understanding and the applications of liquid phase non-thermal plasma formed by pulsed electrical corona discharge in liquids or in liquid-gas hybrid environments. The discharges generated in or in contact with water (H2O) initiate a variety of physical and chemical processes such as the formation of intense ultraviolet radiation, shock waves, and most importantly, chemically active species. From the previous experimental and simulation results, pressure and temperature were shown to be two of the most important environmental parameters that directly affect the plasma physical properties and the chemical reactions. However, there is not much information known on the effects of temperature and pressure changes (without phase change) on liquid phase discharge. Moreover, there is an increasing interest in combining electrical discharge with other advanced oxidation technologies which operate under higher temperature and pressure conditions (e.g. wet oxidation and supercritical oxidation).
It is therefore necessary and important to determine the fundamental effects of pressure and temperature on liquid phase pulsed corona discharge to improve the reactor design and efficiency for various applications. The goal of this study is to chemically and physically investigate the influences of pressure and temperature on liquid phase pulsed corona discharge. Other factors which directly or indirectly are related to pressure and temperature (i.e. energy input, solution conductivity and medium phases) are also evaluated.
In the first part, liquid phase pulsed corona discharge was operated under high external pressure (up to 14 atmospheres) and in high solution temperature (near boiling), respectively. Discharge channel images, electrical properties (initiation voltage, current and voltage waveforms) and chemical measurements (H2, O2, and H2O2 generation rates and energy yields) were carefully evaluated under different experimental conditions. The important breakdown criteria and the mechanisms of discharge propagation under different discharge phases in non-ambient conditions were proposed based on experimental analysis. In summary, this study has demonstrated the capability of operating liquid phase discharge under high pressure. Most importantly, the unique chemical and physical properties of non-thermal plasma are shown to not be affected by external pressure once the discharge is initiated. The key criterion for non-ambient condition discharge, the presence of pre-existing bubbles, is also identified.
The effects of bulk solution temperature on liquid phase discharge was also investigated and the results showed that increasing of the liquid solution temperature (below boiling) does not significantly change the discharge properties because the heat requirement for pre-existing bubble formation was dominated by the latent heat. However, when the solution is raised to boiling or near boiling, a special underwater discharge is formed where the plasma channels are now initiated inside the steam bubbles and these channels propagate along the gas-liquid interface. The discharges within bubbles were also shown to be formed when externally supplying gas bubbles under ambient conditions. The mechanisms of discharge formation under different discharge phase configurations were proposed by comparing discharge images and the chemical measurements (H2, O2, H2O2).
From the analysis of the above experiments, a special underwater discharge was found when the plasma channels are initiated inside the large size (centimeter) gas phase bubbles (from heating or external gas supply) and these channels propagate along the gas-liquid interface. A new discharge configuration, discharge within large gas bubbles along long a needle electrode in water, was also discovered. The advantages and disadvantages of operating a discharge within bubbles in a liquid are discussed and compared with those in direct liquid phase discharge.
Experiments utilizing emission spectroscopy to optically diagnose the generation of radical species in the plasma in water and bubbles were also conducted. A calibration method which uses argon as a chemical actinometer without disturbing the liquid phase discharge was developed. With this method, we are able to compare the radical intensity measured under different conditions (i.e. various solution conductivities). The capability to analyze radicals, in addition to electrical and chemical diagnostics and mathematical simulation, provide new insights into plasma chemical processes.
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