Dielectric barrier discharge fluid purification system

Details Dielectric barrier discharge fluid purification system Dielectric barrier discharge fluid purification system

2002-03-29

2004-11-02

Mayekar, Kishor (Department: 1753)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

With means applying electromagnetic wave energy or...

C204S164000

Reexamination Certificate

active

06811757

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to corona reactors, and more particularly, to a plasma reactor of the dielectric barrier discharge type and its use in plasma-based gas and liquid purification.
BACKGROUND OF THE INVENTION
Plasma may be defined as an electrically conducting medium in which there are roughly equal numbers of positively and negatively charged particles, produced when the atoms in a gas become ionized. It is sometimes referred to as the fourth state of matter, distinct from the solid, liquid and gaseous states.
When energy, such as heat, is continuously applied to a solid, it first melts, then it vaporizes and finally electrons are removed from some of the neutral gas atoms and molecules to yield a mixture of positively charged ions and negatively charged electrons, while overall neutral charge density is maintained. When a significant portion of the gas has been ionized, its properties will be altered so substantially that little resemblance to solids, liquids and gases remains. A plasma is unique in the way in which it interacts with itself, with electric and magnetic fields and with its environment. A plasma can be thought of as a collection of ions, electrons, neutral atoms and molecules, and photons in which some atoms are being ionized simultaneously with other electrons recombining with ions to form neutral particles, while photons are continuously being produced and absorbed.
Plasma may be produced in a discharge tube, which is a closed insulating vessel containing a gas at low pressure through which an electric current flows when sufficient voltage is applied to its electrodes.
Normally, air consists of neutral molecules of nitrogen, oxygen and other gases, in which electrons are tightly hound to atomic nuclei. On application of an electric field above a threshold level, some of the negatively charged electrons are separated from their host atoms, leaving them with a positive charge. The negatively charged electrons and the positively charged ions are then free to move separately under the influence of the applied voltage. Their movement constitutes an electric current. This ability to conduct electrical current is one of the more important properties of plasma
Plasma has been widely studied, different technologies have been developed to obtain different types of plasma and industrial applications have emerged.
The use of plasma as an inducer of chemical reactions and its application for treating gaseous, fluid pollutants and biological contaminants has been widely known for the past couple of decades. The catalyzing performance of plasma depends on its characteristics, which in turn depend on the type of discharge. The discharge itself depends on the shape of electrodes, on the nature of the inter-electrode region, on the voltage and current waveforms used for producing the plasma.
There are four known types of plasma production:
1. Electron beam.
2. Pulsed corona discharge.
3. Surface discharge.
4. Silent discharge (dielectric barrier corona discharge).
Treatment of air streams by dielectric barrier corona discharge is being developed as a cost effective and environmentally friendly alternative to conventional methods of air purification against a wide range of chemical and biological contaminants. Controlled reduction of the contaminant content is achieved by varying the discharge power and the contact time.
An electrical discharge is the passage of electrical current through a material that does not normally conduct electricity, such as air. On application of a high voltage source, the normally insulating air is transformed into a conductor, a process called electrical breakdown, and sparks, which are a form of electrical discharge, fly.
There are several types of electrical discharges:
1. The corona, which is a ‘partial’ discharge occurring when a highly heterogeneous electric field is imposed. Typically, a very high electric field is present adjacent to a sharp electrode, and a net production of new electron-ion pairs occurs in this vicinity. The corona typically has a very low current and very high voltage.
2. The glow discharge, which typically has a voltage of several hundred volts, and currents up to 1 Amp. A small electron current is emitted from the cathode by collisions of ions, excited atoms and photons, and then multiplied by successive electron impact ionization collisions in the cathode fall region.
3. The arc discharge, which is a high current, low voltage discharge, in which electron emission from the cathode is produced by thermionic and/or field emission.
Gas phase corona reactor (GPCR) technology enables the use of electrical discharges in order to accelerate (heat up) electrons to very high energies, while the rest of the gas stays at room temperature. The energized electrons attack background gas molecules producing highly reactive radicals such as [O], [OH], [N], etc., which in turn decompose various air contaminants.
Volatile organic compounds (VOCs) are an example of common air pollutants released in a number of industrial processes. Emission of VOCs is conventionally controlled by techniques such as thermal oxidation, catalytic oxidation, activated carbon adsorption, bio-filtration, etc. These technologies are generally expensive and have high energy requirements. Growing world concern for environmental protection has promoted testing and evaluation of a number of alternate techniques for abatement of VOCs.
Non-thermal plasma generated by GPCRs has developed as a cost effective and environmentally friendly method for destroying VOCs. The majority of the electrical energy applied to the reactor goes into the production of energetic electrons rather than into producing ions and heating the ambient gas, which is a more efficient and cost-effective method of decomposing toxic compounds than conventional methods.
Non-thermal plasma is highly effective in promoting oxidation, enhancing molecular dissociation and producing free radicals that cause the enhancement of chemical reactions, thereby converting pollutants to harmless by-products.
GPCRs of the dielectric barrier discharge (DBD) type have historically been used to produce industrial quantities of ozone, which have been used in the air and water purification fields. In ozone-based air purification, contaminated fluid is brought into contact with ozone (produced by various methods) while in plasma-based air purification the contaminated fluid is driven through a corona reactor and exposed to plasma. Plasma purification has the advantage of being able to treat extremely difficult compounds such as perfluorocarbons. Plasma purification is also more efficient than ozone purification, providing removal of a significantly greater weight of contaminant per unit energy input.
The conventional design of DBD utilizes a 2-electrode system (grounded tube and inner conducting wire) wherein one or both of the electrodes are covered by an insulating layer preventing arcing across the capacitive barrier by the charge build up. Most of the energized electrons are generated in close proximity to the wire resulting in a small effective plasma volume.
A major factor determining efficiency of a plasma based gas purification device is the structure of the gas flow through the electrodes. The most effective way of increasing efficiency is to lengthen the residence time of the fluid flow within the space between the electrodes in which the electrical discharge occurs. Increasing the time during which the discharge is able to act upon the fluid results in increased detoxification of the fluid, thus improving the quality of purification.
Various methods have been described for lengthening residence time of a gas in an ozone generator. U.S. Pat. No. 5,518,698 to Karlson et al describes an ozone generator in which the resident time for the gas within the generator is increased by lengthening the route for the movement of gas flow between electrodes which are shaped as two coaxial cylinders. The gas is introduced into the annular passageway between the ele

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