Corona reaction method and apparatus

Abstract

corona induced chemical reactions are conducted in a corona discharge zone in which narrow high voltage pulses are applied along with a relatively low voltage bias potential. It is found that for many corona discharge reactions, such as the conversion of oxygen to ozone, the present method increases the electrical efficiency of the reaction.

Description

The present invention relates to corona reaction systems, and more particularly to method and apparatus which may be used to increase the electrical efficiency of corona induced chemical reactions.

It is well known that many chemical reactions, such as the conversion of oxygen to ozone, may be effectively conducted in the presence of an electrical corona discharge. While it is found that high voltage corona is in many instances a convenient means by which to induce chemical reactions, corona discharge processes are extremely inefficient in terms of electrical energy required per unit of desired reaction product produced. For example, in the case of ozone produced from oxygen, the theoretical energy required to produce a Kg of ozone is 0.97 KWH per Kg while in practice it is found that about 6.6 KWH per Kg are needed with oxygen feed and 17.0 KWH/Kg with dry air feed.

Prior workers have attempted to decrease the amount of energy required to produce a corona reaction by varying many of the operational parameters of the corona generation system. For example, it is known that the use of pulsed high voltage energy having a pulse duration of 1 microsecond and a frequency as high as 1 Kh_z will more efficiently decompose carbon dioxide than a conventional 60 Hz AC waveform. However, it has also been shown that the use of an extremely high frequency corona power in the radiofrequency range, that is, 1-20 MH_z, does not result in increased efficiency in the production of ozone.

While the prior art suggests that changes in efficiency may be obtained in corona induced chemical reactions by the manipulation of frequency and waveform, it has been found that these attempts have resulted in processes that show no improvement and are in fact impractical from the commerical standpoint. This is primarily due to the fact that the use of high frequencies and voltages result in the production of excess heat which is unmanageable when large scale operation and high unit capacities which require high power densities per unit of electrode surface are contemplated.

It is therefore an object of the present invention to provide an improved corona discharge system.

It is a further object to provide a method by which the overall electrical energy required to produce a corona discharge chemical reaction may be minimized.

It is still a further object to provide a means by which the amount of excess heat which results from a corona discharge reaction process is substantially decreased.

It is yet another object to provide a method by which the operation and equipment parameters involved in a corona discharge chemical reaction system may be optimized to produce maximum product and minimum waste heat.

It is still a further object to provide an improved corona generator system which will make feasible corona reaction processes which are conducted at high unit capacities and maximum electrical efficiencies.

It is still another object to provide an improved ozone generation system which is capable of efficiently producing ozone in large quantities from an oxygen containing reactant gas, such as air, which contains considerable quantities of moisture and other impurities.

It is still a futher object to provide economical, reliable, high voltage, high frequency narrow pulse power supplies which are capable of driving large size corona generators to produce commercial quantities of ozone.

It is still yet another object to provide a system by which ozone may be safely generated in concentrations of 10% by weight without undue decomposition of ozone.

These and still further objects of the present invention will become readily apparent to one skilled in the art from the following detailed description and drawings wherein:

FIG. 1 represents a cross-sectional view with parts broken away of a typical corona discharge cell which may be used in the practice of the present invention;

FIG. 2 is a graphic representation wherein supplied, applied, dielectric and gap voltages are plotted on the vertical axis versus time on the horizontal axis, and represents the preferred waveform of the electrical energy pulse used to generate corona discharge;

FIGS. 3 and 4 are circuit diagrams of preferred power supplies which may be used to obtain the desired corona discharge;

FIG. 5 is a schematic electrical circuit diagram which depicts a preferred series-parallel connection of a purality of corona cells;

FIG. 6 is a plot in which specific ozone yield is plotted on the vertical scale versus duty factor on the horizontal scale, which may be used to select certain preferred operational parameters of the present system; and

FIG. 7 is a plot in which percent energy loss is plotted on the vertical scale versus duty factor on the horizontal scale, which may be used to select certain preferred operational parameters used in the practice of the present invention.

Broadly, my invention comprises a method for increasing the electrical efficiency of a corona discharge reaction system, and correspondingly substantially reducing the amount of waste heat produced thereby, wherein a narrow pulse high voltage waveform is applied along with a relatively low voltage bias potential.

More specifically, I have invented a corona reaction system wherein the corona is produced in a gas filled gap between opposing electrodes by a high voltage, narrow pulse electrical discharge in which the electrical pulse width is less than the gas ion transit time between the electrodes, and wherein a low voltage bias potential is maintained between the electrodes which is sufficient to substantially remove gas ions from the gap in the time interval between pulses.

In most conventional corona discharge systems the high voltage electrical potential is applied across the discharge gap in the form of relatively wide electrical pulses. Typically, the electrical energy waveform is a conventional sine wave, while in other instances pulse energy having a pulse duration of 1 to 200 microseconds (μ sec) have been utilized.

It is observed that these prior art systems are extremely inefficient and that 90 to 99% of the electrical energy is wasted in the form of excess heat. I have determined that the excess heat generated in these conventional systems may be attributed to the kinetic energy which is imparted to the charged gas molecules (ions) which are present in the corona discharge gap. The electrons which are responsible for the formation of the desired reaction product are formed and accelerated to reaction potential during the initial part of the conventional electrical power pulse. The remainder of the pulse supplies kinetic energy to the charged gas molecules which result from the formation of electrons and which do not contributed to the formulation of reaction product. These ionized gas molecules dissipate their kinetic energy in non-productive collision which appears as waste heat in the reaction system.

In my process I have substantially reduced the amount of waste heat by modifying the applied electrical waveform in two ways.

1. The duration of the electrical power pulse (Tw) is selected to be substantially less than the gas ion transit time across the discharge gap (T+).

2. A relatively low voltage bias potential (Vb) is maintained between the discharge electrodes to remove the majority of the charged gas molecules.

By selecting an extremely narrow energy pulse at the required sparking potential to produce ozone (Vs) it is found that the electrons which are formed are accelerated to an energy level required to do useful work. However, the energy pulse is not of sufficient duration to impart any significant waste energy to the charged gas molecules which are correspondingly formed.

It is recognized, however, that the gas ions which are present in the discharge gap must be removed before the next applied energy pulse, or a substantial portion of the energy pulse will be wasted in further acceleration of the non-productive gas ions. To achieve removal of gas ion "debris", I apply a relatively low voltage bias potential or "debris ion sweeping" potential during the interval between high energy electrical pulses. This low voltage bias potential is applied for a period which is substantially greater than the high energy pulse. However, since the energy applied to a charged particle in an electrical field is a function of only the particle charge and the field potential, the energy required to sweep the ion debris from the discharge gap is relatively minor.

The relationship which defines the preferred low voltage bias potential Vb is as follows:

T_r V_b =T+V_s ##EQU1## wherein T_r represents the pulse repetition period, T+ is the calculated gas ion transit time to sweep substantially all the gas ions from the gap at the sparking (corona discharge) potential V_s.

In order to determine the actual pulse width and frequency which is employed in the production of the high frequency narrow pulse energy it is necessary to select a pulse width which is substantially less than the pulse repetition period (which is the reciprocal of the such as N₂ O₅, H₂ O₂ and NH₃ may be conducted with equivalent increase in efficiencies.

Claims

1. A process for conducting corona discharge reactions between spaced electrodes comprising the steps of:

a. passing a reactant gas through a corona discharge gap formed between said spaced electrodes;

b. periodically establishing a corona discharge by dissipating narrow pulse electrical power of high voltage in said gap with the width of each narrow pulse being a relatively small fraction of the pulse period as well as being less than substantially about the gas ion transit time across said gap and greater than the electron transit time; and

c. maintaining a low voltage bias potential across said gap during the interval between said narrow pulses of a predetermined minimum magnitude sufficient to remove a substantial number of gas ions from said gap being before the termination of each pulse period.

2. The method of claim 1 wherein said reactant gas includes oxygen, and ozone is produced.

3. The method of claim 1 wherein said discharge gap has a width of from about 0.1 to 10.0 mm.

4. The method of claim 1 wherein each of said spaced electrodes is covered with a dieelectric member having a thickness of 0.1 to 10.0 mm and a dielectric constant of 2.0 to 200 relative to vacuum; and a dielectric strength sufficient to withstand applied voltage.

5. The method of claim 1 wherein said gap is maintained at a pressure of -0.1 to 3.0 atmospheres (gauge).

6. The method of claim 2 wherein said gas is maintained at a temperature below about 100°C

7. The method of claim 1 wherein said power has a frequency of 0.1 to 200 KHz.

8. The method of claim 1 wherein said high voltage lies in a range between 2.0 to 200 k volts.

9. The method of claim 1 wherein the narrow pulses have a duration from about 1 to 50 percent of the singlely charged gas ion transmit time.

10. The method of claim 1 wherein said low voltage bias potential is selected from the group consisting of positive, negative and/or alternating potentials having a magnitude of from about 0.1 to 50 percent of said high voltage.

11. The method of claim 1 wherein said pulses are supplied from a square wave power supply through an inductance.

12. The method of claim 11 where said pulses are of the same polarity.

13. The method of claim 11 wherein said pulses are of alternating polarity.

14. The method of claim 1 wherein said pulses are produced by vacuum switch tube power supply in which the filament energy losses are substantially equal to the energy consumed in removing the gas ions with said low voltage bias potential.

15. The method of claim 1 wherein the reactant gas is moist air.

16. A process for producing ozone by corona discharge which comprises:

a. passing oxygen through a corona gap having a width of 0.1 to 10 mm;

b. producing a corona discharge by applying electrical power at a peak voltage of about 2.0 to 200 k volts and a frequency of 0.1 to 200 kHz, said power being applied as high voltage pulses where said pulses have a duration of about 0.1 to 10 percent of the pulse repetition period; and

c. maintaining a low bias potential across said gap between said high voltage pulses of about 0.1 to 50 percent of said peak voltage.

17. The method of claim 16, wherein said bias potential is applied for a period of about 10 to 100 percent of said pulse repetition period.

18. The method of claim 17 wherein said bias potential is substantially equal to the product of the oxygen ion transit time and the peak voltage divided by the pulse repetition time.