Cleaning and liquid contact with solids – Processes – Including work heating or contact with combustion products
Reexamination Certificate
2001-04-12
2004-09-21
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Including work heating or contact with combustion products
C134S022180, C134S022120, C134S034000, C134S020000, C134S040000, C210S192000, C210S760000
Reexamination Certificate
active
06793739
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a method for cleaning electrical discharge ozone generators. More particularly, the present invention relates to circulating a warm fluid within the ozone generator to evaporate solid deposits of nitrogen oxides, including dinitrogen pentoxide, from the tubes and dielectrics of the generator.
BACKGROUND TO THE INVENTION
Ozone (O
3
) is a strong oxidizing agent (2.07V) that is used as a disinfectant in various applications, such as wastewater treatment, cooling towers, air treatment, swimming pool cleaning, food processing, hydroponics, and meat processing. Ozone is particularly effective in aqueous environments. Ozone is, however, very reactive and cannot be stored for any significant period of time. As a result, ozone must be generated at the site where it is to be used. Two common means by which ozone is generated are by subjecting oxygen gas (O
2
) to ultraviolet radiation or to an electrical discharge.
One type of electrical discharge generator is an electrical barrier discharge ozone generator, commonly known as a silent discharge generator. One such generator is a corona discharge generator. Corona discharge generators are commonly used to generate ozone on a large scale. The basic principle of electrical discharge ozone generators is that a feed gas is fed through a high voltage electrical discharge field between two electrodes. The oxygen is then ionized as it passes through the electrical field which will cause at least some oxygen to be converted to ozone. For corona discharge generators, the feed gas, usually dry air or oxygen, is subjected to coronal discharges created by high voltages between two electrodes, one of which is contained within a dielectric material. In a tubular ozone generator, a dielectric is supported within a tube and a central cathode within the dielectric is subjected to a high voltage relative to an outer anode. The anode is often grounded. High voltage phenomena occur inside the dielectric envelope and induce a corona discharge field between the outside of the dielectric envelope (hereinafter referred to as “dielectric”) and the outer anode material. An oxygen-containing feed gas is fed into this space and through this field, and the oxygen (O
2
) molecules are split to form atomic oxygen, which then reacts to form ozone.
3O
2
+Energy→2O
3
(Eq. 1)
The quantity of ozone generated depends on several factors, such as for example the voltage, the frequency of AC current, the gap between the dielectric and the cathode and the concentration of O
2
and other gases in the feed gas. The feed gas may be dry, clean air; dry, clean oxygen; or dry, clean oxygen containing small amounts of other relatively inert gases such as nitrogen (N
2
) or argon (Ar). It is important that dry feed gas be used, as water interferes with the reaction and also reacts with gases in the ozone gas to create contaminants, most notably nitric acid (HNO
3
).
Much of the energy required for the reaction is lost as heat; therefore ozone generators should be cooled to operate more efficiently. One of the ways that cooling the generator increases the efficiency of the generator is by causing fewer O
3
molecules to be lost due to decomposition or collision. A good description of ozone generating equipment can be found in U.S. Pat. No. 4,954,321 by Jensen issued Sep. 4, 1990, and also in “Ozone Technology and Equipment Design”, Ozonia North America, USA 1996, the information in, and contents of, both documents hereby being incorporated herein by reference.
Large amounts of ozone are not easily generated. For example, an ozone generation system employing an electrical discharge and which uses liquid oxygen at ≧99.5% purity, that has been vaporized and has had between 2 and 3% N
2
by weight and a certain amount of argon added, will typically produce 10-13% ozone by weight. Because of the relatively low rate of ozone generation, large plants may require several ozone generators to meet the demand for ozone. In turn, each generator may contain many anode/cathode/dielectric units.
As noted above, the amount of ozone generated depends on several factors, one of which is the amount of N
2
in the feed gas. When oxygen separated from air is used as a feed gas, nitrogen may be present. This is because of the method used to separate the oxygen from the air, e.g. vacuum or pressure swing adsorption or cryogenic separation. Nitrogen may also be present in the feed gas because it has been introduced to act as a catalyst. Nitrogen allows production of a higher ozone concentration or the reduction of the power consumed in generating the ozone. For example, large commercial ozone generators using pure oxygen generally create between 6-10% ozone by weight, instead of the 10-13% available when a small amount of N
2
is added. It is therefore not desirable to remove all of the N
2
.
Unfortunately, it has been discovered that the presence of nitrogen in the feed gas results in a solid residue, mainly composed of dinitrogen pentoxide (N
2
O
5
), with some of it being deposited within the generator system, including on the tubes and the dielectrics. The residue may also contain other solid oxides of nitrogen (NO
x
). The oxide deposits on the support tubes and dielectrics and may eventually clog the passageway between the dielectric and the support tube.
Regular maintenance of ozone generators typically involves an inspection and repair of the electrical connections. However, because of the problems inherent in cleaning the generators described in greater detail below, opening the ozone generator to the atmosphere is avoided whenever possible. From time to time, however, ozone generators may require special or preventative maintenance. Such maintenance may be occasioned by failure of more than approximately 10% of the dielectrics or by deposits that clog the passages between the dielectrics and their support tubes in some systems.
Current methods of cleaning ozone generators consist of turning off the power supply and cooling water and purging the generator by circulating dry oxygen gas at room temperature through the system. The purging continues until the residual ozone has been removed from the inside of the generator for the safety of the workers. Thereafter the system is opened up to the atmosphere.
When the ozone generator is opened for regular maintenance, if it is opened for long enough, the water in the ambient air reacts with any residual solid nitrogen oxides to form nitric acid (HNO
3
). The reaction with N
2
O
5
for example, proceeds as follows:
H
2
O+N
2
O
5
→2HNO
3
(Eq. 2)
Nitric acid is an oily, yellow residue, and any nitric acid in the generator needs to be removed.
The cleaning typically requires that all dielectrics and the tubes holding them be cleaned with a proper solvent. Generally, the dielectrics and tubes are removed from the system, cleaned with water and then with an industrial organic solvent such as acetone or a chlorinated organic solvent such as perchloroethylene, or methanol. This cleaning work is time consuming, and may require more than 14 days for an industrial scale ozone generator. In addition, removal and cleaning of the dielectrics will result in some breakage (perhaps 10%), thereby requiring their replacement. Finally, the chlorine containing solvents and the disposal of the contaminated cleaning solvents represent additional cost and safety issues that must be considered.
It is therefore desirable to have a less onerous cleaning method that would decrease the time and expense required for special maintenance of electrical discharge ozone generators, particularly large-scale corona discharge ozone generators.
SUMMARY OF THE INVENTION
The current invention relates to a method of removing solid deposits of the oxides of nitrogen, including in particular dinitrogen pentoxide, in an ozone generator thereby avoiding the need to open the generator to atmosphere. If it is necessary to open the generator, to replace dielectrics for example, the inventive method w
Casey Gregory E
Epiney J. W. G. Michel
Price Robert J.
Roche François
Sutton Christopher T.
Air Liquide Canada
El-Arini Zeinab
Russell Linda K.
Winter Gentle E
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