Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
1999-03-05
2001-02-20
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S004000, C422S186080, C422S186110, C422S186120
Reexamination Certificate
active
06190436
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the purification of ozone, and more particularly to the separation of ozone from oxygen by membranes. Specifically, the invention involves contacting a gaseous ozone-oxygen mixture with a membrane made from an elastomeric polymer and/or silica, thereby producing an ozone-enriched gas on the permeate side of the membrane.
BACKGROUND OF THE INVENTION
Ozone is generally produced on a commercial scale by subjecting substantially pure oxygen to a high voltage discharge, which causes some of the oxygen to be converted to ozone. Conventional ozone generators produce a product stream containing about 10% by weight ozone, which is satisfactory for many ozone applications. However, because the efficiency of some industrial ozone-based processes, such as waste water treatment and pulp and paper bleaching operations, is directly proportional to the concentration of ozone in the treatment gas fed to the processes, there is a demand for equipment that can produce ozone product gases which contain higher ozone concentrations than do currently available ozone gas products. In response to this demand, equipment manufacturers have made improvements in ozone generators which make it possible to make ozone product gas containing up to about 14% by weight ozone. However, the improved ozone generators are considerably more costly to operate than are earlier ozone generators, since the improved generators consume significantly more power than do the earlier generators.
Waste water treatment and paper and pulp plant operators would like to have available ozone generating equipment that can produce ozone gas products containing up to 20% by weight or more ozone, but equipment having such capability is not currently available. Furthermore, even if such equipment were available, it would be prohibitively expensive to operate because of the very high power consumption that would be required to produce ozone gas of this quality.
Because of the continuing need for product gas containing high concentrations of ozone, various techniques for increasing the ozone concentration of ozone-oxygen gas mixtures by separating ozone from oxygen have been considered. One procedure that has been investigated is distillation. Since ozone has a boiling point of about −112° C. and oxygen has a boiling point of about −190° C., distillation would appear to be an attractive method for separating these gases. U.S. Pat. No. 5,756,054 discloses an ozone generating system in which liquid oxygen from a cryogenic oxygen source is subjected to ozonization to produce an ozone-containing product gas, ozone is separated from the product gas by condensation and the gaseous oxygen fraction is recycled to the cryogenic oxygen source. A major drawback of this method of separation is that it is capital- and energy-intensive.
Another ozone-oxygen separation technique that has been explored is adsorption. Ozone is generally more strongly adsorbed by adsorbent materials than is oxygen and thus it can be readily separated from oxygen by adsorption-based processes. U.S. Pat. No. 5,507,957 discloses an ozone generating system in which oxygen is separated from air in an adsorption vessel and the separated oxygen is subjected to ozonization to produce an ozone-containing stream, which is recycled to the adsorption vessel. Ozone in the recycle stream is adsorbed by a preliminary bed of adsorbent and the oxygen contained in the recycle stream passes through the preliminary adsorbent and is recycled to the ozonizer. A problem associated with the use of adsorption for the separation of ozone and oxygen is that the sorbed ozone component cannot be recovered from the adsorption equipment until the adsorbent regeneration phase of the separation process. As was the case with distillation, it is difficult or impossible to recover the ozone product stream from the adsorption equipment without appreciable decomposition of the ozone. A further complication of ozone-oxygen adsorptive separation processes is the fact that some adsorbents actually catalyze the decomposition of ozone.
Membranes have been investigated for nondispersively introducing ozone into water streams from ozone-oxygen gas mixtures. Shanbhag et al., “Membrane-Based Ozonization of Organic Compounds”, Ind. Eng. Chem. Res., vol. 37, 1998, pp. 4388-4398, describes the ozonation of water which contains organic pollutants by contacting a silicone membrane with an ozone-oxygen gas mixture. Ozone from the gas mixture passes through the membrane and contacts the pollutant-containing water on the permeate side of the membrane.
The present invention provides an efficient and effective method of increasing the ozone concentration of ozone-oxygen gas without significant loss of ozone by decomposition. This result is accomplished by separating an ozone-enriched gas stream from an ozone-oxygen gas mixture using a highly ozone-selective membrane manufactured from an elastomeric polymer or silica.
SUMMARY OF THE INVENTION
In a first broad embodiment, the invention comprises a process comprising the steps:
(a) introducing a gas mixture comprising ozone and oxygen into the feed zone of gas separation means comprising a feed zone and a permeate zone separated by an ozone-permeable membrane comprising an elastomeric polymer, silica or combinations thereof, thereby permeating an ozone-enriched gas into the permeate zone and producing an oxygen-enriched gas in the feed zone;
(b) removing the oxygen-enriched gas from the feed zone; and
(c) removing ozone from the permeate zone.
Step (a) of the process is generally carried out at a temperature in the range of about −120 to about 100° C. and at a pressure in the range of about 0.8 to about 20 bara. Step (a) is preferably carried out at a temperature in the range of about −100 to about 0° C., and it is preferably carried out at a pressure in the range of about 0 to about 10 bara. Step (a) is more preferably carried out at a temperature in the range of about −80 to about −30° C., and it is more preferably carried out at a pressure in the range of about 1.5 to about 5 bara.
Preferred elastomeric polymers used in the process of the invention comprise silicone-based polymers, such as silicone rubber (polydimethylsiloxane); ethylene-propylene terpolymer; fluorocarbon elastomer, polyurethane, or combinations thereof. The most preferred elastomeric membranes are those made from silicone rubber.
In some preferred embodiments, step (c) of the process is carried out by: (1) purging the permeate zone with inert gas; or by (2) evacuating the permeate zone; or by (3) purging the permeate zone with inert gas and evacuating the permeate zone.
In some preferred embodiments, the ozone-enriched gas product removed from the permeate zone in step (c) contacts a fluid stream containing ozone-reactive substances downstream of the permeate zone.
In other preferred embodiments, step (c) of the process comprises contacting the ozone-enriched gas with a gas stream containing at least one ozone-reactive substance in the permeate zone.
The process of the invention is particularly useful for treating fluids containing at least one ozone-reactive substance comprising hydrogen, carbon monoxide, nitrogen compounds, sulfur compounds, organic compounds, microbiological agents or mixtures thereof downstream from the permeate zone.
In some embodiments, the fluid stream being treated is a liquid stream, such as an aqueous stream. The invention is particularly useful for treating drinking water or wastewater. In these embodiments, the ozone-reactive substance contained in the liquid generally comprises organic compounds, viruses, living organisms or mixtures thereof.
In other embodiments, the fluid stream being treated is a gas stream, such as air, a breathable gas or an exhaust gas from a combustion process.
In any of the embodiments of the invention in which a gas stream is contacted with ozone-enriched gas, the ozone-reactive substance contained in the gas stream generally comprises hydrogen, carbon monoxide, organic c
Ferrell Robert James
Ji Wenchang
Shirley Arthur I.
Neida Philip H. Von
Pace Salvatore P.
Spitzer Robert H.
The BOC Group Inc.
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