Abstract:
Methods of providing ozone at a selected pressure above atmospheric pressure include supplying a purge gas supply ( 22 ) pressurized above the selected pressure to at least one ozone adsorption apparatus ( 12 ); desorbing ozone from the ozone adsorption apparatus ( 12 ) with the pressurized purge gas supply ( 22 ); and delivering a mixture of ozone and the purge gas supply ( 24 ) at the selected pressure without further compression. One method includes providing a supply of compressed dry air ( 34 ) at a pressure above the selected pressure; diverting a first portion of compressed dry air ( 34 ) to an oxygen generator ( 23 ); generating an oxygen supply ( 15 ); directing the oxygen supply ( 15 ) to an ozone generator ( 11 ); generating an ozone-rich oxygen supply ( 16 ); passing the ozone-rich oxygen supply ( 16 ) through at least one pressure swing adsorption tower ( 13, 14 ) and adsorbing ozone from the ozone-rich oxygen supply ( 16 ) to provide an ozone-depleted oxygen supply ( 20 ); recycling the ozone-depleted oxygen supply ( 20 ) to the ozone generator ( 11 ); diverting a second portion of compressed dry air ( 34 ) to the pressure swing adsorption tower ( 13, 14 ) and desorbing ozone with the compressed dry air ( 34 ); and delivering the mixture of ozone and compressed air ( 24 ) at the selected pressure without further compression.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Application Serial No. 60/375,560, filed Apr. 25, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to providing a supply of ozone.  
         BACKGROUND  
         [0003]    Ozone is widely utilized for processes such as drinking water disinfection, and control of gaseous pollutants such as NO x . However, for large systems, the cost of operating ozone production plants becomes a very large part of the overall system cost. Therefore, various methods for reducing the cost of producing ozone have been implemented. For example, in ozone production plants, ozone-depleted oxygen has been recycled to ozone generators and waste gas from oxygen generators has been used to purge ozone during the pressure swing adsorption process. However, there is need for additional improvements to ozone production plants to reduce the overall system cost of operating production plants used for processes such as drinking water disinfection, waste water treatment, and control of gaseous pollutants such as NO x .  
         SUMMARY  
         [0004]    There is provided, a method of providing ozone at a selected pressure above atmospheric pressure comprising:  
           [0005]    supplying a purge gas supply pressurized above the selected pressure to at least one ozone adsorption apparatus;  
           [0006]    desorbing ozone from said ozone adsorption apparatus with said pressurized purge gas supply; and  
           [0007]    delivering a mixture of said ozone and said purge gas supply at the selected pressure without further compression.  
           [0008]    In one embodiment, there is provided a method of providing ozone at a selected pressure above atmospheric pressure comprising:  
           [0009]    providing a supply of compressed dry air at a pressure above the selected pressure;  
           [0010]    diverting a first portion of said compressed dry air supply to an oxygen generator;  
           [0011]    generating an oxygen supply with said oxygen generator;  
           [0012]    directing said oxygen supply to an ozone generator;  
           [0013]    generating an ozone-rich oxygen supply with said ozone generator;  
           [0014]    passing said ozone-rich oxygen supply through at least one pressure swing adsorption tower;  
           [0015]    adsorbing ozone from said ozone-rich oxygen supply in said pressure swing adsorption tower, to provide an ozone-depleted oxygen supply;  
           [0016]    recycling the ozone-depleted oxygen supply to said ozone generator;  
           [0017]    diverting a second portion of said compressed dry air supply to said pressure swing adsorption tower;  
           [0018]    desorbing said ozone from said pressure swing adsorption tower using said second portion of said compressed dry air supply; and  
           [0019]    delivering a mixture of said ozone from said pressure swing adsorption tower and said second portion of said compressed air supply at the selected pressure without further compression. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic representation of ozone production plant having an oxygen generator using nitrogen adsorption beds to generate an oxygen supply and dryer beds to generate a compressed dry air supply.  
         [0021]    [0021]FIG. 2 is a schematic representation of ozone production plant having an oxygen generator using a cryogenic oxygen generator to generate an oxygen supply and a dry nitrogen supply.  
         [0022]    [0022]FIG. 3 is a schematic representation of ozone production plant having an oxygen generator using a cryogenic oxygen generator to generate a dry oxygen supply and a compressed dry air supply.  
         [0023]    [0023]FIG. 4 is a schematic representation of a vaporized liquid oxygen supply for the oxygen purge and pressurization cycles and for compressing the ozone-depleted oxygen supply.  
         [0024]    [0024]FIG. 5 is a schematic representation of a two tower pressure swing adsorption plant.  
         [0025]    [0025]FIG. 6 is a schematic representation of an improvement to the two tower pressure swing adsorption plant.  
         [0026]    [0026]FIG. 7 is a graph showing the experimental results of ozone adsorption-desorption using dry nitrogen during the waste gas purge cycle.  
         [0027]    [0027]FIG. 8 is a graph showing the experimental results of ozone adsorption-desorption using wet nitrogen during the waste gas purge cycle. 
     
    
     DETAILED DESCRIPTION  
       [0028]    A method is provided wherein the operation of ozone production plants are improved. In certain embodiments, the need for additional compression before utilization of a supply mixture of a waste gas and ozone is eliminated. In certain embodiments, various cryogenic oxygen generators, particularly suitable for generating oxygen for use in large ozone use processes may be used. A vaporized liquid oxygen supply may be used to simplify the recycling of an ozone-depleted oxygen supply to the ozone generator. A high purity oxygen supply, rather than the ozone-depleted oxygen supply, may be used during the oxygen purge and pressurization cycles of a pressure swing adsorption process to minimize the build up of inerts such as nitrogen during oxygen recycle. The size of an ozone buffer tank used to reduce pressure fluctuations and concentration fluctuations of ozone may also be reduced. The waste gas supply used during the waste gas purge cycle may be warmed to allow for use of higher pressures during the waste gas purge cycle than afforded during the ozone adsorption cycle. The waste gas supply may remain wet during the waste gas purge cycle to eliminate the need for drying a compressed air supply before use for ozone desorption, and allowing use of adsorbents with higher ozone adsorption capacity.  
         [0029]    As shown in the accompanying Figures, a plant for the production of ozone is generally indicated by the numeral  10 . In certain of the embodiments of this invention, the ozone production plant  10  has an ozone generator  11  and a two tower pressure swing adsorption (PSA) apparatus or plant  12 . The first tower  13  and second tower  14  of the two tower PSA plant  12  each have an adsorption bed (not shown) used for the adsorption and desorption cycles. These adsorption beds may contain adsorbents such as silica gel, high silica mordenites, dealuminated Y zeolite, and other materials that do not destroy a significant amount of ozone. Adsorbents other than silica gel may require some moisture on them to keep ozone destruction below acceptable levels. These adsorbents adsorb and desorb ozone during the process of ozone generation performed by the ozone generation plant  10  described hereinbelow. Furthermore, even though two towers with their adsorption beds are typical for ozone production, additional towers and their corresponding adsorption beds would allow for a more continuous operation of the ozone production plant  10 .  
         [0030]    The adsorption beds of the first tower  13  and second tower  14  pass through pressurization, ozone adsorption, waste gas purge (also called ozone desorption and regeneration), and oxygen purge cycles. The two tower PSA plant  12  of FIGS.  1 - 3  are each configured to facilitate these cycles. For example, the two tower PSA plant  12  is provided with a first switching system  17  and second switching system  18 . These switching systems contain various valves that operate to connect and disconnect the first tower  13  or second tower  14  to different gas supplies. The intermittent connection and disconnection of different gas supplies allows the first tower  13  and second tower  14  to operate out of phase with one another. Such a phase difference allows the two tower PSA plant  12  to operate relatively continuously.  
         [0031]    For illustrative purposes, but not by way of limitation, the general operation of the ozone production plant  10  as shown in FIGS.  1 - 3  will be described. An oxygen supply  15  is provided to the ozone generator  11 . The ozone generator  11  operates to provide an ozone-rich oxygen supply  16  to the first switching system  17 . The ozone-rich oxygen supply  16  is directed by the first switching system  17  to the two tower PSA plant  12 .  
         [0032]    The first tower  13  and second tower  14  both utilize the ozone-rich oxygen supply  16 , and may operate according to the same pressure swing adsorption process. However, the phase difference between either tower dictates that neither are in the same mode of operation at any given time. For example, at a given time, the adsorption bed of the first tower  13  could be undergoing the adsorption cycle, and the adsorption bed of the second tower  14  could be undergoing the waste gas purge cycle. The total time of one cycle of the pressure swing adsorption cycle may range from about 2 to about 30 minutes. For simplicity, the general operation of the first tower  13  is further described below.  
         [0033]    To initiate the operation of the first tower  13 , the first switching system  17  directs the ozone-rich oxygen supply  16  to the first tower  13  where it is used during the adsorption cycle. The ozone-rich oxygen supply  16  may have a pressure ranging from about 5 to about 50 psig, and the adsorption cycle may be performed at a temperature range from about −50° C. to about 50° C. During the adsorption cycle, the ozone from the ozone-rich oxygen supply  16  is adsorbed by the adsorption beds of the first tower  13 . Part of the ozone-depleted oxygen supply  20  is directed by the second switching system  18  to the second tower  14  (for use in the pressure swing adsorption process occurring in the second tower  14 ) and the remainder is directed to the blower  21 . The blower  21  may return a portion of the ozone-depleted oxygen supply  20  to the ozone generator  11  to be recycled.  
         [0034]    During the waste gas purge cycle, the ozone adsorbed from the ozone-rich oxygen supply  16  is desorbed from the adsorption beds by a waste (or purge) gas supply  22  (such as either a compressed dry air supply  34 ,  52  or a nitrogen supply  44 ). The waste gas supply  22  may have a pressure ranging from about 1 to about 30 psig, and the waste gas purge cycle is performed at a temperature range from about −50° C. to about 100° C. The pressure of waste gas supply  22  is typically lower than the oxygen supply  15  entering the ozone generator, but can be adjusted depending on the needs of the application. Furthermore, the mass flow rate of the waste gas supply  22  can be higher or lower than the ozone-rich oxygen supply  16 , however, the relative flow rates must be sufficient to obtain steady state operation of the pressure swing adsorption process. As will be discussed hereinbelow, the waste gas supply  22  may be generated by the oxygen generation apparatus  23 ,  39 , and  49 , and may be provided to the first tower  13  by the second switching system  18 . The waste gas supply  22  purges the ozone from the first tower  13 , and the resulting supply mixture  24  of waste gas and ozone is subsequently directed via the first switching system  17  to the ozone utilization applications, for example, a drinking water disinfection system.  
         [0035]    During the oxygen purge and pressurization cycles, the ozone-depleted oxygen supply  20  from the second tower  14  may be directed by the second switching means  18  to the first tower  13 . The first tower  13  may be sequentially purged of any excess waste gas supply  22  and pressurized using the ozone-depleted oxygen supply  20 . The pressure swing adsorption cycle is subsequently repeated in the first tower  13 . Furthermore, the pressure swing adsorption process continues in both the first tower  13  and the second tower  14  during the ozone generation process.  
         [0036]    As discussed above, the supply mixture  24  of waste gas and ozone may be directed to drinking water disinfection systems (purification), waste water treatment systems, or NO x  abatement systems. Such drinking water disinfection systems and waste water treatment systems require the supply mixture  24  to be provided at pressures of about 10 to about 25 psig. NO x  abatement systems require ozone supply pressures between 10 and 15 psig. However, additional compression of the supply mixture  24  after exiting the first tower  13  and second tower  14  would require a compressor and the power associated with it, and would lead to loss of ozone. This would be the case if the waste gas from the nitrogen adsorbing beds  30  and  31  in FIG. 1, rather than the compressed dry air supply  34 , were used as the waste gas supply  22  for the waste gas purge cycle. In this case, the waste gas from the nitrogen adsorbing beds  30  and  31  would be at close to atmospheric pressure and the supply mixture  24  would be at close to atmospheric pressure and would require compression. The ozone production and methods described above eliminate the need for additional compression of the supply mixture  24 , as discussed further below.  
         [0037]    In the ozone production plant  10 , various oxygen generators  23 ,  39 , and  49  can be used to produce the oxygen supply  15  and the waste gas supply  22 . In each of the embodiments of the oxygen generators  23 ,  39 , and  49  shown in FIGS.  1 - 3 , the waste gas supply  22  supplied to the first tower  13  and second tower  14  during the waste gas purge cycle discussed above, is provided at an elevated pressure, sufficient to provide the supply mixture  24  at the desired selected pressure, taking into account the known pressure drops throughout the system. For example, a compressed air supply  32  is initially compressed to the required pressure by a compressor  33 . The compressed air supply  32  is delivered to the oxygen generators  23  where the oxygen supply  15  and waste gas supply  22  are produced. The waste gas supply  22  is directed to the first tower  13  and second tower  14 , and additional compression is unnecessary because of the initial compression by the compressor  33 .  
         [0038]    Even though each of the oxygen generators  23 ,  39 , and  49  respectively depicted in FIGS.  1 - 3  eliminate the need for additional compression of the waste gas supply  22 , these oxygen generators operate differently. For example, FIG. 1 depicts an oxygen generator  23  using pressure or vacuum swing adsorption (PSA or VSA) to generate the oxygen supply  15 . The oxygen generator  23  is divided into dryer beds  28  and  29  and nitrogen adsorbing beds  30  and  31 . The dryer beds  28  and  29  remove moisture from the compressed air supply  32 . After exiting the dryer beds  28  and  29 , the resulting compressed dry air supply  34  is divided. One part becomes the waste gas supply  22 , and the other part is directed to the nitrogen adsorbing beds  30  and  31 . The nitrogen adsorbing beds  30  and  31  adsorb nitrogen from the compressed dry air supply  34  to produce the oxygen supply  15 .  
         [0039]    [0039]FIG. 2 shows oxygen production using a cryogenic oxygen generator  39  using air prepurification units  40  and  41  in combination with a cryogenic distillation unit  42 . The prepurification units  40  and  41  remove moisture and carbon dioxide from the compressed air supply  32  using either temperature swing adsorption or pressure swing adsorption processes. After being directed to the cryogenic unit  42 , the resulting purified air supply  43  is cooled to cryogenic temperatures, and separated into oxygen for use as oxygen supply  15  and a nitrogen supply  44 . The nitrogen supply  44  is divided, where one part becomes the waste gas supply  22  discussed above, and the other part becomes a regeneration supply  45  for the air prepurification units  40  and  41 . Intermittently, the process in the air prepurification units  40  and  41  is reversed to facilitate regeneration, and the regeneration supply  45  is used to regenerate the air repurification units  40  and  41 . After regeneration, the regeneration supply  45  exits the air prepurification units  40  and  41  as exhaust stream  46 .  
         [0040]    If waste gas supply  22  is required at higher pressures, the cryogenic oxygen generator  39  of FIG. 2 can be modified. As in the case of the oxygen generator  23  of FIG. 1, a separate compressed dry air supply can be produced when oxygen is made using cryogenic distillation. The resulting modified cryogenic oxygen generator  49  is depicted in FIG. 3. In FIG. 3, dryer beds  50  and  51  are used to dry the compressed air supply  32 . The resulting compressed dry air supply  52  is divided. One part becomes the waste gas supply  22  as discussed above, and the other part is directed to the air prepurification units  53  and  54 . As in FIG. 2, the air prepurification units  53  and  54  of cryogenic oxygen generator  49  of FIG. 3 removes other impurities such as carbon dioxide from the compressed dry air supply  52 . Furthermore, after being directed to the cryogenic unit  55 , the resulting purified air supply  56  is cooled to cryogenic temperatures, and separated into oxygen for use as oxygen supply  15  and a nitrogen supply  57 . The nitrogen supply  57  is used to regenerate the air prepurification units  53  and  54 . Intermittently, the process in the air prepurification units  53  and  54  is also reversed to facilitate regeneration, and the nitrogen supply  57  exits the air prepurification units  53  and  54  and dryer beds  50  and  51  as exhaust stream  58 .  
         [0041]    The general operation of the ozone production plant  10  using the various oxygen generators  23 ,  39 , and  49  as described above eliminates the need for additional compression of the waste gas  22 . Furthermore, the cryogenic oxygen generators  39  and  49  are particularly suitable for large ozone use processes. These large use ozone processes include, among others, the LoTO x  process for NO x  abatement. For large ozone users, the oxygen supply  15  generated with the cryogenic oxygen generators  39  and  49  provides a much more cost effective alternative to other forms of oxygen generation.  
         [0042]    Improvements to the ozone production plant  10  can be used to increase the efficiency of the ozone production process, and further reduce costs. For example, when vaporized liquid oxygen, such as being supplied from a liquid oxygen tank, is used to supply the ozone generator  11  as oxygen supply  15 , the vaporized liquid oxygen can also be used to simplify the recycling of the ozone-depleted oxygen supply  20 . As seen in FIG. 4, the ozone-depleted oxygen supply  20  and a vaporized liquid oxygen supply  59  with an elevated pressure (up to 200 psig) are directed to an eductor  60 . At the eductor  60 , the gas supplies are mixed and the ozone-depleted oxygen supply  20  is therefore compressed. The effective compression of the ozone-depleted oxygen supply  20  forces the ozone-depleted oxygen supply  20 , as part of a mixed oxygen supply  61 , to the ozone generator  11 . The effective compression of the ozone-depleted oxygen supply  20  eliminates the need for the blower  21 .  
         [0043]    Efficiency can also be increased by modifying the pressure swing adsorption process in the two tower PSA plant  12 . In other words, the pressure swing adsorption process is not limited to the various cycles described above, and additional or modified cycles can be used to improve efficiency. The pressure swing adsorption process in the two tower PSA plant  12  is described in Table 1 below.  
                                             TABLE 1                           Bed of Second       Time       Bed of First Tower 13   Tower 14   Valves Open   (minutes)                                Oxygen purge using   Ozone   V2, V3, V5, V6,   0.25       ozone-depleted oxygen   adsorption   V10       supply 20       Pressurization with   Ozone   V2, V5, V6, V10   0.25       ozone-depleted oxygen   adsorption       supply 20       Ozone adsorption   Purge using   V1, V4, V8, V9   4.5           waste gas 22       Ozone adsorption   Oxygen purge   V1, V4, V5, V6,   0.25           using ozone-   V9           depleted           oxygen           supply 20       Ozone adsorption   Pressurization   V1, V5, V6, V9   0.25           with ozone-           depleted           oxygen           supply 20       Purge using waste gas 22   Ozone   V2, V3, V7, V10   4.5           adsorption                  
 
         [0044]    [0044]FIG. 5 illustrates the configuration of the two tower PSA plant  12  facilitating the various cycles, and details the valves V 1 -V 4  forming the first switching system  17  and the valves V 5 -V 10  forming the second switching system  18 . Table  1  refers to the valves V 1  through V 10  depicted in FIG. 5, and details which valves are opened during the various cycles of the pressure swing adsorption process. These cycles are effectuated by the intermittent opening and closing of valves V 1  through V 10 , and the actuation of the valves for specified periods of time (exemplified but not limited to those in Tables 1-4) is usually controlled by a programmable logic controller (not shown).  
         [0045]    Although the entirety of the pressure swing process is described in Table 1, the first portion of the process will be described for purposes of illustration. As shown in Table 1, when the second tower  14  is undergoing the ozone adsorption cycle, the first tower  13  is undergoing the oxygen purge cycle. To perform these cycles, valves V 2 , V 3 , V 5 , V 6 , and V 10  are open for a selected period of time, for example, 0.25 minutes. As a result, part of the ozone-rich oxygen supply  16  is directed via valve V 2  to the second tower  14 .  
         [0046]    The ozone adsorption bed of the second tower  14  adsorbs ozone from the ozone-rich oxygen supply  16 . The ozone-depleted oxygen supply  20  exits the second tower  14 , and is subsequently divided. A first part of the ozone-depleted oxygen supply  20  is directed to a first buffer tank  80  via valve V 10 , and is eventually recycled to the ozone generator  11 . A second part of the ozone-depleted oxygen supply  20  is directed to the first tower  13  via valves V 5  and V 6 .  
         [0047]    During the oxygen purge cycle, the second part of the ozone-depleted oxygen supply  20  passes through the first tower  13 . Subsequently, the second part of the ozone-depleted oxygen supply  20 , and any contaminants collected during the oxygen purge cycle, are directed to a second buffer tank  81 . The second buffer tank  81  reduces the pressure fluctuations of gas supplies received therein. Furthermore, the second buffer tank  81  may also be used to reduce ozone concentration fluctuations. The gas supplies received in the second buffer tank  81  are eventually directed to the ozone utilization application, for example, a drinking water disinfection system. At the expiration of the oxygen purge cycle in the first tower  13 , the pressure swing adsorption process continues according to Table 1.  
         [0048]    As discussed hereinabove, the adsorption beds of the first tower  13  and second tower  14  operate out of phase with one another. Such operation increases the efficiency of the process, by allowing the operation of one tower to complement the operation of the other tower. For example, the out of phase operation of the two tower PSA plant  12 , allows the ozone-depleted oxygen supply  20  exiting one tower to be used in the oxygen purge cycle of the other tower.  
         [0049]    However, as seen in FIG. 6, the efficiency of the pressure swing adsorption process can be increased by using a high purity oxygen supply  71 , rather than the ozone-depleted oxygen supply  20  during the oxygen purge and pressurization cycles. High purity oxygen supply  71  could be oxygen from a liquid oxygen tank or gaseous oxygen from a cryogenic oxygen generator such as oxygen supply  15  in FIGS. 2 and 3. When the ozone-depleted oxygen supply  20  is recycled, this eventually results in an unacceptably large inerts concentration (more than 30%) in the ozone-depleted oxygen supply  20  eventually directed to the ozone generator  11 . For purposes of this specification, nitrogen and argon in air are considered inerts. The presence of such inerts can reduce the efficiency of the ozone generator  11  by more than 20%. However, the high purity oxygen supply  71  reduces the inerts concentration to less than about 5% in the ozone-depleted oxygen supply  20  directed to the ozone generator  11 . Such a concentration of nitrogen will have little effect on the efficiency of the ozone generator  11 . The improved pressure swing adsorption process using the high purity oxygen supply  71 , rather than the ozone-depleted oxygen supply  20 , is described in Table 2.  
                                             TABLE 2                       Bed of First           Time       Tower 13   Bed of Second Tower 14   Valves Open   (minutes)                                Oxygen purge   Ozone adsorption   V2, V3, V5, V10   0.25       using high       purity oxygen       supply 71       Pressurization   Ozone adsorption   V2, V5, V10   0.25       with high       purity oxygen       supply 71       Ozone   Purge using waste gas 22   V1, V4, V8, V9   4.5       adsorption       Ozone   Oxygen purge using high   V1, V4, V6, V9   0.25       adsorption   purity oxygen supply 71       Ozone   Pressurization with high   V1, V6, V9   0.25       adsorption   purity oxygen supply 71       Purge using   Ozone adsorption   V2, V3, V7, V10   4.5       waste gas 22                  
 
         [0050]    During the feed of the ozone-rich oxygen supply  16  to the adsorption beds of the first tower  13  and second tower  13 , ozone is adsorbed on the adsorption beds and oxygen passes through. During desorption using the waste gas supply  22 , ozone is desorbed from the adsorption beds and mixed with the waste gas supply  22 . The resulting supply mixture  24  of ozone and waste gas supply  22  is collected in the second buffer tank  81  before being sent to ozone utilization applications such as drinking water treatment. During desorption the concentration of ozone in the supply mixture  24  exiting the adsorption beds is not constant and can vary by a factor of two or more. The pressures and flow rates of the supply mixture  24  coming out of the adsorption beds may also vary. The second buffer tank  81  mixes the supply mixture  24  to provide a nearly constant ozone concentration and flow rate to the ozone utilization application. The required size of the ozone buffer tank can be determined experimentally or through process simulation.  
         [0051]    The efficiency of the process can also be increased by reducing the size of the second buffer tank  81 . Because gas supplies are not directed to the second buffer tank  81  from either the first tower  13  or second tower  13  during their respective pressurization cycles as shown in Table 2, there are large pressure and concentration fluctuations in the second buffer tank  81 . To overcome these fluctuations, the size of the second buffer tank  81  must be increased significantly. However, large buffer tanks as compared to small buffer tanks increase the possibility of ozone decomposition, and as a result, decrease the efficiency of the process. Also large, ozone compatible buffer tanks, can be fairly expensive. If the backfill step is eliminated the size of the ozone buffer tank can be reduced by 50% or more since there is constant ozone flow to the ozone buffer.  
         [0052]    To keep the size of the second buffer tank  81  small, the second buffer tank  81  has to receive a constant supply of gas containing ozone. Replacing the process described in Table 2 with the process described in Table 3 will provide such a constant supply of gas containing ozone.  
                                             TABLE 3                       Bed of   Bed of        Time       First Tower 13   Second Tower 14   Valves Open   (minutes)                                Oxygen purge   Ozone adsorption   V2, V3, V5, V10   0.25       using fresh       high purity       oxygen supply 71       Feed pressurization   Waste gas purge   V1, V4, V8, V9   0.25       with high purity       oxygen supply       71 and ozone       adsorption       Ozone adsorption   Waste gas purge   V1, V4, V8, V9   4.5       Ozone adsorption   Oxygen purge   V1, V4, V6, V9   0.25           using fresh           high purity           oxygen supply 71       Waste gas purge   Feed pressurization   V2, V3, V7, V10   0.25           with high purity           oxygen supply           71 and           ozone adsorption       Waste gas purge   Ozone adsorption   V2, V3, V7, V10   4.5                  
 
         [0053]    During the feed pressurization cycles shown in Table 3, the first tower  13  and second tower  14  will receive the ozone-rich oxygen supply  16  initially for pressurization and then for ozone adsorption and production of the ozone-depleted oxygen supply  20 . Therefore, the first tower  13  and second tower  14  are effectively pressurized without the need for the pressurization cycle of Table 2. Furthermore, during the process shown in Table 3 when one tower is undergoing the feed pressurization and ozone adsorption cycle, the other tower is undergoing either the waste gas purge cycle or oxygen purge cycle, and a constant supply of gas containing ozone is consequently supplied to the second buffer tank  81 .  
         [0054]    The cycle in Table 3 will reduce the size and corresponding cost of the second buffer tank  81 . It will also reduce ozone decomposition inside the second buffer tank  81  through reduction in ozone residence time in the second buffer tank  81 . In addition to the process described in Table 3, other possibilities exist to reduce or eliminate the second buffer tank  81 . For example, if the ozone production plant  10  is used to treat large drinking or waste water supplies, then large basins for contacting ozone and water can themselves act as an ozone buffer to remove the aforementioned concentration fluctuations, and eliminate or substantially reduce the size of the second buffer tank  81 .  
         [0055]    The efficiency of the process can further be increased by warming the waste gas supply  22  used during the waste gas purge cycle. As discussed above, compressed dry air supply  34 ,  52  and nitrogen supply  44  are used as the waste gas supply  22  to desorb the ozone from the adsorption beds. Warming the waste gas supply  22  to about 10° C. to about 30° C. above the ozone-rich oxygen supply  16 , for at least part of the waste gas purge cycle, reduces the amount of waste gas supply  22  required. Furthermore, warming the waste gas supply  22  also allows use of higher pressures during the waste gas purge cycle than afforded during the desorption with waste gas supply  22  having temperatures similar to ozone-depleted supply  16 . As a result, a heater  92  may be provided as shown in FIG. 6 to heat the waste gas supply  22 . Furthermore, the heat of compression generated during the production of compressed dry air supply  34 ,  52  can be used to heat the waste gas supply  22  when the compressed dry air supply  34 ,  52  is generated. A representative cycle using the warmed waste gas supply  22  is described in Table 4.  
                                             TABLE 4                       Bed of First           Time       Tower 13   Bed of Second Tower 14   Valves Open   (minutes)                                Oxygen purge   Ozone adsorption   V2, V3, V5, V10   0.25       using high       purity oxygen       supply 71       Pressurization   Ozone adsorption   V2, V5, V10   0.25       with high       purity oxygen       supply 71       Ozone   Warm waste gas purge   V1, V4, V8, V9   2       adsorption       Ozone   Waste gas purge   V1, V4, V8, V9   2.5       adsorption       Ozone   Oxygen purge using high   V1, V4, V6, V9   0.25       adsorption   purity oxygen supply 71       Ozone   Pressurization with high   V1, V6, V9   0.25       adsorption   purity oxygen supply 71       Warm waste   Ozone adsorption   V2, V4, V8, V10   2       gas purge       Waste gas   Ozone adsorption   V2, V4, V8, V10   2.5       purge                  
 
         [0056]    The efficiency of the process can still further be increased by using a wet waste gas supply  22  during the waste gas purge cycle. Such a wet regeneration gas can be produced by compressing ambient air to the desorption pressure. Using wet waste gas supply  22  during the gas purge cycle results in some loss in adsorption capacity. However, overall ozone recovery may increase because ozone destruction (or decomposition) decreases significantly when using wet adsorbents. Also, significant energy savings can be realized by only drying the ozone-depleted oxygen supply  20  before entering the ozone generator  11 . As a result, the ozone-depleted oxygen supply  20  should be dried by some suitable drying process before going to the ozone generator  11 . These drying processes include, but are not limited to, PSA, TSA, or a suitable membrane. In fact, the amount of moisture in the oxygen supply  15  may be less than 10% of the moisture in the waste gas supply  22  and this results in significant regeneration energy savings.  
         [0057]    The results of an experiment alternately using wet and dry nitrogen supplies  44  as the waste gas supply  22  are seen in FIGS. 7 and 8. For example, ozone from the ozone-rich oxygen supply  16  (10% ozone in oxygen mixture) was adsorbed on an adsorption bed composed of silica gel for approximately 5 minutes. The adsorbed ozone was subsequently desorbed using wet and dry nitrogen supplies  44  for 5 minutes. The flow rates of the ozone-rich oxygen supply  16  and the wet and dry nitrogen supplies  44  were identical. The ozone concentrations at the outlet of the adsorption bed during cyclic adsorption and desorption are shown in FIGS. 7 and 8. Comparison of the resulting ozone concentrations indicates that the adsorption capacity using a wet or dry nitrogen supply  44  are not significantly different.  
         [0058]    Use of the wet waste gas supply  22  during regeneration makes possible the use of other adsorbents such as high silica mordenites and dealuminated Y zeolites. These adsorbents have ozone adsorption capacities two to three times that of silica gel. However, they can not be used when the adsorbent is dry because of significant ozone loss due to decomposition.  
         [0059]    It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from spirit and scope of the invention. The various embodiments may be practiced in the alternative, or in combination, as appropriate. All such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.