Method and apparatus for producing ozone

Abstract: A method of producing ozone comprises passing oxygen rich gas from a PSA plant 2 through an ozoniser 4 and then passing the ozonised oxygen thus formed directly back to the PSA plant 2 where the ozone is adsorbed in a bed 10, 10' of adsorbent material contained within a vessel 6, 8. During a desorption step nitrogen adsorbed in a bed 12, 12' also located in the vessel 6, 8 desorbs the ozone from the bed 10, 10' and a nitrogen-ozone gas leaves the PSA plant 2 via a line 44.

Description: The present invention relates to a method of and apparatus for treating material such as a liquid with ozone. Ozone is an oxidising agent which finds use in the treatment of, for example, water to produce a potable product. It is known to produce ozone from an ozoniser using an oxygen rich feed gas which has been produced from a pressure swing adsorption (PSA) plant. Use of an oxygen rich gas produced by PSA techniques is very attractive for the generation of ozone in as much as it contains 8% of nitrogen and argon with substantially no water vapour which increases the efficiency of the ozone generator. When oxygen rich PSA gas is used as the feed gas for an ozoniser the maximum concentration of ozone that can be achieved is normally less than 10% but often the ozoniser is set to produce ozone at a concentration by volume of 5%. Although, when compared to the use of air as a feed gas for the ozoniser, the ozoniser is decreased in size for the same output of ozone, the remainder of the oxygen rich gas which has not been converted to ozone is sometimes wasted, used in a different part of a subsequent process or needs to be cleaned up and returned to the process. Japanese published Patent Application 52081092 discloses a method of and apparatus for the production of ozone. Feed air is passed through a pressure vessel forming part of a PSA plant in which is located an adsorbent bed which preferentially adsorbs nitrogen. The oxygen rich gas leaving the pressure vessel is then delivered to an ozoniser where it undergoes silent discharge at a high voltage such that a portion of the oxygen is ozonised. The ozonised oxygen then leaves the ozoniser and is introduced into a separate low temperature ozone adsorber where the ozone is separated by being adsorbed while the oxygen leaves the ozone adsorber and can be recycled by means of a blower to blend with the oxygen leaving the pressure vessel before entry into the ozoniser. The nitrogen generated in the pressure vessel is fed directly to the low temperature ozone adsorber where it desorbs the ozone and is discharged in the form of a ozone/nitrogen feed gas for passage to a material to be treated. A disadvantage of the apparatus described in Japanese Published Patent Application 52081092 is that the recycled oxygen rich gas requires recompression before it can be used again. Further, the apparatus requires the provision of a separate low temperature ozone adsorber. It is an aim of the present invention to provide an improved method of and apparatus for treating a material with ozone whereby oxygen gas mixed with ozone is recycled but does not require cleaning and is not passed through rotating machinery such as compressors or vacuum pumps. According to one aspect of the present invention a method of treating a material with ozone comprises passing feed air through a first bed of adsorbent material located in a vessel forming part of a PSA plant which first bed preferentially adsorbs nitrogen and carbon dioxide relative to oxygen, passing the oxygen rich gas thus separated to an ozoniser for the production of ozonised oxygen, passing the ozonised oxygen to a further bed of adsorbent material which preferentially adsorbs the ozone relative to the oxygen, desorbing the ozone from the further bed of adsorbent material with the nitrogen when desorbed from the first bed of adsorbent material and passing the ozone-nitrogen gas mixture to the material to be treated, and is characterised in that the further bed is also located in the vessel, the ozone is passed directly to the further bed and in that the oxygen rich gas remaining after the ozone is adsorbed by the further bed passes immediately with the feed air through the first bed. Preferably, the feed air prior to its passage through the vessel is passed through a layer of dessicant. Preferably, at least some of the ozone-nitrogen gas mixture passes through a layer of dessicant prior to its passage to the material to be treated. In a preferred embodiment the pressure swing adsorption plant comprises at least two vessels each containing a first bed and a further bed, each vessel being subjected to a cycle including an adsorption step, an equalisation step and a desorption step, the cycles being arranged 180° out-of-phase thereby to produce a substantially continuous supply of oxygen. According to a further aspect of the present invention an apparatus for treating material with ozone comprises a PSA plant for the production of oxygen from compressed feed air, the PSA plant including a pressure vessel containing a bed of adsorbent material which preferentially adsorbs nitrogen and carbon dioxide relative to oxygen, means for the passage of the oxygen from the pressure vessel to an ozoniser for the production of ozonised oxygen, means for returning the ozonised oxygen directly to the pressure vessel which contains a further bed of adsorbent material which preferentially adsorbs the ozone, the oxygen remaining together with the compressed feed air passing through the bed of adsorbent material which preferentially adsorbs nitrogen and carbon dioxide relative to oxygen, and means for desorbing the pressure vessel such that a nitrogen-ozone gas mixture passes from the pressure vessel to the material to be treated. An embodiment of the invention will now be described, by way of example, reference being made to the Figure of the accompanying diagrammatic drawing which is a schematic flow diagram of an apparatus for treating a material with ozone. As shown, the apparatus comprises an oxygen PSA plant 2 for the production of an oxygen rich gas and an ozoniser 4. The PSA plant 2 is a two bed PSA plant comprising two substantially identical pressure vessels 6, 8 each containing a first bed 10, 10' of adsorbent, for example, silica gel, for the adsorption of ozone as will be explained and a second bed 12, 12' of adsorbent material which preferentially adsorbs nitrogen and carbon dioxide relative to oxygen. As is well known in the art each bed 12, 12', in operation, passes through a cycle including an adsorption step, an equalisation step and a regeneration step and the cycles of the two pressure vessels 6, 8 are arranged 180° out-of-phase to provide a continuous product gas rich in oxygen. Each pressure vessel 6, 8 has a line 28, 28' controlled by a valve 30, 30'. The lines 28, 28' communicated via a line 32 controlled by a valve 34 with a buffer vessel 14 containing a bed of carbon molecular sieve. A line 35 controlled by a valve 47 communicates at one end with the interior of the vessel 14 and at its opposite end with ambient atmosphere. A further valve 36 is positioned between the vessel 14 and the ozoniser 4. Associated with each pressure vessel 6, 8 is a vessel 16, 16' containing a dessicant, for example, alumina. A compressor 20 is connected via a line 22 and valve 24 to the vessel 16 and via a line 22' and valve 24' to the vessel 16'. Each vessel 16, 16' is connected via a line 26, 26' respectively to its associated pressure vessel 6, 8. Extending between each vessel 16, 16' is a line 40 in which are located valves 42, 42' which line 40 connects with a line 44 communicating with an ozone contactor vessel (not shown). The ozoniser 4 is connected by a line 37 and valve 39 to pressure vessel 6 and line 37 and valve 39' to pressure vessel 8. A buffer vessel 38 communicates with line 37. When in use and assuming the bed 12 of adsorbent in the pressure vessel 6 is undergoing its adsorption step, the compressor 20 compresses feed air and supplies the feed air under pressure via line 22 and open valve 24 through the vessel 16, where it is dried such that dry feed air passes into the pressure vessel 6. The bed 12 of adsorbent material selectively adsorbs nitrogen and carbon dioxide such that an oxygen rich product gas leaves the pressure vessel 6 via line 28 and open valve 30. The oxygen rich product gas passes through line 28, valve 30 and into line 32, open valve 34 and hence into the buffer vessel 14 which is filled with carbon molecular sieve. The oxygen is largely adsorbed by the carbon molecular sieve and the interstitial spaces between the carbon molecular sieve is filled with an argon enriched gas. From the buffer vessel 14 the oxygen passes through the open valve 36 to the ozoniser 4. In the ozoniser 4 ozone is formed by means of a silent electrical discharge in a manner known per se . Ozonised oxygen is then passed to a second buffer vessel 38 via the line 37. The ozonised oxygen then passes from the buffer vessel 38 through open valve 39' into the pressure vessel 8 at the point in its cycle when the bed 12' has been regenerated and after the equalisation step with the bed 12 of pressure vessel 6. Pressure equalisation is effected by shutting valve 24 such that feed air no longer enters the pressure vessel 6 and opening valves 30, 30' so that gas under pressure leaves pressure vessel 6 via line 28 and passes through the valves 30, 30' into line 28' and hence into pressure vessel 8. The oxygen in the ozonised oxygen on entering the pressure vessel 8 passes through the adsorbent 10' to the bed 12' whereas the ozone is adsorbed in the bed 10'. Feed air is now diverted through open valve 24' to vessel 16' and line 26' such that the bed 12' contained within the pressure vessel 8 is now on its adsorption step. When the bed 12' in pressure vessel 8 reaches its regeneration step, nitrogen rich gas is desorbed from the bed 12' as is the ozone from the bed 10' and the nitrogen-ozone mixture passes through the vessel 16' via valve 42' and lines 40, 44 towards the ozone contactor vessel for treating waste water, for example, in a manner known per se . The buffer vessel 14 filled with carbon molecular sieve acts as a means of balancing any argon build-up in the system and a small amount of gas is released via valve 47 from time to time to maintain the product compression that is suitable for the ozoniser 4. Although the vessels 16, 16' containing the dessicant are shown spaced from their respective pressure vessels 6, 8 they could be contained within the pressure vessels 6, 8. The ozone contained in the 'nitrogen-ozone stream' is now at a concentration of about 0.2% by volume. This is advantageous in many instances reducing hazards and dissociation of the ozone. If the ozone is going to be used in a water system it can be readily stripped from the mixture in a tower as the solubility ratio with nitrogen is twice as high as it is with oxygen. In the above described embodiment the ozone/oxygen mixture is dry and is not passed into the alumina layers contained within vessels 16, 16'. However, if for simplicity, the vessels 16, 16' were built to contain a layer of silica gel then the dryness of the oxygen/ozone mixture would ensure that most of the ozone remained trapped on the silica gel. In a modification the ozone could be desorbed from the beds 10, 10' without passing through the dessicant layers in the vessels 16, 16'. However, a portion of the nitrogen would be needed to regenerate the dessicant. In this event an activated ozone destructor would be placed in the vent line.

Claims: A method of treating a material with ozone comprising passing feed air through a first bed (12, 12') of adsorbent material located in a vessel (6, 8) forming part of a PSA plant (2) which first bed preferentially adsorbs nitrogen and carbon dioxide relative to oxygen, passing the oxygen rich gas thus separated to an ozoniser (4) for the production of ozonised oxygen, passing the ozonised oxygen to a further bed (10, 10') of adsorbent material which preferentially adsorbs the ozone relative to the oxygen, desorbing the ozone from the further bed (10, 10') of adsorbent material with the nitrogen when desorbed from the first bed (12, 12') of adsorbent material and passing the ozone-nitrogen gas mixture to the material to be treated, characterised in that the further bed (10, 10') is also located in the vessel (6, 8), the ozone is passed directly to the further bed (10, 10') and in that the oxygen rich gas remaining after the ozone is adsorbed by the further bed (10, 10') passes immediately with the feed air through the first bed (12, 12'). A method as claimed in Claim 1, characterised in that the feed air prior to its passage through the vessel (6, 8) is passed through a layer of dessicant (16, 16'). A method as claimed in Claim 1 or 2, characterised in that at least some of the ozone-nitrogen gas mixture passes through a layer of dessicant (16, 16') prior to its passage to the material to be treated. A method as claimed in any one of Claims 1 to 3, characterised in that the pressure swing adsorption plant (2) comprises at least two vessels (6, 8) each containing a first bed (12, 12') and a further bed (10, 10'), each vessel (6, 8) being subjected to a cycle including an adsorption step, an equalisation step and a desorption step, the cycles being arranged 180° out-of-phase thereby to produce a substantially continuous supply of oxygen. An apparatus for treating material with ozone comprising a PSA plant (2) for the production of oxygen from compressed feed air, the PSA plant (2) including a pressure vessel (6, 8) containing a bed (12, 12') of adsorbent material which preferentially adsorbs nitrogen and carbon dioxide relative to oxygen, means (32, 34, 36) for the passage of the oxygen from the pressure vessel (6, 8) to an ozoniser (4) for the production of ozonised oxygen, characterised by means (37, 39, 39') for returning the ozonised oxygen directly to the pressure vessel (6, 8) which contains a further bed (10, 10') of adsorbent material which preferentially adsorbs the ozone, the oxygen remaining together with the compressed feed air passing through the bed (12, 12') of adsorbent material which preferentially adsorbs nitrogen and carbon dioxide relative to oxygen, and means for desorbing the pressure vessel (6, 8) such that a nitrogen-ozone gas mixture passes from the pressure vessel (6, 8) to the material to be treated. An apparatus as claimed in Claim 5 in which a vessel (16, 16') is provided containing a dessicant, which dries the compressor feed air prior to its entry into the pressure vessel (6, 8).

Inventors: COATES JOHN ROBERT, GARRETT MICHAEL ERNEST, COATES, JOHN ROBERT, GARRETT, MICHAEL ERNEST
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1993
Main classification: C01B13
IPC codes: C02F1, C01B13, B01D53