FEEDGAS FOR MODERN HIGH-PERFORMANCE OZONE GENERATORS. Bruce T. Stanley. Ozonia Ltd Duebendorf, Switzerland 1999

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1 FEEDGAS FOR MODERN HIGH-PERFORMANCE OZONE GENERATORS Bruce T. Stanley Ozonia Ltd Duebendorf, Switzerland 1999 Abstract The traditional way of producing ozone is by means of Dielectric Barrier Discharge or so-called Silent Electrical Discharge. Typically, dry feedgas - either air or oxygen - is passed through a gap formed by a high voltage electrode and an earth electrode. When high voltage at high frequency is applied to the high voltage electrode ozone is produced in the feedgas by the micro-discharges taking place in the gap. The geometry of the gap and the quality of the dielectric material are of paramount importance and are normally closely guarded secrets by the ozone generator manufacturer. The make-up of the feedgas is, however, equally important and has become a much-debated topic. Introduction All high-performance ozone generators, which function on the Dielectric Barrier Discharge principle, require nitrogen in the feedgas to ensure optimum performance and constant, longterm, ozone generation. Obviously, this requirement is not of any consequence when the ozone generator is being fed with dry air, but it does need particular attention when the generator is being fed with dry oxygen from a liquid oxygen source (LOX). Reactions The formation of ozone involves a reaction between an oxygen atom, an oxygen molecule and a collision partner such as O 2, N 2 or, possibly, other molecules. Assuming the collision partner is nitrogen, the nitrogen molecules are able to transfer their excitation energy, after impact, to the oxygen molecules resulting in dissociation. Nitrogen radicals may also dissociate oxygen or react with nitrogen oxides to liberate oxygen atoms. The above thesis is substantiated by the traces of nitrogen oxides that can be detected in the output of air-fed ozone generators. Measurements of the sum parameter: NO x = NO + NO 2 + NO 3 + 2N 2 O 5 have been presented by different authors and detailed measurements of the nitrogen oxides NO, NO 2, N 2 O 5, N 2 O and nitrogen trioxide have been published. The complex kinetics of NO x formation can be summarised as follows: The nitrous oxide N 2 O and the nitric oxide NO are the initial oxides formed within 100ns after the initiation of a micro discharge.

2 The main reaction leading to N 2 O formation involves the meta-stable excited N 2 (A 3 Σ u + ) molecule: N 2 (A) + O 2 N 2 O + O Nitric oxide is produced mainly from two reactions involving nitrogen atoms: N + O 2 NO + O N + O 3 NO + O 2 The first of the above 2 reactions results in additional oxygen atoms that can eventually form ozone. There are two more such additional reaction paths: N + NO N 2 O N 2 (A,B) + O 2 N 2 + 2O In the presence of ozone and oxygen atoms, NO is oxidised via NO 2 and NO 3 to the highest oxidation state N 2 O 5. The presence of nitrogen therefore provides many reaction paths for the formation of ozone molecules, which greatly enhances the efficiency of the ozone generator. Air-Fed Ozone Generators The actual chemical reactions that take place when ozone is being generated from dry air are far more complex than when ozone is generated from oxygen feedgas. This complexity is due to additional ionic structures (N + and N 2 + ) plus the excitation and dissociation of the nitrogen molecules in the air feedgas. Figure 1. Micro-discharges in air (20% O 2 / 80% N 2 ) VS/BS/H:FEEDGAS.DOC 2

3 Despite the more complicated reactions in air, operators are spared the problems associated with the introduction of trace quantities of nitrogen to enhance ozone production. However, special attention must be given to other trace components in the feedgas: 1) Water : Although it is possible to produce ozone from untreated air it is essential for longterm, high-output service to remove as much moisture from the feedgas as possible. Typically, air feedgas for an ozone generator should be dried in a desiccant type dryer unit down to a dew point of at least minus 60 C measured at atmospheric pressure. Any water vapour entering the ozone generation process could lead to the formation of nitric acid. 2) Hydrocarbons : Special attention must be given to the fact that the hydrocarbon content (expressed as CH 4 ) is kept within limits. Large amounts of hydrocarbons entering the ozone generator are converted by the process into water vapour, which could, again, lead to minute amounts of nitric acid forming. 3) Freons, solvents, etc. : Freons, solvents, etc. should be avoided because they have a negative effect of the electrode surfaces which, subsequently, reduces production efficiency 4) Dust : The dust particles in the feed gas must be limited in both size and number in order to prevent build-up in the ozone generator or the associated valves and instrumentation. A typical air preparation system for an ozone generator will be made-up from a compressor, pressure receiver and a double column desiccant drier. Filter Cooler Receiver Filter Compressor Separator Dryer Figure 2. Typical arrangement of an air dryer system for feedgas preparation VS/BS/H:FEEDGAS.DOC 3

4 These systems are normally located in a convenient room close to the ozone generator. During operation the compressor sucks-in the ambient air - including all impurities, contamination, pollution, etc.. Most of the undesirable components are removed by the filters incorporated in the air preparation unit and the drier unit proper; however, there are other components, which detract from an ozone generator's efficiency. Operators are advised to consult the ozone generator's feedgas specification for more detailed information. Oxygen-Fed Ozone Generators The latest generation of high-performance ozone generators run on oxygen feedgas. The advantages of oxygen feedgas are manifold: a given ozone generator will produce at least twice as much ozone when fed with oxygen than with air; ozone concentrations up to 15 wt% can be economically produced in oxygen feedgas; investment costs are considerably lower for an oxygen-fed unit; etc.. There are several possibilities of obtaining suitable oxygen feedgas for use in ozone generators: 1) Pressure Swing Adsorption : Pressure swing adsorption (PSA) represents a convenient option for operators because the oxygen is produced on-site in the quantities required by the ozone generator. This solution also has certain safety aspects, which prove attractive to clients. The main advantage of oxygen from a PSA unit is the fact that the oxygen produced is ideally suited for use as feedgas in ozone generators. The equipment itself is physically very similar to the air dryer system shown above apart from the fact that the columns are larger. 2) Liquid oxygen storage : LOX is more suitable for the larger applications where investment costs play an important role and additional equipment will have an impact on the plant price. The purity level from a LOX source is very high and lacks the nitrogen content required by high efficiency ozone generators. 3) High pressure storage : High-pressure (gaseous) oxygen storage is only suitable for laboratory type applications requiring very low amounts of ozone per hour. In some cases it is possible to have the required gas composition filled in to the HP container by the gas supplier. VS/BS/H:FEEDGAS.DOC 4

5 In general, the parameters regarding moisture content, etc. are the same as for air, however, once again, operators are advised to consult the ozone generator's feedgas specification. As mentioned above, oxygen from a LOX storage tank has a high purity and is lacking in the required nitrogen content. In such cases it is standard practice to mix a minute quantity of dry nitrogen gas or dry air to the main ozone generator feed to overcome this shortcoming. There are several methods dosing nitrogen to the main gas feed to the ozone generator - the actual method chosen depends on the size of the plant and the level of operator comfort required. 2 PI PI FI ) H.P. nitrogen source 2) Pressure reduction valve 3) Regulating needle valve 4) Flow meter (range 25 to 250 l/h) 5) Solenoid valve 6) Non-return valve 7) Gas feed line to ozone generator Figure 3. Nitrogen dose equipment for smaller applications Figure 1 shows the typical arrangement of a nitrogen dosing system, which relies on HP nitrogen obtained from a gas supplier. Arrangements of this nature are particularly suitable for smaller ozone generator plants where only a very small amount of spiking gas is required per day. Particular attention has to be given to the fact that the supplementary gas does not lower the dew point of the main gas flow to below the specified limit given by the ozone generator manufacturer. 2 4 FI ) Compressor unit with receiver 2) Air dryer 3) Regulating needle valve 4) Flow meter (range 25 to 250 l/h) 5) Solenoid valve 6) Non-return valve 7) Gas feed line to ozone generator Figure 4. Air dose equipment for larger applications VS/BS/H:FEEDGAS.DOC 5

6 Figure 2 depicts a second solution more suited to larger plants where it would be inconvenient to replace HP storage cylinder on a regular basis and the costs involved justifies the incorporation of an air dosing system. In some cases the large plants have sufficiently dry instrument air available on-site, which would mean that neither the compressor unit with receiver nor the dryer unit would be necessary. Conclusion The graph in figure 5 shows the typical effect of the nitrogen content in feedgas on efficiency. Efficiency in % ppm N 2 Figure 5. Graph showing the effect of nitrogen on generator efficiency Although the actual nitrogen content, needed for an optimal production efficiency, may not be identical for each and every ozone generator available, there is indisputable evidence clearly demonstrating the importance of collision partners in the feedgas for all high output ozone generators. VS/BS/H:FEEDGAS.DOC 6

7 Keywords Feedgas; Ozone; Dry air; Dry oxygen; Barrier Discharge; LOX; Advanced Technology. References 1) Eliasson, M. Hirth & U. Kogelschatz, "OZONE SYNTHESIS FROM OXYGEN IN DIELECTRIC BARRIER DISCHARGES", J. Phys. D: Appl. Phy. 20 (1987). 2) U. Kogelschatz, B. Eliasson & M. Hirth, "OZONE GENERATION FROM OXYGEN AND AIR: DISCHAREGE PHYSICS AND REACTION MECHANISMS", Ozone Science & Engineering (1988). 3) S. K. Mehlman, P. Uhlig & P. Serpry, "DEVELOPMENT AND COMMERCIALISATION OF NEW TRAILIGAZCONCEPT OZONE TECHNOLOGY", IOA PAG Annual Conference (1998). 4) S. K. Mehlman, "OXYGEN SUPPLY FOR OZONE GENERATION", Praxair, Inc., Atlanta, USA (1995). 5) H. V. Lang et al., "OZONIA RESEARCH & DEVELOPMENT DEPARTMENT", Ozonia Ltd, Duebendorf, Switzerland. VS/BS/H:FEEDGAS.DOC 7

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