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Elimination of corona generated ozone T
he management of ozone generated during corona discharge treatment has been a concern to the industry for a long
time, and as health and safety and environmental pressures grow, the necessity to contain and destroy ozone close to the source intensifies. Ozone (O3) is a highly toxic and corrosive
oxidising agent that is a major health risk. The recommended human exposure limit is 0.1ppm (0.2mg/m3) — exposure to 50ppm for 30 minutes can be fatal. This gas is a protective necessity in the stratosphere, however is it an unwelcome pollutant in the workplace. Ozone occurs naturally in the atmosphere and is
produced whenever ultraviolet radiation or electrical discharge occurs. Background concentrations vary, but normally these low concentrations do not cause concern. However, ozone generated by certain industrial processes — either deliberately as a vital constituent of the process application, or as a by-product — are high in concentration and therefore do cause concern. Corona discharge treatment for adhesion promotion is an industrial process that generates
pollutive levels of ozone. Therefore, tight management of ozone is required to prevent it from entering the workplace environment. Analysis of the problem has led to a solution:
Ozone Destruct Units, which are increasingly being supplied as standard with corona treatment machinery.
OZONE DESTRUCT UNIT The conventional method of destroying waste gases is thermal incineration. At high temperatures oxygen combines with hydrocarbons to form carbon dioxide and water, (and also with carbon monoxide to form carbon dioxide). The use of high temperatures for long time periods is the basis of thermal incineration as a method of oxidation. Catalytic incineration similarly combines
oxygen with hydrocarbons (or carbon monoxide) to form harmless end products, but the oxidising reaction occurs at much lower temperatures and consequently with lower energy costs. Over the years of catalyst development, two
particular types of catalyst have emerged as being most suitable for the conversion of ozone to oxygen: Activated Carbon and Manganese Dioxide.
ACTIVATED CARBON The main advantages of carbon are that it is easy to source and relatively inexpensive. However, as carbon is an absorbent for oil mist vapours and hydrocarbons, there is a strong possibility that the localised exotherm will initiate combustion when in the presence of a strong oxidising agent, such as ozone, as hot spots will occur in the centre of the carbon granules. With this type of catalyst there is also the disadvantage that the chemical reaction between ozone and carbon results in the carbon being consumed, and this exhausts the filter bed during a period of operation. Thus, activated carbon filters must under no
circumstances be exposed to ozone concentrations higher than 20g/m3 (EH38 Guidance Notes).
MANGANESE DIOXIDE With an inert absorbent such as Manganese Dioxide, hot spots do not represent a hazard as any rise in temperature is dissipated. The inert absorbent is a mix of manganese
dioxide and copper oxide (MnO2, CuO) with proprietary promoters to assist the transformation of ozone to oxygen, this material functions through
8 November 2017
a continuous oxidation-reduction cycle. The manganese supplies a source of active oxygen for the reaction and accelerates the decomposition of ozone into oxygen without itself undergoing any change in composition. Therefore, in theory, the expected life of the catalyst is indefinite, as it is not consumed in the reaction and is independent of the ozone concentration passing through. In practice, the catalytic activity deteriorates
over time due to contamination of the active sites from materials other than ozone present in the gas stream. Some forms of deactivation are readily controlled, for example particulates will mask the catalytically active surface and cause a reduction in ozone destruction. This form of deactivation is controlled by pre-filters, which remove the particulates from the gas stream prior to contact with the main catalyst. Catalysts function by lowering the active energy
requirements for a given chemical transformation, which results in a low capital investment with low operating expenses and maintenance costs. This is why catalytic decomposition of ozone is usually preferable to non-catalytic alternatives. To prolong the lifetime of the main manganese
dioxide catalyst, it is essential that the contaminants from the gas stream be removed before they come in contact with the catalyst. Larger particles are removed by the fibre glass filter prior to the catalyst, but this filter will not remove moisture content, acidic gases or oil mist. A pre-catalyst with a high surface area absorbent is necessary in order to refine the incoming gas stream. An Alumina pre-catalyst is found in a wide range of industrial applications as a catalyst support that removes hydrocarbons, odorous organic compounds, metal carbonyls and oil mists from gas streams, making it a suitable candidate for removing contaminants prior to using an ozone destruct catalyst bed. Waste products, including ozone, are
increasingly being converted to non-toxic, non- polluting elements inside of a plant before being expelled into the atmosphere. After undergoing years of development, the
ozone destruct unit — with the correct choice of ozone resistant and corrosion proof materials, filters, pre-catalyst and main catalyst —will provide years of trouble-free operation whilst preventing an element of atmospheric pollution.
www.coronasupplies.co.uk
www.convertermag.co.uk
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