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OXIDATION AND DISINFECTION 10.45

In ozone applications, the feed gas dew point must be extremely low. Excess moisture
will adversely affect ozone production and may react with nitrous oxides within the ozone
generator to form nitric acid, causing subsequent damage to the generator. Current prac-
tice is to design for -60 ° C as the minimum acceptable dew point. Most systems oper-
ate at below -80 ° C dew point.
Particulates and Other Contaminants. Particulates can potentially cause problems at
a number of locations within an ozone system. Compressors with close running clearances
must be protected from particles that would score the impeller, lobe, or rotor surfaces.
Desiccant dryers and molecular sieve material must be protected from dust and hydro-
carbons that would block the active pores, thereby reducing the material's effectiveness.
Because the ozone generator acts as an electrostatic precipitator, particulates and hydro-
carbons will attach to the dielectric surface. In some cases, if particulates accumulate in
the unit, hot spots will develop and eventually lead to dielectric failure. Fine bubble ozone
diffusers must be protected from particulates that would eventually clog the diffuser.
Particulates may be inorganic or organic and may be present in the ambient air or gen-
erated from within the ozone system itself. Typical inorganic materials include sand, dust,
lime dust, coal dust, construction debris, and moisture droplets. Common organic mate-
rials include pollen, cottonwood seeds, and the like. From within an air feed ozone sys-
tem, the most common particulate is desiccant dust.
Filters are the best method of controlling particulates and in air feed systems should
be placed ahead of the compressor, ahead of the desiccant dryers, and after the desiccant
dryers. In LOX-based systems particulate filters should be placed downstream of the va-
porizers. The goal in any system should be a final feed gas entering the ozone generator
free of all particulates larger than 0.3 ~m.
In addition, hydrocarbons may be present in the atmosphere in large metropolitan ar-
eas and in the oil- and gas-producing areas of the country. Although not commonly used,
lubricated-type compressors will also introduce unwanted hydrocarbons into the system.
Hydrocarbons in the feed gas can be controlled with coalescing and carbon-absorber fil-
ters. For installations with lubricated screw-type compressors, the coalescing filter will
typically remove oil droplets larger than 0.05/zm. However, a carbon absorber is neces-
sary for capturing and removing hydrocarbon vapors. LOX purchase specifications should
limit total hydrocarbons to less than 20 ppm.
Temperature. At elevated temperatures, the rate of ozone decomposition increases.
Consequently, feed gas temperatures should be relatively cool to avoid rapid decomposi-
tion of ozone as it is produced within the generator. Current practice limits entering gas
temperature to below 90 ° F (33 ° C).
Pressure. The system pressure at the point of delivery from the feed gas preparation
system to the generator is dictated by two factors. First is the pressure required to over-
come all downstream pressure losses through the ozone contact basin, including those
from control valves, the generator, line losses, diffusers, and the static head of the water
above the diffuser.
The second is the design pressure at which the manufacturer has optimized generator
performance. If the manufacturer's optimum design pressure exceeds the downstream
losses, a pressure-maintaining valve will be placed downstream of the generator. Low-
frequency generators typically operate at low pressures, 8 to 12 lb/ft 2 (55 to 83 kPa);
medium-frequency generators operate at 18 to 25 lb/ft 2 (124 to 172 kPa).
Mass Flow Rate. Feed gas preparation systems must be designed to provide suffi-
cient mass flow to achieve the design ozone production at the desired ozone concentra-
tion. The mass flow required can be calculated as follows:
Required feed gas mass flow (lb/h) = design ozone production (lb/d)
24 × ozone concentration