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10.54 CHAPTER TEN

The main sources of noise in a PSA separation process are the main air compressors.
Noise emissions from these compressors can range from 80 to 90 dBa. Noise attenuation
should be provided for these units. Typical installations are in an enclosed area, with in-
let, surge, and purge silencers.
The sources of noise in a VSA system include the main air blower, the product com-
pressor, the switching valves, and the vacuum pump used for regeneration. The vacuum
pump is the major source of noise for this system, with noise levels above 90 dBa. Ex-
haust silencers are usually necessary.
Cooling water requirements range from 2 to 5 gpm (8 to 19 L/min) per ton (907 kg)
of system capacity for a VSA process. Several manufacturers have suggested that high-
quality water (total dissolved solids less than 200 ppm) be used as the seal water for the
vacuum pumps. The manufacturer of the most popular vacuum pump used in VSA sys-
tems has indicated that city tap water is generally used for seal water, with no other spe-
cial requirements. A PSA process requires between 4 and 5 gpm (15 and 19 L/min) per
ton (907 kg) of system capacity, with the major portion being used by the compressor's
intercooler and aftercooler.
Space requirements for VSA and PSA systems are roughly the same as for a cryogenic
facility.

Ozone Contactors
Ozone contact basins provide for transfer of ozone gas into the liquid, promote ozone con-
tact throughout the liquid, and serve to retain the ozonated liquid for a period of time as
required to accomplish the desired reactions. The specific process objective and corre-
sponding reactions should dictate contact basin design. Reactions that are rapid relative
to the ozone mass-transfer rate from gas to liquid phase are best served by contactors that
promote the maximum transfer of ozone in the shortest time. For these applications, such
as oxidation of iron, manganese, or simple organics, contact time is often less important,
and contactors that rely on single points of application may be suitable. For reactions that
are slow relative to the ozone mass-transfer rate, such as disinfection or oxidation of com-
plex organics (including the very persistent herbicides and pesticides), contact time is crit-
ical and favors contactors with extended detention time and multiple application points,
such as the conventional multistage fine bubble diffuser design.
Factors Affecting Transfer Efficiency. The mass transfer of ozone into water has been
described by the two-film theory of gas transfer. However, the calculations are complex,
and designers have usually avoided them in favor of conservative estimates for transfer
efficiency based on past experience with similar designs. With the continued development
and use of ozone technology, it becomes important for the designer to understand the ba-
sic factors that affect transfer efficiency, including contactor characteristics, feed gas char-
acteristics, and source water characteristics.
Contactors have been developed in many configurations, and mass transfer will vary
with any characteristic that affects the driving force between the gas and liquid. For the
conventional fine bubble diffuser design, the essential factor is depth of water over the
diffusers, with efficiency increasing with increasing depth. Additional factors of less im-
portance include hydraulic detention time, liquid flow direction relative to gas flow di-
rection, and number of stages.
Feed gas characteristics that influence mass transfer include ozone dose, feed gas con-
centration, and bubble size. Mass-transfer efficiency will decrease with increasing ozone
dose or bubble size but increase with increased ozone concentration. Recent developments
in ozone generation technology with resulting ozone concentrations well over 10% have