Teledyne API - Model T200H/T200M Operation Manual
Principles of Operation
281
8.3.7. PERMEATION DRYER
The air supplied to the O
3
generation system needs to be as dry as possible. Normal room
air contains a certain amount of water vapor, which greatly diminishes the yield of ozone
produced by the ozone generator. Also, water can react with other chemicals inside the
O
3
Generator to produce chemicals that damage the optical filter located in the reaction
cell (Table 10-1) such as ammonium sulfate or highly corrosive nitric acid.
To accomplish this task the T200H/M uses a single tube permeation dryer. The dryer
consists of a single tube of Nafion
®
, a co-polymer similar to Teflon
®
that absorbs water
very well but not other chemicals. The Nafion
®
tube is mounted within an outer, flexible
plastic tube. As gas flows through the inner Nafion
®
tube, water vapor is absorbed into
the membrane walls. The absorbed water is transported through the membrane wall and
evaporates into the dry, purge gas flowing through the outer tube, countercurrent to the
gas in the inner tube (Figure 8-10).
Figure 8-10:
Semi-Permeable Membrane Drying Process
This process is called per-evaporation
and
is driven by the humidity gradient between the
inner and outer tubes as well as the flow rates and pressure difference between inner and
outer tubing. Unlike micro-porous membrane permeation, which transfers water through
a relatively slow diffusion process, per-evaporation is a simple kinetic reaction.
Therefore, the drying process occurs quickly, typically within milliseconds. The first step
in this process is a chemical reaction between the molecules of the Nafion
®
material and
water, other chemical components of the gases to be dried are usually unaffected. The
chemical reaction is based on hydrogen bonds between the water molecule and the Nafion
material. Other small polar gases that are capable of hydrogen bonds can be absorbed this
way, too, such as ammonia (NH
3
) and some low molecular amines. The gases of interest,
NO and NO
2
, do not get absorbed and pass the dryer unaltered.
To provide a dry purge gas for the outer side of the Nafion tube, the T200H/M returns
some of the dried air from the inner tube to the outer tube (Figure 8-11). When the
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