Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 292)

Model 9110TH NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 272
8.3.7. PERMA PURE ® 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 9110TH/M uses a Perma Pure ® 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 9110TH/M returns
some of the dried air from the inner tube to the outer tube (Figure 8-11). When the
analyzer is first started, the humidity gradient between the inner and outer tubes is not