Teledyne 9110T - Nitrogen Oxides Analyzer - Manual (Page 334)

Model 9110T NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 334
Figure 13-11: Semi-Permeable Membrane Drying Process
The process by which the water vapor molecules are collected and transported through Nafion ® material
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.
Because this chemical reaction is based on hydrogen bonds between the water molecule and the Nafion ®
material most other chemical components of the gas to be dried are usually unaffected. Specifically, the
gases of interest for the 9110T, NO and NO
2
, do not get absorbed and pass the dryer unaltered.
On the other hand, other small polar gases that are capable of hydrogen bonds such as ammonia (NH
3
)
can be absorbed this way, too. This is an advantage since gases such as NH
3
can cause interference for
the measurement of NO
x
, NO and NO
2
(see Table 13-1).
Figure 13-12: 9110T Perma Pure ® Dryer