PD-Series Gas Dryers Setup
The Perma Pure PD™-Series gas dryers are
shell and tube moisture exchangers that allow the transfer of water
vapor between two countercurrent flowing gas streams. The devices
consist of an inner tube of Nafion® polymer surrounded by an outer tube
of various inert materials. The water vapor is selectively absorbed into
the walls of the inner Nafion tubes and transferred to a purge gas
stream. This transfer is driven by the difference in the partial
pressures of the water vapor on opposing sides.
Typically the sample flows through the inner tubes and
the purge gas flows in the opposite direction within the shell. The
purge gas stream should be -40°C dew point air, nitrogen or other gas.
The dry gas entering the dryer at the sample outlet
performs two functions:
-
Provides a medium to carry away water vapor from the
sample.
-
Creates a temperature gradient along the length of
the dryer. For more information see
Temperature Effects on Nafion.
When installing the PD-Series gas dryer, the following
rules apply:
-
The sample pressure must be higher than the purge
pressure to prevent the tubing from collapse. Note that the dryer
may be operated with the sample under vacuum as long as a larger
vacuum is pulled on the purge (shell) side.
-
The inlet third of the dryer must be maintained at 5
to 10° above the sample dew point to prevent condensation.
There are
several ways to operate the PD-Series gas dryers.
-
Supply dry purge
air from a desiccant air dryer or nitrogen. The flow should be 2-3
times the sample flow rate. This is the most efficient method. If no dry purge gas is available, one of following methods may be
used.
-
The reflux setup returns
the dry sample back to the dryer for use as the purge after it has
gone through the analyzer. Since the reflux method uses all of the
sample as the purge gas, only the sample flow required for the
analysis passes through the dryer. This results in a very high
drying efficiency. The vacuum on the purge gas should be at least
15" of Hg, with a higher vacuum preferable. This vacuum level is
necessary to provide the desired 2:1 purge-to-sample flow ration
based on the actual volumetric flow.
-
The split sample method diverts some of the
sample from the main stream to be used as the purge gas. As a
result, more sample passes through the dryer than is required for
the analysis and somewhat lower drying efficiency will occur. The
flow diagrams below shows how to operate with the sample under
positive pressure, setup A or under vacuum, setup B.
To determine the purge flow rate required for the split
sample method, the following equation can be used.
Vp = Vs / [(Ps/2Pv)-1]
Where: Vp = Purge flow rate, Vs = Sample flow rate, Ps =
Sample pressure (in absolute units), Pv = Purge pressure (in absolute
units). Any units may be used as long as they are consistent.
For a print version, click on
pdf manual.
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