Application Note: Trace Moisture Measurement with Aluminum Oxide Sensor in X

Application Note: Trace Moisture Measurement with Aluminum Oxide Sensor in X
Application Note
Gas Production & Distribution
Trace Moisture Measurement with Aluminum
Oxide Sensor in X-STREAM Process Gas Analyzers
With an Aluminum Oxide sensor, the X-STREAM Process Gas
Analyzer family is capable of measuring trace moisture in many
challenging applications. Key features include:
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-100 °C (-148 °F) to -10 °C (14 °F) dew point
In combination with other measurements in one housing
Factory calibrated / no field calibration needed
Cryogenic Gases, Natural Gas (LNG), Utility Power Generators,
Heat Treating
The sensor measures water in the range from -100 °C (-148 °F) to
-10 °C (14 °F) dew point with an accuracy of +/-2 °C. A ppm sensor is
also available with corresponding water concentrations of
0–100 ... 3000 ppm. Each sensor comes with a factory calibration.
Trace Moisture
Measurement range
Measurement accuracy
Repeatability
Response time (t95)
Operating humidity
Sensor operating temperature
Temperature coefficient
Operating pressure
Flow rate
X-STREAM Process Gas Analyzers
The moisture sensor can be combined with any other X-STREAM
Analyzer technology, including: non-dispersive infrared, ultraviolet,
and visible photometry (NDIR/UV/VIS), paramagnetic and
electrochemical oxygen (pO2 / eO2), thermal conductivity (TCD)
sensor and trace oxygen (tO2). If combined with other measurements
in series, the moisture sensor is always first in the series and
completely tubed with stainless steel to avoid any diffusion of
ambient air moisture to the sensor. A calibration exchange program
(annual interval) is available. The sensor can be used in various
background gases such as CO, CO2 and natural gas.
Trace Moisture (tH2O)
-100 to -10 °C dew point (0–100 ... 3000 ppm)
±2 °C dew point
0.5 °C dew point
5 min (dry to wet)
0 to 100 % r.h.
-40 to +60 °C
Temperature compensated across operating temperature range
Depending on sequential measurement system, see analyzer specification (1)
max. 1500 hPa abs / 7 psig
Depending on sequential measurement system, see analyzer specification (1)
0.2 to 1.5 l/min
*NOTE: 1psi = 68.95 hPa
(1) If installed in series to another measurement system, e. g. IR channel
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Gas Production & Distribution
Natural Gas Production/Transmission
Natural gas is found in vast underground wells and is often
a petroleum byproduct. Ethylene Glycol (anti-freeze) dehydrators
are used to dry the gas. In some special applications, molecular sieve
may be used. The gas industry uses a particular terminology when
measuring water vapor in the gas stream. It is specified in
lbs/mmft3 (pounds of H2O/million cubic foot of gas) at
15.5 °C (60 °F). This is an absolute measurement such as dew point
or ppm. The normal transfer specification is 7 lbs/mmft3. This is
equivalent to -38.5 °C dew point, -37 °F dew point or 150 ppm.
Should the moisture level increase, the BTU (British Thermal Unit)
value of the gas drops and the potential for corrosion increases.
In LNG plants, the trace moisture sensor is often used to monitor
moisture levels at the dehydrators for mercury removal.
Notes: The sensor should be in a bypass after the vaporizer and not
directly in the flow. This will warm the sample and insure that the
flow past the sensor is not excessive.
Example: With X-STREAM and MLT analyzer Emerson Process
Management is able to provide complete analytical solutions for
Air Separation Unit (ASU)
Notes: Gas lines can be high pressure. Glycol carryover is possible.
The sensor should be protected by a coalescing element.
Example: Combining trace H2O, pO2 and CO2 measurements
in Natural Gas in one flameproof analyzer
Utility Power Generation
Cryogenic Gases
Cryogenic gases are normal atmospheric components which have
been liquefied, separated and purified. Although they may be
supplied as a high pressure gas, most are shipped to the customer
in a liquid state and vaporized on site. The primary cryogenics are
O2, N2 and Ar. H2 and He are supplied in much smaller quantities.
Cryogenic gases are purchased for their particular properties. They
may be used as an inert blanket, in chemical reactions or to protect
a catalyst. Due to this wide range of usage, they are sold in various
degrees of purity. Moisture (H2O) is obviously an impurity, although
on a very small scale. Typically, moisture levels are in the 0–5 ppm
range.
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Generator windings are cooled with hydrogen gas (H2). Hydrogen
is used due to the large heat absorption capacity, approximately
2.7 times that of ambient air. This gas must be kept moderately dry
for several reasons. First, generators with a steel retaining ring having
a composition of 18 % Chromium (Cr) and 5 % Manganese (Mn), are
susceptible to stress failure in a high moisture environment. In this
case, the problem can be solved by an expensive ring replacement
(these rings are 18 % Cr and 18 % Mn, and are claimed to be resistant
to high moisture levels) or by monitoring and drying the gas.
Second, high moisture levels are suspected in the formation of lead
carbonate. The lead is present in solder joints in the generator. Lead
carbonate is hygroscopic and may provide a potential for arcing.
Notes: Due to the potential for oil carryover, all in-line, continuous
systems should be filtered with a good coalescer. In the event of
significant oil vapor, an activated carbon bed should be installed
after the filter and before the sensor.
Example: The moisture measurement can be combined with the
H2 purity measurement (Utility Application) in one flameproof
housing.
Gas Production & Distribution
Utility Power Transmission
Heat Treating Furnaces
Utility transformers are periodically taken out of service for
maintenance. These transformers are filled with an insulating oil,
then topped off with a pressurized nitrogen blanket. Prior to refilling,
the transformers must be purged with dry nitrogen for a period of
time, until the moisture inside the transformer windings has been
removed. The nitrogen source must be verified as dry, then the
transformer exhaust must be monitored until a certain dryness has
been achieved.
Heat treating furnaces may be found in numerous industries. They
are used by manufacturers of semiconductors, electronic parts,
aircraft and aerospace components, metal products, glass products,
ceramics and various types of hermetic seals. In a typical application,
the product moves down a belt into the oven. Depending on the
requirements, one or more of the following gases may be present:
N2, Ar, O2, H2. The dew point may range from ultra-dry
-73 °C (-100 °F) or lower to wet 20 °C (+68 °F), possibly greater
in some situations. The goal is to have a conditioning atmosphere
which has an effect on the product surface (i.e., formation of,
or reduction of an oxide layer, etc...).
Notes: Sampling N2 from transformers fueled with oil can lead to
carry over. A coalescing filter may be required.
Notes: Most heat treating is performed on metals, or metal products.
Prior to treatment, they may have been welded or machined,
then cleaned. Most cleaning and degreasing compounds contain
chlorides. During heat treating, these may be given off as Cl2, HCl
or Cl - radicals. All are highly aggressive and will attack an aluminum
oxide sensor. Also, the treating gases themselves may combine to
form NH3 (if N2 and H2 are used) or they may combine with chemicals
given off by the metals and degreasers to form other aggressive
compounds. Heat treating is done at very high temperatures,
possibly up to 2000 °C (3632 °F). This temperature must be reduced
for sampling. Therefore, a cooling coil or appropriate length of
tubing should be installed before the sample cell. Also, due to the
low process pressures, a vacuum pump may be required.
Dry Compressed Air
Glove Boxes
There are different qualities of compressed air with different
moisture levels:
■■ Deliquescent (typically 20 °F depression from ambient)
■■ Refrigerated (with approx. 2 °C (35 °F) dew point)
■■ Desiccant (-40 to -73 °C (-40 to -100 °F ) dew point)
■■ CDA (-73 °C (-100 °F) dew point)
Glove boxes are used to perform manual manufacturing processes
where exposure to the components may be harmful to personnel,
or exposure to the room atmosphere may be harmful to the
product. Glove boxes, generally, are purged with N2, Ar or CDA
(see Dry Compressed Air application) to form an inert boundary.
They are widely used in the semiconductor/electronic component
metalworking, nuclear and crystal manufacturing industries.
Compressed air must remain dry to avoid pneumatic failure and
winter freeze-up. CDA (compressed dry air) must remain below
2–3 ppm to avoid semiconductor manufacturing defects.
Notes: Although compressed air is one of the easiest applications,
there can be some problems. In several instances -40 °C (-40 °F)
dryers with wide dew point swings were found to spike during
changeover up to 10 °C to 21 °C (50 °F to 70 °F). If the air temperature
drops below this temperature, condensate can form in the pipe. This
is especially troublesome with refrigerated air lines outside in winter.
For these applications, special sampling techniques utilizing sample
systems or heated sample cells may be required.
Notes: Gases and residual products can produce aggressive
chemicals.
Example: Combination of trace H2O and trace O2 in one analyzer is a
smart solution for glove boxes.
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Gas Production & Distribution
Hopper Desiccant Dryers
Used for drying hygroscopic and non-hygroscopic plastic resins, twin
tower hopper dryers operate in the following way: a fan circulates
dry air through the hopper containing plastic resins at slightly above
atmospheric pressures. The moisture laden air is then sent through
a desiccant bed where the moisture is absorbed from the air stream
by the desiccant. An external heater then takes the -40 °C (-40 °F) dry
air and raises the temperature as high as 230 °C (450 °F). This air is
sent back through the hopper of resin to remove additional moisture
in a closed loop.
Notes: Excessive temperatures may reach the sensor. The use of
a cooling coil can bring this temperature close to ambient. Second,
many polymers give off chemical vapors when heated. One family
of compounds are acetaldehydes. These are aggressive and will
attack the moisture sensor. However, dryer desiccant removes most
of these compounds. Therefore, return (wet) leg sampling cannot be
recommended on a continuous basis.
The quality of the dry air is monitored by a dew point sensor set
as required for the desired degree of drying. When the moisture
absorbing capacity of the desiccant reaches its limit, the air flow is
automatically switched to a second desiccant chamber to maintain
the drying process. Meanwhile the first desiccant chamber is
regenerated by heating it to remove moisture and then cooling it so
it can resume absorbing water.
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