Moisture analysis covers a variety of methods for measuring moisture content
in both high level and trace amounts in solids, liquids, or gases. Moisture in
percentage amounts is monitored as a specification in commercial food
production. There are many applications where trace moisture measurements are
necessary for manufacturing and process quality assurance. Trace moisture in
solids must be controlled for plastics, pharmaceuticals and heat treatment
processes. Gas or liquid measurement applications include dry air,
hydrocarbon processing, pure semiconductor gases, bulk pure gases,
dielectric gases such as those in transformers and power plants, and
natural gas pipeline transport. Loss on Drying
The classic laboratory method of measuring high level moisture in solid
or semi-solid materials is loss on drying. In this technique a sample of
material is weighed, heated in an oven for an appropriate period, cooled in the
dry atmosphere of a desiccator, and then reweighed. If the volatile content of
the solid is primarily water, the LOD technique gives a good measure of
moisture content. Because the manual laboratory method is relatively slow,
automated moisture analyzers have been developed that can reduce the time
necessary for a test from a couple hours to just a few minutes. These analyzers
incorporate an electronic balance with a sample tray and surrounding heating
element. Under microprocessor control the sample can be heated rapidly and a
result computed prior to the completion of the process, based on the moisture
loss rate, known as a drying curve. Karl Fischer titration
An accurate method for determining the amount of water is the Karl Fischer
titration, developed in 1935 by the German chemist whose name it bears. This
method detects only water, contrary to loss on drying, which detects any
volatile substances. Techniques used for natural gas
Natural gas poses a unique situation since it can have very high levels of
solid and liquid contaminants as well as corrosives in varying concentrations.
Water measurements are made in parts per million, pounds of water per million
standard cubic feet of gas, mass of water vapor per unit volume, or mass of
water vapor per unit mass of dry gas. That is, humidity is the amount of
"vapor-phase" water in a gas. If there are liquids present in the gas, they are
often filtered out before reaching a gas analyzer to protect the analyzer from
damage. Measurements of moisture in natural gas
are typically performed with one of the following techniques:
Color indicator tubes Chilled mirrors
Electrolytic Piezoelectric sorption, also known as
Quartz Crystal Microbalance Aluminum oxide and silicon oxide
Spectroscopy Other moisture measurement techniques
exist but are not used in natural gas applications for various reasons. For
example, the Gravimetric Hygrometer and the “Two-Pressure” System used by the
National Bureau of Standards are precise “lab” techniques but are not practical
for use in industrial applications. = Color indicator tubes=
The color indicator tube is a device many natural gas pipelines use for a
quick and rough measurement of moisture. Each tube contains chemicals that react
to a specific compound to form a stain or color when passed through the gas.
The tubes are used once and discarded. A manufacturer calibrates the tubes, but
since the measurement is directly related to exposure time, the flow rate,
and the extractive technique, it is susceptible to error. In practice, the
error can be as high as 25 percent. The color indicator tubes are well suited
for infrequent, rough estimations of moisture in natural gas; for example, if
the tube indicates 30 pounds of water, there is a high degree of certainty that
it is over 10 pounds. = Chilled mirrors=
When gas flows over a chilled surface, or chilled mirror, the moisture will
condense on it. The exact temperature at which this condensation begins is known
as the dew point. The temperature of this mirror is reduced from high to low,
and the temperature is read exactly when the dew is observed. By obtaining the
dew point temperature, one can calculate moisture content in the gas. The mirror
temperature is controlled by the flow of a refrigerant over the mirror or by
using a thermoelectric cooler. The detection of condensation on the
mirror can be achieved using visual or optical means. For example, a light
source can be reflected off the mirror into a detector and condensation
detected by changes in light reflected. The observation can also be done
visually; however the exact point at which condensation begins is not
discernible to the eye. Since the temperature is passing through the dew
point rather than stopping exactly at the dew point, the measurement tends to
be high. Additionally, the condensation of moisture can be confused with
condensation of other condensable such as heavy hydrocarbons, alcohol, and
glycol. Automated on-line systems are not able to make these distinctions.
= Electrolytic= The Electrolytic sensor uses two closely
spaced, parallel windings coated with a thin film of phosphorus pentoxide. As
this coating absorbs incoming water vapor, an electrical potential is
applied to the windings that electrolyzes the water to hydrogen and
oxygen. The current consumed by the electrolysis determines the mass of
water vapor entering the sensor. The flow rate and pressure of the incoming
sample must be controlled precisely to maintain a standard sample mass flow
rate into the sensor. The method is fairly inexpensive and can
be used effectively in pure gas streams where response rates are not critical.
Contamination from oils, liquids or glycols on the windings will cause drift
in the readings and damage to the sensor. The sensor cannot react to
sudden changes in moisture, i.e., the reaction on the windings’ surfaces takes
some time to stabilize. Large amounts of water in the pipeline will wet the
surface and requires tens of minutes or hours to “dry-down.” Effective sample
conditioning and removal of liquids is essential when using an electrolytic
sensor. = Piezoelectric sorption=
The piezoelectric sorption instrument compares the changes in frequency of
hydroscopically coated quartz oscillators. As the mass of the crystal
changes due to adsorption of water vapor, the frequency of the oscillator
changes. The sensor is a relative measurement, so an integrated
calibration system with desiccant dryers, permeations tubes and sample
line switching is used to correlate the system on a frequent basis.
The system has success in many applications including natural gas. It
is possible to have interference from glycol, methanol, and damage from
hydrogen sulfide which can result in erratic readings. The sensor itself is
relatively inexpensive and very precise. The required calibration system is not
as precise and adds to the cost and mechanical complexity of the system. The
labor for frequent replacement of desiccant dryers, permeation components,
and the sensor heads greatly increase the operational costs. Additionally,
slugs of water render the system nonfunctional for long periods of time
as the sensor head has to “dry-down.” = Aluminum oxide and silicon oxide=
The oxide sensor is made up of an inert substrate material and two dielectric
layers, one of which is sensitive to humidity. The moisture molecules pass
through the pores on the surface and cause a change to a physical property of
the layer beneath it. An aluminum oxide sensor has two metal
layers that form the electrodes of a capacitor. The number of water molecules
adsorbed will cause a change in the dielectric constant of the sensor. The
sensor impedance correlates to the water concentration. A silicon oxide sensor
can be an optical device that changes its refractive index as water is
absorbed into the sensitive layer or a different impedance type in which
silicon replaces the aluminium. In the first type when light is
reflected through the substrate, a wavelength shift can be detected on the
output which can be precisely correlated to the moisture concentration. Fiber
optic connector can be used to separate the sensor head and the electronics.
This type of sensor is not extremely expensive and can be installed at
pipeline pressure. Water molecules do take time to enter and exit the pores,
so some wet-up and dry down delays will be observed, especially after a slug.
Contaminants and corrosives may damage and clog the pores causing a “drift” in
the calibration, but the sensor heads can be refurbished or replaced and will
perform better in very clean gas streams. As with the piezoelectric and
electrolytic sensors, the sensor is susceptible to interference from glycol
and methanol, the calibration will drift as the sensor’s surface becomes inactive
due to damage or blockage, so the calibration is reliable only at the
beginning of the sensor’s life. In the second type the device is often
temperature controlled for improved stability and is considered to be
chemically more stable than aluminium oxide types and far faster responding
due to the fact they hold less water in equilibrium at an elevated operating
temperature. Whilst most absorption type devices can
be installed at pipe line pressures traceability to international Standards
is compromised. Operation at near atmospheric pressure does provide
traceability and offers other significant benefits such enabling
direct validation against known moisture content.
= Spectroscopy= Absorption spectroscopy is a relatively
simple method of passing light through a gas sample and measuring the amount of
light absorbed at the specific wavelength. Traditional spectroscopic
techniques have not been successful at doing this in natural gas because
methane absorbs light in the same wavelength regions as water. But if one
uses a very high resolution spectrometer, it is possible to find
some water peaks that are not overlapped by other gas peaks.
The tunable laser provides a narrow, tunable wavelength light source that can
be used to analyze these small spectral features. According to the Beer-Lambert
law, the amount of light absorbed by the gas is proportional to amount of the gas
present in the light’s path; therefore this technique is a direct measurement
of moisture. In order to achieve a long enough path length of light, a mirror is
used in the instrument. The mirror may become partially blocked by liquid and
solid contaminations, but since the measurement is a ratio of absorbed light
over the total light detected, the calibration is unaffected by the
partially blocked mirror. The Tunable Diode Laser Absorption
Spectroscopy analyzer has a higher upfront cost compared to the analyzers
above. However, the TDLAS technology is the only one that can meet any one of
the following: the necessity for an analyzer that will not suffer from
interference or damage from corrosive gases, liquids or solids, or an analyzer
that will react very quickly to drastic moisture changes, or an analyzer that
will remain calibrated for very long periods of time.
See also Spectrasensors
Karl Fischer titration