Moisture analysis
Encyclopedia
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 gas
es such as those in transformer
s and power plants, and natural gas
pipeline transport
.
method of measuring high level moisture in solid or semi-solid materials is loss on drying (LOD). 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.
, 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.
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:
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.
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.
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, and training is required to use the manual systems.
, 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 (called slugs) 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 this sensor
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.”
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 (optical) 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 (in-situ). 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 (silicon oxide sensor) 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 (up to 130 Barg) 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.
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 (if the mirror is totally blocked, it must be cleaned).
The Tunable Diode Laser Absorption Spectroscopy (TDLAS
) 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.
Quality Assurance
Quality assurance, or QA for short, is the systematic monitoring and evaluation of the various aspects of a project, service or facility to maximize the probability that minimum standards of quality are being attained by the production process...
. Trace moisture in solids must be controlled for plastics, pharmaceuticals and heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
processes. Gas or liquid measurement applications include dry air, hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
processing, pure semiconductor gases, bulk pure gases, dielectric gas
Dielectric gas
A dielectric gas, or insulating gas, is a dielectric material in gaseous state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric gases are used as electrical insulators in high voltage applications, e.g...
es such as those in transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s and power plants, and natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
pipeline transport
Pipeline transport
Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....
.
Loss on Drying (LOD)
The classic laboratoryLaboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...
method of measuring high level moisture in solid or semi-solid materials is loss on drying (LOD). In this technique a sample of material is weighed, heated in an oven
Oven
An oven is a thermally insulated chamber used for the heating, baking or drying of a substance. It is most commonly used for cooking. Kilns, and furnaces are special-purpose ovens...
for an appropriate period, cooled in the dry atmosphere of a desiccator
Desiccator
thumb|right|A desiccator and a vacuum desiccator - note the stopcock which allows a vacuum to be applied. The blue [[silica gel]] in the space below the platform is used as the [[desiccant]]....
, 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
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
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 titrationKarl Fischer titration
Karl Fischer titration is a classic titration method in analytical chemistry that uses coulometric or volumetric titration to determine trace amounts of water in a sample. It was invented in 1935 by the German chemist Karl Fischer.-Coulometric 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
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
, or mass of water vapor per unit mass of dry gas
Dry gas
Dry gas is an alcohol-based additive used in automobiles to prevent any water in the fuel from freezing, or to restore combustive power to gasoline spoiled by water. The name Drygas is actually a brand name, owned by Cristy. It is a liquid that is added in to the fuel tank, that absorbs the water...
. That is, humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
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
Hygrometer
A hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity. Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed...
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 (also referred to as the Drager Tube or Stain 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 compoundChemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
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 temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
at which this condensation begins is known as the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
. 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
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
of moisture can be confused with condensation of other condensable such as heavy hydrocarbons, alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, and glycol. Automated on-line systems are not able to make these distinctions, and training is required to use the manual systems.
Electrolytic
The Electrolytic sensor uses two closely spaced, parallel windings coated with a thin film of phosphorus pentoxide (P2O5). As this coating absorbs incoming water vaporWater vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
, an electrical potential is applied to the windings that electrolyzes the water to hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. 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 (called slugs) 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 this sensor
Piezoelectric sorption
The piezoelectric sorptionSorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
instrument compares the changes in frequency of hydroscopically coated quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of 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 oxideOxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
sensor is made up of an inert substrate material and two dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing 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
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
. 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
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
. A silicon oxide
Silicon oxide
Silicon oxide may refer to either of the following:*Silicon dioxide, SiO2, very well characterized*Silicon monoxide, SiO, not very well characterized...
sensor can be an optical device that changes its refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
as water is absorbed into the sensitive layer or a different impedance type in which silicon replaces the aluminium.
In the first type (optical) when light is reflected through the substrate, a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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 (in-situ). 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
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
, 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 (silicon oxide sensor) 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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
.
Whilst most absorption type devices can be installed at pipe line pressures (up to 130 Barg) 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 spectroscopyAbsorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
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
Beer-Lambert law
In optics, the Beer–Lambert law, also known as Beer's law or the Lambert–Beer law or the Beer–Lambert–Bouguer law relates the absorption of light to the properties of the material through which the light is travelling.-Equations:The law states that there is a logarithmic dependence between the...
, 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 (if the mirror is totally blocked, it must be cleaned).
The Tunable Diode Laser Absorption Spectroscopy (TDLAS
TDLAS
Tunable diode laser absorption spectroscopy is a technique for measuring the concentration of certain species such as methane, water vapor and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry...
) 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.