Hydrocarbon dew point
Encyclopedia
The hydrocarbon dew point is the temperature
(at a given pressure
) at which the hydrocarbon
components of any hydrocarbon-rich gas mixture, such as natural gas
, will start to condense
out of the gaseous phase. It is often also referred to as the HDP or the HCDP. The maximum temperature at which such condensation
takes place is called the cricondentherm. The hydrocarbon dew point is a function of the gas composition as well as the pressure.
The hydrocarbon dew point is universally used in the natural gas industry as an important quality parameter, stipulated in contractual specifications and enforced throughout the natural gas supply chain, from producers through processing
, transmission and distribution companies to final end users.
The hydrocarbon dew point of a gas is a different concept from the water dewpoint, the latter being the temperature (at a given pressure) at which water vapor present in a gas mixture will condense out of the gas.
, the hydrocarbon dewpoint of processed, pipelined natural gas is related to and characterized by the term GPM which is the gallons of liquifiable hydrocarbons contained in 1000000 cubic feet (28,316.8 m³) of natural gas at a stated temperature and pressure. When the liquifiable hydrocarbons are characterized as being hexane
or higher molecular weight components, they are reported as GPM (C6+).
However, the quality of raw produced natural gas is also often characterized by the term GPM meaning the gallons of liquifiable hydrocarbons contained in 1000 cubic feet (28.3 m³) of the raw natural gas. In such cases, when the liquifiable hydrocarbons in the raw natural gas are characterized as being ethane
or higher molecular weight components, they are reported as GPM (C2+). Similarly, when characterized as being propane
or higher molecular weight components, they are reported as GPM (C3+).
Care must be taken not to confuse the two different definitions of the term GPM.
Although GPM is an additional parameter of some value, most pipeline operators and others who process, transport, distribute or use Natural Gas are primarily interested in the actual HCDP, rather than GPM. Furthermore, GPM and HCDP are NOT interchangeable and one should be careful not to confuse what each one exactly means.
Methods of HCDP determination
There are primarily two classes of HCDP determination. One category is "theoretical" methods, and the other is "experimental" methods.
"Theoretical" methods of HCDP determination
The Theoretical methods, use the component analysis of the gas mixture (usually via Gas Chromatography GC) and then use equation of state (EOS) to calculate what the dew point of the mixture should be at a given pressure. The Peng-Robinson and Kwong-Redlich-Soave equations of state are the most commonly used for determining the HCDP in the natural gas industry.
The theoretical methods suffer from two sources of error. The first source is the error on the analysis of the gas mix components. When using a C6+ GC these errors can be as high as 50F depending on the gas mixture and the assumptions made regarding the composition of the C6+ fraction. For "pipeline quality" natural gas, a C9+ GC analysis will reduce the uncertainty to approximately 10-15F. Using a laboratory C12+ GC analysis can reduce the error further to less than 1C. However, using a C12 laboratory system can introduce additional errors, namely sampling error. If the gas has to be collected in a sample bottle and shipped to a laboratory for C12 analysis, sampling errors can be significant. Obviously there is also a lag time error between the time the sample was collected and the time it was analyzed. . The second source of error relates to the errors embedded in the equation of state model use to calculate the dew point. Different models are prone to varying amounts error at different pressure regimes and gas mixes.
The significant advantage of using the theoretical models is that the HCDP at several pressures (as well as the cricondentherm) can be determined from a single analysis. This provides for operational uses such as determining the phase of the stream flowing through the flow-meter, determining if the sample has been affected by ambient temperature in the sample system, and avoiding Amine foaming from liquid hydrocarbons in the Amine contactor.
GC vendors with a product targeting the HCDP analysis include ABB, Thermo-fisher, Emerson, as well as some other companies.
"Experimental" methods of HCDP determination
In the "experimental" methods, one actually cools a surface on which gas condenses and then measures the temperature at which the condensation takes place. The experimental methods can be divided into manual and automated systems. Manual systems, such as the Bureau of Mines Dewpoint Meter, depend on an operator to manually cool the chilled mirror slowly and to visually detect the onset of condensation. The automated methods use automatic mirror chilling controls and sensors to detect the amount of light reflected by the mirror and detect when condesation occurs through a drop in measured light. The chilled mirror technique is a first principle measurement which is not suspect to "calculation" or "choice of model" errors; however, some liquids must condense before the detection of condensation on the mirror can occur, and therefore reports values lower than the theoretcial methods.
The experimental method also suffers from two sources of error. The first error is in the detection of condensation. A manual chilled mirror device relies on the operator to determine when a mist has formed on the mirror, and is highly subjective. The automated chilled mirror devices are significantly more repeatable but can be affected by contaminants that condense on the mirror before the hydrocarbons. The second source of error is the speed of the cooling of the mirror and the measurement of the temperature of the mirror when the condensation is detected. This error can be minimized by controlling the cooling speed, or having a fast condensation detection system.
The experimental methods only provide a HCDP at the pressure at which the measurement is taken, and cannot provide the cricondentherm or the HCDP at other pressures. However, most operators like to know the HCDP at their current line pressure anyways. Given that the input pressure of the experimental systems can be adjusted by a regulator, one can also measure the HCDP at other pressures by adjusting the input pressure.
Companies who offer an automated chilled mirror system include, Ametek, Michell Instruments, and SpectraSensors.
Temperature
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 a given pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
) at which the 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....
components of any hydrocarbon-rich gas mixture, such as 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...
, will start to condense
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....
out of the gaseous phase. It is often also referred to as the HDP or the HCDP. The maximum temperature at which such 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....
takes place is called the cricondentherm. The hydrocarbon dew point is a function of the gas composition as well as the pressure.
The hydrocarbon dew point is universally used in the natural gas industry as an important quality parameter, stipulated in contractual specifications and enforced throughout the natural gas supply chain, from producers through processing
Natural gas processing
Natural-gas processing is a complex industrial process designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.-Background:...
, transmission and distribution companies to final end users.
The hydrocarbon dew point of a gas is a different concept from the water dewpoint, the latter being the temperature (at a given pressure) at which water vapor present in a gas mixture will condense out of the gas.
Relation of the term GPM to the hydrocarbon dew point
In the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the hydrocarbon dewpoint of processed, pipelined natural gas is related to and characterized by the term GPM which is the gallons of liquifiable hydrocarbons contained in 1000000 cubic feet (28,316.8 m³) of natural gas at a stated temperature and pressure. When the liquifiable hydrocarbons are characterized as being hexane
Hexane
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...
or higher molecular weight components, they are reported as GPM (C6+).
However, the quality of raw produced natural gas is also often characterized by the term GPM meaning the gallons of liquifiable hydrocarbons contained in 1000 cubic feet (28.3 m³) of the raw natural gas. In such cases, when the liquifiable hydrocarbons in the raw natural gas are characterized as being ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
or higher molecular weight components, they are reported as GPM (C2+). Similarly, when characterized as being propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
or higher molecular weight components, they are reported as GPM (C3+).
Care must be taken not to confuse the two different definitions of the term GPM.
Although GPM is an additional parameter of some value, most pipeline operators and others who process, transport, distribute or use Natural Gas are primarily interested in the actual HCDP, rather than GPM. Furthermore, GPM and HCDP are NOT interchangeable and one should be careful not to confuse what each one exactly means.
Methods of HCDP determination
There are primarily two classes of HCDP determination. One category is "theoretical" methods, and the other is "experimental" methods.
"Theoretical" methods of HCDP determination
The Theoretical methods, use the component analysis of the gas mixture (usually via Gas Chromatography GC) and then use equation of state (EOS) to calculate what the dew point of the mixture should be at a given pressure. The Peng-Robinson and Kwong-Redlich-Soave equations of state are the most commonly used for determining the HCDP in the natural gas industry.
The theoretical methods suffer from two sources of error. The first source is the error on the analysis of the gas mix components. When using a C6+ GC these errors can be as high as 50F depending on the gas mixture and the assumptions made regarding the composition of the C6+ fraction. For "pipeline quality" natural gas, a C9+ GC analysis will reduce the uncertainty to approximately 10-15F. Using a laboratory C12+ GC analysis can reduce the error further to less than 1C. However, using a C12 laboratory system can introduce additional errors, namely sampling error. If the gas has to be collected in a sample bottle and shipped to a laboratory for C12 analysis, sampling errors can be significant. Obviously there is also a lag time error between the time the sample was collected and the time it was analyzed. . The second source of error relates to the errors embedded in the equation of state model use to calculate the dew point. Different models are prone to varying amounts error at different pressure regimes and gas mixes.
The significant advantage of using the theoretical models is that the HCDP at several pressures (as well as the cricondentherm) can be determined from a single analysis. This provides for operational uses such as determining the phase of the stream flowing through the flow-meter, determining if the sample has been affected by ambient temperature in the sample system, and avoiding Amine foaming from liquid hydrocarbons in the Amine contactor.
GC vendors with a product targeting the HCDP analysis include ABB, Thermo-fisher, Emerson, as well as some other companies.
"Experimental" methods of HCDP determination
In the "experimental" methods, one actually cools a surface on which gas condenses and then measures the temperature at which the condensation takes place. The experimental methods can be divided into manual and automated systems. Manual systems, such as the Bureau of Mines Dewpoint Meter, depend on an operator to manually cool the chilled mirror slowly and to visually detect the onset of condensation. The automated methods use automatic mirror chilling controls and sensors to detect the amount of light reflected by the mirror and detect when condesation occurs through a drop in measured light. The chilled mirror technique is a first principle measurement which is not suspect to "calculation" or "choice of model" errors; however, some liquids must condense before the detection of condensation on the mirror can occur, and therefore reports values lower than the theoretcial methods.
The experimental method also suffers from two sources of error. The first error is in the detection of condensation. A manual chilled mirror device relies on the operator to determine when a mist has formed on the mirror, and is highly subjective. The automated chilled mirror devices are significantly more repeatable but can be affected by contaminants that condense on the mirror before the hydrocarbons. The second source of error is the speed of the cooling of the mirror and the measurement of the temperature of the mirror when the condensation is detected. This error can be minimized by controlling the cooling speed, or having a fast condensation detection system.
The experimental methods only provide a HCDP at the pressure at which the measurement is taken, and cannot provide the cricondentherm or the HCDP at other pressures. However, most operators like to know the HCDP at their current line pressure anyways. Given that the input pressure of the experimental systems can be adjusted by a regulator, one can also measure the HCDP at other pressures by adjusting the input pressure.
Companies who offer an automated chilled mirror system include, Ametek, Michell Instruments, and SpectraSensors.
External links
- Using a C9+ Gas Chromatograph to determine Hydrocarbon Dew Point
- Natural Gas Processing: The Crucial Link Between Natural Gas Production and Its Transportation
- Hydrocarbon Dew-point – A Key Natural Gas Quality Parameter
- (ISO 6570:2001) Natural gas -- Determination of potential hydrocarbon liquid content