Hydrocarbon dew point

Last updated May 14, 2024

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.^[1] The hydrocarbon dew point is a function of the gas composition as well as the pressure.

Relation to the term GPM

In the United States, the hydrocarbon dew point of processed, pipelined natural gas is related to and characterized by the term GPM which is the gallons of liquefiable hydrocarbons contained in 1,000 cubic feet (28 m³) of natural gas at a stated temperature and pressure. When the liquefiable hydrocarbons are characterized as being hexane or higher molecular weight components, they are reported as GPM (C6+).^[2]^[3]

However, the quality of raw produced natural gas is also often characterized by the term GPM meaning the gallons of liquefiable hydrocarbons contained in 1,000 cubic feet (28 m³) of the raw natural gas. In such cases, when the liquefiable 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+).^[4]

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 categories of HCDP determination. One category involves "theoretical" methods, and the other involves "experimental" methods.

Theoretical methods

The theoretical methods use the component analysis of the gas mixture (usually via gas chromatography, GC) and then use an 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 using GC analysis suffer from four sources of error:

The first source of error is the sampling error. Pipelines operate at high pressure. To do an analysis using a field GC, the pressure has to be regulated down to close to atmospheric pressure. In the process of reducing pressure, some of the heavier components may drop out, particularly if the pressure reduction is done in the retrograde region. Therefore, the gas reaching the GC is fundamentally different (usually leaner in the heavy components) than the actual gas in the pipeline. Alternatively, if a sample bottle is collected for delivery to a laboratory for analysis, significant care must be taken not to introduce any contaminants to the sample, to make sure that the sample bottle represents the actual gas in the pipeline, and to extract the complete sample correctly in to the laboratory GC.
The second source is the error on the analysis of the gas mix components. A typical field GC will have at best (under ideal conditions and frequent calibration) ~2% (of range) error in the quantity of each gas analyzed. Since the range for most field-GCs for C6 components is 0-1 mol%, there will be about 0.02 mol% uncertainty in the quantity of C6+ components. While this error does not change the heating value by much, it will introduce a significant error in the HCDP determination. Furthermore, since the exact distribution of C6+ components is an unknown (the amount of C6, C7, C8, ...), this further introduces additional errors in any HCDP calculations. When using a C6+ GC these errors can be as high as 100 °F or more, 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 may reduce the uncertainty, because it eliminates the C6-C8 distribution error. However, independent studies have shown that the cumulative error can still be very significant, in some cases in excess of 30 C. A laboratory C12+ GC analysis using a Flame Ionization Detector (FID) can reduce the error further. 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.^[5]
The third source of errors is calibration errors. All GCs have to be calibrated routinely with a calibration gas representative of the gas under analysis. If the calibration gas is not representative, or calibrations are not routinely performed, there will be errors introduced.
The fourth source of error relates to the errors embedded in the equation of state model used to calculate the dew point. Different models are prone to varying amounts error at different pressure regimes and gas mixes. There is sometimes a significant divergence of calculated dew point based solely on the choice of equation of state used.

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. However, recent developments in combining experimental methods and software enhancements have eliminated this shortcoming (see combined experimental and theoretical approach below).

GC vendors with a product targeting the HCDP analysis include Emerson,^[6] ABB, Thermo-fisher, as well as other companies.

Experimental methods

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 tester, 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 condensation occurs through changes in the reflected light. The chilled mirror technique is a first principle measurement. Depending on the specific method used to establish the dew point temperature, some correction calculations may be necessary. As condensation must necessarily have already occurred for it to be detected, the reported temperature is lower than when using theoretical methods.^[5]

Similar to GC analysis, the experimental method is subject to potential sources of error. The first error is in the detection of condensation. A key component in chilled mirror dew point measurements is the subtlety with which condensate can be detected — in other words, the thinner the film is when detected, the better. A manual chilled mirror device relies on the operator to determine when a mist has formed on the mirror, and, depending on the device, can be highly subjective. It is also not always clear what is condensing: water or hydrocarbons. Because of the low resolution that has traditionally been available, the operator has been prone to under report the dew point, in other words, to report the dew point temperature as being below what it actually is. This is due to the fact that by the time condensation had accumulated enough to be visible, the dew point had already been reached and passed. The most modern manual devices make possible greatly improved reporting accuracy. There are two manufacturers of manual devices, and each of their devices meet the requirements for dew point measurement apparatus as defined in the ASTM Manual for Hydrocarbon Analysis. However, there are significant differences between the devices – including the optical resolution of the mirror and the method of mirror cooling – depending on the manufacturer.

Automated chilled mirror devices provide significantly more repeatable results, but these measurements can be affected by contaminants that may compromise the mirror's surface. In many instances it is important to incorporate an effective filtration system that prepares the gas for analysis. On the other hand, filtration may alter the gas composition slightly and filter elements are subject to clogging and saturation. Advances in technology have led to analyzers that are less affected by contaminants and certain devices can also measure the dew point of water that may be present in the gas. One recent innovation is the use of spectroscopy to determine the nature of the condensate at dewpoint. Another device user laser interferometry to register extremely tenuous amounts of condensation. It is asserted that these technologies are less affected by interference from contaminants. Another 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.

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. As the cricondentherm of natural gas is typically around 27 bar, there are gas preparation systems currently available which adjust input pressure to this value. Although, as pipeline operators often wish to know the HCDP at their current line pressure, the input pressure of many experimental systems can be adjusted by a regulator.

There are instruments that can be operated in either manual or automatic mode from the Vympel^[7] company.

Companies who offer an automated chilled mirror system include: Vympel,^[7] Ametek, Michell Instruments, ZEGAZ Instruments^[8] and Bartec Benke (Model: Hygrophil HCDT).

Combined experimental and theoretical approach

A recent innovation is to combine the experimental method with theoretical. If the composition of the gas is analyzed by a C6+ GC, and a dewpoint is experimentally measured at any pressure, then the experimental dewpoint can be used in combination with the GC analysis to provide a more exact phase diagram. This approach overcome the main shortcoming of the experimental method which is not knowing the whole phase diagram. An example of this software is provided by Starling Associates.

Related Research Articles

Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.

<span class="mw-page-title-main">Dew point</span> Temperature at which air becomes saturated with water vapour during a cooling process

The dew point of a given body of air is the temperature to which it must be cooled to become saturated with water vapor. This temperature depends on the pressure and water content of the air. When the air is cooled below the dew point, its moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew. When this occurs through the air's contact with a colder surface, dew will form on that surface.

Flow measurement is the quantification of bulk fluid movement. Flow can be measured using devices called flowmeters in various ways. The common types of flowmeters with industrial applications are listed below:

<span class="mw-page-title-main">Hygrometer</span> Instrument for measuring humidity

A hygrometer is an instrument which measures the humidity of air or some other gas: that is, how much water vapor it contains. Humidity measurement instruments usually rely on measurements of some other quantities such as temperature, pressure, mass and mechanical or electrical changes in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use the temperature of condensation, or they sense changes in electrical capacitance or resistance to measure humidity differences. A crude hygrometer was invented by Leonardo da Vinci in 1480. Major leaps came forward during the 1600s; Francesco Folli invented a more practical version of the device, while Robert Hooke improved a number of meteorological devices including the hygrometer. A more modern version was created by Swiss polymath Johann Heinrich Lambert in 1755. Later, in the year 1783, Swiss physicist and Geologist Horace Bénédict de Saussure invented the first hygrometer using human hair to measure humidity.

Natural-gas condensate, also called natural gas liquids, is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields. Some gas species within the raw natural gas will condense to a liquid state if the temperature is reduced to below the hydrocarbon dew point temperature at a set pressure.

Psychrometrics is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.

Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift. The name is a misnomer because the cores of the vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes named trailing or lift-induced vortices because they also occur at points other than at the wing tips. Indeed, vorticity is trailed at any point on the wing where the lift varies span-wise ; it eventually rolls up into large vortices near the wingtip, at the edge of flap devices, or at other abrupt changes in wing planform.

A wet gas is any gas with a small amount of liquid present. The term "wet gas" has been used to describe a range of conditions varying from a humid gas which is gas saturated with liquid vapour to a multiphase flow with a 90% volume of gas. There has been some debate as to its actual definition, and there is currently no fully defined quantitative definition of a wet gas flow that is universally accepted.

Moisture analysis covers a variety of methods for measuring the moisture content in solids, liquids, or gases. For example, moisture is a common 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 known in processes involving plastics, pharmaceuticals and heat treatment. Fields that require moisture measurement in gasses or liquids include hydrocarbon processing, pure semiconductor gases, bulk pure or mixed gases, dielectric gases such as those in transformers and power plants, and natural gas pipeline transport. Moisture content measurements can be reported in multiple units, such as: 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.

The Twister supersonic separator is a compact tubular device which is used for removing water and/or hydrocarbon dewpointing of natural gas. The principle of operation is similar to the near isentropic Brayton cycle of a turboexpander. The gas is accelerated to supersonic velocities within the tube using a De Laval nozzle and inlet guide vanes spin the gas around an inner-body which creates the "ballerina effect" and centrifugally separates the water and liquids in the tube. Hydrates do not form in the Twister tube due to the very short residence time of the gas in the tube. A secondary separator treats the liquids and slip gas and also acts as a hydrate control vessel. Twister is able to dehydrate to typical pipeline dewpoint specifications and relies on a pressure drop from the inlet of about 25%, dependent on the performance required. The fundamental mathematics behind supersonic separation can be found in the Society of Petroleum Engineers paper entitled "Selective Removal of Water from Supercritical Natural Gas". The closed Twister system enables gas treatment subsea.

Natural-gas processing is a range of industrial processes designed to purify raw natural gas by removing contaminants such as solids, water, carbon dioxide (CO₂), hydrogen sulfide (H₂S), mercury and higher molecular mass hydrocarbons (condensate) to produce pipeline quality dry natural gas for pipeline distribution and final use. Some of the substances which contaminate natural gas have economic value and are further processed or sold. Hydrocarbons that are liquid at ambient conditions: temperature and pressure (i.e., pentane and heavier) are called natural-gas condensate (sometimes also called natural gasoline or simply condensate).

Custody Transfer in the oil and gas industry refers to the transactions involving transporting physical substance from one operator to another. This includes the transferring of raw and refined petroleum between tanks and railway tank cars; onto ships, and other transactions. Custody transfer in fluid measurement is defined as a metering point (location) where the fluid is being measured for sale from one party to another. During custody transfer, accuracy is of great importance to both the company delivering the material and the eventual recipient, when transferring a material.

Airport weather stations are automated sensor suites which are designed to serve aviation and meteorological operations, weather forecasting and climatology. Automated airport weather stations have become part of the backbone of weather observing in the United States and Canada and are becoming increasingly more prevalent worldwide due to their efficiency and cost-savings.

Supersonic gas separation is a technology to remove one or several gaseous components out of a mixed gas. The process condensates the target components by cooling the gas through expansion in a Laval nozzle and then separates the condensates from the dried gas through an integrated cyclonic gas/liquid separator. The separator is only using a part of the field pressure as energy and has technical and commercial advantages when compared to commonly used conventional technologies.

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

Instrumentation is used to monitor and control the process plant in the oil, gas and petrochemical industries. Instrumentation ensures that the plant operates within defined parameters to produce materials of consistent quality and within the required specifications. It also ensures that the plant is operated safely and acts to correct out of tolerance operation and to automatically shut down the plant to prevent hazardous conditions from occurring. Instrumentation comprises sensor elements, signal transmitters, controllers, indicators and alarms, actuated valves, logic circuits and operator interfaces.

Michell Instruments consists of a group of eight operating companies located in the UK, France, Netherlands, Germany, Italy, US, China and Japan. The group is involved in the design, manufacture and sale of a wide variety of industrial instrumentation including relative humidity, dew point, moisture in gases and liquids and oxygen analysis.

In the petroleum industry, allocation refers to practices of breaking down measures of quantities of extracted hydrocarbons across various contributing sources. Allocation aids the attribution of ownerships of hydrocarbons as each contributing element to a commingled flow or to a storage of petroleum may have a unique ownership. Contributing sources in this context are typically producing petroleum wells delivering flows of petroleum or flows of natural gas to a commingled flow or storage.

Headspace gas chromatography uses headspace gas—from the top or "head" of a sealed container containing a liquid or solid brought to equilibrium—injected directly onto a gas chromatographic column for separation and analysis. In this process, only the most volatile substances make it to the column. The technique is commonly applied to the analysis of polymers, food and beverages, blood alcohol levels, environmental variables, cosmetics, and pharmaceutical ingredients.

References

↑ Hydrocarbon Dew Point
↑ White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, October 15, 2004)
↑ White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure Archived 2008-10-10 at the Wayback Machine (NGC+ Liquid Hydrocarbon Dropout Task Group, September 28, 2005)
↑ A. J. Kidnay and William Parish (2006). Fundamentals of Natural Gas Processing (1st ed.). CRC Press. ISBN 0-8493-3406-3. (See page 110)
1 2 Andrew Brown et al (May 2007). "Comparison of Methods for the Measurement of Hydrocarbon Dew Point of natural gas", UK National Physical Laboratory Report AS 3, ISSN 1754-2928.
↑ "Automation Solutions | Emerson US".
1 2 Vympel Instruments (Hygrovision BL Hydrocarbon Dew Point Analyzer)
↑ ZEGAZ Instruments (HCD5000(TM) Hydrocarbon Dewpoint Analyzer)

https://www.bartec.de/en/products/analyzers-and-measurement-technology/trace-moisture-measurement-for-gases/hygrophil-hcdt/

External links

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[1] Hydrocarbon Dew Point

[NGC-2] White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, October 15, 2004)

[NGC2-3] White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure Archived 2008-10-10 at the Wayback Machine (NGC+ Liquid Hydrocarbon Dropout Task Group, September 28, 2005)

[4] A. J. Kidnay and William Parish (2006). Fundamentals of Natural Gas Processing (1st ed.). CRC Press. ISBN 0-8493-3406-3. (See page 110)

[NPL2007-5] 1 2 Andrew Brown et al (May 2007). "Comparison of Methods for the Measurement of Hydrocarbon Dew Point of natural gas", UK National Physical Laboratory Report AS 3, ISSN 1754-2928.

[6] "Automation Solutions | Emerson US".

[Hygrovision_BL(TM)-7] 1 2 Vympel Instruments (Hygrovision BL Hydrocarbon Dew Point Analyzer)

[HCD4000(TM)-8] ZEGAZ Instruments (HCD5000(TM) Hydrocarbon Dewpoint Analyzer)

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]