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Some words about Hydrocarbon Dew Point

Hydrocarbon Dew Point is becoming a critical tariff parameter but it has always been a vital

operational parameter for the pipeline industry. Measuring it must be done properly or serious

errors can jeopardize tariff compliance resulting in shut-ins. If liquids build up, especially if

water condensates are present allowing hydrate formation to occur, in the pipeline, that can

damage compressors, valves and other in-pipe devices. Overcompensation for poor analysis techniques, or a less than optimal choice of instrumentation, will also add significantly to

operational costs. The focus of this paper is to identify the major factors that contribute to best

practices for measuring the hydrocarbon dew point (HCDP) in natural gas. The three most

popular methods for measuring this parameter will be discussed. These three techniques are:

• Manual visual method with a chilled mirror dew point instrument

• Equation of state calculations from constituent analysis by gas chromatography

• Automatic optical condensation dew point instrument

Each technique will be described along with the inherent issues that can enhance the use of the

specific technique along with

results that have been achieved by independent laboratory testing. Pros and cons for each

technique will be listed that will

provide a basis for comparing these three methods of measurement for this important

parameter.

Dew Point of a gas is a reference temperature/pressure at which condensation starts.

It is also explained as the temperature to which the gas has to be cooled at a given pressure in

order for it to change it state to liquid.

It’s much like the cold glass sweating on a humid day only the term is used for air while a

natural gas is a multi component mixture. It is the heavier weighted compounds in the natural

gas that condense first and dictate a dew point of the multi component system.

The dew point temperature is also dependant on pressure.

• There is a better value for the gas if hydrocarbon dew point (HCDP) controlled.

• Gas when HCDP controlled will not produce liquids in pipeline transportation.

(Note: Liquids in pipeline will move as a slug, collect in low area, enter into compressor,

increases pumping costs, create fires in burners, increases pressure drop, etc.)

• HCDP controlled gas will eliminate liquid collection and thus corrosion in pipelines.

Understanding the phase diagram:

Let’s understand a phase diagram of a multi-component system (see figure on the right). Move

from point ‘A’ to point ‘B’ - it is increasing the temperature at constant pressure. This is a

process of evaporation of the liquids. The bubble point is reached when the line touches the

curve. Increasing the temperature further increases vaporization and comes to 100% vapors

when the line intersects the curve again. Consider traveling down on line X-Y. Decreasing the

pressure at a constant temperature the vapors change to liquids. At a point of intersection of

both the A-B and X-Y there is maximum amount of liquids formed. Further decrease in pressure

changes the liquids back to vapors. A term called retrograde condensation. This is a phenomena

taking place in the reservoir that liquids are lost on lowering the pressure resulting in gas. Other

points on the curve are ‘critical point’ which is the maximum temperature the component can

exist as liquid. The point ‘cricondenbar’ is the highest pressure for the gas and liquids to co-

exist. Similarly the point “cricondentherm” is the highest temperature at which gas and liquids

can co-exist.Let’s understand a phase diagram of a multi-component system (see figure). Move

from point ‘A’ to point ‘B’ - it is increasing the temperature at constant pressure. This is a

process of evaporation of the liquids. The bubble point is reached when the line touches the

curve. Increasing the temperature further increases vaporization and comes to 100% vapors

when the line intersects the curve again. Consider traveling down on line X-Y. Decreasing the

pressure at a constant temperature the vapors change to liquids. At a point of intersection of

both the A-B and X-Y there is maximum amount of liquids formed. Further decrease in pressure

changes the liquids back to vapors. A term called retrograde condensation. This is a phenomena

taking place in the reservoir that liquids are lost on lowering the pressure resulting in gas. Other

points on the curve are ‘critical point’ which is the maximum temperature the component can

exist as liquid.

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The point ‘cricondenbar’ is the highest pressure for the gas and liquids to co-exist. Similarly the

point “cricondentherm” is the highest temperature at which gas and liquids can co-exist.

Hydrocarbon Dew Point may be:

- calculated from gas chromatography composition using an equation of state;

- measured directly using a chilled mirror method.

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Typical HCDP curves were calculated using ecuation of state: RKS (Redlich-Kwong-Soave),

LRS(London Research Station), PR (Peng-Robinson)

Dr. Ion Irimia ZECHERU

ECO-PETROL Consulting Ltd.

Bucharest-Romania

e-mail: ionzecheru@yahoo.com

Mobile: 0040-731599522

0040-21-8098539