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What Gases are Burnt Off in a Flare Stack?

written by: guruvignesh • edited by: Lamar Stonecypher • updated: 9/29/2010

Why is it more cost efficient to burn off the gases in a flare stack rather than recovering the gases produced in off-shore platforms and in shoreline refineries? This article will give you reasons for it.

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    Gases Burnt off in Offshore Platforms

    The products obtained from offshore platforms are mainly natural gas and crude oil. Before removing the crude oil from the well, they will remove pockets of natural gas located on the sea bed. The obtained natural gas is then processed and transported for industrial purposes and power generating stations.

    Natural gas may occur in:

    • Undergrounds wells are mainly gas bearing
    • Condensate reservoirs (pentanes and heavier)
    • Large oil fields (associated gas)

    In the case of oil wells, natural gas may present either in solution with the crude oil or as a gas cap above it.

    Natural gas comes from underground deposits and contains smaller quantities of heavier hydrocarbons (collectively known as natural gas liquids, or NGL). This is in addition to varying amounts of water, carbon dioxide, nitrogen, and other non-hydrocarbon substances.

    Natural Gas Liquids (NGL)

    Associated gas found in combination with crude oil comprises mainly methane and NGL. NGL is mainly made up of ethane, which is LPG gasoline. NGL has an atmospheric boiling point in the range of -80o C at atmospheric pressure or -45o C at a vapor pressure of 5 bars.

    Liquefaction of Gases (Atmospheric Boiling Point):

    • Methane (LNG) is stored at -162o C
    • Propane is stored at -45o C
    • Butane is stored at -0.5o C

    During the process of separating the LNG from the raw feed, gas obtained from the crude oil bed cap is burned in the flare stake (i.e., NGLs are burnt). The raw feed gas is first stripped of condensates. This is followed by removal of acid gases like carbon dioxide and hydrogen sulphide. Carbon dioxide must be removed as it freezes at temperatures above the atmospheric boiling point of LNG, and toxic compounds like hydrogen sulphide are removed as they cause atmospheric pollution when burnt in fuel. Acid gas removal saturates the gas stream with water vapor, and this is then removed by the dehydration unit.

    The gas then passes to a fractionating unit where NGL is removed and further split into propane and butane. Finally, the main gas flow is now mostly methane, which is liquefied into the end product, liquefied natural gas (LNG).

    Normally in the offshore platform, the fractionating column of pure dry LNG is then passed into the liquefaction plant situated in the platform. Once the LNG is liquefied by using the compressors, it is then stored in tanks at refrigerated condition -162o C- well below its atmospheric boiling point and maintained in it.

    The amount of NGL obtained from the LNG fractionating units is considerably negligible, and it is not profitable to install low-duty compressors in the offshore platforms in order to liquefy the NGL like propane and butane, which have a high atmospheric boiling point when compared to LNG.

    And if at all we install low-duty compressors for liquefying NGL, before liquefaction we have to carry out a separation process. The NGL will contain propane and butane that has to be separated first. So in order to separate the two gases having the different atmospheric boiling points, we need to install a fractionating unit to separate the two gases. So it is better to burn the NGL in the flare stack rather than liquefying it for storage.

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    Gases Burnt in the Shoreline Refinery

    LPG Production

    Liquefied natural gas (LPG) is the general name given for propane, butane, and a mixture of the two. These products can be obtained from the refining of crude oil, and they are usually manufactured in pressurized form.

    Butane is stored in cylinders and is known as bottled gas. It has widespread use as a fuel for heating and cooking in remote locations. Propane can also be utilized as a bottled gas especially in cold climates in which its vapor pressure is more suited.

    During the process of fractional distillation of crude oil in shoreline refineries, the crude petroleum is distilled into basic fractions and the volatile vapors rise to the top of the tower where they are extracted. In this process the first volatile gas obtained will be methane and will be produced in lower quantity. Just as on the offshore platform, it is not cost efficient to install the liquefaction plant will light-duty compressors, so the volatile vapors are burnt in the flare stack.

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    Boil-off Gases from Storage Tanks

    Normally the natural gas and the LPG are stored as a liquid in a refrigerated condition below their atmospheric boiling point in order to store them in large quantities. Boil-off is the vapor produced above the surface of a boiling liquid due to evaporation. It is caused by heat ingress or a drop in pressure.

    So we cannot have an ideal insulated condition in the storage tanks; there may be heat gain from the surroundings of the tank. Due to the heat gain, some quantity of liquid will try to boil-off and produce vapors in the tank. So when the vapor starts to increase in the tank, it builds up the tank pressure and the relief valve present in the tank tries to operate.

    Once the relief valve lifts at the set pressure, the vapor tries to escape and tank pressure is brought back to normal condition. The escaped vapor is burnt in the flare stack both in the offshore platforms and in shoreline refinery storages.

    It is not cost effective to run the re-liquefaction plant in order to convert the small amount of vapor produced due to boil-off and put it back into the storage tanks. The cost of installation and running of the re-liquefaction plant will be expensive especially for LNG where we need multi-stage liquefaction in order to convert the vapors into liquid below their separate atmospheric boiling points.

    Image credit:

    Wikipedia Images - Off shore Oil platform

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