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Natural Gas Refining

written by: Willie Scott • edited by: Willie Scott • updated: 11/23/2011

The gas will have been partially processed offshore on the production platform, the extent of which will depend upon the gas processing equipment installed. The gas is mostly methane but has traces of ethane, propane, CO2, butane and pentene which are removed during natural gas refining onshore.

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    Introduction to Natural Gas Refining

    We are nearly at the end of the series of articles on offshore oil and gas and in this article we look at the final refining of the gas before it is piped into our homes. Some of the gas production platforms that I worked on had a lot of the processing equipment that an onshore gas refinery has. I was mechanical engineer on the construction of a jack-up platform for Elf which had all the right gear installed to put the gas ashore for immediate use.

    However we shall look at the refining of the gas as it comes ashore from the offshore sub-sea pipeline. This refining consists of the separation of all the fluids, and hydrocarbon condensates from the oil to produce dry natural gas

    The gas is mostly methane with traces of other gasses which will have to be removed, but we will start at the stage where the gas has entered the refinery, and assume that the basic processing has been carried out offshore where condensate and water removal has taken place.

    Now the gas has entered the refinery from the offshore pipeline, but these pipelines tend to clog up with condensates and have to be pigged, so we shall start here…

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    Pigging the Subsea Pipeline

    The condensates build up in these pipes and they have to be pigged every so often. This entails sending a neoprene pig, somewhat like a bullet down the inside of the pipeline. The pig is a neat fit and is propelled along by the pressure of the gas behind it as it removes the accumulation of condensate from inside the pipe. The pig is sent from the platform by a pig launcher and is collected at the refinery by a pig receiver.

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    Slug Catcher

    The slug catcher is a horizontal pressure vessel which ensures that any remaining globules of liquid hydrocarbons or condensates are removed before the gas proceeds to the next stage. The condensates and liquids are exported usually via pipeline to the nearest oil refinery for further fractionalizing.

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    Acid Gas Removal Unit

    This unit consists of a dryer to remove the water, an amine tower to remove contaminates of CO2, water vapor, hydrogen sulphide and oxygen, also a nitrogen rejection unit which removes the nitrogen.

    Dehydrator

    The dehydrator removes the water moisture content from the gas by injection of glycol liquid into the stream. This is carried out in the glycol tower and as the glycol particles absorb the water they fall to the bottom, the dry gas rising to the top for the next process.

    The glycol is regenerated by boiling of the water which has a lower flash point, leaving the glycol for reuse.

    Amine tower

    The gas now passes to the amine tower where it has the amine injected into its stream removing the sulphur contaminates and CO2. The gas now passes through a nest of filter pipes where it looses velocity and the remaining contaminates fall to the bottom sections. The gas if finally passed through cyclones where centrifugal force knocks out any remaining water vapour or small particles.

    The hydrogen sulphide, CO2 and amine solution can be removed from the bottom of the tower and sent for processing to form sulphur which has commercial value. This is a yellow powdery substance much like the element produced in several parts of the world. Whilst an engineer at sea I served on a sulphur tanker, shipping the stuff from Port Sulphur in Louisiana down the Mississippi through the gulf of Mexico and across the Atlantic Ocean to Rotterdam. There were steam coils in the sulphur tanks and every so often a charge of powdered sulphur would erupt from the vent goosenecks onto the deck. What a reek! It also turned all the brass black, I was glad I was an engineer and not working on deck.

    Anyway, after the gas sweetening process the residue is pumped from the bottom of the amine tower to the sulphur reclaiming plant. The heart of this is the Claus Process which uses catalytic and thermal reaction to reclaim the element.

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    Nitrogen Rejection Unit

    This unit removes the nitrogen from the gas by one of several processes. We shall examine the cryogenic process

    Methane and nitrogen condense from gas to liquid at different temperatures. The vapour is supercritically cooled; the methane in the vapour liquefies and is collected as it drops out from the nitrogen. The nitrogen is vented at the top of the vessel and the methane liquid is then returned to a gas minus the nitrogen

    This unit removes the nitrogen from the gas by one of several processes. We shall examine the cryogenic process

    Methane and nitrogen condense from gas to liquid at different temperatures. The vapour is supercritically cooled; the methane in the vapour liquefies and is collected as it drops out from the nitrogen. The nitrogen is vented at the top of the vessel and the methane liquid is then returned to a gas minus the nitrogen

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    Methane Separation

    Natural Gas Liquids (NGLs) separation from the methane is required for two reasons

    • It is a requirement for the sale of natural gas to the networks
    • The NGLs themselves can be further refined into usable products.

    The separation is carried out either by absorption or cryogenically. As we have examined a cryogenic method earlier we shall have a look at the absorption method. This is similar to the method used for dehydration only instead of using glycol to collect water vapour this process uses an absorption oil to collect the NGLs.

    The gas is passed through a tower where the oil is injected into the stream and the oil particles absorb the NGLs and fall to the bottom leaving the methane to proceed to the next process.

    The oil rich in NGLs now passes to a fractionalisation column where the different hydrocarbons are separated out into fractions, condensed and stored for commercial sale or used in plant utilities.

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    Mercury Removal Process

    Mercury removal from the methane is important both from a health viewpoint and as a precaution against damage to the different catalysts used throughout the process.

    This unit consists of a vertical pressure vessel which contains a combined catalyst of zeolitic sieved or honeycombed panels impregnated with silver commercially known as HgSIL.

    The gas enters the vessel passing upwards through the sieves, the mercury element being arrested by the dual catalyst.

    The gas proceeds upwards and out of the top of the vessel and once a odour is added is now ready for distribution.

    For information on crude oil refining, please click on link

    A sketch showing natural gas refining is shown below.

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References

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