exporting gas from a platform 1

[eng9901375]

Why is it essential to dehydrate gas before it is compressed and exported from the platform?

Also, what special considerations must be given to the design and operation of the gas export pipeline if the gas contains substantial amounts of carbon dioxide?

 

[Cockroach]

Well, think about it.

As something is compressed, thermodynamics suggests it heat up. Compressing gas also makes it "sweat"; water vapor naturally condenses out of a gas volume being compressed. Therefore, yes it is natural to dehydrate a gas prior to compression, unless you're into exporting water and rusting out your containers!

Carbon dioxide compositions typically need metallugical considerations for pipeline design. The phenonema known as pitting is a natural form of pipeline failure with high content carbon dioxide. Valves on the pipeline, particularly ball types, sieze due to explosive decompression in the rubber elements of the "soft seat" design. The explosive decompression is the result of carbon dioxide (or equally free hydrogen, nitrogen during purging operations) migrating into the interstital voids of the rubber compound, then naturally "expanding" as the valve is blown down to ambient conditions. You need to address the equipment design to mitigate carbon dioxide content, you will NEVER eliminate these problems totally.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Philtech]

Gas is assumed to come from reservoirs in a saturated state of water (that is, water is in a vapor form and the gas contains water molecules). Water content is normally removed or reduced substantially (and required in many pipeline tranport contracts to around 7 lb H20/mcf). Gas holds more water at higher temperatures and lower pressures (and the reverse). Thus, as gas is compressed, it heats up due to compression, and then is cooled down in heat exchangers or in the discharge line. Unfortunately, this drop in temperature (across pipeline friction, chokes, heat exchanges, etc..) causes water to drop out and will lead to corrosion (CO2 in the line makes carbonic acid when mixed with water). Another problem with water in the gas (if not removed) is the formation of hydrates (essentially ice plugs that can form above 32 deg Fahrenheit). Thus, Pipelines do not want their pipelines corroding, water forming liquid holdups in the pipelines (which increases pressure drop and thereby requires more compression at the next pump station), and do not wish to have their lines freezing up due to hydrates (even at 50 deg F). In addition, some gas is further processed in a Gas Plant to remove ethane, propane and heavier components from the gas stream (which is predominantly methane). These gas plants require the water removal to frigerate the gas to enable the separation of the hydrocarbon components to take place. More water has to be removed for LNG (Liquified natural gas), than it does for LPG (Liquified petroleum gases, which are heavier than methane).