2013: Advent Oil & Gas: Processing 3 – gas gathering and treatment

by on December 10, 2013

Natural gas is processed in a broadly similar way to crude oil, but with a number of important differences.

Significantly, the first stage is the cooling of the gas to separate condensate – the liquids that are naturally present as vapours in natural gas.  These are typically light to medium weight hydrocarbons, though some contaminants including sulphure compounds are generally present.  Water vapour also condenses in the cooling process and is removed as waste; the useful condensate is then sent to an oil refinery.

The next stage is acid gas removal, where hydrogen sulphide and other contaminants are removed using any of a variety of processes.  One of the most common is amine gas treating, where a liquid amine (a class of organic nitrogen compound) dissolves and washes away hydrogen sulphide and carbon dioxide.  The amine wash is regenerated, and the hydrogen sulphide converted into saleable elemental suphur.

The gas will still contain other contaminants that need particular processes to remove.  Remaining water is taken out either by absorption into a dehydrating chemical such as triethylene glycol (a component of brake fluid) or by adsorption onto the surface of a solid such as siliga gel (the stuff you might see in dessicator packs, for instance in beef jerky).  Mercury, a particularly nasty heavy metal contaminant, is removed using activated carbon molecular sieves.  The inert gas nitrogen is sometimes separated out in a process called nitrogen rejection, which may use absorption, adsorption, or sometimes cryogenic processes (which also allow valuable helium to be recovered).

After this, the gas is almost entirely hydrocarbons, but is still a mix of compounds.  A turbo-expander cools the gas by reducing its pressure, allowing the methane to be drawn off from the demethaniser and either injected to the gas grid or converted to liquefied natural gas (LNG).  The compounds left over are referred to, confusingly, as natural gas liquids (NGL) although they are gasses at room temperature; they are ‘sweetened’ to remove any remaining sulphur, separated out using a specially adapted fractional distillation process, and can then be used as fuel – liquefied petroleum gas (LPG) – or as industrial feedstocks.

Normally all of this processing takes place at an onshore ‘central processing facility’ (CPF), but in recent years gas production companies have started building them on ships

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