2.4.1 Gas Plants
Gas processing consists of
separating the various
hydrocarbons and fluids
from the pure natural gas to
produce what is known as
“pipeline quality” dry natural
gas. Major transportation
pipelines usually impose
restrictions on the makeup
of natural gas that is
allowed into the pipeline.
Before the natural gas can
be transported it must be
purified.
Whatever the source of the
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natural gas, once separated from crude oil (if present) it commonly exists in
mixtures with other hydrocarbons, principally ethane, propane, butane and
pentanes. In addition, raw natural gas contains water vapor, hydrogen
sulfide (H2S), carbon dioxide, helium, nitrogen and other compounds.
Associated hydrocarbons, known as “natural gas liquids” (NGL), are used as
raw materials for oil refineries or petrochemical plants and as sources of
energy.
2.4.1 Gas compression
Gas from a pure natural
gas wellhead might have
sufficient pressure to feed
directly into a pipeline
transport system.
Gas from
separators has generally
lost so much pressure that
it must be recompressed to
be transported. Turbinedriven compressors gain
their energy by using a
small proportion of the
natural gas that they
compress. The turbine
itself serves to operate a centrifugal compressor, which contains a type of
fan that compresses and pumps the natural gas through the pipeline. Some
compressor stations are operated by using an electric motor to turn the
centrifugal compressor. This type of compression does not require the use of
any natural gas from the pipe; however, it does require a reliable source of
electricity nearby. The compression includes a large section of associated
equipment such as scrubbers (to remove liquid droplets) and heat
exchangers, lube oil treatment, etc.
