Page 297 - Hydrocarbon Exploration and Production Second Edition

284 Oil and Gas Processing

1000
1st Stage 2nd Stage 3rd Stage
500
Power Requirement W / cu.m / h 100

50

10
1 5 10 25 50 100
Compression Ratio
Figure 11.17 Compression power.

maintenance than reciprocating compressors. Compression facilities are generally
the most expensive item in an upstream gas process facility.

11.1.4. Downstream gas processing

The gas processing options described in the previous section were designed
primarily to meet on-site usage or evacuation specifications. Before delivery to the
customer further processing would normally be carried out at dedicated gas
processing plants, which may receive gas from many different gas and oil fields. Gas
piped to such plants is normally treated to prevent liquid dropout under pipeline
conditions (dew point control) but may still contain considerable volumes of NGLs
and also contaminants.
The composition of natural gas varies considerably from lean non-associated gas
which is predominantly methane to rich associated gas containing a significant
proportion of NGLs. NGLs are those components remaining once methane and all
non-hydrocarbon components have been removed, that is (C 2 –C 5+ )(Figure 11.18).
Butane (C 4 H 10 ) and propane (C 5 H 12 ) can be further isolated and sold as liquefied
petroleum gas (LPG). This is commonly seen as bottled gas and is a useful method
of distributing energy to remote areas. LPG is further discussed at the end of
Section 11.1.4.
Sales gas, which is typically made up of methane (CH 4 ) and small amounts of
ethane (C 2 H 6 ), can be exported by refrigerated tanker rather than by pipeline and
has to be compressed by a factor of 600 (and cooled to 1601C). This is then
termed liquefied natural gas (LNG).

11.1.4.1. Contaminant removal
Although gas may have been partially dried and dew point controlled (for
hydrocarbons and water) prior to evacuation from the site of production, some