Page 298 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 285

Methane (C )
1
Ethane (C 2 )
)
Propane (C 3
Natural NGL LPG Butane (C )
Gas 4

Pentanes (C )
5
and heavier fractions
Non-Hydrocarbons
(Water, CO , H S, etc)
2
2
Figure 11.18 Terminology of natural gas.

heavier hydrocarbons, water and other non-hydrocarbon components can still be
present. Gas arriving at the gas plant may pass through a ‘slug catcher’, a device
which removes any slugs of liquid which have condensed and accumulated in
the pipeline during the journey. Following this, gas is dehydrated, processed to
remove contaminants and passed through a demethaniser to isolate most of the
methane component (for ‘sales’ gas). Specifications for sales gas may accommodate
small amounts of impurities such as CO 2 (up to 3%), but gas feed for either LPG or
LNG plants must be free of practically all water and contaminants (Figure 11.19).
Gases which are high in H 2 S are subject to a de-sulphurisation process in which
H 2 S is converted into elemental sulphur or a metal sulphide. There are a number of
processes based on absorption in contactors, adsorption (to a surface) in molecular
sieves or chemical reaction (e.g. with zinc oxide).
Carbon dioxide (CO 2 ) will solidify at the temperatures required to liquefy
natural gas, and high quantities can make the gas unsuitable for distribution.
Removal is usually achieved in contacting towers.
Water can be removed by adsorption in molecular sieves using solid desiccants
such as silica gel. More effective desiccants are available and a typical arrangement
might have four drying vessels: one in adsorbing mode, one being regenerated
(heating to drive off water), one cooling and a fourth on standby for when the first
becomes saturated with water.

11.1.4.2. Natural gas liquid recovery
When gases are rich in ethane, propane, butane and heavier hydrocarbons and
there is a local market for such products, it may be economic to recover these
condensable components. NGLs can be recovered in a number of ways, some of
which have already been described in the previous section. However, to maximise
recovery of the individual NGL components, gas would have to be processed in a
fractionation plant.