Page 300 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 287

ethane
(pipeline)

propane

cooling
NGL butane
input

gasoline
de-ethaniser de-butaniser
de-propaniser
Figure 11.21 NGL fractionation plant.

In the fractionation plant, the first column (de-ethaniser) removes ethane which,
after treatment for storage, may be used as feed for an ethylene plant. The heavier
hydrocarbons pass to the next fractionating column (or de-propaniser) where
propane is removed and so on, until butane has been separated and the remaining
NGLs can be stored as natural gasoline. The lighter components can only be
recovered at very low temperatures – ethane, for example, has to be reduced to
1001C. Propane can be stored as a liquid at about 401C and butane at 01C.
Natural gasoline does not require cooling for storage (Figure 11.21).
Gas fractionation plants require considerable investment and in many situations
would not be economic. However, less complete NGL recovery methods may still
prove cost-effective.
The component factor gives the unit yield for each component and includes a
volume conversion factor. The factors can be obtained from tables.

11.1.4.3. Liquefied natural gas
Where the distance to the customer is very large or where a gas pipeline would have
to cross too many countries, gas may be shipped as a liquid. To condition the gas for
liquefaction any CO 2 ,H 2 S, water and heavier hydrocarbons must be removed,
by the methods already discussed. The choice of how much propane and butane
to leave in the LNG depends upon the heating requirements negotiated with the
customer. At a LNG plant consisting of one or more ‘trains’, gas is condensed by
chilling it to around 120 to 1701C and compressing it up to 60 atm/870 psi.
Once the gas has been condensed the pressure is reduced for storage and shipping.
In order to keep the gas in liquid form, the LNG must be kept at temperatures
below 831C independent of pressure.