Page 46 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

4.2.4 Third stage separator

The final separator here is a two-phase separator, also called a flash drum.
The pressure is now reduced to atmospheric pressure of around 100 kPa, so
that the last heavy gas components will boil out. In some processes where
the initial temperature is low, it might be necessary to heat the liquid (in a
heat exchanger) again before the flash drum to achieve good separation of
the heavy components. There are level and pressure control loops.

As an alternative, when the production is mainly gas, and remaining liquid
droplets have to be separated out, the two-phase separator can be a Knock-
Out Drum (K.O. Drum).

4.2.5 Coalescer

After the third stage separator, the oil can go to a coalescer for final removal
of water. In this unit the water content can be reduced to below 0.1%. The
coalescer is completely filled with liquid: water at the bottom and oil on top.
Internal electrodes form an electric field to break surface bonds between
conductive water and isolating oil in an oil water emulsion. The coalescer
field plates are generally steel, sometimes covered with dielectric material to
prevent short-circuits. The critical field strength in oil is in the range of 0.2 to
2 kV/cm. Field intensity and frequency as well as the coalescer grid layout
are different for different manufacturers and oil types.

4.2.6 Electrostatic desalter

If the separated oil
contains unacceptable
amounts of salts, they
can be removed in an
electrostatic desalter
(not used in the Njord
example) The salts,
which may be sodium,
calcium or magnesium
chlorides come from the reservoir water and are also dissolved in the oil.
The desalters will be placed after the first or second stage separator
depending on Gas Oil Ratio (GOR) and water cut. Photo: Burgess Manning Europe
PLC

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