Gas treatment and compression
The gas train consists of several stages, each taking gas from a suitable
pressure level in the production separator's gas outlet, and from the previous
stage.
A typical stage is
shown on the right.
Incoming gas (on
the right) is first
cooled in a heat
exchanger. It then
passes through the
scrubber to remove
liquids and goes into
the compressor. The
anti-surge loop
(thin orange line)
and the surge valve
(UV0121 23) allow
the gas to
recirculate. The
components are described below.
4.3.1 Heat exchangers
For the compressor to operate efficiently, gas temperature should be low.
The lower the temperature, the less energy will be used to compress the gas
for the given final pressure
and temperature. However,
both gas from separators
and compressed gas are
relatively hot. When gas is
compressed, it must remain
in thermodynamic balance,
which means that the gas
pressure times the volume
over the temperature
(PV/T) must remain
constant. (PV = nkT).
This
ends up as a temperature
increase.
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Heat exchangers of various forms are used to cool the gas. Plate heat
exchangers (upper picture) consist of a number of plates where the gas and
cooling medium pass between alternating plates in opposing directions.
Tube and shell exchangers (next picture) place tubes inside a shell filled with
cooling fluid. The cooling fluid is often pure water with corrosion inhibitors.
When designing the
process, it is important to
plan the thermal energy
balance. Heat should be
conserved, e.g., by using
the cooling fluid from the
gas train to reheat oil in the
oil train. Excess heat is
dispersed, e.g., by
seawater cooling.
However, hot seawater is
extremely corrosive, so
materials with high
resistance to corrosion,
such as titanium must be
used. Photo: SEC Shell and
Tube Heat Exchanges
