Page 287 - Hydrocarbon Exploration and Production Second Edition

274 Oil and Gas Processing

Multistage separation may also be constrained by low wellhead pressures. The
separation process involves a pressure drop, therefore the lower the wellhead
pressure the less scope there is for separation.

11.1.2.2. Separation design
Although there are many variations in separator design, certain components are
common.
The inlet section is designed to separate out most of the liquid phase such as large
slugs or droplets in a two-phase stream. These simple devices redirect the inlet flow
towards the liquid at the bottom of the vessel, separating the stream without
generating a mist (Figure 11.9).
As small droplets of liquid are usually still present in the gas phase, demisting
sections are required to recover the liquid mist before it is ‘carried over’ in the gas
stream leaving the separator. The largest liquid droplets fall out of the gas quickly
under the action of gravity but smaller droplets (less than 200 mm) require more
sophisticated extraction systems.
Impingement demister systems are designed to intercept liquid particles before the
gas outlet. They are usually constructed from wire mesh or metal plates and liquid
droplets impinge on the internal surfaces of the ‘mist mats’ or ‘plate labyrinth’ as the
gas weaves through the system. The intercepted droplets coalesce and move
downward under gravity into the liquid phase. The plate type devices or ‘vane
packs’ are used where the inlet stream is dirty as they are much less vulnerable to
clogging than the mist mat.
Centrifugal demister (or cyclone) devices rely on high velocities to remove liquid
particles and substantial pressure drops are required in cyclone design to generate
these velocities. Cyclones have a limited range over which they operate efficiently;
this is a disadvantage if the input stream flowrate is very variable.
In addition to preventing liquid ‘carry over’ in the gas phase, gas ‘carry under’
must also be prevented in the liquid phase. Gas bubbles entrained in the liquid phase
must be given the opportunity (or residence time) to escape to the gas phase under
buoyancy forces.
The ease with which small gas bubbles can escape from the liquid phase is
determined by the liquid viscosity; higher viscosities require longer residence times.

gas

gas
feed
water
oil
weir

demister
water oil
Figure 11.9 A basic three-phase separator.