Scrubbers and reboilers
The separated gas may contain mist and other liquid droplets. Drops of
water and hydrocarbons also form when the gas is cooled in the heat
exchanger, and must be removed before it reaches the compressor. If liquid
droplets enter the compressor, they will erode the fast rotating blades. A
scrubber is designed to remove small fractions of liquid from the gas.
There are various types of gas-drying equipment available, but the most
common suction (compressor) scrubber is based on dehydration by
absorption in triethylene glycol (TEG)
. The scrubber consists of many levels
of glycol layers.
A large number of gas traps (enlarged detail) force the gas to bubble up
through each glycol layer as it flows from the bottom to the top of each
section.
Processed glycol is pumped in at the top from the holding tank. It flows from
level to level against the gas flow as it spills over the edge of each trap.
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During this process, it absorbs liquids from the gas and comes out as rich
glycol at the bottom. The holding tank also functions as a heat exchanger for
liquid, to and from the reboilers.
The glycol is recycled by removing the absorbed liquid. This is done in the
reboiler, which is filled with rich glycol and heated to boil out the liquids at
temperature of about 130-180 °C (260-350 °F) for a number of hours.
Usually there is a distillation column on the gas vent to further improve
separation of glycol and other hydrocarbons. For higher capacity, there are
often two reboilers which alternate between heating rich glycol and draining
recycled processed glycol. On a standalone unit, the heat is supplied from a
burner that uses the recovered vaporized hydrocarbons. In other designs,
heating will be a combination of hot cooling substances from other parts of
the process and electric heaters, and recycling the hydrocarbon liquids to the
third stage separator.
Figure 7, Glycol regeneration
4.3.3 Compressors, anti-surge and performance
Compressors are used in many parts of the oil and gas process, from
upstream production to gas plants, pipelines, LNG and petrochemical plants.
The overview given here will therefore be referenced from other sections.
Several types of compressors are used for gas compression, each with
different characteristics such as operating power, speed, pressure and
volume:
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• Reciprocating compressors, which use a piston and cylinder design
with 2-2 cylinders are
built up to about 30
MW power, around
500-1,800 rpm (lower
for higher power) with
pressure up to 5MPa
(500 bars). Used for
lower capacity gas
compression and
high reservoir
pressure gas injection. Photo: Ariel corp.
• Screw compressors
are manufactured up to
several MW,
synchronous speed
(3,000/3,600 rpm) and
pressure up to about
2.5 MPa (25 bar). Two
counter-rotating screws
with matching profiles
provide positive
displacement and a
wide operating range.
Typical use is natural
gas gathering.
Photo: Mycom/Mayekawa
mfg.
• Axial blade and fin
type compressors
with up to 15 wheels
provide high volumes at
a relatively low
pressure differential
(discharge pressure 3-5
times inlet pressure),
speeds of 5,000-8,000
rpm, and inlet flows up
to 200,000 m3
/hour.
Applications include air compressors and cooling compression in LNG
plants. Axial rotor photo: Dresser Rand
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• Larger oil and gas
installations use
centrifugal compressors
with 3-10 radial wheels,
6,000–20,000 rpm
(highest for small size), up
to 80 MW load at
discharge pressure of up
to 50 bars and inlet
volumes of up to 500,000
m3
/hour. Pressure
differential up to 10.
Photo:
Dresser Rand
Most compressors will not
cover the full pressure range
efficiently. The lowest pressure is atmospheric, for gas to pipeline, some 3 to
5 MPa (30-50 bar) pressure is used, while reservoir reinjection of gas will
typically require 20 MPa (200 bar) and upwards, since there is no liquid in
the tubing and the full reservoir pressure must be overcome. Therefore,
compression is divided into several stages to improve maintenance and
availability.
Also due to single unit power limitations, compression is often divided in
several parallel trains. This is not the case in this example, since gas is not
exported and reinjection can be interrupted during maintenance periods.
Compressors are driven by gas turbines or electrical motors (for lower power
also reciprocating engines, steam turbines are sometimes used if thermal
energy is available). Often, several stages in the same train are driven by the
same motor or turbine.The main operating parameters for a compressor are
the flow and pressure differentials. The product defines the total loading, so
there is a ceiling set by the maximum design power. Furthermore, there is a
maximum differential pressure (Max Pd) and choke flow (Max Q),
the
maximum flow that can be achieved. At lower flow, there is a minimum
pressure differential and flow before the compressor will "surge" if there is
not enough gas to operate.
If variations in flow are expected or differences between common shaft
compressors occur, the situation will be handled with recirculation. A high
flow, high pressure differential surge control valve will open to let gas from
the discharge side back into the suction side. Since this gas is heated, it will
also pass through the heat exchanger and scrubber so as not to become
overheated by circulation.
