Oil and gas production handbook An introduction to oil and gas production

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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49 
 
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
 
 
4.3.2 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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50 
 
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.