Page 295 - Hydrocarbon Exploration and Production Second Edition
P. 295
282 Oil and Gas Processing
dry gas
outlet
glycol
mist mat
lean glycol
inlet
inlet
separator
gas
wet gas
inlet
rich glycol
outlet
Figure 11.15 Glycol contacting tower.
11.1.3.3. Heavy hydrocarbon removal
Condensable hydrocarbon components are usually removed from gas to avoid liquid
dropout in pipelines, or to recover valuable NGLs where there is no facility for
gas export. Cooling to ambient conditions can be achieved by air or water heat
exchange, or to sub-zero temperatures by gas expansion or refrigeration. Many
other processes such as compression and absorption also work more efficiently at
low temperatures.
If high wellhead pressures are available over long periods, cooling can be
achieved by expanding gas through a valve, a process known as Joule Thomson ( JT)
throttling. The valve is normally used in combination with a liquid gas separator
and a heat exchanger, and inhibition measures must be taken to avoid hydrate
formation. The whole process is often termed ‘low temperature separation’ (LTS)
(Figure 11.16).
If gas compression is required following cooling, a turbo-expander can be used. A
turbo-expander is like a centrifugal compressor in reverse, and is thermodynami-
cally more efficient than JT throttling. Pressure requirements and hydrate
precautions are similar to those of LTS. When high pressures are not available,
refrigeration can be used to cool gas. Propane or freon is compressed, allowed to cool
and then expanded across a valve, cooling the gas as it passes through a chiller.
Temperatures as low as 401C can be achieved. Gas dehydration or glycol injection
must precede the operation to avoid hydrate formation.
11.1.3.4. Contaminant removal
The most common contaminants in produced gas are carbon dioxide (CO 2 ) and
hydrogen sulphide (H 2 S). Both can combine with free water to cause corrosion and