Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
4.1.3 Dimethylether of polyethylene glycol (Selexol process)
The Selexol process uses dimethyl ethers of polyethylene glycol as the solvent for
absorption of carbon dioxide. The Selexol solvent has a high capacity for absorption
of impurities such as sulfur impurities, mercaptans, as well as ammonia, HCN
and other higher hydrocarbons.
H2S is up to nine times more soluble in the Selexol solvent than CO2, which
makes it suitable for selective removal of H2S. Hydrocarbons are also very soluble
in the solvent and the solubility increases with increasing molecular weight. Water
is also highly soluble in the Selexol solvent. Due to this quality the Selexol process
is often used for simultaneous hydrocarbon and water dew point control [34].
4.1.4 Water scrubbing
Water-scrubbing uses the different solubilities of carbon dioxide and methane in
water, especially at low temperatures and high pressures. An example of a waterscrubber
process is given in Figure 12.
Figure 12. An overview of the water scrubbing process. The streams connected to
the main gas flow, the recirculating water flow, the stripping gas, and the recirculating
gas streams are represented by solid, dashed, dotted, and dash/dotted
lines, respectively. The most important components are: 1 Raw gas fan, 2 Compressor,
3 Absorber, 4 Adsorber (drying), 5 Flash tank, 6 Stripper, 7 Pump, 8
Cooler [36].
The removal of carbon dioxide takes place in an absorption column where the gas
is introduced in the bottom of the column and the water is sprinkled from the top
over the packing in the column which is there to enhance the contact area between
the gas and the liquid phases. The process takes place at a low temperature
and at elevated pressure. Assuming raw biogas fed to the system, the gas
leaving the scrubber has very high methane content, up to 99% dry volume, but is
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