Moreover, the solvents can be regenerated using the depressurising technique. In
addition to this, water, nitrogen, hydrogen sulphide and oxygen can be separated
easily along with carbon dioxide. Some of the brand names for the liquids applied
in this technique include Rektisol
® , Genosorb
® , Sepasolv
® , Purisol
® and Selexol
®
(Petersson and Wellinger 2009; Report 2012). Selexol
® absorbs more carbon
dioxide than water and so requires small upgrading unit. Regeneration of the
organic solvent is hard because the solubility of carbon dioxide is high and also
the hydrogen sulphide gas solubility is also high, requiring an additional amount of
heat (Persson 2003).
The operational parameters include 7–8 bars for biogas compression, 20
C
temperature while injecting the biogas. Also, the solvent used must be cooled
down before adding to the unit since temperature affects the Henry’s constant
(Bauer et al. 2013a). Increasing the temperature to 80
C regenerates the organic
solvent (Bauer et al. 2013b; Sun et al. 2015). About 98% methane purity is obtained
(Bauer et al. 2013a; Angelidaki et al. 2018).
9.3.2 Chemical Scrubbing
Chemical scrubbing involves the absorption of gases in the scrubbing liquid
followed by chemical treatment. The reaction is highly specific in not absorbing
the methane gas followed by high methane recovery and purity. Amine scrubbing is
an example of chemical scrubbing. Amine solutions, dimethylethanolamine,
triethanolamine,
mixture
of
piperazine
and
monoethanolamine,
methyldiethanolamine, diglycolamines (Patterson et al. 2011) and
monoethanolamine, are used widely to absorb the carbon dioxide. The carbon
dioxide loading capacity is high for tertiary amines (Kadam and Panwar 2017).
The mixture of piperazine and monoethanolamine has greater absorbing capacity,
and hence less energy is required during the regeneration step (Bauer et al. 2013a, b;
Sahota et al. 2018).
Amine scrubbing gains importance due to the reaction of amine solution with the
carbon dioxide gas in addition to its absorption, leading to the reduction of methane
emission to <0.1%. The liquid that is lost due to evaporation should be replaced, and
regeneration of carbon dioxide is possible by increasing the temperature. The
hydrogen sulphide gas should be removed before the process of absorption since it
gets absorbed with the amine scrubber solutions (Petersson and Wellinger 2009).
Amine scrubbing system is made up of a stripper and an absorbing unit. The
operating pressure required is very low when compared to the water scrubbing
process. The pressure required for the injection of biogas into the bottom of the
absorbing column is about 1–2 bars, and the amine scrubbing solutions are injected
from the top of the unit in countercurrent flow method. The regeneration of the
carbon dioxide and the hydrogen sulphide takes place in the stripping unit with a
pressure of 1.5–3 bars and a temperature of 120–160
C. Addition of heat to the unit
breaks the chemical bonds in the absorber phase (Kapdi et al. 2005), and at last, the
carbon dioxide is condensed for recirculation and the entrapped carbon dioxide is
250
B. S. Dhanya et al.
addition to this, water, nitrogen, hydrogen sulphide and oxygen can be separated
easily along with carbon dioxide. Some of the brand names for the liquids applied
in this technique include Rektisol
® , Genosorb
® , Sepasolv
® , Purisol
® and Selexol
®
(Petersson and Wellinger 2009; Report 2012). Selexol
® absorbs more carbon
dioxide than water and so requires small upgrading unit. Regeneration of the
organic solvent is hard because the solubility of carbon dioxide is high and also
the hydrogen sulphide gas solubility is also high, requiring an additional amount of
heat (Persson 2003).
The operational parameters include 7–8 bars for biogas compression, 20
C
temperature while injecting the biogas. Also, the solvent used must be cooled
down before adding to the unit since temperature affects the Henry’s constant
(Bauer et al. 2013a). Increasing the temperature to 80
C regenerates the organic
solvent (Bauer et al. 2013b; Sun et al. 2015). About 98% methane purity is obtained
(Bauer et al. 2013a; Angelidaki et al. 2018).
9.3.2 Chemical Scrubbing
Chemical scrubbing involves the absorption of gases in the scrubbing liquid
followed by chemical treatment. The reaction is highly specific in not absorbing
the methane gas followed by high methane recovery and purity. Amine scrubbing is
an example of chemical scrubbing. Amine solutions, dimethylethanolamine,
triethanolamine,
mixture
of
piperazine
and
monoethanolamine,
methyldiethanolamine, diglycolamines (Patterson et al. 2011) and
monoethanolamine, are used widely to absorb the carbon dioxide. The carbon
dioxide loading capacity is high for tertiary amines (Kadam and Panwar 2017).
The mixture of piperazine and monoethanolamine has greater absorbing capacity,
and hence less energy is required during the regeneration step (Bauer et al. 2013a, b;
Sahota et al. 2018).
Amine scrubbing gains importance due to the reaction of amine solution with the
carbon dioxide gas in addition to its absorption, leading to the reduction of methane
emission to <0.1%. The liquid that is lost due to evaporation should be replaced, and
regeneration of carbon dioxide is possible by increasing the temperature. The
hydrogen sulphide gas should be removed before the process of absorption since it
gets absorbed with the amine scrubber solutions (Petersson and Wellinger 2009).
Amine scrubbing system is made up of a stripper and an absorbing unit. The
operating pressure required is very low when compared to the water scrubbing
process. The pressure required for the injection of biogas into the bottom of the
absorbing column is about 1–2 bars, and the amine scrubbing solutions are injected
from the top of the unit in countercurrent flow method. The regeneration of the
carbon dioxide and the hydrogen sulphide takes place in the stripping unit with a
pressure of 1.5–3 bars and a temperature of 120–160
C. Addition of heat to the unit
breaks the chemical bonds in the absorber phase (Kapdi et al. 2005), and at last, the
carbon dioxide is condensed for recirculation and the entrapped carbon dioxide is
250
B. S. Dhanya et al.