Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
The amount of carbon dioxide that needs to be removed is described by the total
flow rate and the gas composition, while the solubility is determined by Henry's law
(Eq. 4). This gives the following expression
Q
water
Q
( l / h)
K
biogas
H
* % CO ( mol / h)
* p
tot
2
* % CO ( M )
2
Eq. 6
where Qbiogas is the total biogas flow, %CO2 is the percentage of carbon dioxide in
the raw biogas and Ptot is the pressure in the absorption column. The percentage
of carbon dioxide in the incoming biogas can be removed from this expression,
showing that the needed water flow is independent of the percentage CO2 in the
incoming biogas.
The value of Henry's constant for a specific gas is only valid at one specific temperature.
When the temperature is increased, the solubility usually decreases and
vice versa. The following example of the van't Hoff equation is one example that
can be used to get an approximation of how the solubility varies with the temperature
(Sander 2011).
ܭ ு (ܶ ଶ ) = ܭ ு (ܶ ଵ ) exp ቂܥ ቀ ଵ
்మ
− ଵ
ቁቃ Eq. 7
்భ
In Eq. 7, T1 and T2 are the absolute temperatures for which the constant is known
and searched respectively, while C is a specific coefficient which is defined as
C=dln(kH))/d(1/T). For CO2 in water, the value of this constant is 2400.Figure 19
shows how the solubility of CO2 changes between 10°C and 40°C according to Eq.
7. As can be seen in the figure, the solubility is more than 50% higher at 10°C than
at 25°C. A similar graph has also been published earlier (Petersson & Wellinger
2009).
Relative solubility of CO 2 in water
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
10 15 20 25
Temperature [°C ]
30 35 40
Figure 19 Relative solubility of CO2 in water in the temperature interval between
10°C and 40°C. Solubility normalized to the value at 25°C.
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