Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
exit the stripper column in the top (6) and is cooled in a condenser. The condensate
(mainly steam but with traces of amine) is returned to the stripper (7). The
cooled gas stream will consist mainly of CO2 and if H2S is entered into the system
it will leave it here. In the simplified flowsheet, there is no integration but the cooling
loops may naturally be integrated. As the pH of the solution is quite high with
the basic amines in solution, there is little to no risk of bacterial growth. Therefore
any contactor may be used; trays, structured or random packing alike.
As per the figure the generic amine system may be represented for systems
ranging from small scale, such as biogas applications, to applications in the oil,
gas and chemical industry with units several meters in diameter. The pressure is
however most likely higher in the large industrial applications. More specific to the
biogas case, there is usually gas sweetening (H2S removal) upstream of the system
to avoid smell and material issues downstream. The product gas will also
have to be dried before being used in an automotive or indeed any other application.
This is done using temperature swing adsorption, pressure swing adsorption
or freeze drying.
2.1.2 Theoretical background
The degree of purification of the amine-based systems may be viewed as follows.
Under normal operating conditions, the system is operating close to what
may be described as an ideal plug flow, with little or no back-mixing (at least for
the purpose of describing an actual system). This means that one gas segment
which is entered in the bottom of the column will pass through the column and
shrink in size as the CO2 is removed from the gas segment, but it does not mix
with other gas segments. The driving force of the absorption may be mainly ascribed
to the level of CO2 in the gas as there is a surplus of amine in the system.
Using this line of thinking it may be realized that, as there is no back-mixing, the
purity of the exiting gas is based on the column height alone. It also explains why
much temperature increase is seen in the bottom part of the column.
With respect to throughput this is limited in the lower end by enabling contact between
the gas and liquid through distributers etc. In the upper end, the throughput
is limited by the lifting force of the flowing gas compared to the weight of the flowing
liquid. This point is called the flooding point and may be expressed as
ܨ = ௅
ீ
∗ ටቀఘಸ ቁ Eq. 1
ఘಽ
G is packing specific but is a function of the gas velocity, L is the liquid flow and it
is weighted using the square root of the gas density divided by the liquid density.
Using the flooding point of the packing, the gas velocity of flooding may be determined
for the intended liquid flow rates. This maximum gas velocity is then used
for setting the design gas velocity, usually 50-80% of the flooding gas velocity.
Looking at a molecular level, there are several reactions that may take place (X.
Zhang et al. 2001), they may however be summarized using the following equilibrium
reactions
ܥܱ ଶ + ܼܲ ∙ ܪ ଶܱ ↔ ܼܲܪ ା + ܪܥܱ ଷ ି Eq. 2
ܥܱ ଶ + ܯ ܦܧܣ ∙ ܪ ଶܱ ↔ ܯ ܦܧܣܪ ା + ܪܥܱ ଷ ି Eq. 3
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