Proceedings World Bioenergy 2010

chemical scrubbing, amines are regenerated with steam,
indicating the need of high temperatures and
consequently high energy demand. Regeneration within
the temperature swing adsorption process requires only
low temperature heat.
The presented temperature swing adsorption process
shows some advantages compared to the other biogas
upgrading processes mentioned above. Among them are,
e.g. high methane content in the enriched biogas - typical
values are 98% or even above, and very low methane
losses during the whole process. Moreover, low energy
demand represents another advantage since for ad- and
desorption processes low temperature heat is used instead
of electricity or process steam. The heat is mainly
achieved from combined heat and power processes which
are located at a biogas plant in most cases. Considering
the point of compression of upgraded biogas, it would
take place after carbon dioxide removal. Hence, the mass
flow of the enriched gas is noticeable lower than that of
raw biogas and consequently less compression power is
needed.
3 TEMPERATURE SWING ADSORPTION
The process of temperature swing adsorption is based
on the correlation of different equilibrium loads of the
adsorbent to different temperatures. At low or ambient
temperatures equilibrium load of the adsorbent is high
and therefore the adsorbent is able to bind high quantities
of gases. On the contrary, at elevated temperature levels
equilibrium load decreases, the amount of gas possible to
be bound by the adsorbent decreases too and this may
lead to desorption of already adsorbed gases [5].
Adsorption of gases on the adsorbent is performed at
ambient temperatures whereas for desorption of already
adsorbed gases higher temperatures are required.
During the adsorption process, the temperature in the
column rises due to the exothermic characteristic of the
adsorption leading to lower gas uptake of the adsorbent
due to lower equilibrium load at this elevated
temperature. In order to prolong adsorption time and thus
enlarge the amount of gas adsorbed, one possibility is to
cool the adsorbent during the adsorption phase by an
integrated cooling system. This system should keep
temperature at a quite low level where equilibrium load is
high.
At the end of the adsorption process the adsorbent is
saturated with gas components. Hence, it has to be
regenerated by desorbing the bound gas components. In
the case of temperature swing adsorption processes,
desorption is performed at elevated temperature levels.
Therefore, increasing the temperature is necessary
because of the endothermic characteristic of desorption
and in order to achieve higher temperature levels. This
could be done either by direct heating, which means that
a hot purge gas passes through the column and heats the
adsorbent, or by indirect heating. In the latter case a
heating system is built inside or outside the column and a
heating medium such as hot water warms the adsorbent
indirectly. As mentioned above, elevated temperature
levels are correlated to low equilibrium load causing
desorption of the adsorbed gas (see figure 1). After all the
desorbed gas left the column, the column needs to be
cooled down to ambient temperatures in order to assure
optimal adsorption conditions for the next cycle.
80 world bioenergy 2010
4 ADSORBENT
In the present work the adsorbent Diaion WA21J
provided by Mitsubishi Chemical Corporation was used.
This adsorbent consists of amine groups integrated into a
polymeric matrix made of polystyrene (see Table I).
Table I: Properties of Diaion WA21J [6]
Matrix
DVB-crosslinked
copolymer of styrene
Functional group Ternary amine
Operating temperature 100°C max
Particle size 300 – 1180 µm
Apparent density approx. 643 g/l
The weakly basic property due to the functional
amine group enables the adsorbent to selectively and
reversibly bind sour gases such as carbon dioxide or
hydrogen sulphide.
The adsorbents adsorption capacity for carbon
dioxide was determined by means of thermo gravimetric
analysis. Thermo gravimetric analysis measures changes
in weight in a material under a controlled atmosphere as a
function of temperature and time [7]. The analyzer,
roughly outlined, consists of a high-precision balance
connected to a dish filled with the sample which is, in
this case, the adsorbent. The dish is placed in an oven and
the atmosphere within the oven can be purged with
different gases. Analysis is carried out by increasing the
temperature and/or purging with gases leading to changes
in weight of the sample. During the analysis, weight
against temperature and time is measured.
Adsorption isotherms of carbon dioxide on Diaion
WA21J are shown in figure 1.
Figure 1: Equilibrium adsorption isotherms of carbon
dioxide on Diaion WA21J [8]
The adsorbent exhibits high selectivity for carbon
dioxide whereas its affinity for methane can be
considered as negligible. Moreover, investigations have
shown that the adsorption capacity is not reduced by
simultaneous adsorption of water.
5 LABORATORY TEST RIG
TSA process experiments were carried out using a
laboratory test rig shown in figure 2. First of all, the
desired gas mixture is adjusted with the help of mass
flow controllers, thereby the single gases are provided by
pressurized gas bottles. Afterwards the gas mixture flows