Proceedings World Bioenergy 2010

in the adsorber is separated, carbon dioxide is adsorbed
entirely and pure methane is obtained. Adsorption lasts
for about 950 sec and is ended when breakthrough of
carbon dioxide occurs.
Figure 3: TSA process with desorption by indirect
heating with hot water
Desorption is achieved by heating the adsorbent
indirectly with hot water. Thereby, water is heated and is
let through tube bundles placed inside the adsorber. As a
consequence, heat exchange between the hot water and
the cold adsorbent takes place and heats the adsorbent.
During the desorption step methane is desorbed at
first. Methane comes to the most part from the space
between the adsorbent grains (porosity approx. 45%) as
well as from the space under the adsorber, since
adsorption capacity of the adsorbent with regard to
methane is quite low. The peak of carbon dioxide
desorption occurs after methane desorption, but after this
peak the flow rate of carbon dioxide decreases steadily
and in the end remains at a very low level. Desorption
lasts about 2000 s what is twice as long as adsorption
time.
Referring to temperature, the temperature increase
during the adsorption step indicates the arrival of the
adsorption layer. Adsorption is an exothermic process, so
heat is released and it heats the adsorbent. This
phenomenon can be clearly seen with the steep rise of the
temperature of 30°C. Although cold water is applied
during the entire adsorption step, its coolness is not
sufficient in order to carry off the released heat.
Nevertheless, cold water application has an effect
because after the peak the temperature decreases very fast
and this allows adsorption to take place for a prolonged
time period. Temperature during desorption increases
steadily but it takes about 2000 s in order to get to a
temperature of 70°C.
Regarding the following cycles of this experiment,
adsorption capacity in each of them decreased to one
third of the capacity in the first cycle. The reason for this
lies in the only partial desorption of carbon dioxide.
Desorption by heating with hot water does not allow
complete desorption of the entire adsorbed gas
components; only partial desorption, correlated to the
equilibrium and adsorption capacities at certain
temperatures, is possible.
Desorption by indirect heating with water is
considered as a very inefficient way of desorption due to
long desorption time as well as low desorption efficiency.
7.2 Desorption by indirect heating with hot water and
afterwards purging
Figure 4 depicts the TSA process with desorption
82 world bioenergy 2010
carried out by indirect heating with hot water and
afterwards purging with nitrogen.
This process is very similar to that described above
with one difference occurring not until the end of the
desorption step.
During the adsorption step separation performance is
excellent as well and also here adsorption lasts for about
950 s. Within the first 2000 s of desorption, desorption is
also carried out indirectly by application of hot water
functioning as heat exchange medium. The characteristics
of desorbed gases are similar too, with methane
desorbing at first followed by carbon dioxide.
Figure 4: TSA process with desorption by indirect
heating with hot water and afterwards purging
After 2000 s of indirect heating, no more desorption
of methane or carbon dioxide takes place. As already
explained above, complete desorption is not achievable
by indirect heating and therefore purge gas (N 2) was
applied, for about 200 s. Desorption with purge gas
allows complete desorption due to changes in partial
pressure and the correlated adsorption capacity at
equilibrium. As a consequence, still adsorbed gas
components should theoretically be desorbed. Figure 4
shows clearly that application of purge gas leads to
further desorption of gas components. Obviously, the
desorbed gas consists for the most part of carbon dioxide
but also contains small amounts of methane.
Regarding temperature, during the desorption step of
indirect heating with hot water, temperature increases
steadily but during purging temperature decreases again,
although the purge gas is preheated to 75°C. The reason
for temperature decrease is the desorption process itself
which is of endothermic nature. Desorption of this
remarkable amount carbon dioxide and methane needs
energy and this causes temperature decrease.
Referring to the following cycles of this experiment,
adsorption capacity in each of them decreased only by
approximately 1 w% compared to the capacity in the first
cycle.
Desorption by indirect heating with water is
considered as a rather inefficient. However, application
of purge gas led to further desorption of methane and
carbon dioxide, what causes extended adsorption
capacities (compared to desorption only with hot water)
in the following cycles and therefore improves the entire
process efficiency.
7.3 Desorption by indirect heating with hot water and
simultaneous direct heating with purge gas
Figure 5 illustrates the TSA process with desorption
carried out by indirect heating with hot water and
simultaneous direct heating with purge gas.