Page 35 - Impiantistica industriale

to get the so-called biomethane. This process al- the exhausted adsorbent has be performed.
lows increasing the calorific value of the gas, hence In order to favor the reverse process of desorption
obtaining a product that meets fuel gas specifica- it is possible to proceed by increasing the tem-
tions and is capable of fully replacing natural gas perature, with the Temperature Swing Adsorption
derived from fossil fuels. (TSA) technique, or by decreasing the pressure.
Pressure decreasing can be atmospheric (PSA) or
Between 2000 and 2017, global biogas production below atmospheric pressure (VSA). The combina-
from anaerobic digestion has quadrupled, pas- tion Adsorption and Vacum Regeneration defines
sing from 78 to 364 TWh, mainly between Europe the Vacum Swing Adsorption (VPSA) technology.
(54%), Asia (31%), and the Americas (14%) . In Eu-
3
rope, the number of biomethane plants has grown
exponentially as well. According to the European
Biomethane Map 2020 published by the European VPSA Technology
Biogas Association (EBA) and Gas Infrastructure PSA (or VPSA) technology has an enormous poten-
Europe (GIE), the number of biomethane plants in tial for gas separation and purification, this is due
Europe has increased by 51% in 2 years, passing to the high removal efficiency achievable with low
from 483 in 2018 to 729 in 2020. energy requirements, low capital cost, equipment
5
The increase, on a global scale, of biogas and bio- compactness, safety, and ease of operation .
methane production plants, is the result of ener- Adsorption process for gas separation exploits the
gy policies developed, mainly between 2000 and different types of interactions between the gas mo-
2015, by the different member states based on lecules and the adsorbent, which selectively retain
state directives and incentive systems. The Renew- specific molecules, under different conditions of
able Energy Directive sets a new binding target for pressure and temperature.
the EU for 2030 of at least 32% of renewable ener- According to the adopted plant configuration, the
gy . In Italy, the “Biomethane Decree”, promotes gas stream that has to be purified flows through a
4
the production and use of biomethane and other column containing a fixed bed of porous material.
advanced biofuels. CO molecules in the raw biogas stream are retai-
2
ned by the adsorbent material, allowing their sepa-
Biogas initial processing consists of different tre- ration, and thus enriching the methane content of
atments aimed at removing impurities, mainly H O, the gaseous stream leaving the column.
2
H S, and VOC’s, for each of which various solu- When the adsorbent material approches satura-
2
tions are currently available. tion, CO molecules must be desorbed from the
2
Similarly, for the subsequent conversion of bio- column through a bed regeneration process.
gas to biomethane (upgrading process), several Therefore, the process proceeds with cycles in
technologies are commercially available, which ex- which the adsorption and desorption steps alter-
ploit the different chemical and physical properties nate in the column (“swing”).
of carbon dioxide, as compared to the other com- The largest part of engineering in a PSA process
ponents of the raw mixture, to obtain its efficient relies on designing a regeneration protocol for the
separation. adsorbent able to spend small amount of energy
(reduce energetic penalty) in the fastest possible
VPSA technology has an way (increase productivity) .
6
enormous potential for biogas The adsorption process is favored under high pres-
sure and low temperature conditions. In typical
“upgrading because of its high biogas upgrading processes, adsorption occurs at
efficiency, low capital cost, and a slightly elevated pressure of 4-10 bar, while de-
sorption is performed with the use of vacuum pum-
ease of operation ps, achieving a minimum pressure of 0.1-0.2 bar .
7
PSA (or VPSA) separation processes are cyclic in
The most used technologies are physical absorp- nature (adsorption-desorption) and typically re-
tion, i.e. scrubbing with water or organic solvents; quire the use of multiple columns for continuous
chemical adsorption, mainly by amines scrubbing; production, operated on a sequence of interacting
systems based on gas permeation, such as mem- steps. In this way, when the adsorbent material be-
brane technology, and physical adsorption proces- comes saturated, the raw biogas flow is directed to
ses, based on different porous adsorbents. another column where the adsorbent material has
In addition, cryogenic processes can be used, been regenerated.
especially when a liquefied biomethane is the fi-
nal target, but this technology is still under deve- The basic PSA configuration is represented by the
lopment due to high investment costs. “Skarstrom cycle”, which involves the sequence of
four phases in each column (Figure 1):
Adsorption is one of the promising technologies 1. Adsorption (AD): The CH -CO mixture is fed
4
2
for biogas upgrading, thanks to its high efficiencies to the fixed bed, at high pressure, where the
and easy operation. adsorbent is placed. Selective adsorption of
Because of high CO concentration in biogas are CO takes place, obtaining a gas enriched
2
2
(usually 40%), a thorough regeneration process of with the light component (CH up to 99%).
4
Impiantistica Italiana - Maggio-Giugno 2022 33 33