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2. Blowdown (BD): Immediately before CO bre- zation step results in a direct improvement in pro-
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aks through, the column should be regenera- duct recovery.
ted. The feed step is stopped, and the pressu- This means that part of the gas that will normally
re of the column reduced, counter-currently to get lost in the blowdown step can be exploited to
the feed step. In this step, the lowest pressure pressurize another column, losing less purifi ed gas.
of the system is achieved. Having multiple columns helps in increasing the
3. Purge (PG): When the low pressure is achie- methane recovery because more pressure equali-
ved, the column is fed with a stream rich in zation steps are possible in each cycle.
the light component (feedstock biogas and/ Commonly, three, four or six columns working in
or recycling of part of the product, i.e. puri- parallel are installed to maintain the continuous
fi ed methane) at low pressure, to reduce the process, where each column operates on a diffe-
amount of CO in both the adsorbed and gas rent step of the cycle.
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phases. The purge gas is recycled to the inlet The primary task for VPSA process design is the
bed, recovering at the outlet an off-gas rich in choice of the a dsorbent that should have high CO
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the heavy component (CO ). adsorption capacity, high selectivity, as well as easy
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4. Pressurization (PR): In order to restart a new regeneration and low costs.
cycle, the pressure should be increased. Pres-
surization can be carried out co-currently with
the feed stream (Feed Pressurization, FP) or
counter-currently with purifi ed CH (Backfi ll, Studies of adsorbents used
BF). 4 in the PSA processes

The PSA process is very fl exible in design, the Many studies of adsorbents used in PSA process
separation effi ciency varies strongly with the type for biogas upgrading focus on carbon molecular
and structure of the adsorbent, cycle confi guration sieves (CMS), activated carbons, zeolites, and me-
and the associated operating conditions (e.g., feed tal-organic frameworks (MOFs).
composition, adsorption pressure, cycle time, pur-
ge fl ow rate). Many studies of adsorbents
To indicate the overall performance of a process
and allow for its quantifi able assessment, the key used in PSA process for biogas
performance indicators (KPIs) commonly used are “upgrading focus on carbon
product purity and methane recovery, productivity molecular sieves (CMS),
and energy usage.
These KPIs are dependent on each other and can- activated carbons, zeolites,
not be individually optimized due to the nature of and metal-organic frameworks
adsorption processes, so it is necessary to fi nd a
trade-off between them. (MOFs)
Compared to the basic cycle, many variations can
be introduced in the cycle confi guration, with the Rodrigues’s group performed an investigation re-
introduction of new steps. lated to the removal of CO from biogas by VPSA
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The fi rst improvement that can be introduced is process using CMS and zeolite 13X . Comparisons
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vacuum regeneration, hence turning the process of experimental results showed that VPSA process
into a VPSA, which results in higher adsorption ef- with CMS adsorbent for biogas purifi cation could
fi ciency, at the cost of higher energy consumption. achieve a better product recovery and productivi-
Furthermore, the introduction of a pressure equali- ty at a specifi c CH purity, as compared with that
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using zeolite 13X.
The energy consumption of PSA upgrading bio-
gas plant is ranging from 0.15 to 0.3 kWh/Nm3 of
feed biogas. Wu et al. (2015) compared the energy
consumption of a VPSA process employing me-
tal-organic framework 508b (MOF-508b), carbon
molecular sieve 3 K (CMS-3 K) and zeolite 13X as
adsorbents to separate CO from biogas . Simu-
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lation results revealed that energy consumption of
VPSA process with MOF-508b and CMS-3 K was
56% and 50% lower than the corresponding va-
lues obtained with zeolite. The feed biogas can be
upgraded up to 96-98% methane, with a loss in
methane that can reach up to 4% through the off-
gas stream.
Abd and co-workers investigated the removal of
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CO from biogas mixture using PSA over carbon
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molecular sieve (CMS-FB) using four beds. They
found that the PSA operating conditions such as
Figure 1 - Schematic representation of the different steps in a Skarstrom cycle adsorption pressure, purge ratio, and desorption

34 34 Impiantistica Italiana - Maggio-Giugno 2022