of so-called permanent gases such as nitrogen and oxygen, the air temperature must
be lowered to a point such that nitrogen and oxygen separate based on their boiling
points. When considering the critical temperature of nitrogen (À147
C), an indication of the associated operating conditions to air cryogenic distillation is immediately obtained. Although high-quality levels of separation can be achieved, the
economical drawback (from the point of view of both capital and operational
expenditures) opens the door to the use of adsorption-based technologies. In this
way, different combinations of process concepts (PSA, RPSA, VSA, or VPSA) with
different types of zeolites have been proposed. In particular, A, X, and mordenite
topologies have been commercially implemented. The production of oxygen constitutes an excellent example of the integration of adsorbent and technology
[11]. The different types of process concepts are tailored in order to match the
adsorbent properties and then maximizing both productivity and recovery. When
looking to the properties of the different implemented zeolites, moderately steep
nitrogen isotherms are prioritized. This kind of shapes is characterized by low
nitrogen adsorbed amounts at low pressures and high nitrogen adsorption capacities
at saturation. This combination provides maximized working capacities and consequently associated low bed size factors (BSF). The last property describes the ratio of
amount of adsorbent to amount of product gas per cycle and drives the selection
between the main different technologies. The typical values for the specific energy
consumptions associated with the different technologies follow the order PSA
(525 kWh/ton) > VSA (265 kWh/ton) > VPSA (245 kWh/ton) [12].
In a PSA system, the inlet air is compressed to a given level by a compressor. The
adsorption phase operates at the pressure imposed by the compressor, while desorption phase takes place at atmospheric pressure. The implemented zeolites present
higher affinities for nitrogen thanks to the polarity of the system. During the
adsorption phase, while nitrogen is adsorbed due to the presence of a moderate
quadrupole moment, oxygen is recovered at the outlet of the adsorption vessel. For
oxygen enrichment, the PSA adsorption phase takes generally place at a minimum
pressure of 1.5 atm. Once the zeolite reaches nitrogen saturation, the vessel needs to
be regenerated. This is done by decreasing the pressure of the tank back to atmospheric pressure, thus liberating the adsorbed nitrogen.
Compared to cryogenic distillation, PSA systems are best suited for processes that
do not require purities of oxygen higher than 95%. Although purities as high as
99.9% can be reached, considering purities beyond 99.5% leads to a very fast
increase in the cost of the PSA device. When considering the production rates,
adsorption-based devices (in their different forms) are best adapted to relatively low
volumes of oxygen production (typically 20–100 tons/day). Due to the fact that the
oxygen productivity is mainly governed by the bed size in the adsorption-based
systems, the capital investments rapidly increase when high productivities are
considered. A drawback of the oxygen adsorption concerns the global productivity
decrease associated with the low usability of nitrogen as a by-product. Due to the
intrinsic trade-off between purity and recovery, the nitrogen contains significant
levels of oxygen.
204
J. Pérez-Pellitero and G. D. Pirngruber
be lowered to a point such that nitrogen and oxygen separate based on their boiling
points. When considering the critical temperature of nitrogen (À147
C), an indication of the associated operating conditions to air cryogenic distillation is immediately obtained. Although high-quality levels of separation can be achieved, the
economical drawback (from the point of view of both capital and operational
expenditures) opens the door to the use of adsorption-based technologies. In this
way, different combinations of process concepts (PSA, RPSA, VSA, or VPSA) with
different types of zeolites have been proposed. In particular, A, X, and mordenite
topologies have been commercially implemented. The production of oxygen constitutes an excellent example of the integration of adsorbent and technology
[11]. The different types of process concepts are tailored in order to match the
adsorbent properties and then maximizing both productivity and recovery. When
looking to the properties of the different implemented zeolites, moderately steep
nitrogen isotherms are prioritized. This kind of shapes is characterized by low
nitrogen adsorbed amounts at low pressures and high nitrogen adsorption capacities
at saturation. This combination provides maximized working capacities and consequently associated low bed size factors (BSF). The last property describes the ratio of
amount of adsorbent to amount of product gas per cycle and drives the selection
between the main different technologies. The typical values for the specific energy
consumptions associated with the different technologies follow the order PSA
(525 kWh/ton) > VSA (265 kWh/ton) > VPSA (245 kWh/ton) [12].
In a PSA system, the inlet air is compressed to a given level by a compressor. The
adsorption phase operates at the pressure imposed by the compressor, while desorption phase takes place at atmospheric pressure. The implemented zeolites present
higher affinities for nitrogen thanks to the polarity of the system. During the
adsorption phase, while nitrogen is adsorbed due to the presence of a moderate
quadrupole moment, oxygen is recovered at the outlet of the adsorption vessel. For
oxygen enrichment, the PSA adsorption phase takes generally place at a minimum
pressure of 1.5 atm. Once the zeolite reaches nitrogen saturation, the vessel needs to
be regenerated. This is done by decreasing the pressure of the tank back to atmospheric pressure, thus liberating the adsorbed nitrogen.
Compared to cryogenic distillation, PSA systems are best suited for processes that
do not require purities of oxygen higher than 95%. Although purities as high as
99.9% can be reached, considering purities beyond 99.5% leads to a very fast
increase in the cost of the PSA device. When considering the production rates,
adsorption-based devices (in their different forms) are best adapted to relatively low
volumes of oxygen production (typically 20–100 tons/day). Due to the fact that the
oxygen productivity is mainly governed by the bed size in the adsorption-based
systems, the capital investments rapidly increase when high productivities are
considered. A drawback of the oxygen adsorption concerns the global productivity
decrease associated with the low usability of nitrogen as a by-product. Due to the
intrinsic trade-off between purity and recovery, the nitrogen contains significant
levels of oxygen.
204
J. Pérez-Pellitero and G. D. Pirngruber