5.2 Balancing Capacity, Selectivity, and Regenerability
Many academic papers dealing with the development of new adsorbents focus only
on maximizing the adsorbate loading under adsorption conditions and then fully
regenerate the adsorbent under unrealistic conditions, i.e., very high purge gas flows
and/or heating times. However, what really counts in an adsorption process is not the
absolute adsorption capacity (the amount that can be adsorbed on a virgin solid), but
the so-called working capacity (or cyclic capacity or still delta loading), i.e., the
amount that can be adsorbed repeatedly in hundreds or thousands of adsorptiondesorption cycles. In cyclic steady, the cyclic capacity, i.e., the amount, which can be
adsorbed in each cycle, is equal to the amount that is desorbed in each cycle,
knowing that desorption is subject to constraints: minimal use of purge gas to
avoid dilution of the extract, limited temperature, and/or pressure change to minimize energy consumption. The regenerability of the adsorbent is, therefore, a key
issue.
If we want to discuss the adsorbent properties offering the best compromise
between adsorption and desorption, it is advisable to distinguish pressure and
temperature swing adsorption processes. Temperature swing adsorption is usually
characterized by long cycle times, because heat transfer limitations generally do not
allow a rapid heating or cooling of the adsorbent. Temperature swing adsorption is
often used for the adsorption of trace components, i.e., for purification purposes. If
the adsorption capacity is high and the concentration of the adsorbate in the feed very
low, the adsorption time (i.e., the time before breakthrough) is also long. The long
times needed for heating the adsorbent for desorption are, therefore, usually not an
issue. In purification processes, the key objective is to achieve a high purity of the
raffinate, i.e., a complete retention of the trace component even at very low concentrations. In that case, a very high affinity of the adsorbate with the adsorbent is
favorable. This renders desorption difficult, but if the energy consumption for
regeneration is not an issue, then maximizing the adsorbent-adsorbate interaction
and the adsorption capacity is indeed the right strategy. This is the case, for instance,
of siloxanes removal from biogas. As an extreme case, the purification technologies
employing non-regenerable guard beds can be mentioned. Note that if we speak
about adsorption capacity here, we refer to maximum adsorption capacity that can be
reached when the solid is saturated (we will use the term saturation capacity in the
following). Whether this capacity is reached depends on the process conditions
(partial pressure, temperature).
The critical parameters for other applications are not necessarily the same. In
temperature swing processes for CO 2 capture from flue gas, for example, the
complete removal of CO 2 is not at all critical, but minimizing the energy consumption necessary for regeneration is the key objective. This has a big impact on the
adsorbent selection criteria. A high saturation capacity associated with conditions
that allow approaching saturation is still important. If too little CO 2 is adsorbed,
heating up the solid for regeneration is not efficient. Concerning the interaction of
CO 2 with the sorbent, i.e., the heat of adsorption, a compromise between two factors
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
217
Many academic papers dealing with the development of new adsorbents focus only
on maximizing the adsorbate loading under adsorption conditions and then fully
regenerate the adsorbent under unrealistic conditions, i.e., very high purge gas flows
and/or heating times. However, what really counts in an adsorption process is not the
absolute adsorption capacity (the amount that can be adsorbed on a virgin solid), but
the so-called working capacity (or cyclic capacity or still delta loading), i.e., the
amount that can be adsorbed repeatedly in hundreds or thousands of adsorptiondesorption cycles. In cyclic steady, the cyclic capacity, i.e., the amount, which can be
adsorbed in each cycle, is equal to the amount that is desorbed in each cycle,
knowing that desorption is subject to constraints: minimal use of purge gas to
avoid dilution of the extract, limited temperature, and/or pressure change to minimize energy consumption. The regenerability of the adsorbent is, therefore, a key
issue.
If we want to discuss the adsorbent properties offering the best compromise
between adsorption and desorption, it is advisable to distinguish pressure and
temperature swing adsorption processes. Temperature swing adsorption is usually
characterized by long cycle times, because heat transfer limitations generally do not
allow a rapid heating or cooling of the adsorbent. Temperature swing adsorption is
often used for the adsorption of trace components, i.e., for purification purposes. If
the adsorption capacity is high and the concentration of the adsorbate in the feed very
low, the adsorption time (i.e., the time before breakthrough) is also long. The long
times needed for heating the adsorbent for desorption are, therefore, usually not an
issue. In purification processes, the key objective is to achieve a high purity of the
raffinate, i.e., a complete retention of the trace component even at very low concentrations. In that case, a very high affinity of the adsorbate with the adsorbent is
favorable. This renders desorption difficult, but if the energy consumption for
regeneration is not an issue, then maximizing the adsorbent-adsorbate interaction
and the adsorption capacity is indeed the right strategy. This is the case, for instance,
of siloxanes removal from biogas. As an extreme case, the purification technologies
employing non-regenerable guard beds can be mentioned. Note that if we speak
about adsorption capacity here, we refer to maximum adsorption capacity that can be
reached when the solid is saturated (we will use the term saturation capacity in the
following). Whether this capacity is reached depends on the process conditions
(partial pressure, temperature).
The critical parameters for other applications are not necessarily the same. In
temperature swing processes for CO 2 capture from flue gas, for example, the
complete removal of CO 2 is not at all critical, but minimizing the energy consumption necessary for regeneration is the key objective. This has a big impact on the
adsorbent selection criteria. A high saturation capacity associated with conditions
that allow approaching saturation is still important. If too little CO 2 is adsorbed,
heating up the solid for regeneration is not efficient. Concerning the interaction of
CO 2 with the sorbent, i.e., the heat of adsorption, a compromise between two factors
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
217