Sub-ambient Fiber Sorbent PSA
Analysis of the PSA separation process thus far has focused on the
effects of a single parameter in the CO2 capture
process. This analysis can help guide decisions when considering a
particular sorbent. But to better understand the process more broadly,
it is imperative to understand how changing multiple variables
simultaneously effects the process simultaneously. The separation
performance of three sorbent materials was considered.
UiO-66 and MIL-101(Cr) are two MOFs that show some promise for
sub-ambient CO2 capture.36,41 Both
materials are stable in liquid water, which is a prerequisite for their
spinning into heat managed fiber sorbent structures. In addition, both
materials have shown the ability to be scaled up to the 10-1000 g batch
scale.62 The use of UiO-66 as a sorbent for
sub-ambient CO2 capture has been briefly discussed
elsewhere, where an operating capacity of 4 mmol/g at 243 K was
shown.31 While MIL-101(Cr)’s sub-ambient performance
has not been reported, its crystalline structure with very large pores
(1.4 and 2.2 nm)63 meets a material selection criteria
as suggested elsewhere41 that makes it promising for
having high working capacity. Enabling these sorts of high working
capacities is expected to be one of the key benefits to this sub-ambient
process, enabling the very high productivities shown to be highly
desirable in Figure 5.
UiO-66 has been spun into fiber sorbents
previously31,64 while MIL-101(Cr) shows promise for
such spinning, with its good aqueous and chemical
stability.65 Proof of concept sorbent spinning with
microencapsulated phase change material, μPCM, was carried out with
UiO-66 sorbents previously,31 and the methods applied
should be easily generalizable to other solid sorbent systems.
The performance of these two MOF sorbents and zeolite 13X, a common
sorbent considered for post-combustion CO2 capture via
PSA23,66,67, for a sub-ambient PSA was investigated
using dynamic process simulations. To confirm the necessity of the
downstream liquefaction, this was first carried out at feed conditions
consistent with a process without downstream liquefaction and recycle
(14:86 CO2:N2). The Pareto frontiers
calculated for these materials at two temperatures (273 K and 298 K for
13X, and 243 K and 273 K for the MOFs) are reported in Figure S15 with
the corresponding process and fiber parameters are reported in Tables
S15-S20. None of the sorbents considered were capable of reaching the
performance goal of 95% purity and 90% recovery without liquefaction,
so a hybrid system including liquefaction as a secondary purification
step appears to be necessary for these sorbents.
Zeolite 13X (Figure S15) showed superior performance at 298 K than at
273 K. The results at 273 K were unable to reach the required
CO2 product recovery for the PSA of 92% to enable the
required overall system CO2 recovery of 90%, making
this material undesirable for the sub-ambient PSA process. It is worth
noting that at 298 K condition the productivities of the thermally
managed 13X fiber sorbent PSA are expected to be 6-7 times higher than
that of comparable pellet based systems.19,66,68 With
this performance improvement in mind, the flowsheet discussed throughout
the prior sections of the manuscript was reconfigured to enable an
ambient PSA process while still allowing for the removal of water and
liquefaction of the CO2 rich product via Joule-Thompson
expansion. The relevant flowsheet and preliminary economic estimates for
the best-case scenario are reported in the Supporting Information
(S4.4). Stated briefly, zeolite 13X fiber sorbents with thermal
modulation operating at 298 K appears to be economically competitive
with the sub-ambient MOF fiber sorbents, showing somewhat higher cost of
capture of ~$71/tonne CO2. We leave
more in depth analysis of this alternative approach to ambient, elevated
pressure CO2 capture to future considerations, as the
object of this work has been to understand the possibilities and
limitations of the sub-ambient approach. Still, the preliminary analysis
in the Supporting Information leads us to believe there is potential in
the application of Joule-Thompson cooling combined with water removal,
CO2 liquefaction, and PSA processes operating at ambient
temperatures and elevated pressures.