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.