Conclusions

A framework for CO2 capture utilizing extensive flue gas pretreatment and CO2 post-treatment focusing on sub-ambient operating temperatures and elevated operating pressures with a post-separation liquefaction column has been considered. As expected, the most significant effect on the performance of the capture system energetics and economics (not accounting for the separation unit) comes from the compression of the flue gas and the efficiencies of the rotating equipment. Pressure swing adsorption (PSA) processes show promise for the first separator in the system, as the cost of the PSA unit tends to be controlled primarily by the productivity of the PSA system, and operating at sub-ambient conditions may enable significantly higher productivity with appropriate sorbent selection. The addition of the liquefaction column downstream of the PSA unit allows for sorbents and operating conditions to be considered that would otherwise be eliminated on account of their inability to reach the purities required by pipeline specification.
Structured contactors are an important option for the management of pressure drop and thermal effects, which would otherwise adversely affect performance of the PSA unit. Sorbents considered for traditional CO2 capture via PSA at room temperature such as zeolite 13X may be used in this process, although in the case of 13X performance is actually reduced when operating at sub-ambient conditions. Two MOF sorbents, MIL-101(Cr) and UiO-66, were considered for application as thermally managed fibers in the sub-ambient PSA and showed costs of capture as low as $61/tonneCO2. The tradeoff between PSA product purity and recovery proved to be the key economic parameter of the system once high productivities were reached, as the downstream liquefaction process becomes costly from a capital and energy perspective when high recoveries are combined with low purities.
Our analysis shows there are viable paths to sub-ambient hybrid CO2 capture processes, but high CO2capture productivities (>0.015 mol kg-1s-1 at >75% CO2 purity and 92% CO2 recovery) are required to make the sub-ambient hybrid separation process economically competitive with other alternatives. Our models of sub-ambient PSA shows that sorbent materials with the potential to show enhanced capacities at the desired high-pressure and low-temperature conditions would show the most improvement, while sorbents like zeolite 13X, which shows more potential at traditional operating conditions, may prove incapable of reaching the require purities and recoveries within the bounds of feasible vacuum levels.
Present Address: Department of Materials Process Engineering, Nagoya University, Nagoya, Japan