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