https://doi.org/10.1016/j.egypro.2017.03.1289
“The first step in evaluating the hybrid intercooling design was to perform a detailed analysis of the performance
compared to the recycle design for a specific set of conditions from the range outlined above with the goal of
illustrating the benefits of the hybrid design and providing an explanation of the benefits.
Specifically, the case defined by 90% CO2 removal with a lean loading of 0.27 mol CO2/mol alkalinity was selected
because, as detailed in Figure 3 and the associated discussion, the baseline recycle intercooling design (with DCC)
approaches the isothermal limit of solvent capacity. Therefore, for the hybrid intercooling design to be attractive at
this condition, it must provide benefits beyond solvent capacity. The results of this comparison are summarized in
Table 2.”
“The hybrid intercooling design provides a significant packing reduction (~40%) over the recycle design, indicating
superior mass transfer performance. Both configurations benefit from high intensity contacting with the operating
conditions selected to provide similar liquid loads, or maximum L/G, in each case (see Equation 5). However, the
hybrid design enhances intercooling performance by providing multiple points of intercooling throughout the
absorption process (i.e., more continuous intercooling method) and also provides a more countercurrent contacting
design.
To illustrate the intercooling benefits of the hybrid design, Figure 7 compares the solvent temperature in the recycle
intercooling configuration and the 2×3 hybrid contacting configuration.”
“As Figure 7 shows, the average temperature of the solvent contacting each of the 3 gas paths in the hybrid design
(see Figure 6) is lower than the temperatures in the recycle intercooled absorber for most of the mass transfer area in
the contactors. The recycle intercooling design only has one point of solvent cooling, and has countercurrent sections
where the L/G is low, so the performance of the design is strongly influenced by the CO 2 transferred in the recycle
section, and therefore, strongly dependent on the recycle rate. By contrast, the same maximum L/G ratio in the hybrid
contactor is much more effective due to the multiple intercooling points and the fact that the entire contactor can
operate with the higher L/G while providing a better approximation of countercurrent contacting.
To further illustrate the benefit of the hybrid design, Figure 8 presents the CO2 removal by section in the hybrid
contactor.”
“Figure 8 highlights a general pattern associated with the hybrid configuration – the two rows of beds and the liquid
flow path ensure the CO2 removed in any gas flow path is balanced. For example, gas flow path A contacts the richest
solvent (bottom left bed) and the leanest solvent (top left bed) in the configuration. The CO2 removal rate follows this
solvent loading pattern. Each of the 3 gas flow paths experiences similar CO 2 removal rates. The balancing of CO2
removal across rows allows the hybrid design to approach countercurrent contacting. Additional rows of beds would
improve the countercurrent nature of the configuration. The figure also confirms that the introduction of hot flue gas
does not have a significant detrimental effect on CO2 removal performance of the bottom beds. T he highest CO2
removal occurs in a bottom bed (bottom right). The combination of intercooling and large solvent rate mitigate the
effects of the hot flue gas. This explains why a 1×3 hybrid design can be operated without a DCC as well.
The intercooling duties in Table 2 are similar, as both designs are removing the heat of CO2 absorption and cooling
the incoming flue gas (DCC duty included in the recycle case). The hybrid design does not remove heat generated in
the final liquid pass (bottom, left bed in Figure 5). Since this rich solvent also cools incoming gas performing the DCC
function, this may limit the solvent capacity to some extent compared to a design with a DCC.
The pressure drop in the hybrid intercooling design is also significantly lower than in the recycle design. The overall
gas flow path length is reduced for the hybrid design: the gas only passes through 16 meters of packing (2 x 8 m
sections) rather than the 24 m of packing in the recycle design.
Finally, the pump power is slightly higher for the hybrid design, but this is negligible compared to the benefits
outlined above, and the pumping power for the DCC has not been included for the recycle design, which may offset
the advantage in Table 2.”