Capturing CO2 from ambient air - David Keith

average surface area [m 2 -spray / m 3 reactor]
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
no-coalescence prediction
coalescence model
0 0.1 0.2 0.3 0.4 0.5
spray flow rate [L-solution / (s · m 2 0
cross-section)]
Figure 3.10: Spray surface area as a function of liquid flow rate in a hypothetical contactor. The deviation
from a no-coalescence prediction increases with flow rate (i.e. spray density). Likewise, higher efficiencies
(reactive surface area per unit liquid) are achieved at lower flow rates. Assumed contactor height is 120 m
and air flow velocity is 2 m/s.
Perhaps the most obvious parameter to adjust is the spread of the spray distribution. However, moving
from the average to the narrow distribution (see Figure 3.6) has no significant effect on surface area. This
may be an artifact of the model. Since it is “well-mixed”, large drops are distributed evenly throughout
the volume. With the bi-model distribution in Figure 3.9, the width of the smaller mode may not matter
much since most collection is probably due to drops in the large mode striking drops in the small mode. If
the model were resolved with height, we would see the spray coalescing more slowly near the top of the
tower, and bringing the average surface area up. This behavior was noticed in the Lagrangian model. The
narrow distribution is used in the following calculations although, again, it doesn’t seem to matter.
The way the curves cross in Figure 3.7 suggests there may be an optimal liquid flowrate for maximum
surface area. Figure 3.10 shows S as a function of flowrate. With this model, a peak is not obtained,
but the diminishing returns are obvious. In the absence of coalescence, we would expect S to increase
proportionally to F.
Contactor height also has an effect on spray density. Without height resolution, the model only partly
captures this effect through the larger change rates for shorter contactors. The results are shown in Figure
3.11. The effect is substantial, though only impractically short towers begin to approach the nocoalescence
efficiency.
The last parameter we will vary in the model is mean drop size. Modest changes can be achieved with
choice of nozzle, but dramatically smaller drops are only possible by moving to an air-assist nozzle. We
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