Capturing CO2 from ambient air - David Keith

spray mass density [kg / ln(μm)]
0.02
0.015
0.01
0.005
t=0
t=1s
0
10 100 1000 10000
drop diameter [μm]
Figure 3.8: Size distributions over time for model calculation in Figure 3.7 (full scale density). Drops
rapidly converge on the largest size bin (D = 1cm).
is a very complex, empirical function which is difficult to code precisely. Instead, we will approximate
it by breaking the collision space (diameter of first drop by diameter of second drop) into 5 regions, and
applying a constant which is the approximate average Ecoal for each region.
The results of the CFSTR model with spontaneous breakup are presented in Figure 3.9 for the adjusted
and unadjusted collection kernel. The steady-state distribution is bimodal, reflecting the small drops of
the input spray combined with the aggregated drops which approach the largest stable size. The sharp
downturn the edge of the steady state distributions is at the largest stable size. Addition of spontaneous
breakup had a negligible impact on steady state surface area. This may be because the largest drops have
about the same collection efficiency as the 0.5–2 mm size drops they break into when accounting for the
difference in residence time (1 mm drops collide with less frequency but stay in the contactor longer).
Adjustment of the kernel has an effect, but perhaps not as large as would be expected: S is increased by a
little over 20%. Even a blanket reduction of the collection kernel by 0.5 (halving the collision rate) only
increases S by 45%, to 0.84 m 2 /m 3 (case not shown).
Parameter optimization
Given that coalescence appears to be a very serious problem, we consider contactor design parameters that
may mitigate its effects. Clearly, if coalescence is as bad as the previous results suggest, a dense spray
from the top of a 120 m contactor is not the optimal solution. To explore this issue further, we continue
with the CFSTR model with spontaneous breakup and adjusted collection kernel, varying several contactor
parameters.
32
t=4s
t=16s