Capturing CO2 from ambient air - David Keith

Multistage spray
One possible solution to the coalescence problem is to add the spray in multiple stages along the tower,
as is common in cooling towers. This has the effect of refreshing the spray distribution as it falls (and
as drops from further up coalesce and become less efficient). The energetics are in many respects similar
to a collection of short tower systems with height equal to the height of a stage. Except in this case the
air is passed from one stage to the next so that the pressure drop across the particle trap need only be
overcome once for all stages. That is, the problem of high ˙E f an is solved compared with a short tower
system. A drawback is that solution must be pumped to the upper stages even though it only participates
in absorption for a fraction of the total height, thus ˙Eli fting can be larger in a multistage system compared
with an equivalent collection of short towers. Perhaps a structure which collects spray between each stage
can be included so that solution is mostly only lifted the height of one stage. In any case, the problem of
high ˙Enozzle is shared with short tower systems. A multistage spray system would require a low ΔPnozzle.
Upward flow
In forced-draft cooling towers, air flow is typically from bottom to top with spray nozzles at the bottom.
This increases residence time of the drops since gravitational settling works against the flow rather than
with it, (flow in our prototype was downward). With this design, the sign of vt in Equation 3.6 changes to
give a longer drop residence time of
τup = H
.
vair − vt
Following equation 3.13, we get a higher spray density for the same F or, alternately, we require a smaller
F for the same spray density. For an idealized case with monodisperse spray and no coalescence, we can
calculate the reduction in F and thus the (proportional) reduction in ˙Eli fting and ˙Enozzle by switching to an
upward draft system. Rearranging Equation 3.13 and substituting in τup, we have:
Fdown − Fup
Fdown
= 2vt
.
vair + vt
This is the fractional change in flowrate between upward and downward flow systems with all else equal. It
would translate to the same fractional reduction in pumping energy. Assuming vt = 0.6 m/s (D = 150µm)
and vair = 2 m/s, we would have an increase in drop residence time of 86%, reducing pumping energy by
46%. This is, however, an idealized case. With growing drop size (and growing vt) due to coalescence, the
change in τ and reduction in ˙Eli fting between an upward flow and downward flow system would be more
modest. No simple relationship can be described.
With upward flow there can also be a drop size sorting effect, where the largest drops settle faster
than vair, leaving smaller drops to continue upward. It is not clear how much this would mitigate the
coalescence problem but sorting the largest drops out of the initial distribution would seem to help.
It appears that upward flow would provide at least modest improvement to the energetics of the contactor.
It was not considered in the cost scenarios largely out of practical concerns about having the particle
trap at the top of the structure, placing the fan in the path of the spray, and about applicability of prototype
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