Capturing CO2 from ambient air - David Keith

The reactive surface area during trials in the prototype cannot be precisely known, but we can makes estimates
using available information about nozzles, flow rates, and other parameters. Using our experimental
data with these estimates we calculate values of kspray of 1–3×10 −3 . The range is from the low molarity
solution to the high molarity solution, as expected. We may note by comparison with Equation 3.3, that
theoretically:
kspray,theory = C0�
Dlk[OH
C
− ]
Calculated values of kspray using this equation are about twice those calculated from data, reflecting that
absorption in the contactor was about half of what was expected from theory and available drop size
information. The discrepancy may be due to some mechanistic inefficiency in mass transfer, less-thanexpected
drop surface area, or some other effect. We will use the highest empirical value in the cost
calculations.
We approximate S as uniform with height (this is not strictly accurate, but the distribution of S will not
affect the average mass transfer significantly), so that Equation 3.14 can be integrated to yield:
C = Cine −Sksprayt
To get the outlet concentration, Cout, we evaluate this at t = H
vair :
The rate of CO2 capture of the contactor (denoted ˙M) is then:
3.4.2 Capital cost
Cout = Cine −SksprayH/vair (3.15)
˙M = (Cin −Cout) · vair · A = Cin(1 − e −SksprayH/vair ) · vair · A (3.16)
A contactor can be constructed by modest modification of a power plant cooling tower. This may not be
the optimal form of an air capture contactor, but such structures are assured to be possible and we can draw
capital cost estimates from the power industry. Other examples of spray-based reactors exist in industry,
like SO2-scrubbers for power plants, but cooling towers are the largest, most closely approaching the large
scale desired for air capture.
There are two basic types of cooling towers: natural draft and forced draft. Natural draft towers often
have a hyperbolic profile, are constructed of concrete, and can be very tall, as high as 160 m, though
90–120 m is more typical. They make use of the convective forces generated by their shape, height, and
temperature gradient created by the spray to move air through without a fan (hence the “natural” draft).
Structures include a foundation, spray collection basin, pumps and piping, and often particle filtering
(“plume abatement”) mechanisms. The main differences between a conventional cooling tower and a
version used for air capture would be: (1) our liquid flow rate would be smaller by an order of magnitude,
requiring fewer pumps and smaller piping, (2) we will add a fan or bank of fans to control the air flow, and
(3) we will add a high-efficiency particle filter (“demister”).
For simplicity, we suggest a single, large fan essentially identical to a wind turbine without the tower
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