Capturing CO2 from ambient air - David Keith

Figure 3.13: Simple diagram of a horizontal flow contactor
data, which was collected for a downward flow system.
Horizontal air flow
Another design variation that may reduce contactor cost is a system with horizontal air flow. A diagram is
shown in Figure 3.13. Here H would be relatively short, perhaps 20 m, but L may be much longer, perhaps
100 m. A fan would likely be placed at the inlet and a particle trap at the outlet. Spray nozzles would be
spaced along the length, leaving some distance at the end for drops to settle before reaching the outlet.
The energetics are similar to a short tower system but with two advantages. The first advantage is that
the residence time of the spray is longer compared with a vertical system, given simply by τ = H . Longer
vt
τ for a given height reduces pumping energy as described in the previous section. The other advantage is
that a new parameter is introduced, L, which determines the residence time of the air independent of the
residence time of the spray. Thus L can be adjusted to keep ˙E f an small. The system shares the drawback
with short towers of high ˙Enozzle, though this is somewhat dampened by the effect of longer residence time.
A low ΔPnozzle would be required.
A horizontal system may offer reduced capital cost compared with a vertical system. Although the
area of wall per unit contactor volume required is larger than for a very large cylindrical cooling tower,
the walls bear a much lighter load and so less material may be required overall. On the other hand, more
land area and a sealed roof would be required. It is not obvious how the capital costs would compare to an
equivalent vertical system.
3.5.3 Water loss
We have measured and calculated a rate of water loss that in volume terms is quite high. We have discussed
how the rate of water loss can be managed with high NaOH concentrations, but it may be desirable for
other reasons to run at lower NaOH concentrations. In that case, water loss is highly dependent on the
meteorology of the site. Paying for water at a rate typical of power plant cooling towers does not raise
overall costs significantly, however in a world where air capture is widely deployed, the demand for
water would be large on the scale of developed use, and could upset already overburdened water systems.
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