Capturing CO2 from ambient air - David Keith

data on, coalescence and breakup.
This chapter has explored spray-based contactors as opposed to packed or wetted-surface contactors.
This approach was chosen because the simplicity and similarity to industrial cooling towers of the example
design allowed for relatively simple cost estimates. Additionally, the success of large cooling towers offers
a convincing proof-of-concept for large spray-based contactors. However, some of the experimental data
suggest a filled or wetted-surface contactor would be a better approach. In the prototype, about half of the
CO2 absorption was by the wetted walls of the contactor. This is not surprising, considering that the inside
wall area of the prototype was comparable to the calculated total surface area of suspended spray. But
since the walls received only 10–20% of the liquid flow, CO2 absorption there was much more efficient
with respect to pumping energy. This is a general feature of packed or wetted-surface contactors: liquid
pumping energy is lower than spray-based contactors and tends to be unimportant compared with fan
energy. The geometry of the contactor, if extended to a full scale height, would yield a low air pressure
drop. However, just as in the spray case, we would want a higher rate of CO2 absorption per unit crosssectional
area in order to have low capital cost per unit CO2. In a spray system, we proposed to do this
by increasing liquid flowrate and thus spray density. In a wetted surface system, we would do this by
adding walls or fill material. This, in turn, would impede the flow of air and increase the air pressure drop.
The challenge in designing a wetted surface contactor is to maintain both a low air pressure drop and low
capital cost per unit CO2 absorbed. We recommend further investigation of wetted-surface contactors.
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