Capturing CO2 from ambient air - David Keith

CO 2 absorption [mmol/pass]
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14
12
10
8
6
4
2
0
Low-flow nozzle
High-flow nozzle
1 10
Initial NaOH concentration [mol/l]
Figure 3.4: CO2 absorption for several solution concentrations of NaOH and two nozzles. Nozzle pressure
is held constant. Dotted lines indicate the theoretical effect of [NaOH] if all else were constant, i.e.,
Equation 3.10 fitted to the first point for each nozzle. Absorption does not rise as quickly with hydroxide
concentration as would be expected in the ideal case. A likely explanation is that the higher viscosity of
more concentrated solutions resulted in larger drops being formed at the nozzle.
drop size as a function of nozzle pressure (Allspray, 2002) to produce the circles in Figure 3.2. Higher
nozzle pressures clearly improved absorption, with a trend and absolute value on the order of the model
prediction discussed above.
With other parameters fixed, changing the solution concentration seemed to have competing effects.
Higher concentration solutions should absorb better due to a faster reaction rate (Equation 3.10), but it
is also known that higher viscosity solutions produce larger drops from typical nozzles (Lefebvre, 1989).
Figure 3.4 shows absorption for 3 different NaOH concentrations. For the low-flow nozzle, absorption
increases with concentration, but not as quickly as would be expected for sprays of constant drop size.
The high flow nozzle actually peaks with the 1.3 M solution, and the 5 M solution absorbs even less than
the 0.33 M solution. The effect of viscosity is highly dependent on nozzle geometry, and this may explain
the discrepancy.
3.2.2 Energy requirements
The energy requirements of operating the contactor consist of mechanical work for pumping solution and
for forcing air flow. Pumps must lift solution from the reservoir at the bottom of the tower to the nozzles at
the top and overcome pressure at the nozzle (friction losses are comparatively small with sufficiently-sized
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