Capturing CO2 from ambient air - David Keith

B.1.5 Measurement
The primary goal of the experiments was to calculate pumping energy, fan energy, and water loss per unit
CO2 captured. Accordingly, the quantities we meant to measure were:
1. CO2 uptake
2. liquid flow rate
3. liquid line pressure
4. air flow rate
5. air pressure drops
6. inlet temperature
7. inlet humidity
8. outlet temperature
9. outlet humidity.
CO2 uptake was primarily measured by real-time monitoring of the CO2 concentration in air, alternately
at the inlet and outlet of the reaction chamber. Measurement points are shown in Figure B.7. Air was
pumped continuously from the end of a 1/4 in PE tube inserted at the measuring point through a LiCOR
infrared CO2 analyzer. A computer connected to the LiCOR recorded the CO2 reading in ppm every
second. This yields a (∼ 3 second delayed) time series of CO2 concentration at the sampling point.
Sampling background at the tower inlet was straightforward. Some concerns, however, were raised about
whether outlet concentration at any given sample point is representative of the average rate of CO2 capture.
Hence, six different points at the outlet were tried. Sampling directly in the reaction chamber proved
problematic as drops would get pulled into the sampling tube and bias the measurement. Sampling in the
donut appeared to be the most reliable approach. Agreement between the two sampling points and among
different depths of tube insertion (5, 10, and 20 cm) was good (the differences were not distinguishable
from noise). Agreement between the donut and duct measurements was also good. Measurements from
the box and blower tended to run higher (indicating less CO2 absorption) than donut measurements in
some trials. This is probably explained by dilution of the process air with room air through leaks in the
box and duct connections. Overall, the data used in final calculations appear to be valid representations
of average outlet concentration. As a double-check on the LiCOR readings, the quantity of CO2 absorbed
into solution for some trials was measured also with a Total Organic Carbon analyzer so that results could
be compared.
Total liquid flow rate was measured with an inline digital turbine flowmeter for some trials. When
this was not available, total flow was estimated from manufacture specifications for the nozzle at the
given pressure. Flow of the active spray was taken to be the difference between total flow and dribbler
flow. Dribbler flow was measured for some trials by diverting the flow to a 2 L graduated cylinder for
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