Capturing CO2 from ambient air - David Keith

The prototype has three particle filters. Process air leaves the main body of the prototype through four
4x8 in rectangular openings in the top of the donut, spaced evenly around the perimeter. The primary
particle trap is housed in each opening. Most of the volume of entrained liquid is caught here and drips
down to the cone to join the bulk of the return flow. This filter is considered analogous in function to the
particle trap that would be required in a full-scale system.
Past the primary particle trap, air enters 8 in diameter flexible ducts attached to each opening and
then a custom-built filter box. The filter box houses two additional filters for safety reasons. The first
is a standard fabric home furnace filter which acts as an inexpensive pre-filter. The second is a microglass
media, mini-pleat duct filter rated for > 95% removal of PM2.5 (Filter Group catalog # 40102). Air
leaving the filter box travels through a pair of ducts to the blower, which vents to the room. Considering
the efficiencies of the particle traps and a conservative estimate for the concentration of entrained fine
particles, air leaving the blower should have caustic particle content well below (by perhaps an order of
magnitude) OSHA standards.
B.1.4 Liquid handling
The design goal of the liquid system provides the necessary flowrate of NaOH solution to the nozzle at
sufficient and adjustable pressure, allowing measurement of the flowrate and pressure at the nozzle. Also,
since CO2 absorption by the portion of fluid hitting the walls of the reaction chamber is not counted in
most calculations, the flow on the walls should be measurable so that it can be subtracted out.
A schematic of the liquid system is shown in Figure B.6. For most trials, the liquid handling system
was arranged as shown in a simple loop with a reservoir at the bottom, so that the working solution is
recirculated. Reservoir residence times were 3–15 minutes. Most of the system is comprised of 3/8 in
nylon tubing with stainless steel Swagelok valves and fittings. Additional branches of the system (not
shown) allowed for in-line sample collection, rinsing, and solution transfer between reservoirs.
A pump was required to provide up to 6 L/min of flow at a total head of up to about 650 kPa (lifting
6 m, a target maximum nozzle pressure of 550 kPa, and frictional losses). A centrifugal pump was first
tried. Though it was rated for far higher flows, it could not reach pressures at the nozzle above 300 kPa.
A rotary-vane pump was then used and achieved higher pressures. However, pump performance was not
consistent, and with the high-flow nozzle, the highest pressure reliably achieved was 380 kPa. This limited
the range of pressures tested on the high-flow nozzle, although the pump was rated for more than 800 kPa.
Air entrained in solution seemed to cause vibration and reduced performance. Measures such as arranging
the return flow line to minimize splashing in the reservoir improved this.
Eight different nozzles were tested for use in the prototype. Nozzles were visually observed spraying
water from a height of 3 m, and seven of the eight were also characterized with a Malvern laser diffraction
spray analyzer. The two with the smallest mean drop size were selected for use in experiments. Both
nozzles have full-cone spray patterns.
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