Capturing CO2 from ambient air - David Keith

fastened together and lined, the reaction chamber was lifted by crane and placed onto the frame, a sparse
truss-like structure of 1x3 in (nominal) pine boards (see Figure B.5). The donut, also suspended from the
frame, is plywood and pine-board frame with 1/8 in PVC interior walls. The cone was constructed of two
custom-cut PVC sheets folded into shape and fixed together with stainless steel bolts and sealant. The very
tip of the cone is a standard polyethylene funnel fitted with a flexible 1 in diameter tube by hose clamp.
The bottom of the reaction chamber is positioned 4 in below the top of the donut so that air leaving
the reaction chamber is forced to make a sharp U-turn and travel upwards a short distance, shedding most
of the entrained drops, before entering the particle trap. The cone is set at a 45 ◦ angle with respect to
the horizontal to minimize splashing (and creation of fine mist which may bias the rate of CO2 uptake)
at the bottom of the reaction chamber. Also, since the flow lines of process air pass into the donut above
the cone, we reason that edge effects as the spray hits the bottom will have a negligible impact on mass
transfer.
A lip on the bottom of the inside wall of the reaction chamber collects solution running down the walls
and channels it to a separate return-flow tube that exits through a hold in the cone. The lip is a 1 in diameter
flexible tube cut in half to form an open-top channel, and slightly sloped so that collected fluid drains to a
single dedicated return-flow tube attached to the bottom of the channel. The separate return flow for fluid
hitting the walls allows for measurement of the relative fraction of spray hitting the walls as opposed to
remaining as drops through the reaction chamber.
B.1.2 Materials compatibility
Since the working solution would be strong caustic (up to 20 wt%, pH 14.7), we required materials that
would stand up to caustic for several cumulative days of running time. Fortunately, most plastics have
excellent caustic compatibility, including the most common and inexpensive varieties, like polyvinyl chloride
(PVC) and polyethylene (PE). Most of the wetted surfaces in the prototype were constructed of PVC.
Portable PE gasoline cans served as convenient solution reservoirs and storage containers. Most tubing
was PE or nylon, and fittings were stainless steel Swagelok. The wire mesh particle trap was constructed
of stainless steel wool and the spray nozzles and some miscellaneous fasteners were also stainless steel.
A chemical-resistant pump head and an off-the-shelf silicon-based sealant completes the list of wetted
materials (the sealant was not explicitly resistant to caustic but it was tested and did not visually degrade
after several days of submersion).
B.1.3 Air handling and air safety
As in a full-scale contactor, the prototype has a forced-air system which is meant to move air uniformly
through the tower co-current with the spray and employs a particle trap to keep a significant fraction of the
working solution (in the form of very fine droplets) from leaving the system with the outlet air. See Figure
B.6 for a diagram. The prototype had the additional constraint that experimenters working and breathing
close to the prototype should not be exposed to hazardous levels of caustic particles. OSHA standards for
airborne concentration of caustic solution were used as a guide.
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