Capturing CO2 from ambient air - David Keith

Appendix B
Experimental details and procedure
This section documents many experimental decisions and design details related to the construction and
testing of the prototype contactor. It is meant to allow future interested researchers to reproduce or critique
the techniques.
B.1 Physical apparatus
B.1.1 Basic size and structural design
In setting out to build a prototype contactor based on NaOH drops, the nature of drop generation was
central to determining the overall form. Though there are various means of generating small drops at very
small flow rates (electrostatic augmentation of flow from an orifice, sonic disruption of falling streams or
sheets), we reasoned that any full-scale contactor would rely on spray nozzles. To realistically simulate the
conditions in the full-scale system, the prototype would need to accommodate a spray nozzle. Additionally,
since the prototype is meant to simulate the average conditions in a very wide, very tall tower, edge
effects should be minimized. That is, distortions to the rate of mass transfer caused by liquid on the walls,
transient spray characteristics (escape velocity and angle from the nozzle), and the collection mechanism
at the bottom should be minimized. Without significant edge effects, the prototype results can more easily
be compared with theory and more justifiably scaled up to a full size contactor.
So the use of a spray nozzle introduces several primary constraints on the structural form of the prototype:
(1) The tower should be wide enough that most of the spray falls through without hitting the walls,
(2) the length of the fall should be long enough that initial transient conditions as fluid leaves the nozzle
(escape angle and velocity) are relatively unimportant compared with time drops spend falling at terminal
velocity in a uniform pattern, and (3) since some spray inevitably hits the walls, this portion of the flow
should be measurable for analytical reasons.
Though nozzles with narrow (15-30 ◦ ) spray patterns are available, wider patterns are much more common,
starting with 60 ◦ patterns (both of the nozzles ultimately used for data collection were 60 ◦ nozzles).
All told, we seemed to require a tower diameter of at least 1 m and fall distance of several meters. Additionally,
smooth, straight walls would aid measurement of flow hitting the walls.
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