Capturing CO2 from ambient air - David Keith

Chapter 3
Contactor
In this chapter we propose a form of the contactor modeled after a power plant cooling tower and functioning
similarly to a power plant sulfur-scrubbing tower, and then estimate the cost and energy requirements
of that system. We build a prototype of the contactor which illustrates the feasibility of the process and
assists estimation of the cost and energy requirements of the full-scale analogue.
In the Section 3.1 we first discuss the theoretical modeling and calculations that motivated our contactor
and experimental design, then describe the prototype contactor we developed and methods of its
testing. In Section 3.2 we present our findings from a combination of numerical techniques and experimental
data on (1) the characteristics of CO2 absorption in the contactor, (2) spray droplet collision and
coalescence as it relates to scale-up of prototype results, (3) the energy requirements in the prototype contactor
and scaling to a full-sized system, (4) the quantity of water lost to evaporation in the prototype and a
full-sized system, and (5) an engineering-economic analysis of the cost of a full scale contactor. In Section
3.5, we identify the factors that may significantly affect the cost and feasibility of a NaOH spray-based
contactor positively or negatively. Appendix B, “Experimental Details and Procedure”, supplements the
information in this chapter, giving additional description and documentation of the prototype experiments.
3.1 Materials and Methods
3.1.1 Theoretical methods
To determine the feasibility and inform the design of a NaOH spray system, we applied several theoretical
models to estimate the mass transfer to a drop of NaOH solution falling through air at terminal velocity.
In principle, mass transfer may be limited by gas-phase transport, liquid phase transport, liquid phase
reaction, or a combination. We will find (in Section 3.2) that liquid phase transport and reaction should be
limiting on theoretical grounds, and experimental data confirm this. This section describes the theoretical
models that we applied.
We first assumed gas-phase limitation to mass transfer, so that the flux of CO2 to a drop surface, JCO2 ,
is given by (Seinfeld and Pandis, 1998):
JCO2 = kg(C∞ −Cs) (3.1)
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