Capturing CO2 from ambient air - David Keith

the CO2 capture rate in a spray system.
The equations presented so far address mass transport of CO2. In principle, numerical methods may
be used to estimate evaporative water loss in a contactor as well. As discussed in Section 3.2, it turns
out that Equation 3.1 is not the correct model for CO2, which appears to instead follow Equation 3.3
(CO2mass transfer is liquid-side limited). However, Equation 3.1 can be applied to the evaporation of
water (Seinfeld and Pandis, 1998). If we define RH2O/CO2 as the molar ratio of water evaporated to CO2
captured, then we can estimate it by the ratio of mass transfer equations:
RH2O/CO2 = kg(C∞,H2O −Cs,H2O)
�
Dlk[OH− ]
C0
The calculation is complicated because the vapor pressure of water at the drop surface, Cs,H2O, is a
function of drop temperature, [OH − ], and [CO 2−
3 ] (the concentration of CO2 already absorbed in mol/L).
For a sense of scale, we plug in initial conditions for temperature, relative humidity, and apply other
reasonable parameters. Equation 3.7 then evaluates to the order of 10 3 , which is quite large (though it is
clearly an overestimate because it does not account for changing temperature and other dynamic effects).
It is large enough to suggest that the air leaving a full-scale contactor will be saturated with water vapor.
Thus we can estimate evaporative water loss by assuming the air leaving the contactor has water partial
pressure equal to the vapor pressure of water at the surface of the drops.
For simplicity we assume that air and liquid reach the bottom of the contactor at the same temperature,
so that an energy balance yields the temperature at the outlet:
Tout = Tin − ΔCH2OΔHvap
ρaircp,air + ρlcl
where mvap is the mass of water evaporated per units contactor volume, cp,air and cl are the heat capacities
of air and the liquid solution (assumed equivalent to water), and ρair and ρl are the bulk densities of air
and suspended solution. The quantity of water evaporated, ΔCH2O, is the difference between inlet water
concentration and outlet vapor pressure of the solution:
ΔCH2O = Cs,H2O(T = Tout) −CH2O,in
Equations 3.8 and 3.9 can be solved simultaneously by iteration to yield ΔCH2O. Assuming an overall
capture efficiency for the contactor of CO2 from air allows a calculation of RH2O/CO2 . The results of this
calculation for reasonable conditions in the contactor are presented in the next section.
3.1.2 Experimental methods
We constructed a prototype contactor in order to demonstrate the feasibility of CO2 capture by NaOH
spray and to allow us to measure the energy requirements in a way such that the results could be scaled up
to a full size contactor. Details of the design choices and experimental procedure are given in Appendix
B along with additional photographs of the structure. We required (1) a tower diameter large enough to
19
(3.7)
(3.8)
(3.9)