Capturing CO2 from ambient air - David Keith

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fraction of spray [μm -1 ] 0.014 0.012 0.01 0.008 0.006 0.004 0.002 average distribution (number) narrow distribution (number) narrow distribution (mass) average distribution (mass) 0 0 50 100 150 200 250 300 350 400 450 500 550 600 drop diameter [μm] Figure 3.6: Size distribution of sprays used in coalescence modeling. Both distributions are log-normal with a volume-mean diameter of 150 µm. The “average” distribution has σ = 1.6 and the “narrow” distribution has σ = 1.4. By running the model in a CFSTR mode, we can account for the smaller residence time of larger drops. The largest drops fall quickly enough that they have a residence time of about a fifth of the average size input drops, so this change should have a significant effect. In this mode, the model is seeded with the initial spray distribution as in the earlier calculations. Then at each time step, drops are removed from each size bin according to the settling velocity of that size and the air flow rate through the contactor. Drops are then added from the spray distribution according to the liquid flow rate. The contactor height is assumed to be 120 m and vair is 2 m/s so that the overall change rate is 0.017 s −1 . The model is then run to steady state (about 2 minutes). The total surface area of the steady state distribution, S, represents the average efficiency of the contactor. It turns out that moving to the CFSTR mode moves S from 0.3 to 0.57 m 2 /m 3 -contactor – a significant improvement, but still only 15% of what would be expected in the absence of coalescence. Next we add a provision to the code for spontaneous breakup. We can approximate the process well by assuming that every drop larger than a diameter of 5.1 mm breaks apart (Pruppacher and Klett, 1997). Thus, at each time step the mass of drops in the three largest size bins is removed and the fragments are redistributed according an empirical size table given by Pruppacher and Klett (1997, p. 415). The third modification is to adjust the collection kernel so that it does not count breakup as coalescence. A full treatment of collisional breakup would require substantial rewriting of the core solution mechanism in the model, and is beyond the scope of this research, but we can make adjustments to the kernel to prevent some of the coalescence we know should not be occurring. In principle we would like to multiply the kernel by Ecoal, and we would end up with about half as many coalescence events overall. Ecoal, however 30
surface area [m 2 -surface / m 3 -reactor] 4 3 2 1 full-scale spray density prototype spray density 0 0 10 20 30 reaction time [s] 40 50 60 Figure 3.7: Total surface area of a parcel of spray over time in the contactor. Surface area decreases dramatically due to drops colliding and coalescing. Results are for the “average” input distribution in 3.6 with (1) prototype spray density, (1.5 × 10−5 m3spray m3reactor ) and (2) a larger spray density (7.5 × 10−5 ) more desirable for a full-scale system. Typical target residence times for the spray in full size contactor are 20–50 s. 31
Page 1 and 2: Capturing CO2 from ambient air: a f
Page 3 and 4: For my family, friends, and everyon
Page 5 and 6: etween the National Science Foundat
Page 7 and 8: Contents Acknowledgments iv Abstrac
Page 9 and 10: List of Figures 1.1 Global CO2 emis
Page 11 and 12: Chapter 1 Introduction The climate
Page 13 and 14: egeneration at a high enough concen
Page 15 and 16: 1.4 Routes to air capture 1.4.1 Org
Page 17 and 18: The NaOH approach is the primary su
Page 19 and 20: there is no compelling reason to ca
Page 21 and 22: 7Na2CO3(l)+5Na2Ti3O7(s) ⇋ 4Na8Ti5
Page 23 and 24: changing the process design to acco
Page 25 and 26: Component / source GJ/t-CO2 electri
Page 27 and 28: Chapter 3 Contactor In this chapter
Page 29 and 30: the CO2 capture rate in a spray sys
Page 31 and 32: CO 2 absorption per pass [mol/l] 10
Page 33 and 34: CO 2 absorption [mmol/pass] 16 14 1
Page 35 and 36: Figure 3.5: Water loss measured in
Page 37 and 38: 3.3.1 CO2 depletion in air With lon
Page 39: where vt,1 and vt,2 are the termina
Page 43 and 44: spray mass density [kg / ln(μm)] 1
Page 45 and 46: average surface area [m 2 -spray /
Page 47 and 48: As a bound on the effects of coales
Page 49 and 50: Draft type Height [m] Crosssectiona
Page 51 and 52: 2. Median assumptions, no coalescen
Page 53 and 54: capture system. We can also see tha
Page 55 and 56: Multistage spray One possible solut
Page 57 and 58: Although, the cost of water loss ma
Page 59 and 60: 3.5.6 Solids formation - scaling an
Page 61 and 62: Chapter 4 Cost of air capture Altho
Page 63 and 64: Capital Capacity Electric a Thermal
Page 65 and 66: Component Capital + O&M Energy cost
Page 67 and 68: Chapter 5 Discussion Overview In th
Page 69 and 70: 5.2 Lessons for assessing of future
Page 71 and 72: Bibliography Adams, P. J. and Seinf
Page 73 and 74: IPCC (2001). Climate Change 2001: M
Page 75 and 76: Tzivion, S., Feingold, G., and Levi
Page 77 and 78: F Liquid flow rate in the contactor
Page 79 and 80: Appendix B Experimental details and
Page 81 and 82: Figure B.2: Photograph of completed
Page 83 and 84: Figure B.4: Two cranes lift the rea
Page 85 and 86: fastened together and lined, the re
Page 87 and 88: The prototype has three particle fi
Page 89 and 90: Figure B.7: Sampling points of CO2
Page 91 and 92:
In order to calculate CO2 absorbed,
Page 93 and 94:
Absorbed CO 2 [mol/l] 0.3 0.25 0.2
Page 95:
periodic switching of the spray on
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Capturing CO2 from ambient air - David Keith

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