Modeling and Simulations of NOx and SO2 Seawater Scrubbing in Packed-Bed Columns for Marine Applications
Round 1
Reviewer 1 Report
The authors developed a comprehensive sound model for describing the scrubbing performance of an absorption column including inclination, heaving and oscillation. It is well presented and also validated as far as feasible and therefore deserves publication. Due to the complexity of the model is was unfortunately not possible to check it thoroughly within a reasonable amount of time, but since the model seems to covers aspects and since all results do not show any weird things and I therefore quite confident the model is sound.
Some minor comments.
Line 192: it seems that 'negatively' should be 'positively' (i.e. rate increases at higher T).
Line 208: is k4,5 equal to k1 in Line 187 ? If not, please explain how to get k4,5 from k1.
Line 294: Should the Hj be heat of absorption (in water) instead of molar enthalpy as explained in the notation section ?
Figure 3 and 4: It would be more convenient for the reader if the x-axis is reversed, resulting in a decrease of the CO2 when going to the right (which matches with the intuitive logic of CO2 absorption).
Figure 12, 13: Are the dips around R=-0.16 and +0.16 m realistic for this packing or is it likely the result of e.g. poor convergence (or another artefact) ? Please briefly comment on this in the paper. Also the small dip at R=+0.01m with 15 deg. inclination rises some concern.
Line 534: Please make clear that this describes only the first seconds of the simulation (by explicitly pointing at the first seconds in Fig. 19a). Otherwise, the reader gets confused with the idea that the liquid velocity in the column is the lowest when the scrubber is at its highest position (in the heaving motion), which is explained correctly in Lines 539 and 540.
Author Response
Reviewer 1:
Line 192: it seems that 'negatively' should be 'positively' (i.e. rate increases at higher T).
Agree. The text was updated.
Line 208: is k4,5 equal to k1 in Line 187 ? If not, please explain how to get k4,5 from k1.
k4,5 is the pseudo first-order reaction rate constant for N2O4 hydrolysis (Reaction 19).
Action taken. In Notation section this reaction rate constant was redefined.
Line 294: Should the Hj be heat of absorption (in water) instead of molar enthalpy as explained in the notation section?
is the molar enthalpy of species j in a-phase.
Figure 3 and 4: It would be more convenient for the reader if the x-axis is reversed, resulting in a decrease of the CO2 when going to the right (which matches with the intuitive logic of CO2 absorption).
Figures 3 and 4 are correct.
Figure 12, 13: Are the dips around R=-0.16 and +0.16 m realistic for this packing or is it likely the result of e.g. poor convergence (or another artefact) ? Please briefly comment on this in the paper. Also the small dip at R=+0.01m with 15 deg. inclination rises some concern.
The dips in Figures 12, 13 are the result of both graphic representation and inadequate convergence in the wall region.
Line 534: Please make clear that this describes only the first seconds of the simulation (by explicitly pointing at the first seconds in Fig. 19a). Otherwise, the reader gets confused with the idea that the liquid velocity in the column is the lowest when the scrubber is at its highest position (in the heaving motion), which is explained correctly in Lines 539 and 540.
Does not agree. The waves in two-phase countercurrent flow are developed along the entire period of heaving motion. Interstitial liquid axial velocity decreases and becomes smaller than its value in the static state preceding the heaving motion when the scrubber moves up. When scrubber moves down the local liquid axial velocity increases and become higher than their values in the vertical scrubber static state.
Reviewer 2 Report
The paper deals with NOx and SOx seawater scrubbing. This technology provides substantial advantages, and the present paper deals with an interesting and useful topic for the scientific community. I simply recommend the following minor modifications:
- The literature review must be extended with more papers about NOx and SOx scrubbing using seawater.
- Explain why turbulence effects can be omitted.
- Diffusivity coefficients are not provided. At least their sources should be indicated.
- Indicate possible reasons of the discrepancies between numerical and experimental results indicated in Fig. 2.
Author Response
Reviewer 2:
The literature review must be extended with more papers about NOx and SOx scrubbing using seawater.
Agree. The literature review was extended with 2 paper about SO2 scrubbing in seawater.
Explain why turbulence effects can be omitted.
Effective viscosities, integrating viscous and pseudo-turbulence stress tensors, were formulated using the simple model suggested by Dankworth et al. [35].
Diffusivity coefficients are not provided. At least their sources should be indicated.
H+ and HSO3- molecular diffusion coefficients in seawater were taken from Hikita et al. [36] and Chang and Rochelle [32]. The additional molecular diffusion coefficients were estimated via Wilke-Chang method. (pag. 45 – Word document)
Indicate possible reasons of the discrepancies between numerical and experimental results indicated in Fig. 2.
The discrepancies between the numerical and experimental results indicated in Figure 2 can be generated by the approximated estimation of the mass transfer enhancement factor via the surface renewal model and H+ and HSO3- molecular diffusion coefficients in seawater. However, theoretical SO2 removal efficiency matches reasonably well the experimental SO2 removal efficiency generated at different gas and liquid flow rates and SO2 concentrations.
