Modelling the absorption of nitrogen oxides by ferrous chelate solution in rotating packed bed

ABSTRACT

With mushrooming growth of coal-based industry, the discharged tail gas containing nitrogen oxides had given significant effect on air quality and people’s health in China. Therefore, more stringent NOx emission standards were proposed. Among many technologies, wet scrubbing technology may be an effective way to solve the above problem, but it was limited by absorption of nitrogen oxides. Therefore, rotating packed bed (RPB) was used to intensify the absorption process of nitrogen oxides by ferrous chelate solution. To speed up its industrialization, a theoretical model was proposed by considering chemical reactions, gas and liquid mass transfer rate, liquid holdup and gas–liquid effective interfacial area. The effects of high gravity factor, gas velocity, gas–liquid ratio, temperature and concentration of ferrous chelate solution on NOx removal efficiency were conducted by model and also compared with experimental results. With high absorption efficiency and small equipment size in RPB, it had a great potential to apply in the absorption other harmful matters.

KEYWORDS:

• RPB
• nitrogen oxides
• removal efficiency
• model

Nomenclature

aGL	=	Gas-liquid interfacial area (m2 m−3)
at	=	Specific area per unit volume of packing (m2 m−3)
Bi	=	Constant
CL	=	Concentration of NOx (mol L−1)
CL,e	=	Equilibrium concentration of NOx (mol L−1)
CL,0	=	Initial concentration of NOx (mol L−1)
CNOx,out	=	Concentration of NOx at outlet (mg m−3)
CNOx,in	=	Concentration of NOx at inlet (mg m−3)
D	=	Diameter of packing (m)
DG	=	Gas diffusivity of NOx in the gas (m2 s−1)
DL	=	Liquid diffusivity of NOx in the liquid(m2 s−1);
dp	=	Effective diameter of packing, 6(1-ε)/ap (m)
△Ei	=	Active energy (kcal kmol−1)
H	=	Henry’s constant (Pa m3 mol−1).
Hi	=	Liquid i (NO, NO2, N2O3, N2O4 and etc) diffusivity (Pa m3 mol−1)
k	=	Constant
kG	=	Gas-side mass transfer coefficient (mol Pa−1 m−2 s−1);
kL	=	Liquid-side mass transfer coefficient (m s−1)
KG	=	Overall gas-liquid mass transfer coefficient (mol Pa−1 m−2 s−1)
NNOx	=	Molar mass transfer flux of NOx at the gas-liquid interface (mol m−2 s−1)
p	=	Pressure (kPa)
Pg	=	Partial pressure of NOx in the liquid phase (kPa)
R	=	Universal gas constant (kJ kmol−1 K−1)
Ri	=	Reaction consumed NOx (mol L−1 s−1)
r	=	Radial axis of the RPB (m)
ro	=	Outer diameter of the RPB (m)
t	=	Reaction time (s)
TG	=	Gas temperature (K)
TL	=	Liquid temperature (K)
U	=	Liquid flow rate per unit area (m s−1)
U0	=	Characteristic flow rate per unit area (=1 cm s−1)
v	=	Kinematic viscosity of the liquid (m2 s−1)
v0	=	Characteristic kinematic viscosity (=10−6 m2 s−1)

Dimensionless number

FrL	=	Liquid Froude number (L2at/gc)
ReL	=	Liquid Reynolds number, ρLQLln(r0/ri)/[2πZB(r0-ri)atμL]
ReG	=	Gas Reynolds number, ρGQGln(r0/ri)/[2πZB(r0-ri)atμG]
Scg	=	Gas Schmidt number (νG/DG)
WeL	=	Liquid Webber number (L2ρL/atδ)

Greek letters

β	=	High gravity factor
εL	=	Liquid hold-up
σL	=	Liquid surface tension (N m−1)’
σt	=	Surface tension (N m−1)
η	=	NOx removal efficiency

Acknowledgements

This work was supported by the Teaching reform and innovation project of Luliang University (JXGG202036), Science and Technology Plan Project in Lu Liang (No. GXZDYF2019085), High Level Scientific and Technological Talents Program in Lu Liang (No. Rc2020-115) and Scientific and Technological Innovation Programs of Higher Education Institution in Shanxi (No. 2020L0708).

Author contributions

Z.Y. Zhao reedited and revised the manuscript.

L. Liu. (lecturer) conducted all the simulation and wrote the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).