Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2, 1979
275
Absorption of Lean NO, in Aqueous Solutions of NaCIO, and NaOH Eizo Sada" and Hldehiro Kumazawa Departmeni of Chemical Engineering, Nagoya Universiiy, Nagoya, 464, Japan
Ichibei Kudo and Takashi Kondo Department of Industrial Engineering, Aichi Institute of Technology, Toyoda, 470-03, Japan
The absorption of such lean NO, as encountered in flue gases in aqueous mixed solutions of NaCIO, and NaOH was carried out using a stirred vessel with a plane interface at 25 "C and atmospheric pressure. The rate of NO, absorption was analyzed by the chemical absorption theory under the fast-reaction regime. The reaction which prevailed was found to be the parallel reactions involving oxidation and hydrolysis, and to b e second order with respect to NO2. The second-order rate constant for the hydrolysis was evaluated as 3.09 X IO' L/mol s. The order of reaction relative to CI0,- was derived as unity for chlorite concentrations greater than 1.O M. The third-order rate constant for the oxidation was derived as 7.32 X 10' (L/mol)2/sat [NaOH] = 0.20 M. For the absorption of NO, there appears to be a gradual jump in absorption rate at the interfacial concentration of NO ranging from 5 X lo-' to 2 X lo-@mol/L. Above this transition region, the order of reaction in NO approaches 2, whereas below the transition region, the order of reaction becomes unity.
Introduction More than 90% of the nitrogen oxides (NO,) emitted from stationary combustion facilities is nitrogen monoxide. In order to effectively remove NO, by wet scrubbing methods it is desirable to oxidize NO to NOz in either the gas or liquid phase, because the physical solubility of NO in aqueous absorbent is much smaller than that of NO2. With respect to wet scrubbing with liquid-phase oxidation, the alkaline solution of NaC1OZ as well as the alkaline solution of KMn04 has been found to be a practically promising absorbent for NO. Not only from the standpoint of the degree of NO removal but also the chemical absorption mechanism, the process of absorption with liquid-phase oxidation has been assessed and analyzed (Teramoto et al., 1976a, b; Sada et al., 1977). In our previous paper (Sada et al., 1978),the absorption of NO in aqueous mixed solutions of NaC10, and NaOH was performed using a semi-batch stirred vessel with a plane gas-liquid interface at 25 "C and atmospheric pressure. The rate of absorption was analyzed on the basis of chemical absorption theory under the fast-reaction regime. The overall reaction involved was found to be second order in NO and first order in NaC10, in the range of NaC102 concentration greater than 0.8 M. The reaction rate constants evaluated were exponentially decreased with the NaOH concentration and correlated by k = k , exp(3.73[NaOH]). In our previous work, the gas-phase composition of solute gas (NO) was restricted to percent-order of magnitude. The concentration of NO in the flue gases from stationary combustion sources is generally less than several hundred parts per million. Therefore, it is necessary to discuss the absorption mechanism of such lean NO by oxidative absorbents. The present work was undertaken to establish the chemical absorption mechanism of such lean NO as encountered in the flue gases. To this end, the absorption of NO in aqueous mixed solutions of NaC102 and NaOH was carried out using the stirred vessel with a plane *Address correspondence to this author a t the Department of Chemical Engineering, Kyoto University, Kyoto, 606, Japan.
gas-liquid interface. Moreover, to obtain kinetic information on the second step reaction in the reaction between NO and aqueous mixed solution of NaC102 and NaOH 2 N 0 + C1022N02 + C1(1) 4N02 + ClOz- + 40H-
-
-
4N03-
+ C1- + 2H20
(11) the absorption of lean NO2 was performed into the same absorbent. Experimental Section All experimental runs were made using a stirred vessel with a plane gas-liquid interface at 25 "C and 1 atm. The absorber used (i.d., 80 mm; liquid volume, 500 cm3) is the same one as in our previous work (Sada et al., 1978). The absorber was operated continuously with respect to the gas phase and batchwise with respect to the liquid phase. The stirring speeds of liquid-phase and gas-phase stirrers were kept at 162 and 500 rpm, respectively. Solute gas NO or NO2 was supplied frQm cylinder of 1% concentration balanced by N2. Both these gases were further diluted with N2 to the desired concentration. All the inlet and outlet gas-phase compositions were determined by UV derivative spectrophotometer analyzer (Yanaco UO-1 derivative spectrophotometer). The absorption rate was calculated from the difference between inlet and outlet gas-phase compositions and total gas flow rate. Further details for experimental apparatus and procedure can be found elsewhere (Sada et al., 1978). Experimental Results and Discussion NOz-NaC102/NaOH System. The reaction between NO2 and CIOz- in alkaline solution is considered to be 4NO2 + ClOz- + 40H4N03- + C1- + 2H20 (11)
-
and corresponds to the second-step reaction in the reaction of NO with the same liquid reactant. The reaction involved is assumed to be mth order relative to NO2 and nth order relative to C102-. When the process of chemical absorption lies under the fast-reaction regime, the absorption rate of NO2 can be derived as
7d NA2
=
m + l kC"B0
0019-7882/79/1118~0275$01.00/0 0 1979 American Chemical Society
Cm+'AZi DA2
(1)
276
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 2 , 1979
.-
0.1
0.2
a5 I CBO, mol/l.
Figure 2. Effect of [NaC102] on NaC102/NaOH system.
1
l
IO-^
,
:!:::,
CE0.0.20
Cn,,,
M
4
2
NA2/(C3A2jDA2)1'2
for NOz-
c EO = 0.2
mol/l
Figure 1. Relationship between NA2and CA, for N02-NaC102/NaOH system; effect of [NaC102] on NA2.
In Figure 1, the absorption rate of NO2,NAP,is plotted against the interfacial concentration of NO2, CAZI. The interfacial concentration of NO2 can be converted from the physical solubility and the gas-phase concentration at the interface. The physical solubility of NOz in aqueous mixed solutions of NaC10, and NaOH was calculated from the correlation (Onda et al., 1970) 0.1
a2
05
I
2
CBo, m o l /
where KB and KE are the salting-out parameters for the electrolytes B (NaC102) and E (NaOH), respectively, and depend on the ion species and gas present. The contribution of NO2to the salting-out parameter, however, is not available in the literature. Here the contribution of NO2, x , is assumed to be the same as that of NzO. The solubility of NO2 in water was evaluated from the equilibrium concentration reported by Andrew and Hanson (1961). Figure 1shows that there is a linear relationship between log NAz and log CAZ1with a slope of 1.5. The enhancement factors evaluated from the data falling on the straight lines are ranged from 60 to 450 with an increase in the chlorite concentration, whereas the value of CBO/C&, is varied from 2 X IO3 to 4 X lo5. So the data on the straight lines are believed under the fast-reaction regime of pseudo mth order. Therefore, the order of reaction in NO2 is determined as 2. The crosses in this figure denote data with absorption of NO2 in water. The relationship between log NABand log CAz1also gives a straight line with a slope of 1.5 and the order of reaction (hydrolysis) with respect to NO2 is determined as 2. For the hydrolysis of NO2 (2N02 + H 2 0 HN02 + "OB), the second-order rate constant is derived as 3.09 x los L/mol s from eq 1. In addition, it can be seen from the figure that for lower concentration of NaClO,, the hydrolysis predominates over the oxidation. In general, both the hydrolysis and the oxidation are considered to proceed in parallel and thus, eq 1should be rewritten
Figure 3. Effect Of [NaC102]On ~ A , / ( C ~ A , , ~ -A ,(2/3)khrd. )
As a result, Figures 2 and 3 were obtained. From these figures, the following information can be derived. When the chlorite concentration is lower than 0.2 M, the accompanying reaction is dominated by the hydrolysis. In this case, the absorbent contains 0.2 M NaOH, but the neutralization with NaOH can be neglected in comparison with the hydrolysis. Figure 3 indicates that for CROgreater than 1 M, there appears a linear relationship with a slope of unity. Hence for such high concentrations of NaC102, the oxidation is first order with respect to NaC102. The third-order ((2,l)th order) rate constant k can be calculated by eq 3 with an estimate of khyd. The averaged value of k was 7.32 X 10' For the chlorite concentrations ranging from 0.5 to 1.0 M, the dependence of CBO on iVA,/(C3A,iDA ) - (2/3) is remarkable because the oxidative power of cklorite increases with a decrease in pH value in the solution. The influence of alkaline concentration on the rate of absorption was investigated at a constant chlorite concentration equal to 1.0 M. The concentration of NaOH was varied from 0.2 to 1.5 M. Experimental results were shown in Figure 4 as a plot of NA2vs. CAZI.The secondorder dependence on NO2 (that is, slope 1.5) a t different NaOH concentrations, on the one hand, remains unchanged. On the other hand, the apparent absorption rate complicatedly changed with the NaOH concentration. Assuming that the prevailing reaction is regarded as second order in NO2, the effect of NaOH concentration on the NAp = m + l ( h +kc",,) Cm+1~21 D A ~ (3) absorption rate was calculated. As a result, Figure 5 was obtained. For the sake of comparison, experimental data with the N02-NaOH system were plotted in the same On the basis of above experimental evidence, the dependence O f CBOOn NAP/( C 3 ~ 2 1 D ~ and 2 ) 1 'P2A , / ( C 3 ~ 2 1 D ~ 2 )figure. For an NaOH concentration smaller than 0.4 M,
-
dT,
- (2/3)khydwere assessed.
Ind. Eng. Chem. Process Des. Dev., Vol. 18,
No. 2, 1979 277
10-7
IO"
E
Y
z
A
0.25
:
