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Ind. Eng. Chem. Res. 1998, 37, 4418-4423
Effect of Temperature on NOx Absorption into Nitric Acid Solutions Containing Hydrogen Peroxide D. Thomas* and J. Vanderschuren Department of Chemical Engineering, Faculte´ Polytechnique de Mons, B 7000 Mons, Belgium
A mathematical model previously developed by us for the absorption of NOx into nitric acid solutions containing hydrogen peroxide at 20 °C was adapted to take the effect of temperature into account. It was used to determine at 10 and 30 °C the overall kinetic parameters relative to the absorption of the different NOx species, for increasing HNO3 molarities (up to 2 M) and a low concentration of H2O2 (0.02 M), from test runs performed in a small packed column. The interpretation of the experimental results obtained at 10 and 30 °C according to the model confirmed the previous findings: hydrolysis is the main controlling step for tetravalent nitrogen oxides, and nitrous acid is likely to contribute for the most part to the absorption of trivalent species. Introduction The removal of nitrogen oxides (NOx) by means of a wet technique using aqueous solutions containing an oxidizing agent such as hydrogen peroxide appears to be very engaging, as it results in the production of valuable HNO3, without generating any other polluting byproducts. This abatement technique can be achieved to treat nitrous flue gases arising from industrial sources such as nitric acid towers, organic nitration reactors, catalysts calcinators, metals and minerals processing units, etc. (Deo, 1988; Buck et al., 1991; McIntyre and Wyborn, 1997). Some important effects due to the addition of hydrogen peroxide to the scrubbing solution were presented and discussed in a previous paper (Thomas et al., 1996). A mathematical model for packed tower simulation (Thomas, 1996; Thomas and Vanderschuren, 1997) was also developed, based on the two-film theory of absorption with chemical reaction for the three NOx species NO2, N2O4, and N2O3*, a fictitious component grouping the trivalent components N2O3 and HNO2. The model includes the kinetics of all the liquid-phase reactions involved in the form of overall kinetic parameters (OKPs) (see definitions in Table 2) that do not contain the liquid mass-transfer coefficients and are therefore independent of the hydrodynamic conditions. Numerous experiments conducted at 20 °C in a laboratory column led to the determination of the OKP values relative to the various NOx species (Thomas and Vanderschuren, 1997). These values and the experimental results showed that the dissolution of NO2 and N2O4 is likely to be controlled by hydrolysis and the absorption of N2O3* is regularly growing with the acidity of the solution due to the important contribution of gaseous HNO2, which undergoes fast oxidation with H2O2, enhanced by H+ ions, during dissolution. The model developed has worked with fair success in predicting the NOx removal efficiency of a pilot-scale column (Thomas and Vanderschuren, 1996). Nitrous flue gases can be emitted in various temperature ranges in industrial practice, extending from ambient conditions for some reactors to more than 30 °C for nitric acid towers. Moreover, it is well-known that temperature has a marked effect on NOx absorp-
tion performance, especially for the manufacture of HNO3. Therefore, the aim of this paper is to investigate and get a broad understanding of the influence of temperature on the absorption efficiency of NOx in the presence of H2O2. Studies at temperatures of 10 and 30 °C are reported here. Experimental Method The absorption apparatus, including a small glass laboratory column of inside diameter 0.045 m and height 0.455 m, packed with 10-mm glass Raschig rings, and the experimental procedure have been described previously (Thomas et al., 1996). For the present study, the liquid temperature was controlled by means of a heat exchanger operated countercurrently with glycol or water supplied from a thermostatic bath and placed in the feed line. The absorption column was furthermore insulated with a 20mm-thick sheet of polyurethane foam. The liquid and gas temperatures at both ends of the column were checked by thermometers. Taking into account the heat of absorption, as very small quantities of NOx are absorbed, the increase of the temperature always remains less than 0.1 °C. Flow rates were maintained at constant values for all the experiments, 0.175 m3/(s m2) for the gas and 5.34 × 10-3 m3/(s m2) for the liquid, respectively. The gaseous mixture containing NOx was analyzed by chemiluminescence at the inlet and the outlet, giving the total NOx and nitric oxide contents. Absorption of dilute NOx up to partial pressures of 550 Pa was performed into aqueous and nitric acid solutions containing a small, but in excess, concentration of H2O2 (0.02 M) at atmospheric pressure and temperatures of 10 and 30 ( 0.5 °C. The absorption rate was measured for a large range of gas-phase compositions (total partial pressures and oxidation ratios of NOx in the gas at the inlet of the column) and different concentrations of HNO3 (0-2 M) in the scrubbing liquid. Effect of Temperature in the Model The mathematical model, previously developed (Thomas and Vanderschuren, 1997), is based on the theory
10.1021/ie980118c CCC: $15.00 © 1998 American Chemical Society Published on Web 09/24/1998
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4419
of absorption accompanied by fast irreversible hydrolysis reactions, with Hatta numbers greater than 3, second order with respect to NO2 and first order with respect to N2O4 and N2O3. The following expressions for the liquid-phase mass-transfer rates were then used:
RNO2 )
x
2 3
k2NO DNO2 2
HNO23 Rj )
piNO 1.5 × 2
xk1 Dj p j
ij
Hj
j ) N2O3*, N2O4 (1)
The absorption of NO is given by the expression
RNO ) kLNOciNO
(2)
with a zero bulk concentration, due to the presence of H2O2. To take into account the effect of temperature in the model, the following modifications have to be introduced: (1) The operating temperature has a negative influence on the kinetic constant of NO oxidation with oxygen (constant k1) and on the gas-phase equilibrium constants (K2 and K3) used to compute the partial pressures of the NOx species from given values of the chemical nitric oxide and nitrogen dioxide contents. (2) The effective interfacial area of the packing was considered to vary with the temperature and the HNO3 concentration as a function of the kinematic viscosity of the liquid (Rizzuti et al., 1981); an increased temperature results in a smaller value of a. (3) The physical solubility of NO increases (the Henry constant, HNO, decreases) with temperature. All these temperature functions are summarized in Table 1. Relations 3, 4, 5, and 7 are taken from Joshi et al. (1985). (4) The liquid-phase mass-transfer coefficients vary through the power -0.5 of the liquid-phase Schmidt number.
Figure 1. Effect of the oxidation ratio and of the nitric acid concentration on the fractional absorption of NOx: (pNOx)in ) 500 Pa. (a, top) t ) 10 °C; (b, bottom) t ) 30 °C.
of 500 Pa and different nitric acid solutions containing 0.02 mol/L of hydrogen peroxide. It can be observed that the NOx oxidation ratio in the gas-phase OR has a pronounced favorable influence on A. This effect can be explained by the occurrence in the gas of the most oxidized NOx species, more soluble and more reactive after dissolution into water. Moreover, in both cases, the values of the fractional absorption of NOx are enhanced markedly by the acidity (0.5-2 kmol/m3), especially for intermediate oxidation ratios. Actually, in this range of OR, the trivalent NOx species (N2O3 and HNO2) form maximum concentrations in the gas phase and absorption of HNO2 is enhanced by the fast and H+-catalyzed reaction with hydrogen peroxide. The effect of the total partial pressure of NOx in the feed gas (0-550 Pa) is also confirmed, as illustrated in
Results The effects of the experimental conditions are quite similar to those obtained at 20 °C (Thomas and Vanderschuren, 1997). The NOx absorption efficiencies measured at 10 and 30 °C are shown in parts a and b of Figure 1, respectively, for a total NOx partial pressure Table 1. Functions of Temperature Used in the Model rate constant 2NO + O2 f 2NO2
k1
log k1 )
652 - 4.7356 (kPa-2 s-1) T
(3)
log K2 )
2993 - 11.232 (kPa-1) T
(4)
log K3 )
2072 - 9.240 (kPa-1) T
(5)
equilibrium constants 2NO2 T N2O4
K2 )
NO + NO2 T N2O3
K3 )
pN2O4 pNO22 pN2O3 pNOpNO2
interfacial area
a
aHNO3(T) ) aH2O(293)
Henry constant of NO
HNO
log HNO )
(
νHNO3(T)
νH2O(293)
)
0.7
with aH2O(293) ) 114 m-1
1463.3 - 0.178 (kPa m3/kmol) T
(6)
(7)
4420 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Figure 4. Influence of temperature on the OKP values.
Figure 2. Influence on the NOx absorption efficiency of the NOx inlet partial pressure, at different nitric acid molarities: OR ) 90-95%. (a, top) t ) 10 °C; (b, bottom) t ) 30 °C.
at temperatures of 10 and 30 °C, using the same procedure and mathematical treatment of experimental results as for 20 °C (Thomas and Vanderschuren, 1997). The results are given in parts a and b of Figure 3, which are plots of the OKP values against nitric acid concentrations, for 10 and 30 °C, respectively. Grouping the three series of OKP data, the following conclusions can be drawn for all temperatures investigated: (1) The OKPs of tetravalent oxides NO2 and N2O4 (NOxIV) remain quite constant with the acidity, which means that the rate of dissolution of these NOx species is controlled by hydrolysis, not influenced by the oxidation rate of HNO2. (2) The factor relative to N2O3* is regularly increasing with the acidity due to the important contribution of gaseous HNO2 which undergoes during dissolution a fast oxidation with hydrogen peroxide, enhanced by the HNO3 molarity. It was thought to be desirable to develop Arrheniustype correlations for OKPs in the ranges, covered in our work, of 10-30 °C for the temperature and 0-2 kmol/ m3 for the nitric acid concentration of the scrubbing solution. The following relationships plotted in Figure 4 were found to hold (Thomas, 1996):
ln OKPNO2 ) -7.156 -
1847 (kmol/(m2 s kPa1.5)) (8) T
ln OKPN2O4 ) -2.183 -
2609 (kmol/(m2 s kPa)) (9) T
3328.2 + 1.8988c T 0.7642c2 + 0.101 22c3 (kmol/(m2s kPa)) (10)
ln OKPN2O3* ) 3.104 -
Figure 3. Variation of the OKP values with the HNO3 concentration of the scrubbing solution. (a, top) t ) 10 °C; (b, bottom) t ) 30 °C.
Figure 2, for an oxidation ratio of 90-95% and various nitric acid concentrations. The OKP values relative to the various NOx species were determined from numerous experiments conducted in the laboratory column operated continuously in a countercurrent way, with aqueous and nitric acid solutions (0.5-2 kmol/m3) containing a small excess of H2O2,
It can be seen that the overall activation energies in the process are positive and increasing in the species order NO2, N2O4, and N2O3*. Table 2 compares for NO2 and N2O4 the mean values of OKP determined in this work with those obtained by different authors for NOxIV absorption into water. The same orders of magnitude confirm that the absorption phenomenon of tetravalent NOx in the presence of H2O2 appears to be controlled by the hydrolysis of the species NO2 and N2O4. The table also includes, for these two species, the Hatta numbers derived from the values of kL and of the kinetic constants k estimated from the determined OKPs. Moreover, Hatta numbers of 234 (10 °C) and 353 (30 °C) were found for the absorption of N2O3* into aqueous solutions containing H2O2, and these values are obviously increasing with the acidity
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4421 Table 2. Comparison of the Overall Kinetic Parameters for Tetravalent NOx ref Takeuchi et al., 1977 Sada et al., 1979 Komiyama and Inoue, 1980 Thomas and Vanderschuren, 1997 this work
ref
type of absorbera
t, °C 25 25 15 20 10 30
Hoftyzer and Kwanten, 1972 Kameoka and Pigford, 1977 Komiyama and Inoue, 1980 Weisweiler and DeiB, 1987 Suchak et al., 1991 Pirkl et al., 1993 Thomas and Vanderschuren, 1997 this work
25 25 15 25 30 25 20 10 30
HaNO2c
10-7
27.7 × 22.6 × 10-7 8.39 × 10-7 15.4 × 10-7 11.7 × 10-7 17.0 × 10-7
BC SC BC PC PC PC t, °C
OKPNO2,b kmol/(m2 s kPa1.5)
type of absorbera LJA SA BC LJA SC SC PC PC PC
>3 if piNO2 > 4.7 Pa >3 if piNO2 > 9.8 Pa >3 if piNO2 > 3.6 Pa
OKPN2O4,d kmol/(m2 s kPa)
HaN2O4e
10-6
9.2 × 6.8 × 10-6 14 × 10-6 4.7 × 10-6 7.6 × 10-6 7.2 × 10-5 14.2 × 10-6 11.5 × 10-6 21.1 × 10-6
12 6.7 22.3
a BC, bubble column; SC, stirred cell; PC, packed column; LJA, laminar jet absorber; SA, spherical absorber. b OKP 2 NO2 ) [( /3k2NO2DNO2)/ HNO23]1/2. c HaNO2 ) (2/3k2NO2DNO2ciNO2)1/2/kLNO2. d OKPN2O4 ) (k1N2O4DN2O4)1/2/HN2O4. e HaN2O4 ) (k1N2O4DN2O4)1/2/kLN2O4.
Figure 5. Effect of temperature on the absorption efficiency of NOx into aqueous solutions containing hydrogen peroxide: (pNOx)in ) 500 Pa.
Figure 6. Variation with temperature of the absorption rates of NOx into nitric acid solutions (1 M) containing H2O2: (pNOx)in ) 250 Pa.
of the scrubbing solution. Thus, for the whole range of experimental conditions, the Hatta numbers greater than 3, for all components, allow the confirmation that the reactions between NOx and H2O2 are fast, according to the assumption made in the model. As to the gas-phase mass-transfer resistance of which the contribution to the overall mass-transfer resistance was evaluated for the total NOx by the ratio (pNOx piNOx)/pNOx, its remains always nonlimitative and lower than 13% for all experiments.
On one hand, for NO2 and N2O4, the increase of OKP values corresponds essentially to the enhancement with temperature of the kinetics of the hydrolysis reactions in the liquid phase. Similarly to tetravalent NOx species, one may assume that the hydrolysis of dinitrogen trioxide (N2O3), which is the rate-controlling step for that species, is enhanced with temperature too. The increasing OKP value of N2O3* is likely to be due, moreover, to the effect of temperature on the oxidation kinetics of HNO2 by H2O2. On the other hand, two negative influences of a rising temperature on NOx absorption performance can be mentioned: First, the value of the effective interfacial area is reduced, due to the decrease of the kinematic viscosity (relation 6 in Table 1); the value of a decreases by 23% as the temperature rises from 10 to 30 °C. Second, the gas-phase equilibria lead to lower contents in the gas of N2O3 and N2O4 (see Figure 7) which are the more soluble and reactive NOx species. One can see, nevertheless, that the partial pressures of NO and NO2 increase only scarcely. It appears that the increase of the OKP values is not sufficient to counterbalance these two effects which lower NOx absorption rates. Figure 8 illustrates the influences of the oxidation ratio and temperature on the calculated contributions of the various NOx species to the overall absorption into
Discussion of the Effect of Temperature Actually, it can be seen in Figure 5 that, for a total partial pressure of 500 Pa and in the whole range of oxidation ratios (2-95%), the fractional absorption of NOx into aqueous solutions containing hydrogen peroxide decreases as the temperature increases. This effect is also observed in Figure 6 for other operating conditions and can be confirmed from comparison between parts a and b of Figure 1 presented above. This negative effect of temperature is also well-known in the absorption of NOx into water, for the manufacture of HNO3. The discussion needs to be focused on the explanation of this negative effect of temperature because OKP values for all NOx species were found to rise appreciably with temperature.
4422 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Figure 7. Influence of temperature (10, 20, and 30 °C) on the gas-phase equilibrium concentrations of the different NOx species: (pNOx)in ) 500 Pa.
Figure 9. Predicted outlet NOx partial pressures versus the height of packing: 25-mm polypropylene Pall rings.
pressure differences as obtained at the ends of all elementary volumes of the column. It is worth noticing that, due to their high solubility and reactivity, the compounds which exist in lower concentrations in the gaseous phase (N2O4 and N2O3) take part substantially in the overall absorption; the contribution of N2O4 is found to increase with OR, while the contribution of N2O3* reaches a maximum value at intermediate OR. As to the influence of temperature, it appears that the contribution of NO2 is increasing, while the contributions relative to N2O3* and N2O4 are decreasing with a temperature rise. For these last two species, the shift effect of the gas-phase equilibria due to temperature is therefore predominant, whereas for NO2, the major influence remains of a kinetic type. The model can be used to predict the influence of temperature in the design of absorption columns, namely, the height of the packing material. Examples illustrated in Figure 9 are relative to the absorption of NOx into a nitric acid solution (2 M) containing hydrogen peroxide, for constant liquid and gas flow rates. Simulations are achieved for various NOx inlet partial pressures and oxidation ratios and a column packed with 25-mm polypropylene Pall rings. In all cases, a decrease in absorption temperature (from 30 to 10 °C) results in lower (25% or more) heights of the packing material required to reach the same performance. Conclusions
Figure 8. Calculated contributions to the absorption of NOx species into aqueous solutions containing H2O2 as a function of the oxidation ratio: (pNOx)in ) 500 Pa. (a, top) NO2; (b, middle) N2O4; (c, bottom) N2O3*.
aqueous solutions containing H2O2, for a total NOx partial pressure of 500 Pa. These were computed from the local individual fluxes by summing up the partial
NOx absorption into aqueous and nitric acid solutions containing H2O2 was studied at 10 and 30 °C in a small packed laboratory column. The mathematical model previously developed by us for absorption at 20 °C was adapted and used to determine the overall kinetic parameters for the absorption of different NOx species as functions of temperature. Taking into account the catalytic effect due to HNO3 on the NOx absorption in the presence of H2O2, the variation of these parameters with the HNO3 concentration was studied up to 2 M. The interpretation of the experimental results obtained at 10 and 30 °C according to the model was similar to that made previously for 20 °C. It confirmed that hydrolysis is the main controlling step for tetravalent nitrogen oxides and that, among the trivalent species, nitrous acid is likely to play a major role. The model allows us to predict packed column performance as functions of the height of the packing, for various operating conditions (acidity of the liquid phase,
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4423
superficial flow rates, etc.), and, given the present study, for different temperatures. Acknowledgment Dr D. Thomas is grateful to the FNRS (National Fund for Scientific Research of Belgium) for a grant of Aspirant. Nomenclature a: effective gas-liquid specific interfacial area, m-1 A: overall NOx absorption efficiency ) [(pNOx)in - (pNOx)out]/ (pNOx)in c: nitric acid concentration of the solution, kmol m-3 D: liquid-phase diffusivity, m2 s-1 G: volumetric gas flow rate, m3 s-1 H: Henry’s constant, m3 Pa kmol-1 Ha: Hatta number k1: first-order rate constant, s-1 k2: second-order rate constant, m3 kmol s-1 kL: liquid-phase mass-transfer coefficient, m s-1 L: liquid flow rate, m3 s-1 M: molarity of nitric acid solution, kmol m-3 NO*: nitric oxide: pNO* ) pNO + pN2O3* NO2*: nitrogen dioxide: pNO2* ) pNO2 + 2pN2O4 + pN2O3* N2O3*: fictitious trivalent NOx: pN2O3* ) pN2O3 + 0.5pHNO2 OKP: overall kinetic parameter (see Table 2 for definition) OR: experimental oxidation ratio of NOx in the inlet gas ) pNO2*/pNOx p: bulk gas partial pressure, Pa R: absorption rate, kmol m-2 s-1 T: temperature, K Subscript in/out: inlet/outlet of the column
Literature Cited Buck, M.; Clucas, J.; McDonogh, C.; Wood, S. NOx removal in the stainless steel pickling industry with hydrogen peroxide. U. S. Chem. Oxid. 1991 (Proceedings International Symposium), 78. Deo, P. V. The use of hydrogen peroxide for the control of air pollution. Chem. Prot. Environ. 1988, 34, 275. Hoftyzer, P. J.; Kwanten, J. G. Absorption of nitrous gases. In Gas Purification Processes for Air Pollution Control, 2nd ed.; Nonhebel, G., Ed.; Newnes: London, 1972; p 164.
Joshi, J. B.; Mahajani, V. V.; Juvekar, V. A. Absorption of NOx gases. Chem. Eng. Commun. 1985, 33, 1. Kameoka, Y.; Pigford, R. L. Absorption of nitrogen dioxide into water, sulfuric acid, sodium hydroxide, and alkaline sodium sulfite aqueous solutions. Ind. Eng. Chem. Fundam. 1977, 16 (1), 163. Komiyama, H.; Inoue, H. Absorption of nitrogen oxides into water. Chem. Eng. Sci. 1980, 35, 154. McIntyre, G.; Wyborn, P. J. NOx Process Improvement and Pollution Prevention with Hydrogen Peroxide, Communication A16. In Comptes-Rendus des Journe´ es d’Etudes: Epuration des Effluents Gazeux Industriels; Vanderschuren, J., Bruxelmane, M., Thomas, D., Eds.; FPMs: 1997; pp 13-15. Pirkl, H. G.; Nussel, C.; Hofmann, H. Kinetic der Absorption von Stickstoff(IV)-oxid bei kleinen Partialdrucken. Chem. Ing. Tech. 1993, 65 (11), 1350. Rizzuti, L.; Augugliaro, V.; Cascio, G. The influence of the liquid viscosity on the effective interfacial area in packed columns. Chem. Eng. Sci. 1981, 36, 973. Sada, E.; Kumazawa, H.; Butt, M. A. Single and simultaneous absorptions of lean SO2 and NO2 into aqueous slurries of Ca(OH)2 or Mg(OH)2 particles. J. Chem. Eng. Jpn. 1979, 12 (2), 111. Suchak, N. J.; Jethani, K. R.; Joshi, J. B. Modeling and simulation of NOx absorption in pilot-scale packed columns. AIChE J. 1991, 37 (3), 323. Takeuchi, H.; Ando, M.; Kizawa, N. Absorption of nitrogen oxides in aqueous sodium sulfite and bisulfite solutions. Ind. Eng. Chem. Process. Des. Dev. 1977, 16 (3), 303. Thomas, D. Absorption des oxydes d′azote dans des solutions contenant des agents oxydantssApplication au peroxyde d′hydroge`ne. Ph.D. Thesis, Faculte´ Polytechnique de Mons, March 1996. Thomas, D.; Vanderschuren, J. The absorption-oxidation of NOx with hydrogen peroxide for the treatment of tail gases. Chem. Eng. Sci. 1996, 51 (11), 2649. Thomas, D.; Vanderschuren, J. Modeling of NOx absorption into nitric acid solutions containing hydrogen peroxide. Ind. Eng. Chem. Res. 1997, 36 (8), 3315. Thomas, D.; Brohez, S.; Vanderschuren, J. Absorption of dilute NOx into nitric acid solutions containing hydrogen peroxide. Trans. Inst. Chem. Eng., Part B 1996, 74, 52. Weisweiler, W.; DeiB, K. H. Influence of electrolytes on the absorption of nitrogen oxide components N2O4 and N2O3 in aqueous absorbents. Chem. Eng. Technol. 1987, 10, 131.
Received for review February 24, 1998 Revised manuscript received July 24, 1998 Accepted August 2, 1998 IE980118C
