Ind. Eng. Chem. Res. 2000, 39, 5003-5011
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Absorption of Nitrogen Oxides in Columns Equipped with Low-Pressure Drops Structured Packings Edoardo Decanini, Giuliano Nardini, and Alessandro Paglianti* Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, I-56126 Pisa, Italy
The absorption of nitrogen oxides was investigated experimentally in a column equipped with HelieR structured packing for different values of operative conditions (NOx inlet concentration, specific gas, and liquid flow rate). The experimental data were compared with the absorption efficiency predicted by three models, the first obtained by the open literature and the others developed in this work. Some conclusions about the relative importance of the different masstransfer mechanisms involving the absorption process were deduced. Introduction The study of absorption into water of mixtures of various nitrogen oxides, which for the sake of the brevity, are named NOx, has attracted considerable interest because this process is fundamental in the production of nitric acid and in other important processes. Furthermore, the removal of NOx from industrial gas streams has received increasing attention because of a stringent body of legislation for air pollution control and reduction: for instance, the abatement of so-called NOx fuel and NOx thermal generated in a burning plant, the scrubbing of gas produced in the recovery of precious metals by treatment with nitric and hydrochloric acid solutions, and the NOx recovery from gas produced in organic nitration. The removal of NOx from industrial gas streams can also be achieved by means of alternative technologies such as selective catalytic reduction (SCR), which employs ammonia, and nonselective thermic reduction (NSTR), which employs gaseous hydrocarbons; these technologies, however, are not as advantageous as absorption. Moreover, the study of NOx absorption is really important also from a theoretical point of view because it is surely one of the most complex absorption process: in fact, the NOx absorption involves numerous masstransfer mechanisms with reactions in both liquid and gas phases. Several authors, who have studied the mechanisms of NO and NO2 absorption in water and in nitric acid solutions at different concentrations, have contributed to the study of the process to clarify its mechanisms (Andrew and Hanson,1 Corriveau and Pigford,2 Dekker et al.,3 Koval and Peters,4 Kramers et al.,5 Lee and Schwartz,6 Lefers and van den Berg,7 Weisweiler and Deiss,8); these authors valued the mass-transfer kinetics between liquid and gas phases of two oxides and of their mixtures. Bodestein9 studied the oxidation in the gas phase of NO to NO2 while Crawford and Counce10 and Komiyama and Inoue11 investigated the decomposition kinetics of HNO2 produced by hydration of the nitrogen oxides absorbed in water; the latter authors also studied * To whom correspondence should be addressed: Department of Chemical Engineering, University of Pisa, Via Diotisalvi n. 2, I-56126 Pisa, Italy. E-mail:
[email protected]. Phone: +39-050-511225. Fax: +39-050-511266.
the transport through the liquid phase of NO produced from HNO2 decomposition. In the literature special attention has also been given to the development of absorption models that allow us to simulate the process in plate columns, in packed columns, and in spray towers (Jethani et al.,12 Ramanand and Phaneswara,13 Suchak et al.,14 Suchak and Joshi,15) developing detailed models to take into account all possible absorptions. Finally, some other authors, such as Counce and Perona16 and Selby and Counce,17 evidenced the importance of the mechanisms of absorption that involves NO2 and N2O4, in comparison to the other mechanisms, giving a simplified model of the process. The aim of the present work has been to study experimentally the process of absorption of NO2 in water working with columns filled with low-pressure drop structured packing (Launaro and Paglianti18). Experimental Apparatus and Procedures The experimental loop used in this work is schematically described in Figure 1. The air has been supplied by a compressor and, before entering a column, has been mixed in a NO2 stream extracted from a cylinder: the NO2 stream has been heated and diluted with technical air to avoid its condensation (Teb ) 21 °C). The air flow rate has been measured with a rotameter and has been regulated with a manual valve while the dioxide flow rate has been measured and controlled, employing a mass flowmeter and controller. In present experiments only NO2 (in equilibrium with its dimer N2O4), mixed with air, has been fed to the absorption column without introducing, since the beginning, others kind of nitrogen oxides. A manual valve (V1) has been set on the pipe outlet to regulate the absolute pressure in the equipment. The gas temperature has been measured with two thermometers on the inlet and the outlet flow lines; also, the relative humidity was measured by means of two hygrometers. The water for absorption has been pumped from the tank D1 by means of a centrifugal pump and has been fed to the column C; the water has been discharged from the bottom after flowing countercurrently through the column with respect to gas. The water flow rate has been measured by means of a rotameter and has been regulated by means of a manual valve, V2. The inlet and outlet liquid temperatures have been measured with two thermometers.
10.1021/ie000270q CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/2000
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Figure 3. Experimental absorption data at various NOx inlet concentrations and for different values of specific liquid flow rates (kg/m2‚h) at constant specific gas flow rate ug ) 0.415 m3/m2‚s.
Figure 1. Experimental loop. Symbols: C, absorption column; D1, D2, storage tank; FC, flow controller; FI, flow indicator; P1, centrifugal pump; P2, fan; PI, pressure indicator; V1, V2, valve.
An analysis of sampling solutions has been done employing an ionic chromatograph liquid-phase DIONEX 2000: the analysis equipment allows one to measure the concentrations of sodium nitrites and nitrates; knowing the solution volume and the sampling time, it has been possible to evaluate nitrogen oxides concentrations and their partial pressures in the gas phase. Therefore, to quantify the absorption for each experiment, the following indices have been estimated:
η)
(in) (out) - pNO pNO x x (in) pNO x
NTU/OG
∫
1 ) Z
(out) pNO x (in) pNO x
0 dpNO x 0 -pNO x
(in) pNO x 1 ) log (out) (1) Z p NOx
Figure 2. Single HelieR structured packing element.
The first index expresses the efficiency of absorption, while the second one, tightly related to the first one, is a modification of the number of transfer unity definition NTUOG because, for the absorption of nitrogen oxides, the current definition is not applicable because the process takes places with different mass-transfer mechanisms.
Table 1. Geometric Characteristics of HelieR Packing
Experimental Work and Results Obtained
property
value
diameter thickness weight specific area void fraction elements per unit volume
1.5 in. 1.2 mm 7.34 g 210 m2/m3 0.936 17511 1/m3
The absorption column has been filled with 25 elements of HelieR structured packing: the overall height of packing has been 0.985 m, and the elements have been connected in the 3 × 3 configuration according to the indications furnished by Launaro and Paglianti.18 A single element of structured packing is represented in Figure 2 while Table 1 shows some geometric details. Measurements of absorption efficiency have been performed using two sampling points on the gas line: each gas sample has been drawn off the sampling point, after being measured, and has been sent to a bubbler containing an absorption solution of 0.1 M NaOH in water: in this way the NOx in the gas has been fixed in solution like sodium nitrites and nitrates.
The experiments of absorption have been conducted at room conditions (P = 1 atm and T ) 293 K) and the specific flow rates of gas and liquid phases have been chosen in the range of values commonly employed in industrial practice, assuming ug ) 0.197-1.26 m3/m2‚ s) and ul ) 4900-39 500 kg/m2‚h. The concentrations of NOx introduced into column have been assumed instead in the range cNOx ) 233-60 676 mg/m3. During the experimental work no significant temperature variations have been measured between inlet and outlet gas and liquid streams for each run so it can be considered that the column operated at isothermal conditions: it can be deduced that heat effects caused by absorption and evaporation inside the column are not significant in the range of operative conditions used in this work. The experimental results are reported in Figures 3 and 4: it is important to point out that the abatement efficiency of nitrogen oxides depends sensitively on the working conditions; particularly, it has been found that the absorption efficiency increases with decreasing specific gas flow rate and increases with increasing both
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All the reactions and all the mass-transfer mechanisms that have been taken into account to describe the whole process of absorption are shown in Figure 5. Mass Transfer between the Phases and Reactions in the Liquid and Gas Phase The molar flow through gas film for a cubic meter of column volume can be written as
Figure 4. Experimental absorption data at various NOx inlet concentrations and for different values of specific gas flow rates (m3/m2‚s) at constant specific liquid flow rate ul ) 19 800 kg/m2‚ h.
Figure 5. NOx absorption mechanisms into water.
the specific liquid flow rate and concentration of nitrogen oxides at the column inlet. The reduction of the abatement efficiency with increasing specific gas flow rate can be attributed to the reduction of contact time between the two phases: in this case this effect prevails on the increase of gas-side mass-transfer coefficients due to higher volumetric specific flow rate. The increase of absorption efficiency with increasing the specific liquid flow rate depends both on the increase of the liquidside mass-transfer coefficients but, mainly, on the increase of the effective interfacial area. Finally, the increase of the abatement efficiency with nitrogen oxides concentration at the column inlet is a characteristic of the absorption process of nitrogen oxides: in fact, increasing the overall nitrogen oxides concentration in the gas phase, the partial pressures of more soluble species, and particularly N2O4, increase (Counce and Perona16). Process Model Developing a model of the process is necessary to introduce some simplifying hypotheses as follows: i. The gas and liquid phases flow countercurrently through the apparatus approaching a plug flow. ii. The liquid holdup is uniform and mass-transfer coefficients are constant throughout the column. iii. The gas follows the ideal gas law. iv. The absorption in the column is isothermal: the raising of temperature owed to the dissolution and the reaction with the water of the chemical species absorbed is neglected; this hypothesis is entirely allowed because of the low concentrations of NOx in present absorption experiments. vi. The column operates at steady state.
JNO,g ) kg,NOa[piNO - poNO]
(2)
o i - pNO ] JNO2,g ) kg,NO2a[pNO 2 2
(3)
o i JN2O4,g ) kg,N2O4a[pN - pN ] 2O4 2O4
(4)
o i JN2O3,g ) kg,N2O3a[pN - pN ] 2O3 2O3
(5)
o i JHNO3,g ) kg,HNO3a[pHNO - pHNO ] 3 3
(6)
o i JHNO2,g ) kg,HNO2a[pHNO - pHNO ] 2 2
(7)
i o - pH ] JH2O,g ) kg,H2Oa[pH 2O 2O
(8)
It is important to point out that NO is desorbed from the liquid phase (see eq 8) contrarily to all other oxides. In fact, in the present experimental work, the gas phase fed to the column is constituted by air mixed only with NO2 (in equilibrium with N2O4) and NO is produced only by the reaction nitrous acid decomposition. In liquid-phase NO2, N2O4 and N2O3 react with water as follows:
2NO2(l) + H2O(l) f HNO2(l) + HNO3(l)
(9)
N2O4(l) + H2O(l) f HNO2(l) + HNO3(l)
(10)
N2O3(l) + H2O(l) f 2HNO2(l)
(11)
while HNO2 and HNO3 are absorbed only physically. The flow of these species through the liquid film can be written as i JNO2,l ) ENO2kl,NO2acNO ) 2
a(HNO2)3/2
x23k
i 3/2 idr,NO2DNO2,l(pNO2)
(reaction order n ) 2) (12)
i JN2O4,l ) EN2O4kl,N2O4acN ) 2O4 i aHN2O4xkidr,N2O4DN2O4,l(pN ) 2O4
(reaction order n ) 1) (13)
JN2O3,l )
i EN2O3kl,N2O3acN 2O3
)
i ) aHN2O3xkidr,N2O3DN2O3,l(pN 2O3
(reaction order n ) 1) (14)
b i JHNO2,l ) kl,HNO2a(cHNO - cHNO ) 2 2
(15)
i b - cHNO ) JHNO3,l ) kl,HNO3a(cHNO 3 3
(16)
JNO,l ) kl,NOa(cbNO - ciNO)
(17)
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Table 2. Kinetic Constants for Hydrolysis Reactions in the Liquid Phase
concentration in the gas and liquid phases that can be expressed by means of Henry’s law as follows:
kidr,NO2 ) 1 × 105 m3/mol‚s (Lee and Schwartz6)
ciNO ) HNOpiNO
HN2O4xDl,N2O4kidr,N2O4 ) 3.9 × 10-5 mol/m2‚s‚Pa (Schifano20)
HN2O3xDl,N2O3kidr,N2O3 ) 1.57 × 10-5 mol/m2‚s‚Pa (Corriveau2)
In the present work the acceleration factors of the equations (12)-(14) have been computed as suggested by Doraiswamy and Sharma,19
Ej )
xΦj = Φ x j tanhxΦj
(18)
xΦj )
i i cN ) HN2O4pN 2O 4 2O4 i i cN ) HN2O3pN 2O 3 2O3 i i cHNO ) HHNO2pHNO 2 2 i i cHNO ) HHNO3pHNO 3 3
where for pseudo n-order reactions is
i pH ) f(T) 2O
x
(n-1)/2 (Hj pij)
i i cNO ) HNO2pNO 2 2
2 D k n + 1 j,l idr,j
kl,j
The approximation in (18) is valid because for the three oxides it can be verified that xΦj > 3 so that tanhxΦj = 1. The kinetic constants used in the present work are shown in Table 2. It is important to point out that it has been supposed that the mass transfer of NO2, N2O4, and N2O3 through liquid film depends only on the concentration, and so on the partial pressure, that it is assumed to be close to the interface, neglecting possible presence inside the liquid phase. To verify that this hypothesis is correct, it is enough to evaluate the equilibrium partial pressures of three oxides when the concentration of nitric and nitrous acids in the liquid phase is the same as that in experimental work. The concentration of two acids in the liquid phase is known by chromatographic analysis and the equilibrium partial pressures of three oxides can be evaluated by means of constants of heterogeneous equilibrium for reactions of the formation of nitric and nitrous acids in the liquid phase [(9)-(11)] (see Joshi et al.21): the resulting equilibrium pressures are
