ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS IN TWIN SPRAY COLUMNS TSUNG WEN CHIEN, HSIN CHU∗ and YARN-CHI LI Department of Environmental Engineering, National Cheng Kung University, Tainan 701, Taiwan (∗ author for correspondence, e-mail:
[email protected], Tel: 6 2080108, Fax: 6 2752790)
(Received 3 August 2004; accepted 4 May 2005)
Abstract. NO X is a major pollutant that causes acid deposition and photochemical smog. A large amount of NO X is emitted from combustion sources such as power plants. As some articles have indicated, NO X removal with NaClO2 solution in an absorption column is an effective control approach. In this approach, first insoluble NO is oxidized into water-soluble NO2 under acidic conditions, then NO2 is removed in alkaline NaClO2 solutions. The results indicate that the reaction rate constant is 9.1 × 104 (L)4.4 /cm6 /s/mol2.4 in the first absorption column with acidic conditions, and the reaction orders with respect to NO and NaClO2 are 1.4 and 2, respectively. In the second absorption column with alkaline conditions, the reaction rate constant is 3.2 × 107 (L)5.2 /cm6 /s/mol3.2 and the reaction orders with respect to NO and NaClO2 are 1.6 and 2.5, respectively. The activation energies in the first and second absorption column are 71.8 and 139.6 kJ/mol, respectively. Keywords: absorption, flue gas, nitrogen oxides, sodium chlorite, spray column
Notation The following symbols are used in this paper: C = concentration in liquid phase (mol l−1 ) D = diffusivity in liquid phase (cm2 s−1 ) D = diffusivity in gas phase (cm2 s−1 ) E = enhancement factor kG = gas-side mass transfer coefficient (mol s−1 cm−2 atm−1 ) k L = liquid-side mass transfer coefficient (cm s−1 ) N = absorption rate (mol s−1 cm−2 ) P = partial pressure (atm) PT = operating pressure of the system (atm) PW = saturated vapor pressure of water at operating temp. (atm) Subscripts A = dissolved gas A (NO) B = liquid-phase reactant B (NaClO2 ) E = liquid-phase reactant E (NaOH) i = gas–liquid interface w = water 0 = initial value Water, Air, and Soil Pollution (2005) 166: 237–250
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1. Introduction To reduce the amount of SO2 and NO X emitted from the stationary combustion sources, several dry, wet and bio-treatment processes have been discussed (Chou and Lin, 2000; Livengood and Markussen, 1994; Mochida et al., 2000) and implemented. The major dry processes for NO X removal include low NO X burners, overfire air, reburning, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), non-selective catalytic reduction (NSCR) and adsorption. Bioprocesses for air pollution control are categorized as bioscrubber, biofilter, and biotrickling filter systems. The major wet processes for NO X removal are gas phase oxidation followed by absorption, absorption with liquid phase reduction, and absorption with liquid phase oxidation. Several liquid absorbents including FeSO4 /H2 SO4 , Fe(II)EDTA, Na2 S/NaOH, Na2 S2 O4 /NaOH, H2 O2 , Na2 SO3 , FeSO4 /Na2 SO3 , Urea, NaOH, Na2 CO3 (Jethani et al., 1990; Joshi et al., 1985), KMnO4 /NaOH (Chu et al., 1998, 2001a), NaClO2 /NaOH (Chien and Chu, 2000; Chien et al., 2001, 2002; Chu et al., 2001b, 2003), P4 (Lee and Chang, 1992) and molybdenum blue solution (Zhao et al., 1998) have been tested for NO X absorption. Although NO could be removed from flue gases effectively, the P4 process has remained at a demonstration stage by the safety concerns of yellow phosphorus. Molybdenum blue solution can also remove NO2 from sour gases effectively, but conversion of NO is slow. Among these absorbents, NaClO2 was the most effective reagent. From the study of Chien (2000), the costs of NO X wet scrubbing by P4 , H2 O2 , and NaClO2 were 2.1, 2.8, and 3.0 US$ /kg NO X removal, respectively. The absorption of NO X in NaClO2 solution was studied by Teramoto et al. (1976) and Sada et al. (1978a, b, 1979). These studies investigated NO absorption kinetics in mixed alkaline aqueous solutions of NaClO2 and NaOH, using a semibatch agitated vessel with a flat gas–liquid interface. More recently, Brogren et al. (1998) and Hsu et al. (1998) performed similar tests with alkaline NaClO2 and NaOH solutions by using a packed column and stirred tank, respectively. The above literature is related to the absorption rate of NO and/or SO2 in solutions with higher concentrations of NaClO2 and NaOH. The absorption of NO X on a packed column has been carried out by Chan (1991), Brogren et al. (1998), and Hsu et al. (1998). Yang et al. (1996, 1998) performed similar studies on a bubble column, spray scrubber, and packed column. A summary of the previous studies on NO X absorption kinetics in NaClO2 /NaOH solutions is listed in Table I. The operating pH value of traditional wet flue gas desulfurization system (FGD) is between 4–6, but most of the above investigations are strongly alkaline. The aim of this work is to investigate the reaction kinetics of individual and simultaneous removal of lean SO2 and NO in an aqueous solution of acidic sodium chlorite using bench-scale twin spraying columns.
Sada et al. (1978a) Sada et al. (1978b) Sada et al. (1979) Brogren (1998)
200–1000 300–800
8,000–15,000 5,000–75,000 1,500–150,000 290
Ref.
Hsu et al. (1998) This work for 1st column This work for 2nd column
pN O (ppm)
0.05–1.0 0.02
0.2–1.5 0.29–1.64 0.25–2.0 0.1–1.0
[NaClO2 ] (M) 0.05–0.5 0.015 0.2–1.5 – – – – – 0–2.0
[NaOH] (M) – – – 8 9 10 11 9.2–9.9 5
pH 25 25 25 20 20 20 20 30 25–70
Temp. (◦ C) 2 2 1 1.766 1.665 1.553 1.346 2 1.4 1.6
Reaction order of NO (m) 1 1 1 0.693 0.603 0.668 0.908 1 2 2.5
Reaction order of NaClO2 (n)
TABLE I The relevant kinetic studies of NO absorption with NaClO2 solutions
3.8 × 1012 exp(−3.73[NaOH]) 2.1 × 1012 7.32 × 108 1.55 × 106 1.40 × 106 3.80 × 105 1.22 × 104 6.55 × 108 9.14 × 104 3.2 × 107
Reaction rate constant, k (L/mol)m+n−1 s−1
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Figure 1. A schematic diagram of a bench-scale spray scrubber.
2. Material and Methods 2.1. MATERIALS The absorption experiments were performed by using a bench-scale spraying scrubber. The whole system includes a flue gas simulation system, a scrubber, and a sampling & analysis system as shown in Figure 1. The flue gas simulation system was composed of an air compressor (Swan, 1/4 hp), a pure NO cylinder (99.7%, IWATANI), a pure N2 cylinder (99.9%, San Fu), four mass flow controllers (Teledyne Hasting-Raydist HFC-202), a custom-made two stage mixer filled with glass beads, and a custom-made electrical temperature controlled heater (Shimaden). Flow rates of air, pure NO and pure N2 were controlled by three mass flow meters. Before adding compressed air, NO had to be diluted by N2 in a plug flow mixer in order to avoid the production of huge amount of NO2 . Diluted NO was further diluted, in another plug flow mixer by the mass flow-controlled compressed air, to the desired concentrations. The simulated flue gas was then heated by a electrical heater to the operating temperature before entering to the scrubber. The material of pipings, valves, regulators, and fittings was either SS-316 or Teflon. The scrubber was a custom-made Lucite spraying absorber. The length of the reaction zone from the point at gas inlet to the point at spraying nozzle was
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10 cm and the internal diameter of the absorber was 5 cm. The spray nozzle, made by System Spraying Co., was a Unijet 1/4TT-SS+TG-SS0.4+W6051SS100. The liquid recirculation pump (K33MYFY-233) was made by Micropump Co. and had a maximum capacity of 250 mL/min. A rotameter (AALBORG T54/1-102-5S,
