SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANKREACTOR WITH NACLO2/NAOH SOLUTIONS

HSIN CHU∗, TSUNG-WEN CHIEN and BOUR-WEI TWUDepartment of Environmental Engineering, National Cheng Kung University, Tainan, Taiwan

(∗ author for correspondance, e-mail: chuhsin@mail.ncku.edu.tw; fax: +886 6 275 2790)

(Received 7 June 2000; accepted 11 July 2002)

Abstract. The wet scrubbing combined SOx/NO removal system is one of the advanced air pollutioncontrol devices. This study tries to understand the kinetics of the absorption in the system. Theabsorption of SO2 and simultaneous absorption of SO2 and NO, whose concentrations are typicalfor flue gases emitted from coal-fired power plants, in a stirred tank reactor with NaClO2/NaOHsolutions were carried out at 50 ◦C. The liquid-side and gas-side mass transfer coefficients of thesystem were determined. The results indicate that the absorption of SO2 is completely gas-filmcontrolled if the NaOH concentration is greater than 0.1 M or the NaClO2 concentration is greaterthan 0.2 M. Adding SO2 would decrease the absorption rate of NO; however, the addition of NOhas no effect on the absorption rate of SO2. The existence of O2 has no significant effect on theabsorption rate of SO2 and NO in the combined SOx/NO removal tests.

Keywords: absorption, flue gas, nitrogen oxides, sodium chlorite, sodium hydroxide, stirred tankreactor, sulfur dioxide

Nomenclature

C = concentration in liquid phase [mol L−1]

D = diffusivity in liquid phase [cm2 sec−1]

D = diffusivity in gas phase [cm2 sec−1]

E = enhancement factor

I = ionic strength [g-ion L−1]

kG = gas-side mass transfer coefficient [mol sec−1 cm−2 atm−1]

kL = liquid-side mass transfer coefficient [cm sec−1]

N = absorption rate [mol sec−1 cm−2]

P = partial pressure [atm]

PT = operating pressure of the system [atm]

PW = saturated vapor pressure of water at operating temp. [atm]

Xa, Xc, Xg = contribution to K of anions, cations and the gas, respectively

[L g-ion−1].

Water, Air, and Soil Pollution 143: 337–350, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

338 HSIN CHU ET AL.

Subscripts

A1 = dissolved gas A1 (SO2)

A2 = dissolved gas A2 (NO)

B = liquid-phase reactant B (NaClO2)

E = liquid-phase reactant E (NaOH)

i = gas-liquid interface

w = water

0 = initial value.

1. Introduction

Acid rain is one of the major air pollution problems nowadays. Besides SO2, NOx

are also the major pollutants which causes acid rain. Conventionally, engineersusually design one air pollution control device for each pollutant, which costs muchand requires spaces. The combined SO2/NO removal systems are therefore findingtheir niches. Among them, Makansi (1990) indicated that wet scrubbing combinedSO2/NO removal system was one of the best technologies. The mechanism of SO2

removal in the wet scrubbing is not new to us. However, the mechanism of NOremoval in the wet scrubbing has plenty of unknown areas to be developed. Ingeneral, there are two groups of chemicals used in the absorption of NOx and SO2

from flue gases. One group of them are oxidative additives such as O3 (Li, 1993),P4 (Chang and Lee, 1992), or NaClO2 (Chien and Chu, 2000; Chien et al., 2001;Yang, 1994). They are added into the scrubbing system to convert insoluble NO tosoluble NO2. The other group are metal chelates such as ferrous chelates (Shi et al.,1996a,b,c, 1997; Pham and Chang, 1994). They react with NO to form a complex,which can be removed then.

Kobayashi, et al. (1977) investigated the removal of nitrogen oxides using anumber of inorganic and organic reagents. Their results showed that the strongoxidizing reagents such as KMnO4 and NaClO2 removed NO efficiently. Chienand Chu (2000) used NaClO2/NaOH to perform a preliminary test on a semi-continuous spraying sieve tray combined SO2/NO removal system and a continu-ous spraying sieve tray column test. The results showed that NaClO2/NaOH workedfine for the combined SO2/NO removal system. Teramoto et al. (1976) used a semi-batch stirred tank reactor to absorb NO in a NaClO2/NaOH solution to investigatethe effect of various operating parameters on the absorption rate of NO. They foundthat the absorption rate of NO increased with the increasing NO concentrationin the gas phase and the increasing NaClO2 concentration in the liquid phase.However, their experiments were performed at 25 ◦C, which is not the commonoperating temperature – 50 ◦C for wet scrubbers. Sada et al. (1978a, b) also did aseries of NO, SO2, combined SO2/NO absorption studies using NaClO2 in a semi-

SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANK REACTOR 339

batch stirred tank reactor. The concentrations of NO and SO2 in the gas streamranged from 0.5 to 15 Vol% and from 1.1 to 9.6 Vol%, respectively. Again, theexperiments were performed at 25 ◦C. They pointed out that NaClO2 would bedecomposed in an acid solution as follows:

NaClO2 + H2O ↔ HClO2 + NaOH (1)

8 HClO2 ↔ Cl2 + H2O (2)

2 NaClO2 + Cl2 ↔ 2 HaCl + 2 ClO2 (3)

Some ClO2 may evolve from the solution into the gas phase and oxidize SO2 tohelp the absorption of SO2 in the system as follows:

5 SO2 + 2 ClO2 + 6 H2O ↔ 5 H2SO4 + 2 HCl (4)

They suggested that this would help the absorption rate of SO2 exceeding thetheoretical rate of completely gas-film controlled conditions and result in a de-crease of NO absorption rate. However, reactions (1) to (4) were suppressed un-der alkaline conditions and the rate of SO2 absorption depended mainly on OH−concentration as follows:

SO2 + 2 OH− ↔ SO2−3 + H2O (5)

2 SO2−3 + ClO−

2 ↔ 2 SO2−4 + Cl− (6)

The rate of SO2 absorption under this situation was found below the theoreticalrate of completely gas-film controlled conditions.

This study focused on the individual absorption of SO2 and simultaneons ab-sorption of SO2 and NO, as occurred in flue gases, in a stirred tank reactor withNaClO2/NaOH solutions at 50 ◦C. The liquid-side and gas-side mass transfer coef-ficients of the system were determined. The effects of concentrations of NaClO2,NaOH, O2, and other parameters were tested on the absorption kinetics of thesystem.

2. Materials and Methods

The experimental setup and procedure were basically the same as in our previ-ous investigation on the absorption kinetics of NO in the KMnO4/NaOH solutions(Chu et al., 1998, 2001a) and in the NaClO2/NaOH solutions (Chu et al., 2001b);therefore, only a few essential features are repeated here.

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2.1. EXPERIMENTAL SETUPS

The absorption kinetics of this study was carried out in a bench scale stirred tankreaction system with a plane gas-liquid interface. The experimental system shownas Figure 1 can be divided into three parts: a flue gas simulation system, an ab-sorption reactor, and a gas sampling and analyzing system. Flow rates of pure N2,pure SO2, and 2075 ppm NO were controlled by three mass flow meters. SO2 andNO were diluted by N2 in a plug flow mixer and further diluted, by the mass flowcontrolled compressed air in another plug flow mixer, to the desired concentrationsand O2 contents. The simulated flue gas was then heated to 50 ◦C by a electricalheating tape before entering into the absorption reactor. The liquid volume andthe gas volume in the tank were 770 cm3 and 580 cm3, respectively. The gas andliquid impellers were driven by two countercurrent variable speed DC motors,respectively. The heated simulation gas entered a saturator first to make sure itbe saturated with water vapor. The tank was immersed in a water bath to keepthe gas phase and liquid phase all at 50 ◦C. An ice box impingement condenser(Pyrex) and two particle filters (Balston, 95S6 and 45G) were installed betweenthe sampling port and various gas analyzers. Four rotameters were installed beforethe O2 analyzer (Signal Model 800 magneto dynamic type), SO2 analyzer (Signalseries 1,000 UV type), NOX analyzer (Analysis Automation Limited model 441chemiluminescent type), and bypass exit, respectively.

2.2. MATERIALS

Standard gases included zero gas (N2, 99.995%, San Fu), O2 span gas (19%, SanFu), SO2 span gas (4510 ppm, Sam Fu), and NO span gas (1010 ppm, San Fu).NaOH used was a product of Merck Chemical Inc. (purity > 99%) and NaClO2

was a product of Nihon Isitu Chemical Inc. (purity > 87%).

2.3. OPERATIONAL CONDITIONS AND PROCEDURES

The simulated gas continuously flowed through the system with a flow rate of 2 Lmin−1 and the liquid was batchwise. Prior to the chemical absorption experiments,the physical absorption of pure CO2 (99.8%, San Fu) into water was performed todetermine the liquid-side mass transfer coefficients of the system. The absorptionof SO2 (1000–3000 ppm) into an aqueous solution of 1 M NaOH was carried outto determine the gas-side mass transfer coefficients of the system. In addition tothese two experiments, four absorption experiments were done for single SO2 andcombined SO2/NO to test the effects of various operating parameters, includingSO2 concentration, NO concentration, O2 concentration, NaClO2 concentration,NaOH concentration, and agitation speeds in liquid phase (nL) and gas phase (nG).The operating conditions of the six experiments are given in Table I. Around 20ml each of the liquid sample of the first experiment was taken at the fifth and tenth

SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANK REACTOR 341

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342 HSIN CHU ET AL.

TABLE I

The operating conditions for various experiments

No. SO2 conc. NO conc. O2 conc. [NaClO2] [NaOH] nL nG

(ppm) (ppm) (%) (mol L−1) (mol L−1) (rpm) (rpm)

1 0 0 0 0 0 80–330 237

2 1000–3000 0 0 0 1 208 160–300

3 1000–3000 0 0 0–0.5 0 208 237

4 1000–3000 0 0 0–0.5 0.01–0.1 208 237

5 1000–2000 300–800 0 0.5 0.1 208 237

6 1000–1500 300–800 3–6 0.5 0.1 208 237

minutes, respectively, for analysis. The analysis of the CO2 concentration in theaqueous solution followed the standard titrimetric method.

After a preliminary test was performed, the inlet and outlet gas samples wereanalyzed for all other experiments at the tenth minute because that the absorptionrate of the system remained stable after six to seven minutes. Each data point shownin this manuscript is obtained by averaging the values from two runs under the sameoperating condition.

3. Results and Discussion

3.1. DATA ANALYSIS

The absorption rate of gas A in the system, NA, measured in the experiments canbe represented by:

NA = kG (PA0 - PAi) = EKL (CAi – CA0) (7)

where PA0 can be obtained by:

PA0 = PA (PT – PW ) (8)

For a completely gas phase controlled reaction, PAi = 0, NA can be simplified to:

NA = kGPA0 (9)

CAiw, the interfacial concentration of gas A in pure water, can be estimated fromthe Henry’s law. However, the interfacial concentration of gas A in an electrolytesolution, CAi, is related to the ionic strength of the solution. Onda, et al. (1970a,b) indicated that this was observed by Van Krevelen and Hoftijzer. The relation ofthem can be represented by:

log(CAi /CAiw) = – (KB IB + KEIE) (10)

SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANK REACTOR 343

where KB and KE are the salting-out parameters for the electrolyte B (NaClO2) andE (NaOH), respectively. The salting-out parameter depends on the anions, cations,and gas present and is expressed by:

K = Xa + Xc + Xg (11)

Assuming the value of X is independent of temperature, the values of X for variousspecies are available in the literatures of Sada, et al. (1986) and Onda, et al. (1970a,b): Xa(ClO2–) = 0.3497, Xa(OH−) = 0.3875, Xc(Na+) = – 0.0183, Xg(SO2) = –0.3145, and Xg(NO) = – 0.1825.

kL,CO2 can be obtained from equation (7) with the data of the first experiment.kG,SO2 can be obtained from Equation (9) with the data of the second experiment.Sada, et al. (1978b) indicated that kL,NO and kG,NO can be estimated by:

kL,NO = kL,CO2 (DNO−H20/DCO2−H20)2/3 (12)

kG,NO = kG,SO2 (DNO−N2 /DCO2−N2 )2/3 (13)

where DCO2−H2O and DNO−H2O can be calculated from the Wilke-Chang equationas shown in Bird et al.’s book (1960) and the values of them are 3.59 × 10−5 cm2

sec−1 and 4.47 × 10−5 cm2 sec−1, respectively, at 50 ◦C. DNO−N2 and DSO2−N2

can be calculated from the Chapman-Enskog equation as shown in Bird et al.’sbook (1960) and the values of them are 0.234 cm2 sec−1 and 0.147 cm2 sec−1,respectively, at 50 ◦C.

3.2. THE LIQUID-SIDE MASS TRANSFER COEFFICIENT, kL,CO2 AND GAS-SIDE

MASS TRANSFER COEFFICIENT, kG,SO2

The liquid-side mass transfer coefficient, kL,CO2, for absorption of pure CO2 intowater is correlated with nL (rpm) in this study by:

kL,CO2 = 7.12 × 10−5 n0.578L (14)

The gas-side mass transfer coefficient, kG,SO2 for absorption of known concen-trations of SO2 into a solution with 1 M NaOH is correlated with nG (rpm) in thisstudy by:

kG,SO2 = 3.94 × 10−6 n0.186G (15)

3.3. SO2-NaClO2 EXPERIMENT

Figure 2 shows the measurements of SO2 absorption rate over a range of theNaClO2 concentrations. NA1 represants the SO2 absorption rate, PA10 is the par-tial pressure of inlet SO2, CB0 stands for the initial NaClO2 concentration, andCE0 is the initial NaOH concentration. The dotted line represents the theoretical

344 HSIN CHU ET AL.

Figure 2. Effect of SO2 partial pressure on absorption rate of SO2 in the NaClO2 solution.

prediction of absorption rate in a completely gas-film controlled region for thisstudy. The observed absorption rates for the cases that the NaClO2 concentrationsare greater than 0.2 M slightly exceed the absorption rate under completely gas-film controlled conditions. Sada, et al. (1978b) mentioned that this implied thepossibility of existence of gas-phase oxidation of SO2 by the desorbed ClO2 fromthe solution. The higher the NaClO2 concentration is, the more ClO2 is expectedto be desorbed from the liquid phase into the gas phase and the absorption rate ofSO2 is higher.

3.4. SO2-(NaClO2 + NaOH) EXPERIMENT

Figure 3 shows the measurements of SO2 absorption rate over a range of theNaClO2 concentrations with 0.01 M NaOH. The observed absorption rate of thisexperiment is less than that of the SO2-NaClO2 experiment for each corresponded

SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANK REACTOR 345

Figure 3. Effect of SO2 partial pressure on absorption rate of SO2 in the NaClO2/0.01 M NaOHsolution.

NaClO2 concentration except for the zero [NaClO2] case. This may be due to thatthe presence of diluted NaOH inhibits the production of ClO2 followed reactions(1) and (2). Therefore, no conditions in this experiment have the absorption ratesthat exceed the theoretical rate of completely gas-film controlled conditions.

Figure 4 shows the measurements of SO2 absorption rate over a range of theNaClO2 concentrations with 0.1 M NaOH. In this experiment, reactions (1) to (4)are believed to be suppressed under the stronger alkaline situation and the ratesof SO2 absorption depend mainly on OH− concentration. The rates of SO2 ab-sorption in this experiment are close to the theoretical rates of completely gas-filmcontrolled conditions no matter what NaClO2 concentrations are.

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Figure 4. Effect of SO2 partial pressure on absorption rate of SO2 in the NaClO2/0.1 M NaOHsolution.

3.5. (SO2 + NO)-(NaClO2 + NaOH) EXPERIMENT

As shown in Figure 4, the rates of SO2 absorption in a 0.1 M NaOH solution areclose to the theoretical rates of completely gas-film controlled conditions. How-ever, the absorption rates of SO2 in the presence of NO slightly exceed the com-pletely gas-film controlled rates as shown in Figure 5. This suggests the presenceof NO may consume more NaOH in the solution. Therefore, the gas-phase reactionbetween ClO2 and SO2, as stated in the SO2-NaClO2 section, is very possible tohappen.

However, the overall effect of the presence of NO on the SO2 absorption rateis trivial. The effect of the presence of SO2 on the NO absorption rate, NA2, in asolution with 0.5 M NaClO2 and 0.1 M NaOH is shown in Figure 6. CA2i is the in-terfacial concentration of NO. It reveals that NA2 decreases with the increasing SO2

SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANK REACTOR 347

Figure 5. Effect of NO partial pressure on absorption rate of SO2.

concentration and the effect is quite significant. This may be due to the solubilityof NO in water is 20 times less than that of SO2.

3.6. (SO2 + NO + O2) – (NaClO2 + NaOH) EXPERIMENT

The effect of oxygen concentration on the simultaneous SO2 and NO absorptionrate has not been investigated by the other researchers. A set of experiments wereperformed for the simulated flue gases with 3–6% O2. Figure 7 indicates that theeffect of O2 concentration on the absorption rates of NO and SO2 is trivial in acombined NO/SO2 system. As indicated in the article of Chien and Chu (2000),the effect of oxygen concentration on the NO removal efficiency is not significanttoo.

348 HSIN CHU ET AL.

Figure 6. Effect of SO2 partial pressure on absorption rate of NO.

4. Conclusions

The absorption of SO2 and simultaneous absorption of SO2 and NO, as occuredin flue gases, in a stirred tank reactor with NaClO2/NaOH solutions were carriedout at 50 ◦C. In the SO2-NaClO2 experiment, the absorption rates of SO2 for thecases that the NaClO2 concentrations are greater than 0.2 M slightly exceed thecompletely gas-film controlled rates. This implies the possibility of existence ofgas-phase oxidation of SO2 by the desorbed ClO2 from the solution. In the SO2

– (NaClO2 + NaOH) experiment, the absorption of SO2 is completely gas-filmcontrolled where the NaOH concentration is greater than 0.1 M. In the (SO2 +NO) – (NaClO2 + NaOH) experiment, the effect of the presence of NO on the SO2

absorption rate is trivial and the NO absorption rate decreases quite significantlywith the increasing SO2 concentration. In the (SO2 + NO + O2) – (NaClO2 +

SIMULTANEOUS ABSORPTION OF SO2 AND NO IN A STIRRED TANK REACTOR 349

Figure 7. Effect of O2 partial pressure on absorption rate of SO2 and NO.

NaOH) experiment, the effect of O2 concentration on the absorption of NO andSO2 is trivial.

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