ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUMCHLORITE SOLUTIONS IN TWIN SPRAY COLUMNS

TSUNG WEN CHIEN, HSIN CHU∗ and YARN-CHI LIDepartment of Environmental Engineering, National Cheng Kung University, Tainan 701, Taiwan(∗author for correspondence, e-mail: chuhsin@mail.ncku.edu.tw, Tel: 6 2080108, Fax: 6 2752790)

(Received 3 August 2004; accepted 4 May 2005)

Abstract. NOX is a major pollutant that causes acid deposition and photochemical smog. A largeamount of NOX is emitted from combustion sources such as power plants. As some articles have indi-cated, NOX 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, thenNO2 is removed in alkaline NaClO2 solutions. The results indicate that the reaction rate constant is9.1 × 104 (L)4.4/cm6/s/mol2.4 in the first absorption column with acidic conditions, and the reactionorders with respect to NO and NaClO2 are 1.4 and 2, respectively. In the second absorption columnwith alkaline conditions, the reaction rate constant is 3.2 × 107 (L)5.2/cm6/s/mol3.2 and the reactionorders with respect to NO and NaClO2 are 1.6 and 2.5, respectively. The activation energies in thefirst 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 factorkG = gas-side mass transfer coefficient (mol s−1 cm−2 atm−1)kL = 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)SubscriptsA = dissolved gas A (NO)B = liquid-phase reactant B (NaClO2)E = liquid-phase reactant E (NaOH)i = gas–liquid interfacew = water0 = initial value

Water, Air, and Soil Pollution (2005) 166: 237–250 C© Springer 2005

238 T. W. CHIEN ET AL.

1. Introduction

To reduce the amount of SO2 and NOX emitted from the stationary combustionsources, several dry, wet and bio-treatment processes have been discussed (Chouand Lin, 2000; Livengood and Markussen, 1994; Mochida et al., 2000) and im-plemented. The major dry processes for NOX removal include low NOX burners,overfire air, reburning, selective catalytic reduction (SCR), selective non-catalyticreduction (SNCR), non-selective catalytic reduction (NSCR) and adsorption. Bio-processes for air pollution control are categorized as bioscrubber, biofilter, andbiotrickling filter systems. The major wet processes for NOX removal are gasphase oxidation followed by absorption, absorption with liquid phase reduction,and absorption with liquid phase oxidation.

Several liquid absorbents including FeSO4/H2SO4, Fe(II)EDTA, Na2S/NaOH,Na2S2O4/NaOH, H2O2, Na2SO3, FeSO4/Na2SO3, Urea, NaOH, Na2CO3 (Jethaniet 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 NOX absorption. Although NO could be removed from fluegases effectively, the P4 process has remained at a demonstration stage by the safetyconcerns 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 costsof NOX wet scrubbing by P4, H2O2, and NaClO2 were 2.1, 2.8, and 3.0 US$ /kgNOX removal, respectively.

The absorption of NOX in NaClO2 solution was studied by Teramoto et al.(1976) and Sada et al. (1978a, b, 1979). These studies investigated NO absorptionkinetics in mixed alkaline aqueous solutions of NaClO2 and NaOH, using a semi-batch 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 andNaOH 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 so-lutions with higher concentrations of NaClO2 and NaOH. The absorption of NOX

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 abubble column, spray scrubber, and packed column. A summary of the previ-ous studies on NOX absorption kinetics in NaClO2/NaOH solutions is listed inTable 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 aimof this work is to investigate the reaction kinetics of individual and simultaneousremoval of lean SO2 and NO in an aqueous solution of acidic sodium chlorite usingbench-scale twin spraying columns.

ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS239

TAB

LE

I

The

rele

vant

kine

ticst

udie

sof

NO

abso

rptio

nw

ithN

aClO

2so

lutio

ns

Rea

ctio

nR

eact

ion

p NO

[NaC

lO2]

[NaO

H]

Tem

p.or

der

ofor

der

ofR

eact

ion

rate

cons

tant

,kR

ef.

(ppm

)(M

)(M

)pH

(◦ C)

NO

(m)

NaC

lO2

(n)

(L/m

ol)m

+n−1

s−1

Sada

etal

.(19

78a)

8,00

0–15

,000

0.2–

1.5

0.05

–0.5

–25

21

3.8

×10

12ex

p(−3

.73[

NaO

H])

Sada

etal

.(19

78b)

5,00

0–75

,000

0.29

–1.6

40.

015

–25

21

2.1

×10

12

Sada

etal

.(19

79)

1,50

0–15

0,00

00.

25–2

.00.

2–1.

5–

251

17.

32×

108

Bro

gren

(199

8)29

00.

1–1.

0–

820

1.76

60.

693

1.55

×10

6

–9

201.

665

0.60

31.

40×

106

–10

201.

553

0.66

83.

80×

105

–11

201.

346

0.90

81.

22×

104

Hsu

etal

.(19

98)

200–

1000

0.05

–1.0

–9.

2–9.

930

21

6.55

×10

8

Thi

sw

ork

for

1stc

olum

n30

0–80

00.

020–

2.0

525

–70

1.4

29.

14×

104

Thi

sw

ork

for

2nd

colu

mn

1.6

2.5

3.2

×10

7

240 T. W. CHIEN ET AL.

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 scrub-ber. The whole system includes a flue gas simulation system, a scrubber, and asampling & analysis system as shown in Figure 1.

The flue gas simulation system was composed of an air compressor (Swan, 1/4hp), 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-madetwo stage mixer filled with glass beads, and a custom-made electrical temperaturecontrolled heater (Shimaden). Flow rates of air, pure NO and pure N2 were con-trolled by three mass flow meters. Before adding compressed air, NO had to bediluted by N2 in a plug flow mixer in order to avoid the production of huge amountof NO2. Diluted NO was further diluted, in another plug flow mixer by the massflow-controlled compressed air, to the desired concentrations. The simulated fluegas was then heated by a electrical heater to the operating temperature before en-tering to the scrubber. The material of pipings, valves, regulators, and fittings waseither SS-316 or Teflon.

The scrubber was a custom-made Lucite spraying absorber. The length of thereaction zone from the point at gas inlet to the point at spraying nozzle was

ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS241

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+W6051SS-100. The liquid recirculation pump (K33MYFY-233) was made by MicropumpCo. and had a maximum capacity of 250 mL/min. A rotameter (AALBORGT54/1-102-5S, <150 mL/min) was used to control the flow rate of the sprayingsolution.

2.2. ANALYTICAL METHODS

NO and O2 concentrations in the gas samples were measured by a SO2/NOX an-alyzer (Model ZRF2-FGG11G2, California Analytical Instruments, Inc., NDIRtype) and a O2 analyzer (Model 300, California Analytical Instruments, Inc., fuelcell type), respectively. Gas samples with known NO and O2 contents were usedfor the calibration. The size distribution of spray droplets and gas–liquid surfacearea were measured by a Malvern 2600 Laser sizer. The range of gas–liquid surfaceareas were between 139.25–733.2 cm2 (Chien and Chu, 2000). The liquid compo-nents were analyzed using UV-visible spectroscopy and Dionex DX-100 IC (IonChromatography).

2.3. OPERATION

The ranges of operating parameters are listed in Table II. The contact time of thegas was defined as the ratio between the height of the gas–liquid contact zone andthe superficial velocity of the gas, where the height of gas–liquid contact zone isthe distance between the gas inlet of scrubbing column and the spray nozzle. Theinitial pH values of NaClO2 solutions used in the first column were adjusted by HClsolution. In each run, the simulated flue gas flowed continuously, and the scrubbingsolution was also sprayed continuously without recirculation.

TABLE II

NO/NOX /SO2–NaClO2 experimental conditions

Operating condition Range

NO conc. (ppm) 300, 500, 800

NaClO2 conc. (M) 0.002

NaOH conc. (M) 0, 0.05, 1.0, 2.0

pH0 of solution used in the first column 5.0

Retention time (s) 1, 2, 3

Temp. (◦C) 26, 50, 70

O2 (%) 9

242 T. W. CHIEN ET AL.

2.4. DATA ANALYSIS

The absorption rate of gas A in the system, NA, can be represented by Danckwert(1970):

NA = kG(PA0 − PAi ) = EkL (CAi − CA0) (1)

where PA0 can be obtained by:

PA0 = PA (PT − PW ) (2)

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

NA = kG PA0 (3)

However, the interfacial concentration of gas A in an electrolyte solution, CAi ,is related to the ionic strength of the solution.

NA =√

2

m + 1DAkmnCm+1

Ai CnB0 (4)

The reaction order NO (m) and NaClO2 (n) could be found according to theabove equation. One can calculate the reaction order m and n from the slope oflines in the log–log plots of CAi and CB0 vs. NA, respectively. As the values ofCAi , CB0, and DA are given, we substitute of m and n into equation (4) to find thereaction rate constant km,n .

3. Results and Discussion

3.1. THE REACTION KINETICS OF INDIVIDUAL NOX ABSORPTIONS

The effects of operating parameters on NOX absorption rate are illustrated as below:

3.1.1. The Effect of NOX Concentration on Absorption RateAs shown in Figure 2, the absorption rate increased with NOX concentration due tothe driving force of absorption increasing with the concentration gradient betweenthe gas phase and liquid phase. A similar observation was reported by Teramotoet al. (1976), Sada et al. (1978a,b, 1979), Brogren et al. (1998), and Hsu et al.(1998). However, the absorption rates in Figure 2 are slightly higher than thoseof Sada et al. (1978a,b, 1979). This may result from the differences of pH valuesbetween this study and that of Sada et al. (1978a,b, 1979).

ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS243

Figure 2. Effect of NOX concentration on the NOX absorption rate.

Figure 3. Effect of NaOH concentration on the NOX absorption rate.

3.1.2. The Effect of NaOH Concentration on Absorption RateFigure 3 shows that the absorption rate for the second column and twin columnsslightly increased with NaOH concentration, however, the absorption rate for firstcolumn slightly decreased with NaOH concentration. A similar observation was

244 T. W. CHIEN ET AL.

Figure 4. Effect of operating temperature on the NOX absorption rate.

reported by Sada et al. (1978a,b, 1979). As shown in our previous studies (Chienand Chu, 2000; Chien et al., 2001) and other investigations (Teramoto et al., 1976;Sada et al., 1979), the oxidative power of NaClO2 decreases with an increase ofpH. However, the absorption rate will normally increase with an increase of pH asshown in column 2.

3.1.3. The Effect of Operating Temperature on Absorption RateFigure 4 shows that the NOX absorption rates in column 2 increased with operatingtemperature over the range of operating parameters used in this study. However,the NOX absorption rates in column 1 and twin columns had a minimum valueat 50◦C since the solubility of NOX decreases but the reaction rate increases withincreasing temperature.

3.1.4. The Effect of Liquid Flow Rate on Absorption RateAs shown in Figure 5, the absorption rate decreased with increasing liquid flowrate.The absorption rate is calculated on a basis of molar transfer rate per surface area.Although the variation of liquid flowrate used in this study has a slight effect onNO conversion and NOX removal efficiency, the surface area of the spray dropletsincreases with the liquid flowrate.

A comparison of the NOX absorption kinetics in NaClO2/NaOH solutions be-tween this study and other investigations is listed in Table I.

ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS245

Figure 5. Effect of liquid flowrate on the NOX absorption rate.

Figure 6. Effect of NOX concentration on the NOX absorption rate in first column.

3.2. THE REACTION ORDER OF NO AND NaClO2 CONCENTRATION

UNDER ACIDIC CONDITIONS

The reaction order of NO (m) can be calculated from the results of Figure 6. Theaverage slope of the lines ((m + 1)/2) was 1.2. This implies that the reaction order

246 T. W. CHIEN ET AL.

Figure 7. Effect of NaClO2 Concentration on the NOX absorption rate under fixed NOX film con-centration in first column.

of NO (m) was 1.4. This value is similar to the results of Sada et al. (1978a,b),Brogren et al. (1998) and Hsu et al. (1998).

The reaction order of NaClO2 (n) could be calculated from the result of Figure 7.The slope of line (n/2) was close to 1.0. The reaction order of NaClO2 (n) wasabout 2.0. This value is higher than the results of Sada et al. (1978a,b, 1979),Brogren et al. (1998) and Hsu et al. (1998) due to differences in that pH of NaClO2

solutions.

3.3. THE REACTION ORDER OF NO AND NaClO2 CONCENTRATION

UNDER BASIC CONDITIONS

The reaction order of NO (m) can be calculated from the results of Figure 8. Theaverage slope of the lines ((m +1)/2) was 1.3, implying that the reaction order was1.6.

The reaction order of NaClO2 (n) can be calculated from the results of Figure 9.The slope of line (n/2) was close to 1.25, and so the reaction order was about 2.5.This value is also higher than the results of Sada et al. (1978a,b, 1979), Brogrenet al. (1998) and Hsu et al. (1998).

3.4. THE ACTIVATION ENERGY

The reaction rate constant usually varies with temperature. The relationship betweenreaction rate constant and temperature is expressed by the Arrhenius equation shown

ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS247

Figure 8. Effect of NOX Concentration on the NOX absorption rate in second column.

Figure 9. Effect of NaClO2 concentration on the NOX absorption rate under fixed NOX film concen-tration in second column.

as below:

k = A · exp

(− Ea

RT

)(5)

248 T. W. CHIEN ET AL.

Figure 10. The activation energy of the NO absorption with acidic NaClO2 solution.

Figure 11. The activation energy of NO absorption with basic NaClO2 solution.

where k is reaction rate constant ((L/mol)m+n−1/s), A is frequency factor((L/mol)m+n−1/s), Ea is the activation energy (Kcal/mol), R is universal gas constant(1.987 × 10−3 Kcal/mol/K), and T is the temperature (K).

Based on the results in Figure 10, the activation energy in the first column underacidic condition is 71.8 kJ/mol, whilst from Figure 11, that in the second columnunder basic condition is 139.6 kJ/mol.

ABSORPTION KINETICS OF NITROGEN OXIDES USING SODIUM CHLORITE SOLUTIONS249

4. Conclusion

All experiments were performed in bench-scale custom-made acrylic twin spraycolumns. The operating parameters included 300–800 ppm NO, 0.01–0.05 MNaClO2, 25–70 ◦C, and 1–3 s retention time. The results indicate that the reac-tion rate constant is 9.1 × 104 (L)4.4/cm6/s/mol2.4 in the first absorption columnwith acidic conditions, and the reaction orders with respect to NO and NaClO2 were1.4 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 orderswith respect with NO and NaClO2 were 1.6 and 2.5, respectively. The activationenergies in the first and second absorption column are 71.8 and 139.6 kJ/mole,respectively.

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