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Mass transfer in the absorption of SO2 and NOx using aqueous euchlorine

Journal of Environmental Sciences 21(2009) 155–161

Mass transfer in the absorption of SO2 and NO x using aqueous euchlorine scrubbing solution
DESHWAL Bal-Raj1 , LEE Hyung-Keun2, ?
1. Department of Chemistry, A.I.J.H.M. College, Rohtak 124001, Haryana, India. E-mail: [email protected] 2. Green-house Gas Research Center, Korea Institute of Energy Research, Daejeon-305 343, Korea Received 29 November 2007; revised 08 January 2008; accepted 01 April 2008

Abstract Attempts have been made to generate euchlorine gas by chlorate-chloride process and to utilize it further to clean up SO2 and NO x from the ?ue gas in a lab scale bubbling reactor. Preliminary experiments were carried out to determine the gas and liquid phase mass transfer coe?cients and their correlation equations have been established. Simultaneous removal of SO2 and NO x from the simulated ?ue gas using aqueous euchlorine scrubbing solution has been investigated. Euchlorine oxidized NO into NO2 completely and the later subsequently absorbed into the scrubbing solution in the form of nitrate. SO2 removal e?ciency around 100% and NO x removal e?ciency around 72% were achieved under optimal conditions. Mass balance has been con?rmed by analyzing the sulfate, nitrate, euchlorine and chloride ion using ion chromatograph/auto-titrator and comparing it with their corresponding calculated values. Key words: mass transfer; sulfur dioxide; nitric oxide; bubbling reactor; euchlorine DOI: 10.1016/S1001-0742(08)62244-5

Introduction
The combustion of fossil fuels leads to the emission of sulfur oxides (SO x ) and nitrogen oxides (NO x ). Besides, sulphuric acid industry and roasting of sul?de ore in metallurgy release substantial amount of sulfur oxides into atmosphere. Volcanic eruption is yet another natural source of SO2 emission. The emission of SO2 and NO x has been a major environmental concern because of their hazardous e?ects on human health and the ecosystems. SO2 is the most pervasive air pollutant and is the main cause of acid rain. NO x are particularly responsible for atmospheric ozone depletion, smog and visibility problems. Due to the stringent regulations in the recent years, an e?cient technology for the abatement of SO2 and NO x emissions from both stationary and mobile sources is thus highly desirable. Recently, considerable attention has been focused on the simultaneous removal of SO2 and NO x in a single reactor considering the capital investment, operation cost and space for equipment (Adewuyi et al., 1999; Harriott et al., 1993; Lee et al., 2005). Flue gas desulphurization (FGD) is the most widely used process which can remove SO2 e?ciently. If minor adjustment in wet FGD process may work for combined removal of SO2 and NO x , it may prove a compact and cost e?ective technology for the future. However, NO can not be as easily absorbed as SO2 .
* Corresponding author. E-mail: [email protected]

Technologies for NO x removal include combustion control and post-combustion treatment. Combustion control aims at reducing the NO x formation during combustion of fossil fuel. Post-combustion methods include selective non-catalytic reduction (SNCR), selective catalytic reduction (SCR) and scrubbing etc. Among these technologies, scrubbing methods are economically competitive and have advantage of controlling other acid gases and particulates at the same time (Yang et al., 1996). In general, additives are added into scrubbing system ?rst to convert relatively inert NO into NO2 which can be subsequently removed by alkaline absorbents. Aqueous solutions of numerous oxidative absorbents such as hydrogen peroxide (Baveja et al., 1979), per acid (Littlejohn and Chang, 1990), organic tertiary hydro peroxides (Perlmutter et al., 1993), sodium chlorite (Sada et al., 1978; Brogen et al., 1998; Chu et al., 2001; Lee et al., 2005), KMnO4 (Brogen et al., 1997; Chu et al., 1998) and chlorine dioxide (Jin et al., 2006; Deshwal et al., 2008) have been investigated to determine their e?ciency in the removal of NO x . Sodium chlorite has proved the most e?cient oxidant among them. However, the drawback with sodium chlorite is that it has good oxidizing ability at lower pH while the absorbing capability is good only at higher pH. Therefore, pH is a crucial parameter to oxidize NO into NO2 and to absorb NO2 thereafter. In addition, sodium chlorite is relatively unstable and quite expensive chemical. In recent years, chlorine-dioxide has attracted considerable attention due to its wide applications in the ?elds

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of bleaching, oxidation, disinfection, and gas absorption. It can clean up both NO x and SO2 simultaneously in a wide range of pH (Jin et al., 2006; Deshwal et al., 2008). Although chlorine-dioxide can be produced from acid solutions of either sodium chlorite (Deshwal et al., 2004a, Deshwal and Lee, 2005a, White et al., 1942) or sodium chlorate (Ni and Wang, 1997; Burke et al., 1993, Deshwal and Lee, 2004b, Deshwal and Lee, 2005b), yet sodium chlorate is better cost e?ective raw material for chlorine-dioxide generation. In fact, it is very di?cult to generate 100% pure chlorine-dioxide. The chloratechloride process gives highest yield of ClO2 at the lowest cost among all other commercial processes. Although this process is extremely simple to operate, responds immediately but theoretically, it gives euchlorine, i.e., a mixture of chlorine dioxide and chlorine in the molar ratio of 2:1. The general stoichiometry of chlorate-chloride process may be expressed as Reaction (1) (Deshwal and Lee, 2004b):
– 4H+ + 2ClO– 3 + 2Cl ?→ 2ClO2 + Cl2 + 2H2 O

schematic diagram of the experimental system is shown in Fig.1. 1.1 Euchlorine generation unit This unit is composed of a reactor which is well stirred and sealed vessel having total volume of 2.5 L. Concentrated sodium chloride solution (1–2 mol/L) was continuously injected into reactor at a suitable ?ow rate (0.5–1.5 mL/min) by a syringe pump (Model-200, KDS Scienti?c Inc., USA). The reactor was ?lled with 1.5 L solution of sodium chlorate (0.2–0.4 mol/L) in a relatively concentrated sulfuric acid (11–12 mol/L). Continuous stirring was provided by a mechanical agitator. Temperature of the reaction vessel was controlled within (45 ± 0.1)°C by water thermostat (WBC-1506D, JEIO TECH, Korea). Nitrogen gas was purged through the reaction mixture using a bubbling device at a ?ow rate of 2 L/min. Euchlorine thus carried by N2 gas was further introduced into bubbling reactor. 1.2 Flue gas treatment unit Flue gas cleansing unit composed of simulated ?ue gas supply system, bubbling reactor, pH control system, euchlorine absorber, data acquisition system, and sampling cum analysis system. The bubbling reactor is a well stirred and sealed vessel (ID-15 cm, Height-45 cm) having internal volume of 8 L. The simulated ?ue gas was obtained by controlled mixing of SO2 , NO, and N2 using mass ?ow controllers (MFC). Air was introduced into reactor using air pump to maintain the dissolved O2 concentrations about 30% to 40% of the saturated O2 concentrations. Continuous stirring was provided by a mechanical agitator (4 blades, disc turbine type impeller) with a speed of 250 r/min. Temperature of the reaction vessel was controlled within (45 ± 0.1)°C, a typical scrubbing temperature. The

(1)

The standard oxidation potential of chlorine-dioxide and chlorine in the solution phase is –1.188 and –1.396 V, respectively (Dean, 1999), which implies that both have potential to oxidize NO into NO2 . Hence euchlorine, generated from chlorate-chloride process, can be utilized directly to oxidize NO. It will not only reduce the cost but also solve the problem of pH adjustment. Therefore, with this view, aqueous euchlorine solution has been chosen for the combined removal of NO x and SO2 from the ?ue gas.

1 Materials and methods
The experimental system is divided into two parts: euchlorine generation unit and ?ue gas treatment unit. A

Fig. 1

Schematic diagram of the bubbling reactor.

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pH of reaction solution was controlled by using an autopH control system (KFC-MK-250, Korea) by continuous addition of NaOH (0.2 mol/L) solution with peristalsis pump (Cole-Palmer Co., USA). The euchlorine absorber (2 L vessel) consisted of ca. 2% carbonate bu?ered potassium iodide solution (1.5 L). Samples from reactor and absorber were analyzed by either ion chromatograph (IC) or by iodometric titration using auto-titrator (Metrohm-Swiss). The inlet and outlet gas concentrations were analyzed after removing its moisture in the sample conditioner by the SO2 analyzer (Model-Ultramat 23, IR type, Siemens, Germany), NO analyzer (Model-42C, Chemiluminescent type, Thermo Environmental Instruments Inc., USA), and DO meter (835A, Thermo Orion, USA). 1.3 Materials Standard gases included N2 (99%), SO2 span gas (99%), and NO span gas (99.9%). N2 and SO2 were the products of Anjeon Gas Co., Korea and NO was the product of Mathieson Co., New Zealand, Sodium chlorate (98%, Junsei Chemical Co., Ltd., Japan), sodium chloride (99.5%, Aldrich Chem. Co., Inc., USA), sulfuric acid (98%, PFP, Osaka, Japan), potassium iodide (99.5%, Samchun Pure Chem. Co., Ltd., Korea), and sodium thiosulfate (99%, Shinyo Pure Chem. Co., Ltd., Japan) used in the present study were the analytical grade reagents.

Because SO2 is absorbed completely in 0.2 mol/L NaOH solution, the product HSO2 × C(SO2 )aq can be ignored, and Eq. (2) can be rewritten as: rSO2 = kg × a × P(SO2 )av (5)

The experimental absorption rate of sulfur dioxide rSO2 , can be calculated as follows:
- /VR rSO2 = QL × CSO2 3

(6)

? (mol/L) where, QL (L/min) is the liquid ?ow rate, CSO2 3 is the concentration of sul?te ions in the reactor, and VR (cm3 ) is the reactor volume. Now comparing Eqs. (5) and (6), can get: ? QL × CSO2 3

kg × a =

VR × P(SO2 )av

(7)

2 Results and discussions
2.1 Mass transfer characteristics A number of preliminary experiments were carried out to assess the physical characteristics, particularly the gas phase and liquid phase mass transfer coe?cients. The product of gas phase mass transfer coe?cients kg (mol/105 (cm2 ·s·Pa)) and interfacial area per unit volume a (cm?1 ) were measured by absorbing sulfur dioxide from SO2 /N2 mixture into NaOH solutions (0.2 mol/L). Since in relatively concentrated NaOH solution, the dissolved SO2 reacted instantaneously and irreversibly with the liquid phase reactant at the gas-liquid interface, as a result, the liquid phase mass transfer resistance is considered to be negligible. The values of kg × a were calculated from the absorption rate (rSO2 , mol/(cm3 ·s)) of SO2 for SO2 /NaOH system as follows (Lancia et al., 1997): rSO2 = kg × a × ΔPSO2 (2)

Experiments were carried out at di?erent SO2 gas ?ow rates and input SO2 concentrations and the values of kg × a were determined from P(SO2 )in , P(SO2 )out , and the concentration of sul?te in the reaction vessel. The concentrations of sul?te were determined by iodometric titration of the reaction sample against standard sodium thiosulfate solution. The kg × a can be correlated to gas ?ow rate (Qg , L/min), partial pressure of SO2 (P(SO2 )av ), and agitation speed (N, r/min) as follows:
k3 k4 2 kg × a = k1 × Qk g × P(SO )av × N
2

(8)

The agitation speed was varied from 60 to 360 r/min, and absorption rate of SO2 was found independent of the agitation speed. Thus neglecting the term N k4 and taking logarithm on both sides, Eq. (9) can be get: log(kg × a) = logk1 + k2 logQg + k3 logP(SO2)av (9)

The values of k1 , k2 , and k3 have been calculated by plotting the graph log(kg ×a) vs. logQg or logP(SO2 )av and the correlation equation was obtained as:
.26 .2811 × p0 kg × a = 3.80231 × 10?6 × Q1 g (SO )av
2

(10)

where, ΔPSO2 (1×105 Pa) is the gas-liquid driving force for absorption, and can be evaluated by: ΔPSO2 = P(SO2 )av ? HSO2 × C(SO2) aq (3)

where, HSO2 (1×105 (Pa·L)/mol) is the Henry’s constant for SO2 , C(SO2 )aq (mol/L) is concentration of SO2 in liquid bulk, and P(SO2 )av (1×105 Pa) is the logarithmic average of inlet P(SO2 )in (1×105 Pa). Outlet SO2 partial pressure P(SO2 )out (1×105 Pa) and can be calculated as: P(SO2 )av P(SO2 )in ? P(SO2 )out = ln(P(SO2 )in /P(SO2 )out ) (4)

Further, the data for liquid phase mass transfer coe?cients kL (cm/s), were obtained by absorbing O2 from air into distilled water with a low concentration of O2 . The O2 in the feed water was removed by blowing N2 into the reactor. The gas phase mass transfer resistance was assumed to be negligible because O2 has a low solubility in water and the O2 concentration in atmospheric air is as high as 20 percentage of volume. The dissolution rate of O2 in the distilled and degassed water can be expressed as Eq. (11): dC = kL × a × (CS ? C ) dt Integrating the Eq. (11), can get: ln(Cs ? Ct ) = kL × a × t + ln(CS ? C0 ) (12)

(11)

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where, CS (mg/L) is the saturated dissolved O2 concentration, C0 (mg/L) and Ct (mg/L) are the dissolved O2 concentration in the initial stage and after time t respectively. Experiments were carried out at di?erent O2 gas ?ow rates and agitation speeds and the values of kL × a were obtained from Eq. (12). The kL × a in O2 /H2 O system can be correlated to Qg and N as follows:
3 kL × a = k1 × N k2 × Qk g

(13)

Taking logarithm on both sides, we get: log(kL × a) = logk1 + k2 logN + k3 logQg (14)

where, H (1×105 (Pa·L)/mol) is the Henry’s constant for gas and all other terms have the usual signi?cance. The absorption rate of SO2 or NO are plotted against absorption driving force, i.e., logarithmic mean of SO2 or NO partial pressure (Fig. 3), where a couple of straight lines are reported which represent the upper and lower limits for the gas absorption rate. The upper line describes conditions of gas ?lm control (no liquid-side resistance); however, the lower line is the representative of conditions in which only physical absorption occurs. The rate of mass transfer of a gas increases when reaction occurs within the liquid ?lm and it can be expressed as: R =( H 1 + )?1 (Pav ? H × Caq ) kg × a Φ × k L × a (17)

where, the values of k1 , k2 , and k3 have been calculated by plotting the graph log(kL ×a) vs. logQg or logN and the correlation equation was obtained as:
.3198 kL × a = 6.395 × 10?4 × N 0.452 × Q0 g

(15)

2.2 Gas ?lm control and physical absorption According to the two ?lm theory (Fig. 2), the rate of straight mass transfer of a gas (R) in water (only via absorption) can be expressed as Eq. (16) (Levenspiel, 1999): R=( H ?1 1 + ) (Pav ? H × Caq ) kg a kL a (16)

where, R is the rate of mass transfer of a gas when reaction occurs within the liquid ?lm and Φ is the enhancement factor due to chemical reaction of gas with the aqueous euchlorine solution. The absorption rate of a gas in the aqueous euchlorine scrubbing solution (RA ) was calculated experimentally as follows: RA = QA × (CA(in) ? CA(out) ) × 10?6 × 1 1 × 22.4 VR (18)

Fig. 2 Absorption of sulfur dioxide as visualized by the two ?lm theory.

where, QA (L/s) is gas ?ow rate, CA(in) (ppmv) and CA(out) (ppmv) are input and output concentration of gas respectively. The liquid-side resistance is quite signi?cant in the absorption of NO as is apparent from the Fig. 3b, whereas Fig. 3a shows that the absorption rate of SO2 is mainly controlled by the gas-side resistance. In case of SO2 , almost no liquid-phase mass transfer resistance has been noticed if there is su?cient euchlorine in the scrubbing solution. Thus, the absorption rate of SO2 is mainly controlled by gas ?lm resistance. On the other hand, the solubility of NO in water (1.25 × 10?3 mol/105 (Pa·L)) is about 560 times lesser than that of SO2 (7.03 × 10?1 mol/105 (Pa·L)) at 50°C. Moreover, the dissolved NO is ?rst oxidized into NO2 by euchlorine, and later is absorbed by the scrubbing solution. Therefore, liquid-phase mass transfer resistance mainly controls the absorption of NO. Although the physical absorption of NO occurs but its absolute value is smaller than that of gas ?lm control.

Fig. 3

Gas ?lm control and physical absorption in the euchlorine-NO-NaOH system (a) and euchlorine-SO2 -NaOH system (b).

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Φ can be calculated by Φ = R /R. Enhancement factors of SO2 and NO x increased as evident from Fig. 4. SO2 being more soluble, thus a small increase in enhancement factor occurred. However, NO and NO2 being less soluble, are di?cult to absorb and have the main resistance in the liquid phase; thus enhancement factor increased signi?cantly by their chemical reaction with the aqueous euchlorine scrubbing solution. 2.3 Removal of SO2 and NO x from ?ue gas by euchlorine solution Simultaneous removal of SO2 and NO was studied at 45°C, pH 3.5. The input SO2 and NO concentrations were 500 and 350 ppmv respectively, and were introduced by passing euchlorine gas into the scrubbing solution. The SO2 and NO x removal e?ciencies at di?erent euchlorine feeding rates are presented in Fig. 5. Euchlorine ?rstly cleaned up the more reactive and soluble SO2 gas and thereafter the surplus euchlorine oxidized NO gas. At the feeding rate of 0.825 mmol/min, all of the euchlorine was utilized in SO2 absorption and NO x removal was found negligible. However, at higher

euchlorine feeding rates, it was observed that euchlorine cleaned up both SO2 and NO quite e?ciently. Euchlorine oxidized NO into NO2 completely. NO2 absorption e?ciency increased with the increasing euchlorine feeding rates. A consistent and reproducible SO2 and NO x absorption e?ciencies around 100% and 72% were observed at euchlorine feeding rates of 3.045 mmol/min. Furthermore, euchlorine proved a remarkably e?cient oxidant as well as absorbent in a wide pH range of 3.5 to 8 as is evident from Fig. 6. In addition, the oxidizing as well as absorption ability of euchlorine was not a?ected by pH, thus making it a superior oxidative absorbent compared to sodium chlorite. Flue gases mainly contain NO and NO2 but the major component of NO x is NO (ca. 90%). Euchlorine is a mixture of chlorine-dioxide and chlorine at the molar ratio of 2:1, where both the constituents have the ability to oxidize NO. Chlorine-dioxide, a major constituent of euchlorine is believed to clean up NO and SO2 via following reactions (Jin et al., 2006): 5SO2 + 2ClO2 + 6H2 O ?→ 5H2 SO4 + 2HCl (19) (oxidation) 5NO + 2ClO2 + H2 O ?→ 5NO2 + 2HCl (20) 5NO2 + ClO2 + 3H2 O ?→ 5HNO3 + HCl (absorption) (21)

The overall reaction for the NO x removal can be written as Reaction (22) (Jin et al., 2006; Deshwal et al., 2008): 5NO + 3ClO2 + 4H2 O ?→ 5HNO3 + 3HCl (22)

Fig. 4 Enhancement factor of SO2 and NO x at various euchlorine feeding rates.

Chlorine gas, another constituent of euchlorine is also a strong oxidant. It is already reported that chlorine is capable of oxidizing NO into NO2 and nitrate (Yang et al., 1996). The stoichiometry of reaction between chlorine with NO can be expressed as: NO + Cl2 + H2 O ?→ NO2 + 2HCl 2NO + 3Cl2 + 4H2 O ?→ 2HNO3 + 6HCl (23) (24)

Fig. 5 Simultaneous removal of SO2 and NO x with time at various euchlorine feeding rates. Conditions: temperature 45°C; pH 3.5; input NO concentration 350 ppmv; input SO2 concentration 500 ppmv.

Fig. 6 NO x removal e?ciency and output concentrations of NO and NO2 at various pH. Conditions: temperature 45°C; input SO2 concentration 250 ppmv; input NO concentration 350 ppmv; euchlorine feeding rate 1.66 mmol/min.

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Fig. 7 Comparison between experimental and calculated sulfate concentrations (a) and of input euchlorine and output euchlorine plus accumulated chloride concentrations (b).

The presence of sulfate, nitrate and chloride in the samples of the bubbling reactor con?rmed the mechanism proposed above. 2.4 Mass balance for removal of SO2 and NO x The formation of sulfate, nitrate and chloride ions as suggested above in the Reactions (19)–(24) is con?rmed by analyzing the sample from bubbling reactor using ion chromatograph. The samples from euchlorine absorber were quantitatively analyzed iodometrically using autotitrator to examine the euchlorine evolved unreacted. The relation between calculated and experimental sulfate is illustrated in Fig. 7a. The mass balance for euchlorine is demonstrated in Fig. 7b. Both ?gures have demonstrated a fair mass balance.

3 Conclusions
The present study deals with the simultaneous removal of SO2 and NO x using aqueous euchlorine scrubbing solution in the lab-scale bubbling reactor. Mass transfer characteristics have been examined critically by determining the gas and liquid phase mass transfer coe?cients. Removal e?ciency around 100% and 72% has been achieved for SO2 and NO x respectively at a typical scrubbing temperature of 45°C. The byproducts of the reaction in the combined removal of SO2 and NO x using euchlorine solution are sulphate, nitrate and chloride which are not hazardous materials, thus causing no secondary pollution. The mass balance for sulfate, nitrate and chloride ions has been con?rmed. The oxidizing as well as absorption ability of euchlorine was found una?ected by pH, thus making it a superior oxidative absorbent compared to sodium chlorite.

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