Absorption of Nitrogen Oxides into Sodium Hydroxide Solution in a

Sep 12, 2017 - Results indicated that preoxidation of NO by O3 significantly improved NOx removal efficiency and the removal efficiency increased with...
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Absorption of Nitrogen Oxides into Sodium Hydroxide Solution in a Rotating Packed Bed with Preoxidation by Ozone Baochang Sun,†,‡ Miaopeng Sheng,‡ Wenlei Gao,† Liangliang Zhang,†,‡ Moses Arowo,‡ Yan Liang,§ Lei Shao,†,‡ Guang-Wen Chu,†,‡ Haikui Zou,*,†,‡ and Jian-Feng Chen†,‡ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, P. R. China § Beijing Urban Drainage Monitoring Center Co., Ltd., Beijing 100061, P. R. China ‡

ABSTRACT: This study employed a rotating packed bed (RPB) to enhance the absorption performance of nitrogen oxides (NOx) into sodium hydroxide (NaOH) solution with the preoxidation of NO by ozone. The absorption performance of NOx was evaluated in terms of its removal efficiency (η) from a gas stream under various operating conditions including O3/NOx molar ratio (MR), rotation speed of the RPB (N), liquid flow rate (L), NaOH concentration (CNaOH), inlet NOx concentration (CNOx), and using time (t). Also, the corresponding effect of adding oxidants (NaClO, H2O2, and KMnO4) and a reductant (CO(NH2)2) into the NaOH solution on NOx removal efficiency was investigated. Results indicated that preoxidation of NO by O3 significantly improved NOx removal efficiency and the removal efficiency increased with increasing O3/NOx molar ratio, NaOH concentration, and liquid flow rate but decreased with increase in inlet NOx concentration and using time. Additionally, NOx removal efficiency first increased and then decreased with increasing rotation speed of the RPB. Both the oxidation and reduction additives enhanced NOx removal efficiency, and the order of enhancement was found to be KMnO4 > H2O2 > CO(NH2)2 > NaClO. These results further indicate that the hydrolysis reactions of NOx are the rate-determining steps in the NOx absorption process and, thus, the main factors hindering NOx removal during the wet scrubbing process. This work demonstrates that RPB has great potential for removal of NOx by a wet scrubbing process in view of the small size of the RPB, low temperature, and the short gas retention time of 0.27 s applied in this work.

1. INTRODUCTION Continued rise in world population and economic growth have led to rapid increase in atmospheric nitrogen oxides (NOx) concentration in the last two decades, especially in developing countries like China and India.1−3 The presence of NOx in the atmosphere can lead to severe environmental problems such as acid rain and photochemical smog which is detrimental to both human and animal health.4 It is therefore necessary to eliminate NOx from industrial gas streams which is the main pollution source prior to disposal into the atmosphere. Many technologies, mostly classified as either dry or wet processes, have been developed for NOx emission control.5 Contrary to the dry processes, wet processes involve the use of chemical absorbents, and have the advantages of low investment cost as well as can remove NOx alongside other air pollutants such as SO2 from flue gas.6 NO and NO2, respectively, occupy 90% and 5−10% of total NOx emission,4,7 and since NO is less soluble in water than NO2, its oxidation to NO2 both via liquid- and gas-phase oxidation processes is beneficial to NOx removal.6 Oxidants such as hydrogen peroxide (H2O2),8,9 potassium permanganate (KMnO4),10,11 sodium hypochlorite (NaClO),12 and ozone (O3)13−15 can either be injected in the flue gas or directly added into liquid scrubbers to accelerate the absorption of NOx. Among these oxidants, O3 can oxidize NO to NO2 or even to a more water-soluble component N2O5 through homogeneous gas-phase reaction with high reaction rate and © XXXX American Chemical Society

oxidation efficiency as well as high selectivity by controlling the O3/NOx molar ratio (MR).13,15 Nonetheless, wet scrubbing processes for removal of NOx require intensification of mass transfer of NOx (especially for NO) from gas phase into liquid phase in order to enhance its removal efficiency as well as reduce the initial investment cost by minimizing the size of scrubbing reactors.6 A rotating packed bed (RPB), originally introduced by Ramshaw for mass-transfer intensification processes,16 is widely acknowledged as an efficient process-intensification technology and has been successfully applied in distillation,17 absorption,18 bromination,19 wastewater treatment,20 and other reaction processes. The high gravity environment in the RPB caused by the high-speed rotating packing leads to violent collisions between liquid and packing, and splits liquid into very fine liquid elements (including droplets, films, and threads), resulting into huge contact area and surface renewable rate between gas and liquid phases and, consequently, improves the mass-transfer process. Li et al.21 adopted an RPB to treat high concentration NOx (NO2) tail gas of 200−240 g m−3 by dilute nitric acid solution in the presence of O3 and found that high gravity level accelerated the oxidation of HNO2 to HNO3 by enhancing the mass transfer of O3 from gas to liquid and Received: May 16, 2017 Revised: September 8, 2017 Published: September 12, 2017 A

DOI: 10.1021/acs.energyfuels.7b01417 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels thereby improving the removal of NOx. Zhang et al.22 introduced RPB to improve the NO removal efficiency by using FeII (EDTA) solution as absorbent and found that increasing gravity level is beneficial to the removal of NO and a high removal efficiency of 87% could be attained with a gravity level beyond 150g. Previous studies indicate that the hydrolysis processes of NOx species are the rate-controlling steps,6,8,9 and thus, the technique is still limited by the low solubility of NO in view of the short gas−liquid contact time in the RPB. This study employed a rotating packed bed (RPB) to intensify the absorption performance of NOx into NaOH solution. NO, which is a major component of NOx emission and is less soluble in water than NO2, was first preoxidized to NO2 by ozone at 298 K and 104 kPa to enhance the removal efficiency of NOx. The effects of various operating conditions including O3/NOx molar ratio (MR), rotation speed of the RPB (N), liquid flow rate (L), NaOH concentration (CNaOH), inlet NOx concentration (CNOx), and using time (t) on NOx removal efficiency (η) were explored. Additionally, common oxidants (NaClO, H2O2, and KMnO4) and a reductant (CO(NH2)2) were separately added into the NaOH solution and their corresponding effects on NOx removal efficiency were investigated.

oxidation products including N2O3 and N3O4 are also generated according to reactions 4 and 5 due to the high oxidation ability of O3.8,16 However, under typical conditions of 295 K and 3.33 × 102 Pa, they are easily converted to N2O5 according to reactions 5 and 6 due to their shorter lifetimes of 0.0009 and 0.0025 s for N2O3 and N3O4, respectively, as compared to 300 s for N2O5.24 Also, the equilibrium concentrations of N2O3 and N2O4 in the gas phase are very small according to the equilibrium constants of reactions 2 and 4 as shown in Table 1. Therefore, the main oxidation products are NO2 and N2O5 under the condition of MR > 1. However, if there is water vapor in the gas phase, HNO2 and HNO3 are generated according to the following reactions:8,13,26

(2)

NO2 + O3 → NO3 + O2

(3)

NO + NO2 ↔ N2O3

(4)

N2O3 + O3 ↔ N2O4 + O2

(5)

N2O4 + O3 ↔ N2O5 + O2

(6)

NO2 + NO3 ↔ N2O5

(7)

9

references

(10)

N2O4 + H 2O → HNO2 + HNO3

(11)

N2O5 + H 2O ↔ 2HNO3

(12)

(13)

(14)

Hoftizer and Kwanten25

NO(l) + NO2(l) + H 2O(l) → 2HNO2(l)

(15)

Paiva and Kachan9

N2O3(l) + H 2O(l) → 2HNO2(l)

(16)

N2O4(l) + H 2O(l) → HNO2(l) + HNO3(l)

(17)

k1 = 2.59 × 10 exp(− 3.176/RT )

Mok

2

K 2 = 6.98 × 10−15 exp(6866/T )

4

K4 = 4.12 × 10−13 exp(4869/T ) K4 = 1.0 × 10

N2O3 + H 2O → 2HNO2

2NO2(l) + H 2O(l) → HNO2(l) + HNO3(l)

23

1

2072/ T − 12.240

(9)

There is a disagreement about mechanism and kinetics of reaction 9 as discussed by Newman and Carta,27 but the reaction should be considered in view of its importance in both chemical processing and the chemistry of the atmosphere. Equilibrium concentration of HNO3 is very small under typical operation conditions if partial pressure of NO is close to or larger than that of NO2.27 It also can be inferred that the equilibrium of HNO3 is much lower than that of HNO2 from Table 1. Additionally, Yoon et al. has found that no HNO3 was detected when MR < 1. Thus, reaction 8 may be neglected if MR < 1. When MR > 1, HNO3 in gas phase generates mainly via reaction 12. Moreover, N2O3 and N2O4 are easily converted to HNO3 according to reactions 10, 11, and 13. Therefore, in the presence of water vapor, the main oxidation products are NO2 and HNO2 while MR < 1, and NO2, N2O5, HNO2 and HNO3 while MR > 1. Since gaseous HNO2, HNO3, and N2O5 are more easily soluble in water than NO, the oxidation of NO is beneficial to the absorption of NOx. In this work, MR was maintained at 0.6 mol mol−1, and thus, the main oxidation products in the presence of water vapor include NO2 and HNO2. 2.2. Liquid-Phase Reactions. In NaOH solution, dissolved NOx first reacts with H2O to form HNO2 and HNO3, and thereafter, both HNO2 and HNO3 are neutralized with NaOH. Generally, the following reactions may take place:9,13

Table 1. Reaction Rate and Equilibrium Constants rate and equilibrium constants

NO + NO2 + H 2O → 2HNO2

HNO2 + O3 → HNO3 + O2

When MR < 1, reaction 1 is the main reaction and NO is majorly oxidized to NO2.23 When 1 < MR < 1.5, the major oxidation products are NO2 and N2O5 via reactions 1, 3, and 7.13,15 Although NO2 may thoroughly convert to N2O5 under high MR, NO2 always exists even if MR equals 2.5. Others

reaction no.

(8)

HNO2 is also easily oxidized by O3 according to the following reaction:13

2. CHEMICAL REACTIONS IN GAS AND LIQUID PHASES 2.1. Gas-Phase Reactions. The reaction between NO and O3 has high selectivity and reaction rate constant (see Table 1). When NO is mixed with O3 in the gas stream, the following reactions occur:8,13 NO + O3 → NO2 + O2 (1) NO2 + NO2 → N2O4

2NO2 + H 2O → HNO2 + HNO3

Joshi et al.

26

8

K8 = 9.36 × 10−7 (at 25 °C)

Joshi et al.26

9

K 9 = 1.825 × 10−12 exp(4723/T )

Hoftizer and Kwanten25

N2O5(l) + H 2O(l) → 2HNO3(l)

(18)

K 9 = 1.67 × 10−5 (at 25 °C)

Joshi et al.26

3HNO2(l) ↔ 2NO(l) + HNO3(l) + H 2O(l)

(19)

B

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Energy & Fuels HNO2(l) + NaOH(l) → NaNO2(l) + H 2O(l)

(20)

HNO3(l) + NaOH(l) → NaNO3(l) + H 2O(l)

(21)

Although NO2 is more soluble in water than NO, its solubility (HNO2 = (7.0 ± 0.5) × 10−3 L mol−1 atm−1 at 22 °C) is still relatively low.28 At a low partial pressure, reaction 14 with a reaction constant of k14 = (1.0 ± 0.1) × 108 L mol−1 s−1 is considered as a slow irreversible second-order reaction.28 On the other hand, reactions 16 and 17 are fast and irreversible;9 nonetheless, still they can be neglected since, in this study, the main oxidation species in the gas stream is NO2. Despite the fact that HNO2(l) is unstable and decomposes according to reaction 19 to release NO, the reaction can be prevented by the instantaneous reaction 20.8,9 Furthermore, owing to the instantaneous reactions 20 and 21, reactions 14−17 are the rate-controlling steps in this process.8 In view of the very short lifetime of N2O3 and N3O4 in the gas and the relatively low MR used in this study, the main reactions in the liquid phase are assumed to be 14, 15, 20, and 21, whereas reactions 14 and 15 are the rate-controlling steps.

Figure 2. Schematic diagram of the RPB. (Limicen Ozone R&D Center, Guangzhou, China, detection range of 0−1000 ppm and an accuracy of 1 ppm). When the O3/NOx molar ratio reached a steady preset value, NaOH solution was pumped into the RPB through a liquid inlet and then sprayed radially on the inner edge of the packing. The liquid (NaOH solution) split into fine liquid elements of droplets, films, and threads under the influence of high centrifugal force created by the rotating packing. NOx was absorbed into NaOH solution by counter-current contact of the gas stream with the liquid stream. Finally, both gas and liquid streams exited from the RPB through a gas outlet and a liquid outlet, respectively. The operating conditions were varied as follows: MR = 0−1.3, N = 200− 1200 rpm, CNaOH = 0−0.1 mol L−1, L = 60−300 L h−1, CNOx = 200− 1200 mg m−3, and using time = 5−30 min. The experiments were carried out at 298 K and about 104 kPa. Gas stream was transferred by the pressure-driving force, and the gas pressure drop with a value lower than 200 Pa could be ignored. In this research, each experiment was repeated, and the results showed that the repeated error was no larger than 4%. Therefore, the average value was used in this paper. It should be noted that the NO analyzer employed in this study had an electrochemical transducer of lead−lead oxide electrode to detect NO. Since O3 is harmful to this kind of electrode, the NO analyzer was removed prior to introduction of O3 into the gas mixer. The total NOx concentration in the gas inlet and outlet was detected by Chinese national standard HJ 479−2009.29

3. EXPERIMENTAL SECTION 3.1. Materials. NaOH, KMnO4, and CO(NH2)2, all of analytical grade, were purchased from Beijing Chemical Works, while H2O2 and NaClO were obtained from Xilong Chemical Co., Ltd., and Sinopharm Chemical Reagent Co., Ltd., respectively. NO (purity ⩾ 99.9%) and N2 (purity ⩾ 99.9%) were supplied by Beijing Huayuan Gas Chemical Industry Co., Ltd. All the chemicals were used without further purification. 3.2. Experimental Procedure. Figure 1 shows the experimental setup for absorption of NOx into NaOH solution in RPB with

4. RESULTS AND DISCUSSION The NOx removal efficiency (η) was calculated by the following equation ⎛ c NOx ,out ⎞ ⎟ × 100% η = ⎜⎜1 − c NOx ,in ⎟⎠ ⎝

Figure 1. Experimental setup for absorption of NOx into NaOH solution in RPB with preoxidation by ozone.

(22)

where CNOx,in and CNOx,out are inlet NOx concentration and outlet NOx concentration, respectively. 4.1. Effect of MR. Figure 3 shows that NOx removal efficiency increased significantly with increase in MR, but it changed insignificantly when MR exceeded 0.6. Due to the really low solubility of NO, oxidation of NO by O3 to NO2 is a key step in NOx absorption. As aforementioned, the main oxidation products were NO2 and HNO2 when MR was less than 1, whereas the main oxidation products were NO2, N2O5, HNO2, and HNO3 when MR was greater than 1. Consequently, the removal efficiency increased with an increase in MR due to the large solubilities of NO2, N2O5, HNO2, and HNO3. However, there was little change in η with a rise in MR beyond 0.6. The similar results have also been observed by Thomas and Vanderschuren,8 Liu,30 and Cui.31 Thomas and

preoxidation by ozone. The packing of the RPB comprised stainless wire mesh with a voidage of 0.97 and surface area of 522 m3 m−2. The inner diameter, outer diameter, and axial height of the packing were 0.08, 0.2, and 0.012 m, respectively, and the diameter of the shell of the RPB was 0.3 m (see Figure 2). A simulated flue gas with a flow rate of 4.2 m3 h−1 was obtained by mixing N2, NO, and O3. First, N2 and NO were separately introduced into the gas mixer, and thereafter O3 as soon as NO concentration reached a steady preset value. NO was partly oxidized by O3 in the premixer. O3 was generated from pure O2 by an ozone generator (GFG10, Shanmeishuimei Environmental Technologies Co. Ltd., Beijing, China). The concentration of NO was determined by a NOx detector (Ecom-CN, RBR Analytical Instrument Institution, Germany, NO volume fraction ranging from 0 to 2000 ppm and an accuracy of 1% of the range) while that of O3 was monitored by an ozone analyzer C

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NOx and NaOH (reactions 23−25) may take place in liquid phase.26,32,33 2NO2(l) + 2NaOH(l) → NaNO2(l) + NaNO3(l) + H 2O(l) (23)

N2O3(l) + NaOH(l) → 2NaNO2(l)

(24)

N2O4(l) + 2NaOH(l) → NaNO2(l) + NaNO3(l) + H 2O (25) 34

Kameoka and Pigford inferred that reaction 25 takes place in parallel with the hydrolysis of N2O4 and this could interpret their experimental results. However, they also pointed out that there is no direct evidence concerning this reaction mechanism. Takeuchi and Yamanaka,35 and Gu et al.36 believed that NaOH cannot react with NOx directly, but it will neutralize HNO2 and HNO3 formed from the hydrolysis of NOx, i.e., reactions 20 and 21. A small amount of NaOH could accelerate reactions 20 and 21, and thus lead to the acceleration of reactions 14 and 15. Consequently, the liquid-phase driving force increases and thus boosts the absorption process of NOx, leading to a considerable increase in removal efficiency. Nonetheless, reactions 14 and 15 are the controlling steps, and NaOH cannot directly consume NO or NO2. Meanwhile, viscosity and ionic strength of the solution increase with increasing CNaOH, which lowers the solubilities and diffusivities of NOx species into solution. Therefore, too high CNaOH may hinder the absorption process. In our work, the removal efficiency of NOx became fairly stable when CNaOH exceeded 0.05 mol L−1. 4.3. Effect of Rotation Speed. Figure 5 shows that removal efficiency increased with increase in rotation speed of

Figure 3. Effect of MR on NOx removal efficiency.

Vanderschuren8 believed that trivalent NOx species, particularly HNO2 (i.e., reactions 9 and 20), play an important role in NOx absorption into NaOH solution at intermediate MR around 0.6 and lead to the highest removal efficiency of NOx. On the basis of previous studies and the analysis of experimental data, it can be deduced that, when MR ≤ 0.6, NO2 concentration increased with increasing MR to generate HNO2 through the reactions 9 and 15, and then improved the absorption of NOx via the subsequent reaction 20, which led to an increase in removal efficiency. When MR > 0.6, even though both reactions 8 and 14 were promoted, hydrolysis reaction 14 was still the controlling step, and the redundant NO2 could not be absorbed completely due to its low reaction rate and the short gas−liquid contact time in the RPB, which lead to an insignificant effect on removal efficiency. Therefore, a relatively steady removal efficiency was observed when MR exceeded 0.6, and an MR of 0.6 was adopted for the further investigations in this work. 4.2. Effect of NaOH Concentration. Figure 4 shows that NOx removal efficiency increased considerably with increase in CNaOH but fairly remained stable with a rise in CNaOH beyond 0.05 mol L−1. In an alkaline solution, direct reactions between

Figure 5. Effect of rotation speed on NOx removal efficiency.

RPB, reaching a maximum at a rotation speed of 800 rpm, and thereafter declined with further rise in rotation speed. Higher rotation speed splits liquid into finer liquid elements and thereby improves the degree of wetness of gas in RPB. As a result, gas-phase reaction (reaction 9) is enhanced and thus generates more gaseous HNO2 to neutralize with NaOH according to reaction 20, resulting into increased absorption of NOx. Also, higher rotation speed leads to more fine liquid elements which provide more effective gas−liquid contact area and accelerates gas−liquid surface renewal rate. This consequently leads to higher mass-transfer efficiency and by

Figure 4. Effect of NaOH concentration on NOx removal efficiency. D

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there is a limit to which increasing liquid flow rate alone can increase NOx removal efficiency. 4.5. Effect of Inlet NOx Concentration. Figure 7 shows that removal efficiency slightly decreased with increasing inlet

promoting the absorption of nitrogenous species (mainly NO and NO2) into the liquid mainly via reactions 14 and 15. However, higher rotation speed also leads to reduced liquid retention time in RPB,37 and therefore hinders the absorption process in view of the low solubility of NOx. This factor overrode the aforementioned benefits of higher rotation speed beyond 800 rpm in this study, leading to the decline in removal efficiency. Yoon et al.15 adopted an ultrasonic humidifier to generate fine liquid droplets (droplet size distribution of 1−5 μm) to enhance mass-transfer efficiency, which aims to ensure high removal efficiency under a very low gas retention time of 3.1 s. In the RPB, liquid elements have an average size of 10−5−10−4 m,37,39 which is relatively larger than those in Yoon’s work.15 Moreover, their velocities keep continuously increasing along the radial direction of packing38 and high velocity can hold the liquid elements in the RPB rather than being blown away by gas, resulting in a high flooding point to avoid severe liquid entrainment. These mean satisfied removal efficiency with short gas retention time can be achieved. Therefore, one can see that RPB has the potential to enhance the absorption of NOx in view of the short gas retention time (approximately 0.27 s in this work) as compared to the commonly used wet processes.15 4.4. Effect of Liquid Flow Rate. Figure 6 shows that removal efficiency increased with increasing liquid flow rate.

Figure 7. Effect of inlet NOx concentration on NOx removal efficiency.

Figure 6. Effect of liquid flow rate on NOx removal efficiency.

NOx concentration (i.e., increasing partial pressure of NOx). Although higher inlet NOx concentration enhances gas-phase driving force and thus improves reactions 14 and 15 which are beneficial to NOx absorption, the restricted diffusivities of NOx species in the liquid impede the mass transfer of NOx from gas to liquid. This means increasing inlet NOx concentration had a limited effect on the rate-controlling steps of reactions 13−16. Additionally, since the denitration capacity of the solution was constant, an increase in inlet NOx concentration caused a reduction in the amount of absorbed NOx per volumetric gas. As a result, an increase in inlet NOx concentration caused a slight decrease in NOx removal efficiency. 4.6. Effect of Using Time. 5 L of NaOH solution was cycled for NOx absorption in the RPB in order to investigate the effect of using time on NOx removal efficiency. It is evident from Figure 8 that removal efficiency decreased with increasing using time. Cycling of the solution in the RPB over time led to accumulation of HNO2 and HNO3 in the solution, resulting in reduced alkalinity via hydrolysis reaction of HNO2 and HNO3,

Higher liquid flow rate means extra fresh NaOH solution to absorb NO x according to reactions 20 and 21, and consequently gives a rise in mass-transfer driving force. Also, higher liquid flow rate resulted into increased liquid holdup and larger effective gas−liquid contact area. All of these factors enhanced the absorption of NOx, resulting into higher NOx removal efficiency. However, there was insignificant increase in removal efficiency at liquid flow rates beyond 180 L h−1. This may be attributed to a drop in the effective gas−liquid contact area as a result of increase in both the average diameter of liquid droplets and the thickness of liquid film.39,40 The fact that NOx removal efficiency only increased by 13.9% when liquid flow rate increased from 60 to 300 L h−1 implies that the hydrolysis reactions 13−16 are still the ratedetermining steps in the absorption process of NOx. Thus,

Figure 8. Effect of using time on NOx removal efficiency. E

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Energy & Fuels and thus hindering the hydrolysis of NOx species. NaOH concentration in the solution also decreased with cycling of the solution over time, and 20% NaOH was consumed after 30 min. All of these factors led to reduction in NOx removal efficiency. 4.7. Effect of Additives. Three common oxidants (NaClO, H2O2, and KMnO4) and a reductant (CO(NH2)2) were separately added into the solution, and the corresponding effects of different additive concentration and different MR on NOx removal efficiency were investigated as shown in Figure 9.

NO(g) + NO2(g) + CO(NH 2)2 → 2N2(g) + CO2(g) + 2H 2O

(26)

Although KMnO4 greatly promoted the absorption of NOx by NaOH solution, H2O2 and CO(NH2)2 may be the better choice in view of the high cost and secondary pollution of KMnO4. Additionally, the effects of MR and additive concentration on removal efficiency in Figure 9 was not obvious when MR and additive concentration exceeded 0.6 and 0.01 mol L−1, respectively, which are similar to those in Figures 3 and 4. Thus, it may be concluded that the hydrolysis reactions 14 and 15 are still the rate-controlling steps and that both oxidants, and the reductant have limited effect on the whole absorption process of NOx into liquid in view of the shorter gas and liquid retention time in RPB compared to other common reactors.

5. CONCLUSION This study investigated the absorption performance of NOx into NaOH solution in an RPB with preoxidation by ozone at 298 K and 104 kPa. The absorption performance was evaluated in terms of NOx removal efficiency (η) as a function of various operating conditions including O3/NOx molar ratio (MR), rotation speed of the RPB (N), liquid flow rate (L), NaOH concentration (CNaOH), inlet NOx concentration (CNOx), using time (t), and the presence of oxidation and reduction additives. The removal efficiency increased with increasing MR and reached the highest level at MR of 0.6. Alkaline environment favored neutralization of HNO2 and HNO3 and thus improved the absorption of NOx, but NaOH concentration beyond 0.05 mol L−1 had an insignificant effect on NOx removal efficiency. Increasing rotation speed of the packing enhances the gas− liquid mass-transfer process but reduces liquid retention time. Consequently, NOx removal efficiency increased first and then decreased with increase in rotation speed. The removal efficiency increased with increasing liquid flow rate, but the increase became insignificant at liquid flow rates beyond 180 L h−1. Both increasing inlet NOx concentration and using time resulted into a reduction in NOx removal efficiency. Both oxidation and reduction additives enhanced the NOx removal process in the order of KMnO4 > H2O2 > CO(NH2)2 > NaClO. However, in view of the high cost and secondary pollution of KMnO4, H2O2 and CO(NH2)2 may be the better choice. These results indicate that the hydrolysis reactions of NOx are still the rate-determining steps in the NOx absorption process and are the main obstacles hindering NOx removal by wet scrubbing processes. This work demonstrates that RPB has great potential for removal of NOx by a wet scrubbing process in view of the low temperature and the short gas retention time of 0.27 s applied in this work.

Figure 9. Effect of additives on NOx removal efficiency: (a) different O3/NOx molar ratio; (b) different additive concentration.

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It is clear that a small addition of either oxidant or reductant enhanced the absorption of NOx even if at a low MR, and the order of enhancement was found to be KMnO4 > H2O2 > CO(NH2)2 > NaClO. In the liquid-phase oxidation process, the oxidants convert HNO2 to HNO3 and also directly react with NOx,6 and thus favor the absorption of NOx. CO(NH2)2 is a strong reducing agent which can convert NOx to N2 via the overall reaction 26,41 but the reaction may be prevented by the absorption reaction between NaOH and CO2.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 64449453. E-mail: [email protected]. ORCID

Baochang Sun: 0000-0002-3435-1250 Haikui Zou: 0000-0003-0681-9036 Notes

The authors declare no competing financial interest. F

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(25) Hoftizer, P.; Kwanten, F. Absorption of Nitrous Gases. In Gas Purification Processes for Air Pollution Control; Nonhebel, G., Ed.; Newnes-Butterworths: London, 1972. (26) Joshi, J.; Mahajani, V.; Juvekar, V. Invited review absorption of NOx gases. Chem. Eng. Commun. 1985, 33, 1. (27) Newman, B. L.; Carta, G. AIChE J. 1988, 34, 1190. (28) Lee, Y.; Schwartz, S. J. Phys. Chem. 1981, 85, 840. (29) Ministry of Environment Protection of China. Ambient air Determination of nitrogen oxidesN-(1-Naphthyl)ethylene diamine dihydrochloride spectrophotometric method; HJ 479−2009; Nov 2009. http://kjs.mep.gov.cn/hjbhbz/bzwb/dqhjbh/jcgfffbz/200910/ W020111114528130751641.pdf (accessed Jan 15, 2017). (30) Liu, H. Studies on NOx Removal by NO Catalytic Oxidation and Alkali Solution Absorption at Ambient Temperature. Ph.D. Thesis, Zhejiang University, Zhejiang, China, 2011 (in Chinese). (31) Cui, J. Experimental Research on Simultaneous Removal of SO2 and NOx using Advanced Oxidation Process. Master Thesis, North China Electric Power University, Beijing, China, 2012 (in Chinese). (32) Pradhan, M.; Joshi, J. AIChE J. 1999, 45, 38. (33) Pradhan, M.; Joshi, J. Chem. Eng. Sci. 2000, 55, 1269. (34) Kameoka, Y.; Pigford, R. L. Ind. Eng. Chem. Fundam. 1977, 16, 163. (35) Takeuchi, H.; Yamanaka, Y. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 389. (36) Gu, Y.; Tan, T. J. Chem. Eng. Chin. Univ. 1990, 2, 006 (in Chinese). (37) Guo, K.; Guo, F.; Feng, Y.; Chen, J.; Zheng, C.; Gardner, N. C. Chem. Eng. Sci. 2000, 55, 1699. (38) Yang, K. Micromixing and Gas-Liquid Mass Transfer Characteristic in Rotating Packed Bed. Ph.D. Thesis, Beijing University of Chemical Technology, Beijing, China, 2010 (in Chinese). (39) Zhang, J. An Experimental and Simulation Study on Liquid Flowing and Mass Transfer in RPB. Ph.D. Thesis, Beijing University of Chemical Technology, Beijing, China, 1996 (in Chinese). (40) Guo, F.; Zheng, C.; Guo, K.; Feng, Y.; Gardner, N. C. Chem. Eng. Sci. 1997, 52, 3853. (41) Fang, P.; Cen, C.; Tang, Z.; Zhong, P.; Chen, D.; Chen, Z. Chem. Eng. J. 2011, 168, 52−59.

ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (No. 2016YFC0205205), the National Natural Science Foundation of China (Nos. 21406009, U1607114), and Beijing Municipal Natural Science Foundation (No. 8162020).

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REFERENCES

CNaOH=NaOH concentration (mol L−1) Cadditive=concentration of additive (mol L−1) CNOx=NOx concentration (mg m−3) CNOx,in=inlet NOx concentration (mg m−3) CNOx,out=outlet NOx concentration (mg m−3) G=gas flow rate (m3 h−1) ki=rate constant for reaction i (L mol−1 s−1) Ki=equilibrium constant for reaction i (Pa−1) L=liquid flow rate (L h−1) MR=O3/NOx molar ratio (mol mol−1) N=rotation speed of the RPB (rpm) t=using time (min) η=NOx removal efficiency (%)

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DOI: 10.1021/acs.energyfuels.7b01417 Energy Fuels XXXX, XXX, XXX−XXX