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Keywords:

  • 1,2,3-TCB;
  • molten salt oxidation (MSO);
  • chlorine retention;
  • reaction pathway

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

This work focused on the destruction of 1,2,3-trichlorobenzene (1,2,3-TCB) in a binary molten salt oxidation (MSO) system. The effects of MSO reactor temperatures, additive amounts of TCB, and salt mixture on destruction and removal efficiency (DRE) of TCB and chlorine retention efficiency (CRE) were investigated. The drained salt was characterized by X-ray diffraction (XRD) analysis, and the reaction mechanism and pathway were proposed as well. The results showed that the DRE and CRE reached more than 99 and 95%, respectively, at temperatures above 900°C. The reaction by-products included C6H6, CH4, and CO, while chlorinated species were not detected. Larger amounts of salt mixture meant prolonging the gas residence time and were helpful for reaction. When TCB loading increased from 5 to 25 g, the DRE decreased by 6, <1, and <1%, and the CRE also decreased. The XRD analysis of drained salt confirmed the capture of chlorine in TCB by the molten salt. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 65–69, 2014


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

Chlorobenzenes are widely used in chemical industry as organic synthesis intermediates, pesticides, dye carrier, dielectric fluids, degreasing solvents, and synthetic transformer oils [1, 2]. However, their increasing use leads to a concomitant increase released to the environment. These toxic compounds are environmentally stable and easily distributed due to their volatility and resistance to degradation, therefore they have been ranked as prior pollutants by USEPA [3, 4]. Numerous studies have certified their existence in soil [5-11]; therefore, it is highly necessary to develop appropriate treatment and remediation technologies.

Molten salt oxidation (MSO) is a versatile and promising technology for the destruction of chlorinated organics. It is capable of trapping chlorine during the destruction of organics and has been tested for the destruction of polychlorinated biphenyls, hexachlorobenzene, carbon tetrachloride, and poly(vinyl chloride) (PVC) with high destruction efficiency [12]. Yang et al. [13] investigated the destruction of chlorobenzene and trichloroethylene in a two-stage molten salt reactor. Incomplete products (PIs) such as C6H6, C6H5Cl, CCl4, and C2H2 were produced in the first reactor and substantially destroyed in the second reactor. A 2,4 dichlorophenol-spiked organic was also selected and tested. The results showed that off-gas quality was very good with CO and NOx less than 10 and 110 ppm, respectively. Other PIs such as polychlorinated dibenzo-p-dioxins and dibenzofurans were not detected. Yang et al. [14] also confirmed the capture of chlorine and fluorine during the destruction of PVC plastics and polytetrafluoroethene (PTFE) tubes in a bench-scale MSO reactor. These reports indicated that MSO is a robust, controllable technology and has good prospects for the treatment of chlorinated organics. Thus, it can be used as a second chemical scrubbing reactor following the first reactor, where organic pollutants desorbed from contaminated soils.

Trichlorobenzene (TCB) is one of the most widely used chlorobenzenes and has often been used as a model pollutant for developing efficient treatment technologies and probing the reaction mechanisms. In this study, 1,2,3-TCB was selected as a model pollutant to investigate its destruction in a MSO reactor system. The effects of MSO reactor temperatures, additive amounts of TCB and salt mixture on destruction and removal efficiency (DRE) of TCB and chlorine retention efficiency (CRE) were investigated. The drained salt was characterized by X-ray diffraction (XRD) analysis, and the reaction mechanism and pathway were proposed as well.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

Reactor System

The bench-scale reactor in this work was produced by Ocean Glory GH (Beijing, China) Technology and its schematic diagram is shown in Figure 1. It consists of a thermal desorption reactor with a capacity of 3 L and a MSO reactor a 2-L capacity. They are made of 304 stainless steel and 310S stainless steel to withstand both high temperatures and chlorine attack. Both reactors are heated by external ceramic fiber heaters. The thermal desorption reactor could be used to model the desorption of TCB from contaminated soils. The volatilized TCB from the first reactor is then introduced into MSO reactor and oxidized. The top of the thermal desorption reactor cover is connected to an air reservior. The off-gas leaving the MSO reactor is cooled down by a vertical air-to-gas heat-exchanging condenser, and then scrubbed by NaOH solution, n-hexane and subsequently vented. The NaOH solution and n-hexane were successively used mainly for absorbing the acid gas and organic components in off-gas.

image

Figure 1. The schematic diagram of reactor system.

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Materials

1,2,3-TCB used was obtained from Chengdu Xiya Chemical (China). A binary salt mixture consisting of 71 mol % Na2CO3 and 29 mol % K2CO3 with eutectic temperature of 573°C was selected [15]. Powdered anhydrous sodium carbonate and potassium carbonate were of analytical grade from Beijing Chemical Reagents Company.

Procedures

Various amounts (10, 12, and 15 mol) of salt mixture were measured and placed in the MSO reactor and then heated to different fixed temperatures (750, 800, 850, 900, and 950°C) to achieving melting. Thereafter, different amounts of TCB (5, 15, and 25 g) were placed in the thermal desorption reactor, heated and the air was simultaneously introduced with a flow rate of 1.5 L/min. When the temperature of thermal desorption reactor reached above the boiling point of TCB, the off-gas from the MSO was collected and its composition analyzed. After each batch experiment, the salt was drained, cooled down and analyzed for chloride content. The amounts of unreacted TCB in n-hexane and chloride ion in NaOH solution were analyzed.

Characterization and Test

The XRD analysis was used to determine the crystalline phases of drained salt. A Rigaku D/max-2550 X-ray power diffractometer operated at 40 kV and 30 mA, with Cu Kα as the radiation source. The amount of choride in drained salt and NaOH solution were determined by ion chromatography (IC). The contents to the concentration of CO and CO2 were measured by a Perkin-Elmer Instruments Auto System XL GC, equipped with thermal conductivity detector. C6H6 and CH4 were identified and measured by gas chromatography–mass spectrometry.

The dechlorination efficiency of TCB can be assessed by two indexes, DRE and CRE. The calculated formulae are as follows:

  • display math(1)

where Min—the amount of TCB placed in the thermal desorption reactor (g); Mout—the amount of TCB absorbed in n-hexane (g).

  • display math(2)

where SCl—the amount of chlorine atoms in TCB (g); YCl—the amount of chlorine in drained salt/g; JCl—the amount of chloride in alkaline solution (g); WCl—the amount of chlorine atoms in TCB absorbed in n-hexane solution (g).

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

Effect of MSO Reactor Temperatures

The DRE of TCB and the CRE at different temperatures are depicted in Table 1. It is shown that as temperatures increased, both DRE and CRE increased. A similar trend was followed for different amounts of TCB (5–25 g). This increasing trend was noted in the temperature range of 800–900°C and thereafter became steady. At higher temperatures above 900°C, the DRE and CRE reached more than 99 and 95%, respectively. This indicated that most of the chlorine atoms in TCB were captured by the salt and a small portion had not reacted completely and was subsequently absorbed by n-hexane.

Table 1. The DRE, CRE, and off-gas compositions at different temperatures (salt mixture: 10 mol)
C6H6 conc. (×102 mg·m−3)CH4 conc. (×102 mg·m−3)CO content (%)CO2 content (%)DRE (%)CRE (%)Add. TCB (g)Temp. (°C)
  1. Add. TCB: additive amount of TCB; conc.: concentration; Temp.: temperature.

0.320.133.75494.5289.795750
0.350.13.424.998.9994.04800
0.150.072.657.599.0297.04850
0.030.031.910.399.6298.62900
0.020.011.441299.9198.91950
2.171.156.5310.490.5886.0515750
2.060.276.2810.991.6887.09800
1.480.314.161199.4195.43850
0.360.12.6112.1599.6196.62900
0.230.071.9213.2599.6896.69950
5.64.16.2912.788.9784.5225750
3.61.165.6913.190.8486.30800
3.951.184.9413.598.2394.30850
0.70.474.3814.999.2195.24900
0.410.232.8117.4999.2896.30950

It was found that the reaction by-products included C6H6, CH4, and CO. The chlorinated species were not detected, which was consistent with the report of chlorobenzene destruction [16]. The concentrations and contents of C6H6, CH4, and CO decreased with increasing temperatures, while CO2 levels increased. This indicated that elevating temperatures was helpful for reaction and the PIs such as C6H6, CH4, and CO were further oxidized and transformed into CO2.

Effect of Additive Amounts of TCB

Table 2 illustrates the DRE and CRE with different additive amounts of TCB. It showed that both DRE and CRE decreased with an increasing TCB addition. For each fixed temperature 750, 850, and 950°C, when the TCB amount increased from 5 to 25 g, the DRE decreased by 6, <1, and <1%, and the CRE decreased by 5, 3, and 3%, respectively. This indicated that increasing TCB amounts could enlarge the test scale at the cost of DRE and CRE, but controlling the temperature could relieve this dilemma.

Table 2. The DRE, CRE, and off-gas compositions with different amounts of TCB (salt mixture: 10 mol)
C6H6 conc. (×102 mg·m−3)CH4 conc. (×102 mg·m−3)CO content (%)CO2 content (%)DRE (%)CRE (%)Add. TCB (g)Temp. (°C)
  1. Add. TCB: additive amount of TCB; conc.: concentration; Temp.: temperature.

0.020.010.741299.9198.915950
0.050.031.6812.9499.6298.6210
0.230.071.9213.2599.6896.6915
0.360.112.1814.6199.3196.3320
0.410.232.8117.4999.2896.3025
0.150.072.657.599.0297.045850
0.810.13.57.899.7496.7510
1.480.314.161199.4195.4315
2.230.394.1911.399.0995.1320
3.951.184.9413.598.2394.3025
0.320.133.75394.5289.795750
1.310.614.256.891.9487.3410
2.171.155.5310.490.5886.0515
3.53.595.4612.589.4084.9320
5.64.16.2912.788.9784.5225

From Table 2, it can be seen that the concentrations and contents of C6H6, CH4, CO, and CO2 emitted increased with addition of TCB. This trend was significant when the temperature was lower. At a temperature of 750°C and with TCB loading increased from 5 to 25 g, the concentrations of C6H6 increased significantly from 32 to 560 mg·m−3 and CH4 increased from 13 to 410 mg·m−3. This indicated that PIs levels also increased with TCB increasing, which was accompanied by a decrease of DRE and CRE.

Effect of Additive Amounts of Salt Mixture

Table 3 shows the DRE and CRE with different amounts of salt mixture and revealed that both DRE and CRE increased with temperature increases, when the TCB loading increased from 10 to 15 mol. Increasing the amount of salt mixture can be interpreted as prolonging the gas residence time, which would be expected to increase the destruction process [16].

Table 3. The DRE, CRE, and off-gas compositions with different amounts of salt mixture (TCB: 15 g)
C6H6 conc. (×102 mg·m−3)CH4 conc. (×102 mg·m−3)CO content (%)CO2 content (%)DRE (%)CRE (%)Add. salt (mol)Temp. (°C)
  1. Add. salt: additive amount of salt mixture; conc.: concentration; Temp.: temperature.

2.171.155.539.490.5886.0510750
1.670.955.1310.191.6887.0912
1.170.754.3311.493.8389.5015
2.060.476.2810.9  10800
1.360.344.5211.3  12
0.960.273.4812.1  15
1.480.314.161199.4195.4310850
0.780.223.612.4399.5396.5812
0.580.162.641499.5697.8215
0.3640.1012.6112.15  10900
0.290.092.2113.75  12
0.1640.061.7316.35  15
0.230.071.9213.2599.6896.6910950
0.130.051.6815.6599.8097.8512
0.030.021.0418.7599.8399.1115

Table 3 also showed that the concentrations and contents of C6H6, CH4, and CO decreased with salt mixture increase while CO2 levels increased as expected. This indicated that increasing the amount of salt mixture could lead to complete reaction and thus eliminate the PIs, including C6H6, CH4, and CO to CO2.

MECHANISM AND REACTION PATHWAY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

The XRD patterns of salt mixture before and after reaction are given in Figure 2. It showed that the major crystalline phases of salt mixture before reaction were sodium carbonate hydrate (Na2CO3·H2O, JPCDS card no. 70-2148), natrite (Na2CO3, JPCDS card no. 37-451) and potassium carbonate (K2CO3, JPCDS card no. 01-071-3954). After reaction, the major phases of drained salt were sylvine (KCl, JPCDS card no. 75-296) and sodium carbonate hydrate. The sylvine may be formed by the reaction of carbonate ions and the destruction product of TCB. The sodium carbonate hydrate was the remainder of unreacted molten salt.

image

Figure 2. The XRD patterns of salt mixture before (a) and after (b) reaction.

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There are many earlier studies that describe the treatment efficiency of MSO but they do not attempt to explain the mechanisms. The only attempt to probe the mechanism was that by Stelman and Gay [17]. It was based on studies of the reaction between graphite and molten sodium carbonate [18-20]. At that time, these workers did not appreciate the molten salt chemistry involved and assumed that the molten salt was acting as a catalyst. In developing catalyst-enhanced molten salt oxidation, Griffiths et al. [21] explained that the oxygen dissolved chemically in molten carbonates and formed oxidizing species, O22− and O2. They also proved that the addition of nitrate ion as catalyst could sustain higher concentrations of superoxide ions.

The reaction pathway assumed of this study was that (see Figure 3), first, oxygen was chemically dissolved in molten salt and formed oxidizing species, O22− and O2. These strong oxidizing agents attacked TCB and thus a hydrogenation/dechlorination reaction occurred. When TCB was completely dechlorinated, C6H6, and chloride ion were produced simultaneously. Hydrogen atoms detached from TCB by the oxidizing species will react to form hydroxyl radicals, which could be further oxidized to form water molecules. Under severe reaction conditions, C6H6 was further oxidized forming CH4, CO, CO2, and H2O. The resulting chloride ion will be retained in the salt, thus reducing the emission of chlorinated pollutants.

image

Figure 3. The reaction pathway of 1,2,3-TCB.

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CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

MSO is a promising alternative technology that can effectively treat 1,2,3-TCB without emission of chlorinated pollutants. The detected by-product contained C6H6, CH4, and CO but chlorinated species were not found. The effects of MSO reactor temperatures, additive amounts of TCB, and salt mixture on DRE and CRE were investigated. The results showed that the DRE and CRE reached more than 99 and 95%, respectively, at temperatures above 900°C. Larger amounts of salt mixture were helpful for the reaction. Within the temperature range of 750–950°C, when TCB loading increased from 5 to 25 g, the DRE decreased by 6, <1, and <1%, and the CRE also decreased. The XRD analysis confirmed the capture of chlorine by the molten salt. It is worth noting that the air flow rate may be another factor, so in the further studies the effect of air flow rate at which the air is bubbled through the carbonate melt could be investigated for evaluating the optimum reaction conditions.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED

We gratefully acknowledge financial support from the Chinese National 863 High Technology (Grant no. 2009AA064001) and the Scientific Research Foundation of Hangzhou Dianzi University (Grant no. KYS205612029). We also would like to thank the anonymous referees for their helpful comments on this article.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. MECHANISM AND REACTION PATHWAY
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. LITERATURE CITED
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