Hydrothermal Dechlorination and Denitrogenation of Municipal-Waste

The hydrothermal processing of municipal-waste-plastics-derived fuel oil (kerosene fraction; Cl content ) 62 ppm, N content ) 1150 ppm) under sub- and...
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Ind. Eng. Chem. Res. 2002, 41, 5393-5400

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Hydrothermal Dechlorination and Denitrogenation of Municipal-Waste-Plastics-Derived Fuel Oil under Sub- and Supercritical Conditions Masamichi Akimoto,* Kiyoshi Ninomiya, Shigeo Takami, Masahiro Ishikawa, Masahiro Sato, and Katsuhiro Washio Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki, Niigata 945-1195, Japan

The hydrothermal processing of municipal-waste-plastics-derived fuel oil (kerosene fraction; Cl content ) 62 ppm, N content ) 1150 ppm) under sub- and supercritical conditions has been investigated so as to demonstrate the possible use of water and aqueous solutions of metal salts and hydroxides for the dechlorination and denitrogenation of the fuel oil. The hydrothermal processing was carried out in a small SUS316 stainless steel batch reactor under nitrogen atmosphere. Although the two reactions took place in water, they proceeded much more readily under basic conditions, especially in aqueous solutions of alkaline metal hydroxides. That is, the nitrogen content in the product oil decreased to 297 ppm upon processing with water for 15 min at 425 °C, whereas it decreased to 49 ppm when 0.10 mol/L NaOH was used instead of water at 375 °C. Under these hydrothermal conditions, the chlorine content in the product oil was always nearly 0 ppm. Organic acids such as benzoic acid and phthalic acid in the fuel oil could also be removed. Introduction In Japan, the thermal and/or catalytic degradation of waste plastics into fuel oil is one of the more promising of various methods for recycling municipal waste plastics. However, the fuel oil produced from municipal waste plastics by the present commercial plants in Japan contains organic and inorganic chlorides and nitrogen compounds. For example, the total chlorine and nitrogen contents in the kerosene fraction are 50740 and 1000-2000 ppm, respectively, depending on the composition of the municipal waste plastics and the degradation process employed.1 Upon burning of the oil, these chlorine and nitrogen atoms can damage the combustion furnace and, at the same time, produce hazardous materials such as dioxins and nitrogen oxides. Therefore, dechlorination and denitrogenation processes for the waster-derived fuel oil should be established as soon as possible. The formation of organic chlorides and nitrogen compounds in the thermal decomposition of mixed plastics into fuel oil and the removal of heteroatoms (Cl and N) from the resulting fuel oil by use of silicaalumina catalyst and iron oxides have been reported.2-6 However, neither the chlorine content nor the nitrogen content was low enough for the refined oils to be used as fuel oil. The chlorine and nitrogen contents in the refined oils should be lower than 10 and 100 ppm, respectively, from the viewpoint of environmental protection. Recently, the application of supercritical water for the liquefaction of heavy fossil fuels such as coal, oil shale, and oil sand has attracted attention.7 To obtain cleanerburning fuel oils through the removal of heteroatoms, * To whom all correspondence should be addressed. Telephone and Fax: +81-257-22-8138. E-mail: akimoto@ acb.niit.ac.jp.

the reactivities of a large number of different organic compounds in supercritical water have also been investigated.8-10 In addition, many studies have been reported on the application of supercritical water in the destruction of environmentally hazardous chlorinated hydrocarbons.11-14 The present work aims to show the possible use of water for the dechlorination and denitrogenation of fuel oil produced from municipal waste plastics. The promotional effects of various metal salts and hydroxides on these two hydrothermal reactions are also investigated. Experimental Section Materials. Fuel oil produced by the thermal degradation of municipal waste plastics was supplied from Rekisei Koyu Co. Ltd. of the Niigata Plastics Liquefaction Center (Niigata, Japan). In this center, waste plastic vessels, wrappings, and bags are sorted and cut into small pieces, and the waste plastic fluffs obtained (PE, 21 wt %; PP, 24 wt %; PS, 37 wt %; PVC, 5 wt %; PET, 6 wt %; and others, 7 wt %) are thermally degraded into fuel oil at 400 °C after being thermally dehydrochlorinated at 340 °C. The resulting fuel oil is then fractionated into gasoline, kerosene, and heavy oil fractions. The total chlorine contents in these three fractions are 550, 45, and 43 ppm, respectively, and the nitrogen contents are 0.085, 0.12, and 0.11 wt %, respectively. These three fractions also contain benzoic acid, phthalic acid, and telephthalic acid.15 The fuel oil used for the hydrothermal processing in the present work was the kerosene fraction. It was composed of 32 wt % aromatic hydrocarbons, 27 wt % unsaturated hydrocarbons, and 41 wt % saturated hydrocarbons. The reported15 physical and physicochemical properties of the fuel oil used are summarized in Table 1. The nitrogen compounds in the fuel oil that could be extracted with cold water are summarized in Table 2.

10.1021/ie020338x CCC: $22.00 © 2002 American Chemical Society Published on Web 10/04/2002

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Table 1. Physical and Physicochemical Properties of Fuel Oil Used C elemental analysis (wt %) organic acid content specific gravity (15 °C/4 °C) kinematic viscosity (50 °C)

86.5

H 11.8

O

N

0.2

0.12

benzoic acid, 1100 ppm o- and p-phthalic acids, 210 ppm 0.8362 1.74 cSt

S

(0.115)b

0.0043

boiling point ignition point pour point

Cl 0.0045a

(0.0062)b

109.5-502.0 °C 46.0 °C 0.0 °C

a Total chlorine content (inorganic chlorine content, 0.0015 wt %; organic chlorine content, 0.0033 wt %). b Values obtained by our analytical methods.

Table 2. Nitrogen Compounds in the Fuel Oil Extracted with Watera compound

content (ppm)

N content (ppm)b

NH4+ benzonitrile -caprolactam unextracted

2.0 1607 238 -

1.6 218 29.4 901

a Extracted by water at room temperature and analyzed by use of an ion chromatograph and a gas chromatograph-mass spectrometer (see Experimental Section). b Corresponding nitrogen content in the fuel oil.

The combined nitrogen content of NH4+, benzonitrile, and -caprolactam was 249 ppm (Table 2). The organic nitrogen compounds that could not be extracted with cold water remained structurally unknown. Various metal salts and hydroxides, sulfuric acid, diethyl ether, and other organic compounds used in the present work were purchased from Kanto Chemicals (Tokyo, Japan) or Wako Pure Chemicals Industries, Ltd., (Osaka, Japan) and were of the highest purity available. Deionized water was always used. Processing and Analyses. Hydrothermal processing of the fuel oil was carried out in a small batch reactor that was not equipped for the collection of gaseous products for analysis. The reactor consisted of a SUS316 stainless steel tube (8.7 mm i.d., 6.7 cm length, 2 mm thickness, 4.0 cm3 inner volume), a cap union, a reducing union, and a full port reducer equipped with a sheath for a thermocouple (Hoke Inc., Cresskill, NJ). The cap union, reducing union, full port reducer, and sheath were made of SUS316 stainless steel. The reactor, closed at one end, was loaded with 1.0 g of fuel oil. Then, 0.25-1.0 g of water or an aqueous solution of metal compound was added, and the other end of the reactor was closed after the air inside the reactor had been replaced with nitrogen. The reactor was suspended in a preheated electric furnace and was heated for the required reaction time (15-60 min). The reaction temperature was measured by a thermocouple inserted into the reactor. Heating times of 5-15 min were required to reach the desired reaction temperatures (100-425 °C). Below the critical temperature of water (374.2 °C), an aqueous liquid phase was always present in the reactor during the course of hydrothermal processing. Following the reaction, the reactor was cooled in air and opened, and the liquid reaction product was collected. The reactor was rinsed three times with 3 mL of diethyl ether and then with 3 mL of water, and the organic and aqueous portions obtained were added to the liquid reaction product. After complete mixing, the organic and aqueous layers were separated. The organic layer obtained was dried overnight with anhydrous sodium sulfate. Diethyl ether in the organic layer was then evaporated under vacuum at room temperature to obtain the product oil, i.e., the processed fuel oil. The yield of the product oil was always 100% within experi-

mental errors. The product oil was then analyzed for nitrogen content by means of elemental analysis. A Sumigraph NCH-21 NCH analyzer (Sumika Chemical Analysis Service, Ltd., Osaka, Japan) was used. By the elemental analysis, the total nitrogen content in the fuel oil was determined to be 1150 ppm (Table 1). On the other hand, the aqueous layer obtained above was analyzed for chloride ions using nitrate ions as the internal standard. A Shimadzu LC-10A ion chromatograph, equipped with a CDD-6A electric conductivity detector and a Shimadzu C-R6A integrator, was used. From the measured chloride ion concentration, the chlorine content in the product oil was determined. When aqueous alkaline solutions were used instead of water, the aqueous layer obtained was neutralized with 0.025 mol/L H2SO4 before analysis of chloride ions. The total chlorine content in the fuel oil, which was determined by reaction with 0.05 mol/L NaOH (1.0 g of fuel oil/1.0 g of alkaline solution) for 30 min at 375 °C, was 62 ppm (Table 1). Previous experiments revealed that all of the chlorine atoms contained in the fuel oil could be evolved as chloride ions by this alkaline hydrothermal treatment. Ammonium and lithium ions in the aqueous layer were analyzed using a Shimadzu PIA1000 personal ion analyzer with magnesium ions as the internal standard. Organic compounds such as carboxylic acids and amides extracted into the aqueous phase during the course of hydrothermal processing were also analyzed. That is, the aqueous layer that was separated after the reaction as described above was acidified with 0.50 mol/L H2SO4 and then extracted with diethyl ether (extract 1). The aqueous raffinate obtained was evaporated to dryness under vacuum at ca. 50 °C, alkalized with 1 mL of 1.0 mol/L NaOH, and then extracted with diethyl ether (extract 2). Extract 1 was found to contain benzonitrile, phenol, benzoic acid, and phthalic acid, whereas extract 2 contained benzamide and -caprolactam. These two diethyl ether extracts were separately analyzed using a Shimadzu QP-5000 gas chromatograph-mass spectrometer with 2-phenyl ethanol as the internal standard. Here, the identification of the organic compounds in these ether extracts was confirmed using standard samples. Physical and Physicochemical Properties of Water. The densities and vapor pressures of saturated water under subcritical temperatures were obtained from the reported steam table.16 The water pressures in the reactor under supercritical conditions were calculated using the 38-coefficient equation of state proposed by Saul and Wagner.17 These calculations were carried out neglecting the liquid volume of fuel oil loaded. The ion products of water were calculated using the equation of Marshall and Franck.18

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Figure 1. Effect of reaction temperature on the hydrothermal processing of fuel oil with pure water (reaction time ) 30 min). Processing conditions: fuel oil, 1.0 g; water, 1.0 g. Vapor pressure of water ) 1.56 MPa (200 °C), 8.59 MPa (300 °C), 22.1 MPa (374 °C), 27.8 MPa (400 °C), 33.2 MPa (425 °C).

Results and Discussion Processing with Water. Figure 1 shows the chlorine and nitrogen contents in the product oil obtained upon hydrothermal processing of the fuel oil with pure water at various reaction temperatures. When the fuel oil was contacted with pure water at 25 °C, the chlorine content in the product oil decreased to 54 ppm. This shows that the inorganic chlorine content in the fuel oil was 62 54 ) 8 ppm. The chlorine content in the product oil decreased markedly at higher reaction temperatures and was nearly 0 ppm at 400 °C. Thus, the organic chlorides in the fuel oil could be dechlorinated by pure water above 100 °C. On the other hand, the nitrogen content in the product oil was nearly constant, 902904 ppm, at 25-250 °C. However, it then decreased at higher reaction temperatures and was 131 ppm at 425 °C (Figure 1). Thus, 89% of the amount of nitrogen originally contained in the fuel oil could be removed at 425 °C. The decrease in the nitrogen content of ca. 250 ppm from the amount present at 25-250 °C (Figure 1) can be attributed to the removal of NH4+, benzonitrile, and -caprolactam through extraction by and/or the reaction with water (Table 2). In contrast, the hydrothermal denitrogenation of organic nitrogen compounds by pure water seems to occur above 275 °C. At any rate, these results clearly show the possible use of water for the dechlorination and denitrogenation of waste-plasticsderived fuel oil, especially under supercritical conditions. Processing with Aqueous Solutions of Alkaline and Alkaline Earth Metal Compounds. The effects of the acid-base properties of aqueous solutions on the hydrothermal processing of the fuel oil were investigated at the fixed reaction temperature of 350 °C. In these experiments, aqueous solutions of alkaline and alkaline earth metal salts and hydroxides were used instead of water. Figure 2 shows the correlation of the nitrogen content in the product oil with the pH of the aqueous solutions of these metal compounds at room temperature. When pure water (pH 6.80) was used, the nitrogen content was 690 ppm. However, the nitrogen content decreased markedly when the pH of the aqueous solutions used decreased or increased (Figure 2). This shows that hydrothermal denitrogenation occurs readily under acidic or basic conditions. The nitrogen content in the product oil was 369 ppm for the aqueous solution of NaHSO4 (pH 1.65), whereas it was 143-166 ppm for

Figure 2. Correlation of the nitrogen content in the product oil obtained at 350 °C with the pH of the aqueous solutions of alkaline and alkaline earth metal compounds at room temperature (reaction time ) 30 min). Processing conditions: fuel oil, 1.0 g; aqueous metal compound solution, 1.0 g. Concentration of aqueous metal compound solutions ) 0.05 mol of metal ion/L for alkaline metal compounds, 0.025 mol of metal ion/L for alkaline earth metal compounds. 1, NaHSO4; 2, KH2PO4; 3, Na2SO4; 4, CH3COONa; 5, NaHCO3; 6, Na2CO3; 7, Na3PO4; 8, BaO; 9, Mg(OH)2; 10, Mg(OH)2‚ MgCO3; A, alkaline earth metal hydroxides and CaO; B, alkaline metal hydroxides.

the aqueous solutions of alkaline metal hydroxides (pH 12.16-12.56, Figure 2). Thus, of the alkaline and alkaline earth metal compounds studied, alkaline metal hydroxides were most effective for the hydrothermal denitrogenation of the fuel oil. Furthermore, the nitrogen content decreased when the basicity of the alkaline metals increased. That is, the nitrogen contents were 163 ppm (LiOH), 166 ppm (NaOH), 155 pm (KOH), 148 ppm (RbOH), and 143 ppm (CsOH). A similar result was observed for the aqueous solutions of alkaline earth metal hydroxides. The nitrogen contents in the product oil were 230 ppm [Ca(OH)2], 206 ppm [Sr(OH)2], and 183 ppm [Ba(OH)2]. The relatively high nitrogen contents for Mg(OH)2 and Mg(OH)2‚MgCO3 (Figure 2) are attributed to their insolubility in water. On the other hand, the chlorine content in the product oil was 1-3 ppm for these acidic aqueous solutions, whereas it was 0 ppm for these basic aqueous solutions. (Data are not shown.) The promotional effect of bases on the removal of organic nitrogen has already been observed in the production of liquid hydrocarbons by the hydrothermal processing of oil shale in a molten NaOH-KOHLiOH.19 Paspek and Klein20 reported the high catalytic activity of HCl for the removal of organic nitrogen from shale oil in supercritical water. Processing with Aqueous Solutions of Transition Metal Sulfates. The promotional effect of transition metal sulfates on the hydrothermal processing of fuel oil was also investigated under the same processing conditions as those used in Figure 2. The transition metal sulfates studied were aluminum, chromium(II), iron(II), cobalt(II), nickel(II), copper(II), zinc, cadmium, and tin(II) sulfates, and the concentration of aqueous transition metal sulfate solutions was 0.05 mol of metal ions/L. All of these metal sulfate solutions were acidic (pH ) 2.10-6.24 at room temperature). The chlorine content in the product oil was relatively high when the pH of these aqueous metal sulfate solutions at room temperature was low. That is, the chlorine content was 7-10 ppm for FeSO4, SnSO4, and Al2(SO4)3 (pH 2.103.56), whereas it was 1-4 ppm for the other six metal sufates (pH 4.36-6.24). The chlorine content (1 ppm)

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Figure 3. Effect of reaction temperature on the hydrothermal processing of fuel oil with 0.05 mol/L NaOH (reaction time ) 30 min). Processing conditions: fuel oil, 1.0 g; 0.05 mol/L NaOH, 1.0 g.

observed for the aqueous CuSO4, CdSO4, and ZnSO4 solutions equaled that of pure water. (Data are not shown.) In contrast to hydrothermal dechlorination, hydrothermal denitrogenation of the fuel oil in these aqueous transition metal sulfate solutions took place more readily at higher H+ concentrations, as in the case of the aqueous solutions of alkaline metal salts (Figure 2). However, the lowest nitrogen content obtained was 303 ppm for the aqueous CuSO4 solution. This nitrogen content is two times higher than that observed for the aqueous KOH solution (155 ppm, Figure 2). Thus, aqueous solutions of alkaline metal hydroxides were most effective for the hydrothermal refining of fuel oil produced from municipal waste plastics. Processing with Aqueous NaOH Solution (Temperature Effect). Figure 3 shows the effect of reaction temperature on the hydrothermal processing of the fuel oil with 0.05 mol/L NaOH. The chlorine and nitrogen contents in the product oil obtained at 25 °C, 52 and 900 ppm, respectively, were nearly equal to those obtained with pure water at 25 °C (54 and 904 ppm, respectively, Figure 1). At higher reaction temperatures, the chlorine content decreased markedly and reached 0 ppm at 300 °C. Denitrogenation also occurred much more readily under these conditions than with pure water. In pure water, removal of the organic nitrogen in the fuel oil occurred above 275 °C, whereas in 0.05 mol/L NaOH, it took place even at 150 °C (Figures 1 and 3). At higher reaction temperatures, the nitrogen content in the product oil decreased markedly, reaching as low as 40 ppm at 400 °C (Figure 3). Thus, 96.5% of the amount of nitrogen originally contained in the fuel oil was removed at 400 °C. Processing with Aqueous NaOH Solution (Reaction Time and Concentration Effects). Figure 4 shows the effect of the reaction time on the hydrothermal denitrogenation of fuel oil. In these three hydrothermal processing systems, the nitrogen content in the product oil decreased dramatically within 15 min and then more gradually at longer reaction times. In the processing with 0.05 mol/L NaOH at 350 °C, the nitrogen content was 205 ppm after 15 min of reaction, whereas it was 129 ppm after 60 min of reaction. In the processing with 0.10 mol/L NaOH at 375 °C, the nitrogen content was 49 ppm after 15 min of reaction, whereas it was 24 ppm after 60 min of reaction (Figure 4). Thus, in these two alkaline processing systems, 92.6

Figure 4. Effect of reaction time on the nitrogen content in the product oil obtained under various hydrothermal conditions. Processing conditions: fuel oil, 1.0 g; water or aqueous alkaline solutions, 1.0 g.

Figure 5. Effect of reactant weight ratio on the nitrogen content in the product oil obtained at 350 °C (reaction time ) 30 min). Processing conditions: fuel oil, 1.0 g.

and 97.8%, respectively, of the amounts of nitrogen that could be removed within 60 min could be removed within 15 min. In the processing with pure water at 425 °C, the nitrogen content was 297 ppm after 15 min of reaction, . Thus, according to a similar calculation, the amount of nitrogen that could be removed within 15 min was 78.1% (Figure 4). The chlorine content in the product oil was always 0 ppm in these three hydrothermal processing systems. (Data are not shown.) Processing with Aqueous NaOH Solution (Reactant Ratio Effect). Figure 5 shows the effect of the reactant weight ratio on the hydrothermal processing of fuel oil at 350 °C. In the processing with 0.10 mol/L NaOH, the nitrogen content in the product oil was 506 ppm at the reactant weight ratio of 0.25, whereas it was 112 ppm at the reactant weight ratio of unity. The nitrogen contents obtained with 0.05 mol/L NaOH were higher than those obtained with 0.10 mol/L NaOH. In these two alkaline reaction systems, the chlorine content in the product oil was 0-3 ppm for the range of reactant weight ratios employed. (Data are not shown.) Thus, the use of high weight ratios of aqueous NaOH solution to fuel oil was effective for the hydrothermal refining of the fuel oil. Removal of Organic Acids. Table 3 summarizes the organic compounds found in the aqueous phase after hydrothermal processing of the fuel oil. When the fuel oil was contacted with water at 25 °C, the aqueousphase product contained phenol, phthalic acid, benzoic acid, -caprolactam, and benzonitrile (Table 3). The contents of phthalic acid (1.3 × 10-3 mmol/g of fuel oil)

Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5397 Table 3. Organic Compounds Found in the Aqueous-Phase Product after Hydrothermal Processing of Fuel Oila processing conditions

H2O 25 °C, 30 min

phenol phthalic acidb benzoic acid 3M benzoic acidc diM benzoic acidd benzamide 3M benzamidee -caprolactam benzonitrile

4.9 13 100 neg neg neg neg 21 156

0.05 mol/L LiOH 250 °C, 30 min amount (mmol × 104) 6.7 10 109 17 neg 100 7.5 8.8 neg

0.05 mol/L LiOH 350 °C, 30 min

0.10 mol/L NaOH 375 °C, 60 min

7.0 negf 165 28 neg 11 1.6 8.2 neg

5.7 neg 83 96 9.5 neg neg 12 neg

a Fuel oil, 1.0 g; alkaline solution, 1.0 g. b o- and p-phthalic acids. c 3-Methyl benzoic acid. Methyl benzamide. f Negligible.

Table 4. Nitrogen Balance in the Hydrothermal Processing of Fuel Oil with Aqueous Solutions of Alkaline Metal Hydroxidesa NH4+ in N NH3 N in N in fuel oil product aq phase recovery selectivity (%)e (%)d fed (mg) oil (ppm)b (ppm)c 0.05 mol/L LiOH 1.15 570 5.2 83.5 34.9 250 °C, 30 min 0.05 mol/L LiOH 1.15 158 20.4 82.6 80.0 350 °C, 30 min 0.10 mol/L NaOH 1.15 28 27.9 98.2 96.6 375 °C, 60 min processing conditions

a

Fuel oil, 1.0 g; alkaline solution, 1.0 g. b Yield of product oil was 1.0 g. c Volume of aqueous phase was 50 mL. d Organic nitrogen compounds found in the aqueous-phase product (Table 3) are taken into account. e Yield of NH4+/amount of nitrogen in the fuel oil removed.

and benzoic acid (1.00 × 10-2 mmol/g of fuel oil) in the aqueous-phase product were nearly equal to the analytical values reported (Table 1). The amount of phenol found in the aqueous-phase product did not change much as a result of hydrothermal processing. The amount of -caprolactam found after processing with aqueous alkaline solutions at 250-375 °C was smaller than that found after water processing at 25 °C. The combined amount of phthalic acid; benzoic acid; and benzoic acid’s methyl-substituted compounds benzamide, 3-methyl benzamide, and benzonitrile decreased at higher temperatures. That is, the amount was 0.0269, 0.0244, 0.0206, or 0.0189 mmol/g of fuel oil at 25, 250, 350, or 375 °C, respectively. The amount of phthalic acid was negligible above 350 °C (Table 3). These quantitative variations in the organic compounds in the aqueous-phase product indicate that all of these compounds were not additionally produced from the fuel oil during the course of hydrothermal processing, although conversion of these compounds did take place. The mechanisms of the reactions are discussed later in this paper. At any rate, we believe that the fuel oil could be greatly refined by the extraction of these organic compounds into the aqueous phase as well (Table 3). Nitrogen and Alkaline Metal Ion Balances. Tables 4 and 5 summarize the mass balances for nitrogen and alkaline metal ions in the hydrothermal processing of the fuel oil. These mass balance analyses were conducted on the same runs as summarized in Table 3. When the fuel oil was processed with 0.05 mol/L LiOH for 30 min at 250 or 350 °C, the percent recovery of nitrogen remained at 83-84%, possibly as a result of the formation of unknown intermediate nitrogen compounds. However, the percent recovery of nitrogen increased markedly to 98.2% when 97.6% of the nitrogen originally present in the fuel oil had been removed (0.10 mol/L NaOH at 375 °C for 60 min, Table 4).

d

3,4- and 3,5-dimethyl benzoic acids. e 3-

Table 5. Alkaline Metal Ion Balance in the Hydrothermal Processing of Fuel Oil with 0.05 mol/L LiOHa Li+ content in LiOH soln fed (mg) 0.05 mol/L LiOH, 0.332 250 °C, 30 min 0.05 mol/L LiOH, 0.332 350 °C, 30 min processing conditions

Li+ content in aq phase (ppm)b 6.7

Li+ recovery (%) 100.9

6.6

99.4

a Fuel oil, 1.0 g; alkaline solution, 1.0 g. b Volume of aqueous phase was 50 mL.

The selectivity to NH3, i.e., the yield of NH4+ divided by the amount of nitrogen in the fuel oil removed, increased dramatically from 34.9% in the processing with 0.05 mol/L LiOH at 250 °C to 96.6% in the processing with 0.10 mol/L NaOH at 375 °C (Table 4). This indicates that the organic nitrogen in the fuel oil was removed through the ultimate formation of NH3. Evolution of NH3 has been reported for the reaction of organic nitrogen compounds in supercritical water.21-23 Table 5 shows that the recovery of Li+ ions in the hydrothermal processing of fuel oil with 0.05 mol/L LiOH was completely established within the range of processing conditions employed. This indicates that no alkaline metal ions remained in the product oil obtained under alkaline hydrothermal conditions. Reaction Mechanism. At higher temperatures, water participates in the conversion of organic compounds as a catalyst, a reactant, and a solvent. Product distributions suggested that classical acid- and basecatalyzed organic reactions take place in liquid water under subcritical conditions,24-28 whereas under supercritical conditions, free-radical organic reactions also occur.23,29-34 Water can act as an acidic or basic catalyst, and its reactivity is often reinforced in the presence of acids, bases, or solid acid catalysts.24-26,28,30 In the thermal degradation of mixed plastics containing PVC, organic chlorides such as 2-chloro-2-methylpropane, R-chloroethylbenzene, and 2-chloro-2-phenylpropane are formed.2 They are formed by the reaction of HCl with hydrocarbons evolved from the degradation of PE, PP, and PS.2 In the present study, however, the organic chlorides contained in fuel oil were not structurally identified. These organic chlorides must have relatively high boiling points. At any rate, the majority of the hydrothermal processing of the fuel oil was carried out in the presence of liquid water under subcritical conditions (Figures 1-5). Hence, we believe that hydrothermal dechlorination by pure water occurred through hydrolysis, as reported previously.12,14,35 H+ and/or OH- ions evolved by the dissociation of water

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Scheme 1

must have catalytically participated in this hydrolysis reaction. The ion products of pure liquid water under saturated conditions, as calculated by the equation of Marshall and Frank,18 are 3.85 × 10-12 (200 °C), 4.11 × 10-12 (250 °C), 1.98 × 10-12 (300 °C), and 1.65 × 10-13 mol2/L2 (350 °C). The concentrations of H+ and OH- ions in saturated liquid water at these temperatures thus range from 4.06 × 10-7 to 2.03 × 10-6 mol/L. These concentrations are 4-20 times larger than those obtained in pure liquid water under ambient conditions. The results of hydrothermal dechlorination with aqueous solutions of transition metal sulfates that are reported elsewhere in this work suggest the higher catalytic activity of OH- over H+ ions. For aqueous solutions of alkaline metal hydroxides, we believe that the dechlorination occurs through a classical nucleophilic mechanism, i.e., through nucleophilic addition of OH- to the carbon atom with which chlorine atom combines. On the other hand, the formation of aliphatic and aromatic nitriles (e.g., propionitrile and 4-phenylbutyronitrile) and amines (e.g., 2-phenylethylamine and dibenzylamine) and N-containing heterocylic compounds (e.g., methylquinoline and benzylpyrrole) was reported in the thermal degradation of ABS.4 The municipal waste plastics used for the production of fuel oil at the Niigata Plastics Liquifaction Center includedN-containing polymers such as ABS. Hence, such N-containing compounds as those described above must have been formed during the course of the thermal degradation of the municipal waste plastics used to produce the fuel oil examined here. In this study, however, the organic nitrogen compounds contained in the fuel oil remained structurally unknown except for benzonitrile and -caprolactam (Table 2). The quantitative variations of phthalic acid, benzoic acid, benzamide, benzonitrile, and methyl-substituted compounds in the aqueous-phase product (Table 3) suggest that the reactions of Scheme 1 are operative during the course of hydrothermal processing of fuel oil. In reality, when 0.05 mmol of o-phthalic acid was contacted with 2 mL of 0.10 mol/L NaOH at 350 °C for 30 min, 12% of the phthalic acid reactant was converted to benzoic acid. Houser et al.22 observed the formation of benzene in the reaction of benzonitrile in supercritical water and proposed the same reaction path as shown in Scheme 1 (C6H5CN f C6H6). The promotional effect of alkaline compounds on hydrothermal denitrogenation (Figures 2-5) shows that OH- ions serve as a catalyst for hydrolysis of benzonitrile and benzamide. In contrast, -caprolactam was hardly denitrogenated even under the alkaline hydrothermal conditions employed (Table 3). As for the alkylation of organic compounds containing heteroatoms in supercritical water, freeradical and ionic reaction paths have been postulated.22,30-34 On the other hand, the hydrothermal denitrogenation of organic nitrogen compounds other than benzonitrile and -caprolactam remains mechanistically unclarified.

However, hydrothermal denitrogenation occurred in pure water (Figures 1 and 4), and it occurred much more readily in aqueous alkaline solutions (Figures 2-5). Hence, it could be postulated that, in aqueous alkaline solutions, hydrothermal denitrogenation occurs mainly through an ionic reaction path catalyzed by OH- ions. Houser et al.21-23 reported that water serves as an oxidant and as a source of hydrogen in the denitrogenation of benzylamine, quinoline, and isoquinoline in supercritical water. Katritzky et al.31 found that aromatic heterocyclic nitrogen compounds such as pyridine, acridine, indole, and carbazole are essentially unreactive in pure supercritical water at 460 °C. Organic nitrogen that could not be removed above 400 °C (Figures 1 and 3) must be of the aromatic heterocyclic type. Houser et al.21 pointed out the need for a catalyst such as ZnCl2 for the removal of nitrogen from 3-phenylpyridine in supercritical water at 450 °C. Implications for Scale-Up to Larger Reactors. In the present work, no corrosion of the batch reactor was observed. The yield of the product oil was always 100% within experimental errors. By the hydrothermal processing used in the present work, the physical properties of the fuel oil were altered slightly but advantageously. For the refined oil obtained at 275 °C (Cl content ) 3 ppm, N content ) 73 ppm), the density and kinematic viscosity decreased by 0.2 and 1.8%, respectively, and also the ignition point decreased by 2 °C. However, the pour point remained unchanged.36 In the present work, the hydrothermal processing of fuel oil was carried out by use of a small batch reactor having no devices to promote reactant mixing. Hence, the dechlorination and denitrogenation reactions must have been diffusion-limited. This view can be supported by the relatively long reaction times employed (Figures 1-5). When this hydrothermal process is scaled up to industrial size, the use of flow-type reactors having devices to promote reactant mixing is recommended. By using this type of reactors, the reaction time (or, more precisely, the retention time) can be shortened markedly. In the hydrothermal process studied in this work, the aqueous portion of the products was neutralized and drained away. The aqueous-phase products obtained above 350 °C contained phenol, benzoic acid, benzamide, and -caprolactam (Table 3). In the hydrothermal processing with 0.05 mol/L LiOH at 350 °C for 30 min, for example, the concentrations of these four compounds in the aqueous-phase product were 0.007, 0.239, 0.016, and 0.009 wt %, respectively. The combined concentration of these four compounds was 0.271 wt %, and the main component was benzoic acid (Table 3). This aqueous portion of the products can be cleaned by an activated sludge or chemical oxidation process. In the latter case, an aqueous solution of sodium hypochloride and supported nickel are used at 20-25 °C as the oxidant and catalyst, respectively.37 At any rate, the present work clearly indicates the possible use of aqueous solutions of alkaline metal hydroxides for the hydrothermal dechlorination and denitrogenation of fuel oil produced by the thermal degradation of municipal waste plastics. We are now performing hydrothermal processing of the fuel oil using a flow-type reaction system. Conclusions The hydrothermal processsing of fuel oil derived from municipal waste plastics (kerosene fraction; Cl content

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) 62 ppm, N content ) 1150 ppm) in water and aqueous solutions of various metal salts and hydroxides under sub- and supercritical conditions has been investigated in a small SUS316 stainless steel batch reactor for the purpose of removing the Cl and N heteroatoms from the fuel oil. Although hydrothermal dechlorination and denitrogenation occurred in pure water, they proceeded much more readily under basic conditions, especially in aqueous solutions of alkaline metal hydroxides. Dechlorination occurred more readily than denitrogenation. The nitrogen content in the product oil decreased to 49 ppm after 15 min of reaction with 0.10 mol/L NaOH at 375 °C, whereas the chlorine content decreased to 0 ppm under the same processing conditions. Also, both the phthalic acid and the benzoic acid contained in the fuel oil could be removed by the hydrothermal processing. Thus, the present work clearly shows the possible use of aqueous solutions of alkaline metal hydroxides for the refining of fuel oil produced by the thermal degradation of municipal waste plastics. Acknowledgment The authors gratefully acknowledge the supply of municipal-waste-plastics-derived fuel oil from Rekisei Koyu Co. Ltd. and the financial support of the Sasaki Environmental Technology Foundation in 1999. Nomenclature ABS ) acrylonitrile-butadiene-styrene copolymer PE ) polyethylene PP ) polypropylene PS ) polystyrene PVC ) poly(vinyl chloride) PET ) poly(ethylene glycol telephthalate)

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Received for review May 6, 2002 Revised manuscript received August 4, 2002 Accepted August 19, 2002 IE020338X