Hydrothermal Denitrogenation of Fuel Oil Derived from Municipal

Kubota, H.; Sekizawa, T. Chemical Reaction Engineering; The Nikkan Kogyo Shimbun, Ltd.: Tokyo, Japan, 1995. There is no corresponding record for this ...
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Ind. Eng. Chem. Res. 2003, 42, 2074-2080

KINETICS, CATALYSIS, AND REACTION ENGINEERING Hydrothermal Denitrogenation of Fuel Oil Derived from Municipal Waste Plastics in a Continuous Packed-Bed Reactor Masamichi Akimoto,* Toshifumi Sato, and Tatsuro Nagasawa Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki, Niigata 945-1195, Japan

The hydrothermal processing of fuel oil produced from municipal waste plastics (kerosene fraction: Cl content ) 27 ppm, N content ) 927 ppm) was investigated under subcritical conditions so as to remove the organic nitrogens contained in the fuel oil. The hydrothermal processing was carried out in a small SUS316 stainless steel continuous packed-bed reactor (packed with Raschig rings) under liquid-phase conditions. Although the denitrogenation took place in pure water, the reaction proceeded much more readily in aqueous solutions of NaOH. That is, the nitrogen content in the product oil decreased to 73 ppm upon processing with 0.05 mol/L NaOH for 4.0 min at 275 °C and at the reactant weight feed ratio of unity, whereas it decreased to 16 ppm when 0.10 mol/L NaOH was similarly used at 325 °C. The chlorine content in the product oil also decreased to 3 ppm as a result of the processing at 275 °C. The rate of hydrothermal denitrogenation could well be described in terms of the homogeneous first-order reaction kinetics with the equilibrium nitrogen content under the hydrothermal processing conditions employed. Upon hydrothermal processing at 275 °C, the density, kinematic viscosity, and ignition point of the fuel oil decreased slightly. Introduction The degradation of waste plastics into fuel oil is one of the methods that have been proposed for recycling waste plastics as an energy source. 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 50-740 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 compounds can damage the combustion furnace and at the same time produce hazardous materials such as dioxins and nitrogen oxides. The chlorine and nitrogen contents in the oil should be lower than 10 and 100 ppm, respectively, from the viewpoint of environmental protection. The dechlorination of fuel oil produced from municipal waste plastics has been investigated by Sakata and coworkers. They used Fe3O4/carbon composite or CaCO3/ carbon composite as a chlorine sorbent at 320-350 °C.2-4 On the other hand, the application of supercritical water for the liquefaction of heavy fossil fuels such as coal, oil shale, and oil sand has attracted attention.5 To obtain clean fuel oils through the removal of heteroatoms (Cl, N, and S), the reactivities of a large number of different organic compounds in sub- and supercritical water have also been investigated.6-9 We have investi* To whom correspondence should be addressed. Tel. and Fax: +81-257-22-8138. E-mail: [email protected].

gated the hydrothermal dechlorination and denitrogenation of the fuel oil produced from municipal waste plastics under sub- and supercritical conditions. In our previous paper, the hydrothermal processing of the fuel oil was carried out in a small batch reactor under a nitrogen atmosphere, and the following findings were obtained.10 1. The hydrothermal dechlorination and denitrogenation proceed readily even under subcritical conditions, and aqueous acidic or basic solutions, especially aqueous alkaline solutions, are much more effective than pure water. 2. Organic nitrogens are removed in the ultimate form of NH3. 3. Upon hydrothermal processing, -caprolactam and organic acids such as benzoic acid and phthalic acid present in the fuel oil can also be removed through extraction by water and/or hydrothermal decomposition reactions. 4. No alkaline metal ions remain in the refined oil obtained under the alkaline hydrothermal conditions. The present work aims at demonstrating the feasibility of scaling up this hydrothermal process to industrial size. For this purpose, the hydrothermal processing of the fuel oil was carried out in a continuous packed-bed reactor under subcritical conditions. Because the removal of the organic chlorines proceeded more readily than that of the organic nitrogens,10 the present work is focused on the removal of organic nitrogens in the fuel oil. Reaction kinetics and the physical and physicochemical properties of the refined fuel oil are also investigated.

10.1021/ie020909l CCC: $25.00 © 2003 American Chemical Society Published on Web 04/10/2003

Ind. Eng. Chem. Res., Vol. 42, No. 10, 2003 2075 Table 1. Physical and Physicochemical Properties of Fuel Oil Useda S elemental analysis (ppm) organic acid content (ppm)

e100

Nb 1200

(927)e

benzoic acid e 100 (122)e

boiling point: 196.0-325.0 °C kinematic viscosity (30 °C): 2.553 cSt ignition point: 81 °C

Clc 20

(27)e

Cld e5 (2)e

o- and p-phthalic acids e 300 (207)e density (15 °C): 0.8402 g/cm3 pour point: -27.5 °C

a Reported by Sapporo Plastic Recycle Corp. b Total nitrogen content. c Total chlorine content. d Inorganic chlorine content. e Values in parentheses were obtained by our analytical methods. Inorganic chlorine (Cl-) and organic acids were extracted with cold water and analyzed by use of an ion chromatograph and a gas chromatograph-mass spectrometer (see ref 10).

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

NH4 -caprolactam unextracted

content (ppm)

N content (ppm)b

6.5 1461 -

5 180 742

a Extracted by water at room temperature and analyzed by use of an ion chromatograph and a gas chromatograph-mass spectrometer (see ref 10). b Corresponding nitrogen content in the fuel oil.

Experimental Section Materials. Fuel oil produced by the thermal degradation of municipal waste plastics was supplied from Sapporo Plastic Recycle Corp., Sapporo, Japan. In this corporation, municipal waste plastics (vessels, wrappings, and bags) are cut into small pieces and pelleted, and the resulting waste plastic pellets (PE, 39 wt %; PP, 16 wt %; PS, 22 wt %; PVC, 5 wt %; PVDC, 2 wt %; PET, 8 wt %; ABS, 0.4 wt %; and others, 8 wt %) are thermally degraded into fuel oil at 400-450 °C after being thermally dehydrochlorinated at 300-350 °C. The resulting fuel oil is then fractionated into gasoline, kerosene, and heavy oil fractions.11 The fuel oil used for the hydrothermal processing in the present work was the kerosene fraction. It was composed of 65.2 vol % saturated hydrocarbons, 27.7 vol % aromatic hydrocarbons, 7.1 vol % olefinic hydrocarbons, and 0.2 vol % asphaltenes. Table 1 gives the reported physical and physicochemical properties of the fuel oil. Table 2 gives the nitrogen compounds in the fuel oil that could be extracted with cold water. The combined nitrogen content of NH4+ and -caprolactam in the fuel oil was 185 ppm. In the thermal degradation of mixed plastics containing PVC, organic chlorides such as 2-chloro-2methylpropane and R-chloroethylbenzene are formed.12 On the other hand, the formation of aliphatic and aromatic nitriles and amines and N-containing heterocyclic compounds was reported in the thermal degradation of ABS.13 However, such organic chlorides and nitrogen compounds as described above were hardly found in the aqueous extract of the fuel oil. The organic nitrogen compounds that could not be extracted with cold water remained structurally unknown. The analytical values such as nitrogen and organic acid contents that were determined by our analytical methods (Tables 1 and 2) are used in the present work. Aqueous NaOH solutions (0.05 and 0.10 mol/L), a 0.025 mol/L aqueous H2SO4 solution, diethyl ether, and other organic and inorganic compounds used in the present work were purchased from Kanto Chemicals (Tokyo, Japan) or Wako Pure Chemicals Industries, Ltd. (Osaka, Japan). They were of guaranteed reagent grade. Deionized water was always used.

Figure 1. Schematic diagram of the flow-type reaction apparatus.

Processing and Analyses. Figure 1 shows a schematic diagram of the experimental apparatus used. The reactor consisted of a SUS316 stainless steel tube (1/2 in. o.d. and 0.083 in. wall thickness) that was packed with 2 mm Raschig rings (void volume ) 58-60%) to promote the reactant mixing. Two Shimadzu LC-10A pumps (Shimadzu, Kyoto, Japan), a Balston 95S6 inline filter (Whatman Inc., Tewksbury, MA) and a Tescom 26-1700 back-pressure regulator (Tescom Corp., Elk River, MN), were used to maintain a constant flow rate through the reaction system under elevated temperatures and pressures. These instruments were connected using 1/4 in. o.d. SUS316 stainless steel pipes (0.083 in. i.d.). Fuel oil and water or fuel oil and an aqueous NaOH solution were separately preheated to ca. 200 °C, mixed, and then fed into the reactor immersed in the molten salt bath [NaNO3/KNO3 ) 1/1 (w/w)]. The effluent from the reactor was externally cooled in a cold-water bath to terminate the reaction. The fluid temperature inside the reactor was measured using a thermocouple K in a SUS316 stainless steel sheath. The reaction temperature was controlled within (1 °C. The hydrothermal processing was carried out at 250-325 °C under liquid-phase conditions. Hence, the total reaction pressure used was always 0.5 MPa higher than the saturated vapor pressure of water at the reaction temperature used. The residence time of the reactant mixture was determined on the basis of the liquid densities of the fuel oil and pure water under the reaction conditions used. Here, no volume change was assumed upon mixing of the fluid reactants. The liquid densities of the fuel oil at elevated temperatures and pressures were estimated by the method standardized by American Petroleum Institute.14 The densities of aqueous NaOH solutions (0.05 and 0.10 mol/L) were assumed to equal that of saturated liquid water at the same temperature. The residence time was varied by changing the reactor length. The effluent from the back-pressure regulator was neutralized with 0.025 mol/L H2SO4, and the organic

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Table 3. Effect of the Linear Velocity of Reactant Mixture inside the Reactor on the Nitrogen Content of the Product Oila void volume in the reactor (cm3)

linear velocity (cm/min)

N content (ppm)

3.0 5.8 11.6

4.2 8.4 16.8

137 142 139

a Processing conditions: 275 °C; 6.5 MPa; residence time ) 1.0 min; 0.05 mol/L NaOH/fuel oil weight feed ratio ) 1.

layer was separated, washed with water, and dried overnight with anhydrous Na2SO4. The resulting organic layer is called ” product oil” throughout this work. The nitrogen content in the product oil was determined 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 content of nitrogen originally contained in the fuel oil was determined to be 927 ppm (Table 1). The chlorine content in the product oil was determined from the measured amount of chloride ions evolved upon treatment of the product oil (1.0 g) with 0.05 mol/L NaOH (1.0 g) at 375 °C for 30 min.10 A Shimadzu LC-10A ion chromatograph, equipped with a CDD-6A electric conductivity detector and a Shimadzu C-R6A integrator, was used. Physical and Physicochemical Properties of Water. The densities and vapor pressures of saturated water under subcritical temperatures were obtained from the reported steam table.15 The dielectric constants of water were calculated using the equation of Archer and Wang.16 The dielectric constants of organic solvents were obtained from a handbook.17 The equilibrium constants for the hydrolysis of NH3 in water, the molar association constants of aqueous NaOH solutions, and the activity coefficients of free ions in aqueous NaOH solutions were obtained from Quist and Marshall,18 Ho et al.,19 and Zarembo et al.,20 respectively. The density, kinematic viscosity, pour point, and ignition point of the fuel oil and the product oil were determined by the methods of Japanese Industrial Standards (JIS). The plotting software package “Delta Graph Pro.3.5” (Delta Point, Monterey, CA) was used for the curve-fitting calculations. Results and Discussion Effect of Processing Conditions. Table 3 gives the effect of the linear velocity of the reactant mixture inside the reactor on the nitrogen content of the product oil at 275 °C. Although the linear velocity of the reactant mixture increased by 4 times from 4.2 to 16.8 cm/min at the fixed residence time of 1.0 min, the nitrogen content in the product oil remained nearly constant, 137-142 ppm. This indicates that at these linear velocities of the reactant mixture the hydrothermal denitrogenation is not diffusion-limited.21 Hence, the fixed fluid linear velocity of 8.4 cm/min inside the reactor was used throughout this work. Figure 2 shows the effect of reaction temperature on the nitrogen content of the product oil at the residence time of 1.0 min. In the processing with pure water, the nitrogen content was 277 ppm at 250 °C and it decreased to 230 ppm at 325 °C. When aqueous NaOH solutions were used instead of pure water, the nitrogen content decreased more markedly. That is, with 0.05 mol/L NaOH, the nitrogen content was 164 ppm at 250

Figure 2. Effect of the reaction temperature on the nitrogen content of the product oil (residence time ) 1.0 min). Processing conditions: water or aqueous NaOH/fuel oil weight feed ratio ) 1; reaction pressure ) 4.5 MPa (250 °C), 6.5 MPa (275 °C), 9.5 MPa (300 °C), and 13.5 MPa (325 °C).

Figure 3. Effect of the reaction temperature on the nitrogen content of the product oil (residence time ) 2.0 min). Processing conditions are the same as those in Figure 2 except for the residence time.

°C, and it decreased to 109 ppm at 325 °C. With 0.10 mol/L NaOH, the nitrogen content decreased to 115 and 61 ppm, respectively, at 250 and 325 °C. The nitrogen content further decreased at the residence time of 2.0 min (Figure 3). With pure water, the nitrogen content was 222 ppm at 250 °C and it decreased to 170 ppm at 325 °C. With 0.05 mol/L NaOH, the nitrogen content obtained ranged from 119 ppm at 250 °C to 67 ppm at 325 °C. With 0.10 mol/L NaOH, the nitrogen contents obtained were as low as 69 ppm at 250 °C and 20 ppm at 325 °C. By use of 0.10 mol/L NaOH, thus, the nitrogen content could be decreased to be lower than 100 ppm even at 250 °C. At the residence time of 4.0 min, the nitrogen content decreased much more markedly, especially in the hydrothermal processing with aqueous NaOH solutions (Figure 4). That is, the nitrogen content decreased to 99 ppm at 250 °C and reached 38 ppm at 325 °C when 0.05 mol/L NaOH was used. With 0.10 mol/L NaOH, the nitrogen content decreased to 56 ppm at 250 °C and reached as low as 16 ppm at 325 °C. With 0.10 mol/L NaOH, thus, 98% of the amount of nitrogen originally contained in the fuel oil could be removed at 325 °C. In contrast, the nitrogen content in the product oil was relatively high when pure water was used. The nitrogen content was 133 ppm even at 325 °C (Figure 4). Figure 5 shows the effect of the reactant weight feed ratio on the nitrogen content of the product oil at 275 °C. As an aqueous NaOH solution, 0.05 mol/L NaOH

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Figure 4. Effect of the reaction temperature on the nitrogen content of the product oil (residence time ) 4.0 min). Processing conditions are the same as those in Figure 2 except for the residence time.

Figure 5. Effect of the reactant weight feed ratio on the nitrogen content of the product oil in the hydrothermal processing with 0.05 mol/L NaOH at 275 °C and 6.5 MPa (residence time ) 4.0 min). Table 4. Comparison of the Physical and Physicochemical Properties of the Refined Fuel Oil with Those of the Original Fuel Oil

density at 15 °C (g/cm3) kinematic viscosity at 30 °C (cSt) pour point (°C) ignition point (°C) total nitrogen content (ppm) total chlorine content (ppm)

original fuel oil

refined fuel oila

0.8384 2.370 -27.5 82.0 927 27

0.8368 2.327 -27.5 80.0 73 3

a Processing conditions: 275 °C; 6.5 MPa; residence time ) 4.0 min; 0.05mol/L NaOH/fuel oil weight feed ratio ) 1.

was used and the hydrothermal processing was carried out at the residence time of 4.0 min. When the weight feed ratio of the aqueous NaOH to fuel oil was 0.25, the nitrogen content in the product oil was 154 ppm. However, the nitrogen content decreased at higher reactant weight feed ratios and reached 73 ppm at the reactant weight feed ratio of unity. Thus, the use of high weight feed ratios of an aqueous NaOH solution to fuel oil was effective for the removal of nitrogen in the fuel oil. Physical and Physicochemical Properties of Refined Fuel Oil. The density, kinematic viscosity, pour point, and ignition point of the refined fuel oil were determined and compared with those of the original fuel oil (Table 4). The refined fuel oil was prepared by the hydrothermal processing at 275 °C and the residence time of 4.0 min using 0.05 mol/L NaOH. The nitrogen and chlorine contents in the refined fuel oil were 73 and

Figure 6. Miscibility of the fuel oil with various organic solvents at room temperature. Miscibility test was carried out at room temperature by mixing 0.50 mL each of the fuel oil and an organic solvent in a test tube. Organic solvents used were 11 alcohols (C1C12), 3 hydrocarbons (C6 and C7), 3 nitriles (C2-C7), 2 ketones (C3), 2 chlorohydrocarbons (C6), 2 aromatic nitrogen compounds (C6 and C8), and so on (diethyl ether, o-cresol, 1,4-dioxane, and mnitrotoluene): O, completely miscible; 4, partially miscible; b, immiscible.

3 ppm, respectively, and -caprolactam and organic acids such as benzoic and phthalic acids were hardly found in the refined oil. As a result of the hydrothermal processing, the density, kinematic viscosity, and ignition point of the fuel oil decreased slightly but the pour point remained unaltered (Table 4). By the hydrothermal processing, thus, the physical and physicochemical properties of the fuel oil were altered slightly but were improved as fuel oil. Reaction Kinetics. At elevated temperatures, water is relatively nonpolar because of lower hydrogen bonding. Hence, water may be miscible with the fuel oil and react with organic nitrogen compounds present in the fuel oil. Then, the miscibility of pure water with the fuel oil under the hydrothermal processing conditions was investigated as follows. That is, the miscibility of the fuel oil with a large number of organic solvents that have a wide range of dielectric constants () was investigated under ambient conditions, and the obtained result was compared with the dielectric constants of saturated liquid water at elevated temperatures. Under ambient conditions, the fuel oil was immiscible with glycerol ( at 25 °C ) 42.5), ethylene glycol ( ) 37.7), and acetonitrile ( ) 36.0), but the fuel oil was partially miscible with methanol ( ) 32.6) and chloroacetone ( ) 30.0). However, the fuel oil was completely miscible with propionitrile ( ) 27.2) and the other organic solvents whose dielectric constants at 25 °C were lower than 27.2 (Figure 6). The dieletric constants of saturated liquid water that were calculated using the equation of Archer and Wang16 were 34.8 (200 °C), 27.0 (250 °C), 20.1 (300 °C), 13.0 (350 °C), and 5.5 (374 °C ) Tc). Hence, it can be postulated that the fuel oil was completely miscible with the saturated liquid water above 250 °C. Assuming a plug-flow reactor and a homogeneous liquid-phase reaction, the reaction kinetics for the hydrothermal denitrogenation of the fuel oil was investigated. When a small decrease in the nitrogen content of the fuel oil (dx) takes place upon passing the reactant mixture through a small volume of the reactor (dV), the following nitrogen balance is obtained.

FC0 dx ) r dV

(1)

In the present study, the nitrogens originally present in the fuel oil were not completely removed even at 325 °C, and at longer residence times the nitrogen content seemed to reach a constant value depending on the

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Table 5. Kinetic Parameters Obtained at 275 °C and 6.5 MPaa reactant

xe (ppm) (166.3)c

H2O 168.9 0.05 mol/L NaOH 81.5 (80.6) 0.10 mol/L NaOH 37.1 (36.5)

k (min-1) (1.83)c

2.14 2.61 (2.35) 2.81 (2.56)

R2 b 0.9988 (0.9984)c 0.9997 (0.9995) 0.9999 (0.9999)

a Experimental data shown in Figures 2-4 were used for the calculations with x0 ) 927 ppm. b R ) regression factor. c Values in parentheses were obtained with x0 ) 742 ppm.

hydrothermal processing conditions used. When 0.10 mol/L NaOH was used, for example, the nitrogen contents obtained at 325 °C were 61, 20, and 16 ppm, respectively, at the residence times of 1.0, 2.0, and 4.0 min (Figures 2-4). Here, we tentatively call this constant value an equilibrium nitrogen content. For denitrogenation, we assume a homogeneous first-order reaction kinetics with an equilibrium nitrogen content (xe) under the hydrothermal processing conditions employed.

r ) -kC0(x - xe)

(2)

By combination of eqs 1 and 2 and integration of the resulting equation with the boundary conditions x ) x0 at V ) 0 and x ) x at V ) V, the following equation is derived.

x ) xe + (x0 - xe) exp[-k(V/F)]

(3)

The nitrogen contents obtained at the various residence times (V/F) of the hydrothermal processing at 275 °C (Figures 2-4) were fitted to eq 3, and the kinetic parameters were determined (Table 5). The obtained values of R2 (R ) regression factor), 0.9988-0.9999, indicate that eq 3 fitted the experimental data obtained at 275 °C well. Equation 3 also fitted well the similar experimental data obtained at 250, 300, and 325 °C. The obtained values of R2 were 0.9978-1.0000. (Kinetic parameters obtained are not shown.) As given in Table 2, however, the fuel oil used in the present work contained NH4+ and -caprolactam. These two nitrogen compounds could be extracted with water upon hydrothermal processing.10 Hence, the values of the kinetic parameters given in Table 5 that were determined with x0 ) 927 ppm involve the contribution of the extraction by water to the hydrothermal denitrogenation. The nitrogens that were removed by the hydrothermal reaction are those in the organic nitrogen compounds that could not be extracted with water. The nitrogen content in the fuel oil that was attributed to the unextracted organic nitrogen compounds was 742 ppm (Table 2). Even when we assume x0 to be 742 ppm, eq 3 could fit well the results of the hydrothermal denitrogenation at 250-325 °C shown in Figures 2-4 (R2 ) 0.9968-0.9999) and the kinetic parameters could be determined similarly. The values of k and xe at 275 °C thus determined are also given in Table 5. These values of k and xe do not involve the contribution of the extraction by water to the hydrothermal denitrogenation. Reaction Mechanism. At elevated 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 occur in liquid water under subcritical temperatures.22-26

As shown in Figures 2-4, the catalytic role of OHions in the hydrothermal denitrogenation of the fuel oil is evident. Here, we discuss the contribution of OH- ions to the hydrothermal denitrogenation of the fuel oil in pure water. As reported previously, the organic nitrogens in the fuel oil are removed in the ultimate form of NH3.10 Comparison of the amount of NH3 evolved by the denitrogenation with the content of organic acids in the fuel oil indicates that the effluents from the reactor were always basic in the hydrothermal processing with pure water. In the hydrothermal processing with pure water at 275 °C for 4.0 min (Figure 4), for example, the concentration of OH- ions in the effluent from the reactor is calculated to be 7.7 × 10-5 mol/L when the equilibrium hydrolysis constant of pK ) 6.3 for NH3 in pure water18 can be used. In contrast, the concentrations of NaOH in the effluent at 275 °C and the residence time of 4.0 min were 0.017 and 0.036 mol/ L, respectively, in the hydrothermal processing with 0.05 and 0.10 mol/L NaOH (Figure 4). Comparison of these OH- and NaOH concentrations with the observed values of the rate constant (Table 5) suggests that the hydrothermal denitrogenation in pure water proceeded mainly through the pathway not catalyzed by OH- ions. The authors believe that the hydrothermal denitrogenation in pure water is initiated mainly by the direct attack of a water molecule on the organic nitrogen compounds present in the fuel oil. In pure water, of course, the hydrothermal denitrogenation catalyzed by OH- ions that were formed as a result of the reaction also takes place, but the contribution of this OH-catalyzed reaction is small. Houser et al.27-29 reported that the water molecule served as an oxidant and as a source of hydrogen in the reaction of organic nitrogen compounds such as quinoline in supercritical water. At any rate, the authors believe that in aqueous NaOH solutions the hydrothermal denitrogenation comprises both the pathway catalyzed by OH- ions and the pathway initiated by the direct attack of a water molecule. We then propose the following relation for the rate constant k of the hydrothermal denitrogenation in aqueous NaOH solutions.

k ) kw[H2O] + kOH[OH-]

(4)

where kw is the rate constant for the pathway initiated by the direct attack of a water molecule whereas kOH is the rate constant for the pathway catalyzed by OHions. [H2O] and [OH-] are the concentrations of the water molecule and OH- ions, respectively, in the reaction phase. The concentration of the water molecule in the reaction phase was calculated based on the liquid densities of water and the fuel oil under the hydrothermal conditions used. In the calculation of the concentration of OH- ions, the molar association constants of Na+ and OH- ions in water19 and the activity coefficients of these ions in aqueous NaOH solutions20 were used. The values of the rate constant k obtained from eq 3 at the different concentrations of OH- ions were fitted to eq 4, and the values of the rate constants kw and kOH were determined (Table 6). The value of log kOH fitted linearly that of the reciprocal reaction temperature (K-1) with R2 ) 0.9988. The activation energy for the pathway catalyzed by OH- ions was thus found to be 48.6 kJ/ mol. Li and Houser30 reported the activation energy of 112 kJ/mol in ZnCl2-catalyzed denitrogenation of quino-

Ind. Eng. Chem. Res., Vol. 42, No. 10, 2003 2079 Table 6. Kinetic Parameters Obtained in the Hydrothermal Denitrogenation of the Fuel Oila reaction temp (°C)

kw [L/(mol‚min)]

kOH [L/(mol‚min)]

250 275 300 325

0.102 0.100 0.0985 0.0976

11.6 19.2 29.6 47.7

a Experimental data shown in Figures 2-4 were used for the calculations with x0 ) 742 ppm.

line in supercritical water. The value of the rate constant kw decreased slightly at higher reaction temperatures (Table 6). The reason for this alteration of the rate constant remains unknown. Townsend et al.31 found the correlation of the hydrolysis rate constant with the solvent dielectric constant in the reactions of dibenzyl ether, phenethyl phenyl ether, and guaiacol in supercritical water. In the present work, a detailed reaction mechanism of the hydrothermal denitrogenation of the fuel oil remained unclarified. The treatment of wastewater from this hydrothermal process was discussed in our previous paper.10 Conclusions The major conclusions that are derived from this work are as follows: 1. The denitrogenation proceeded much more readily in aqueous NaOH solutions than in pure water. The nitrogen content in the product oil decreased to 73 ppm upon processing with 0.05 mol/L NaOH for 4.0 min at 275 °C and at the reactant weight feed ratio of unity, whereas the nitrogen content decreased to 16 ppm when 0.10 mol/L NaOH was similarly used at 325 °C. As a result of the processing at 275 °C, the chlorine content in the product oil decreased to 3 ppm as well. 2. As a result of the hydrothermal processing at 275 °C presented above, the density, kinematic viscosity, and ignition point of the fuel oil decreased slightly but its pour point remained unaltered. 3. The rate of the hydrothermal denitrogenation could well be described in terms of the homogeneous firstorder reaction kinetics with the equilibrium nitrogen content under the hydrothermal conditions used. Thus, these findings demonstrate the feasibility of scaling up this hydrothermal process to industrial size. Acknowledgment The authors gratefully acknowledge the supply of municipal-waste-plastics-derived fuel oil from Sapporo Plastic Recycle Corp. Nomenclature ABS ) acrylonitrile-butadiene-styrene copolymer PE ) polyethylene PET ) poly(ethylene glycol terephthalate) PP ) polypropylene PS ) polystyrene PVC ) poly(vinyl chloride) PVDC ) poly(vinylidene chloride) C0 ) concentration of fuel oil in the reactant mixture (g of fuel oil/mL of the reactant mixture) F ) feed rate of the reactant mixture (mL/min) k ) rate constant of denitrogenation (min-1) kOH ) rate constant of denitrogenation catalyzed by OHions [L/(mol‚min)]

kw ) rate constant of denitrogenation initiated by the direct attack of a water molecule [L/(mol‚min)] r ) rate of denitrogenation [g of nitrogen/(mL‚min)] V ) void volume inside the reactor (mL) x ) nitrogen content in the product oil (g of nitrogen/g of oil) xe ) tentative equilibrium nitrogen content in the product oil (g of nitrogen/g of oil) x0 ) nitrogen content in the original fuel oil (g of nitrogen/g of oil)

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Received for review November 12, 2002 Revised manuscript received March 3, 2003 Accepted March 4, 2003 IE020909L