756
Ind. Eng. Chem. Res. 2001, 40, 756-767
Depolymerization of Styrene-Butadiene Copolymer in Near-Critical and Supercritical Water Yoonkook Park,† James N. Hool,‡ Christine W. Curtis,† and Christopher B. Roberts*,† Chemical Engineering Department and Industrial and Systems Engineering Department, Auburn University, Auburn, Alabama 36849
Many conventional solvents do not sufficiently dissolve cross-linked polymers such as styrenebutadiene rubber (SBR) to allow efficient depolymerization. Supercritical and near-critical water provides an alternative benign solvent for this application. Supercritical water oxidation and thermal degradation under supercritical water conditions provide a means to break down rubbery materials into organic compounds that can then be recovered as a chemical feedstock. In this study, depolymerization reactions of styrene-butadiene copolymer are examined in a semicontinuous reactor. A statistical experimental analysis technique was used to investigate the effect of various operating conditions: temperature (300-450 °C), pressure (135 and 170 bar), and the presence of hydrogen peroxide as an oxidant (0-5 wt %). The experimental results demonstrate the ability of supercritical and near-critical water to break down the SBR into a range of lower molecular weight organic compounds for potential recovery. Analysis of variance (ANOVA) shows that the temperature and oxidant concentration are significant at the 1% level for destruction efficiency. Benzene, toluene, ethylbenzene, styrene, phenol, acetophenone, benzaldehyde, and benzoic acid are identified as liquid products using gas chromatography in both batch and semicontinuous reactors. The gas products were comprised of carbon monoxide, carbon dioxide, and water as determined by Fourier transform infrared spectroscopy. The destruction efficiency and a semiquantitative analysis of the liquid products show that both pyrolysis and oxidation products are observed, and low molecular weight oxidation products are observed to be primary. 1. Introduction Separation and reaction processes that utilize supercritical and near-critical water as a solvent, such as supercritical water oxidation (SCWO), are effective and environmentally attractive techniques for the remediation of certain waste materials.1-3 The SCWO process is conducted at temperatures and pressures above the critical temperature and pressure of water, 374 °C and 22.1 MPa, respectively. Organic compounds are readily oxidized by an oxidizing agent, such as oxygen or hydrogen peroxide, in the presence of water during the SCWO process. This paper addresses the use of supercritical and near-critical water with and without the addition of the oxidizing agent hydrogen peroxide as a means of degrading styrene-butadiene rubber (SBR), a primary component in tires. SCWO has been applied to many different wastes, from pure components4-6 to complex industrial waste.1-3 As an example, Funazukuri and co-workers7 showed that supercritical water extraction of tire waste was very effective in producing lower molecular weight organics. Additionally, Dhawan et al.8,9 have shown that under supercritical conditions aromatic solvents react with either styrene-butadiene or used tires to produce hundreds of volatile products. Recently, Park et al.3 examined the conversion of tire waste material to lower * To whom correspondence should be addressed: Phone: 334-844-2036. Fax: 334-844-2063. E-mail: croberts@ eng.auburn.edu. † Chemical Engineering Department. ‡ Industrial and Systems Engineering Department.
molecular weight organics using SCWO. Other recent work in this area has focused on the feasibility of using supercritical fluids to treat other polymer materials, such as polyolefins10 and natural rubber.11 The interpretation of the experimental results in studies that utilize tire material can be complicated because of the very complex compositions of the tire material. Therefore, several research groups have studied thermal and oxidative degradation processes of simple, single rubber polymers. Delman et al.12 studied the degradation of SBR in the presence of ozone and oxygen. The ozonization was performed at room temperature, employing toluene solutions of SBR. They observed that the intrinsic viscosity (η), which was defined by the viscosity of a polymer in solution obtained through measurements of solution viscosity, initially decreased rapidly with ozonization time and then decreased with time at a slower rate until a limiting η was attained. Rode and co-workers13 studied the thermooxidative degradation of SBR films in the presence of benzoyl peroxide at 125-300 °C. The primary products identified were ketones, aldehydes, carboxylic acids, water, carbon monoxide, and carbon dioxide. In addition to oxidative degradation processes, thermal depolymerization processes of many polymers, in the absence of oxidizing agents, have also been examined. The thermal, noncatalytic depolymerization of polystyrene, as an example, is considered to be a radical process, which includes initiation, depolymerizing propagation, and radical coupling.14 In the low-temperature range of 200-300 °C, the thermal depolymerization of polystyrene results only in the reduction of chain
10.1021/ie000502l CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
Ind. Eng. Chem. Res., Vol. 40, No. 3, 2001 757
Figure 1. Semicontinuous SCWO apparatus: (1) nitrogen cylinder, (2) hydrogen peroxide solution, (3) HPLC pump, (4) Omega pressure gauge, (5) pressure-relief valve, (6) reactor, (7) furnace, (8) heat exchanger, (9) back-pressure regulator, (10) liquid-gas separator, (11) liquid product, (12) gas product, and (13) preheater.
molecular weight, without the formation of volatile products.15 At temperatures higher than 300 °C, volatile products and oligomers (dimers, trimers, etc.) are observed.16 Larger amounts of volatile compounds, mainly toluene, ethylbenzene, cumene, styrene, naphthalenes, and indanes, were found at temperatures above 350 °C.17 McNeil and Stevenson18 studied the thermal depolymerization of styrene-butadiene diblock copolymer under vacuum, using programmed heating conditions. They showed that the initial reaction, which occurs between 300 and 400 °C, was cyclization in the polybutadiene section of the polymeric chain, while at about 400 °C, a limited amount of volatile products, mainly 1,3-butadiene and 4-vinylcyclohexene, was formed. At higher temperature or with prolonged isothermal heating, the primary products were styrene and toluene. In these previous studies, the conversion of polymer waste material, such as SBR and tires, to valuable chemical compounds through thermal depolymerization and oxidative degradation in supercritical water has been demonstrated. In particular, our previous examination of the conversion of waste tire material using SCWO showed conversion of waste tire to lower molecular weight products.3 The objective of the work presented in this paper was to investigate the thermal and oxidative degradation of SBR, the major polymeric component of waste tire, under near-critical and supercritical water conditions. 2. Experimental Section SBR (Mw ) 235 000, Mn ) 98 000, and styrene ) 23.5 wt %) was received from Firestone Synthetic Rubber and Latex Co. (Akron, OH). The material was used as received and chopped into pieces that were less than 3 mm3 before being loaded in the reactor. Distilled water was used as the supercritical fluid media in all experiments. Hydrogen peroxide (Fisher, 30 wt %) was used as the oxidizing agent, and methylene chloride (Fisher,
ACS grade) was used to extract and recover the oxidation products from the aqueous product phase. 2.1. Apparatus. SBR degradation experiments in near-critical and supercritical water were performed in two custom-built SCWO units constructed in our laboratory. Batch reaction studies were performed by charging water, hydrogen peroxide (H2O2), and the SBR into an 11 mL Inconel 625 high-pressure reactor (Hi-P, Erie, PA). The total charge of water and hydrogen peroxide was ca. 4.5 g, with the hydrogen peroxide weight percentages ranging from 0 to 5%. The amount of solid SBR charged into the reactor was ca. 0.1 g. The reactor was maintained at near-critical or supercritical water conditions in a high-temperature furnace controllable to (2 °C. The reactor was agitated using a custom-built shaker attachment to the furnace. Temperatures (300450 °C), oxidant concentrations (0-5 wt %), and reaction times (20, 40, and 60 min) were varied to investigate the effect of each factor. After the desired reaction time, the reaction was quenched by placing the reactor in a water bath. The mass of water and peroxide solution charged to the reactor was sufficient to produce a reaction pressure in excess of 40 MPa (400 bar) in these batch studies. In addition to the batch studies, semicontinuous SCWO reactions were performed at several temperatures using the semicontinuous reactor system shown in Figure 1. For semicontinuous operation, SBR was charged initially into a 5 mL Inconel 625 high-pressure reactor; the furnace was brought to the specified reaction temperature; and then the solution of hydrogen peroxide (H2O2) and water which consisted of 0-5 wt % H2O2 was continuously pumped at a constant flow rate (2.5 and 5 cm3/min) through the reactor using an Accuflow Series II high-pressure liquid chromatography (HPLC) pump. The reaction temperatures studied were 300, 350, 400, and 450 °C at a pressure of 13.5 and 17.0 MPa, measured using an Omega test pressure gauge. This pressure, which is somewhat below the critical
758
Ind. Eng. Chem. Res., Vol. 40, No. 3, 2001
pressure of pure water, was chosen because of the apparatus limitations. The product stream was subsequently cooled using a heat exchanger (shell and tube) and fed to a liquid-gas separator, where the liquid and gas fractions were collected and analyzed. 2.2. Analysis Technique. Gaseous and liquid products from the batch and semicontinuous reactions were analyzed to determine major products. Unfortunately, in the case of the batch reactor, it is not possible to collect the gas product because of limitations of our reactor design. However, the gas products were collected from the semicontinuous system from the liquid-gas separator in Figure 1 using a gas bag (Calibrated Instruments Inc., Hawthorne, NY). The gaseous phase was collected and analyzed by a Fourier tranform infrared (FTIR) spectroscope (Perkin-Elmer Spectrum 2000 FTIR) equipped with a gas cell. The ratio of CO2/ CO was determined from peak area comparisons to calibration standards (CO2, 2280-2400 cm-1; CO, 20002250 cm-1). The relative absorptivities of CO2 to CO were ca. 5/1.19 A commercial software package obtained from Perkin-Elmer was used to obtain the area of infrared peaks from the gas products. In addition to using FTIR spectroscopy to analyze the gaseous products, FTIR was also used to analyze the solid residuals obtained from the reactions of SBR for the SBR content. The samples were prepared by grinding 20-30 mg of the solids using an agate mortar and pestle and adding the solvent of tetrahydrofuran (THF). The solution was spread evenly on a NaCl salt plate and analyzed by FTIR. Most of the THF evaporated during the process, because the sample was prepared in the fume hood. Each spectrum was obtained in the same manner: 100 spectral scans were obtained with a spectral resolution of 8 cm-1 in the 4000-600 cm-1 spectral domain. The absorbance band near 965 cm-1 has been attributed to the in-phase, out-of-plane bending of dCH in SBR.20 Identification of products in the aqueous phase of some representative experiments was performed by gas chromatography using a Varian 3400 gas chromatograph, equipped with an AT-5 column (30 mm × 0.25 mm × 0.25 µm; Alltech), a flame ionization detector, and a Varian 8100 autosampler/injector. The gas chromatographic analysis was optimized to provide the required degree of separation based on the resolution of target compounds. Thus, the GC was operated in the temperature-programming mode with an initial column temperature of 50 °C for 2 min, then increased linearly to 250 °C at a rate of 20 °C, and held at the upper temperature for 2 min. The detector was maintained at 260 oC, and the injector port was set to a temperature of 250 oC. The organic products were extracted from the aqueous product phase from the reactor using methylene chloride, and an internal standard, sec-butylbenzene, was added to the methylene chloride solution for GC analysis. For the development of calibration curves, the error of internal standard response was less than 10% except for benzoic acid, whose error was approximately 20%. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that many organic compounds were formed during the reactions of SBR in supercritical water. In a previous study, Park et al.3 showed that SCWO of waste tire production material (of which SBR is the primary component) produced volatile liquid products as well as gaseous products. The amount of
liquid products obtained was calculated by quantitative analysis of GC-MS results. The reaction temperature played a key role in determining the amount of liquid products produced. At the lowest temperature of 300 °C, liquid products coming from the SBR depolymerization reactions are around 35 wt % of the starting waste tire material. At the intermediate temperatures of 350 and 400 °C, the liquid products obtained are more than 23 wt % of the starting material, and only 10 wt % of the starting material was converted to liquid products at the highest reaction temperature of 450 °C. Furthermore, nearly identical destruction efficiencies and amounts of liquid products were obtained from both reactions of the waste tire material and the pure SBR at similar operating conditions. Unfortunately, because the number of liquid products detected by GC-MS are innumerable, it was not possible to perform quantitative analysis of the liquid products produced for all of the SBR depolymerization reactions. Only the prevalent peaks of benzene, toluene, ethylbenzene, styrene, benzaldehyde, phenol, acetophenone, and benzoic acid were analyzed quantitatively because most compounds were present in very small amounts. The hydrocarbon products, benzene, toluene, ethylbenzene, and styrene, which did not contain oxygen, may have been formed through a thermal degradation mechanism. These compounds were designated as simple hydrocarbon products (HyP). The latter four compounds, benzaldehyde, phenol, acetophenone, and benzoic acid, which are oxygenates, were formed through oxidative degradation. These compounds were designated as oxygen-containing products (OxP). The mass produced of these eight compounds typically constituted less than 5 wt % of the SBR starting mass. In the cases of high degradation efficiency, a significant fraction of the SBR is converted to gaseous products. Similarly, in the case of low degradation efficiency, a significant fraction of the SBR remained as a solid either as SBR or a high molecular weight degradation product. 3. Results and Discussion 3.1. SBR Batch Reaction SCWO Experiments. The depolymerization of SBR was studied at several different temperatures and pressures in a batch reactor. Table 1 presents the experimental conditions used in 34 SBR degradation experiments performed in the SCWO batch reactor. The destruction efficiency, which is defined as the change of the SBR mass as a result of reaction (initial SBR - final SBR mass) divided by the initial SBR mass, strongly depended upon the reaction conditions. The effect of temperature on the destruction efficiencies was investigated by performing experiments in which the temperature was varied at constant H2O2 concentration and reaction time. At 0 wt % H2O2, the destruction efficiency increased dramatically by changing the temperature from 300 to 450 °C. At 300 °C and 60 min (run S3), low thermal energy resulted in a destruction efficiency of 5%, as shown in Table 2. In contrast, above the critical temperature (374 °C) of water, a destruction efficiency of about 90% was obtained because the thermal energy was sufficient to destroy the SBR structure (runs S27 and S39). The effect of H2O2 concentration on SBR degradation was also studied at constant reaction temperature and time. As the H2O2 concentration was increased in the batch SCWO reaction, at a given temperature, the destruction efficiency of SBR increased until it reached
Ind. Eng. Chem. Res., Vol. 40, No. 3, 2001 759 Table 1. Experimental Conditions and Results for SBR Degradation in Supercritical Water in a Batch Reactora,b mass of the obtained liquid product [µg] organic reaction destruction benzoic expt reaction H2O2 concn [wt %] time [min] efficiencyc pHd benzene toluene ethylbenzene styrene benzaldehyde phenol acetophenone acid no. temp [°C] S3 S6 S9 S12
300 300 300 300
0.0 0.6 2.5 5.0
60 60 60 60
5.1 20.7 25.0 54.0
6.0 4.0 4.0 4.0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 180.7 303.4 390.3
0 0 102.7 123.6
0 86.3 167.5 207.8
0 75.8 202.1 385.0
S13 S14 S16 S17 S18 S19 S20 S21 S22 S23 S24
350 350 350 350 350 350 350 350 350 350 350
0.0 0.0 0.6 0.6 0.6 2.5 2.5 2.5 5.0 5.0 5.0
20 40 20 40 60 20 40 60 20 40 60
0.0 5.9 4.3 6.1 16.3 17.9 36.3 24.6 24.5 49.0 61.0
7.0 5.5 4.0 4.0 4.0 3.0 4.0 4.0 3.0 3.5 4.0
0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0
0 0 76 146.3 158.1 210.6 272.8 438.3 430.1 509.5 405.7
0 0 0 0 186.9 0 85.7 308.6 105.5 184.8 280.8
0 0 0 82.1 109.2 112.7 144.5 342.6 226.4 252.3 272.4
0 0 156.3 175.6 71.9 310.4 262.3 431.8 673.2 311.8 505.2
S26 S27 S29 S30 S31 S32 S33 S34 S35 S36
400 400 400 400 400 400 400 400 400 400
0.0 0.0 0.6 0.6 2.5 2.5 2.5 5.0 5.0 5.0
40 60 40 60 20 40 60 20 40 60
30.3 88.1 39.8 >90.0 5.1 43.6 >90.0 42.5 >90.0 >90.0
6.0 7.0 4.0 4.0 4.0 4.0 4.0 3.0 4.0 4.0
0 0 0 0 0 0 58.1 0 0 0
53.3 63.8 52.7 102.1 0 40.2 230.3 0 0 58.3
0 0 0 0 0 0 144.1 0 0 0
98.2 53.6 74.9 0 0 49.9 245.3 0 0 0
0 0 119 121.8 91.1 325.8 169 609.5 235.8 244.4
0 0 0 121.1 0 262.5 155.2 260.9 236.9 283.9
0 0 78.9 103.8 64.6 240.9 127 391.3 124.9 193.1
0 0 0 0 68.6 392.3 386.2 820.8 467.3 273.1
S38 S39 S41 S42 S43 S44 S45 S47 S48
450 450 450 450 450 450 450 450 450
0 0 0.6 0.6 2.5 2.5 2.5 5.0 5.0
40 60 40 60 20 40 60 40 60
>90.0 >90.0 >90.0 >90.0 52.7 >90.0 >90.0 >90.0 90.0
5.0 4.0 4.0 3.5 4.0 4.0 4.0 4.0 4.0
0 0 35.3 0 0 44.7 0 92.8 70.7
103.4 88.9 205.7 133.4 45.7 152.3 85.8 141.5 120.9
43.3 17.6 124.0 95.5 0 82.8 25.2 104.3 23.7
0 0 30.6 0 128.8 0 0 0 0
0 0 50.6 0 364.6 72.8 0 272.8 77.0
87.4 108.8 180.9 241.8 128.3 178.7 214.7 426.8 451.0
0 0 94.1 95 211.5 113.8 105.3 303.6 226.1
170.1 0 101.2 0 460.2 148.3 31.1 366.8 149.2
a Initial mass of SBR and the total water and hydrogen peroxide solution in each experiment kept at constants of 0.1 and 4.5 g, respectively. b The reaction pressure was >400 bar in all runs. c (Initial SBR - final SBR mass)/initial SBR mass × 100 (%). d The pH is estimated with pH paper.
Table 2. Destruction Efficiency as a Function of the H2O2 Concentration and Reaction Time 20 min 300 °C 350 °C 400 °C 450 °C a
40 min
60 min
0
0.6
2.5
5.0
0
0.6
2.5
5.0
0
0.6
2.5
5.0
a 0 a a
a 4.3 a a
a 17.9 5.1 52.7
a 24.5 42.5 a
a 5.9 30.3 >90.0
a 6.1 39.8 >90.0
a 36.3 43.6 >90.0
a 49.0 >90.0 >90.0
5.1 a 88.1 >90.0
20.7 16.3 >90.0 >90.0
25.0 24.6 >90.0 >90.0
54.0 61.0 >90.0 >90.0
The reaction was not performed.
>90%. At 300 °C, the destruction efficiencies were 20, 25, and 54% at 0.6, 2.5, and 5.0 wt % H2O2 at a reaction time of 60 min, respectively, as shown in Table 2. The effect of the H2O2 concentration was also evident at 350 °C: at 40 min the destruction efficiency increased from 5.9% at 0 wt % H2O2 to 49.0% at 5.0 wt % H2O2. Because the destruction efficiency had already reached nearly 90% at 400 and 450 °C without H2O2, the effect of the H2O2 concentration could not be observed in terms of the destruction efficiency. However, the addition of H2O2 changed the liquid product distribution dramatically at these higher temperatures. The detectable compounds consisted of only the OxPs at the lower temperatures; however, at the higher temperatures, both OxPs and HyPs were present. This will be discussed in more detail in the next section. In addition, the reaction time is also very important in terms of the destruction efficiency. For instance, at the conditions of 400 °C and 2.5 wt % H2O2, the destruction efficiencies were 5.1, 43.6, and >90% (Table 2) at 20, 40, and 60 min, respectively. 3.1.1. Analysis of Liquid Products. A semiquantitative liquid product analysis of the SBR depolymer-
ization reaction in supercritical water is shown in Tables 1 and 3. The effect of temperature is evident in the liquid product distribution obtained from experiments S3, S27, and S39, all of which were reacted without H2O2 for 60 min. At 300 °C, none of the eight aforementioned compounds were observed, as shown in Table 3 (run S3). Although the SBR polymer may have been broken down to some oligomers based upon the destruction efficiency of 5%, the thermal energy was insufficient to produce identifiable lower molecular weight compounds at the low reaction temperature of 300 °C. However, when the reaction temperature was raised to 400 °C, SBR reacted and broke down into identifiable lower molecular weight compounds. All of the detectable organic compounds were HyPs. When the reaction temperature was further increased to 450 °C, the OxPs of phenol and benzoic acid were detected as well as HyPs, which strongly suggests that the reaction medium of water in supercritical condition also served as an oxidant in the reaction. Helling and Tester also showed that supercritical water served as an oxidant in the oxidation of carbon monoxide.6
760
Ind. Eng. Chem. Res., Vol. 40, No. 3, 2001
Table 3. Selected Liquid Product Distribution and Destruction Efficiency of the SBR Batch Reaction Runs of Reaction Time at 60 min H2O2 concn 0 wt % run no. 300 °C
S3 none observed
DEa
5%
run no. 350 °C
N/Ab
DE
>6%
run no. 400 °C
S27 toluene, styrene
0.6 wt %
S12 benzaldehyde, phenol, acetophenone, benzoic acid 54%
S18 benzaldehyde, phenol, acetophenone, benzoic acid 16%
S21 benzaldehyde, phenol, acetophenone, benzoic acid 25%
S24 benzaldehyde, phenol, acetophenone, benzoic acid 61%
S33 benzene, toluene, ethylbenzene, styrene, benzaldehyde, phenol, acetophenone, benzoic acid >90%
S36 toluene, benzaldehyde, phenol, acetophenone, benzoic acid >90%
S45 toluene, ethylebenzene, phenol, acetophenone, benzoic acid >90%
S48 benzene, toluene, ethylbenzene, benzaldehyde, phenol, acetophenone, benzoic acid >90%
DE
88%
run no. 450 °C
S39 toluene, ethylbenzene, phenol, benzoic acid >90%
S42 toluene, ethylbenzene, phenol, acetophenone >90%
a
5.0 wt %
S9 benzaldehyde, phenol, acetophenone, benzoic acid 25%
S30 toluene, benzaldehyde, phenol, acetophenone >90%
DE
2.5 wt %
S6 benzaldehyde, phenol, acetophenone, benzoic acid 21%
DE: destruction efficiency. b N/A: not available.
The effect of H2O2, the oxidant, on the SCWO was evident in the products that were produced at different reaction temperatures. When H2O2 was added to the reaction of SBR in supercritical water, the detectable compounds consisted of only OxPs of benzaldehyde, acetophenone, phenol, and benzoic acid at 300 and 350 °C (runs S6, S9, S12, and S16-S24). By contrast, at 400 and 450 °C, both OxPs and HyPs comprised of benzene, toluene, ethylbenzene, and styrene were present. The specific compounds present at each condition are noted in Table 3. When SBR reacted at low temperatures (300 and 350 °C), forming OxPs, the oxidative mechanism of the hydroxyl radical generated from H2O2 appeared to be the predominant mechanism based upon the liquid product analysis in Table 3. However, at higher reaction temperatures of 400 and 450 °C, two mechanisms appeared to be operative. An oxidative degradation occurred, resulting in OxPs, and a thermal degradation occurred, resulting in the production of HyPs, such as toluene and ethylbenzene, as presented in Table 3. Rice et al.21 showed that hydrogen peroxide decomposition plays a key role during the oxidation of methanol in supercritical water experiments over a temperature range of 440-500 oC at 24 MPa. They identified that the dissociation of H2O2 is not well characterized quantitatively but appeared to be rate controlling during much of the reaction.21 Hydrogen peroxide decomposition in supercritical water was also studied by several groups.22,23 For example, Croiset et al.22 conducted experiments at pressures ranging from 5.0 to 34.0 MPa and for temperatures up to 450 oC. Their results showed that H2O2 decomposition in water followed first-order kinetics in the aqueous, vapor, and supercritical phases. They also stated that the water density was the most important factor determining the homogeneous rate of H2O2 thermal decomposition in water. Akiya and Savage23 have recently performed some very detailed studies, quantifying the decomposition rate constant of H2O2 as a function of the reduced density using the density functional theory. They showed that the rate constant for H2O2 decomposition goes through a maximum with increasing water density and that the rate constant in
supercritical water is larger than the rate constant in the gas phase. Given that the kinetics of oxidation is very dependent on the •OH radical concentration, the extent of H2O2 decomposition will be a very important factor. This density-dependent H2O2 decomposition may explain the high destruction efficiencies and predominant oxidation products obtained at the liquidlike densities and the lack of oxidation products at gaslike densities. In the SCWO reactions at almost all reaction temperatures (300, 350, and 400 °C), a small amount of H2O2 altered the liquid products distribution, as presented in Table 3. When only 0.6 wt % H2O2 was added to the reaction, the destruction efficiency and liquid product distribution were different from those in reaction without H2O2. The oxygenated products of benzaldehyde, phenol, acetophenone, and benzoic acid were obtained through oxidative degradation by H2O2, especially at 300 and 350 °C, as shown in Table 3. However, when no H2O2 was present, none of the eight compounds was observed. At 400 °C, toluene and styrene were observed without H2O2, while with 0.6 wt % H2O2, toluene was still observed in conjunction with three oxygenated products, benzaldehyde, phenol, and acetophenone. The higher temperature of 450 °C increased the number and variety of products in the reaction without H2O2, resulting in toluene, ethylbenzene, phenol, and benzoic acid being formed. When 0.6 wt % H2O2 was added, the benzoic acid was replaced by phenol but the other products remained the same. On the basis of the reaction products observed during the batch reaction at different temperatures and pressures, two parallel primary reaction pathways of thermal degradation and oxidative degradation appeared to be operative during the degradation of SBR. Unfortunately, to the authors’ knowledge, detailed reaction pathways for the depolymerization of polymer in supercritical water by using H2O2 as the oxidant are not available in the literature to date. At low reaction temperatures, an oxidative degradation mechanism, with hydroxyl radicals being produced from H2O2, may be preferred. When H2O2 was present at low tempera-
Ind. Eng. Chem. Res., Vol. 40, No. 3, 2001 761 Table 4. Experimental Conditions and Results for SBR Degradation in Near-Critical Water in Semicontinuous Unit Runs of Reaction Time at 60 min pH
run no.
temp [°C]
pressure [bar]
H2O2 concn [wt %]
flow rate [cm3/min]
destruction efficiency [%]
CO2/CO ratio
20 min
40 min
60 min
A1 A2a A3a A4 A5a A6 A7 A8a A9 A10 A11a A12 A13 A14a A15a A16 A17 A18 A19 A20 A21 A22 A23 A24 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10a B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21a B22 B23 B24
300 300 300 350 350 350 300 300 300 350 350 350 400 400 400 450 450 450 400 400 400 450 450 450 300 300 300 350 350 350 300 300 300 350 350 350 400 400 400 450 450 450 400 400 400 450 450 450
135 135 135 135 135 135 170 170 170 170 170 170 135 135 135 135 135 135 170 170 170 170 170 170 135 135 135 135 135 135 170 170 170 170 170 170 135 135 135 135 135 135 170 170 170 170 170 170
0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5 0 2.5 5
2.5 2.5 2.5 2.5 2.5 2.5 5 5 5 5 5 5 5 5 5 5 5 5 2.5 2.5 2.5 2.5 2.5 2.5 5 5 5 5 5 5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 5 5 5 5 5 5
4.6 100 100 13.2 17.8 79.7 2.6 100 100 7.4 100 100 41.7 30 44.8 100 100 100 100 86.9 96 100 100 100 0.5 100 100 13.2 81.2 93.8 2.5 100 100 19.2 73.1 98.9 100 81.2 100 100 100 100 44.5 55.1 81.3 100 100 100
n/a 17.6 7.6 n/a 10.2 16.3 n/a 31.5 21.0 n/a 11.2 15.3 n/a n/a 6.3 n/a 66.3 6.2 n/a 5.0 11.3 n/a 2.3 8.2 n/a 6.8 4.4 n/a 6.8 7.2 n/a 30.8 11.1 n/a 8.0 19.0 n/a 3.6 3.3 n/a 6.8 2.5 n/a 12.6 6.4 n/a n/a 4.8
7 3 3 7 4 4 7 3 3 7 4 3 7 4 3 7 4 6 7 4 4 7 4 4 7 4 4 7 4 4 7 4 4 7 4 4 7 4 4 7 4 4 7 4 4 7 6 6
7 3 4 7 5 4 7 4 5 7 5 5 7 5 4 7 5 7 7 4 5 7 6 7 7 5 6 7 4 4 7 5 5 7 4 4 7 4 5 7 6 6 7 4 4 7 7 7
7 5 4 7 6 5 7 4 6 7 6 6 7 6 4 7 5 7 7 5 6 7 7 7 7 7 7 7 4 4 7 5 6 7 4 4 7 4 6 7 7 7 7 5 5 7 7 7
a The experiments were performed at least twice to check the reproducibility, and repeated runs yielded nearly identical destruction efficiencies in all cases with variation between runs of less than 10% at the same nominal operating conditions.
tures, all of the primary liquid products from SBR were comprised of OxPs. At higher reaction temperatures, in contrast, a thermal degradation mechanism may also be occurring because at 400 and 450 °C HyPs as well as OxPs were obtained in the analyzed liquid products. 3.2. Semicontinuous SCWO Unit. The SBR depolymerization reactions using a batch reactor gave results in terms of both the destruction efficiency and liquid product distribution. To obtain additional information concerning the effect of reaction parameters on the destruction efficiency and liquid product distribution, the depolymerization of SBR was also performed in a semicontinuous reactor in which the effects of reaction parameters, temperature, pressure, oxidant concentration, and flow rate were studied. As in the batch reactor studies, two likely reaction mechanisms were apparent where thermal degradation was predominant at higher temperatures (>400 °C) and oxidative degradation was predominant at lower temperatures (
