Energy & Fuels 2002, 16, 109-118
109
Thermal Pyrolysis of Poly(vinyl alcohol) and Its Major Products Je-Lueng Shie,† Yi-Hung Chen,† Ching-Yuan Chang,*,† Jyh-Ping Lin,‡ Duu-Jong Lee,§ and Chao-Hsiung Wu| Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106, Taiwan, Department of Environmental Engineering, Lan-Yang College of Technology, Tou-Cheng, I-Lan 261, Taiwan, Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan, and Department of Environmental Engineering, Da-Yeh University, Chang-Hwa 515, Taiwan Received April 10, 2001. Revised Manuscript Received September 13, 2001
Poly(vinyl alcohol) (PVA) can be used to manufacture the immobilizing support of microorganisms in wastewater treatment systems. The withdrawn activated sludge contains lots of PVA. Before considering the thermal treatment of activated sludge containing PVA, one should investigate the behavior of PVA alone during the thermal treatment. The pyrolysis of PVA is thus examined with a thermal gravimetric analyzer (TGA). The kinetics of the thermal pyrolysis of PVA is conducted using nitrogen as the carrier gas in 378-1073 K and at various constant heating rates (HRs) of 5, 10, and 25 K/min. Major products are collected in 378-1073 K and at the constant HR of 5 K/min. The results indicate that the entire pyrolysis process of PVA under the experimental conditions of this investigation consists of two distinct pyrolysis stages. The corresponding activation energies E, reaction orders n, and frequency factors A of the reactions of two distinct pyrolysis stages are 148.35 and 129.4 kJ/mol for E, 1.04 and 1.48 for n, and 4.319 × 1011, and 5.169 × 106 1/s for A. The liquid products (condensates of gas at 298 K) consist of two layers (upper and lower layers). The distillation characteristics of the oil portions of both liquid products of upper and lower layers from the pyrolysis of PVA are between gasoline and diesel oil; meanwhile, the oil quality of the lower layer is better than that of the upper one. However, the upper level liquid product does not contain water, but the lower level one contains about 97.37-98.63 wt % water. The major gaseous products (noncondensable gases at 298 K) excluding N2 are HCs (hydrocarbons, 40.36 wt %), H2O (20.93 wt %), CO2 (19.77 wt %), and CO (18.94 wt %). The HCs mainly consist of C6 (14.3 wt %) and C1 (12.53 wt %). All this information is useful not only to the proper design of a pyrolysis system but also to the better utilization of liquid products and understanding of gaseous emission. The oil portions of liquid products can be obtained by the prefractional condensation or postdistillation.
Introduction Some wastewater treatment processes are based on naturally immobilized microorganisms. Aggregated cells in activated sludge systems as well as cells attached to the rock or plastic surfaces in trickling filters are good examples of exploitation of immobilized cells in wastewater treatment.1 Among the immobilization techniques, poly(vinyl alcohol) (PVA) can be used to manufacture the immobilizing support of microorganisms in wastewater treatment systems. The purposes of the immobilization of microorganisms are (1) preventing the wash-out of microorganisms, (2) protecting the microorganisms from the toxicity of toxic materials and removing the toxic pollutants, and (3) enhancing the †
Graduate Institute of Environmental Engineering. Department of Environmental Engineering, Lan-Yang College of Technology. § Department of Chemical Engineering. | Department of Environmental Engineering, Da-Yeh University. (1) Webb, C. Cell Immobilization, in Environmental Biotechnology; Forster, C. F., Wase, D. A. J., Eds.; Ellis Horwood: Chichester, U.K., 1987; pp 347-376. ‡
efficiencies of wastewater treatment systems. Immobilization of activated sludge microorganisms in PVA dropped in saturated boric acid solution or refrigerated for gel formation is a useful immobilization technique.2 The optimal pH value of enzyme activity is not changed by immobilization, though the activity profile of immobilized cell is broader than that of free cell.3 Cell immobilization in PVA by PVA-freezing method has also been considered for enhancing the nitrification in wastewater treatment.4-5 The withdrawn activated sludge contains lots of PVA. Before considering the (2) Matsui, T.; Kyosai, S.; Takahashi, M. Application of Biotechnology to Municipal Wastewater Treatment; Water Sci. Technol. 1991, 23, 1723-1732. (3) Ariga, O.; Kato, M.; Sano, T.; Nakazawa, Y.; Sano, Y. Mechanical and Kinetic Properties of PVA Hydrogel Immobilizing Beta-Galactosidase. J. Fermentation and Bioeng. 1993, 76 (3), 203-206. (4) Ariga, O.; Takagi, H.; Nishizawa, H.; Sano, Y. Immobilization of Microorganisms with PVA Hardened by Iterative Freezing and Thawing. J. Ferment. Technol. 1987, 65, 651-658. (5) Myoga, H.; Asano, H.; Nomura, Y.; Yoshida, H. Effects of Immobilization on the Nitrification Treatability of Entrapped Cell Reactors Using the PVA Freezing Methodol. Water Sci. Technol. 1991, 23, 1117-1124.
10.1021/ef010082s CCC: $22.00 © 2002 American Chemical Society Published on Web 12/18/2001
110
Energy & Fuels, Vol. 16, No. 1, 2002
thermal treatment of activated sludge containing PVA, one should examine that of PVA alone. Thermal decomposition of PVA in a nitrogen flow was investigated in the presence of an Fe-containing catalysts and the iron-carbon composites formed were characterized.6-7 Carbon composites with large amounts of fine iron particles were produced by the pyrolysis of Fe-PVA complexes with different iron contents.6 Fecontaining catalyst greatly affects both liquid and solid products of thermal decomposition of PVA.7 There were also a lot of investigations about the thermal, γ-radiation and photo degradation, and about the stabilities of PVA alone, PVA/PVC (poly(vinyl chloride)), or PVA/ miscible cellulose blends.8-13 For the PVA degradation, the effect with PVA irradiated with γ-rays at elevated temperatures and then subjected to the thermal treatment is greater than that with PVA preirradiated at ambient temperature.12 In the thermal degradation of PVA/PVC blends, strong interaction occurs in the degrading blend wherein hydrogen chloride from PVC caused substantial acceleration in the dehydration of PVA.8 Also, the thermal stability and decomposition behavior of PVA are improved by the addition of PVP (poly(N-vinylpyrrolidone). Thus, there is also a strong interaction between PVA and PVP. PVA forms miscible blends with PVP throughout the entire composition range.13 In the metal-chloride treatment of PVA, it was founded that γ-irradiation of ZnCl2-treated PVA produced a remarkable change in its crystallinity and the treatment of PVA with ZnCl2 helped the oxidative degradation of PVA.9 The above studies provided some useful information of the thermal treatment of PVA but gave no data about the detailed kinetic model and products distribution. It is thus the aim of the present work to deal with the thermal pyrolysis of PVA with the viewpoint of providing a kinetic model and product distribution of solid, liquid and gas. The pyrolysis is performed by the use of a dynamic thermogravimetric (TG) reaction system at the temperature-programmed heating rates (HRs) of 5, 10, and 25 K/min in nitrogen atmosphere. The corresponding activation energies E, frequency factors A, and reaction order n of reactions are determined. Also, the solid residues, liquid oils, and noncondensable gases at 298 K are collected and analyzed by the elemental (6) Ohtsuka, Y.; Watanabe, T.; Nishiyama, Y.; Matsuda, M. Iron Dispersed Carbon Composites Formed from Iron-Poly(vinyl alcohol) Complexes. J. Mater. Sci. 1994, 29, 877-882. (7) Maksimova, N. I.; Krivoruchko, O. P. Study of Thermocatalytic Decomposition of Polyethylene and Polyvinyl Alcohol in the Presence of an Unsteady-State Fe-containing Catalyst. Chem. Eng. Sci. 1999, 54, 4351-4357. (8) Fukatsu, K. Study of Thermal Degradation of Poly(vinyl alcohol)/ (Poly(vinyl chloride) Blend with Flame Retardants. J. Fire Science. 1986, 4, 204-215. (9) Rabie, S. M.; Nagwa, A. H.; Moharram, M. A. Effect of γ-Irradiation and Temperature on the Structure of Metal-Chloride-Treated Poly(vinyl alcohol). J. Applied Polymer Science. 1990, 40, 1163-1176. (10) Baimuratov, E.; Saidov, D. S.; Kalontarov, I. Y. Thermal, Photo and γ-radiation Degradation of Mechanically Loaded Poly(vinyl alcohol) (PVA). Polym. Degrad. Stab. 1993, 39, 35-39. (11) Nishioka, N.; Hamabe, S.; Murakami, T.; Kitagawa, T. Thermal Decomposition Behavior of Miscible Cellulose/Synthetic Polymer Blends. J. Appl. Polym. Sci. 1998, 69, 2133-2137. (12) Zhao, W.; Yamamoto, Y.; Tagawa, S. Radiation Effects on the Thermal Degradation of Poly(vinyl chloride) and Poly(vinyl alcohol). J. Polym. Sci.: Part A: Polym. Chem. 1998, 36, 3089-3095. (13) Rao, V.; Latha, P.; Ashokan, P. V.; Shridhar, M. H. Thermal Degradation of Poly (N-vinylpyrrolidone)-Poly(vinyl alcohol) Blends. Polym. J. 1999, 31 (10), 887-889.
Shie et al. Table 1. Typical Properties of Polyvinyl Alcohol (PVA) Sample Used in This Study item
property
viscosity (4 wt % sol., 20 oC) saponification value volatile matter ash pH
35-45 cps 99-100 vol % max 5.0 wt % max 1.0 wt % 5-7.5
Particle Size Distribution mesh range
wt %
0.84 mm) 20-25 (0.71 ∼ 0.84 mm) 25-30 (0.596 ∼ 0.71 mm) 30-40 (0.42 ∼ 0.596 mm) 40-45 (0.35 ∼ 0.42 mm) 45-80 (0.177 ∼ 0.35 mm) >80 (811 K mass/total mass wt %
38.62 (1.07)
∼0 4.65
4
0.36 (0.27)
2.77 (2.11)
1.78 (1.36)
8.49 (2.67)
5.36 (0.32)
6.7 (1.90)
10.07 (1.91)
0.04 (0)
HL21
∼0e
GL
DL
FL21
(0.14)d 0.97c
92 unleaded GL 95 unleaded GL 98 unleaded GL TPLmixed1073 (F) TPL-L1073 (E) TPL-U1073 (D) TPL-mixed637 (C) TPL-L637 (B) TPL-U637 (A)
tive analyses are made by GC-FID (for HCs), GC-TCD (for H2, CO, CO2) and psychrometer (for H2O). Retention times of major compounds and methods of the data treatment are the same as the study of Chang et al.14 The instantaneous concentrations of gaseous products at various temperature ranges are shown in Figures 3 and 8. From Figure 8, the maximum instantaneous concentrations occur at about 598 K for HCs, 648 K for H2O, 948 K for CO2, and 998 K for CO, respectively. Obviously, the depolymerization and dehydration are the main reactions in the first stage, while gasification is prominent in the second stage. The accumulated masses of four major products (HCs, H2O, CO2, and CO) relative to total mass of gaseous products, in wt %, are 40.36 (121,081 µg/L) for HCs, 20.93 (62,786 µg/L) for H2O, 19.17 (59,313 µg/L) for CO2, and 18.94 (56,830 µg/ L) for CO, respectively. It is noted that the CO2 and CO are not detected before 548 K. Among the seven groups of HCs (Figure 3), the accumulated masses of major hydrocarbons relative to the total HCs gases, in wt %, are about 35.43 for C6, 31.04 for C1, 15.49 for C4, 8.94 for C5, 4.41 for C3, 4.26 for C7, and 0.42 for C8, respectively. The HCs mainly consist of C6 compounds and CH4. Quantitative analyses for the emissions of benzene (B), toluene (T), ethylbenzene (E), and xylene (X) are made and shown in Figure 4 at various temperatures. The accumulated masses of four BTEX gaseous products relative to total mass of BTEX gaseous products, in wt %, are 80.28 (4,170.8 µg/L) for benzene, 19.23 (73.75 µg/L) for toluene, 0.04 (20.56 µg/L) for isoxylene, and 0.01 (5.19 µg/L) for ethylbenzene, respectively. Among the BTEX, the compounds with higher concentrations are benzene and toluene. The maximum instantaneous concentrations occur at about 598 K for benzene and 748 K for toluene, respectively. (iii) Liquid Products. The liquid products are mostly collected in the first condensing tube immersed in a 298 K water bath. Since water exists in the liquid products, the liquid products exhibit two layers (upper and lower) after staying still. They are collected by six ways so as to investigate the effects of the final temperature on their qualities. TPL-U-637 (A), TPL-L-637 (B), and TPLmixed-637 (C) are the upper and lower layers of liquid products, and their mixture at 378-637 K, respectively. In the mixed liquid of C, the upper and lower layers are about 17.71 and 82.29 wt %, respectively. TPL-U1073 (D), TPL-L-1073 (E) and TPL-mixed-1073 (F) are the upper and lower layers of liquid products, and their mixture at 378-1073 K. In the mixed liquid of F, the upper and lower layers are about 41 and 59 wt %, respectively. The liquid products and some commercial oils are analyzed for the different boiling points (bps) by GC, according to the Standard Test Method for Boiling Range Distribution of Petroleum Fractions, proposed by the ASTM D-2887 method. Excluding water contents, one notes that five portions of the organic components of liquid products are cut apart as listed in Table 5. Including water contents, the corresponding five portions and water content of liquid products are presented in Table 6. From Table 5, the oil portion of liquid A contains, in wt %, about 0.97 light naphtha, 38.62 heavy naphtha, 57.24 light gas oil, 3.18 heavy gas oil, and 0.01 vacuum residue, respectively. Thus, its quality is between
Energy & Fuels, Vol. 16, No. 1, 2002 115 Table 5. Distillation Characteristics of Oil Portions of PVA Pyrolysis Liquid Products without Counting Water Content in This Study and of Some Commercial Oilsa,b
Pyrolysis of Poly(vinyl alcohol)
TPL-L637 (B)
TPL-mixed637 (C)
TPL-U1073 (D)
TPL-L1073 (E)
TPL-mixed1073 (F)
98 unleaded GL
95 unleaded GL
92 unleaded GL GL
DL
FL21
HL21
ND
ND
1.3412
2.1437
7.2789
2709.95 (1882.25) 117.34
1343.78
NDe
12006.27
12068.63
11716.49c (380.65)d 9057.87 (1505.44) 662.58 (94.35) ND 1.1371
7603.22 (230.74) 3304.85 (1828.32) 462.65 (13.74) ND
TPL-U1073 (D)
0.9761
ND
8891.06 (817.77) 869.44 (705.11) ND
TPL-L1073 (E)
1.0421
ND
8363.05 (577.09) 1867.96 (1165.63) 189.69
TPL-mixed1073 (F)
68623.31 (6546.22) 26681.42 (3256.5) 50318.16 (3229.71) 16.51
19477.11 (1424.07)
98 unleaded GL
20539.5 (1866.42) 70556.06 (18200.94) 28518.36 (3893.52) 48812.97 (13016.27) 16.8427
95 unleaded GL
80695.64 (5112.16) 56262.62 (2180.78) 66046.73 (3364.8) 32586.1 (376.17) 23.5591
92 unleaded GL
GL 87934.59 (2089.98) 64980.5 (6612.01) 56523.15 (24912.93) 34913.2 (336.57) 24.4351
DL 348.64 (12.05) 1740.11 (210.86) 801.70 (296.6) 1829.1 (706.53) 0.472
TPL, GL, DL, FL, HL: Total pyrolysis oil, gasoline, diesel oil, fuel oil, and heavy oil. A, B: Upper and lower layers of TPL at 378-637 K. C: Mixed TPL at 378-637 K. D, E: Upper and lower layers of TPL at 378-1073 K. F: Mixed TPL at 378-1073 K. b Heating rate HR ) 5 K/min. c Unit: wt %. d Values in parentheses are standard deviations (σn-1). e ND: Nondetectable concentration.
a
benzene (mg/Kg) toluene (mg/Kg) ethylbenzene (mg/Kg) isoxylene (mg/Kg) total wt %
TPL- mixed637 (C)
TPL-L637 (B)
TPL-U637 (A)
Table 7. BTEX Concentrations in PVA Pyrolysis Oils (Oil Portions without Counting Water) in This Study and Some Commercial Oilsa,b
a TPL, GL, DL, FL, HL: Total pyrolysis oil, gasoline, diesel oil, fuel oil, and heavy oil. A, B: Upper and lower layers of TPL at 378-637 K. C: Mixed TPL at 378-637 K. D, E: Upper and lower layers of TPL at 378-1073 K. F: Mixed TPL at 378-1073 K. b Heating rate HR ) 5 K/min. c Unit: wt %. d Values in parentheses are standard deviations (σn-1). e Negligible. f ND: Nondetectable concentration.
light naphtha 0.97c(0.14)d 0.12 0.27 0.36 (0.27) 0.037 (0.029) 0.169 (0.128) 8.49 (2.67) 5.36 (0.32) 6.7 (1.90) 10.07 (1.91) 0.04 (0) ∼0e ∼0 343-366 K heavy naphtha 38.62 (1.07) 1.74 8.27 29.67 (0.13) 0.816 (0.037) 12.64 (0.075) 64.37 (2.21) 70.95 (4.28) 67.9 (5.16) 62.93 (0.24) 7.84 (2.34) 4.03 0.29 366-477 K light gas oil 57.24 (0.23) 0.73 10.74 59.38 (0.17) 0.46 (0.051) 24.62 (0.1) 26.59 (3.4) 23.31 (4.40) 24.88 (5.22) 26.15 (2.65) 87.27 (1.42) 46.01 23.88 477-616 K heavy gas oil 3.18 (1.44) 0.03 0.59 10.81 (0.24) 0.057 (0.04) 4.45 (0.122) 0.54 (0.51) 0.38 (0.19) 0.52 (0.39) 0.85 (0.25) 4.78 (0.99) 49.59 75.19 616-811 K vacuum residue 0.01 (∼0) ∼0 ∼0 0.02 (0.02) ∼0 0.01 (0.01) ∼0 ∼0 ∼0 ∼0 ∼0 0.38 0.64 >811 K water < 0.001 97.37 (4.44) 80.13 (16.7) NDf 98.63 (4.77) 58.19 (11.01) ND ND ND ND ND ND ND
TPL-U637 (A)
Table 6. Distillation Characteristics of Oil Portions of PVA Pyrolysis Liquid Products Counting Water Content in This Study and of Some Commercial Oilsa,b
116 Energy & Fuels, Vol. 16, No. 1, 2002 Shie et al.
Pyrolysis of Poly(vinyl alcohol)
gasoline and diesel oil but more close to diesel oil. As for the oil portion of liquid B, it contains, in wt %, about 4.65 light naphtha, 66.33 heavy naphtha, 27.69 light gas oil, and 1.33 heavy gas oil, respectively. The comparison of distillation results indicates that it is close to gasolines (such as 98, 95, 92 unleaded gasolines and gasoline). Thus, the oil quality of the oil portion of the lower layer is better than that of the upper layer. The oil portion of liquid C (mixture of A and B) contains, in wt %, about 4 light naphtha, 61.42 heavy naphtha, 32.92 light gas oil, and 1.66 heavy gas oil, respectively. Its quality is close to gasoline. Under the conditions of 378-1073 K, the oil portion of liquid D contains, in wt %, about 0.36 light naphtha, 29.67 heavy naphtha, 59.38 light gas oil, 10.81 heavy gas oil, and 0.02 vacuum residue, respectively. Its quality is between gasoline and diesel oil but again more close to diesel oil. Comparison with liquid A indicates that the quality of the oil portion of liquid D is worse than that of liquid A, reflecting that increasing the pyrolysis temperature does not promote the oil quality of liquid products. As for the liquid E, its oil portion contains, in wt %, about 2.77 light naphtha, 59.54 heavy naphtha, 33.55 light gas oil, and 4.14 heavy gas oil, respectively. Its oil quality is close to gasoline. The oil quality of the oil portion of the lower layer is better than that of the upper layer. The oil portion of liquid F (mixture of D and E) contains, in wt %, about 1.78 light naphtha, 47.29 heavy naphtha, 44.14 light gas oil, 6.88 heavy gas oil, and 0.01 vacuum residue, respectively, with oil quality between gasoline and diesel. Again, the comparison of liquids B and E indicates that with the increase of temperature decreases the qualities of oil portions. Further, from Table 6, liquids B, C, E, and F contain water of about 97.37, 80.13, 98.63, and 58.19 wt %, respectively. However, the liquids A and D contain no water. Thus, for liquid products with two layers, the upper and lower layers mainly contain oil and water, respectively. In the first stage pyrolysis, liquid product C contains about 80.13 wt % water, nevertheless, including both first and second stages, liquid product F contains about 58.19 wt % water. Obviously, as the pyrolysis temperature increases, the water content in liquid product decreases. It also indicates that in the first stage, dehydration is the main reaction, therefore, in the second stage, the production of water decreases while that of oil portion increases. These results support the postulate that the reactions in the first stage are mainly dehydration and depolymerization, and in the second stage, the reactions are mainly depolymerization and thermal decomposition. The oil qualities of liquid products of A and D (upper layers) are between gasoline and diesel oil. Also they do not contain water. Therefore, the upper layers of liquid products of pyrolysis of PVA have potential for commercial uses after simple separation. (iv) Benzene (B), Toluene (T), Ethylbenzene (E), and Isoxylene (X) in the Liquid Products. The amounts of benzene (B), toluene (T), ethylbenzene (E), and isoxylene (X) in the oils affect the qualities of oils. Without counting the water contents, Table 7 lists the BETX concentrations in the oil portions of pyrolysis liquid products and some commercial oils. From Table 7, the total BETX concentrations of the oil portions of pyrolysis liquid products are between 0.9761 and 7.2789 wt %.
Energy & Fuels, Vol. 16, No. 1, 2002 117
Liquid C (mixed, 378-637 K) has the highest total concentration of BETX in the oil portions, while liquid E (lower layer, 378-1073 K) the lowest. From these results, without counting the water contents, the total BETX concentrations decrease as the pyrolysis temperature increases. Thus, the BETX concentrations of liquid products are higher in the first stage than in the second stage. Among the BETX in liquid products, the major portion is benzene contributing the C6 hydrocarbons in the gas products of HCs in the first stage. According to Table 7, the total BETX concentrations of commercial gasolines (98, 95, 92 unleaded gasolines, and gasoline) and diesel oil are between 16.51 and 24.4351 and 0.472 wt %, respectively. From the B, E, T, and X concentrations, 98 unleaded gasoline is close to 95 unleaded gasoline, while 92 unleaded gasoline is close to gasoline. The total concentrations of BETX of four commercial gasolines are higher than those of pyrolysis liquid products containing water. However, the total BETX concentrations of the oil portions of liquids A, B, C, D, E, and F are higher than the concentration of commercial diesel oil (0.472 wt %). From these results, all oil portions of the BETX concentrations of liquid products are between gasoline and diesel oil. Mass Balance of Solid Residues, Gaseous Products and Liquid Products. The products of PVA pyrolysis consist of solid residues (char), gaseous, and liquid products. At 1073 K, the final products relative to the initial dry PVA, in wt %, are about 31.37 liquid products (13.11 wt % of oil portions and 18.26 wt % of water), 42.37 gaseous products, and 4.58 solid residues, respectively. The total recovery is 78.32 wt % with 21.68 wt % uncollected. The 21.68 wt % deficiency of recovery may include (1) uncollected substances coated on the walls of collection line, (2) unidentified gaseous substances, and (3) experimental errors. In terms of relative percentages to the sum of collected gaseous products, liquid oils, and solid residues, the final product distributions, in wt %, are about 40.05 liquid products (16.74 wt % of oil portions and 23.31 wt % of water), 54.1 gaseous products, and 5.85 solid residues, respectively. Implication of Kinetic Models and Variation of Products with Temperature. The two-stage kinetic model proposed herein for the pyrolysis of PVA is simple and for the engineering use. The detailed intrinsic reaction scheme should be rather complex and would need further study to elucidate it. Restated, in this study, the pyrolysis gases are passed through two condensing tubes immersed in 298 K water bath before collection of gas. The condensates of condensable gases at 298 K are referred as the liquid products. In the pyrolytic temperature range of 378-1073 K, the compositions of the pyrolytic products of solid residues, condensate liquid products and noncondensable gases, which are associated with each other, vary with the temperature according to the two-stage kinetic model proposed in this study. Further from this study, it is seen that a higher heating rate HR gives a higher value of the residual mass fraction M at the same reaction temperature T and thus a lower conversion X (X ) 1 - M). A higher HR also results in a higher peak value of reaction rate (r ) dX/dt) and a higher temperature for its occurrence. Noting that X and dX/dt correspond to the yields MV
118
Energy & Fuels, Vol. 16, No. 1, 2002
Shie et al.
and formation rate dMV/dt of products, one may reasonably expect that, at the same T, a higher HR also results in a lower MV but a higher dMV/dt. For the PVA initially subjected to a high temperature (say, 1073 K), one may regard the situation as a special case of employing extremely high HR with a set ceiling temperature. Thus, according to the two-stage kinetic model and the role of HR on MV and dMV/dt, the product spectrum would be different for the cases with different heating rates and preset ceiling temperatures. Hence, the proposed two-stage kinetic model and temperature-dependent products spectrum provide useful information for the proper design and operation of the pyrolysis system of PVA. Recently, Lou et al.22 investigated the gas-product formation (CO, CO2, and HCs) in thermal decomposition of styrene-butadiene rubber (SBR) under various heating conditions (2, 5, 10, and 20 K/min) in oxygennitrogen atmospheres (5, 10, and 20 vol % O2). It is seen that the reaction zone shifts to the high-temperature region and that the peak height increases with an increasing HR. That is, an increase of HR causes the temperature with vigorous reaction to increase and the gas formation curve to move toward a high-temperature region. The gas formation rate also increases with a higher HR. Thus, the results of the present study of PVA system are consistent with those of Lou et al.22 of SBR system.
The pyrolysis of PVA is carried out by a TGA reaction system in nitrogen atmosphere. A two-stage kinetic model is proposed to predict the experimental results. The first stage may be described by a global reaction of devolatilization producing volatiles V1. The second stage may consist of two competitive (parallel) reactions, producing other volatiles V2 and char, respectively. The activation energies, frequency factors and reaction orders are determined for this two-stage reactions under experimental conditions. For the major products analysis, the accumulated masses of the four major gaseous products (HCs, H2O, CO2, and CO) relative to total mass of gaseous products, in wt %, are 40.36 (121081 µg/L) for HCs, 20.93 (62786 µg/L) for H2O, 19.17 (59,313 µg/ L) for CO2, and 18.94 (56830 µg/L) for CO, respectively. The HCs mainly consist of C6 compounds and CH4. The oil qualities of upper layer liquid products (liquids A and D) are between gasoline and diesel oil. Also, they do not contain water. Therefore, the upper layer liquid products of the pyrolysis of PVA have potential for commercial uses after simple separation. This study greatly assists the resource recovery of PVA as an energy resource.
(21) Chen, K. C. An Investigation of Major Products for Pyrolysis of Tire Tread in Nitrogen. M. S. Thesis, Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Taiwan, 1996.
(22) Lou, J. C.; Lee, G. F.; Chen, K. S. Incineration of Styrenebutadiene Rubber: The Influence of Heating Rate and Oxygen Content on Gas Products Formation; J. Hazard. Mater. 1998, 58, 165178.
Conclusions
EF010082S
