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The catalytic effect of Red Mud on the degradation of poly (vinyl chloride) containing polymer mixture into fuel oil
Polymer Degradation and Stability 73 (2001) 335–346 www.elsevier.nl/locate/polydegstab
The catalytic effect of Red Mud on the degradation of poly (vinyl chloride) containing polymer mixture into fuel oil Jale Yanika,*, Md. Azhar Uddinb, Kazuo Ikeuchib, Yusaku Sakatab a
b
Department of Chemistry, Ege University, 35100 Izmır, Turkey Department of Applied Chemistry, Okayama University, Okayama 700-8530, Japan Received 29 November 2000; accepted 23 February 2001
Abstract Thermal and catalytic degradation of poly (vinyl chloride) (PVC) containing polymer mixtures, PVC/PE, PVC/PP and PVC/PS, into fuel oil was investigated. In the catalytic degradations, Red Mud (a waste from alumina production) was tested as both cracking and dechlorination catalyst. For comparison, g-Fe2O3 as a chlorine sorbent and SA-1 (silica alumina) as a solid acid catalyst were also used. The effect of degradation conditions, such as nitrogen gas flow, stepwise pyrolysis, catalyst contact mode, on the dechlorination was also investigated. The use of N2 gas flow suppressed partially the reaction between HCl gas from the degradation of PVC and polymer degradation products. By stepwise pyrolysis, over 90% chlorine in the feed plastic was recovered as HCl gas. SA1 catalyst accelerated the rate of polymer degradation and lowered the boiling point of liquid products, but the chlorine content of oil over SA1 was also the highest. Red Mud (RM) and iron oxides sorbents showed good effect on the fixation of evolved HCl. However, they had no effect on the cracking of polymers. The oils derived from PVC containing pure polymer mixtures by thermal degradation contained a lower amount of chlorine than the oils obtained using RM and other catalysts. From this result we conclude that the formation of some organic chlorine compounds may be promoted by the interaction of the HCl and the catalysts. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Plastic degradation; Dechlorination; Red Mud
1. Indroduction The pyrolysis of plastic waste into useful fuel oil has been considered to be a promising recycling method. The main problem causes during the pyrolysis of waste plastics are related to Poly (vinyl chloride) (PVC). The pyrolysis products of PVC mixed waste plastics contain both inorganic and organic chlorine compounds [1–4]. The production of fuels having no chlorine content must be a main aim in waste plastics pyrolysis. The degradation mechanisms of PVC are well known. Recently, attempts have focused on the dechlorination of PVC containing polymer mixtures (especially binary mixture). Very little is known about the catalytic degradation of PVC containing polymer mixtures. Uddin et al have studied the thermal and catalytic degradation of PE/PVC, PP/PVC and PS/PVC mixtures [5]. Their data showed that by using iron oxides, FeOOH and Fe3O4, * Corresponding auhtor. Fax: +90-232-388-8264. E-mail address: [email protected] (J. Yanik).
the chlorine compounds in oil could be decreased to a very low level. They suggested that the chlorine removed from the products of degradation of PVC mixed plastics is fixed in the form of iron compounds such as ferrous chloride. They have also reported that the ferrous chloride acts as a catalyst for the dechlorination of organic chlorine compounds [6] In this study, the thermal and catalytic degradation of PVC/HDPE, PVC/PP and PVC/PS mixture was carried out by stepwise pyrolysis. In the catalytic degradations, Red Mud (RM) was used as a cracking catalyst and as a sorbent to fix the HCl formed by PVC degradation. RM is a by-product of alumina manufacture process (Bayer process). It contains mainly oxides of iron, aluminium, titanium, silicon, calcium and sodium. It has been used for the liquefaction of coal [7–9] and biomass [10] and in the hydrogenation of anthracene oil [11] as hydrogenation catalyst. To our knowledge, there are no published reports on the polymer degradation in the presence of RM. Use of a waste material as a catalyst for the recovery of other wastes is an important point of this study.
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00095-7
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2. Experimental 2.1. Materials The Red Mud, as a sludge, was supplied by Seydisehir Aluminium Company, Turkey. It was filtered and then dried at 110 C. It was analyzed by classical chemical analysis. Surface properties were measured by nitrogen adsorption, BET (Bel Japan, Belsorp 28SP). The surface properties and chemical compositions of RM are shown in Table 1. Polypropylene (PP) was obtained from UBE Chemical Industries; polystyrene (PS) from Asahi Kasei Industries; High-density polyethylene (PE) from Mitsui Chemical; and poly(vinyl chloride)(PVC) from Geon Chemical. The chlorine content of the PVC was 52.8 wt.%. 2.2. Methods
The amount of gaseous products was determined by subtracting the weight of liquid and residue from the plastic sample feed. The liquid products were analyzed by gas chromatograph with a flame ionization detector (Yanaca: G6800-FID). A gas chromatograph with a thermal conductivity detector (Yanaca: G180-TCD) was used to analyze the gases from single type polymers. Organic chlorine compounds in the liquid product were analyzed by two gas chromatographs with either an atomic emission detector (HP: G2350A-AED) or a mass selective detector (HP: 5973-MSD). Gaseous organic chlorine products were not detected.
3. Results and discussion 3.1. Thermal and catalytic degradation of PE/PVC mixture
The degradation and dechlorination experiments were carried out in a glass reactor (35 mm i.d. and 350 mm l) under atmospheric pressure at 430 C under N2 flow by semi-batch operation. A fixed amount of single type plastic (10 g) or mixed plastics [weight ratio [(PE, PP or PS)/PVC=8/2] and Red Mud (weight ratio: RM/pure polymer=1/10 or RM/mixed polymer=2/10) was loaded into the reactor. The reactor was heated to 120 C at 3 C/min and held at 120 C for 60 min to remove the physically absorbed water from the catalyst and the plastic sample. Then the temperature was increased to 300 C at a heating rate of 3 C/min. and the reactor was held at this temperature for 60 min to evolve the HCl. After the dechlorination, the reactor temperature was increased to 430 C at a heating rate of 3 C/min. The hydrogen chloride evolved from the degradation of PVC was trapped in a flask containing an aqueous solution of NaOH and analyzed by ion chromatography. The schematic diagram of experimental set up was given previously [5]. A few experiments was carried out without the dechlorination step or without N2 stream to compare the effect of RM with other catalyst used in previous studies. The products of degradation were classified into three groups: gases, liquid and residue. Table 1 Properties of Red Mud Element
Wt.%
Fe2O3 SiO2 Al2O3 TiO2 CaO MgO K2O Na2O
37.72 17.10 17.27 4.81 4.54 0.40 0.29 7.13
Surface area (m2 g 1)
16
In this group of experiments, the dechlorination ability of RM and the effect of its contact mode [liquidphase contact (LP), vapor-phase contact (VP)] were investigated. Besides RM, TR-99701 (g-Fe2O3) as a chlorine sorbent and SA-1 (SiO2.Al2O3) as a solid acid catalyst were also used. In addition to catalytic effects, the effect of degradation procedure on the dechlorination was also investigated. 3.1.1. Effect of the type of degradation procedure In order to investigate the effect of degradation procedure on the dechlorination, three different methods of degradation in presence of RM were examined. In the first method (method 1), a mixture of PE/PVC was degraded under the same conditions used in the previous works [12]. That is, the reactor was heated to 120 C at 3 C/min and held at this temperature for 60 min in N2 flow. Nitrogen flow was then cut off and the temperature was increased to 430 C at a heating rate of 3 C/min. In the second method (method 2), the reactor contents were heated to 120 C at 3 C/min and held at this temperature for 60 min. Then the temperature was increased to 300 C (at 3 C/min) and held at that temperature for 60 min to dehydrochlorinate the PVC with N2 flow. After the nitrogen flow was cut off, the temperature was increased to 430 C. In the third method (method 3); both dechlorination and degradation were carried out at 430 C in nitrogen flow as described in Section 2.2. Tables 2 and 3 show the product distribution and chlorine distribution from the degradation of PVC/ HDPE mixture for all three degradation methods. The comparison of methods 1 and 2 shows that the dechlorination step increased the gas yield and decreased the liquid yield. Although gas products have not been analyzed, we can say that this is due to greater HCl evolution. In addition the larger quantity of chlorine (as HCl)
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adsorbed in traps in method 2 also supports this idea. It is known that the dechlorination of PVC occurs at lower temperatures than degradation of other polymers. During the dechlorination step, only gas evolution was observed, but no liquid products were formed. The use of N2 flow during the whole degradation process (method 3) led to more gas products and less residue. Since degradation products are more easily removed from the reactor in presence of N2 flow, secondary reactions were partially hindered. However, N2 flow was passed only from the top of the reactor, so some of the volatile products condensed near the upper parts of the reactor and returned to the reactor to give further secondary reactions. The type of degradation procedure had no effect on the degradation rate of PVC/HDPE mixture (Fig. 1). In this figure, lapse time was counted from beginning of dechlorination in methods 2 and 3 and after 120 C in method 1. The dechlorination step had no effect on the composition of liquid hydrocarbon products. However, in case of N2 flow (method 3), C5–C8 Table 2 Product yields (wt.%) for degradation of HDPE/PVC mixture at 430 C by different degradation procedure Catalyst
Method Liquid Gas Residue Liquid density (g cm 3)
Red Mud Red Mud Red Mud Red Mud, VP TR-99701(g-Fe2O3) SA-1(SiO2.Al2O3
1 2 3 3 3 3
60.2 56.1 57.3 29.3 56.0 47.0
15.4 18.7 21.5 22.5 19.6 35.8
24.4 25.2 21.2 48.3 24.4 17.3
0.766 0.761 0.766 0.756 0.779 0.761
hydrocarbons decreased and C16–C20 hydrocarbons increased than in the absence of N2 (Fig. 2). The main effect of degradation procedure was on the amount of chlorine and its distribution in the products. After the dechlorination step, 47% of chlorine in the feed accumulated in traps as HCl. This amount was only 27.6% when the dechlorination step was not carried out. Without the dechlorination step, the amount of chlorine in the oil was very high, 4316 ppm. Although the dechlorination takes place quantitatively at low temperatures, it did not finish before the degradation of PE at a heating rate of 3 C/min (in method 1). N2 flow also affected the amount of chlorine in the oil. The chlorine contents of oils were 678 and 543 ppm in methods 2 and 3, respectively. This shows that after the dechlorination step, residual HCl could be removed by N2 flow in method 3. Table 3 Distribution of Cl in the products obtained from the degradation of HDPE Catalyst
Red Mud Red Mud Red Mud Red Mud (vapor phase) TR-99701 (g-Fe2O3) SA-1 (SiO2.Al2O3) None
Method
1 2 3 3 3 3 3
Cl content in oil (ppm)
4316 678 543 940 361 1591 144
Cl distrubition (wt.%) Liquid
Gas
2.40 0.35 0.29 0.26 0.20 0.70 0.10
27.60 47.00 47.80 23.10 34.70 82.70 96.20
Fig. 1. Cumulative volume of liquid products from catalytic degradation using RM of HDPE/PVC mixture at different degradation procedures.
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Fig. 2. C-NP gram of liquid products from catalytic degradation using RM of HDPE/PVC mixture at different degradation procedures.
Fig. 3. Cl-NP gram of liquid products from catalytic degradation using RM of HDPE/PVC mixture at different degradation procedures.
The boiling point distribution of organic chlorine compounds in oil is called Cl-NP gram [5]. The Cl-NP gram in Fig. 3 was obtained by plotting the Cl content in the oil against the carbon number of normal paraffins. These compounds were mainly distributed in the boiling point range of 36–174 C (equivalent to the bps of n-C6 to n-C10). The organic chlorine compounds in oil could not be identified by GC–MS. The previous study [5] related to the thermal degradation of PVC/ HDPE under the same conditions as method 1 showed that the main organic chlorine compounds were 2chloro-2-methylpropane and 2-chloro-2-methylpentane.
They suggested that chlorine containing compounds were formed by the addition of HCl to the tertiary carbon of the branched olefins. All other degradation experiments were carried out with the dechlorination step and using N2 flow. 3.1.2. Effect of the catalyst contact mode In order to investigate the effect of catalytic contact mode, two kinds of catalyst contact mode, in which the catalyst (RM) was kept in contact with either melted polymer mixtures (liquid phase contact-LP) or degraded hydrocarbon vapors from polymer mixtures (vapor
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Fig. 4. Cumulative volume of liquid products from thermal and catalytic degradation of HDPE/PVC mixture at 430 C.
phase contact-VP), were compared. For degradation in vapor phase contact, RM was placed on a stainless steel net located 12 cm from the bottom of reactor where the temperature was 350 C. Degradation was carried out in method 3. As expected, in the case of vapour phase contact the degradation rate and liquid yield of PVC/HDPE mixture were very low (Table 1 and Fig. 1). In the case of VP contact, the liquid yield was 29.3%, compared to 57.3% for LP contact. 74% of residue was adsorbed on the catalyst bed in liquid form. Since the temperature of the catalyst bed was 350 C, lighter liquid hydrocarbons could leave the reactor. The composition of liquid product from VP contact (Fig. 2) supports this idea. Liquid compounds were mainly distributed in the range of C5– C15 (equivalent to hydrocarbons with boiling points ranging over 36–270 C), whereas they were distributed in a wide range of carbon numbers (n-C5 to n-C25) equivalent to boiling point ranges of 36–405 C in case of LP-contact. The yield of gaseous products did not differ from that of LP contact, but the residue was much greater. This shows that RM has no catalytic activity in VP-contact, it hindered the removing of volatiles by adsorption. However previous work [13] showed that in VP contact, thermally degraded hydrocarbons could be further decomposed into gaseous and lighter liquid products over a cracking catalyst. In the same study, the evolved HCl was fixed on the chlorine sorbents such as iron oxides, FeOOH and Fe2O3 and the oil containing less chlorine was obtained in VP-contact than LP-contact. In our study, RM in VP-contact adsorbed more HCl evolved from the degradation of PVC than in LPcontact. 23.1% of the feed chlorine could be removed as HCl, whereas this amount was 47.8% in case of LPcontact. In the dechlorination process, the adsorption of
HCl inside the reactor on the sorbent and/or catalysts is the main goal. However the higher chlorine amount in the oil (940 ppm) shows that adsorbed HCl reacted with the thermal degradation products of HDPE at temperatures as low as 350 C. Since RM is a mixture of several metal compounds, one or some of metal compounds could catalyze the reaction between fixed HCl on catalyst and the degradation products of HDPE. From a consideration of product yields and chlorine content of oil, we can conclude that LP-contact is more effective than VP-contact in the degradation of PVC/ HDPE mixture with RM. 3.1.3. Effect of types of catalyst on PVC/HDPE degradation In this section, we compare the results of thermal and catalytic degradation of PVC/HDPE mixture over various types of catalyst such as silica-alumina (SA-1) having a SiO2/Al2O3 mole ratio of 83.3/16.17, g-Fe2O3 (TR-99701) and RM. All experiments were carried out in LP-contact mode and in method 3. The previous studies [5,12–15] showed that the SA1 catalyst accelerated the rate of polymer degradation and lowered the boiling point of liquid products, and TR-99701 decreased the chlorine content in oil to a very low level. It fixed the chlorine from the products of degradation of PVC mixed plastics in form of iron compounds such as ferrous chloride. Fig. 4 shows the cumulative volume of liquid products against lapsed time. Lapsed time was counted from the beginning of the dechlorination step. As expected, the use of a solid acid catalyst increased the degradation rate, but this increase was not much more than that of thermal or catalytic degradation with TR-99701 or RM, but it was effective for increasing the gaseous
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Fig. 5. C-NP gram of liquid products from thermal and catalytic degradation of HDPE/PVC mixture at 430 C. Table 4 Yields of products obtained from the degradation of PS/PVC or PP/PVC at 430 C
PS/PVC PS/PVC PP/PVC PP/PVC
Catalyst
Liquid (wt.%)
Gas (wt.%)
Residue (wt.%)
Liquid density (g cm 3)
None Red Mud None Red Mud
67.7 69.1 57.4 56.7
14.9 11.8 25.3 22.0
17.5 19.2 17.4 21.4
0.880 0.891 0.740 0.747
hydrocarbons and for producing liquid hydrocarbons with lower molecular weight (Fig. 5). SA1 had a more catalytic effect for the degradation of single PE at 430 C [13]. The amount and composition of the liquid products were very similar for both thermal and catalytic degradation with RM or TR-99701. But in case of thermal degradation the amount of residue increased and gas amount decreased. The oil having the lowest amount of chlorine compounds was obtained by thermal degradation (Table 3). 96.2% of feed chlorine was in gas, and only 0.1% was in oil. The fact that the distribution of chlorine in gas was 47.8% in case of RM and 34.7% in the case of TR99701 indicating that the HCl from degradation of PVC was fixed on the RM as well as on the TR-99701. We do not know whether the HCl was only absorbed on RM or was fixed in the form of metal chloride. Since RM is a complex mixture of inorganic materials and is not a synthesized catalyst, it is difficult to suggest any adsorption mechanism. The chlorine amount in oil from both RM and TR-99701 was higher than that of thermal degradation. We suggested above that a part of the
fixed HCl could also react with degradation products of HDPE due to the catalytic effect of metals in RM. In case of RM, the amount of chlorine in the oil was 543 ppm compared to 361 ppm in the case of TR-99701.This result shows that the some of the metals in RM have a catalytic effect. Over SA-1, the HCl amount in gas was greater than those of the other catalyst, as well as the chlorine content in oil was also the highest (1591 ppm). In previous works [5,12], chlorine content of oil over SA-1 was lesser than that of oil from thermal degradation. In our case, the dechlorination step seems very effective. We can mention that during the dechlorination step, PE was molten form and HCl could be easily removed from the polymer mixture within an hour. However in the presence of a solid material such as a cracking catalyst, part of HCl adsorbed in the reactor and (following the degradation step) it reacted with the degradation products of polymers. Although SA-1 is not a chlorine sorbent or dechlorination catalyst, the HCl amount in gas was lower than that of thermal degradation. Distributions of organic chlorine compounds derived from
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Fig. 6. Cl-NP gram of liquid products from thermal and catalytic degradation of HDPE/PVC mixture at 430 C.
the catalytic and thermal degradation of PVC/PE are shown in Fig. 6 as Cl-NP gram. The organic chlorine compounds were distributed in carbon number range of n-C6 to n-C11 for the HCl sorbed catalyst (TR-99701 and RM) and in the range of n-C6 to n-C10 for SA-1 and thermal process. The types of chlorine compounds were similar for all catalyst type, but their contents differed. Briefly, in the absence of a catalyst, we obtained oil having very little chlorine from the degradation of PVC/ PE mixtures with the dechlorination step and N2 flow. The use of a cracking catalyst increased the light hydrocarbons in the oil, but also increased the chlorine content of the oil. RM is as effective as g-Fe2O3 to sorbed chlorine during degradation of PVC/PE mixtures. 3.2. Thermal and catalytic degradation of PS/PVC and PP/PVC mixture In the previous study [5], we investigated the degradation of PP/PVC mixture at 380 C using different types of catalyst and the degradation of PS/PVC at 360 C without a catalyst. In the present work, the dechlorination and cracking behavior of these polymer mixtures were investigated. Experiments were carried out at 430 C with a dechlorination step and in N2 flow with and without RM. Even if the degradation temperatures of PP and PS are lower than that of PE, the degradation temperature of 430 C was chosen after consideration of the fact that all polymers will be treated together as municipal waste plastics.
Table 5 Distribution of Cl in the products obtained from the degradation of PS/PVC or PP/PVC at 430 C Catalyst
PS/PVC PS/PVC PP/PVC PP/PVC
None Red Mud Red Mud None
Cl content in oil (ppm) 81 381 764 365
Cl distribution (wt.%) Liquid
Gas
0.05 0.25 0.39 0.19
92.3 22.1 29.9 98.0
Table 4 shows the yield of products obtained from PVC mixed plastics. For both polymer mixtures, liquid yields from catalytic degradation did not differ significantly from that of thermal degradation, whereas the use of RM increased the amount of residue and decreased the amount of gas. This change in the yields of residue and gas was not due to the effect of RM on the polymer degradation mechanism: the adsorption of HCl gas from the degradation of PVC on the RM led to an increase in residue amount and a decrease in the gas yield. The degradation rate of polymer mixtures as well as the degradation yields was similar for both thermal and catalytic degradation (Fig. 7). Fig. 8 shows the composition of liquid products as a C-NP gram (normal paraffin gram for hydrocarbons). Degradation of PS/ PVC mixture both with and without catalyst gave the well-known PS pyrolysis products containing styrene monomer,a-methylstyrene, toluene and small quantities
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Fig. 7. Cumulative volume of liquid products from thermal and catalytic degradation of PS, PP/PVC mixture at 430 C.
Fig. 8. C-NP gram of liquid products from thermal and catalytic degradation of PS, PP/PVC mixture at 430 C.
of dimer. The liquid hydrocarbon products from PP/ PVC degradation had a carbon number distribution of n-C5 to n-C25 with a sharp peak at n-C9 (2,4-dimethyl-1heptene). RM had a big effect on the chlorine content and distribution in products for both PP/PVC and PS/PVC mixtures as in the case of PE/PVC (Table 5). In the absence of RM, over 90% of chlorine in PVC recovered in traps as HCl. In the presence of RM, this amount was
22.1% for PS and 29.9% for PP. Also in this group of experiments, the chlorine sorption properties of RM could be seen clearly. As expected, the amount and composition of chlorine compounds in liquid products differed according to the type of degraded plastic. In the presence of RM, the chlorine content of the oil was 318 ppm for PS/PVC and 764 ppm for PP/PVC. In the previous work [5] relating to fixing of HCl to a suitable sorbent (such as a-FeOOH, Fe3O4, calcined a-FeOOH),
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Fig. 9. Cl-NP gram of liquid products from thermal and catalytic degradation of PS, PP/PVC mixture at 430 C. Table 6 Yields of products obtained from the degradation of PE, PS or PP at 430 C
PE PE PS PS PP PP
Catalyst
Liquid (wt.%)
Gas (wt.%)
Residue (wt.%)
Liquid density (g cm 3)
None Red Mud None Red Mud None Red Mud
68.2 71.6 85.2 85.6 72.6 72.2
14.4 15.0 1.0 2.2 12.7 15.4
16.8 13.4 13.8 12.2 14.7 12.4
0.760 0.771 0.903 0.903 0.737 0.744
an oil having 1100 ppm organic chlorine content was obtained from the dechlorination of PVC/PP mixture at 380 C. In the same work, the thermal degradation of PP/PVC and PS/PVC was carried out at 380 and 360 C, respectively. The oil recovered contained 17 600 and 10 500 ppm chlorine for PS/PVC and PP/PVC, respectively. However, in the present process, we obtained the oil containing 365 and 81 ppm chlorine from PP/PVC and PS/PVC, respectively. It is interesting to note that the oils from both PP/PVC and PS/PVC contained only organic chlorine compounds. This is due to the dechlorination step. Organic chlorine compounds were distributed in hydrocarbons with light boiling points (< 174 C) in both the absence and presence of RM. The compositions of the chlorine containing liquid products are given in Fig. 9. In the case of PS/PVC degradation, 2-chloro-2-phenylpropane was formed in the absence of RM; a-chloroethylbenzene and 2-chloro-2-phenylpropane compounds were formed in the presence of RM. In the case of PP/PVC degradation, organic chlorine compounds of C6 and C8 were formed during thermal degradation, whereas chlorine compounds were distributed
comparatively in the range of n-C6 to n-C10 in the presence of RM. These chlorine compounds could not be identified by GC-MS. However a previous work [3] showed that the organic chlorine compounds in oil from the degradation of PP/PVC were 2-chloro-2-methylpropane (C6) and 2-chloro-2-methylpentane (C8). RM had an effect on the type of organic chlorine compounds in both PP/PVC and PS/PVC mixtures. It is thought that RM catalyzed the formation of new chlorine compounds in the reaction between HCl and degradation products of PP or PS. However, we can not say which compound/compounds in RM led to the form the new organic chlorine compounds. It is interesting to note that in the catalytic degradation of polymer mixtures, the amount of recovered chlorine (as HCl) in traps was 22.2% for PS/PVC and 29.9% for PP/PVC whereas it was 47.8% for PE/PVC. This shows that the amount of sorbed HCl on the RM varied depending on the polymer types. This can be explained thus: because the polymer melt gets into contact with catalyst surface, the fixation effect of metal compounds in RM might be changed with the type of polymer melt.
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Fig. 10. Cumulative volume of liquid products from thermal and catalytic degradation of PS, PE and PP at 430 C.
Fig. 11. Composition of gas products from thermal and catalytic degradation of PS, PE and PP at 430 C.
3.3. Effect of Red Mud on the degradation of single compound plastic In order to investigate the behaviour of RM on the degradation of single compound plastic, the degradation of PE, PS or PP was carried out with and without
RM at 430 C under nitrogen stream. Table 6 shows the yields of products from thermal and catalytic degradation. For catalytic degradation, the yield of liquids did not differ significantly from that of thermal degradation. In the cases of PP and PS, RM had effect on the yields of residue and gas. In the presence of RM, the
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Fig. 12. C-NP gram of liquid products from thermal and catalytic degradation of PS, PE and PP at 430 C.
residue amount decreased by 11 and 15%, gas amount increased by 100 and 21% for PS and PP, respectively. However, these differences are not considerable on the total product base. Fig. 10 shows the cumulative volume of liquid products as a function of lapsed time. The count of lapsed time was started from beginning of 300 C. As expected the rate of degradation of PS and PP were much faster than that of PE. The degradation rates of polymers were same in presence and absence of RM. The composition of gaseous products is shown in Fig. 11. In both the thermal and catalytic degradation of PP, gaseous products were same. The main gas products were C3. In thermal degradation of PS, the gaseous products were mainly C2, C3, and C4, whereas in catalytic degradation C3 increased and C4 was absence, and also unsaturated hydrocarbons became more. For the thermal and catalytic degradation of PE, the gaseous products were C1, C2 and C3. Composition of the liquid products from catalytic and thermal degradation of PE, PS and PP is shown in Fig. 12 as a Normal Paraffin gram (NP-gram). As shown in Fig. 12, RM had no effect on the carbon number distribution of liquid products. In the case of PE, the liquid hydrocarbon products were distributed in a wide range carbon numbers (n-C5 to n- C25) equivalent to boiling point ranges of 36–405 C, and were composed of linear olefins and paraffins. Similar results for the thermal degradation of PE at 430 C have been reported in studies (13–15). The liquid hydrocarbon products from PP degradation also had a carbon number distribution of n-
C5 to C25 with peaks at n-C6, C9, C11, C14, C16 and C18. However, in the case of PS, the liquid hydrocarbons were distributed sharply in the range of n-C8 to n-C10 with a peak at n-C9 and C18 and composed of styrene, amethylstyrene and toluene. In the previous study [13] related to the catalytic degradation of PE and PP over various types of solid catalysts, such as silica-alumina, zeolite and non-acidic mesoporous silica catalyst, it was mentioned that the silica-alumina and mesoporous silica catalysts accelerated the rate of degradation of PE and PP; and they degraded the heavier waxy compounds into lighter liquid hydrocarbons. If we compare our results to the above literature: in the thermal degradation of PP at 430 C, the degradation rate of PP and yields of products was similar to the results that of silica-alumina, but silica-alumina catalyst led to lighter hydrocarbons in oil. In the PE degradation over silica-alumina and mesoporous silica catalysts, the yield and composition of liquid hydrocarbons and the degradation rate were better than our results. As a result we concluded that RM had no effect on the degradation of PE, PP and PS.
4. Conclusion The results can be summarized as follows. . During the thermal degradation of the polymer mixtures containing PVC, over 90% of feed chlorine was recovered as HCl by dechlorination step at 350 C for 1 h.
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. The use of N2 flow during the both dechlorination and degradation steps decreased the reaction between HCl and degradation products of polymer due to the effective removal of HCl from reactor before the degradation of other polymers occurs. . Red Mud had a good effect on the fixation of evolved HCl as well as iron oxides sorbents. . Red Mud had an effect on the composition of organic chlorine compounds derived from the degradation of PS/PVC and PP/PVC mixtures. . The use of RM in VP contact mode led to more HCl fixation, but VP-contact mode increased the chlorine amount of oil, because of the reaction between fixed HCl and adsorbed degradation products on RM. . Red Mud showed no effect on the cracking of both PVC containing polymer mixture and single polymer. . The oils derived from PVC containing polymer mixtures by thermal degradation contained lower amount chlorine than the oils obtained by using of Red Mud. Unfortunately, we do not suggest any mechanism on the Red Mud effect, because RM is a complex mixture of inorganic materials and is not a synthetic catalyst.
Acknowledgements We would like to thank the Venture Business Laboratory of Okayama University, Japan for financial support for Dr. Jale Yanik to visit Japan and perform
this work at Okayama University. We would also like to thank Dr. Katsuhide Murata of Japan Eco Environment Products for his suggestions in this work.
References [1] Blazso M, Zelei B, Jakab E. J Anal Appl Pyrolysis 1995;35:221. [2] Blazso M. Rapid Commun Mass Spectrometry 1981;12:1. [3] Sakata Y, Uddin MA, Muto A, Narazaki K, Murata K, Kaji M. Polym Recycling 1996;2(4):309. [4] Sakata Y, Uddin MA, Koizumi K, Murata K. Polym Degrad Stab 1996;53(1):111. [5] Uddin MdA, Sakata Y, Shiraga Y, Muto A, Murata K. Ind Eng Chem Res 1999;38:1406. [6] Uddin Md.A, Shiraga Y, Muto A, Sakata Y, Murata K. Liquefaction of wasteplastics by catalytic degradation and fixation of chlorine in waste plastics derived oil with iron oxide sorbents. In: 5th ARAB Int Conf on Polym Sci Technol 18–22 September 1999, Aswan, Egypt. [7] Eamsiri A, Jackson R, Pratt KC, Chiristov V, Marshall M. Fuel 1992;71:449. [8] Garg D, Givens EN. Ind Eng Chem Process Des Dev 1985;24:66. [9] Sato S, Morita M, Hashimoto T, Mitunori I, Chiba K, Tagaya H. Fuel 1989;68:622. [10] Klopties B, Hodek W, Bandermann F. Fuel 1990;69:448. [11] Llano JJ, Rosal R, Sastre H, Diez FV. Fuel 1994;73:688. [12] Shiraga Y, Uddin MA, Muto A, Narazaki M, Sakata Y. Energy and Fuels 1999;13:428. [13] Sakata Y, Uddin MA, Muto A. J Anal Appl Pyrolysis 1999;51:135. [14] Uddin MA, Sakata Y, Muto A, Shiraga Y, Koizumi K, Kanada Y, Murata K. Microporous and Mesoporous Materials 1998;21:557. [15] Sakata Y, Uddin MA, Muto A, Koizumi K, Kanada Y, Murata K. J Anal Appl Pyrolysis 1997;43:15.
The catalytic effect of Red Mud on the degradation of poly (vinyl chloride) containing polymer mixture into fuel oil Jale Yanika,*, Md. Azhar Uddinb, Kazuo Ikeuchib, Yusaku Sakatab a
b
Department of Chemistry, Ege University, 35100 Izmır, Turkey Department of Applied Chemistry, Okayama University, Okayama 700-8530, Japan Received 29 November 2000; accepted 23 February 2001
Abstract Thermal and catalytic degradation of poly (vinyl chloride) (PVC) containing polymer mixtures, PVC/PE, PVC/PP and PVC/PS, into fuel oil was investigated. In the catalytic degradations, Red Mud (a waste from alumina production) was tested as both cracking and dechlorination catalyst. For comparison, g-Fe2O3 as a chlorine sorbent and SA-1 (silica alumina) as a solid acid catalyst were also used. The effect of degradation conditions, such as nitrogen gas flow, stepwise pyrolysis, catalyst contact mode, on the dechlorination was also investigated. The use of N2 gas flow suppressed partially the reaction between HCl gas from the degradation of PVC and polymer degradation products. By stepwise pyrolysis, over 90% chlorine in the feed plastic was recovered as HCl gas. SA1 catalyst accelerated the rate of polymer degradation and lowered the boiling point of liquid products, but the chlorine content of oil over SA1 was also the highest. Red Mud (RM) and iron oxides sorbents showed good effect on the fixation of evolved HCl. However, they had no effect on the cracking of polymers. The oils derived from PVC containing pure polymer mixtures by thermal degradation contained a lower amount of chlorine than the oils obtained using RM and other catalysts. From this result we conclude that the formation of some organic chlorine compounds may be promoted by the interaction of the HCl and the catalysts. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Plastic degradation; Dechlorination; Red Mud
1. Indroduction The pyrolysis of plastic waste into useful fuel oil has been considered to be a promising recycling method. The main problem causes during the pyrolysis of waste plastics are related to Poly (vinyl chloride) (PVC). The pyrolysis products of PVC mixed waste plastics contain both inorganic and organic chlorine compounds [1–4]. The production of fuels having no chlorine content must be a main aim in waste plastics pyrolysis. The degradation mechanisms of PVC are well known. Recently, attempts have focused on the dechlorination of PVC containing polymer mixtures (especially binary mixture). Very little is known about the catalytic degradation of PVC containing polymer mixtures. Uddin et al have studied the thermal and catalytic degradation of PE/PVC, PP/PVC and PS/PVC mixtures [5]. Their data showed that by using iron oxides, FeOOH and Fe3O4, * Corresponding auhtor. Fax: +90-232-388-8264. E-mail address: [email protected] (J. Yanik).
the chlorine compounds in oil could be decreased to a very low level. They suggested that the chlorine removed from the products of degradation of PVC mixed plastics is fixed in the form of iron compounds such as ferrous chloride. They have also reported that the ferrous chloride acts as a catalyst for the dechlorination of organic chlorine compounds [6] In this study, the thermal and catalytic degradation of PVC/HDPE, PVC/PP and PVC/PS mixture was carried out by stepwise pyrolysis. In the catalytic degradations, Red Mud (RM) was used as a cracking catalyst and as a sorbent to fix the HCl formed by PVC degradation. RM is a by-product of alumina manufacture process (Bayer process). It contains mainly oxides of iron, aluminium, titanium, silicon, calcium and sodium. It has been used for the liquefaction of coal [7–9] and biomass [10] and in the hydrogenation of anthracene oil [11] as hydrogenation catalyst. To our knowledge, there are no published reports on the polymer degradation in the presence of RM. Use of a waste material as a catalyst for the recovery of other wastes is an important point of this study.
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00095-7
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2. Experimental 2.1. Materials The Red Mud, as a sludge, was supplied by Seydisehir Aluminium Company, Turkey. It was filtered and then dried at 110 C. It was analyzed by classical chemical analysis. Surface properties were measured by nitrogen adsorption, BET (Bel Japan, Belsorp 28SP). The surface properties and chemical compositions of RM are shown in Table 1. Polypropylene (PP) was obtained from UBE Chemical Industries; polystyrene (PS) from Asahi Kasei Industries; High-density polyethylene (PE) from Mitsui Chemical; and poly(vinyl chloride)(PVC) from Geon Chemical. The chlorine content of the PVC was 52.8 wt.%. 2.2. Methods
The amount of gaseous products was determined by subtracting the weight of liquid and residue from the plastic sample feed. The liquid products were analyzed by gas chromatograph with a flame ionization detector (Yanaca: G6800-FID). A gas chromatograph with a thermal conductivity detector (Yanaca: G180-TCD) was used to analyze the gases from single type polymers. Organic chlorine compounds in the liquid product were analyzed by two gas chromatographs with either an atomic emission detector (HP: G2350A-AED) or a mass selective detector (HP: 5973-MSD). Gaseous organic chlorine products were not detected.
3. Results and discussion 3.1. Thermal and catalytic degradation of PE/PVC mixture
The degradation and dechlorination experiments were carried out in a glass reactor (35 mm i.d. and 350 mm l) under atmospheric pressure at 430 C under N2 flow by semi-batch operation. A fixed amount of single type plastic (10 g) or mixed plastics [weight ratio [(PE, PP or PS)/PVC=8/2] and Red Mud (weight ratio: RM/pure polymer=1/10 or RM/mixed polymer=2/10) was loaded into the reactor. The reactor was heated to 120 C at 3 C/min and held at 120 C for 60 min to remove the physically absorbed water from the catalyst and the plastic sample. Then the temperature was increased to 300 C at a heating rate of 3 C/min. and the reactor was held at this temperature for 60 min to evolve the HCl. After the dechlorination, the reactor temperature was increased to 430 C at a heating rate of 3 C/min. The hydrogen chloride evolved from the degradation of PVC was trapped in a flask containing an aqueous solution of NaOH and analyzed by ion chromatography. The schematic diagram of experimental set up was given previously [5]. A few experiments was carried out without the dechlorination step or without N2 stream to compare the effect of RM with other catalyst used in previous studies. The products of degradation were classified into three groups: gases, liquid and residue. Table 1 Properties of Red Mud Element
Wt.%
Fe2O3 SiO2 Al2O3 TiO2 CaO MgO K2O Na2O
37.72 17.10 17.27 4.81 4.54 0.40 0.29 7.13
Surface area (m2 g 1)
16
In this group of experiments, the dechlorination ability of RM and the effect of its contact mode [liquidphase contact (LP), vapor-phase contact (VP)] were investigated. Besides RM, TR-99701 (g-Fe2O3) as a chlorine sorbent and SA-1 (SiO2.Al2O3) as a solid acid catalyst were also used. In addition to catalytic effects, the effect of degradation procedure on the dechlorination was also investigated. 3.1.1. Effect of the type of degradation procedure In order to investigate the effect of degradation procedure on the dechlorination, three different methods of degradation in presence of RM were examined. In the first method (method 1), a mixture of PE/PVC was degraded under the same conditions used in the previous works [12]. That is, the reactor was heated to 120 C at 3 C/min and held at this temperature for 60 min in N2 flow. Nitrogen flow was then cut off and the temperature was increased to 430 C at a heating rate of 3 C/min. In the second method (method 2), the reactor contents were heated to 120 C at 3 C/min and held at this temperature for 60 min. Then the temperature was increased to 300 C (at 3 C/min) and held at that temperature for 60 min to dehydrochlorinate the PVC with N2 flow. After the nitrogen flow was cut off, the temperature was increased to 430 C. In the third method (method 3); both dechlorination and degradation were carried out at 430 C in nitrogen flow as described in Section 2.2. Tables 2 and 3 show the product distribution and chlorine distribution from the degradation of PVC/ HDPE mixture for all three degradation methods. The comparison of methods 1 and 2 shows that the dechlorination step increased the gas yield and decreased the liquid yield. Although gas products have not been analyzed, we can say that this is due to greater HCl evolution. In addition the larger quantity of chlorine (as HCl)
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adsorbed in traps in method 2 also supports this idea. It is known that the dechlorination of PVC occurs at lower temperatures than degradation of other polymers. During the dechlorination step, only gas evolution was observed, but no liquid products were formed. The use of N2 flow during the whole degradation process (method 3) led to more gas products and less residue. Since degradation products are more easily removed from the reactor in presence of N2 flow, secondary reactions were partially hindered. However, N2 flow was passed only from the top of the reactor, so some of the volatile products condensed near the upper parts of the reactor and returned to the reactor to give further secondary reactions. The type of degradation procedure had no effect on the degradation rate of PVC/HDPE mixture (Fig. 1). In this figure, lapse time was counted from beginning of dechlorination in methods 2 and 3 and after 120 C in method 1. The dechlorination step had no effect on the composition of liquid hydrocarbon products. However, in case of N2 flow (method 3), C5–C8 Table 2 Product yields (wt.%) for degradation of HDPE/PVC mixture at 430 C by different degradation procedure Catalyst
Method Liquid Gas Residue Liquid density (g cm 3)
Red Mud Red Mud Red Mud Red Mud, VP TR-99701(g-Fe2O3) SA-1(SiO2.Al2O3
1 2 3 3 3 3
60.2 56.1 57.3 29.3 56.0 47.0
15.4 18.7 21.5 22.5 19.6 35.8
24.4 25.2 21.2 48.3 24.4 17.3
0.766 0.761 0.766 0.756 0.779 0.761
hydrocarbons decreased and C16–C20 hydrocarbons increased than in the absence of N2 (Fig. 2). The main effect of degradation procedure was on the amount of chlorine and its distribution in the products. After the dechlorination step, 47% of chlorine in the feed accumulated in traps as HCl. This amount was only 27.6% when the dechlorination step was not carried out. Without the dechlorination step, the amount of chlorine in the oil was very high, 4316 ppm. Although the dechlorination takes place quantitatively at low temperatures, it did not finish before the degradation of PE at a heating rate of 3 C/min (in method 1). N2 flow also affected the amount of chlorine in the oil. The chlorine contents of oils were 678 and 543 ppm in methods 2 and 3, respectively. This shows that after the dechlorination step, residual HCl could be removed by N2 flow in method 3. Table 3 Distribution of Cl in the products obtained from the degradation of HDPE Catalyst
Red Mud Red Mud Red Mud Red Mud (vapor phase) TR-99701 (g-Fe2O3) SA-1 (SiO2.Al2O3) None
Method
1 2 3 3 3 3 3
Cl content in oil (ppm)
4316 678 543 940 361 1591 144
Cl distrubition (wt.%) Liquid
Gas
2.40 0.35 0.29 0.26 0.20 0.70 0.10
27.60 47.00 47.80 23.10 34.70 82.70 96.20
Fig. 1. Cumulative volume of liquid products from catalytic degradation using RM of HDPE/PVC mixture at different degradation procedures.
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Fig. 2. C-NP gram of liquid products from catalytic degradation using RM of HDPE/PVC mixture at different degradation procedures.
Fig. 3. Cl-NP gram of liquid products from catalytic degradation using RM of HDPE/PVC mixture at different degradation procedures.
The boiling point distribution of organic chlorine compounds in oil is called Cl-NP gram [5]. The Cl-NP gram in Fig. 3 was obtained by plotting the Cl content in the oil against the carbon number of normal paraffins. These compounds were mainly distributed in the boiling point range of 36–174 C (equivalent to the bps of n-C6 to n-C10). The organic chlorine compounds in oil could not be identified by GC–MS. The previous study [5] related to the thermal degradation of PVC/ HDPE under the same conditions as method 1 showed that the main organic chlorine compounds were 2chloro-2-methylpropane and 2-chloro-2-methylpentane.
They suggested that chlorine containing compounds were formed by the addition of HCl to the tertiary carbon of the branched olefins. All other degradation experiments were carried out with the dechlorination step and using N2 flow. 3.1.2. Effect of the catalyst contact mode In order to investigate the effect of catalytic contact mode, two kinds of catalyst contact mode, in which the catalyst (RM) was kept in contact with either melted polymer mixtures (liquid phase contact-LP) or degraded hydrocarbon vapors from polymer mixtures (vapor
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339
Fig. 4. Cumulative volume of liquid products from thermal and catalytic degradation of HDPE/PVC mixture at 430 C.
phase contact-VP), were compared. For degradation in vapor phase contact, RM was placed on a stainless steel net located 12 cm from the bottom of reactor where the temperature was 350 C. Degradation was carried out in method 3. As expected, in the case of vapour phase contact the degradation rate and liquid yield of PVC/HDPE mixture were very low (Table 1 and Fig. 1). In the case of VP contact, the liquid yield was 29.3%, compared to 57.3% for LP contact. 74% of residue was adsorbed on the catalyst bed in liquid form. Since the temperature of the catalyst bed was 350 C, lighter liquid hydrocarbons could leave the reactor. The composition of liquid product from VP contact (Fig. 2) supports this idea. Liquid compounds were mainly distributed in the range of C5– C15 (equivalent to hydrocarbons with boiling points ranging over 36–270 C), whereas they were distributed in a wide range of carbon numbers (n-C5 to n-C25) equivalent to boiling point ranges of 36–405 C in case of LP-contact. The yield of gaseous products did not differ from that of LP contact, but the residue was much greater. This shows that RM has no catalytic activity in VP-contact, it hindered the removing of volatiles by adsorption. However previous work [13] showed that in VP contact, thermally degraded hydrocarbons could be further decomposed into gaseous and lighter liquid products over a cracking catalyst. In the same study, the evolved HCl was fixed on the chlorine sorbents such as iron oxides, FeOOH and Fe2O3 and the oil containing less chlorine was obtained in VP-contact than LP-contact. In our study, RM in VP-contact adsorbed more HCl evolved from the degradation of PVC than in LPcontact. 23.1% of the feed chlorine could be removed as HCl, whereas this amount was 47.8% in case of LPcontact. In the dechlorination process, the adsorption of
HCl inside the reactor on the sorbent and/or catalysts is the main goal. However the higher chlorine amount in the oil (940 ppm) shows that adsorbed HCl reacted with the thermal degradation products of HDPE at temperatures as low as 350 C. Since RM is a mixture of several metal compounds, one or some of metal compounds could catalyze the reaction between fixed HCl on catalyst and the degradation products of HDPE. From a consideration of product yields and chlorine content of oil, we can conclude that LP-contact is more effective than VP-contact in the degradation of PVC/ HDPE mixture with RM. 3.1.3. Effect of types of catalyst on PVC/HDPE degradation In this section, we compare the results of thermal and catalytic degradation of PVC/HDPE mixture over various types of catalyst such as silica-alumina (SA-1) having a SiO2/Al2O3 mole ratio of 83.3/16.17, g-Fe2O3 (TR-99701) and RM. All experiments were carried out in LP-contact mode and in method 3. The previous studies [5,12–15] showed that the SA1 catalyst accelerated the rate of polymer degradation and lowered the boiling point of liquid products, and TR-99701 decreased the chlorine content in oil to a very low level. It fixed the chlorine from the products of degradation of PVC mixed plastics in form of iron compounds such as ferrous chloride. Fig. 4 shows the cumulative volume of liquid products against lapsed time. Lapsed time was counted from the beginning of the dechlorination step. As expected, the use of a solid acid catalyst increased the degradation rate, but this increase was not much more than that of thermal or catalytic degradation with TR-99701 or RM, but it was effective for increasing the gaseous
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Fig. 5. C-NP gram of liquid products from thermal and catalytic degradation of HDPE/PVC mixture at 430 C. Table 4 Yields of products obtained from the degradation of PS/PVC or PP/PVC at 430 C
PS/PVC PS/PVC PP/PVC PP/PVC
Catalyst
Liquid (wt.%)
Gas (wt.%)
Residue (wt.%)
Liquid density (g cm 3)
None Red Mud None Red Mud
67.7 69.1 57.4 56.7
14.9 11.8 25.3 22.0
17.5 19.2 17.4 21.4
0.880 0.891 0.740 0.747
hydrocarbons and for producing liquid hydrocarbons with lower molecular weight (Fig. 5). SA1 had a more catalytic effect for the degradation of single PE at 430 C [13]. The amount and composition of the liquid products were very similar for both thermal and catalytic degradation with RM or TR-99701. But in case of thermal degradation the amount of residue increased and gas amount decreased. The oil having the lowest amount of chlorine compounds was obtained by thermal degradation (Table 3). 96.2% of feed chlorine was in gas, and only 0.1% was in oil. The fact that the distribution of chlorine in gas was 47.8% in case of RM and 34.7% in the case of TR99701 indicating that the HCl from degradation of PVC was fixed on the RM as well as on the TR-99701. We do not know whether the HCl was only absorbed on RM or was fixed in the form of metal chloride. Since RM is a complex mixture of inorganic materials and is not a synthesized catalyst, it is difficult to suggest any adsorption mechanism. The chlorine amount in oil from both RM and TR-99701 was higher than that of thermal degradation. We suggested above that a part of the
fixed HCl could also react with degradation products of HDPE due to the catalytic effect of metals in RM. In case of RM, the amount of chlorine in the oil was 543 ppm compared to 361 ppm in the case of TR-99701.This result shows that the some of the metals in RM have a catalytic effect. Over SA-1, the HCl amount in gas was greater than those of the other catalyst, as well as the chlorine content in oil was also the highest (1591 ppm). In previous works [5,12], chlorine content of oil over SA-1 was lesser than that of oil from thermal degradation. In our case, the dechlorination step seems very effective. We can mention that during the dechlorination step, PE was molten form and HCl could be easily removed from the polymer mixture within an hour. However in the presence of a solid material such as a cracking catalyst, part of HCl adsorbed in the reactor and (following the degradation step) it reacted with the degradation products of polymers. Although SA-1 is not a chlorine sorbent or dechlorination catalyst, the HCl amount in gas was lower than that of thermal degradation. Distributions of organic chlorine compounds derived from
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Fig. 6. Cl-NP gram of liquid products from thermal and catalytic degradation of HDPE/PVC mixture at 430 C.
the catalytic and thermal degradation of PVC/PE are shown in Fig. 6 as Cl-NP gram. The organic chlorine compounds were distributed in carbon number range of n-C6 to n-C11 for the HCl sorbed catalyst (TR-99701 and RM) and in the range of n-C6 to n-C10 for SA-1 and thermal process. The types of chlorine compounds were similar for all catalyst type, but their contents differed. Briefly, in the absence of a catalyst, we obtained oil having very little chlorine from the degradation of PVC/ PE mixtures with the dechlorination step and N2 flow. The use of a cracking catalyst increased the light hydrocarbons in the oil, but also increased the chlorine content of the oil. RM is as effective as g-Fe2O3 to sorbed chlorine during degradation of PVC/PE mixtures. 3.2. Thermal and catalytic degradation of PS/PVC and PP/PVC mixture In the previous study [5], we investigated the degradation of PP/PVC mixture at 380 C using different types of catalyst and the degradation of PS/PVC at 360 C without a catalyst. In the present work, the dechlorination and cracking behavior of these polymer mixtures were investigated. Experiments were carried out at 430 C with a dechlorination step and in N2 flow with and without RM. Even if the degradation temperatures of PP and PS are lower than that of PE, the degradation temperature of 430 C was chosen after consideration of the fact that all polymers will be treated together as municipal waste plastics.
Table 5 Distribution of Cl in the products obtained from the degradation of PS/PVC or PP/PVC at 430 C Catalyst
PS/PVC PS/PVC PP/PVC PP/PVC
None Red Mud Red Mud None
Cl content in oil (ppm) 81 381 764 365
Cl distribution (wt.%) Liquid
Gas
0.05 0.25 0.39 0.19
92.3 22.1 29.9 98.0
Table 4 shows the yield of products obtained from PVC mixed plastics. For both polymer mixtures, liquid yields from catalytic degradation did not differ significantly from that of thermal degradation, whereas the use of RM increased the amount of residue and decreased the amount of gas. This change in the yields of residue and gas was not due to the effect of RM on the polymer degradation mechanism: the adsorption of HCl gas from the degradation of PVC on the RM led to an increase in residue amount and a decrease in the gas yield. The degradation rate of polymer mixtures as well as the degradation yields was similar for both thermal and catalytic degradation (Fig. 7). Fig. 8 shows the composition of liquid products as a C-NP gram (normal paraffin gram for hydrocarbons). Degradation of PS/ PVC mixture both with and without catalyst gave the well-known PS pyrolysis products containing styrene monomer,a-methylstyrene, toluene and small quantities
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Fig. 7. Cumulative volume of liquid products from thermal and catalytic degradation of PS, PP/PVC mixture at 430 C.
Fig. 8. C-NP gram of liquid products from thermal and catalytic degradation of PS, PP/PVC mixture at 430 C.
of dimer. The liquid hydrocarbon products from PP/ PVC degradation had a carbon number distribution of n-C5 to n-C25 with a sharp peak at n-C9 (2,4-dimethyl-1heptene). RM had a big effect on the chlorine content and distribution in products for both PP/PVC and PS/PVC mixtures as in the case of PE/PVC (Table 5). In the absence of RM, over 90% of chlorine in PVC recovered in traps as HCl. In the presence of RM, this amount was
22.1% for PS and 29.9% for PP. Also in this group of experiments, the chlorine sorption properties of RM could be seen clearly. As expected, the amount and composition of chlorine compounds in liquid products differed according to the type of degraded plastic. In the presence of RM, the chlorine content of the oil was 318 ppm for PS/PVC and 764 ppm for PP/PVC. In the previous work [5] relating to fixing of HCl to a suitable sorbent (such as a-FeOOH, Fe3O4, calcined a-FeOOH),
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Fig. 9. Cl-NP gram of liquid products from thermal and catalytic degradation of PS, PP/PVC mixture at 430 C. Table 6 Yields of products obtained from the degradation of PE, PS or PP at 430 C
PE PE PS PS PP PP
Catalyst
Liquid (wt.%)
Gas (wt.%)
Residue (wt.%)
Liquid density (g cm 3)
None Red Mud None Red Mud None Red Mud
68.2 71.6 85.2 85.6 72.6 72.2
14.4 15.0 1.0 2.2 12.7 15.4
16.8 13.4 13.8 12.2 14.7 12.4
0.760 0.771 0.903 0.903 0.737 0.744
an oil having 1100 ppm organic chlorine content was obtained from the dechlorination of PVC/PP mixture at 380 C. In the same work, the thermal degradation of PP/PVC and PS/PVC was carried out at 380 and 360 C, respectively. The oil recovered contained 17 600 and 10 500 ppm chlorine for PS/PVC and PP/PVC, respectively. However, in the present process, we obtained the oil containing 365 and 81 ppm chlorine from PP/PVC and PS/PVC, respectively. It is interesting to note that the oils from both PP/PVC and PS/PVC contained only organic chlorine compounds. This is due to the dechlorination step. Organic chlorine compounds were distributed in hydrocarbons with light boiling points (< 174 C) in both the absence and presence of RM. The compositions of the chlorine containing liquid products are given in Fig. 9. In the case of PS/PVC degradation, 2-chloro-2-phenylpropane was formed in the absence of RM; a-chloroethylbenzene and 2-chloro-2-phenylpropane compounds were formed in the presence of RM. In the case of PP/PVC degradation, organic chlorine compounds of C6 and C8 were formed during thermal degradation, whereas chlorine compounds were distributed
comparatively in the range of n-C6 to n-C10 in the presence of RM. These chlorine compounds could not be identified by GC-MS. However a previous work [3] showed that the organic chlorine compounds in oil from the degradation of PP/PVC were 2-chloro-2-methylpropane (C6) and 2-chloro-2-methylpentane (C8). RM had an effect on the type of organic chlorine compounds in both PP/PVC and PS/PVC mixtures. It is thought that RM catalyzed the formation of new chlorine compounds in the reaction between HCl and degradation products of PP or PS. However, we can not say which compound/compounds in RM led to the form the new organic chlorine compounds. It is interesting to note that in the catalytic degradation of polymer mixtures, the amount of recovered chlorine (as HCl) in traps was 22.2% for PS/PVC and 29.9% for PP/PVC whereas it was 47.8% for PE/PVC. This shows that the amount of sorbed HCl on the RM varied depending on the polymer types. This can be explained thus: because the polymer melt gets into contact with catalyst surface, the fixation effect of metal compounds in RM might be changed with the type of polymer melt.
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Fig. 10. Cumulative volume of liquid products from thermal and catalytic degradation of PS, PE and PP at 430 C.
Fig. 11. Composition of gas products from thermal and catalytic degradation of PS, PE and PP at 430 C.
3.3. Effect of Red Mud on the degradation of single compound plastic In order to investigate the behaviour of RM on the degradation of single compound plastic, the degradation of PE, PS or PP was carried out with and without
RM at 430 C under nitrogen stream. Table 6 shows the yields of products from thermal and catalytic degradation. For catalytic degradation, the yield of liquids did not differ significantly from that of thermal degradation. In the cases of PP and PS, RM had effect on the yields of residue and gas. In the presence of RM, the
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Fig. 12. C-NP gram of liquid products from thermal and catalytic degradation of PS, PE and PP at 430 C.
residue amount decreased by 11 and 15%, gas amount increased by 100 and 21% for PS and PP, respectively. However, these differences are not considerable on the total product base. Fig. 10 shows the cumulative volume of liquid products as a function of lapsed time. The count of lapsed time was started from beginning of 300 C. As expected the rate of degradation of PS and PP were much faster than that of PE. The degradation rates of polymers were same in presence and absence of RM. The composition of gaseous products is shown in Fig. 11. In both the thermal and catalytic degradation of PP, gaseous products were same. The main gas products were C3. In thermal degradation of PS, the gaseous products were mainly C2, C3, and C4, whereas in catalytic degradation C3 increased and C4 was absence, and also unsaturated hydrocarbons became more. For the thermal and catalytic degradation of PE, the gaseous products were C1, C2 and C3. Composition of the liquid products from catalytic and thermal degradation of PE, PS and PP is shown in Fig. 12 as a Normal Paraffin gram (NP-gram). As shown in Fig. 12, RM had no effect on the carbon number distribution of liquid products. In the case of PE, the liquid hydrocarbon products were distributed in a wide range carbon numbers (n-C5 to n- C25) equivalent to boiling point ranges of 36–405 C, and were composed of linear olefins and paraffins. Similar results for the thermal degradation of PE at 430 C have been reported in studies (13–15). The liquid hydrocarbon products from PP degradation also had a carbon number distribution of n-
C5 to C25 with peaks at n-C6, C9, C11, C14, C16 and C18. However, in the case of PS, the liquid hydrocarbons were distributed sharply in the range of n-C8 to n-C10 with a peak at n-C9 and C18 and composed of styrene, amethylstyrene and toluene. In the previous study [13] related to the catalytic degradation of PE and PP over various types of solid catalysts, such as silica-alumina, zeolite and non-acidic mesoporous silica catalyst, it was mentioned that the silica-alumina and mesoporous silica catalysts accelerated the rate of degradation of PE and PP; and they degraded the heavier waxy compounds into lighter liquid hydrocarbons. If we compare our results to the above literature: in the thermal degradation of PP at 430 C, the degradation rate of PP and yields of products was similar to the results that of silica-alumina, but silica-alumina catalyst led to lighter hydrocarbons in oil. In the PE degradation over silica-alumina and mesoporous silica catalysts, the yield and composition of liquid hydrocarbons and the degradation rate were better than our results. As a result we concluded that RM had no effect on the degradation of PE, PP and PS.
4. Conclusion The results can be summarized as follows. . During the thermal degradation of the polymer mixtures containing PVC, over 90% of feed chlorine was recovered as HCl by dechlorination step at 350 C for 1 h.
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. The use of N2 flow during the both dechlorination and degradation steps decreased the reaction between HCl and degradation products of polymer due to the effective removal of HCl from reactor before the degradation of other polymers occurs. . Red Mud had a good effect on the fixation of evolved HCl as well as iron oxides sorbents. . Red Mud had an effect on the composition of organic chlorine compounds derived from the degradation of PS/PVC and PP/PVC mixtures. . The use of RM in VP contact mode led to more HCl fixation, but VP-contact mode increased the chlorine amount of oil, because of the reaction between fixed HCl and adsorbed degradation products on RM. . Red Mud showed no effect on the cracking of both PVC containing polymer mixture and single polymer. . The oils derived from PVC containing polymer mixtures by thermal degradation contained lower amount chlorine than the oils obtained by using of Red Mud. Unfortunately, we do not suggest any mechanism on the Red Mud effect, because RM is a complex mixture of inorganic materials and is not a synthetic catalyst.
Acknowledgements We would like to thank the Venture Business Laboratory of Okayama University, Japan for financial support for Dr. Jale Yanik to visit Japan and perform
this work at Okayama University. We would also like to thank Dr. Katsuhide Murata of Japan Eco Environment Products for his suggestions in this work.
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