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Thermal and catalytic degradation of structurally different types of polyethylene into fuel oil
Polymer Degradation and Stabiliry 56 (1997) 37-44 0 1997 Elsevier Science Limited ELSEVIER
PII:
SO141-3910(96)00191-7
Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00
Thermal and catalytic degradation of structurally different types of polyethylene into fuel oil Md Azhar Wddin,” Kazuo Koizumip Katsuhide Murata” & Yusaku Sakata** “Department of Applied Chemistry, Okayama University 3-l-l Tsushima Naka Okayama 700 Japan bMitsui Engineering and Ship Building Co. Ltd. Chiba 290 Japan (Received
31 May 1996; revised 8 August 1996; accepted 23 August 1996)
The degradation of four different types of polyethylene (PE) namely high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and cross-linked PE (XLPE) was carried out at 430°C by batch operation using silica-alumina as a solid acid catalyst and thermally without any catalyst. For thermal degradation, both HDPE and XLPE produced a significant amount of wax-like compounds and the yields of liquid products (58-63 wt%) were lower than that of LDPE and LLDPE (76-77 wt%). LDPE and LLDPE produced a very small amount of wax-like compounds. Thus the structure of the degrading polymers influenced the product yields. The liquid products from thermal degradation were broadly distributed in the carbon fraction of n-C, to n-C& (boiling point range, 36-405°C). With silica-alumina, all of the polyethylenes were converted to liquid products with high yields (77-83 wt%) and without any wax production. The liquid products were distributed in the range of n-C, to n-C& (mostly C,-C,,). A solid acid catalyst indiscriminately degraded the various types of polyethylene into light fuel oil with an improved rate. 0 1997 Elsevier Science Limited. All rights reserved.
1 INTRODUCTION
with polypropylene in a fluidized-bed reactor at 450-530”C).7 The products of degradation were mainly heavy hydrocarcarbons or waxes (boiling point >5OO”C. It seems likely that the operating conditions greatly affect the product yields and compositions. Recently we have demonstrated the effects of the silica-alumina catalyst on the polypropylene (PP) degradation process 1) when the melted PP was brought into contact with the solid catalyst (liquid phase contact) and 2) when the thermally degraded hydrocarbon vapours from PP were brought into contact with the solid catalyst (vapour phase contact).* So far as we know, no report has been published which compares the thermal and catalytic degradation of structurally different types of polyethylene. In this paper we report a process whereby structurally different types of polyethylene viz. high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and cross-linked low density PE (XLPE) are degraded
The thermal and catalytic degradation of plastic polymers is becoming an increasingly important method for the conversion of waste plastics into valuable chemicals and fuel oil. Many recent investigations of waste plastic degradation into fuel oil have reported on the thermal degradation of polyethylene into liquid hydrocarbons.‘-7 Some reports have been published on the catalytic degradation of polyethylene where the thermally degraded hydrocarbon vapors were brought into contact with solid acid catalysts to obtain secondary cracking products.5 Mordi et al. have examined the product distribution from the catalytic degradation of low density polyethylene over various types of zeolite catalysts under batch conditions.6 Kastner et al. have reported the pyrolysis of high density polyethylene, linear low density polyethylene and a mixture of the two * To whom correspondence
should be addressed. 37
38
M. Azhar 1Jddin et al.
into fuel oil both in the absence (thermal) and presence of the silica-alumina catalyst in liquid phase contact. The yields of product gas, liquid, wax-like compounds and carbonaceous residues; recovery rate of liquid products; and boiling point composition of liquid products from the catalytic degradation of various types of PE were compared with those of non-catalytic thermal degradation.
cooling condenser in order to condense the liquid products. Gaseous products passed through a water seal pot were finally collected in a teflon bag. The products of degradation were analyzed using two gas chromatographs: liquid products with an ov-101 capillary column and a FID detector; and gaseous products with a Porapak QS column and a TCD detector. Gel permeation chromatography (GPC) was employed to determine the molecular weight distribution of the residual liquids in the reactor.
2 EXPERIMENTAL 3 RESULTS AND DISCUSSION 2.1 Materials The catalysts employed in this study, silicaalumina (SA-1; SiO,/Al,O, mole ratio: 83*3/16-7 and SA-2; SiO,/Al,O, mole ratio: 21*1/78*9) were supplied by Mizusawa Chemical Industries. High density PE (HDPE) with a long straight carbon backbone was obtained from Mitsui Petrochemical Industries; low density PE (LDPE) with densely spaced branching and sub-branching on the polymer backbone from Mitsubishi Chemical Industries; linear low density PE (LLDPE) with sparsely spaced branching on the polymer backbone from Ube Chemical Industries; and Cross-linked PE (XLPE) having cross-linked straight carbon chains from Furukawa Electrical Industries. 2.2 Method Thermal and catalytic degradations of PE samples were carried out in a glass reactor (35 mm i.d. and 250mL volume) by batch operation. Details of the experimental procedures are given in a previous paper.’ 10 g of PE sample was loaded into the reactor for thermal degradation and 10 g of PE mixed with 1 g of catalyst (1 mm in size) was loaded into the reactor for the catalytic degradation. In a typical run, the air remaining in the reactor was purged with nitrogen and then the reactor was heated to 120°C in 60 min and held for 60 min at 120°C to remove the adsorbed water from the catalyst and the polyethylene sample. The nitrogen flow was then cut off and the temperature increased from 120°C to 430°C at a heating rate of 3”C/min. The outlet of the reactor was connected to a water
3.1 Material balance for thermal and catalytic (SA-2) degradation of various types of polyethylene The degradation products were classified into four groups: gases (products which were not condensable at water cooling temperature), liquid hydrocarbons, wax-like compounds, and carbonaceous residues. The wax-like compounds refer to the hydrocarbon products which are solidified like wax at room temperature. The amount of gaseous products was determined by subtracting the weight of liquid products, wax-like products and residues from the PE sample feed. The term residues refers to the carbonaceous material remaining in the reactor after the degradation run. Table 1 shows the distribution of yields of gases (G), liquids (L), wax-like compounds (IV) and residues (R) produced from the thermal and catalytic (SA-2) degradation of various types of PE at 430°C. In HDPE and XLPE prothermal degradation, duced a significant amount of wax-like compounds and the yields of liquid product were lower than that for LDPE and LLDPE. LDPE and LLDPE also produced a small amount of wax-like compounds (5.7 and 8.7 wt%, respectively). These results suggest that PE having branching on the polymer backbone (LDPE and LLDPE) degrades more easily to liquid hydrocarbon products than the long straight chain PE (HDPE and XLPE). When these PE samples were degraded in the presence of silica-alumina (SA-2) catalyst, no wax-like compounds were obtained and both the yields of liquid and gaseous products were increased over those in the thermal degradation. HDPE and XLPE, which produced a significant amount of heavier
Thermal and catalyticdegradation of polyethylene Table 1. Product yield for the&
39
and catalytic degradation of polyethylene at WC
Thermal
Catalytic (SA-2)
HDPE
LDPE
LLDPE
XLPE
HDPE
LDPE
LLDPE
XLPE
Product yield”’ (wt%) Liquid (L) Waxy compound (W) Gaseous (G) Residues (R)
58.4 26.3 6.3 9.0
75.6 8.7 8.2 7.5
78.9 5.7 7.8 7.6
63.1 20.6 7.2 9.1
77.4 0 11.6 11.0
80.2 0 10.8 9.0
82.5 0 10.4 7.1
78.6 0 11.2 10.2
Bromine number of liquid products (g(BrNOO g (sample)
62.7
63.7
58.4
52.8
90.1
87.5
85.7
88.0
“‘G=lOO-(L+W+R).
hydrocarbon (wax-like) products, also effectively degraded into liquid hydrocarbon products.
from 120°C to the reaction temperature commenced. It is noteworthy that in thermal degradation, wax-like compounds were produced in the initial stages of the reaction and accumulated in the graduated cylinder. In this work, it was assumed that the degradation was completed when no more accumulation of liquid products was observed in the graduated cylinder over a 30 min period. As can be seen in Fig. 1, the initial rate of degradation of all types of PE over the silica-alumina (SA-2) catalyst was about 3-4 times faster than that for thermal degrada-
3.2 Rate of recovery of liquid hydrocarbon products The cumulative volume of both liquid and wax-like compounds (at water cooling temperature) in the graduated cylinder and the temperature inside the reactor as a function of lapsed time is shown in Fig. 1. Lapsed time was counted from when the heating of the sample
0
A
HDPE(catal) LDPE( catal)
n l
500
LLDPE( catal) XLPE(Catal)
420
# Q
0
HDPE(Therm.)
0
LLDPE(’ Therm.:
A
LDPE(Therm.)
o
XLPE(Therm.)
Lapse time / min Fig. 1.
Cumulative
volume of liquid and wax-like products from thermal and catalytic (SA-2) degradation polyethylene at 430°C.
of various types of
M. Azhar Uddin et al.
40
tion. These results suggest that the acid sites of silica-alumina in contact with the melted PE accelerated the degradation of PE significantly. In both thermal and catalytic degradation, HDPE and XLPE required a much longer time for the degradation than LDPE and LLDPE. This indicates the greater difficulty of degrading HDPE and XLPE compared to LDPE and LLDPE. 3.3 Composition of the gaseous products Figure 2 shows the composition of the gaseous products from the thermal and catalytic (SA-2) degradation of polyethylene (HDPE) carried out at 430°C. For thermal degradation, the gaseous products were mainly C, (propane, propylene), C, (ethane, ethylene) and a small amount of C4 (butane, butene) component. In catalytic degradation, the content of C, and C, components decreased and that of C4 increased. The production of a large amount of the C, component is one of the special features of the catalytic degradation of polyethylene. The degradation of LDPE, LLDPE and XLPE also revealed similar results.
3.4 Composition of liquid products The liquid products were characterized by a Normal Paraffin gram (NP-gram) demonstrated by Murata et a1.l’ Figures 3 and 4 show the NP-gram (carbon number distribution and boiling point range) of the liquid products obtained from the thermal and catalytic (SA-2) degradation of various types of PE at 430°C respectively. The carbon numbers of the abscissa in Figs 3 and 4 were obtained by analyzing the gas chromatogram of the liquid products. The gas chromatographic peaks in normal paraffin of C,_l-C, were assigned to the hydrocarbons of (n)th carbon number, and the fractional weight percent of the (n)th carbon component was calculated. These carbon numbers are equivalent to the retention values of the corresponding normal paraffins and indicate a range of boiling points in which the boiling points of hydrocarbons are distributed. For thermal degradation (Fig. 3), the liquid hydrocarbon products were distributed in a wide range carbon numbers (n-C, to nCZs) equivalent to boiling point ranges of 36 to 405°C. For catalytic degradation over silicaalumina (SA-2), the liquid products were
60 m
q
HDqE(Thermal) HDPE(Ca.tal,SA-2)
3
2
Carbon number Fig. 2. Composition
of the gaseous
products
from thermal
and catalytic
(SA-2) degradation
of HDPE
at 430°C.
Thermal and catalyticdegradation of polyethylene
25 -
__o__b_
20
-
41
500 HDPE(Therm.1 LDPE(Therm.)
-
LLDPE(Therm.)
-
XLPE(Therm.)
400 I /
300
200
100
0 25
15 Carbon Fig. 3. Distribution
number
of carbon number (NP-gram) in liquid products from non-catalytic polyethylene at 430°C.
thermal degradation
of various types of
500
25
5
__o--
HDPE(Catal.)
-
LDPE( Catal.)
-
LLDPE( Cat al.)
-
XLPE(Catal.)
15
400 I
25
Carbon number Fig. 4.
Distribution
of carbon number (NP-gram) in liquid products from catalytic degradation polyethylene. Catalyst; silica-alumina (SA-2); Degradation temperature: 430°C.
of various
types of
42
M. Azhar Uddin et al.
*-
HDPE(Catal.
, SA-1)
d-
HDPE(Catal.,
SA-2)
_t_
Gasoline
25
15 Carbon Fig. 5. Distribution
number
of carbon number (NP-gram) in liquid products from catalytic degradation Catalyst (SA-1) and SA-2 at 430°C and in commercial gasoline.
predominant in carbon fraction of n-C, to nCIs (Fig. 4). The bromine number of the liquid products, which is a measure of unsaturation in liquid hydrocarbons, was determined in order to compare the degree of total unsaturation in liquid products from the thermal and catalytic degradation of PE (see Table 1). Bromine liquid products were 53numbers of 64 g(Br,)/lOO g(liquid product) for thermal degradation and 86-90 g(Br,)/lOO g(liquid product), for catalytic degradation. Therefore, catalytically degraded products have a higher degree of total unsaturation than the non-catalytic thermally degraded products. 3.5 The Effects of silica-alumina (SA-1) and silica-alumina (SA-2) catalysts on liquid product composition Two types of silica-alumina catalysts having SiO,/Al,O, ratios of 83*3/16.7 (SA-1) and 21.1/78.9 (SA-2) were used in the catalytic degradation of HDPE in order to investigate the effects of acidity of silica-alumina on liquid product composition. As the SiO,/Al,O, ratio of
of HDPE using silica-alumina
the SA-1 and SA-2 catalysts are different both the acid strength and acid content of the these catalysts may differ. The yield of liquid products was 68 wt% for SA-1 compared to 77 wt% for SA-2. Figure 5 shows the NP-gram of the liquid products obtained from the catalytic degradation of HDPE over SA-1 and SA-2 along with the NP-gram of commercial gasoline. With SA-1, liquid products were distributed in the range of equivalent carbon numbers of n-C, to n-C,, (boiling point range 36 to 216”C), very similar to those of commercial gasoline. As shown in Fig. 4, the liquid products were predominant in the carbon fraction of n-C, to n-C,,. Therefore, the SA-1 catalyst degraded the PE sample into much lighter hydrocarbon fuel oil than the SA-2 catalyst. The yield and composition of the liquid by altering the products can be controlled SiO,/Al,O, ratio i.e. the acidity of the catalyst. 3.6 Molecular weight distribution of residual liquid from polyethylene degradation Figure 6 shows the molecular weight distribution of the residual liquid in the reactor determined
Thermal and catalyticdegradation of polyethylene
43
(a) HDPE 1 Raw sample 2 Thermal
.
(b) LLDPE
1 Raw sample 2 Thermal
,.
I ‘\
tog M /Fig. 6. Molecular weight distribution
of residual liquid obtained in the course of degradation determined by means of GPC.
by gel permeation chromatography (GPC) when the PE samples were heated from 120 to 430°C and held for 60min (total 165 min). In thermal degradation, the molecular weight distribution of the residual liquid from both HDPE (Fig. 6(a)) and LLDPE (Fig. 6(b) was shifted to a lower molecular weight range with respect to the molecular weight distribution of their original (raw) samples. However, the molecular weight distribution of the residual liquid from LLDPE shifted to a much lower molecular weight range than that of HDPE, indicating that LLDPE degraded more easily than HDPE. When HDPE was heated in the presence of the silica-alumina (SA-1) catalyst, the molecular weight distribution of the residual liquid extended to a lower molecular weight range than that for the non-catalytic thermal degradation implying that the solid acid catalyst in direct contact with melted PE promotes the degradation of PE into lower molecular weight compounds. The results of this work strongly suggest that silica-alumina effectively catalyzed the degradation of PE into lighter hydrocarbons, and
of (a) HDPE and (b) LLDPE,
catalytic effects were found both in the rate and product components of the degradation. It is also evident from the results of thermal degradation that the structure of the degrading polymers influenced the product yields significantly.
ACKNOWLEDGEMENT The authors wish to thank, Dr. Yoshihisa Kiso of Mitsui Petrochemical Industries of Japan for performing gel permeation chromatographic (GPC) analyses.
REFERENCES 1. Murata, K. & Makino, T., Nippon Kagaku Kaishi, 1973, 2214.
2. McCaffrey, W. C., Kamal,
M. R. & Cooper,
D. G.,
Polym. Deg. Stab., 47 (1995) 133.
3. Uemichi, Y., Ayame, A., Kashiwaya, Y., Tsukidate, M. & Kano, H., .I. Chromatogr., 259 (1983) 69. 4. Audisio, G., Silvani, A., Beltrame, P. L. & Camity, P., J. Anal. Appl. Pyrolysis, 7 (1984) 83.
5. Mordi, R. C., Field, R. & Dwyer, J., J. Anal. Appl.
44
M. Azhar
Pyrolysis, 29 (1994) 45. H. and Kaminsky, W., Hydrocarbon 6. Kastner, Processing, 1995, 109. 7. Saito, K., Kagaku to Kogyo, 66 (1992) 438. 8. Sakata, Y., Uddin, M. A., Koizumi, K. & Murata, K.,
Uddin
et al.
Chem. Lett., (1996) 245. 9. Sakata, Y., Uddin, M. A., Koizumi, K. & Murata, K., Polym. Deg. Stab., 53 (1996) 111. 10. Murata, K. & Makino, T., Nippon Kagaku Kaishi, 1975,
192.
PII:
SO141-3910(96)00191-7
Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00
Thermal and catalytic degradation of structurally different types of polyethylene into fuel oil Md Azhar Wddin,” Kazuo Koizumip Katsuhide Murata” & Yusaku Sakata** “Department of Applied Chemistry, Okayama University 3-l-l Tsushima Naka Okayama 700 Japan bMitsui Engineering and Ship Building Co. Ltd. Chiba 290 Japan (Received
31 May 1996; revised 8 August 1996; accepted 23 August 1996)
The degradation of four different types of polyethylene (PE) namely high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and cross-linked PE (XLPE) was carried out at 430°C by batch operation using silica-alumina as a solid acid catalyst and thermally without any catalyst. For thermal degradation, both HDPE and XLPE produced a significant amount of wax-like compounds and the yields of liquid products (58-63 wt%) were lower than that of LDPE and LLDPE (76-77 wt%). LDPE and LLDPE produced a very small amount of wax-like compounds. Thus the structure of the degrading polymers influenced the product yields. The liquid products from thermal degradation were broadly distributed in the carbon fraction of n-C, to n-C& (boiling point range, 36-405°C). With silica-alumina, all of the polyethylenes were converted to liquid products with high yields (77-83 wt%) and without any wax production. The liquid products were distributed in the range of n-C, to n-C& (mostly C,-C,,). A solid acid catalyst indiscriminately degraded the various types of polyethylene into light fuel oil with an improved rate. 0 1997 Elsevier Science Limited. All rights reserved.
1 INTRODUCTION
with polypropylene in a fluidized-bed reactor at 450-530”C).7 The products of degradation were mainly heavy hydrocarcarbons or waxes (boiling point >5OO”C. It seems likely that the operating conditions greatly affect the product yields and compositions. Recently we have demonstrated the effects of the silica-alumina catalyst on the polypropylene (PP) degradation process 1) when the melted PP was brought into contact with the solid catalyst (liquid phase contact) and 2) when the thermally degraded hydrocarbon vapours from PP were brought into contact with the solid catalyst (vapour phase contact).* So far as we know, no report has been published which compares the thermal and catalytic degradation of structurally different types of polyethylene. In this paper we report a process whereby structurally different types of polyethylene viz. high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and cross-linked low density PE (XLPE) are degraded
The thermal and catalytic degradation of plastic polymers is becoming an increasingly important method for the conversion of waste plastics into valuable chemicals and fuel oil. Many recent investigations of waste plastic degradation into fuel oil have reported on the thermal degradation of polyethylene into liquid hydrocarbons.‘-7 Some reports have been published on the catalytic degradation of polyethylene where the thermally degraded hydrocarbon vapors were brought into contact with solid acid catalysts to obtain secondary cracking products.5 Mordi et al. have examined the product distribution from the catalytic degradation of low density polyethylene over various types of zeolite catalysts under batch conditions.6 Kastner et al. have reported the pyrolysis of high density polyethylene, linear low density polyethylene and a mixture of the two * To whom correspondence
should be addressed. 37
38
M. Azhar 1Jddin et al.
into fuel oil both in the absence (thermal) and presence of the silica-alumina catalyst in liquid phase contact. The yields of product gas, liquid, wax-like compounds and carbonaceous residues; recovery rate of liquid products; and boiling point composition of liquid products from the catalytic degradation of various types of PE were compared with those of non-catalytic thermal degradation.
cooling condenser in order to condense the liquid products. Gaseous products passed through a water seal pot were finally collected in a teflon bag. The products of degradation were analyzed using two gas chromatographs: liquid products with an ov-101 capillary column and a FID detector; and gaseous products with a Porapak QS column and a TCD detector. Gel permeation chromatography (GPC) was employed to determine the molecular weight distribution of the residual liquids in the reactor.
2 EXPERIMENTAL 3 RESULTS AND DISCUSSION 2.1 Materials The catalysts employed in this study, silicaalumina (SA-1; SiO,/Al,O, mole ratio: 83*3/16-7 and SA-2; SiO,/Al,O, mole ratio: 21*1/78*9) were supplied by Mizusawa Chemical Industries. High density PE (HDPE) with a long straight carbon backbone was obtained from Mitsui Petrochemical Industries; low density PE (LDPE) with densely spaced branching and sub-branching on the polymer backbone from Mitsubishi Chemical Industries; linear low density PE (LLDPE) with sparsely spaced branching on the polymer backbone from Ube Chemical Industries; and Cross-linked PE (XLPE) having cross-linked straight carbon chains from Furukawa Electrical Industries. 2.2 Method Thermal and catalytic degradations of PE samples were carried out in a glass reactor (35 mm i.d. and 250mL volume) by batch operation. Details of the experimental procedures are given in a previous paper.’ 10 g of PE sample was loaded into the reactor for thermal degradation and 10 g of PE mixed with 1 g of catalyst (1 mm in size) was loaded into the reactor for the catalytic degradation. In a typical run, the air remaining in the reactor was purged with nitrogen and then the reactor was heated to 120°C in 60 min and held for 60 min at 120°C to remove the adsorbed water from the catalyst and the polyethylene sample. The nitrogen flow was then cut off and the temperature increased from 120°C to 430°C at a heating rate of 3”C/min. The outlet of the reactor was connected to a water
3.1 Material balance for thermal and catalytic (SA-2) degradation of various types of polyethylene The degradation products were classified into four groups: gases (products which were not condensable at water cooling temperature), liquid hydrocarbons, wax-like compounds, and carbonaceous residues. The wax-like compounds refer to the hydrocarbon products which are solidified like wax at room temperature. The amount of gaseous products was determined by subtracting the weight of liquid products, wax-like products and residues from the PE sample feed. The term residues refers to the carbonaceous material remaining in the reactor after the degradation run. Table 1 shows the distribution of yields of gases (G), liquids (L), wax-like compounds (IV) and residues (R) produced from the thermal and catalytic (SA-2) degradation of various types of PE at 430°C. In HDPE and XLPE prothermal degradation, duced a significant amount of wax-like compounds and the yields of liquid product were lower than that for LDPE and LLDPE. LDPE and LLDPE also produced a small amount of wax-like compounds (5.7 and 8.7 wt%, respectively). These results suggest that PE having branching on the polymer backbone (LDPE and LLDPE) degrades more easily to liquid hydrocarbon products than the long straight chain PE (HDPE and XLPE). When these PE samples were degraded in the presence of silica-alumina (SA-2) catalyst, no wax-like compounds were obtained and both the yields of liquid and gaseous products were increased over those in the thermal degradation. HDPE and XLPE, which produced a significant amount of heavier
Thermal and catalyticdegradation of polyethylene Table 1. Product yield for the&
39
and catalytic degradation of polyethylene at WC
Thermal
Catalytic (SA-2)
HDPE
LDPE
LLDPE
XLPE
HDPE
LDPE
LLDPE
XLPE
Product yield”’ (wt%) Liquid (L) Waxy compound (W) Gaseous (G) Residues (R)
58.4 26.3 6.3 9.0
75.6 8.7 8.2 7.5
78.9 5.7 7.8 7.6
63.1 20.6 7.2 9.1
77.4 0 11.6 11.0
80.2 0 10.8 9.0
82.5 0 10.4 7.1
78.6 0 11.2 10.2
Bromine number of liquid products (g(BrNOO g (sample)
62.7
63.7
58.4
52.8
90.1
87.5
85.7
88.0
“‘G=lOO-(L+W+R).
hydrocarbon (wax-like) products, also effectively degraded into liquid hydrocarbon products.
from 120°C to the reaction temperature commenced. It is noteworthy that in thermal degradation, wax-like compounds were produced in the initial stages of the reaction and accumulated in the graduated cylinder. In this work, it was assumed that the degradation was completed when no more accumulation of liquid products was observed in the graduated cylinder over a 30 min period. As can be seen in Fig. 1, the initial rate of degradation of all types of PE over the silica-alumina (SA-2) catalyst was about 3-4 times faster than that for thermal degrada-
3.2 Rate of recovery of liquid hydrocarbon products The cumulative volume of both liquid and wax-like compounds (at water cooling temperature) in the graduated cylinder and the temperature inside the reactor as a function of lapsed time is shown in Fig. 1. Lapsed time was counted from when the heating of the sample
0
A
HDPE(catal) LDPE( catal)
n l
500
LLDPE( catal) XLPE(Catal)
420
# Q
0
HDPE(Therm.)
0
LLDPE(’ Therm.:
A
LDPE(Therm.)
o
XLPE(Therm.)
Lapse time / min Fig. 1.
Cumulative
volume of liquid and wax-like products from thermal and catalytic (SA-2) degradation polyethylene at 430°C.
of various types of
M. Azhar Uddin et al.
40
tion. These results suggest that the acid sites of silica-alumina in contact with the melted PE accelerated the degradation of PE significantly. In both thermal and catalytic degradation, HDPE and XLPE required a much longer time for the degradation than LDPE and LLDPE. This indicates the greater difficulty of degrading HDPE and XLPE compared to LDPE and LLDPE. 3.3 Composition of the gaseous products Figure 2 shows the composition of the gaseous products from the thermal and catalytic (SA-2) degradation of polyethylene (HDPE) carried out at 430°C. For thermal degradation, the gaseous products were mainly C, (propane, propylene), C, (ethane, ethylene) and a small amount of C4 (butane, butene) component. In catalytic degradation, the content of C, and C, components decreased and that of C4 increased. The production of a large amount of the C, component is one of the special features of the catalytic degradation of polyethylene. The degradation of LDPE, LLDPE and XLPE also revealed similar results.
3.4 Composition of liquid products The liquid products were characterized by a Normal Paraffin gram (NP-gram) demonstrated by Murata et a1.l’ Figures 3 and 4 show the NP-gram (carbon number distribution and boiling point range) of the liquid products obtained from the thermal and catalytic (SA-2) degradation of various types of PE at 430°C respectively. The carbon numbers of the abscissa in Figs 3 and 4 were obtained by analyzing the gas chromatogram of the liquid products. The gas chromatographic peaks in normal paraffin of C,_l-C, were assigned to the hydrocarbons of (n)th carbon number, and the fractional weight percent of the (n)th carbon component was calculated. These carbon numbers are equivalent to the retention values of the corresponding normal paraffins and indicate a range of boiling points in which the boiling points of hydrocarbons are distributed. For thermal degradation (Fig. 3), the liquid hydrocarbon products were distributed in a wide range carbon numbers (n-C, to nCZs) equivalent to boiling point ranges of 36 to 405°C. For catalytic degradation over silicaalumina (SA-2), the liquid products were
60 m
q
HDqE(Thermal) HDPE(Ca.tal,SA-2)
3
2
Carbon number Fig. 2. Composition
of the gaseous
products
from thermal
and catalytic
(SA-2) degradation
of HDPE
at 430°C.
Thermal and catalyticdegradation of polyethylene
25 -
__o__b_
20
-
41
500 HDPE(Therm.1 LDPE(Therm.)
-
LLDPE(Therm.)
-
XLPE(Therm.)
400 I /
300
200
100
0 25
15 Carbon Fig. 3. Distribution
number
of carbon number (NP-gram) in liquid products from non-catalytic polyethylene at 430°C.
thermal degradation
of various types of
500
25
5
__o--
HDPE(Catal.)
-
LDPE( Catal.)
-
LLDPE( Cat al.)
-
XLPE(Catal.)
15
400 I
25
Carbon number Fig. 4.
Distribution
of carbon number (NP-gram) in liquid products from catalytic degradation polyethylene. Catalyst; silica-alumina (SA-2); Degradation temperature: 430°C.
of various
types of
42
M. Azhar Uddin et al.
*-
HDPE(Catal.
, SA-1)
d-
HDPE(Catal.,
SA-2)
_t_
Gasoline
25
15 Carbon Fig. 5. Distribution
number
of carbon number (NP-gram) in liquid products from catalytic degradation Catalyst (SA-1) and SA-2 at 430°C and in commercial gasoline.
predominant in carbon fraction of n-C, to nCIs (Fig. 4). The bromine number of the liquid products, which is a measure of unsaturation in liquid hydrocarbons, was determined in order to compare the degree of total unsaturation in liquid products from the thermal and catalytic degradation of PE (see Table 1). Bromine liquid products were 53numbers of 64 g(Br,)/lOO g(liquid product) for thermal degradation and 86-90 g(Br,)/lOO g(liquid product), for catalytic degradation. Therefore, catalytically degraded products have a higher degree of total unsaturation than the non-catalytic thermally degraded products. 3.5 The Effects of silica-alumina (SA-1) and silica-alumina (SA-2) catalysts on liquid product composition Two types of silica-alumina catalysts having SiO,/Al,O, ratios of 83*3/16.7 (SA-1) and 21.1/78.9 (SA-2) were used in the catalytic degradation of HDPE in order to investigate the effects of acidity of silica-alumina on liquid product composition. As the SiO,/Al,O, ratio of
of HDPE using silica-alumina
the SA-1 and SA-2 catalysts are different both the acid strength and acid content of the these catalysts may differ. The yield of liquid products was 68 wt% for SA-1 compared to 77 wt% for SA-2. Figure 5 shows the NP-gram of the liquid products obtained from the catalytic degradation of HDPE over SA-1 and SA-2 along with the NP-gram of commercial gasoline. With SA-1, liquid products were distributed in the range of equivalent carbon numbers of n-C, to n-C,, (boiling point range 36 to 216”C), very similar to those of commercial gasoline. As shown in Fig. 4, the liquid products were predominant in the carbon fraction of n-C, to n-C,,. Therefore, the SA-1 catalyst degraded the PE sample into much lighter hydrocarbon fuel oil than the SA-2 catalyst. The yield and composition of the liquid by altering the products can be controlled SiO,/Al,O, ratio i.e. the acidity of the catalyst. 3.6 Molecular weight distribution of residual liquid from polyethylene degradation Figure 6 shows the molecular weight distribution of the residual liquid in the reactor determined
Thermal and catalyticdegradation of polyethylene
43
(a) HDPE 1 Raw sample 2 Thermal
.
(b) LLDPE
1 Raw sample 2 Thermal
,.
I ‘\
tog M /Fig. 6. Molecular weight distribution
of residual liquid obtained in the course of degradation determined by means of GPC.
by gel permeation chromatography (GPC) when the PE samples were heated from 120 to 430°C and held for 60min (total 165 min). In thermal degradation, the molecular weight distribution of the residual liquid from both HDPE (Fig. 6(a)) and LLDPE (Fig. 6(b) was shifted to a lower molecular weight range with respect to the molecular weight distribution of their original (raw) samples. However, the molecular weight distribution of the residual liquid from LLDPE shifted to a much lower molecular weight range than that of HDPE, indicating that LLDPE degraded more easily than HDPE. When HDPE was heated in the presence of the silica-alumina (SA-1) catalyst, the molecular weight distribution of the residual liquid extended to a lower molecular weight range than that for the non-catalytic thermal degradation implying that the solid acid catalyst in direct contact with melted PE promotes the degradation of PE into lower molecular weight compounds. The results of this work strongly suggest that silica-alumina effectively catalyzed the degradation of PE into lighter hydrocarbons, and
of (a) HDPE and (b) LLDPE,
catalytic effects were found both in the rate and product components of the degradation. It is also evident from the results of thermal degradation that the structure of the degrading polymers influenced the product yields significantly.
ACKNOWLEDGEMENT The authors wish to thank, Dr. Yoshihisa Kiso of Mitsui Petrochemical Industries of Japan for performing gel permeation chromatographic (GPC) analyses.
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