Ind. Eng. Chem. Res. 1998, 37, 2889-2892
2889
RESEARCH NOTES Spontaneous Degradation of Municipal Waste Plastics at Low Temperature during the Dechlorination Treatment Yusaku Sakata,*,† Md. Azhar Uddin,† Akinori Muto,† Masaya Narazaki,† Kazuo Koizumi,† Katsuhide Murata,‡ and Mitsuo Kaji§ Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima Naka, Okayama 700, Japan, Mitsui Engineering and Shipbuilding Company Ltd., Chiba Laboratory, Ichihara, Chiba 290, Japan, and Plastic Waste Management Institute, Toranomon, Tokyo 105, Japan
This work deals with the investigation of the conditions under which a spontaneous degradation municipal waste plastic containing poly(vinyl chloride) (PVC) occurs during the dechlorination on an experimental scale by batch operation using a glass reactor, paying particular attention to the gas-solid reaction between the hydrogen chloride deriving from the PVC and the aluminum foil contained in the waste plastics. On the basis of observation of the effect of the rate of heating and the mode of contact between the PVC and aluminum foil on the change in the plastic melted level in the reactor, we were able to reproduce the spontaneous degradation of plastics with the production of a waxlike substance at around 593 K and at atmospheric pressure. The reason for the occurrence of spontaneous reaction was concluded to be the formation of “hot spots” (localized heating) originating from the heat of reaction between HCl generated from PVC and Al foil which eventually triggered the drastic thermal degradation of melted plastics. The possible conditions for the occurrence of the unusual reaction were discussed. Introduction
Table 1. Composition (wt %) of Municipal Waste Plastics (MWP1) and Model Mixed Plastics (Mix1)
Dechlorination is a necessary step in the process of degradation of mixed plastics containing chloropolymers such as PVC. The products of degradation of PVC containing a plastics mixture may become crucial in accidental fires during thermal degradation. The presence of chloropolymers in plastics mixtures exposed to heat raised the problem of whether their degradation products affect the well-known thermal degradation reaction of PE (Blazso et al., 1995). Instances of the sudden production of large quantities of waxlike hydrocarbons at relatively low temperatures during the dechlorination of municipal waste plastics by thermal degradation have been known in secret. There is a danger that blockage of the gas line can result from this abnormal production of waxlike substances. In this work, we have investigated the conditions under which this phenomenon occurs on an experimental scale by batch operation using a glass reactor, paying particular attention to the gas-solid reaction between the hydrogen chloride deriving from the PVC in the mixed plastics and the aluminum foil which may be contaminating the raw plastic material. On the basis of observation of the effect of the rate of heating and the mode of contact between the PVC and aluminum foil on the change in * Author to whom all correspondence should be addressed. Tel: +81(86)9251-8081. Fax: +81(86)251-8082. E-mail:
[email protected]. † Okayama University. ‡ Mitsui Engineering and Shipbuilding Company Ltd. § Plastic Waste Management Institute.
sample
PE
PP
PS
PVC
PVDC
PET
ABS
Al foil
MWP1 Mix1
32 37
19 17
27 23
5 8
1 0
14 10
2 5
present addeda
a Kitchen Al foil was added to model mixed plastics in some of the experimental runs.
the plastic melt level in the reactor, we were able to reproduce the abnormal phenomenon of waxlike substance production at around 573 K and to video this for detailed discussion. Experimental Section Plastics samples having two different compositions were used in this study: (1) sorted municipal waste plastics in the form of fluff collected from Niigata City Japan (abbreviated as MWP1), and (2) model mixed plastics (abbreviated as Mix1) having a composition similar to that of the sorted municipal waste plastics containing PET prepared by mixing pellets of various types of plastic polymers (Table 1). In addition, the fluffy MWP1 sample was pressed into a sheet of 0.5 × 10-3 m thickness at a temperature low enough not to produce HCl from the PVC content of the sample by thermal degradation. The MWP1 sheet was cut into chips of 5 × 10-3 m size, and these were used in some of the experimental runs. We have already reported the results of the degradation of the MWP1 sample which was dechlorinated at 593 K into fuel oil performed by batch operation (Sakata
S0888-5885(97)00939-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/16/1998
2890 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 1. Schematic diagram of the experimental setup.
et al., 1996a). It was confirmed that the thermal degradation behaviors of the MWP1 and Mix1 samples were very similar to each other, and we concluded that the Mix1 sample can be used as a model for real municipal waste plastics. The experimental apparatus (Figure 1) and procedure were the same as those previously reported (Sakata et al., 1996a,b, 1997). Experimental runs were carried out in a transparent glass reactor using 50 × 10-3 kg of sample plastic by batch operation. A special, transparent glass furnace was used for heating, and the temperature was monitored and controlled with the thermocouple TC1. The sample was first heated to 393 K and held there for 3.6 × 103 s in a N2 flow in order to remove the physically absorbed water from the sample. The N2 flow was then cut off, and the sample was heated to 573-593 K at a heating rate of (0.05-0.25 K/s) and held at that temperature for a long time. The outlet of the reactor was connected to a water cooling condenser in order to condense the liquid products. The liquid products were collected in a graduated cylinder. Gaseous products passed through a water seal pot were finally collected in a Teflon bag. The hydrogen chloride evolved from the degradation of PVC was trapped in a flask containing an aqueous solution of NaOH, and the amount of HCl trapped was determined by the titration method. The transparent glass wall of the electric heater made it possible to observe the changes in the level of the bed of melted plastic and froth inside the reactor against the course of time and captured in a 8 mm video. Results and Discussion Investigation of the Conditions for the Spontaneous Degradation of Waste Plastics during the Dechlorination Treatment. As shown in Table 2, the experiments were repeated 10 times and conditions for the occurrence of the unusual reaction were investigated.
For run 1 with MWP1 and run 2 with Mix1, the reactor temperature was increased from the sample drying temperature 393 to 573 K with a heating rate of 0.05 K/s. In both cases, melting of the sample plastics starts with a rise in the temperature at around 533 K; frothing with the generation of hydrogen chloride from PVC commenced, and the level of the liquid surface increased slowly. For example, an increase in the plastic melt level from 6.0 × 10-2 to 9.0 × 10-2 m was observed. When the reactor temperature reached the predetermined temperature 573 K, the plastic melt level started to decrease after a while and finally maintained a constant level. During this period the production of liquid was not observed. As the froth was mainly hydrogen chloride entrapped in the alkali solution, no accumulation of gaseous products in the Teflon bag was observed. During run 3 with Mix1, the temperature was rapidly increased from 393 to 573 K at a rate of 0.16 K/s compared with 0.05 K/s in runs 1 and 2. For run 3 the observation was almost the same as for run 2. In run 4, a mixture of PE (80%)-PVC (20%) was used for simplification instead of a Mix1 sample and the temperature was increased from 473 to 593 K at an even faster rate of 0.25 K/s (Sakata et al., 1996c). The results of observations for runs 1-4 were basically the same. There was no drastic generation of gas or froth, and no blowout of the liquid product out of the reactor was observed. The experimental conditions of run 5 were similar to those of run 4 except for the addition of 8.5 × 10-3 kg of kitchen aluminum foil in the form of chips of 10 × 10-3 m size. The experiment was planned bearing in mind the fact that the municipal waste contains some plastic film coated with aluminum foil used for food packaging. Figure 2 shows the level of the melted plastics inside the reactor plotted against the lapse of time for runs 4 and 5. During the run 5 experiment, a drastic frothing was observed at around 573 K and the mixture of melted plastics and carbonaceous particles was blown out of the reactor. In addition, liquid products were accumulated in the graduated cylinder and gaseous products other than HCl were also produced in huge quantities and filled up the Teflon bag. The composition of the gaseous products was found to be C3 component (10 wt %), C4 component (60 wt %), C5 component (30 wt %), and a small amount of hydrogen and methane. The production of gaseous products containing C3-C5 hydrocarbons indicates that the degradation of plastics into gaseous products (gasification) occurred. In other words, the temperature in the reactor might have exceeded the predetermined set temperature, resulting in localized heating which was sufficient enough to trigger the unusual gasification reaction. The following reaction between aluminum (Al) and hydrogen chloride (HCl) shown below can be suggested to be responsible for the localized heating (heat of reaction):
Al(s) + 3HCl(g) f AlCl3(g) + 3/2H2(g) ∆H ) -321.3 kJ/mol (1) 2Al + 6HCl f Al2Cl6 + 3H2
(2)
The predicted product anhydrous aluminum chloride (having a melting point of 463 K at 253 × 105 Pa and a
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2891 Table 2. List of Conditions for the Test of the Occurrence of Spontaneous Degradation of Waste Plastics during Dechlorination Treatment run no.
sample amount (10-3 kg)
PVC content (wt %)
Al content (10-3 kg)
Al foil size (10-3 m)
1 2 3 4 5
MWP1 (50) (fluff) Mix1 (50) Mix1 (50) PE + PVC (50) PE + PVC (50)
5 8 8 20 20
unknown 0 0 0 8.5
unknown
10
6 7 8 9
Mix1 (50) MWP1 (50) (chips) Mix1 (50) Mix1 (50)
8 5 8 8
0.5 0.3 2.0 2.0
5 unknown 5 5
Mix1 (50)
8
2.0
5
10 a
rate of heating 393 K (0.05 K/s) 573 K 393 K (0.05 K/s) 573 K 393 K (0.16 K/s) 573 K 393 K (0.08 K/s) 423 K (0.16 K/s) 473 K (0.25 K/s) 593 K 393 K (0.08 K/s) 423 K (0.16 K/s) 473 K (0.25 K/s) 593 K spontaneous reaction occurred at around 573 Ka 393 K (0.08 K/s) 423 K (0.16 K/s) 473 K (0.25 K/s) 593 K 393 K (0.08 K/s) 423 K (0.16 K/s) 473 K (0.25 K/s) 593 K 393 K (0.08 K/s) 423 K (0.16 K/s) 473 K (0.25 K/s) 593 K 393 K (0.05 K/s) 593 K;’ sample arrangement: PVC 4 × 10-2 kg/Al foil 2 × 10-2 kg/remainder of plastics, total three layers spontaneous reaction occurred at around 573 Ka 393 K (0.05 K/s) 593 K; sample arrangement: uniform mixing of Al and plastics
Spontaneous degradation of plastics occurred with the production of waxlike compounds.
Figure 2. Change in plastic melt level (PE + PVC) inside the reactor with the lapse of time and set temperature for runs 4 and 5.
boiling point of 455.7 K at 100.3 × Pa) can exist as a stable dimer Al2Cl6 below 673 K (Kubo et al., 1987). Run 6 was carried out by (1) using multicomponent model mixed plastics (Mix1), (2) reducing the PVC content from 10 × 10-3 to 4 × 10-3 kg, and (3) reducing the size of the Al strip from 10 × 10-3 m to 5 × 10-3 m size. An increase in the level of liquid and froth was observed with the rise of temperature; however, the blowout of the liquid products from the reactor was not observed. After the experimental run, carbonaceous particles, probably produced from the PVC degradation, were found floating in the wax. Although the rate of heating was comparatively high (0.25 K/s), the actual temperature of the sample plastics inside the reactor increased with some lag from the set temperature. Run 7 was carried out using the MWP1 (Al content: 0.64%) in the form of thin chips of 5 × 10-3 m size; however, the unusual reaction was not observed. Considering that the Al concentration in runs 6 and 7 were not high enough to cause the unusual reaction, run 8 was carried out using 50 × 10-3 kg of the Mix1 sample with the addition of 2 g of Al foil (5 × 10-3 m). In this case also the unusual reaction did not occur. Based on the results of previous runs, the experimental method of run 9 was redesigned with a new approach. This run was planned to investigate the effect of the mixing mode of the sample plastics and Al foil, such as layer by layer mixing instead of uniform mixing. 103
Figure 3. Change in plastic melt level (Mix1) inside the reactor with the lapse of time and set temperature for runs 9 and 10.
In other words, a total of 50 × 10-3 kg of the Mix1 sample and 2 g of Al were used as in run 8, but first of all 4 × 10-3 kg of PVC (8% of the plastic) was placed at the bottom portion of the reactor followed by 2 × 10-3 kg of Al foil (5 × 10-3 m), and finally the rest of the 46 × 10-3 kg plastic mixture (excluding PVC) was placed on the top, forming a total of three layers. In order to carry out the heating of the sample along with the melting of the plastics, the temperature in the reactor was increased slowly from 293 to 593 K at a rate of 0.05 K/s. As a result, the melted plastics sample started expanding at around 573 K and the level of liquid surface at the top portion of the reactor content increased (Figure 3). During this period no evolution of bubbles from the bottom was observed. However, at around 579 K the environment of the reactor inside changed dramatically: The upper part of the reactor was filled with brown gas and at the same time gaseous products were generated vigorously. Subsequently, the transparent reactor wall was blackened and 1.0 × 10-6 m3 of waxlike liquid products accumulated in the graduated cylinder. For run 9, it can be thought that the melted plastics acted as a lid on the gas produced in the bottom layer from PVC and the pressure buildup of the gaseous products finally blew up the liquid products in the top layer. However, a detailed observation of the reactor inside by a video camera suggested that the spontaneous reaction did not occur instantaneously; i.e., it was not
2892 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
a simple physical blowup of the plastic melt by the buildup of gas pressure only. In conclusion, it is suggested that the reason for the occurrence of spontaneous reaction is the formation of “hot spots” (localized heating) originating from the heat of reaction between HCl and Al which eventually triggered the drastic thermal degradation of melted plastics. In order to confirm the effect of localized heating, run 10 was carried out under conditions similar to run 9 except that the pieces of Al foil were uniformly mixed with the plastic sample instead of being placed in a layerwise fashion (PVC/Al/other plastics). In this case the uncontrollable reaction did not occur, as was predicted. Conditions for the Occurrence of the Unusual Reaction. On the basis of the results of the above 10 experiments, the possible conditions for the occurrence of the unusual reaction during the dechlorination treatment of municipal waste plastics can be summarized as follows: (1) A temperature above 573 K. (2) Coexistence of hydrogen chloride and Al foil. (3) Decrease in viscosity of the plastic melt. Concluding Remarks. In conclusion, the main reason for the occurrence of an uncontrollable explosionlike reaction, the subject matter of this study, is the heat of reaction between HCl and metallic Al forming hot spots in the reactor. This suggests that the detection and exclusion of aluminum or aluminum compounds at the inlet of the reactor is absolutely necessary. A similar reaction may occur with metallic Fe and HCl to form FeCl3, and experiments for the confirmation of this speculation will be conducted in the near future. Nomenclature ABS: acrylonitrile-butadiene-styrene copolymer MWP: municipal waste plastics
PE: polyethylene PET: poly(ethylene terephthalate) PP: polypropylene PS: polystyrene PVC: poly(vinyl chloride) PVDC: poly(vinylidene chloride)
Literature Cited Blazso, M.; Zelei, B.; Jakab, E. Thermal decomposition of lowdensity polyethylene in the presence of chlorine-containing polymers. J. Anal. Appl. Pyrolysis 1995, 35, 221-235. Kubo, R., Nagakura, S., Iiguchi, H., Ezawa, H., Eds. In Rikagaku Jiten; Iwanami: Tokyo, 1987; Vol. 4. Sakata, Y.; Uddin, M. A.; Koizumi, K.; Narazaki, M.; Murata, K.; Kaji, M. Dechlorination of Municipal Waste Plastics and Thermal Degradation into Fuel Oil. Proceedings of the 7th Research Meeting of the Japanese Society of Waste Management Experts, Fukuoka, Japan, 1996; The Japan Society of Waste Management Experts: Tokyo, 1996a; pp 378-380. Sakata, Y.; Uddin, M. A.; Koizumi, K.; Murata, K. Catalytic Degradation of Polypropylene into Liquid Hydrocarbons Using Silica-Alumina Catalyst. Chem. Lett., 1996b, 245-246. Sakata, Y.; Uddin, M. A.; Koizumi, K.; Murata, K. Thermal Degradation of Polyethylene mixed with Poly(vinyl chloride) and Poly(ethylene terephthalate). Polym. Degrad. Stab. 1996c, 53, 111-117. Sakata, Y.; Uddin, M. A.; Muto, A.; Koizumi, K.; Kanada, Y.; Murata, K. Catalytic Degradation of Polyethylene into Fuel Oil over Mesoporous Silica (KFS-16) Catalyst. J. Anal. Appl. Pyrolysis 1997, 43, 15-25.
Received for review December 30, 1997 Revised manuscript received May 11, 1998 Accepted May 11, 1998 IE9709392
