Thermal degradation of plastic polymers is becoming an increasingly important method for the conversion of plastic materials into valuable chemicals and oil products. In this work, virgin high-density polyethylene (HDPE) was chosen as a material for pyrolysis. A simple pyrolysis reactor system has been used to pyrolyse virgin HDPE with an objective to optimize the liquid product yield at a temperature range of 400°C to 550°C. The chemical analysis of the HDPE pyrolytic oil showed the presence of functional groups such as alkanes, alkenes, alcohols, ethers, carboxylic acids, esters, and phenyl ring substitution bands. The composition of the pyrolytic oil was analyzed using GC-MS, and it was found that the main constituents were n-Octadecane, n-Heptadecane, 1-Pentadecene, Octadecane, Pentadecane, and 1-Nonadecene. The physical properties of the obtained pyrolytic oil were close to those of mixture of petroleum products.
Plastic materials comprise a steadily increasing proportion of the municipal and industrial waste going into landfill. Owing to the huge amount of plastic wastes and environmental pressures, recycling of plastics has become a predominant subject in today’s plastics industry. Development of technologies for reducing plastic waste, which are acceptable from the environmental standpoint and are cost-effective, has proven to be a difficult challenge because of the complexities inherent in the reuse of polymers. Establishing optimal processes for the reuse/recycling of plastic materials, thus, remains a worldwide challenge in the new century. Plastic materials find applications in agriculture as well as in plastic packaging, which is a high-volume market owing to the many advantages of plastics over other traditional materials. However, such materials are also the most visible in the waste stream and have received a great deal of public criticism as solid materials have comparatively short life-cycles and usually are nondegradable.
Thermal cracking, or pyrolysis, involves the degradation of the polymeric materials by heating in the absence of oxygen. The process is usually conducted at temperatures between 500 and 800°C and results in the formation of a carbonized char and a volatile fraction that may be separated into condensable hydrocarbon oil and a noncondensable high calorific value gas. The proportion of each fraction and its precise composition depend primarily on the nature of the plastic waste and on process conditions as well.
In pyrolytic processes, a proportion of species generated directly from the initial degradation reaction are transformed into secondary products due to the occurrence of inter- and intramolecular reactions. The extent and the nature of these reactions depend both on the reaction temperature and also on the residence of the products in the reaction zone, an aspect that is primarily affected by the reactor design.
In addition, reactor design also plays a fundamental role, as it has to overcome problems related to the low thermal conductivity and high viscosity of the molten polymers. Several types of reactors have been reported in the literature, the most frequent being fluidized bed reactors, batch reactors, and screw kiln reactors [
Characteristics of thermal degradation of heavy hydrocarbons can be described with the following items. High production of C1s and C2s in the gas product. Olefins are less branched. Some diolefins made at high temperature. Gasoline selectivity is poor; that is, oil products have a wide distribution of molecular weight. Gas and coke products are high. Reactions are slow compared with catalytic reactions.
High-density polyethylene (HDPE) is the third-largest commodity plastic material in the world, after polyvinyl chloride and polypropylene in terms of volume. It is a thermoplastic material composed of carbon and hydrogen atoms joined together forming high-molecular-weight products. The effect of temperature and the type of reactor on the pyrolysis of HDPE has been studied, and some of the results are reviewed.
Wallis and Bhatia have done the thermal degradation of high-density polyethylene in a reactive extruder at various screw speeds with reaction temperatures of 400°C and 425°C. A continuous kinetic model was used to describe the degradation of the high-density polyethylene in the reactive extruder. It was found that purely random breakage and a scission rate which had a power-law dependence on molecular size of 0.474 best described the experimental data. The greatest discrepancy between the model prediction and the experimental data was the large molecular size region at short residence times; however, this only accounted for a very small percentage of the total distribution and was attributed to the presence of fast initiation reaction mechanism that was only significant at low conversions [
Conesa et al. studied the production of gases from polyethylene (HDPE) at five nominal temperatures (ranging from 500°C to 900°C) using a fluidized sand bed reactor. HDPE primary decomposition and wax cracking reactions take place inside the reactor. Yields of 13 pyrolysis products (methane, ethane, ethylene, propane, propylene, acetylene, butane, butylenes, pentane, benzene, toluene, xylenes, and styrene) were analyzed as a function of the operating conditions. From the study of HDPE pyrolysis in a fluidized sand bed reactor, they have found that the yield of total gas obtained increases in the range 500°C–800°C from 5.7 to 94.5%; at higher temperatures, the yield of total gas decreases slightly; the formation of methane, benzene, and toluene is favored by high residence times, but ethane, ethylene, propane, propylene, butane, butylenes, and pentane undergo cracking to different extents at increasing residence times and/or temperature; and the maximum yield of total gas obtained at 800°C from HDPE pyrolysis is 94.5% with the following composition: 20% methane, 3.8% ethane, 37% ethylene, 0.2% propane, 4.7% propylene, 0.3% butane, 0.4% butylenes, 2.2% pentane, 24% benzene, 2.1% toluene, 0.01% acetylene, and 0.02% xylenes and styrene [
Walendziewski and Steininger reported the thermal degradation of polyethylene in the temperature range 370–450°C. In the case of thermal degradation of polyethylene, an increase in degradation temperature led to an increase of gas and liquid products, but a decrease of residue (boiling point > 360°C). However, the increase of gas was not too large as compared to the sharp decrease of residue with increase of temperature. The result of analysis of gas products obtained by the pyrolysis of polyethylene at 400°C is summarized in Table
Composition of gas products obtained from pyrolysis of polyethylene at 400°C [
Component | Thermal | Catalytic | Hydrocracking |
---|---|---|---|
Methane | 22.7 | 12.4 | 21.1 |
Ethane | 27.4 | 20.4 | 21.2 |
Ethylene | 1.4 | 2.3 | 0.1 |
C3 | 26.6 | 30.4 | 23.7 |
C4 | 11.0 | 20.3 | 20.7 |
C5 | 6.9 | 5.6 | 7.3 |
C6 | 2.1 | 3.3 | 3.8 |
Walendziewski carried out two series of experiments of waste polymers cracking. The first series of polymer cracking experiments was carried out in glass reactor of 0.5 dm3 volume at atmospheric pressure and in a temperature range 350–420°C, the second one in autoclaves under hydrogen pressure (
Walendziewski carried out the experiments of waste polymers cracking in a continuous-flow tube reactor. The main components of the reactor unit were a screw extruder as a waste plastics feeder and a tube cracking reactor equipped with an internal screw mixer. Cracking process was realized at the temperature range 420–480°C and raw material feeding rate from 0.3 up to 2.4 kg/h. The principal process products, gaseous and liquid hydrocarbon fractions, are similar to the refinery cracking products. They are unstable due to their high olefins content (especially from polystyrene cracking), and their chemical composition and properties strongly depend on the applied feed composition, that is, shares of polyethylene, polypropylene, and polystyrene. The material balance experiments showed that the main products, liquid or solid materials in ambient temperature, contain typically 20–40% of gasoline fractions (range of boiling point 35–180°C) and 60–80% of light gas oil fractions (initial boiling point > 180°C). The solid carbon residues are similar to coal cokes and even contain 50% mineral components. Their calorific values attain 20 MJ/kg and they are solid products of quality similar to brown coals [
A number of studies have been reported in which a range of catalysts and reaction conditions have been employed to convert waste plastics into the hydrocarbon liquid using pyrolysis during the past four decades. The most commonly used catalysts in the catalytic degradation of high-density polyethylene are solid acids (zeolite, silica-alumina) [
This work focuses on characterization of liquid product obtained from thermal pyrolysis of virgin high-density polyethylene at different temperature ranges. Thermal pyrolysis of high-density polyethylene pellets was done in a semibatch reactor at a temperature range of 400°C to 550°C and at a heating rate of 20°C/min. The effect of pyrolytic temperature on reaction time, liquid yield, and volatiles was also studied. The obtained liquid product was characterized by different physical and chemical properties using GC-MS and FTIR.
HDPE pellets (2.5 mm in size) obtained from Reliance Industries Ltd., India, with density 0.945 g/cc3, Melt Flow Index (MFI) value 0.2–15 g/10 min−1 (at 190°C and 2.16 kg load), and melting point 133°C were used for experiments. These plastic pellets were used directly in the thermal pyrolysis reaction. The proximate analysis of HDPE pellets was done by ASTM D3173-75 and ultimate analysis was done using CHNS analyzer (Elementar Vario El Cube CHNSO). Calorific value of the raw material was found by ASTM D5868-10a.
Thermogravimetric analysis of the HDPE sample was carried out with a Shimadzu DTG-60/60H instrument. A known weight of the sample was heated in a silica crucible at a constant heating rate of 293 K/min operating in a stream of nitrogen with a flow rate of 40 mL/min from 32°C to 700°C.
The pyrolysis setup consists of a semibatch reactor made of stainless-steel tube (length: 145 mm, internal diameter: 37 mm, and outer diameter: 41 mm) sealed at one end and an outlet tube at other end as shown in the previous study [
Fourier transform infrared spectroscopy (FTIR) of the pyrolysis oil obtained at optimum condition was taken in a Perkin-Elmer Fourier transformed infrared spectrophotometer with resolution of 4 cm−1, in the range of 400–4000 cm−1 using Nujol mull as reference to know the functional group composition. The components of liquid product were analyzed using GC-MS-QP 2010 (Shimadzu) using flame ionization detector. The GC conditions, column oven temperature progress, column used, and MS conditions are given in the Table
GC-MS conditions.
GC-MS-OP 2010 Shimadzu | ||
---|---|---|
GC conditions | ||
| ||
Column oven temperature | 70°C | |
Injection mode | Split | |
Injection temperature | 200°C | |
Split ratio | 10 | |
Flow control mode | Linear velocity | |
Column flow | 1.51 mL/min | |
Carrier gas | Helium 99.9995% purity | |
| ||
Column oven temperature progress | ||
Rate | Temperature (°C) | Hold time (min) |
| ||
— | 70 | 2 |
10 | 300 | 7.0 |
| ||
Column: DB-5 | ||
| ||
Length | 30.0 m | |
Diameter | 0.25 mm | |
Film thickness | 0.25 µm | |
| ||
MS conditions | ||
| ||
Ion source temperature | 200°C | |
Interface temperature | 240°C | |
Start |
40 | |
End |
1000 |
The proximate and ultimate analyses of virgin HDPE sample are shown in Table
Proximate and ultimate analyses of virgin HDPE.
Properties | Virgin HDPE |
---|---|
Proximate analysis | |
Moisture content | 0.00 |
Volatile matter | 99.92 |
Fixed carbon | 0.00 |
Ash content | 0.08 |
Ultimate analysis | |
Carbon (C) | 83.29 |
Hydrogen (H) | 13.93 |
Nitrogen (N) | 0.20 |
Sulphur (S) | 0.07 |
Oxygen (O)/others | 2.51 |
GCV (MJ/kg) | 47.64 |
Thermogravimetric analysis (TGA) is a thermal analysis technique which measures the weight change in a material as a function of temperature and time in a controlled environment. This can be very useful to investigate the thermal stability of a material or to investigate its behavior in different atmospheres (e.g., inert or oxidizing). TGA is applied in determination of the study of thermal stability/degradation of virgin HDPE in various ranges of temperature.
From the TGA curve as shown in Figure
TGA and DTG curves of virgin HDPE.
The pyrolysis of virgin HDPE yielded four different products, that is, oil, gas, wax, and residue or coke. The distributions of these fractions are different at different temperatures and are shown in Table
Distribution of different fractions at different temperatures in thermal pyrolysis of virgin HDPE.
Temperature (°C) | Oil (wt%) | Residue (wt%) | Wax (wt%) | Gas/volatile (wt%) | Reaction time (min) |
---|---|---|---|---|---|
400 | 31.3 | 5.65 | 7.7 | 45.35 | 680 |
450 | 52.46 | 3.95 | 8.9 | 34.69 | 175 |
500 | 44.32 | 1.29 | 28.99 | 25.4 | 80 |
550 | 8.83 | 0.68 | 52.02 | 38.47 | 50 |
The condensable oil/wax (a mixture of alkanes that falls within the
The effect of temperature on the reaction time of the reaction for the pyrolysis of virgin HDPE plastic is shown in Figure
Effect of temperature on reaction time and product distribution.
Fourier transform infrared spectroscopy (FTIR) is an important analysis technique which detects various characteristic functional groups present in oil. On interaction of an infrared light with oil, chemical bond will stretch, contract, and absorb infrared radiation in a specific wave length range regardless of the structure of the rest of the molecules. Figure
FT-IR spectrometry of virgin HDPE pyrolytic oil.
The GC-MS analysis of the oil sample obtained by the thermal pyrolysis of virgin HDPE was carried out to know the compounds present in the oil (Figure
GC-MS analysis of virgin HDPE pyrolytic oil.
R. time (min) | Area % | Name of compound | Molecular formula |
---|---|---|---|
6.301 | 1.24 | 1-Decene | C10H20 |
6.450 | 1.12 | Decane | C10H22 |
8.105 | 2.04 | 1-Undecene | C11H22 |
8.238 | 1.78 | n-Undecane | C11H22 |
9.735 | 3.50 | 1-Dodecanol | C12H26O |
9.855 | 3.19 | n-Dodecane | C12H26 |
11.541 | 4.62 | 1-Tridecene | C13H26 |
12.615 | 5.30 | 1-Tetradecene | C14H28 |
12.711 | 4.82 | Tetradecane | C14H30 |
12.772 | 0.65 | 7-Tetradecene | C14H28 |
13.909 | 5.40 | 1-Pentadecene | C15H30 |
13.997 | 5.13 | Pentadecane | C15H32 |
15.039 | 0.48 | 1,19-Eicosadiene | C20H38 |
15.130 | 5.36 | 1-Hexadecene | C16H32 |
15.210 | 5.60 | n-Octadecane | C18H36 |
15.261 | 0.51 | Cyclohexadecane | C16H32 |
16.203 | 0.49 | 1,19-Eicosadiene | C20H38 |
16.283 | 5.09 | 1-Nonadecene | C19H38 |
16.357 | 5.52 | n-Heptadecane | C17H36 |
16.406 | 0.51 | 1-Heptadecene | C17H34 |
17.378 | 4.43 | 1-Octadecene | C18H36 |
17.447 | 5.47 | Octadecane | C18H38 |
17.493 | 0.69 | 1-Octadecene | C18H36 |
18.419 | 3.26 | 1-Nonadecene | C19H38 |
18.482 | 4.67 | Nonadecane | C19H40 |
GC plot of oil obtained at 450°C.
Table
Physical properties analysis of virgin HDPE pyrolytic oil.
Tests | Results obtained | Test method |
---|---|---|
Specific gravity at 15°C/15°C | 0.8013 | IS:1448 P:16 |
Density at 15°C in kg/cc | 0.8006 | IS:1448 P:16 |
Kinematic viscosity at 40°C in Cst | 3.3 | IS:1448 P:25 |
Kinematic viscosity at 100°C in Cst | 1.4 | IS:1448 P:25 |
Conradson carbon residue | <0.01% | IS:1448 P:122 |
Flash point by Abel method | 10°C | IS:1448 P:20 |
Fire point | 15°C | IS:1448 P:20 |
Cloud point | 28°C | IS:1448 P:10 |
Pour point | 18°C | IS:1448 P:10 |
Gross calorific value in MJ/kg | 44.27 | IS:1448 P:6 |
Sulphur content | 0.03% | IS:1448 P:33 |
Calculated Cetane Index (CCI) | 70 | IS:1448 P:9 |
Distillation | IS:1448 P:18 | |
Initial boiling point | 72°C | |
Final boiling point | 364°C |
From comparison with other transportation products as shown in Table
Product properties comparison of HDPE pyrolytic oil with commercial transportation products.
Properties |
Specific gravity 15°C/15°C | Kinematic viscosity at 40°C (cst) | Flash point (°C) | Pour point (°C) | GCV (MJ/kg) | IBP (°C) | FBP (°C) | Chemical formula |
---|---|---|---|---|---|---|---|---|
HDPE pyrolytic oil | 0.8013 | 3.3 | 10 | 18 | 44.27 | 72 | 364 | C10–C20 |
Waste HDPE pyrolytic oil [ |
0.7835 | 1.63 | 1 | −15 | 42.81 | 82 | 352 | C19–C24 |
Gasoline [ |
0.72–0.78 | — | −43 | −40 | 42–46 | 27 | 225 | C4–C12 |
Diesel [ |
0.82–0.85 | 2–5.5 | 53–80 | −40 to −1 | 42–45 | 172 | 350 | C8–C25 |
Biodiesel [ |
0.88 | 4–6 | 100–170 | −3 to 19 | 37–40 | 315 | 350 | C12–C22 |
Heavy product oil [ |
0.94–0.98 | >200 | 90–180 | — | −40 | — | — | — |
Thermal pyrolysis of virgin HDPE was performed in a semibatch reactor made up of stainless steel at temperature range from 400°C to 550°C and at a heating rate of 20°C/min. The liquid yield is highest at 450°C, highly volatile products are obtained at low temperature, and the products obtained at 500°C and 550°C are viscous liquid and wax. Reaction time decreases with increase in temperature. The functional group present in the virgin HDPE pyrolytic oil is similar to the other plastic pyrolytic oils given in several literatures. It was found that the pyrolytic oil contains around 25 types of compounds having carbon chain length in the range of C10–C20. The physical properties of pyrolytic oil obtained were in the range of other pyrolytic oils and moderate-quality products. It has been shown that a simple batch pyrolysis method can convert virgin HDPE to liquid hydrocarbon products with a significant yield which varies with temperature.