Chemcon 2013 th
66 Annual Session of Indian Institute of Chemical Engineers Hosted at Institute of Chemical Technology, Mumbai 400 019 27-30 December 2013
Studies on Pyrolysis of Plastic Wastes Polyethylene to Valuable Hydrocarbons to Minimize Plastic Wastes Load to Environment P. Gaurh1 and H. Pramanik2 1,2
Department of Chemical Engineering & Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi -221005, India Email addresses:
[email protected],
[email protected] Abstract: The entire world is now facing tremendous environmental problem due to the increased use of polyethylene in daily lifestyle. Polyethylene is the easily available Plastic material which is used primarily in packaging e.g., plastic bag, plastic films, geomembranes, containers including bottles, etc. The increasing polyethylene load on the earth has led to a number of governments introducing new legislation for its recovery as a resource. Accumulation of enormous amounts of plastic waste produced all over the world has negative impacts on the environment. Pyrolysis of polyethylene waste is a suitable option in converting this waste into economically valuable hydrocarbons, which can be used either as fuels or as feed stock in the petrochemical industry. In this perspective polyethylene waste was collected from Varanasi city municipality dumping zone and converted to valuable hydrocarbons via pyrolysis. In this study, both the catalytic and thermal pyrolysis were performed. The temperature range of study for both the processes were 400-600 oC. The catalyst ZSM-5 of 80:1 and 30:1 (Silica/Alumina) were used. The maximum hydrocarbon yield of 84 % was obtained at a temperature of 600 oC for ZSM-5 (30:1) using plastic to catalysts ratio of 30:1. Whereas, ZSM-5 (80:1) using same condition produced hydrocarbon yield of 81%. The thermal process produced maximum yield of 72% at a temperature of 600 oC. Key Words: Polyethylene, waste, pyrolysis, hydrocarbon 1.
Introduction
Plastics have become indispensable in our society today and the world at large. Due to their light weight, durability, energy efficiency, coupled with a faster rate of production and design flexibility, these plastics are employed in entire gamut of industrial and domestic area [1]. It is mostly used in the production of sachet water bags and the way of controlling its wastes has been very difficult, this lead to pyrolysis which makes these wastes very useful in the production of hydrocarbons (fuel). In recent years the production and consumption of plastics have increased drastically; as a consequence the responsible disposal of plastic wastes has created serious social and environmental arguments. The aim of the paper is to highlight the different option to convert the plastic waste to liquid hydrocarbons using pyrolysis. Plastics are used in our daily lives in a number of applications. Due to the high thermal resistance of plastics, the rapid market changes and introduction of the open loop
recycling concept, energy recovery is limited and might be considered in the developmental stages, especially in the case of PSW [2]. Collection, processing, and marketing are each critical to the success of chemical recycling and energy recovery.
Figure 1 Plastic consumption by application (India)
Chemcon 2013 th
66 Annual Session of Indian Institute of Chemical Engineers Hosted at Institute of Chemical Technology, Mumbai 400 019 27-30 December 2013 Today use of different kind of plastics goods and their demand is increasing rapidly from domestic use to industrial applications also (Figure 1). It is growing at an annual rate of 22% annually. The polymers production has reached to 8.5 million tons in 2007. 1.1
Pyrolysis
The Pyrolysis process consists of the thermal degradation of the wastes in the absence of oxygen/air. In pyrolysis process, the polymeric materials are heated to high temperatures, so their macro-molecular structures are broken down into smaller molecules and a wide range of hydrocarbons are formed. These pyrolytic products can be divided into; a gas fraction, liquid fraction (paraffin’s, olefins, napthenes and aromatics), solid residues. [3] There are two types of pyrolysis – thermal cracking and catalytic cracking.
plastics such as PE and PP have already been tested extensively; the catalysts tested are mainly those used in the petrochemical refinery industry [6].The laboratory experimental set-up in these studies is a mostly flow reactor; it may be useful to distinguish between two modes of catalyst usage: ‘liquid phase contact’ and ‘vapour phase contact’. In ‘liquid phase contact’, the catalyst is contacted with melted plastics and acts mainly on the partially degraded oligomers from the polymer chains; in ‘vapour phase contact’, the polymer is thermally degraded into hydrocarbon vapours which are then contacted with the catalyst. The current project is developing for the production of liquid hydrocarbon fuel by the application of liquid phase contact catalytic cracking.
2.
Materials and Methods
2.1.
Raw Materials
1.1.1 Thermal Cracking Thermal cracking is a process in which hydrocarbons such as crude oil are subjected to high heat and temperature to break the molecular bond and molecular weight of the substance being cracked. This process is used to extract usable components knows as fraction, which are released during the cracking process. Four types of mechanisms of plastics pyrolysis have been proposed: (a) End-chain scission or de-polymerization. The polymer is broken up from the end groups successively yielding the corresponding monomers. (b) Random-chain scission. The polymer chain is broken up randomly into fragments of uneven length. (c) Chain-strippingElimination of reactive substitutes or side groups on the polymer chain, leading to the evolution of a cracking product on one hand, and a charring polymer chain on the other. (d) Cross-linking-Formation of a chain network, which often occurs for thermosetting polymers when heated [4,5] 1.1.2 Catalytic Cracking Catalytic cracking is the process of cracking in the presence of a catalyst. A number of experimental studies have been carried out by various researchers with the objective of improving liquid hydrocarbons yield from plastics pyrolysis by introducing suitable catalysts. Common
The average composition of municipal plastic waste (MPW) is shown in Figure 2.
Figure 2: The composition of municipal plastic waste Based on the composition of this average plastic mixture, a commercial polyethylene (PE) was used as raw materials in our experiments. The waste plastics used for the process consisted mainly of LDPE products in the form of used plastic that are for carrying things.The polyethylene waste was chosen from roads and dustbins. The sample that was collected was subjected to cutting by using scissors manually. This was done to increase the surface area of contact of the material during melting process. A small particle size was chosen to reduce the effect of heat transfer.
Chemcon 2013 th
66 Annual Session of Indian Institute of Chemical Engineers Hosted at Institute of Chemical Technology, Mumbai 400 019 27-30 December 2013 2.2
Experimental Method
Pyrolysis or cracking processes break down polymer chains into useful lower molecular weight compounds. This can be achieved by the application of heat at atmospheric pressure in the absence of oxygen, which can be either thermal or catalytic cracking. Condenser
Silica Tube
Gas
2.2.2 Catalytic Cracking Same process is used in catalytic cracking as used in thermal cracking. In catalytic cracking we used ZSM-5 catalyst with plastic samples. Two types of catalyst are used for catalytic cracking: ZSM-5 (30:1 mole ratio) and ZSM-5 (80:1 mole ratio). Cracking process is been performed at different plastic to catalyst ratio (20:1, 30:1, 40:1).
Tubular Reactor
3. Ice Bath
Results and Discussion
3.1 Thermal cracking 3.1.1 Effect of Temperature on Yield
Condensate Liquid
Figure 3: Schematic Diagram of Experimental set-up Figure 3 shows the experimental set up for Pyrolysis of polyethylene. Reactor was made up of silica tube and it was connected with ice bath to condense the volatile hydrocarbons. The methods of experiments are mentioned in the next section 2.2.1 and 2.2.2.
Figure 4 shows the product yield from the thermal pyrolysis of the LDPE. It is evident that as the temperature is increased the percentage of gas increases. Burton et al, [1] stated that high temperature-high heating rate environments favour increased gas formation as the molecules breakdown and form a wide range of smaller organic molecules. In addition, with the higher amount of energy available at the higher temperature there is a tendency for an increased number of secondary reactions. The amount of oil and wax are decreased with an increase in temperature.
2.2.1 Thermal Pyrolysis First 20g of plastic sample was filled in a Silica tube (29.5 cm height and 21.2 cm diameter). Silica tube was completely closed at the end using cork and a pipe connecting from silica tube to condenser which was in ice bath. Silica tube containing plastic sample was placed inside the Quartz tubular reactor at different temperature (300-600oC) for 30min. Plastic sample was uniformly heated from all the sides of silica tube. The vapors coming from the silica tube were passed through the pipeline connected to the top of the tube from where it passed through a glass condenser inside ice-bath setup as shown in the Figure 3. Here condensation of the vapors takes place. Proper arrangement was made for the condensation by putting wet jute over the pipelines to support further condensation. Then the condensed liquids were collected. The non-condensable gases were very less.
Figure 4: Hydrocarbon yield for thermal cracking at 600 oC Liquid yield from thermal cracking at different temperature is shown in the Figure 4. On increasing temperature
Chemcon 2013 th
66 Annual Session of Indian Institute of Chemical Engineers Hosted at Institute of Chemical Technology, Mumbai 400 019
formation of liquid product slightly increases, waxy product decreases. On the other hand gaseous product firstly increases than it decreases, this is due to attaining desirable temperature for thermal cracking. Liquid yield, gaseous yield and total hydrocarbon yield was found 27.98%, 20.33% and 48.31% respectively. Liquid conversion in thermal cracking was found low because of low operating temperature. It can be further heated for getting better results.
% Liquid Yield
27-30 December 2013
3.2 Catalytic Cracking Catalytic cracking were conducted by using LDPE as the raw material and ZSM-5 as catalyst. The plastic was cracked catalytically at various temperature ranges (400ºC, 450ºC, 500ºC, 550ºC, 600ºC) and at different plastic to catalyst ratio (20:1, 30:1, 40:1). The products obtained were of different composition and the product yield was different for different temperatures.
% Yield
3.2.1 Effect of Plastic to Catalyst (ZSM-5 30:1) Ratio on Yield
90
Liquid
80
Gas
70
Wax
60
Total hydrocarbon
50
45 40 35 30 25 20 15 10 5 0
20:1 Plastic to Catalyst ratio 30:1 Plastic to Catalyst ratio 40:1 Plastic to Catalyst ratio
300
400 500 600 Temperature ( C)
700
Figure 6 : Hydrocarbon yied at 600 0C Liquid yield at different plastic to catalyst ratio(Catalyst ZSM-5, 30:1) Figure 5 shows the hydrocarbon yied at 600 oC for different plastic to catalyst ratio. The plastic to catalyst ratio of 30:1 gives better liquid yield (Figure 6). Where as wax content are less for all the ratio (20:1, 30:1, 40:1) at 600 oC. The hydrocarbon of high boiling point is produced more with the increase of temperature irrespective of temperature. Liquid, gaseous and total hydrocarbon yield for 30:1 plastic to catalyst ratio using ZSM-5 (30:1) was found 38.87%, 44.52% and 84% respectively. 3.2.2 3.2.1 Effect of Plastic to Catalyst (ZSM-5 80:1) Ratio on Yield
40 30 10 0 20.1 30.1 40.1 Plastic to Catalys t Ratio
Figure 5: Hydrocarbon yield at 600 oC for ZSM-5 (30:1) as catalyst
100 90 80 70 60 50 40 30 20 10 0
Liquid
% Yield
20
10.1
20.1 30.1 40.1 Plastic to catalyst ratio
Figure 7: Hydrocarbon yield at 600 oC for ZSM-5 (80:1) as catalyst
Chemcon 2013 th
66 Annual Session of Indian Institute of Chemical Engineers Hosted at Institute of Chemical Technology, Mumbai 400 019 27-30 December 2013 4. Conclusions
Figure 8 : Hydrocarbon yied at 600 0C Liquid yield at different plastic to catalyst ratio (Catalyst ZSM-5, 80:1) Figure 7 shows the hydrocarbon yied at 600 oC for different plastic to catalyst ratio using ZSM-5(80:1) as catalyst. The plastic to catalyst ratio of 20:1 gives better liquid yield (Figure 8). Where as wax content are less for all the ratio (20:1, 30:1, 40:1) at 600 oC. The hydrocarbon (HC) of high boiling point (liquid range HC) are produced more with the increase of temperature irrespective of temperature. Liquid, gaseous and total hydrocarbon yield for 20:1 plastic to catalyst ratio using ZSM-5 (80:1) was found 41%, 42% and 81% respectively.
3.3 Calorific Value Calorific value analysis has been done of the condensate liquids obtained from different plastic to catalyst ratio and at different temperature. CV of Plastic Sample = 10,818.398 Cal/g CV of Wax (thermal cracking) = 9504 Cal/g CV of Condensate liq (thermal cracking) =10,243 Cal/g CV of Condensate liq (Catalytic cracking)= 9,293 Cal/g
3.4 Flash & Fire point Flash point of condensate liquid Fire point of condensate liquid
= =
24.5 °C 25.5 °C
The waste plastic was subjected to various conditions of reactions starting from thermal cracking to catalytic cracking by using different ratio of feed to catalyst. The product obtained in each case varied from the other in terms of yield of liquid product or total hydrocarbon. The plastic which was catalytically cracked by using ZSM-5 as the catalyst, it gave quite good quantity and quality of liquid fuel. The highest yield was obtained when the feed to catalyst ratio was 30:1 for ZSM-5 (mole ratio 30:1). At this condition, it required a temperature of about 600°C and 25 minutes time. From the experiment the optimum value of feed to catalyst ratio was found to be 30:1. While using ZSM-5 (mole ratio 80:1), it also gave good quantity. But here optimum value for feed to catalyst ratio was found to be 20:1 at 600 °C. The liquid product obtained had a Calorific value of 9294 Cal/g and Flash & Fire point of 24.5-25.5 °C, which is quite good and falls in the range of gasoline fuel. From the graph plotted for ZSM-5, it can be observed that as the feed to catalyst ratio increases, the time of reaction decreases but the temperature required is more. Reference [1] Zadgaonkar, Alka, Waste plastics to liquid hydrocarbon fuel project, Management of plastics, polymer wastes and bio polymers and impact of plastics on the ecosystem, 2 (5) 2004. [2] J. DeGaspari, Mechanical Engineering Magagine (ASME), June 1999. [3] K. H. Lee, Thermal and catalytic degradation of pyrolytic oil from pyrolysis of municipal plastic wastes, Journal of Analytical and Applied Pyrolysis, 85 (1-2) 2009, 372-379. [4] W. Kaminsky, G.D. Andrews, P.M. Subramanian, Pyrolysis of polymers In Emerging Technologies in Plastics Recycling. ACS Symp. Ser. 513 1992, 60–70. [5] N. Miskolczia, L. Barthaa, G. Deak, B. Jover, Thermal degradation of municipal plastic waste for production of fuel-like hydrocarbons, Polymer Degradation and Stability 86 2004, 357-366. [6] A.G. Buekens, H. Huang, Catalytic plastics cracking for recovery of gasoline-range hydrocarbons from municipal plastic wastes, Resources, Conservation and Recycling 23 1998, 163-181. [7] A. Angya, N. Miskolczi, L. Bartha, I. Valkai, Catalytic cracking of polyethylene waste in horizontal tube reactor, Polymer Degradation and Stability 94 2009, 1678–1683.
