PLASTIC WASTE MANAGEMENTIntroduction to plasticsPlastics are the materials consisting of wide range of synthetic or semi synthetic organic compounds that are malleable and so can be moulded into different solid objects. Basically they are of high molecular mass and most of them are derived from petrochemicals. They are being used in large scale right from paper clips to spacecraft items because of their low cost, ease of manufacture, versatility and imperviousness to water.
In developed countries about one-third of plastics are being used in packaging, roughly the same in building applications and rest in automobiles, toys and furniture. In developing countries as in India, 42% are being used in packaging. In recent era usage of plastics has tremendously grown and global consumption of individual plastics is shown in below table.Table 1: Global consumption of individual plasticsPlastics Global consumption (%)Polyethylene 33.5Polypropylene 19.
5Polyvinylchloride 16.5Polystyrene 8.5Polyurethane 5.5Polyethylene terephthalate 5.5Styrene copolymers 3.5High performance and special plastics 13Plastic waste management 1Since 1960, use of plastics has increased dramatically and subsequently portion of our garbage made of plastic has also increased from 1% of total municipal solid waste stream to approximately 13%.
Another major concern about plastics in waste stream is their long life span and they are not bio degradable in nature. These wastes directly or indirectly make high impact on ecosystem, particularly on marine life at micro and macro scale.There are various techniques employed in managing the plastic waste and figure 1 depicts the same.
Figure 1: Plastic waste management technologyLand filling: Land filling of plastics result in the formation of carbon sink, which serves as a home for greenhouse gases and these results in environmental pollution. So nowadays land filling is not encouraged.Mechanical recycling: This is a method of reprocessing of used plastic into similar one. It is proved to be disadvantageous as cost involved in segregation, transportation is high and it lacks in durability.
Biological recycling: This refers to conversion of biodegradable polymers into biomass.Incineration: Burning of plastics to destruct it. Since plastics are made with petrochemical products, this results in carbon emissions along with carcinogen resulting in severe health disorders and environmental pollution. So this is not encouraged in most part of the world.The principle involved in incineration technique is employed in pyrolysis process. In the literature A K Panda et al.
showed that calorific value of polyethylene is similar to that of conventional fuels and same is depicted in table 2.Table 2: Calorific values of conventional fuels in comparison with plasticsFuel Calorific value(MJ/Kg)Polyethylene 43Gasoline 46Methane 53Fuel oil 43Coal 30Mixed plastics 30-40Municipal solid waste 10Thermo fuel system suitability for different plastics 1There are varieties of plastics but all plastics are not suitable for producing hydrocarbon fuel. Table 3 shows the thermo fuel system suitability for different plastics.Table 3: Thermo fuel system suitability for different plasticsPlastic Thermo fuel productionPolyethylene Very goodPolypropylene Very goodPolystyrene Very good and gives excellent fuel propertiesABS resin GoodPVC Not suitablePolyurethane Not suitableFRP Fair, pre-treatment is required to remove fibres.PET Not suitableConversion of polyethylene into hydrocarbon fuel through pyrolysis.
PolyethyleneIt is a mixture of similar polymers of ethene (ethylene) with various values of n in (C2H5)n. 80 million tonnes of polyethylene is produced annually and major application is in packaging. There are two types of polyethylene i.e. low density polyethylene (LDPE) and high density polyethylene (HDPE).Applications of polyethyleneContainersDispensing bottlesWash bottlesTubingPlastic bags for computer componentsNormal plastic bagsTraysCorrosion resistant work surfaceKetchup bottles.
Pyrolysis process 2Figure 2: Pyrolysis process setupThe experimental setup shown in figure 2 for pyrolysis process is shown by Cleetus et al. In construction it constitutes of reactor, condenser, nitrogen cylinder and catalytic cracker as main components. Polymer and catalyst at known ratio is taken and they are mixed well such that frictional heat generated is sufficient to remove moisture content. Now this mixture is taken into the reactor and nitrogen gas is introduced to provide inert atmosphere and before switching on the heater nitrogen gas is even passed to heater to remove oxygen content and to provide inert atmosphere. Now heater is switched on, polymer inside the reactor undergo thermal degradation in the presence of catalyst and in the absence of oxygen commonly known as pyrolysis process.
After some time vapours are evolved from the reactor and these vapours are made to pass into the condenser 1 of which water is circulated as a coolant at 28 degree centigrade. Low boiling fractions of vapour will be condensed in condenser 1and remaining uncondensed fraction will go to condenser 2 where water is circulated at 10 degree centigrade. This low temperature of water is maintained by accompanying refrigerator. Uncondensed portion of vapour from condenser 2 escapes into atmosphere.Factors affecting pyrolysis process and yield of fuel.The various factors influencing pyrolysis process is as follows:Type of reactorForm of plastic wasteCatalystPolymer to catalyst ratioCatalyst contact modeEffect of temperature and heating rateEffect of flow rate of nitrogen gasOperating pressureResidence timeType of reactors 3Hita et al. studied the types of reactor influencing pyrolysis process.
Stirred tank reactor: Figure 3 shows stirred tank reactor. Here reactants are continuously added and products are continuously withdrawn. Stirrer is employed for effective mixing of polymer and catalyst.Screw or rotary kiln reactor: Figure 4 shows screw or rotary kiln reactor.
This is employed in large scale operation. It was observed that higher oil yields were obtained at 550 degree centigrade with low gas yield. Usually employed for slow pyrolysis process.Fixed bed reactor: Figure 5 shows fixed bed reactor. It consists of cylindrical vessel packed with catalyst in pellet form. It is easy to design and operate. Inert ceramic balls are placed above the catalyst bed to distribute feed evenly.
The major disadvantage is regeneration and replacement of catalyst is difficult and plugging of bed due to coke deposition.Fluidised bed reactor: Figure 6 shows fluidised bed reactor. It contains catalyst material in a fluid state i.e.
catalyst is either suspended by inert liquid or by gas. Heat transfer is much better resulting in more uniform temperature compared to fixed bed reactor. Typically employed in fast and flash pyrolysis.
Figure 3: Stirred tankFigure 4: Screw or rotary kiln reactor Figure 5: Fixed bed reactorFigure 6: Fluidised bed reactor.Form of plastic waste 45Study was performed on the form of catalyst that needs to be employed in pyrolysis process. Zeatier et al. performed pyrolysis process using solid polyethylene with ZSM 5 catalyst and they obtained larger hydrocarbon molecules ranging from C6-C23. S L Wong et al. performed pyrolysis process using a mixture of polyethylene and benzene with ZSM 5 catalyst and they obtained liquid products containing nearly 90-97% of benzene and lighter hydrocarbons ranging from C4-C8. Thus the key step to obtain longer hydrocarbon chain is intermolecular hydrogen transfer i.
e. transfer of hydrogen from one polymer chain to another polymer chain.Catalysts 1Catalysts play a dominant role in pyrolysis process.
There are different kinds of catalyst which are employed by researchers to know its effect on pyrolysis process. Mainly catalyst emphasises on temperature, type of yield, operating pressure, range of carbon compounds obtained and rate of reaction.Different kinds of catalysts suggested by A K Panda et al. as followsZSM – 5: Yielded higher percentage of gaseous products than oil.Mordenite: Enhanced rate of reaction, produced C11-C13 paraffinREY Zeolite: Suitable for oil conversion with optimum gasoline yield.
Ultra Stable Y Zeolite: Products in gasoline range C3-C15 were observed.Silica Alumina: Liquid products with high yield (77-83%weight) within a range of C3-C15 were produced.Fluid catalytic cracking catalyst (FCC): Produced light hydrocarbon liquid with C6-C15.Synthesised fly ash catalyst: Decreased decomposition temperature and initiation time.Silica Alumina + Zeolite: Produced compounds ranging from C9-C19Lead Sulphide: Produced liquid, gaseous components and wax with nearly 100% efficiencyClay catalysts: More active when they are used above 600KPolymer to catalyst ratio 6Akpanduoh et al. showed the importance of polymer to catalyst ratio. It is important to maintain polymer to catalyst ratio.
They performed experiment using two catalyst with USY 20% and USY 40%. Results are tabulated as shown in table 4 and table 5.Table 4: For catalyst 20% USYPolymer to catalyst ratio Conversion (%) Liquid yield (%)1:1 98 662:1 99 764:1 99 896:1 99 73Table 5: For catalyst 40% USYPolymer to catalyst ratio Conversion (%) Liquid yield (%)1:1 94 412:1 95 664:1 93 78From the above tabular readings it is observed that optimum polymer to catalyst ratio that need to be maintained is 4:1. This proved to be same with research conducted by Cleetus C et al.Catalyst contact mode 27Scheries et al. performed an experiment to know the influence of catalyst contact mode.
There are two types of catalyst contact mode i.e. liquid phase contact and vapour phase contact.Liquid phase contact: Here polymer and catalysts are mixed together with catalyst in fluid state. Typically we make use of fluidised bed reactor.
Reaction is proceed with both polymer and catalyst inside the reactor.Vapour phase contact: Here polymers are burnt and vapours evolved are made to pass through catalyst and then they are taken into condenser 1.It was observed that yield not differed significantly with these two methods.Effect of temperature 48Operating temperature plays an important role in conversion of waste plastic to liquid hydrocarbon fuel. Cleetus et al. and Demirbas et al studied the temperature effect on liquid yield. Figure 7 shows the variation of temperature with conversion rate.
It is observed that as temperature increases conversion rate increased and it remained constant beyond 600 degree centigrade.Figure 8 shows the influence of temperature on liquid yield, it is observed that till 550 degree centigrade temperature increased linearly and after 550 degree centigrade as temperature increased liquid yield decreased.Figure 7: Plot relating temperature and conversion (%) Figure 8: Plot relating temperature and liquid yield (%)Future ScopeIf cheaper catalysts of optimal efficiency is employed, production costs can be decreased considerably.
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