Catalytic Conversion and Chemical Recovery

6 Catalytic Conversion and Chemical Recovery Sanaa Hafeez1, Elena Pallari2,3, George Manos4 and Achilleas Constantinou1,4 1

Division of Chemical & Petroleum Engineering, School of Engineering, London South Bank University, London, United Kingdom; 2Centre for Implementation Science, Health Service & Population Research Department, De Crespigny Park, Denmark Hill, King’s College London, London, United Kingdom; 3Institute of Pharmaceutical Sciences, Academic Centre, Franklin-Wilkins Building, King’s College London, London, United Kingdom; 4Department of Chemical Engineering, University College London, London, United Kingdom

6.1 Introduction A polymer is a large molecule made-up of repeated subunits called monomers (meaning only one part), that form a longer chain-like structure (macromolecule). The reaction where polymers are formed from monomers and/or co-monomers, usually through covalent chemical bonding, is known as polymerization. The structure of the polymer can differ depending on the monomers’ structure, such as ethylene and propylene, thus giving rise to its variable applications, whether biological or synthetic polymer. Examples include thermoplastics, polyethylene bags, containers, and polystyrene (PS) cups. However, unlike natural or biological polymers, synthetic macromolecules are not biodegradable, meaning they cannot be assimilated by microorganisms. Since their introduction in the 1950s, the production and use of plastics has been rapidly accelerating, outpacing any other manufactured material [1]. The commercial success of plastics comes down to its wide use, and hence, increase in production demand by the packaging industry (39.9%), as well as the sports segment medical and healthcare fields (22.4%), the building and construction sectors (19.7%), the transportation and aircraft industries (8.9%), their applications in electrical and electronic appliances (5.8%), and agriculture (3.3%). The European plastics industry is estimated to be the second largest plastics producer (18.5%) after China (27.8%), with 49 million tons of plastic produced in 2015. Consequently, the plastic solid waste (PSW) generation and disposal is trending, while there is a slow increase in their recycling or incineration rates (18% and 24% of nonfiber generated waste in 2014). Since plastic production started, it is estimated that 6300 metric tons (Mt) of

plastic waste have been produced up to the year 2015, of which 12% were incinerated, 9% were recycled, with more than 10% recycled more than once, while 60% were discarded in the environment or in landfills [2]. It was demonstrated that most European countries chose the landfill option over recycling or energy recovery unless there was a landfill disposal ban in place as in Germany, Denmark, and the Netherlands. In 2014, 30.8% of 25.8 million tonnes of plastic that ended in the waste streams of Europe was landfilled, 39.5% was recovered, and 29.7% was recycled [2]. The importance of a municipal solid waste (MSW) disposal policy has been discussed, as it can bring benefits by reducing health and environmental concerns associated with landfilling, such as ground water contamination, fire and explosion risks, or sanitary problems. Furthermore, by placing emphasis on the use of feedstock, energy recovery and other treatment methods of waste valorization toward recovering energy or valuable products from waste, brings about economic and societal benefits. Plastic polymers are the largest contributors to waste, and this has continued to increase due to an expansion in the variety of plastic products currently in operation. Thermosets and thermoplastics are the two most common types of plastics. However, thermoplastics make up approximately 80% of the plastics utilized in Western Europe. This may be due to the susceptibility to molding of the plastic under the application of heat, which can allow thermoplastics to be used for a larger variety of purposes. The six most prevalent plastics in European PSW are polypropylene (PP), PS, high density polyethylene (HDPE), low density polyethylene (LDPE), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). The largest constituents

Plastics to Energy. DOI: https://doi.org/10.1016/B978-0-12-813140-4.00006-6 © 2019 Elsevier Inc. All rights reserved.

147

148

of PSW are the polyethylene (HDPE and LDPE) plastics, which makes them a highly common waste product [3]. Converting plastics into energy is a highly sought after process because of its high calorific value [4]. The problems associated with plastic waste management and the increasing world demand for energy can be improved by the conversion of plastics to energy. Furthermore, the fuels obtained from these processes tend to be cleaner because of the lack of sulfur compounds, and the fact that they exhibit the characteristics of fossil fuels. Polyolefins are organic polymers which only contain carbon and hydrogen [5]. The polymers mentioned have a high carbon content, which enables pyrolysis to be a preferred method of treatment when managing plastic wastes. For example, PP has a carbon content of around 85.5% 86.1%, and PE has been found to have a carbon content of approximately 83.9% 86.1%. In addition, plastics such as PVC and PVA have the potential to generate enhanced yields of aromatics after pyrolysis. This is because aromatics make up most of their chemical structure, and are deemed as polyaromatic plastics in the industries. The products produced from the pyrolysis of plastics are gas, liquid, and char. However, the desired products from the process are the pyrolysis liquid oils. Analyzing the type of feedstocks that can be used for the reaction is imperative in determining the kinds of products produced from it. It has been reported that plastics that are abundant in volatile matter and ash content have a large impact on the synthesis of pyrolysis oil. As the content of the volatile matter increases, the yield of pyrolysis oil also increases. This makes the pyrolysis of plastic wastes more desirable, as a large number of plastics contain a very high volatility content [6 8]. There are four main PSW treatment methods [9]: 1. Primary process: Here, PSW re-enters the heating cycle of the process to produce products of a similar material. This tends to be an unpopular method of recycling, as clean plastic scrap is required. 2. Secondary process (or mechanical recycling): The PSW is re-extruded, processed, converted, and blended with new polymers. Mechanical recycling can only be performed with single-polymer plastics.

PLASTICS

TO

ENERGY

3. Tertiary process (chemical method): The PSW is converted into monomer feedstock and/or basic chemicals for the production of petrochemicals and polymers. These products of chemical recycling have been proven to be useful as fuels. 4. Energy recovery (Quaternary Recycling): This process generates energy in the form of heat, steam, and electricity by the combustion of PSW. This method is usually only used when other material recovery processes fail due to economic restraints. In the waste management industry, processing costs, the amount of industrial waste generated, and requirements associated with the quality control and final product integrity lead to some of the main challenges these treatment methods face [10]. The pyrolysis process breaks down long chained polymer molecules into a wider range of smaller molecules using energy from high temperatures. By controlling the pressure and heat flow of the reaction, smaller chained molecules can be obtained [11]. Pyrolysis also presents a number of benefits such as environmental, operational, and economic advantages. An example of an operational advantage is the requirement of no flue gas clean up, while an environmental benefit is that it provides an alternative solution to landfilling [10]. Pyrolysis of plastics can generate products that can be used as fuels for energy. The three routes that can be taken are hydrocracking, thermal pyrolysis (noncatalytic), and catalytic pyrolysis. Hydrocracking of plastic waste involves the cracking of large polymer molecules in the presence of hydrogen into smaller hydrocarbon molecules that can be used as fuels for energy. The reaction takes place with hydrogen over a catalyst, and it typically uses a batch-stirred reactor under conditions of 150 400°C and 3 10 MPa hydrogen. The catalysts consist of a structured support (alumina, silica-alumina, sulfated zirconia, and zeolites) coated with transition metal solids such as Pt, Fe, Mo, and Ni. The studies conducted on hydrocracking of polymers and polymers from PSW have been centered mainly on generating good quality fuels. The feedstock used for the reaction can typically be polymers from PSW, or pure polymers such as polyethylene, PVC, and mixed polymers. Polymers that have been mixed

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

with materials such as refinery oils, waste rubber tires, or coal can also be used as feedstocks for hydrocracking. The process is intensified by the use of catalysts which lowers temperatures and makes the reaction more efficient. This can be further intensified by adding solvents, such as tetralin, decalin, and 1-methyl naphthalene to enhance mixing [5,12]. Noncatalytic or thermal pyrolysis is a noncatalytic process that represents the degradation of plastics using energy from heat (350 900°C) under anaerobic conditions. The typical products from the pyrolysis process are gases that can be used for combustion, pyrolysis oils, or waxes and char. The process is typically desirable for waste management. MPW can be converted to fuels for energy, and materials that can be safely disposed of [13]. In addition, studies have found that maximum liquid fuel yields from pyrolysis can be obtained at mild temperatures. The liquid fuel yields obtained were around 80% [14,15]. The fact that the process takes place under the absence of oxygen means that dioxins are not produced. This also reduces the amount of toxic greenhouse emissions in the environment, as opposed to conventional plastic waste management methods such as incineration, which generates toxic nitrogenous compounds and gases such as CO and CO2. The pyrolysis process also does not require any flue gas clean up [16,17]. Along with the many environmental benefits to fuel production from pyrolysis, the process also has economic benefits too. This is because many of the feedstock pretreatment steps are not needed, as opposed to other waste management methods [9]. During the catalytic pyrolysis process, a catalyst is used to aid the pyrolysis reaction. The utilization of a catalyst will lower the temperature required for the reaction, and so decreasing the reaction time. Lowering the thermal energy required for the process will make it more economically attractive, and the process can be further optimized by the reuse of the catalysts, or investigating the use of a more effective catalysts in lower quantities. Catalytic pyrolysis seems to be the most attractive, as it combines the advantages of thermal pyrolysis with the benefits of catalysis in a reaction. This can potentially aid the problem of plastic waste around the world [12]. Catalysts have also been found to have a positive impact on the activity and selectivity of certain pyrolysis experiments.

149

6.2 Catalytic Versus Noncatalytic Pyrolysis 6.2.1 Noncatalytic Pyrolysis Thermal pyrolysis of polymers is an endothermic and energy demanding process. The temperatures required for the reaction typically are between 350°C and 500°C. However, in some experimental studies, the temperatures required to obtain desirable product yields have been as high as 700 900°C. The majority of studies that have been conducted have focused mainly on PS, PP, and polyethylene. Pyrolysis studies have also been conducted using other plastics, such as PVC, polymethyl methacrylate, and PET, however these studies are not as extensive. Overall, the thermal noncatalytic pyrolysis of plastics produces liquids that are not suitable for use as alternatives to motor engine fuels. This is due to their low octane/ketene rating and excessive residue content at mild temperatures. In order for the products of thermal pyrolysis of plastics to be used as fuels, they must be further refined. The products generated from thermal pyrolysis often have a large range of hydrocarbon chains which can vary from hydrogen to coke. Therefore, the reaction conditions must be carefully controlled and optimized to obtain desired products such as petroleum and diesel. As the temperature of the reaction increases, the number of gaseous products formed also increases, while the yield of liquid fuels decreases. Furthermore, the composition of the liquid pyrolysis oil generated changes with the reaction temperature. At lower temperatures, there is a higher number of aliphatic compounds, as opposed to higher temperatures which contain a larger aromatic content [12].

6.2.2 Catalytic Pyrolysis Catalytic pyrolysis has been developed to overcome the problems faced by thermal pyrolysis. The addition of a catalyst lowers the activation energy required for the reaction, and so significantly reduces the temperature required for the pyrolysis reaction to take place. The time required for the reaction to reach completion is also reduced. Utilizing a catalyst can increase conversion rates for a large variety of polymers at significantly lower temperatures than with noncatalytic thermal pyrolysis. Studies have also reported an increase in gaseous product yields with catalytic pyrolysis when compared to thermal

150

PLASTICS

pyrolysis with similar reaction conditions. The use of catalysts also provides narrower and enhanced control over the hydrocarbon product distribution in LDPE, HDPE, PP, and PS pyrolysis, as opposed to thermal pyrolysis which generates a wide range of hydrocarbons ranging from C5 C28. In addition, the pyrolysis oil products obtained in the presence of a catalyst have a lower olefin content and a higher branched hydrocarbon and aromatic content [12]. Fig. 6.1 shows the differences between thermal and catalytic pyrolysis in terms of the length of the carbon chains seen in the products [18]. The acidic catalysts used have the ability to donate H1 ions, which allows isomerization and hence, increases the octane/cetane rating and gives rise to a superior fuel quality. Catalysts that tend to have a higher density and are constructed from stronger acids can be more successful in cracking long chained polymers [12]. The high density strong acidic catalysts tend to have a shorter lifetime due to stronger coking, and must be replaced regularly. One of the biggest problems faced with using catalysts for pyrolysis of PSW is the formation of coke, which leads to deactivation of the catalyst. A typical problem that can arise with using a catalyst in a pyrolysis reactor is that the catalyst has a limited lifetime and must be replaced periodically which adds to the operational costs. Furthermore, the catalyst can suffer from 30 After thermal cracking

Weight proportion (wt%)

After catalytic cracking 20

10

0 10 Kerosene Gas

20

30 40 Number of carbon atoms

Gasoline

Figure 6.1 Schematic showing differences between thermal and catalytic pyrolysis in terms of the length of the carbon chains seen in the products [18].

TO

ENERGY

poisoning, and when deactivated, the used catalyst must be disposed of [18].

6.2.3 Differences Between Catalytic and Noncatalytic Pyrolysis Santella et al. carried out a study to investigate the thermal and catalytic pyrolysis of a mixture of plastics from waste electrical and electronic equipment (WEE). Three different temperatures were used in the reaction, 400°C, 600°C, and 800°C. At 400°C, two zeolite catalysts were used: HUSY and HZSM-5, both with a high silica content. The results showed that pyrolysis oil was always the dominant product, and that the higher temperatures (600°C and 800°C) generated the highest yields, which were approximately 94 wt%. At 400°C, the reaction on the HZSM-5 catalyst produced a yield which was 5 wt% greater than the one obtained from thermal pyrolysis at much higher temperatures of 600°C and 800°C. Both of the zeolite catalysts used demonstrated a superior cracking performance. They also lowered the viscous fraction and enhanced the selectivity of shorter chain hydrocarbon molecules in the pyrolysis mixture. The utilization of the zeolite-based catalysts in this study showed benefits such as enhanced selectivity of desired hydrocarbons and an increased liquid fuel yield. However, a substantial amount of solid residue (char), approximately 13%, was found when using HUSY [19]. Other studies have also been conducted to demonstrate the application of a catalyst to the pyrolysis process. Mertinkat et al. [20] studied the effects of fluid catalytic cracking (FCC) catalysts as fluidized bed material on the pyrolysis of PS and PE in a scale of approximately 1 kg/h at 370 515°C. The application of FCC catalysts drastically reduced the temperature required for pyrolysis to take place. The temperature with the catalyst was as low as 370°C when compared with the temperature required without the catalyst, which was 515°C. The catalytic pyrolysis of PS produced a lower amount of the monomer (1 7 wt%) compared to the noncatalytic experimental process which generated 59 61 wt%. The main products of the catalytic pyrolysis reaction were ethylbenzene, benzene, and toluene. The yield to styrene was relatively low. As mentioned previously, with the catalytic processes, coke formation is an issue, with

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

approximately 20% being formed in the reaction. This means that the catalyst would have to be regenerated regularly if used on an industrial scale. The thermal pyrolysis experiment generated a yield greater than 60% for styrene, as well as coke formation. The coked excess FCC catalyst left the reactor into an overflowing vessel. High styrene yields are undesirable as they can cause the formation of resinous products in liquid fractions. The results obtained from the catalytic pyrolysis of PS and PE show that a desirable range of products can be obtained for the recycling of PSW at a mild temperature range of 370 450°C. High yields of BTX (benzene, toluene, and xylene) aromatics can be obtained, as well as light hydrocarbon gases and ethylbenzene. Marcilla et al. [21] investigated the thermal and catalytic pyrolysis of LDPE and HDPE over HZSM-5 and HUSY zeolite catalysts in a batch reactor. The reaction temperature was between 300°C and 550°C at a heating rate of 5°C/min. These conditions allowed the entire polymer to decompose to liquid, gaseous, and solid products, with negligible solid residue (coke deposited on the catalyst). Thermal pyrolysis generated large liquid yields of around 93 and 85 wt% for LDPE and HDPE, respectively. The catalytic pyrolysis generated predominantly gaseous yields, with the highest gaseous yield being around 73 wt% achieved for HDPE degradation using the HZSM-5 zeolite. The thermal pyrolysis products were mainly 1olefins and n-paraffins, some iso-paraffins and aromatics were also present. The researchers concluded that the chemicals generated from catalytic pyrolysis are useful and have important industrial applications. Walendziewski and Steininger [22] investigated the thermal and catalytic pyrolysis of PE and PS at reaction temperatures of 410 430°C and 390°C, respectively. The results showed that the best thermal pyrolysis temperature for the polymers was approximately 410 430°C. However, catalytic pyrolysis only required temperatures of around 280 390°C. The liquid oil yield obtained from catalytic cracking consisted mainly of lighter unsaturated hydrocarbons, as opposed to thermal pyrolysis. Vasile et al. [23] conducted the thermal and catalytic decomposition over a mixture of polymers which closely resemble the polymers found in MPW without PVC. The catalysts used in the experiment were HZSM-5 and orthophosphoric

151

acid modified (PZSM-5) zeolite catalysts. The results showed that thermal pyrolysis generated a wax yield of approximately 87 wt%, and a gaseous yield of 12 wt%. However, the catalytic pyrolysis generated lighter hydrocarbons of approximately 50 wt%, and a gaseous yield of around 30 wt%. Catalytic pyrolysis increases the yield of gaseous products, lowers the condensate, and modifies their composition when compared with thermal pyrolysis. The liquid yield produced was predominantly comprised of C3 hydrocarbons, while the liquid yield was mainly composed of aromatic compounds.

6.2.4 Catalysts The use of catalysts for the pyrolysis of plastics process has shown superiority over thermal pyrolysis, in terms of reaction temperature required, improved selectivity to petroleum, and the encouragement of isomerization [24]. Catalysts can be categorized as homogeneous or heterogeneous. The most dominant type of catalyst to be used for catalytic pyrolysis is heterogeneous catalysts, that is, solid catalysts. This is because the separation processes required can take place with ease due to the difference in phases [9]. Despite this, some researchers have utilized homogeneous catalysts such as ionic liquids for the thermal cracking of polymers. The main advantage of this type of catalysis is that much lower reaction temperatures are required for the pyrolysis to take place. Another advantage is that they have an enhanced selectivity toward lighter liquid alkanes. However, the main issue with the use of homogeneous catalysts is that they have to be separated from the product stream.

6.2.4.1 Homogeneous Catalytic Process Despite the fact that heterogeneous catalysts have been most commonly utilized, some researchers have employed homogeneous catalysts for the catalytic pyrolysis of polymers. These catalysts have mainly been Lewis Acids such as AlCl3, metal tetrachloroaluminates melts, and catalytic systems based on organic ionic liquids [25]. Ivanova et al. [26] conducted the catalytic pyrolysis of PE with AlCl3 and electrophilic complexes at a reaction temperature of 370°C. The results showed that the addition of the homogeneous catalyst to the system generated high gas yields of approximately 88 wt%.

152

Thermal pyrolysis generated a significantly lower yield of 40 wt% at 400°C. The composition of the gaseous yield was predominantly isobutane and isobutene. The number of hydrocarbons that were heavier than C5 were too insignificant to consider. The ionic liquid tetrachloroaluminate melts M (AlCl4)n (M 5 Li, Na, K, Mg, Ca, Ba; n 5 1 2) was also used as a catalyst in the experiment. The results showed that a product yield of approximately 90% 95% of C4 hydrocarbons was achieved. Adams et al. [27] conducted the pyrolysis of HDPE or LDPE to light alkanes using ionic liquids as the catalyst. The reaction took place by suspending the powdered polymer in several ionic liquids. An acidic co-catalyst was added, and the mixture was then stirred at reaction temperatures between 90°C and 250°C for a period of 1 6 days. The yield produced was approximately between 60 and 95 wt %, and predominantly consisted of short chained hydrocarbons (C3 C5), such as propane and butane, for example. The reaction temperature is substantially lower than that of heterogeneous catalysis, however, the reaction time needed to produce this yield is incredibly longer (1 6 days). The utilization of ionic liquids as catalysts for the pyrolysis of polymers demonstrates a high selectivity toward lighter alkane hydrocarbons. In addition, the products from the reaction can be easily separated from the ionic catalysts by using techniques such as solvent extraction or other physical separation processes [28].

6.2.4.2 Heterogeneous Catalytic Process The most common types of catalysts used are the nanosized acidic zeolite catalysts, with the most effective at pyrolysis being HZSM-5, H-ultrastable, and Y-zeolite (H-US-Y). These catalysts tend to show a better performance than the amorphous silica-alumina and mesoporous MCM-41. The large external surface area and high acidity of the acidic zeolite catalyst generates a higher selectivity to shorter chained hydrocarbons. However, the mesoporous HMCM-41 catalyst tends to produce heavier hydrocarbons. The acid site density of the catalyst also has an effect on the products produced. Higher acid densities encourage the thermal degradation of the hydrocarbons, however, they can produce coke, which is undesirable. The catalysts that have a lower and weaker acid density, like clays, are found

PLASTICS

TO

ENERGY

to be able to withstand coke formation. As a result, the acid density of the catalyst must be carefully controlled during preparation and different pretreatment methods. In addition to the chemical properties of a catalyst, the geometric features of the catalyst also play a large role in determining the type of products that are produced. Pore size can affect catalytic activity and product selectivity. For example, zeolite catalysts have a micropore crystalline structure with pore sizes smaller than 1 nm; on the other hand, alumina and amorphous silica-alumina are mesoporous substances with a vast distribution of large pore sizes. The narrow size distribution of zeolite permits different molecules to control a limited diffusion inside the pores, which is referred to as shape selectivity. However, alumina and silicaalumina catalysts have lower surface areas but larger pore sizes and larger pore volumes. These particular catalysts have a lower acid strength when compared to zeolites. However, they have adequate diffusion of large hydrocarbons that have big kinetic diameters through the pores, without the control of different molecules [25,29]. Park et al. [30] investigated the effect of pore shape on the catalytic performance of zeolites in the liquid phase degradation of HDPE. The various zeolite catalysts used were BEA, FAU, MWW, MOR, and MFI. The catalysts all varied in pore shape, and the reaction was carried out at 380°C or 410°C in a batch reactor. The highest activity was observed with the BEA and MFI zeolites due to their bent pore shape, which inhibited carbon deposition. This restricted the formation of longer chained molecules. The MOR zeolite conveyed the lowest activity. This was because of the swift blocking of the one- dimensional pore structure. The zeolite catalyst with the largest pores (FAU) intensified mass transfer in the process, which increased the yield of the liquid product. The MWW zeolite catalyst had large recurrent spaces, which caused a slow diffusion of cracked products, led to further cracking, and also enhanced the yield of the shorter chained hydrocarbons. It was concluded that the geometry of the zeolite catalyst pores was important in defining the selectivity and activity of the products by constraining the production of large molecules and manipulating the diffusion rate of cracked products. Gobin and Manos [31] conducted the catalytic pyrolysis of PE over different microporous

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

materials that were zeolites, zeolite-based commercial cracking catalysts, and clays and their pillared analogs. The process was performed in a semibatch reactor. The results showed that the US-Y catalysts appeared to be the most active, however, it had the highest coke production because of its strongly acidic sites. Furthermore, the ZSM-5 catalyst increased the number of gaseous products being produced and lowered the formation of coke because of its shape selective properties. The commercial cracking catalysts were active during polymer degradation, and generated higher liquid yields and lower coke formation due to the lower acidity content. All the catalysts investigated resulted in liquid products being produced that had a range of boiling points similar to those found in motor engine fuels. The commercial cracking catalysts produced heavier chained hydrocarbon, while the ZSM-5 produced lighter chained hydrocarbons. A further thermogravimetric study of polymer catalytic degradation over microporous materials was conducted to estimate the apparent activation energy. The catalysts analyzed were zeolites, commercial cracking catalysts, clays, and pillared clays. The results showed that the ultrastable Y (USY) zeolite demonstrated the lowest activation energy due to the high acidity of the catalyst. The commercial cracking catalysts demonstrated a higher activation energy than their parent USY zeolite. However, the clays and pillared clays showed the highest apparent activation energy due to their very low acidity [32]. Garforth et al. [33] carried out the pyrolysis of HDPE using various catalysts, for example, HZSM5, HUSY, and HMOR. The results showed that the acidic zeolite catalysts were most effective in cracking the polymer to lighter hydrocarbons than the weaker acidic amorphous catalysts. The best performance was demonstrated by the HZSM-5 catalyst, as it showed a relatively lower deactivation than the other zeolites. Furthermore, the coke formation was significantly smaller with HZSM-5 (around 4.6 wt%) in comparison to 8.9 wt%, and 7.1 wt% residue found by HMOR and HUSY, respectively. Muhammad et al. [34] investigated the effect of zeolite catalysts, Y-zeolite and ZSM-5, on the pyrolysis of plastics from WEE to synthesize pyrolysis oil in the petroleum range. The reaction took place in a two-stage packed bed batch reactor. Pyrolysis took place in the first step, and in the second step, the produced gases from the reaction

153

were further processed using the catalysts. Under thermal pyrolysis, the reaction generated a pyrolysis oil yield of approximately 84 wt%. The yield of oil produced from the catalytic pyrolysis was found to be 78 and 80 wt% for ZSM-5 and Y-zeolite, respectively. There was an approximate decrease of 5 10 wt% in oil yield and increase in gaseous products (mainly ethane and propene). The thermal pyrolysis reaction generated oils that were largely aromatic with increased concentrations of styrene. The catalytic process produced higher concentrations of toluene, benzene, and ethylbenzene in the pyrolysis oil, but had a reduced styrene presence. Marcilla et al. [21] studied the effects of thermal and catalytic pyrolysis on LDPE using zeolite catalysts HZSM-5 and HUSY. The results showed that the thermal pyrolysis of LDPE generated more liquid yields than gases that are largely comprised of olefins and paraffins. The introduction of the catalyst HZSM-5 significantly decreased the number of gaseous products produced. Coke formation and liquid oil was observed more with the HUSY catalyst. Manos et al. [35] studied the catalytic degradation of HDPE to hydrocarbons over different zeolites. The range of hydrocarbon products obtained were typically C3 C15. The results showed that alkanes were the main products obtained with the large pore Y-ultrastable, Y and B zeolite catalysts mainly produced alkanes. Furthermore, the results showed that less alkene and aromatic products were formed with little amounts of cycloalkanes and cycloalkenes. Medium pore mordenite and ZSM-5 catalysts predominantly produced olefins. The medium pore zeolite catalysts generated mainly alkenes. It was found that the medium-pore zeolites synthesized more lighter hydrocarbons than the zeolites with larger pores. The experiment produced a variety of different products, many of which had high values as fuel, which demonstrates that using catalysts for pyrolysis of PSW can be a potential process of recycling plastics. Ate¸s et al. [36] investigated the pyrolysis of MSW and MPW with and without the use of catalysts. The catalysts used in the reaction were Y-zeolite, β-zeolite, equilibrium FCC, MoO3, Ni Mo-catalyst, HZSM-5, and Al(OH)3. It was found that the catalysts increased the yield of the gaseous products and significantly decreased the reaction time. Catalysts also had a superior selectivity and productivity in converting the aliphatic hydrocarbons to the aromatic and

154

aliphatic compounds in oil yields. Serrano et al. [37] investigated the effects of the catalysts HMCM-41, HZSM-5, and amorphous SiO2 Al2O3 on the pyrolysis of PS. The results showed that the HMCM-41 catalyst demonstrated a superior performance in terms of activity in comparison to thermal pyrolysis; the remaining catalysts showed either lower conversions or similar conversions when compared with the noncatalytic process. Both thermal pyrolysis and catalytic pyrolysis over HZSM-5 mainly produced styrene that was approximately 50 wt%. The catalytic degradation over HMCM-41 and SiO2 Al2O3 mainly yielded benzene, ethylbenzene, and cumene. However, these were in quantities of lower than 20 wt%. The catalytic properties of HMCM-41 generated the highest activity and conversion of PS when compared to other catalysts and thermal pyrolysis. This shows that HMCM-41 has the potential to be used for the degradation of plastic wastes. The possibility of using pyrolysis to process waste rubber gloves in the presence of Y-zeolite catalyst was investigated by Hall et al. [38]. The catalyst Y-zeolite was selected to further process the products of the pyrolysis process, and to produce desirable products such as toluene and xylene in high quantities. The results showed that thermal pyrolysis of the latex rubber gloves mainly generated limonene and oligomers of polyisoprene within the pyrolysis oil. The addition of the Y-zeolite catalyst to the process significantly increased the production of aromatics, toluene, naphthalene, xylene, ethylbenzene, and methylbenzene. In addition, the hydrocarbon gas yield also increased. For both thermal pyrolysis and catalytic pyrolysis, it was found that as the temperature increased, the yield of the major compounds also increased. Santella et al. [19] carried out the thermal and catalytic pyrolysis of a synthetic mixture which contained real waste plastics that were demonstrative of the plastics found in WEE. The two zeolite catalysts used were HUSY and HZSM-5 at the reaction temperature conditions of 400°C. The results obtained from the catalytic pyrolysis of plastics from WEE were compared to the results obtained from thermal pyrolysis which took place at 400°C, 600°C, and 800°C. The mass balance produced showed that the pyrolysis oil obtained was always the predominant product (approximately 94 wt% at 600°C and 800°C), regardless of the process conditions used. A higher pyrolysis oil yield was obtained when the catalyst

PLASTICS

TO

ENERGY

HZSM-5 was used at 400°C, in comparison to the oil yields obtained by thermal pyrolysis at much higher temperatures.

6.3 Effect of Operation Variables Operational variables influencing the process included the temperature, catalyst amount, polymer waste composition, pressure of operation, as well as residence and/or reaction time. Besides conversion, these parameters also affected product yield and selectivity.

6.3.1 Reaction Temperature Temperature is one of the operational variables that has the most noteworthy impact on pyrolysis process. This is because temperature governs the reaction that involves the cracking of the polymer chain into smaller molecules. A thermogravimetry analyzer can measure and determine the behavior of thermal degradation in plastics. The analyzer generates two different types of graphs. These are known as the thermogravimetry analysis (TGA) curve and the derivative TGA curve. The former measured the changes in weight of a substance as a function of temperature and time, and the latter provided information on the degradation step which occurred during the process that is often represented by the number of peaks [39]. Scott et al. [40] found that temperatures higher than 700°C would tend to predominantly generate gases, whereas temperatures lower than this would produce a solid yield. Chin et al. [41] conducted a kinetic study on the thermal degradation of HDPE using a TGA under nonisothermal conditions. It was found that the degradation process began at approximately 378 404°C and reached conversion at approximately 517 539°C. The heating rates varied in the range of 10 50°C/min. As expected, as the heating rates increased, the weight loss also increased, which meant the reaction was taking place a lot faster. Another study carried out by Bockhorn et al. [42] showed that the thermal degradation temperature for HDPE was at 325°C. The heating rates varied from 10 K/min between room temperature and 600°C. Marcilla et al. [43] conducted a thermogravimetric study on the decomposition of HDPE and found that the highest thermal degradation rate of

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

HDPE took place at 467°C. This shows that the thermal degradation of HDPE approximately starts at temperatures of higher than 325°C and reaches the maximum decomposition at temperatures higher than 467°C. Mastral et al. [15] investigated the effect of temperature on the pyrolysis of HDPE in a fluidized bed reactor (FBR). A range of temperatures were used from 650°C to 850°C. The results showed that at lower temperatures of 640°C, the yield to wax was around 80 wt%. At the same conditions, the gas yield was around 11 wt%. As the reaction temperature increased, the yield of gaseous products also increased. The gas yield reached 64 wt% at 685°C, and as the temperature increased further to 780°C and beyond, the maximum gas yield was obtained at 86 wt%. However, increasing the temperature any further would encourage the formation of cyclic compounds formed by cyclation reactions. C ¸ epeliogˇullar and Pu¨tu¨n [44] carried out the thermal degradation of PET and found that the main thermal decomposition of PET began at around 400°C, while maximum weight loss due to degradation occurred at around 428°C. Once the temperature reached 470°C, no further changes were seen. Therefore, the thermal degradation of PET occurred at around 350°C and 430°C. Another PET degradation study was carried out by Molto´ et al. [45] at heating rates of 5, 10, and 20 K/min. The first PET weight loss occurred at around 180 230°C. The PET decomposition took place fastest at approximately 370 460°C, and then decomposition occurred at a gentle rate until 800° C. On the contrary, PVC degradation takes place slightly different, as significant weight loss occurs at two distinct temperatures as reported by C¸epeliogˇullar and Pu¨tu¨n [46]. They found that the first substantial weight loss due to decomposition took place in the temperature range of 260 385°C, and that the value obtained for the change in weight loss was a 62 wt% difference. The second significant weight loss occurred in the temperature range of 385 520°C. Here, a 22 wt% weight loss was measured from the previous weight. As the temperature was further increased to 800°C, the weight loss of the PVC decreased and became negligible, which meant that the degradation of PVC took place in the temperature range of 220 520°C. Onwudili et al. [3] investigated the effect of temperature on the pyrolysis of LDPE. The results showed that at 400°C the LDPE decomposed to

155

produce a 95 wt% yield of a viscous wax like material. In addition, the conversion to pyrolysis oil was observed at approximately 410°C, and at these conditions the main product obtained was an oily wax. The complete thermal degradation of LDPE was observed at temperatures approximately 425°C and higher. The process yielded 90 wt% oil and 10.0 wt% at a temperature of 425°C. However, once the temperature increased further, the pyrolysis oil yield decreased due to further cracking and more secondary reactions. This caused an increase in the number of gaseous products formed and char formation. At 450°C, the oil conversion decreased to 72 wt%, while 2 wt% char was produced. Similar results were obtained by Tiikma et al. [47] who reported that the LDPE conversion began at 400°C and that the maximum oil yield was obtained at 450°C. However, beyond this temperature, the oil yield decreased then char formation increased. Marcilla et al. [21] conducted a study on the thermal degradation of LDPE and found that pyrolysis liquid oil yield was obtained at temperatures between 360°C and 385°C. This liquid oil yield increased as the temperature increased, while maximum oil yield was obtained at a temperature range of 469 494°C. Any further increase in temperature would cause a decrease in the liquid oil yield, and instead encourage char formation. Furthermore, another study carried out by Marcilla et al. [39] showed that the temperature at which the maximum oil yield can be obtained was approximately 550°C, and that increasing the temperature any further caused a decrease in yield. Therefore, it can be concluded that the optimum temperature for the pyrolysis of LDPE to obtain a liquid yield would be between 350°C and 500°C. The effect of temperature on the pyrolysis reaction is a great one, as it can affect the product yield and composition, as seen by encouraging secondary reactions to occur.

6.3.2 Polymer-to-Catalyst Ratio The polymer-to-catalyst ratio is another important operational variable that can affect product selectivity and quality of the products obtained from pyrolysis. Mastral et al. [48] carried out the catalytic pyrolysis of HDPE over a nanocrystalline HZSM-5 zeolite catalyst. The experiment was conducted using a laboratory FBR at mild reaction conditions of 350 550°C. They found that the polymer-to-catalyst ratio had a

156

largely significant effect on the yield and composition of every fraction that was obtained. Smaller ratios caused a larger concentration of acid sites per gram of the polymer reacted, and so a higher gas yield was produced. When the catalyst ratios of 0.27 and 1.4 were used, the compositions of the wax yield produced were linear paraffins as expected. As the polymer-to-catalyst ratio increases, the noncatalytic thermal degradation of the polymer also increases. Increasing the polymer-to-catalyst ratio further to 9.2 leads to methane formation, which is not observed from catalytic pyrolysis, and which is characteristic of thermal decomposition β-scission mechanism in the absence of catalyst. Furthermore, the yield of gaseous products is lower than the liquid wax yield due to the milder reaction temperature of 500°C. The liquid wax composition consists mainly of olefins at the 9.2 ratio. This shows that increasing the polymer-to-catalyst ratio can directly affect the way in which pure thermal degradation of the polymer takes place. Similar results were obtained by Jan et al. [49], who studied the catalytic degradation of HDPE into fuel products using BaCO3 as a catalyst. The catalytic pyrolysis took place at reaction conditions of 450°C, at a reaction time of 1 h. Different weights of the catalyst were used: 0.5, 1.5, and 2.0 g for a fixed weight of the HDPE sample that was set at 5.0 g. The results showed that as the mass of the catalyst increases, the total conversion of pyrolysis oil and wax products also gradually increases. It was concluded that a small amount of catalyst is needed to generate the maximum liquid fuel yield, and as a result, the cost of the process was reduced. In addition, Schirmer et al. [50] investigated the catalytic degradation of PE over HZSM-5 and Ytype zeolites using a cycled-spheres-reactor. The results showed that as the mass of the catalyst decreases, the temperature reduction due to catalyst presence also decreases. Furthermore, increasing the amount of the HZSM-5 catalyst did not generate a further reduction in temperature. However, the Y-type catalyst demonstrated some ability in lowering the temperature further when the catalyst amount was increased. This showed that there may not be enough acid sites available for the Y-type catalysts to degrade heavier molecules at the smaller catalyst contents. However, when there are sufficient acid sites available, the decomposed molecules can be degraded further on the internal acid sites of the catalysts.

PLASTICS

TO

ENERGY

Sharratt et al. [51] studied the catalytic cracking of HDPE over the HZSM-5 zeolite catalyst using a laboratory scale FBR. The polymer-to-catalyst ratio was varied from 1:10 to 1:1 at the reaction temperature of 360°C. The product conversions were not so much affected by the different ratios, as the conversions obtained were always greater than 90%. However, the product distributions were affected. As the polymer-to-catalyst mass ratio increased, the yield of lighter hydrocarbons (C1 C4) also increased. The conversion for these products was approximately between 61% and 69%. Also at this ratio, coke formation became more apparent. Increasing the catalyst amount will favor the primary cracking reaction, and so shorter chained hydrocarbons will be produced. The catalytic degradation of linear LDPE over two commercial cracking catalysts was studied by Akpanudoh et al. [52]. The catalysts contained 20% and 40% Y-ultrastable zeolite, respectively. The acidity content of the system was varied by changing the amount of acid sites in the polymer catalyst system and by altering the catalyst to polymer ratio. A drastic increase in the liquid fuel production at a low acidity content was followed by a slight decline at larger values. Lower acidity values generate a lower polymer conversion because the acid sites are not sufficient for the conversion of the whole polymer. However, higher acidity values can lead to overcracking, and as a result, a larger quantity of gaseous products will be produced. Gulab et al. [53] investigated the effects of process conditions on the performance of a zeolite catalyst over the pyrolysis of HDPE. The results showed that thermal degradation of the polymer took place at a small catalyst content. This can benefit an industrial process economically. It was also shown that when increasing the polymer-to-catalyst mass ratio, the system was found to have become less active. However, the higher reaction temperatures compensated for this decrease in activity, which still resulted in high conversion values. Furthermore, reducing the catalyst content favored the reduction of overcracking, and as a result, higher liquid fuel yields were obtained. The experiment demonstrated that temperature also affects the type of products obtained. It was found that higher temperatures had an increased selectivity to middle boiling components (C8 C9), while the lower temperatures demonstrated a selectivity to heavier longer chained components, C14 C18.

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

Manos et al. [54] investigated the catalytic degradation of HDPE on an USY zeolite catalyst. The experiment was conducted in a semibatch reactor at varying heating rates and temperatures. The results showed that when a catalyst was not used for the thermal degradation of HDPE, the decomposition rate showed a rapid increase at around 500°C. However, when the zeolite catalyst was added to the system, thermal decomposition took place gradually. The researchers found that polymer degradation happened at a significantly lower temperature even if the smallest catalyst amount was utilized (polymer-to-catalyst ratio 5 9:1). As the amount of catalyst increases, the reaction rate also increases. Different polymer-to-catalyst ratios were used: 1:2, 1:1 and 2:1, and these ratios generated degradation curves that were very alike. The results obtained were indicative of the existence of a limiting reaction step. The limiting reaction step could be due to large macromolecules reacting first on the external surface of the USY zeolite catalyst, with smaller formed molecules entering the internal catalytic pore structure and undergoing further reactions in the internal pores of the catalyst. Increasing the catalyst amount beyond a certain maximum point had no effect on the degradation rate. The melted plastic resides in the pores of the catalyst bed. Increasing the amount of polymer allows the voids of the catalyst bed to be filled totally. Once this happens, the polymer that is in excess is no longer in contact with the zeolite catalyst. Up to this amount, as the content of the zeolite catalyst increases, the contact between polymer and catalyst also increases, and so a larger proportion of the polymer can take place in the preliminary degradation step.

6.3.3 Polymer Waste Composition The type of plastic feedstock used can have a major influence on the product yield obtained from the process. An approximate analysis can be conducted to determine the chemical properties of a compound. This is based on four parameters which are moisture content, fixed carbon, ash matter, and volatile content [6]. The two factors that have been demonstrated to have the greatest influence on pyrolysis oil yield are ash matter and volatile content. If a polymer has high ash content, then it will favor the production of char and gaseous products and diminish the production of the liquid yield. On the other hand, a higher volatile content will

157

increase the pyrolysis oil yield [4]. The desired products from the pyrolysis process of plastic wastes are the ones that can replace conventional fuels, for example, petroleum, diesel, and vacuum gas oils (VGOs) [55]. The composition of the plastic wastes used for pyrolysis can have a huge influence on catalytic performance too. Catalysts that have been utilized to generate high conversions from pure polymers tend to deactivate faster when thermally cracking a real mixture of polymer wastes [25]. Serrano et al. [56] conducted the pyrolysis of plastic film waste obtained from Spanish greenhouses. It was found that the addition of a 4 wt% EVA copolymer in LDPE caused the deactivation of the mesoporous catalysts used. Furthermore, it can often be difficult to determine the way in which a catalyst behaves. This is because plastic waste composition can vary a lot depending on where it has been obtained from [18]. HDPE is a strong material due to its high degree of crystallinity and small branching. As a result of this, HDPE is often used for milk bottle packaging, oil containers, and detergent bottles. It is the third largest constituent of MSW, with approximately an 18% contribution to PSW [9]. Kumar and Singh [57] conducted the pyrolysis of HDPE at a reaction temperature range of 400 550°C. The products the pyrolysis reaction yielded were oil, wax, gas, and solid residue. The highest liquid yield obtained from the experiment was 79 wt%, and the gaseous product yield was 25 wt%. There was no solid residue formation, and the properties of the oil suggested that they were very similar to that of conventional fuels such as petroleum, kerosene, and diesel [57]. In addition, Marcilla et al. [21] conducted the thermal pyrolysis of HDPE in a batch reactor at 550°C. The results showed that the liquid product yield dominated at 85 wt%, while the gaseous yield was significantly lower at 16 wt%. Increasing thermal degradation produces higher liquid yields. However, increasing it further beyond the maximum will encourage secondary reactions to occur, and so the liquid yield will decrease. Horvat and Ng [58] carried out the thermal pyrolysis of HDPE in a semibatch reactor. The results showed that a liquid oil yield of approximately 90 wt% was obtained at 460° C. The product generated was also reported to have a large quantity of propane present. The results showed that high liquid yields that have similar compositions to conventional liquid fuels can be obtained by the pyrolysis of HDPE. This makes

158

pyrolysis an attractive option to tackle the matter of plastic waste management. LDPE has also been used extensively as a feedstock for pyrolysis. It is most commonly used for plastic bags and wrapping films for packaging. This is due to its higher ductility than HDPE, and lower crystallinity, which allows it to be molded easily. LDPE waste is very common, as it is the second largest polymer waste in MSW [4]. The pyrolysis of LDPE has the potential to be converted into petrochemical feedstock. This has been demonstrated by Williams and Williams [59], whereby LDPE was converted into desirable chemicals. The reaction temperatures used in the experiment were between 500°C and 700°C. The maximum liquid yield obtained was 89 wt% at a low temperature of 500°C. The greatest gaseous yield obtained was 71 wt%, and this was obtained at a higher temperature of 700°C. The results showed that the gas yield was mainly composed of light hydrocarbons such as hydrogen, methane, ethane, and propane. As the pyrolysis temperature increased, the gaseous yield also increased. However, the liquid yield decreased. The liquid oil and wax yield were composed mainly of aliphatic compounds which consisted of alkanes, alkenes, and alkadienes. The aromatic composition increased at maximum temperatures, while at higher temperatures, single ring aromatics and polycyclic aromatic hydrocarbons were observed. The researchers concluded that the liquid oil and wax yield had a large possibility to be converted into new plastics or upgraded to refined fuels in petrochemical industries. Aguado et al. [60] carried out the thermal and catalytic pyrolysis of LDPE into high value hydrocarbons. The temperatures used during the reaction were between 425°C and 475°C. The results showed that reaction temperatures of 450°C allowed conversions of greater than 90 wt% to be achieved. The thermal pyrolysis of LDPE produced mainly α-olefins and n-paraffins, the majority of which (75 wt%) were obtained as liquid products. The catalytic pyrolysis took place at 425°C, and generated mainly gaseous hydrocarbons, which were predominantly made up of olefins. The liquid yield that was produced contained a large amount of desirable aromatic and branched compounds which were within the petroleum hydrocarbon range (C5 C12). This means that it is suitable to be blended with conventional petroleum fuels. PP is a widely used plastic due to its high heat resistance, low density, and hardness. Due to its

PLASTICS

TO

ENERGY

desirable properties, it often has a variety of uses, and as a result, it is a large contributor of MPW. Researchers have investigated the pyrolysis of PP to produce useful chemicals, which can aid the issue of plastic waste accumulation in the environment. Ahmad et al. [61] investigated the pyrolysis of PP to produce liquid fuels over a reaction temperature range of 250 400°C. The results showed that the liquid yield obtained was a maximum of around 70 wt%, while the gaseous yield obtained was 29 wt%, and the solid residue yield was 1.3 wt %. The liquid products consisted mainly of hydrocarbons in the range of C6 C16, and were particularly enriched with naphtha range hydrocarbons. In addition, the pyrolysis of PP generated high yields of paraffinic hydrocarbons. The researchers concluded that the pyrolysis of PP produced products were appropriate to be used as liquid fuels. PS is composed of styrene monomers and is a naturally colorless substance. It has several beneficial properties such as heat resistance, a light weight, and an increased strength. These properties make it useful for food packaging, appliances, and construction alongside many other applications. Onwudili et al. [3] investigated the pyrolysis of PS in a closed batch reactor at reaction conditions of 300 500°C for a reaction time of 1 h. The results showed that PS produces a very high liquid oil yield (97 wt%) at the optimum temperature of 425° C. The oil product consisted mainly of aromatic compounds such as toluene, ethylbenzene, and styrene. The maximum yield of gaseous products produced from the reaction was 2.5 wt%. Scott et al. [40] carried out the pyrolysis of PS (Dow Styron) at a temperature range of 532 708°C. The predominant product under all temperatures was styrene, both as a liquid condensate and absorbed by the cotton wool filter. In addition, other products identified were toluene, ethylbenzene, propenylbenzene, propynyl-benzene, and naphthalene. The reaction generated a monomer yield of approximately 75 wt % or greater, and the complete aromatic liquid yield obtained was approximately between 83 and 88 wt% with no char formation. Liu et al. [62] conducted a study on the pyrolysis of PS in a FBR in temperatures ranging 450 700°C. At 600°C, the highest liquid oil yield was obtained: 99 wt%. The main components of the liquid fraction were styrene monomer, dimer, and trimer. A high liquid yield (98 wt%) was also obtained at a milder reaction temperature of 450°C. Therefore, PS pyrolysis

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

can be carried out at around 400 500°C to obtain significantly high oil yields at lower temperatures. PET is a very common plastic used for food and drink packaging, as well as other applications such as printing sheets and photographic film. This makes it one of the most widely recycled plastics used by consumers. C ¸ epeliogˇullar and Pu¨tu¨n [44] conducted the pyrolysis of PET to produce liquid oil using a FBR under a reaction temperature of 500°C. The main product obtained was gaseous with a yield of 77 wt% with no char formation. The liquid yield was much lower at 23 wt%. The results were due to the low volatility content of PET, which meant that the gas production would be favored more. In addition, almost 50% of the liquid oil yield was composed of benzoic acid based on the GC-MS analysis. As a result, the pyrolysis oil was undesirable to use as a fuel because of its highly corrosive nature and low fuel quality. A similar study conducted by FakhrHoseini and Dastanian [14] reported higher pyrolysis oil yields at 40 wt%, while the gaseous yield obtained was 52 wt%. There was char formation this time that was found to be at 9.0 wt%. If gaseous products are desired, then the thermal degradation of PET would be the most suitable as gases are predominantly produced from this process. PVC is a polymer that consists of chlorine and carbon obtained from hydrocarbon feedstocks. It is most commonly used for electrical insulation and window frames. Miranda et al. [63] investigated the pyrolysis of PVC in a batch reactor. The reaction temperatures used during the experiment were 25 600°C, and a heating rate of 10°C/min was applied. The liquid oil yield was found to be significantly low and varied between 0.45 and 13 wt%, and it appeared to increase with increasing temperatures. The formation of tar was substantially higher than the liquid oil yield obtained, and a maximum value was found to be 20.0 wt%. The main product obtained from the pyrolysis of PVC was hydrogen chloride (HCl). The maximum yield of this was found to be 58 wt%. This was an undesirable product as it is highly corrosive and poisonous when thermal energy is applied, and as a result, experimental equipment can be damaged. These results showed that the pyrolysis of PVC may not be so desirable due to the formation of HCl and the low liquid oil yields obtained. Several researchers have also explored the possibility of conducting the pyrolysis of mixed plastic

159

composition. This is because liquid pyrolysis oil can be obtained from the mixed plastic composition. Kaminsky and Kim [64] conducted the thermal pyrolysis of mixed plastic waste obtained from German households in a FBR at 780°C. The plastic waste consisted of approximately 75% PE and PP, and around 25% PS. Initially, the PVC was separated from the plastic waste, however, approximately 1.0 wt% was still present. The experimental results showed that approximately 48 wt% liquid yields were obtained. Siddiqui and Redhwi [65] conducted the thermal and catalytic pyrolysis of LDPE, HDPE, PP, and PET in a microreactor at 430 440°C. Three reactions of each plastic with PS were performed in the ratio of 1:1, 1:2, and 1:3. The amount of PS present was varied during each experimental run to investigate its role and reactivity. The results showed that a mixed feed ratio of 1:1 generated the optimum results in terms of conversion. The catalytic pyrolysis gave the best yields in terms of pyrolysis oils obtained. The researchers concluded that the thermal and catalytic pyrolysis of plastics with PS was a feasible process. This was because products obtained from the process could be upgraded into desirable liquid fuels and chemical feedstocks.

6.3.4 Reaction and Residence Time Residence time is also another parameter that can have a significant effect on the pyrolysis process. Residence time is essentially the average period of time that a particle spends in a reactor, which can affect the product distribution [15]. As the residence time increases, the conversion of the primary product also increases. As a result, the yield of shorter chained hydrocarbons and noncondensable gases increases. These products are also found to be more thermally stable as well [66]. A similar observation was made by Al-Salem and Lettieri [67], whereby the results of the isothermal pyrolysis of HDPE was carried out using a TGA setup. The reaction temperatures varied between 500 and 600°C, and the results showed that the aromatics and residual char production rate was negligible when compared with the noncondensable gases and liquids. Mastral et al. [15] investigated the effect of residence time on the product distribution and gas composition during the pyrolysis of HDPE in a FBR. The study was conducted at five

160

different temperatures: 650, 685, 730, 780, and 850°C, and the residence time was altered from 0.64 to 2.6 s. The results showed that the residence time had a great impact on the pyrolysis product distribution. At 640°C, the highest yielding product was a cream colored wax. The yield of this product ranged from 80 wt% at 0.8 s to 69 wt% at 1.5 s. During these same process conditions, the gas product composition had a range of 11 wt% at 1 s and 32 wt% at 1.5 s. Higher temperatures (780°C) produced larger gaseous yields: 86 wt% of gas and 10 wt% of oil, both at 1.34 s. The effect of residence time became more prevalent at higher temperatures of 850°C. It was found that with increasing residence times, the yields of methane and hydrogen increased from 12 to 22 wt% and from 1 to 3.6 wt%, respectively. Furthermore, the yield of ethene achieved the greatest value of 41 wt % at a residence time of 0.86 s. The experiment showed that the optimum conditions for the process were longer residence times, and higher temperatures were approximately 750 780°C. Lopez et al. [68] investigated the influence of residence time on the pyrolysis of PSW in a semibatch reactor. The results showed that at 0 min, a substantial conversion of 76 wt% to gases and liquids was achieved. This took place during the heating and cooling stages of the process, as this is where the thermal inertia of the system is highest. After this point, the solid yield rapidly declines and remains at a relatively constant level above 15 min. This represented the total degradation of the PSW sample. The solid yield that is found (approximately 1 wt%) represents char formation and cannot be further degraded despite the reaction time given. In addition, the desired liquid yield was found to have reached a maximum of 65 wt% after 30 min of reaction time. After 30 min, there was no more increase of the liquid yield, which showed that once total thermal degradation of the plastic sample has been reached, the liquid yield would remain constant. Onwudili et al. [3] studied the effect of residence time on the composition of products from the pyrolysis of PS and LDPE in a batch reactor. The experiment took place at a temperature range of 300 500°C. The results showed that the main components in the pyrolysis oil product were aliphatic hydrocarbons, however, as the residence time increased, the aromatic content also increased. It was found that LDPE reached full conversion (91 wt% oil and 8.7 wt% gas) at zero residence

PLASTICS

TO

ENERGY

time at a reaction temperature of 450°C. The oil yield was found to decrease with longer residence times, while the gas yield was found to increase. As the time increased to 120 min, the final oil yield was found to have decreased to approximately 61 wt%, while the gas yield increased to 29 wt%. The amount of char produced at 120 min was 10.1%, however, the production only became noticeable at an experimental time of 60 min and longer.

6.3.5 Mass and Heat Transfer Heat transfer is often a major issue regarding the pyrolysis of plastics. The operating conditions can affect the results of any pyrolysis’ experimental setup. FBRs have been known to demonstrate an excellent mass and heat transfer, as well as maintain an isothermal profile throughout the reactor [64]. Ceamanos et al. [69] conducted an experimental study of the thermal pyrolysis of HDPE, and compared isothermal and dynamic experiments. They found that there were two causes of discrepancies observed in the results obtained for the kinetic parameters of the thermal degradation of HDPE. The first cause was due to an imperfect heat transfer inside the sample as well as between the experimental system and the sample. The second cause was the complex degradation mechanism involved. It was found that heat transfer issues could potentially be minimized using smaller amounts of polymer sample and lower heating rates. However, the complex mechanism of thermal degradation can allow significant differences to occur in the parameters obtained from isothermal and nonisothermal experiments. Aguado et al. [70] studied the kinetics of PS pyrolysis in a conical spouted bed reactor (CSBR) using a temperature range of 450 550°C. The results were compared with those obtained from TGA and in a microreactor. The activation energy obtained from the CSBR experiments was found to be lower than the energy obtained by TGA. This was due to unavoidable mass and heat transfer limitations which arose in the individual particles of the PS and sand at high temperatures. This happens because PS is not a crystalline material, which means that after the glass transition, it is not able to coat the sand particles uniformly with a fine layer. As a result, further mass and heat transfer limitations would be expected. The kinetic results

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

generated from the CSBR are a representation of the maximum rate of PS pyrolysis at this temperature range. Furthermore, a very small plastic/sand ratio was used, and so, any large scale operation in a CSBR or fluidized bed would require reducing the ratio. The conical spouted bed ratio is adaptable for the increase in productivity, however, the reaction rate would decrease because an increase in the plastic layer coating the sand would increase the mass and heat transfer limitations. It was concluded that the CSBR offers isothermal operation and excellent contact between the phases during the kinetic study of pyrolysis of PS.

161

Products and unreacted materials to separation

Catalyst on support

Diffuser

6.4 Reactor Types The type of reactor used in this process has a large influence on important parameters such as mass and heat transfer rates, residence time, mixing ability of reactants and catalysts, and economic efficiency. On a laboratory or pilot scale, the types of reactors used are: batch, semibatch, fixed bed, FBRs, CSBR, and reactors using microwave assisted technology.

6.4.1 Fixed and Fluidized Bed Reactors Fixed bed reactors (Fig. 6.2) are the simplest type pf reactor to design, and consist of solid catalyst particles being loaded and packed into the bed. However, there are often problems faced with the plastic feed such as high viscosities, low thermal conductivities, and irregular shape when being placed inside the reactor. There are also problems faced with small catalyst surface areas inside the reactor. Several researchers have chosen to use the fixed bed reactor for the pyrolysis of plastics [44,71 73]. In certain cases, the fixed bed reactor is used after the first noncatalytic stage of pyrolysis has been completed. This is so the gaseous and liquid products from the first stage can be fed into the fixed bed reactor with more ease [25]. The catalytic upgrading of the pyrolysis gases obtained from the pyrolysis of PE was investigated by Bagri and Williams [74] in a fixed bed reactor using a Yzeolite catalyst. The results showed that a liquid oil yield of 95 wt% was obtained in the absence of a catalyst, with very little or no char, and a very low gas yield at 500°C. However, the results of the

Reactants

Figure 6.2 Schematic reactor [4].

diagram

of

fixed

bed

catalytic pyrolysis showed that the oil yield decreased to approximately 85.0 wt%. Increasing the temperature would cause an increase of the gaseous yield and a constant decrease in pyrolysis oil yield. Several other researchers have also investigated using a fixed bed reactor for the upgrading of products obtained from primary pyrolysis [23,75]. Using a fluidized bed reactor (Fig. 6.3) resolves some of the problems faced with a fixed bed reactor. This particular reactor offers a much more enhanced mass and heat transfer and minimizes temperature gradients throughout the reactor [64]. The mass and heat transfer are improved due to better mixing of the catalyst with the fluid, which provides a greater surface area for the reaction to occur. In addition, it is a continuous flow process, which means the process can run uninterrupted and frequent feedstock charging is not needed, as opposed to batch processes. The FBR can also generate a narrower and more uniform spectrum of products by influencing the residence time of polymer waste in the reactor at a constant temperature. The spent catalysts used in the reaction can also be replaced with new ones without disturbing the process. Therefore, on an industrial scale, the FBR offers lower operating costs and higher efficiencies. However, there can be problems with bed

162

PLASTICS

TO

ENERGY

Products

The flow of gas makes the catalyst particles behave like a fluid

Distributer plate Each catalyst particle is about the size of a grain of sand

Reactants

Fluid bed at rest

Ethene Hydrogen Chloride Oxygen

Fluid bed with gas flow

Figure 6.3 Schematic diagram of FBR [4].

defluidization due to melted plastic feedstocks sticking to the fluidizing media, such as sand. Jung et al. [76] carried out the pyrolysis of waste PP and PE in a FBR. Researchers commonly choose the FBR, as it offers shorter residence times, isothermal behavior, and excellent mass and heat transfer. The pyrolysis of PE and PP generated a maximum liquid oil yield of 60.0 and 43 wt %, respectively [76]. Luo et al. [77] conducted the catalytic pyrolysis of HDPE and PP into a liquid fuel using a FBR. A maximum liquid fuel yield of greater than 86 wt% containing hydrocarbons in the range of C5 C11, was achieved at the mild temperature range of 400 550°C.

HDPE

123 mm

To condenser and analyzers

Reactor 340 mm

28º

205 mm

6.4.2 Conical Spouted Bed Reactor The CSBR reactor (Fig. 6.4) was designed to overcome some of the issues faced with the FBR such as defluidization of the bed due to the melted plastic. The CSBR generates a vigorous contact between the different phases, and a higher collision rate significantly reduces particle agglomeration in the bed matter. This was reported by Aguado et al. [25], that stated that a CSBR reactor can overcome the issue of particle agglomeration behavior. Furthermore, it was found that the cyclic movement of the sand particles led to a more uniform coating with fused polymers. The CSBR has much smaller pressure drops, which is not true for the FBR [78]. On the

Preheater

20 mm Nitrogen Air 20 mm

Figure 6.4 Schematic representation of CSBR [77].

contrary, some issues have arisen during reactor function such as catalyst feeding, catalyst entrainment, and product collection. Furthermore, it has more complicated design process as opposed to

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

the fixed bed reactor and FBR design, where a number of pumps must be installed in the system. This then increases the capital cost [4]. Arabiourrutia et al. [79] investigated the thermal pyrolysis of HDPE, LDPE, and PP in a CSBR, and the temperature range for the reaction was between 450°C and 600°C. The results showed that at the lowest reaction temperature (450°C), liquid wax yields of 80, 80, and 92 wt% were achieved for LDPE, HDPE, and PP, respectively. The liquid products dominated at the lower temperatures, however, as the temperature increased, the liquid yield decreased, and the gaseous products increased. Furthermore, the liquid waxes that were produced at higher temperatures consisted more of lighter hydrocarbons, as opposed to the heavier liquid wax obtained at lower temperatures. This can be explained by further cracking taking place at higher temperatures. This confirms the activity of secondary and tertiary reactions during the thermal degradation of plastics. The waxes obtained at higher temperatures have a heating value of 44 45 MJ/kg, which coincides with the values for conventional liquid fuels. The experiment shows that the CSBR has a higher selectivity to produce waxes at moderate to high temperatures, and these waxes can be further upgraded to conventional fuels such as petroleum in FCC reactors. Elordi et al. [80] conducted the catalytic pyrolysis if HDPE at 500°C using a spent FCC catalyst in a CSBR. The results showed that a high gasoline yield, C5 C11, of 50 wt % was obtained, and a 28 wt% yield of C2 C4 olefins was produced. These results were due to good reactor conditions and enhanced catalyst properties. The researchers found that the CSBR improved the physical steps to melting the plastics and coating the catalyst with melted plastics. The reactor favored the primary cracking reactions and reduced the secondary reactions from the olefins. This significantly reduced the number of gaseous products produced from the reaction.

6.4.3 Batch and Semibatch Reactors A batch reactor is a closed system with no continuous flow of reactants entering the system or products leaving the system while the reaction takes place. In batch reactors, a reaction mixture can react for long time in order to reach high conversions. However, the disadvantages of the reactor

163

can be high labor costs per batch, the inconsistent product quality from batch to batch, and the issues faced with industrial scale production. A semibatch reactor allows reactants to be added and/or products to be extracted simultaneously. Although the semibatch reactor has the same disadvantages as the batch reactor, there are some advantages too. The reactor has the benefit of temperature control by controlling the feed flow rate and reducing undesired side reactions through the maintenance of a low concentration of reactants [81]. Kaminsky et al. [82] conducted the thermal and catalytic pyrolysis of PP in a batch reactor at a temperature range of 300 500°C. At higher temperatures of 500°C, greater than 98 wt% of the PP was decomposed in the absence of a catalyst. The results from the thermal pyrolysis showed that over 60 wt% of the product yield were light oils, and 20 wt% of the yield was gaseous. Lower temperatures of 400°C showed that over 40 wt% of the PP could not be decomposed, even if the reaction time was 1 h. In this case, 35 wt% of liquid yield was obtained, and was composed of heavier hydrocarbon molecules. The lighter hydrocarbon oil yield was 12 wt%, and the gaseous yield was 10 wt%. The catalytic process of PP was carried out over a combination of TiCl4 and AlCl3. This catalytic combination was similar to that of a Ziegler-Natta catalyst. The use of the catalyst significantly reduced the reaction temperature required to decompose PP. The optimum results were obtained at a temperature of 300° C with a ratio of 1:1 of the two catalysts. At these conditions, approximately 80 wt% of the PP was degraded to produce around 42 wt% of light oils, and approximately 16 wt% of gases. It was found that increasing the amount of catalyst in the reaction would lead to an increase in the yield of the lighter oil fraction, as well as the gas fraction. It was concluded that higher catalyst amounts favored secondary reactions and decreased the selectivity of the process. Adding catalysts to the reaction can be beneficial by enhancing the hydrocarbon yield and lowering reaction temperatures. The catalyst is usually mixed with the reactants within the batch reactor. However, this can cause the formation of large amounts of solid residue that can lead to deactivation of the catalyst. In addition, it is often difficult to separate the char from the catalyst at the end of the process. Lopez et al. [68] carried out the thermal pyrolysis of plastic wastes in a semibatch reactor at a

164

temperature range of 460 600°C. The highest liquid yield was obtained at the lowest reaction temperature of 460°C. The product yields for liquids, gases, and solids were 72, 27, and 1.1 wt%, respectively. As the temperature of the reaction increases, the gaseous yield also increases. The liquid oil yield was found to decrease with increasing temperatures. At 600°C, the liquid yield had dropped to 43 wt%, and the gaseous yield had increased to 56 wt%; the solid yield remained approximately the same for all reaction temperatures. Lower temperatures generated highly viscous liquids that contained a large proportion of heavier hydrocarbon molecules; higher temperatures produced smaller liquid yields, but with a higher aromatic content. It was concluded that the liquid products could be used as high HHV alternative fuels, or as a source of useful chemicals such as toluene and styrene. In a further study carried out by Lopez et al. [83], the catalytic pyrolysis of plastic wastes was carried out in a semibatch reactor. The catalysts used in the reaction were ZSM-5 zeolite and Red Mud (Fe2O3), and were tested at 440°C and 550°C. The catalysts had a strong influence on the pyrolysis yield and product composition. The addition of the ZSM-5 zeolite catalyst generates an enhanced gaseous yield of 40 wt%, and a liquid yield of 57 wt% at 440°C. The increased gaseous yield was due to the thermal cracking ability of the catalyst, which was due to its high porosity and strong acidity. The red mud catalyst showed similar results with that of thermal pyrolysis, however the gaseous and liquid yields were slightly higher, 4 and 7 wt% higher, respectively. The use of catalysts significantly reduces the temperature required to achieve the desired yield, hence operating costs can be reduced.

6.4.4 Microwave Assisted Technology Pyrolysis using microwave assisted technology is a new process, and has been effective in recycling waste plastics to produce useful chemicals. During this process, the waste plastic material is mixed with carbon, which is a highly microwaveabsorbent material. The carbon absorbs energy from the microwave radiation, which then generates elevated temperatures (up to 1000°C) to allow the pyrolysis reaction to occur [84]. This thermal energy then transfers to the polymers due to conduction. This generates a highly efficient energy

PLASTICS

TO

ENERGY

transfer and the need for chemicals is significantly reduced. The advantages of using microwave assisted technology for pyrolysis of plastics is an increase in even distribution of heat and a higher degree of control over the heating process. Utilizing microwave radiation enables high temperatures and enhanced heating rates. In addition, exceptional efficiencies for conversion of the electrical energy into heat energy and heat transfer to the load can be achieved. The equipment required for this technology has also been found to be very reliable and compares highly to other conventional heating methods [66]. Although the use of microwave assisted technology appears to be promising, there are still some concerns surrounding the process. There is not a substantial amount of data to measure the dielectric properties of the treated waste feed. The dielectric properties of the polymer can have an impact on the efficiency of the microwave heating. Plastics appear to have a small dielectric constant, and so mixing it with carbon has the potential to improve the heating efficiency of the material [84]. Hussain et al. [85] conducted the microwavemetal interaction pyrolysis of waste PS using a microwave oven at a frequency of 2450 MHz. An aluminum coil was used as an antenna and heat generation media inside the reactor. The reaction temperatures achieved were as high as the melting point of aluminum, and the main products obtained were styrene and aromatic compounds. The results showed that a liquid yield of 88 wt%, a gaseous yield of 9 10 wt%, and solid residues were generated from the process. The liquid yield was mainly composed of substituted benzene, polycyclic aromatics, and condensed ring aromatics. The researchers concluded that the pyrolysis of waste PS is an enhanced alternative to conventional pyrolysis techniques. This is because of shorter reaction times required. The process generated elevated liquid yields of enhanced selectivity in shorter reaction times, and enabled a more economically viable process of recycling waste PS to useful chemicals on an industrial scale.

6.5 Processing Often, catalytic pyrolysis may produce pyrolysis oil that is desirable to use as a fuel or as a chemical feedstock. Here, the advantage of using catalysts is

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

that no further upgrading process step of the pyrolysis products may be required, and so, can improve the economic viability of the process. However, sometimes it may be desirable to conduct the thermal pyrolysis of a plastic and then obtain the pyrolysis products to be passed into a second reactor containing a catalyst in order to further obtain the desired product. This method is used when there are chemicals present in the plastic feedstock which can adversely affect the catalyst productivity and lifespan.

6.5.1 Direct Catalytic Pyrolysis Catalytic pyrolysis poses several advantages over the noncatalytic route. First of all, it drastically reduces the pyrolysis temperature by lowering the reaction activation energy. The use of catalysts has also demonstrated an increased selectivity toward desired pyrolysis liquid products. However, catalytic pyrolysis does suffer from some limitations. The main limitation is related to catalyst performance deterioration with time. Hence, catalyst lifetime and regeneration in the process should be taken into account to maximize economical value [25]. The catalysts should be resilient to the heterogeneous nature of plastic waste feedstock, along with the addition of various additives. Direct catalytic pyrolysis has been shown to be beneficial for the liquid yield obtained from the pyrolysis reaction. Abbas-Abadi et al. [86] conducted the catalytic pyrolysis of PP with an equilibrium FCC catalyst in a stirred tank reactor. The results showed that the utilization of a catalyst improved the economic viability of the process to generate light hydrocarbons. It was concluded that the liquid yield was improved due to the addition of the catalyst. The pyrolysis oil yield obtained from the experiment was 92 wt%. Lopez-Urionabarrenechea et al. [87] explored the combination of catalytic and stepwise pyrolysis of packaging waste. The plastic waste used in the experiment consisted of PE, PP, PS, PET, and PVC. The pyrolysis was conducted in a semibatch reactor at 440°C using a ZSM-5 catalyst. A dechlorination step was carried out in the presence and absence of the catalyst at 300°C. In doing so, the liquid chlorine content was reduced by up to 75 wt % when compared with conventional catalytic pyrolysis. However, it was found that when the catalyst was integrated into this dechlorination step,

165

the catalytic activity decreased. As a result, liquid yields with higher long chained hydrocarbons were produced, and a lower aromatic content was generated. The researchers found that adding the catalyst after the thermal dechlorination step was the most effective in obtaining the same results with the conventional direct catalytic process in regards to product yields and purity. It was concluded that when the pyrolysis feedstock contains PVC, a thermal dechlorination step should be done before the catalytic pyrolysis to prevent catalytic deactivation.

6.5.2 Thermal Pyrolysis With Catalytic Upgrading of Pyrolysis Oil Although direct catalytic pyrolysis has some noteworthy advantages, often thermal pyrolysis with catalytic upgrading of pyrolysis oil may appear to be a more attractive process. This may occur when the type of plastic feedstock used contains a large amount of chemicals that can negatively impact the catalytic performance. The thermal pyrolysis step in the process allows for the removal of these undesirable components [25]. Often the formed product vapors of thermal pyrolysis are upgraded in a catalytic bed following the noncatalytic pyrolysis reactor. Aguado et al. [60] conducted a two-step thermo-catalytic pyrolysis of LDPE. The reaction system consisted of an initial pyrolytic furnace which was subsequently followed by a packed bed reactor containing n-HZSM-5 zeolite where the catalytic upgrading of the pyrolysis vapors took place. The pyrolysis temperatures were between 425°C and 475°C. The results showed that the catalytic reforming caused a significant increase of the gaseous yield. The gaseous yield obtained was 74 wt % at the maximum temperature, and a liquid yield of 22 wt%. The results gathered from this experiment demonstrated a similar trend to that of direct catalytic cracking using the same catalyst. Bagri and Williams [74] investigated the influence of zeolite catalytic upgrading of the pyrolysis gases produced from the pyrolysis of PE. In the first fixed bed reactor, the thermal pyrolysis of PE took place. The gaseous products of the thermal pyrolysis were then passed to a secondary reactor that contained the zeolite catalyst. The results showed that oil yield decreased and gaseous yield

166

increased in the presence of both catalysts. As the reaction temperature of the catalyst increased, the oil yield decreased further, and the gaseous yield increased even more. It was found that the catalytic upgrading of pyrolysis gases produced increased the aromatic content.

6.6 Co-processing of Plastics Co-processing or co-pyrolysis is a process that involves using a mixture of two or more different materials as a feedstock for the reaction. Although the pyrolysis of plastics has demonstrated production of alternative fuels, conducting co-pyrolysis of plastics with other materials such as biomass, can prove to be beneficial in some respects. Plastics contain hydrogen and carbon, and so, this material can be used in pyrolysis to produce hydrocarbon fuels. The liquid products obtained from plastic pyrolysis have a higher calorific value when compared to conventional fuels. As a result, the addition of polymers into biomass pyrolysis can increase the quality of the liquid product generated. The pyrolysis of biomass is a feasible reaction that generates products such as hydrocarbons and hydrogen gas. These products can be used as an energy source. However, the pyrolysis oil has low combustion efficiency, and the high oxygen content leads to a low calorific value, corrosion issues, and instability. Studies have shown that the co-pyrolysis of plastics and biomass have enhanced the quality of the products produced. Furthermore, using copyrolysis can substantially reduce the amount of waste, which reduces the costs required for waste management, as less landfill sites are required. Therefore, co-pyrolysis can be proposed as an effective and economical waste management method [7]. Co-processing of plastics can also take place with VGO, as shown in a study conducted by Ali and Siddiqui [88]. The thermal and catalytic copyrolysis of mixed plastic waste with PVC and petroleum residue was carried out at optimum reaction conditions of 350°C in the presence of N2 gas for 1 h. The results showed that the presence of VGO increased the overall conversion in the copyrolysis reactions in contrast to single pyrolysis. It was concluded that the catalytic co-pyrolysis of PVC and VGO can generate products that can be upgraded to transportation fuels, and this could

PLASTICS

TO

ENERGY

have the potential to be applied on an industrial scale. Passamonti and Sedran [89] conducted the catalytic co-pyrolysis of LDPE and VGO at reaction temperatures of 500, 525, and 550°C, and a contact time of 3 30 s. It was found that recycling waste LDPE by co-pyrolysis, as a part of a typical feedstock to the catalytic cracking of hydrocarbons process, does not hinder the standard operation. As a result, it is possible to take advantage of the benefits of co-processing without developing new technologies to recycle waste plastics in a conventional refinery. This makes the process more costeffective by taking advantage of economies of scale. Co-processing of two materials increases the octane rating of the gasoline obtained in the refinery due to the substantial increase in olefins from the LDPE. The LDPE is converted, without any problems, and favors primary catalytic cracking reactions, thus increasing the olefin concentrations in the petroleum and LPG boiling ranges. Odjo et al. [90] performed the co-pyrolysis of VGO and LDPE in an FCC riser reactor. An equilibrium FCC catalyst was utilized at reaction temperatures of 500 700°C, and catalyst feed ratios of 5:1, 7:1, and 10:1. The results showed that co-processing of the two materials is a useful and feasible method to obtain fuels for energy and chemical feedstocks. It was found that gas yields of approximately 5 and 50 wt% of the total cracking product increased with increasing temperatures. However, the liquid yield decreased as temperature increased. Abnisa et al. [91] conducted the co-pyrolysis of palm shell and waste PS to generate a pyrolysis oil that could potentially be used as a chemical feedstock or fuel. The results showed that a maximum liquid oil yield of around 68 wt% was obtained at a reaction temperature of 600°C, a palm shell/PS ratio of 40:60, and a reaction time of 45 min. The liquid oil yield consisted mainly of aliphatic and aromatic hydrocarbons. The results obtained from the copyrolysis study were significantly better than the ones obtained from a previous study of the pyrolysis of palm shell alone; a liquid yield of 46 wt% at 500° C and a reaction time of 60 min. Furthermore, the high heating value of the pyrolysis of palm oil alone was approximately 11.94 MJ/kg. However, the high heating value for the co-pyrolysis of palm shell and PS was found to be around 40.34 MJ/kg. The researchers concluded that the high heating value and composition of the liquid products obtained

6: CATALYTIC CONVERSION

AND

CHEMICAL RECOVERY

were very similar to those of conventional fuels. Another co-pyrolysis study of plastics with biomass was conducted by Brebu et al. [92] where the feedstocks were pine cone and a mixture of polymers (PE, PP, and PS). The results showed that the highest oil yield was obtained (64 wt%) during the copyrolysis of the pine cone and plastics when compared with thermal pyrolysis of pine cone or plastics individually. In addition, the chars produced from the co-pyrolysis experiments yielded a high calorific value, very low ash content (below 1 wt%), and low sulfur content. This implies the use of char to be beneficial as a solid fuel, or as a feedstock for activated carbon synthesis. It was concluded that the copyrolysis process is allows waste biomass or plastics to be used as a feedstock to produce useful chemicals or fuels for energy. C ¸ epeliogˇullar and Pu¨tu¨n [46] investigated the copyrolysis of biomass and plastic wastes. The biomass feedstocks used were cotton stalk, hazelnut shell, sunflower residue, and arid land plant. The plastic feedstock used was PVC and PET blended together in a 1:1, w/w ratio. The experimental conditions were a heating rate of 10 K/min from room temperature to 800°C. The activation energy required to decompose the biomass feedstocks was lower than the energy needed to decompose the polymers. This demonstrated the difference in kinetic behaviors between the pyrolysis of biomass and plastics. Copyrolysis showed that the mixtures had characteristics of both biomass and plastic materials during the individual components thermal pyrolysis. It can be concluded that the co-pyrolysis of the two different feedstocks can produce desirable products due to the synergistic effects between the feeds. Chattopadhyay et al. [93] investigated the copyrolysis of plastics and biomass using catalysts. The plastic feedstock was a mixture of HDPE, PP, and PET, while the biomass feedstock was writing paper. The reaction took place in a fixed-bed reactor in the presence of cobalt based alumina, ceria, and ceria-alumina catalysts. The results demonstrated the synergistic effect between the two different feedstocks. The liquid oil yield steadily increased with increasing polymer content in the blend. The gases were the predominant product formed in the yield (47 wt% hydrogen). This was achieved at a biomass/ plastics ratio of 5:1 with the presence of 40% Co, 30% CeO2/30%, and Al2O3 catalyst.

167

6.7 Concluding Remarks The pyrolysis of polymers is a sustainable process that overcomes issues faced with solid waste accumulation such as landfill sites. The process itself produces less pollution than other PSW treatment methods, as it does not produce dioxins, carbon monoxide, and greenhouse gases. The pyrolysis process produces desirable liquid oil, gaseous products, and char. The useful hydrocarbons produced can be used as fuel for energy or as chemical feedstocks. This lowers the demand for the nonrenewable fossil fuels, of which there are a finite supply. Catalytic pyrolysis offers shorter residence times and lower reaction temperatures. The catalytic process also demonstrates the ability to produce products of a similar composition to motor fuels, which means further upgrading of the pyrolysis products is not required. However, certain catalysts can be expensive, and should not deactivate or suffer from poisoning over time. Thermal pyrolysis is often coupled with the catalytic upgrading of pyrolysis products to produce useful chemical products and fuels. It has also been found that by varying the operation variables such as reaction temperature and plastic waste composition, a desirable product distribution can be obtained, making this a versatile process. There is a variety of different reactors and experimental setups used for the pyrolysis process. For a large industrial scale operation, continuous processes are recommended due to the disadvantages associated with batch processes such as high labor costs and differences of product compositions produced from batch to batch. Fixed bed reactors are typically used as a secondary reactor unit to treat or upgrade the products from the first reaction unit. More recently, microwave assisted technology has been utilized due to enhanced heating rates. This makes the process efficient and cost effective. Co-processing of plastics with biomass, for example, has been found to enhance the quality of products obtained when compared to pyrolysis of the single material. In addition, the co-pyrolysis process significantly reduces the amount of solid waste in the environment. This process is something that can further reduce the need for other PSW management methods.

168

PLASTICS

References [1] Plastics Europe, The facts: an analysis of European plastics production, demand and waste data. ,http://www.plasticseurope.org/ documents/document/20161014113313-plastics_ 2016 the_facts_2016_final_version.pdf., (accessed 19.12.17). [2] R. Geyer, J.R. Jambeck, K.L. Law, Production, use, and fate of all plastics ever made, Sci. Adv. 3 (7) (2017) e1700782. [3] J.A. Onwudili, N. Insura, P.T. Williams, Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: effects of temperature and residence time, J. Analyt. Appl. Pyrol. 86 (2) (2009) 293 303. [4] S.D.A. Sharuddin, F. Abnisa, W.M.A.W. Daud, M.K. Aroua, A review on pyrolysis of plastic wastes, Energy Convers. Manage. 115 (2016) 308 326. [5] B. Kunwar, H.N. Cheng, S.R. Chandrashekaran, B.K. Sharma, Plastics to fuel: a review, Renew. Sustain. Energy Rev. 54 (2016) 421 428. [6] F. Kreith (Ed.), Mechanical Engineering Handbook Series, CRC Press, Taylor & Francis Group, 1998. [7] F. Abnisa, W.M.A. Wan Daud, A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil, Energy Convers. Manage. 87 (2014) 71 85. [8] M.N. Islam, M.R.A. Beg, Fixed bed pyrolysis of waste plastic for alternative fuel production, J. Energy Environ. 3 (2004) 69 80. [9] S.M. Al-Salem, A. Antelava, A. Constantinou, G. Manos, A. Dutta, A review on thermal and catalytic pyrolysis of plastic solid waste (PSW), J. Environ. Manage. 197 (2017) 177 198. [10] S.M. Al-Salem, P. Lettieri, J. Baeyens, Recycling and recovery routes of plastic solid waste (PSW): a review, Waste Manage. 29 (10) (2009) 2625 2643. [11] H. Qinglan, W. Chang, L. Dingqiang, W. Yao, L. Dan, L. Guiju, Production of hydrogen-rich gas from plant biomass by catalytic pyrolysis at low temperature, Int. J. Hydrogen. Energy 35 (17) (2010) 8884 8890. [12] A.K. Panda, R.K. Singh, D.K. Mishra, Thermolysis of waste plastics to liquid fuel. A

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

TO

ENERGY

suitable method for plastic waste management and manufacture of value added products—a world prospective, Renew. Sustain. Energy Rev. 14 (1) (2010) 233 248. D. Chen, L. Yin, H. Wang, P. He, Reprint of: pyrolysis technologies for municipal solid waste: a review, Waste Manage. 37 (2015) 116 136. S.M. FakhrHoseini, M. Dastanian, Predicting pyrolysis products of PE, PP, and PET using NRTL activity coefficient model, J. Chem. (2013). Available from: https://doi.org/ 10.1155/2013/487676. 2013, Article ID 487676, 5 pages. F.J. Mastral, E. Esperanza, P. Garcıa, M. Juste, Pyrolysis of high-density polyethylene in a fluidised bed reactor. Influence of the temperature and residence time, J. Analyt. Appl. Pyrolysis 63 (1) (2002) 1 15. R.K. Singh, B. Ruj, Time and temperature depended fuel gas generation from pyrolysis of real world municipal plastic waste, Fuel 174 (2016) 164 171. Z. Fodor, J.J. Klemeˇs, Waste as alternative fuel minimising emissions and effluents by advanced design, Process Safety Environ. Protect. 90 (3) (2012) 263 284. J. Scheirs, Overview of commercial pyrolysis processes for waste plastics, in: Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics Into Diesel and Other Fuels, 2006, pp. 381 433. C. Santella, L. Cafiero, D. De Angelis, F. La Marca, R. Tuffi, S.V. Ciprioti, Thermal and catalytic pyrolysis of a mixture of plastics from small waste electrical and electronic equipment (WEEE), Waste Manage. 54 (2016) 143 152. J. Mertinkat, A. Kirsten, M. Predel, W. Kaminsky, Cracking catalysts used as fluidized bed material in the Hamburg pyrolysis process, J. Analyt. Appl. Pyrolysis 49 (1 2) (1999) 87 95. A. Marcilla, M.I. Beltra´n, R. Navarro, Thermal and catalytic pyrolysis of polyethylene over HZSM5 and HUSY zeolites in a batch reactor under dynamic conditions, Appl. Cataly. B: Environ. 86 (1 2) (2009) 78 86. J. Walendziewski, M. Steininger, Thermal and catalytic conversion of waste polyolefines, Cataly. Today 65 (2) (2001) 323 330. C. Vasile, H. Pakdel, B. Mihai, P. Onu, H. Darie, S. Ciocaˆlteu, Thermal and catalytic

6: CATALYTIC CONVERSION

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

AND

CHEMICAL RECOVERY

decomposition of mixed plastics, J. Analyt. Appl. Pyrolysis 57 (2) (2001) 287 303. E. Butler, G. Devlin, D. Meier, K. McDonnell, A review of recent laboratory research and commercial developments in fast pyrolysis and upgrading, Renew. Sustain. Energy Rev. 15 (8) (2011) 4171 4186. J. Aguado, D.P. Serrano, J.M. Escola, Catalytic upgrading of plastic wastes, in: Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley & Sons, Ltd, 2006, pp. 73 110. S.R. Ivanova, E.F. Gumerova, K.S. Minsker, G.E. Zaikov, A.A. Berlin, Selective catalytic degradation of polyolefins, Progress Polym. Sci. 15 (2) (1990) 193 215. C.J. Adams, M.J. Earle, K.R. Seddon, Catalytic cracking reactions of polyethylene to light alkanes in ionic liquids, Green Chem. 2 (1) (2000) 21 24. D. Zhao, M. Wu, Y. Kou, E. Min, Ionic liquids: applications in catalysis, Cataly. Today 74 (1 2) (2002) 157 189. K.H. Lee, Thermal and catalytic degradation of waste HDPE, in: Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley & Sons, Ltd, 2006, pp. 129 160. J.W. Park, J.H. Kim, G. Seo, The effect of pore shape on the catalytic performance of zeolites in the liquid-phase degradation of HDPE, Polym. Degrad. Stability 76 (3) (2002) 495 501. K. Gobin, G. Manos, Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method, Polym. Degrad. Stability 83 (2) (2004) 267 279. K. Gobin, G. Manos, Thermogravimetric study of polymer catalytic degradation over microporous materials, Polym. Degrad. Stability 86 (2) (2004) 225 231. A.A. Garforth, Y.H. Lin, P.N. Sharratt, J. Dwyer, Production of hydrocarbons by catalytic degradation of high density polyethylene in a laboratory fluidised-bed reactor, Appl. Cataly. A: Gen. 169 (2) (1998) 331 342. C. Muhammad, J.A. Onwudili, P.T. Williams, Catalytic pyrolysis of waste plastic from electrical and electronic equipment, J. Analyt. Appl. Pyrolysis 113 (2015) 332 339.

169

[35] G. Manos, A. Garforth, J. Dwyer, Catalytic degradation of high-density polyethylene over different zeolitic structures, Ind. Eng. Chem. Res. 39 (2000) 1198 1202. [36] F. Ate¸s, N. Miskolczi, N. Borsodi, Comparision of real waste (MSW and MPW) pyrolysis in batch reactor over different catalysts. Part I: Product yields, gas and pyrolysis oil properties, Bioresour. Technol. 133 (2013) 443 454. [37] D.P. Serrano, J. Aguado, J.M. Escola, Catalytic conversion of polystyrene over HMCM-41, HZSM-5 and amorphous SiO2 Al2O3: comparison with thermal cracking, Appl. Cataly. B: Environ. 25 (2000) 181 189. [38] W.J. Hall, N. Zakaria, P.T. Williams, Pyrolysis of latex gloves in the presence of Y-zeolite, Waste Manage. 29 (2) (2009) 797 803. [39] A. Marcilla, M.I. Beltra´n, R. Navarro, Evolution of products during the degradation of polyethylene in a batch reactor, J. Analyt. Appl. Pyrolysis 86 (1) (2009) 14 21. [40] D.S. Scott, S.R. Czernik, J. Piskorz, D.S.A. Radlein, Fast pyrolysis of plastic wastes, Energy Fuels 4 (4) (1990) 407 411. [41] B.L.F. Chin, S. Yusup, A. Al Shoaibi, P. Kannan, C. Srinivasakannan, S.A. Sulaiman, Kinetic studies of co-pyrolysis of rubber seed shell with high density polyethylene, Energy Convers. Manage. 87 (2014) 746 753. [42] H. Bockhorn, Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements, J. Analyt. Appl. Pyrolysis 50 (2) (1999) 77 101. [43] A. Marcilla, J.C. Garcı´a-Quesada, S. Sa´nchez, R. Ruiz, Study of the catalytic pyrolysis behaviour of polyethylene polypropylene mixtures, J. Analyt. Appl. Pyrolysis 74 (1 2) (2005) 387 392. ¨ . C¸epeliogˇullar, A.E. Pu¨tu¨n, Utilization of [44] O two different types of plastic wastes from daily and industrial life, J. Selcuk Univ. Nat. Appl. Sci. 2 (2) (2013) 694 706. [45] J. Molto´, R. Font, J.A. Conesa, Kinetic model of the decomposition of a PET fibre cloth in an inert and air environment, J. Analyt. Appl. Pyrolysis 79 (1 2 SPEC. ISS.) (2007) 289 296. ¨ . C¸epeliogˇullar, A.E. Pu¨tu¨n, Thermal and [46] O kinetic behaviors of biomass and plastic

170

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

PLASTICS

wastes in co-pyrolysis, Energy Convers. Manage. 75 (2013) 263 270. L. Tiikma, H. Tamvelius, L. Luik, Coprocessing of heavy shale oil with polyethylene waste, J. Analyt. Appl. Pyrolysis 79 (1 2 SPEC. ISS.) (2007) 191 195. J.F. Mastral, C. Berrueco, M. Gea, J. Ceamanos, Catalytic degradation of high density polyethylene over nanocrystalline HZSM5 zeolite, Polym. Degrad. Stabil. 91 (12) (2006) 3330 3338. M.R. Jan, J. Shah, H. Gulab, Catalytic degradation of waste high-density polyethylene into fuel products using BaCO3 as a catalyst, Fuel Process. Technol. 91 (11) (2010) 1428 1437. J. Schirmer, J.S. Kim, E. Klemm, Catalytic degradation of polyethylene using thermal gravimetric analysis and a cycled-spheresreactor, J. Analyt. Appl. Pyrolysis 60 (2) (2001) 205 217. P.N. Sharratt, Y.H. Lin, A.A. Garforth, J. Dwyer, Investigation of the catalytic pyrolysis of high-density polyethylene over a HZSM-5 catalyst in a laboratory fluidized-bed reactor, Ind. Eng. Chem. Res. 36 (12) (1997) 5118 5124. N.S. Akpanudoh, K. Gobin, G. Manos, Catalytic degradation of plastic waste to liquid fuel over commercial cracking catalysts: effect of polymer to catalyst ratio/acidity content, J. Molec. Cataly. A: Chem. 235 (1 2) (2005) 67 73. H. Gulab, M.R. Jan, J. Shah, G. Manos, Plastic catalytic pyrolysis to fuels as tertiary polymer recycling method: effect of process conditions, J. Environ. Sci. Health Part A 45 (7) (2010) 908 915. G. Manos, A. Garforth, J. Dwyer, Catalytic degradation of high-density polyethylene on an ultrastable-Y zeolite. Nature of initial polymer reactions, pattern of formation of gas and liquid products, and temperature effects, Ind. Eng. Chem. Res. 39 (2000) 1203 1208. B.K. Sharma, B.R. Moser, K.E. Vermillion, K.M. Doll, N. Rajagopalan, Production, characterization and fuel properties of alternative diesel fuel from pyrolysis of waste plastic grocery bags, Fuel Process. Technol. 122 (2014) 79 90. D.P. Serrano, J. Aguado, E. Garagorri, J.M. Rodrı´guez, L. Morselli, G. Palazzi, et al.,

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

[65]

[66]

[67]

[68]

TO

ENERGY

Feedstock recycling of agriculture plastic film wastes by catalytic cracking, Appl. Cataly. B: Environ. 49 (4) (2004) 257 265. S. Kumar, R.K. Singh, Recovery of hydrocarbon liquid from waste high density polyethylene by thermal pyrolysis, Braz. J. Chem. Eng. 28 (4) (2011) 659 667. N. Horvat, F.T. Ng, Tertiary polymer recycling: study of polyethylene thermolysis as a first step to synthetic diesel fuel, Fuel 78 (4) (1999) 459 470. P.T. Williams, E.A. Williams, Fluidised bed pyrolysis of low density polyethylene to produce petrochemical feedstock, J. Analyt. Appl. Pyrolysis 51 (1) (1999) 107 126. J. Aguado, D.P. Serrano, G. San Miguel, M.C. Castro, S. Madrid, Feedstock recycling of polyethylene in a two-step thermo-catalytic reaction system, J. Analyt. Appl. Pyrolysis 79 (1) (2007) 415 423. I. Ahmad, M.I. Khan, H. Khan, M. Ishaq, R. Tariq, K. Gul, et al., Pyrolysis study of polypropylene and polyethylene into premium oil products, Int. J. Green Energy 12 (7) (2015) 663 671. Y. Liu, J. Qian, J. Wang, Pyrolysis of polystyrene waste in a fluidized-bed reactor to obtain styrene monomer and gasoline fraction, Fuel Process. Technol. 63 (1) (2000) 45 55. R. Miranda, J. Yang, C. Roy, C. Vasile, Vacuum pyrolysis of PVC I. Kinetic study, Polymer Degrad. Stability 64 (1) (1999) 127 144. W. Kaminsky, J.S. Kim, Pyrolysis of mixed plastics into aromatics, J. Analyt. Appl. Pyrolysis 51 (1) (1999) 127 134. M.N. Siddiqui, H.H. Redhwi, Pyrolysis of mixed plastics for the recovery of useful products, Fuel Process. Technol. 90 (4) (2009) 545 552. C. Ludlow-Palafox, H.A. Chase, Microwave pyrolysis of plastic wastes, in: Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels, John Wiley & Sons, Ltd, 2006, pp. 569 594. S.M. Al-Salem, P. Lettieri, Kinetic study of high density polyethylene (HDPE) pyrolysis, Chem. Eng. Res. Design 88 (12) (2010) 1599 1606. A. Lopez, I. De Marco, B.M. Caballero, M.F. Laresgoiti, A. Adrados, Influence of time and

6: CATALYTIC CONVERSION

[69]

[70]

[71]

[72]

[73]

[74]

[75]

[76]

[77]

[78]

AND

CHEMICAL RECOVERY

temperature on pyrolysis of plastic wastes in a semi-batch reactor, Chem. Eng. J. 173 (1) (2011) 62 71. J. Ceamanos, J.F. Mastral, A. Millera, M.E. Aldea, Kinetics of pyrolysis of high density polyethylene. Comparison of isothermal and dynamic experiments, J. Analyt. Appl. Pyrolysis 65 (2) (2002) 93 110. R. Aguado, M. Olazar, B. Gaisa´n, R. Prieto, J. Bilbao, Kinetics of polystyrene pyrolysis in a conical spouted bed reactor, Chem. Eng. J. 92 (1 3) (2003) 91 99. J.M. Saad, M.A. Nahil, P.T. Williams, Influence of process conditions on syngas production from the thermal processing of waste high density polyethylene, J. Analyt. Appl. Pyrolysis 113 (2015) 35 40. M.S. Renzini, L.C. Lerici, U. Sedran, L.B. Pierella, Stability of ZSM-11 and BETA zeolites during the catalytic cracking of lowdensity polyethylene, J. Analyt. Appl. Pyrolysis 92 (2) (2011) 450 455. L. Ballice, A kinetic approach to the temperature-programmed pyrolysis of low-and high-density polyethylene in a fixed bed reactor: determination of kinetic parameters for the evolution of n-paraffins and 1-olefins, Fuel 80 (13) (2001) 1923 1935. R. Bagri, P.T. Williams, Catalytic pyrolysis of polyethylene, J. Analyt. Appl. Pyrolysis 63 (1) (2002) 29 41. P. Onu, C. Vasile, S. Ciocılteu, E. Iojoiu, H. Darie, Thermal and catalytic decomposition of polyethylene and polypropylene, J. Analyt. Appl. Pyrolysis 49 (1 2) (1999) 145 153. S.H. Jung, M.H. Cho, B.S. Kang, J.S. Kim, Pyrolysis of a fraction of waste polypropylene and polyethylene for the recovery of BTX aromatics using a fluidized bed reactor, Fuel Process. Technol. 91 (3) (2010) 277 284. G. Luo, T. Suto, S. Yasu, K. Kato, Catalytic degradation of high density polyethylene and polypropylene into liquid fuel in a powderparticle fluidized bed, Polym. Degrad. Stab. 70 (1) (2000) 97 102. S.L. Wong, N. Ngadi, T.A.T. Abdullah, I.M. Inuwa, Current state and future prospects of plastic waste as source of fuel: a review, Renew. Sustain. Energy Rev. 50 (2015) 1167 1180.

171

[79] M. Arabiourrutia, G. Elordi, G. Lopez, E. Borsella, J. Bilbao, M. Olazar, Characterization of the waxes obtained by the pyrolysis of polyolefin plastics in a conical spouted bed reactor, J. Analyt. Appl. Pyrolysis 94 (2012) 230 237. [80] G. Elordi, M. Olazar, P. Castan˜o, M. Artetxe, J. Bilbao, Polyethylene cracking on a spent FCC catalyst in a conical spouted bed, Ind. Eng. Chem. Res. 51 (43) (2012) 14008 14017. [81] H.S. Fogler, Essentials of Chemical Reaction Engineering, Pearson Education, 2010. [82] W. Kaminsky, I.J.N. Zorriqueta, Catalytical and thermal pyrolysis of polyolefins, J. Analyt. Appl. Pyrolysis 79 (1) (2007) 368 374. [83] A. Lo´pez, I. De Marco, B.M. Caballero, M.F. Laresgoiti, A. Adrados, A. Aranzabal, Catalytic pyrolysis of plastic wastes with two different types of catalysts: ZSM-5 zeolite and Red Mud, Appl. Cataly. B: Environ. 104 (3 4) (2011) 211 219. [84] S.S. Lam, H.A. Chase, A review on waste to energy processes using microwave pyrolysis, Energies 5 (10) (2012) 4209 4232. [85] Z. Hussain, K.M. Khan, S. Perveen, K. Hussain, W. Voelter, The conversion of waste polystyrene into useful hydrocarbons by microwave-metal interaction pyrolysis, Fuel Process. Technol. 94 (1) (2012) 145 150. [86] M.S. Abbas-Abadi, M.N. Haghighi, H. Yeganeh, A.G. McDonald, Evaluation of pyrolysis process parameters on polypropylene degradation products, J. Analyt. Appl. Pyrolysis 109 (2014) 272 277. [87] A. Lopez-Urionabarrenechea, I. De Marco, B. M. Caballero, M.F. Laresgoiti, A. Adrados, Catalytic stepwise pyrolysis of packaging plastic waste, J. Analyt. Appl. Pyrolysis 96 (2012) 54 62. [88] M.F. Ali, M.N. Siddiqui, Thermal and catalytic decomposition behavior of PVC mixed plastic waste with petroleum residue, J. Analyt. Appl. Pyrolysis 74 (1 2) (2005) 282 289. [89] F.J. Passamonti, U. Sedran, Recycling of waste plastics into fuels. LDPE conversion in FCC, Appl. Cataly. B: Environ. 125 (2012) 499 506.

172

[90] A.O. Odjo, A.N. Garcı´a, A. Marcilla, Conversion of low density polyethylene into fuel through co-processing with vacuum gas oil in a fluid catalytic cracking riser reactor, Fuel Process. Technol. 113 (2013) 130 140. [91] F. Abnisa, W.W. Daud, S. Ramalingam, M.N. B.M. Azemi, J.N. Sahu, Co-pyrolysis of palm shell and polystyrene waste mixtures to synthesis liquid fuel, Fuel 108 (2013) 311 318.

PLASTICS

TO

ENERGY

[92] M. Brebu, S. Ucar, C. Vasile, J. Yanik, Copyrolysis of pine cone with synthetic polymers, Fuel 89 (8) (2010) 1911 1918. [93] J. Chattopadhyay, T.S. Pathak, R. Srivastava, A.C. Singh, Catalytic co-pyrolysis of paper biomass and plastic mixtures (HDPE (high density polyethylene), PP (polypropylene) and PET (polyethylene terephthalate)) and product analysis, Energy 103 (2016) 513 521.