**5. Chemical recycling of polyethylene (LDPE and HDPE)**

#### **5.1 Introduction**

Under the category of chemical recycling of polyethylenes, advanced process (similar to those employed in the petrochemical industry) appear e.g. pyrolysis, gasification, liquid–gas hydrogenation, viscosity breaking, steam or catalytic cracking (Al-Salem *et al,* 2009). Catalytic cracking and reforming facilitate the selective degradation of waste plastics. The use of solid catalysts such as silica alumina, ZSM-5, zeolites, and mesoporous materials for these purposes has been reported. These materials effectively convert polyolefins into liquid fuel, giving lighter fractions as compared to thermal cracking (Al-Salem *et al*, 2009).

In particular, polyethylene has been targeted as a potential feedstock for fuel (gasoline) producing technologies. PE thermally cracks into gases, liquids, waxes, aromatics and char. The relative amounts of gas and liquid fraction are very much dependent on the type of polymer used. Thus, higher decomposition was observed in PP, followed by LDPE and finally HDPE. It seems that less crystalline or more branched polymers are less stable in thermal degradation (Achilias et al., 2007). Many papers have been published recently on this subject and excellent reviews can be found in the book by Scheirs and Kaminsky, 2006 and Achilias et al., 2006.

Polyethylene (as well as other vinyl polymers) degrade via a four step free radical mechanism: radical initiation, de-propagation (as opposed to propagation in the case of polymerization), intermolecular and intramolecular hydrogen transfer followed by βscission (initial step in the chemistry of thermal cracking of hydrocarbons and the formation of free radicals) and, lastly, radical termination. β-Scission and hydrogen abstraction steps often occur together in a chain propagation sequence. That is, a radical abstracts a hydrogen atom from the reactant to form a molecule and a new radical. A bond β is then broken to the radical centre (β-scission) to regenerate an abstracting radical and to produce a molecule with a double bond (a molecule with a double bond involving the carbon atom that had been the radical centre). Sample size and surface area to volume ratio of the melt have a significant influence on the rate and relative importance of the various mechanisms of polymer degradation. In pyrolysis, which is normally done on micro-scale, only random initiation and intermolecular transfer were reported to be important. Conversely, on milligram scale of polyethylene charges and samples, intermolecular transfer of hydrogen atoms via abstraction by free radicals was considered to be the predominant transfer mechanism to produce volatiles. There is also a growing interest in developing value added products such as synthetic lubricants via PE thermal degradation.

The development of value added recycling technologies is highly desirable as it would increase the economic incentive to recycle polymers. Several methods for chemical recycling are presently in use, such as direct chemical treatment involving gasification, smelting by blast furnace or coke oven, and degradation by liquefaction. The main advantage of chemical recycling is the possibility of treating heterogeneous and contaminated polymers with limited use of pre-treatment. Petrochemical plants are much greater in size (6–10 times) than plastic manufacturing plants. It is essential to utilize petrochemical plants in supplementing their usual feedstock by using plastic solid wastes (PSW) derived feedstock (Al-Salem *et al,* 2009).

### **5.2 Thermolysis schemes and technologies**

#### **5.2.1 Pyrolysis**

26 Material Recycling – Trends and Perspectives

methylnaphthalene, 1-methylnaphthalene, biphenyl, methylbiphenyl, dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, ethylbiphenyl; three ring compounds like

1. Thermal recycling of PS yields higher percentage of styrene monomer, which can be fermented by bacteria to produce polyhydroxyalkanoates (PHA)—the starting material

2. The waste PS can be blended with biodegradable polymers to produce biodegradable

3. Styrene monomer produced by recycling can be grafted onto biodegradable polymers

Under the category of chemical recycling of polyethylenes, advanced process (similar to those employed in the petrochemical industry) appear e.g. pyrolysis, gasification, liquid–gas hydrogenation, viscosity breaking, steam or catalytic cracking (Al-Salem *et al,* 2009). Catalytic cracking and reforming facilitate the selective degradation of waste plastics. The use of solid catalysts such as silica alumina, ZSM-5, zeolites, and mesoporous materials for these purposes has been reported. These materials effectively convert polyolefins into liquid

In particular, polyethylene has been targeted as a potential feedstock for fuel (gasoline) producing technologies. PE thermally cracks into gases, liquids, waxes, aromatics and char. The relative amounts of gas and liquid fraction are very much dependent on the type of polymer used. Thus, higher decomposition was observed in PP, followed by LDPE and finally HDPE. It seems that less crystalline or more branched polymers are less stable in thermal degradation (Achilias et al., 2007). Many papers have been published recently on this subject and excellent reviews can be found in the book by Scheirs and Kaminsky, 2006

Polyethylene (as well as other vinyl polymers) degrade via a four step free radical mechanism: radical initiation, de-propagation (as opposed to propagation in the case of polymerization), intermolecular and intramolecular hydrogen transfer followed by βscission (initial step in the chemistry of thermal cracking of hydrocarbons and the formation of free radicals) and, lastly, radical termination. β-Scission and hydrogen abstraction steps often occur together in a chain propagation sequence. That is, a radical abstracts a hydrogen atom from the reactant to form a molecule and a new radical. A bond β is then broken to the

fuel, giving lighter fractions as compared to thermal cracking (Al-Salem *et al*, 2009).

phenanthrenes and four ring compounds like pyrenes and chrysenes.

Some future prospectus of PS recycling include (Maharana et al., 2007).

**5. Chemical recycling of polyethylene (LDPE and HDPE)** 

for the synthesis of biodegradable polymers.

to give biodegradable polymers.

**4.4 Future prospectus** 

polymers.

**5.1 Introduction** 

and Achilias et al., 2006.

Thermolysis is the treatment in the presence of heat under controlled temperatures without catalysts. Thermolysis processes can be divided into advanced thermo-chemical or pyrolysis (thermal cracking in an inert atmosphere), gasification (in the sub-stoichiometric presence of air usually leading to CO and CO2 production) and hydrogenation (hydrocracking).

Thermal degradation processes allow obtaining a number of constituting molecules, combustible gases and/or energy, with the reduction of landfilling as an added advantage. The pyrolysis process is an advanced conversion technology that has the ability to produce a clean, high calorific value gas from a wide variety of waste and biomass streams. The hydrocarbon content of the waste is converted into a gas, which is suitable for utilisation in either gas engines, with associated electricity generation, or in boiler applications without the need for flue gas treatment. This process is capable of treating many different solid hydrocarbon based wastes whilst producing a clean fuel gas with a high calorific value. This gas will typically have a calorific value of 22–30 MJ/m3 depending on the waste material being processed. Solid char is also produced from the process, which contains both carbon and the mineral content of the original feed material. The char can either be further processed onsite to release the energy content of the carbon, or utilized offsite in other thermal processes (Al-Salem *et al*, 2009).

The main pyrolysis units and technologies on an industrial scale include PYROPLEQ (rotary drum), Akzo (circulating fluidized bed), NRC (melt furnace), ConTherm technology (rotary drum), PKA pyrolysis (rotary drum), PyroMelt (melt furnace), BP (circulating fluidized

Recent Advances in the Chemical Recycling

(Al-Salem *et al,* 2009).

Salem *et al*., 2010).

**5.4 Catalytic degradation** 

2002) and fixed beds (Achilias et al., 2007).

**5.3 Polyolefins thermal cracking** 

in need of appropriate end-product design.

**5.2.4 Other chemical recycling schemes** 

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 29

Degradative extrusion provides an optimum engineering solution especially on a smallindustrial scale (10 kg/h). Τhe advantages of degradative extrusion as (i) achieving molecular breakdown of thermoplastics and hence low viscosity polymer melts, (ii) applying a combination of mechanical and chemical recycling scheme prompts the degradation process by introducing steam, gas, oxygen or catalysts, if needed. Another advantageous technology for chemical treatment is catalytic and steam cracking. The concept for both processes is the employment of either steam or a catalyst in a unit operation

Appropriate design and scale (of operation and economy) are of paramount importance when it comes to thermal treatment plants. Thermal degradation behaviour in laboratory scale enables the assessment of a number of important parameters, such as thermal kinetics, activation energy assessment (energy required to degrade materials treated and product formation) and determining reference temperatures of the half life of polymers and maximum degradation point achievable. It is also important to perform pilot scale experiments utilizing a number of rectors and unit operation before commencing with an alteration on a performance scale This will also aid in the determination of the mode of the material processing of the thermal plant (i.e. pulsating, continuous, batch, etc.). Pyrolysis (depolymerization in inert atmospheres) is usually the first process in a thermal plant, and is

A number of studies have been carried out (Achilias et al., 2006) on polyolefins thermal cracking in inert (pyrolysis) and/or partially oxidized atmospheres (e.g. step pyrolysis, gasification). Previous reports focused on kinetic parameters estimations by means of different techniques and experimental conditions. Thermogravimetry is the most commonly used technique for the determination of kinetic parameters, although the experimental conditions utilized are very different, involving broad ranges of temperature, sample amount, heating rates (in the case of dynamic runs), reaction atmospheres and pressures. Almost all of previously published literature shows a power law equation to describe the thermal cracking of polymers and perform isothermal and/or dynamic experiments (Al-

Studies concerning the use of different catalysts in the pyrolysis of polyolefins have been conducted by many authors (Achilias et al., 2006). Thus, TG and micro-reactors have been widely used to pyrolyse plastics with zeolite-based acid catalysts (Marcilla et al., 2001; 2004). Catalytic pyrolysis of polyethylene samples has also been carried out in the laboratory scale reactors, such as batch reactors (Seo et al., 2003, Van Grieken et al., 2001), semi-batch reactors (with evacuation of volatile products) (Akpanudoh et al.2005, Cardona and Corma,

The catalytic degradation of polymeric materials has been reported for a large range of model catalysts, including amorphous silica–alumina, zeolites Y, mordenite and ZSM-5, the

bed), BASF (furnace) and NKT (circulating fluidized bed). Detailes can be found in Al-Salem *et al*, 2010.

Pyrolysis provides a number of other advantages, such as (i) operational advantages, (ii) environmental advantages and (iii) financial benefits. Operational advantages could be described by the utilisation of residual output of char used as a fuel or as a feedstock for other petrochemical processes. An additional operational benefit is that pyrolysis requires no flue gas clean up as flue gas produced is mostly treated prior to utilisation. Environmentally, pyrolysis provides an alternative solution to landfilling and reduces greenhouse gas (GHGs) and CO2 emissions. Financially, pyrolysis produces a high calorific value fuel that could be easily marketed and used in gas engines to produce electricity and heat. Several obstacles and disadvantages do exist for pyrolysis, mainly the handling of char produced and treatment of the final fuel produced if specific products are desired. In addition, there is not a sufficient understanding of the underlying reaction pathways, which has prevented a quantitative prediction of the full product distribution (Al-Salem *et al.,* 2009).

#### **5.2.2 Gasification**

Air in this process is used as a gasification agent, which demonstrates a number of advantages. The main advantage of using air instead of O2 alone is to simplify the process and reduce the cost. But a disadvantage is the presence of (inert) N2 in air which causes a reduction in the calorific value of resulting fuels due to the dilution effect on fuel gases. Hence, steam is introduced in a stoichiometric ratio to reduce the N2 presence. A significant amount of char is always produced in gasification which needs to be further processed and/or burnt. An ideal gasification process for PSW should produce a high calorific value gas, completely combusted char, produce an easy metal product to separate ash from and should not require any additional installations for air/water pollution abatement (Al-Salem et al, 2009).

Early gasification attempts of plastics, have been reported since the 1970s. The gasification into high calorific value fuel gas obtained from PSW was demonstrated in research stages and results were reported and published in literature for PVC, PP and PET. The need for alternative fuels has lead for the co-gasification of PSW with other types of waste, mainly biomass. Pinto *et al.* (2002, 2003) studied the fluidized bed co-gasification of PE, pine and coal and biomass mixed with PE. Xiao *et al.* (2009) co-gasified five typical kinds of organic components (wood, paper, kitchen garbage, plastic (namely PE), and textile) and three representative types of simulated MSW in a fluidized-bed (400–800 0C). It was determined that plastic should be gasified at temperatures more than 500 0C to reach a lower heating value (LHV) of 10,000 kJ/N (Al-Salem *et al*, 2009).

#### **5.2.3 Hydrogenation (hydrocracking)**

Hydrogenation by definition means the addition of hydrogen by chemical reaction through unit operation. The main technology applied in PSW recycling via hydrogenation technology is the Veba process. Based upon the coal liquefaction technology, Veba Oel AG converted coal by this process into naphtha and gas oil. Major technologies are summarized in Al-Salem *et al*, 2009.

#### **5.2.4 Other chemical recycling schemes**

28 Material Recycling – Trends and Perspectives

bed), BASF (furnace) and NKT (circulating fluidized bed). Detailes can be found in Al-Salem

Pyrolysis provides a number of other advantages, such as (i) operational advantages, (ii) environmental advantages and (iii) financial benefits. Operational advantages could be described by the utilisation of residual output of char used as a fuel or as a feedstock for other petrochemical processes. An additional operational benefit is that pyrolysis requires no flue gas clean up as flue gas produced is mostly treated prior to utilisation. Environmentally, pyrolysis provides an alternative solution to landfilling and reduces greenhouse gas (GHGs) and CO2 emissions. Financially, pyrolysis produces a high calorific value fuel that could be easily marketed and used in gas engines to produce electricity and heat. Several obstacles and disadvantages do exist for pyrolysis, mainly the handling of char produced and treatment of the final fuel produced if specific products are desired. In addition, there is not a sufficient understanding of the underlying reaction pathways, which has prevented a quantitative

Air in this process is used as a gasification agent, which demonstrates a number of advantages. The main advantage of using air instead of O2 alone is to simplify the process and reduce the cost. But a disadvantage is the presence of (inert) N2 in air which causes a reduction in the calorific value of resulting fuels due to the dilution effect on fuel gases. Hence, steam is introduced in a stoichiometric ratio to reduce the N2 presence. A significant amount of char is always produced in gasification which needs to be further processed and/or burnt. An ideal gasification process for PSW should produce a high calorific value gas, completely combusted char, produce an easy metal product to separate ash from and should not require any additional installations for air/water pollution abatement (Al-Salem

Early gasification attempts of plastics, have been reported since the 1970s. The gasification into high calorific value fuel gas obtained from PSW was demonstrated in research stages and results were reported and published in literature for PVC, PP and PET. The need for alternative fuels has lead for the co-gasification of PSW with other types of waste, mainly biomass. Pinto *et al.* (2002, 2003) studied the fluidized bed co-gasification of PE, pine and coal and biomass mixed with PE. Xiao *et al.* (2009) co-gasified five typical kinds of organic components (wood, paper, kitchen garbage, plastic (namely PE), and textile) and three representative types of simulated MSW in a fluidized-bed (400–800 0C). It was determined that plastic should be gasified at temperatures more than 500 0C to reach a lower heating

Hydrogenation by definition means the addition of hydrogen by chemical reaction through unit operation. The main technology applied in PSW recycling via hydrogenation technology is the Veba process. Based upon the coal liquefaction technology, Veba Oel AG converted coal by this process into naphtha and gas oil. Major technologies are summarized

prediction of the full product distribution (Al-Salem *et al.,* 2009).

value (LHV) of 10,000 kJ/N (Al-Salem *et al*, 2009).

**5.2.3 Hydrogenation (hydrocracking)** 

in Al-Salem *et al*, 2009.

*et al*, 2010.

**5.2.2 Gasification** 

et al, 2009).

Degradative extrusion provides an optimum engineering solution especially on a smallindustrial scale (10 kg/h). Τhe advantages of degradative extrusion as (i) achieving molecular breakdown of thermoplastics and hence low viscosity polymer melts, (ii) applying a combination of mechanical and chemical recycling scheme prompts the degradation process by introducing steam, gas, oxygen or catalysts, if needed. Another advantageous technology for chemical treatment is catalytic and steam cracking. The concept for both processes is the employment of either steam or a catalyst in a unit operation (Al-Salem *et al,* 2009).

### **5.3 Polyolefins thermal cracking**

Appropriate design and scale (of operation and economy) are of paramount importance when it comes to thermal treatment plants. Thermal degradation behaviour in laboratory scale enables the assessment of a number of important parameters, such as thermal kinetics, activation energy assessment (energy required to degrade materials treated and product formation) and determining reference temperatures of the half life of polymers and maximum degradation point achievable. It is also important to perform pilot scale experiments utilizing a number of rectors and unit operation before commencing with an alteration on a performance scale This will also aid in the determination of the mode of the material processing of the thermal plant (i.e. pulsating, continuous, batch, etc.). Pyrolysis (depolymerization in inert atmospheres) is usually the first process in a thermal plant, and is in need of appropriate end-product design.

A number of studies have been carried out (Achilias et al., 2006) on polyolefins thermal cracking in inert (pyrolysis) and/or partially oxidized atmospheres (e.g. step pyrolysis, gasification). Previous reports focused on kinetic parameters estimations by means of different techniques and experimental conditions. Thermogravimetry is the most commonly used technique for the determination of kinetic parameters, although the experimental conditions utilized are very different, involving broad ranges of temperature, sample amount, heating rates (in the case of dynamic runs), reaction atmospheres and pressures. Almost all of previously published literature shows a power law equation to describe the thermal cracking of polymers and perform isothermal and/or dynamic experiments (Al-Salem *et al*., 2010).

#### **5.4 Catalytic degradation**

Studies concerning the use of different catalysts in the pyrolysis of polyolefins have been conducted by many authors (Achilias et al., 2006). Thus, TG and micro-reactors have been widely used to pyrolyse plastics with zeolite-based acid catalysts (Marcilla et al., 2001; 2004). Catalytic pyrolysis of polyethylene samples has also been carried out in the laboratory scale reactors, such as batch reactors (Seo et al., 2003, Van Grieken et al., 2001), semi-batch reactors (with evacuation of volatile products) (Akpanudoh et al.2005, Cardona and Corma, 2002) and fixed beds (Achilias et al., 2007).

The catalytic degradation of polymeric materials has been reported for a large range of model catalysts, including amorphous silica–alumina, zeolites Y, mordenite and ZSM-5, the

Recent Advances in the Chemical Recycling

enhancement.

finished.

**5.5.2 Batch reactors** 

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 31

due to their cyclic movement. Vigorous solid flow and the action of the spout avoid the formation of agglomerates. Furthermore, the CSBR has great versatility in terms of gas residence time, which may be reduced to values near 20 ms (Olazar et al., 1999) and, consequently, the yield of polyaromatic compounds is minimized. Besides, the smaller attrition of catalyst particles, due to the absence of a distributor plate, is another advantage over the fluidized bed for its use in catalytic processes. This excellent behaviour of the CSBR has already been recorded in other processes carried out prior to catalytic pyrolysis, such as catalytic polymerization, where a similar problem of fusion of catalyst particles coated with polymer occurs (Olazar et al., 1994). The simple design of a CSBR makes its scaling up straightforward. Furthermore, its throughput by reactor volume unit is higher than that of a bubbling fluidized bed due to the lower amount of sand required for fluidization

Thermal and catalytic degradation of polyethylene was conducted by Seo et al. 2003 at atmospheric pressure in a batch type reactor as is illustrated in Fig. 4. The reactor was a 1.1 liter round shape stainless steel bottle placed in a thermostatic furnace. Experimental procedure is as follows. The reactor system was connected to a nitrogen supply to eliminate air before premixed plastics and catalysts were fed into the reactor. Temperature of the reactor was increased to 450 0C and held for 30 min until the reaction was completely

Fig. 3. Schematic representation of the continuous pyrolysis unit (Elordi et al. 2009).

family of mesoporous MCM-41 materials (Marcilla *et al.,* 2002; 2003) and a few silicoaluminophosphate molecular sieves (Araujo *et al.,* 2002, Fernandes et al., 2002). Catalytic activity is closely related to the amount of acid sites, pore size and also shape of the catalyst (Park *et al.*, 2008, Serrano *et al.,* 2003). Silicoaluminophosphate (SAPO) molecular sieves represent an important class of adsorbents and catalytic materials generated by the introduction of silicon into its aluminophosphate framework. The medium pore SAPOs are attractive for catalytic applications due to the presence of specific acid sites in its structure which can convert the polymer into useful hydrocarbons (Elordi *et al*., 2009, Singhal *et al.,* 2010, Park *et al*. 2008). The use of BaCO3 as a catalyst for the thermal and catalytic degradation of waste HDPE was also reported (Rasul Jan *et al*., 2010).

The catalysts more frequently employed for the cracking of polyolefins are shape-selective zeolites and mesoporous materials, such as HY, HZSM-5, Hβ or MCM-41 (Huang et al., 2009), which undergo inevitable deactivation by coke deposition. Indeed, this deactivation is a major hurdle in the implementation and scale-up of the valorization of plastics by cracking (Marcilla et al. 2007). Microporous zeolites have very high thermal stability and customized acid sites. Thus, the selection of the zeolite should be based on a target selectivity: HZSM-5 zeolite promotes the production of olefins (original monomers), while Hβ and HY zeolites maximize the production of middle distillates (Elordi et al., 2009).

The relevant literature reports well-founded mechanisms for coke formation and protocols for characterizing the coke deposited on zeolites. These studies assay reactions such as the cracking of hydrocarbons (Cerqueira et al., 2005; 2008, Guisnet et al., 2009). Marcilla et al. studied the deactivation of zeolites during the cracking of high-density polyethylene (HDPE), by using mainly a thermobalance as reactor. It should be pointed out that coke formation is strongly affected by the following factors, amongst others: catalyst properties (e.g. shape selectivity, acidity, and concentration of acid sites) (Huang et al, 2009), reactor medium (Aguayo et al., 1997), operating conditions or feedstock properties.

#### **5.5 Reactor types**

#### **5.5.1 Pyrolysis in a fluidized bed reactors**

Pyrolysis in a fluidized bed reactor and similar devices is the one with most possibilities for large-scale implementation for continuous waste plastic upgrading. The thermal degradation of plastic polymers has been studied first (Predel and Kaminsky, 2000, Berrueco et al. 2002, Mastellone et al., 2002, Mastral et al., 2002), but in situ catalytic pyrolysis has become a relevant research topic (Mastral et al., 2006, Hernández et al., 2007). The conical spouted bed reactor (CSBR) presents interesting conditions for catalytic pyrolysis because of the low bed segregation and lower attrition than the bubbling fluidized bed. The good performance of the CSBR has been proven for the selective production of waxes (Aguado et al., 2002), fuel-like hydrocarbons (Elordi et al., 2007) and monomers (Elordi et al., 2007). This good performance is a consequence of the solid flow pattern, high heat transfer between phases and the smaller defluidization problems when sticky solids are handled. Defluidization is due to the agglomeration of soid particles (sand) coated with melted plastic, constituting a severe problem in fluidized bed reactors. In the CSBR, polyolefins melt as they are fed into the reactor and they uniformly coat the sand and catalyst particles due to their cyclic movement. Vigorous solid flow and the action of the spout avoid the formation of agglomerates. Furthermore, the CSBR has great versatility in terms of gas residence time, which may be reduced to values near 20 ms (Olazar et al., 1999) and, consequently, the yield of polyaromatic compounds is minimized. Besides, the smaller attrition of catalyst particles, due to the absence of a distributor plate, is another advantage over the fluidized bed for its use in catalytic processes. This excellent behaviour of the CSBR has already been recorded in other processes carried out prior to catalytic pyrolysis, such as catalytic polymerization, where a similar problem of fusion of catalyst particles coated with polymer occurs (Olazar et al., 1994). The simple design of a CSBR makes its scaling up straightforward. Furthermore, its throughput by reactor volume unit is higher than that of a bubbling fluidized bed due to the lower amount of sand required for fluidization enhancement.

### **5.5.2 Batch reactors**

30 Material Recycling – Trends and Perspectives

family of mesoporous MCM-41 materials (Marcilla *et al.,* 2002; 2003) and a few silicoaluminophosphate molecular sieves (Araujo *et al.,* 2002, Fernandes et al., 2002). Catalytic activity is closely related to the amount of acid sites, pore size and also shape of the catalyst (Park *et al.*, 2008, Serrano *et al.,* 2003). Silicoaluminophosphate (SAPO) molecular sieves represent an important class of adsorbents and catalytic materials generated by the introduction of silicon into its aluminophosphate framework. The medium pore SAPOs are attractive for catalytic applications due to the presence of specific acid sites in its structure which can convert the polymer into useful hydrocarbons (Elordi *et al*., 2009, Singhal *et al.,* 2010, Park *et al*. 2008). The use of BaCO3 as a catalyst for the thermal and catalytic

The catalysts more frequently employed for the cracking of polyolefins are shape-selective zeolites and mesoporous materials, such as HY, HZSM-5, Hβ or MCM-41 (Huang et al., 2009), which undergo inevitable deactivation by coke deposition. Indeed, this deactivation is a major hurdle in the implementation and scale-up of the valorization of plastics by cracking (Marcilla et al. 2007). Microporous zeolites have very high thermal stability and customized acid sites. Thus, the selection of the zeolite should be based on a target selectivity: HZSM-5 zeolite promotes the production of olefins (original monomers), while Hβ and HY zeolites

The relevant literature reports well-founded mechanisms for coke formation and protocols for characterizing the coke deposited on zeolites. These studies assay reactions such as the cracking of hydrocarbons (Cerqueira et al., 2005; 2008, Guisnet et al., 2009). Marcilla et al. studied the deactivation of zeolites during the cracking of high-density polyethylene (HDPE), by using mainly a thermobalance as reactor. It should be pointed out that coke formation is strongly affected by the following factors, amongst others: catalyst properties (e.g. shape selectivity, acidity, and concentration of acid sites) (Huang et al, 2009), reactor

Pyrolysis in a fluidized bed reactor and similar devices is the one with most possibilities for large-scale implementation for continuous waste plastic upgrading. The thermal degradation of plastic polymers has been studied first (Predel and Kaminsky, 2000, Berrueco et al. 2002, Mastellone et al., 2002, Mastral et al., 2002), but in situ catalytic pyrolysis has become a relevant research topic (Mastral et al., 2006, Hernández et al., 2007). The conical spouted bed reactor (CSBR) presents interesting conditions for catalytic pyrolysis because of the low bed segregation and lower attrition than the bubbling fluidized bed. The good performance of the CSBR has been proven for the selective production of waxes (Aguado et al., 2002), fuel-like hydrocarbons (Elordi et al., 2007) and monomers (Elordi et al., 2007). This good performance is a consequence of the solid flow pattern, high heat transfer between phases and the smaller defluidization problems when sticky solids are handled. Defluidization is due to the agglomeration of soid particles (sand) coated with melted plastic, constituting a severe problem in fluidized bed reactors. In the CSBR, polyolefins melt as they are fed into the reactor and they uniformly coat the sand and catalyst particles

degradation of waste HDPE was also reported (Rasul Jan *et al*., 2010).

maximize the production of middle distillates (Elordi et al., 2009).

medium (Aguayo et al., 1997), operating conditions or feedstock properties.

**5.5 Reactor types** 

**5.5.1 Pyrolysis in a fluidized bed reactors** 

Thermal and catalytic degradation of polyethylene was conducted by Seo et al. 2003 at atmospheric pressure in a batch type reactor as is illustrated in Fig. 4. The reactor was a 1.1 liter round shape stainless steel bottle placed in a thermostatic furnace. Experimental procedure is as follows. The reactor system was connected to a nitrogen supply to eliminate air before premixed plastics and catalysts were fed into the reactor. Temperature of the reactor was increased to 450 0C and held for 30 min until the reaction was completely finished.

Fig. 3. Schematic representation of the continuous pyrolysis unit (Elordi et al. 2009).

Recent Advances in the Chemical Recycling

**5.5.3 Fixed bed reactor** 

Polymer Temperature

(oC)

pyrolysis of LDPE, HDPE and PP (Achilias et al., 2007).

Fig. 6. The fixed bed reactor system (Achilias et al., 2007).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 33

Achilias et al. (2007) used a laboratory-scale fixed bed reactor (Fig. 6) to study the thermal and catalytic degradation of polyethylene. The reactor was filled with the FCC catalyst and the piston was filled with the polymer. The time of the experiment was 17 min and the reaction temperature 450 0C. Experimental conditions and product yield from the thermal

Catalyst Gaseous

LDPE 450 - 1.4 22.2 76.4 HDPE 450 - 1.7 21.6 76.7 PP 450 - 4.1 49.3 46.6 LDPE 450 FCC 0.5 46.6 52.9 HDPE 450 FCC 0.5 38.5 61.0 PP 450 FCC 6.2 67.3 26.5

Table 13. Experimental conditions and product yield from the thermal and catalytic

product (wt.%)

Liquid product (wt.%)

Residue (wt.%)

and catalytic pyrolysis of LDPE, HDPE and PP appear in Table 13.

Fig. 4. Schematic diagram of a batch pyrolysis system (Seo et al., 2003).

Another type of laboratory scale batch reactor was used for the catalytic degradation of polyethylene by Van Grieken et al. (2001). The experiments were carried out in a batch reactor provided with a helicoidal stirrer at 120 rpm (Fig. 5). Three temperatures (380, 400 and 420°C) and different reaction times (0–360 min) under nitrogen flow were studied. The effluent from the reactor was connected to a water-cooled trap in order to condense the liquid products, whereas the effluent gas was finally collected in a teflon bag.

Fig. 5. Scheme of the experimental cracking reaction system (Van Grieken et al., 2001).

### **5.5.3 Fixed bed reactor**

32 Material Recycling – Trends and Perspectives

Fig. 4. Schematic diagram of a batch pyrolysis system (Seo et al., 2003).

liquid products, whereas the effluent gas was finally collected in a teflon bag.

Fig. 5. Scheme of the experimental cracking reaction system (Van Grieken et al., 2001).

Another type of laboratory scale batch reactor was used for the catalytic degradation of polyethylene by Van Grieken et al. (2001). The experiments were carried out in a batch reactor provided with a helicoidal stirrer at 120 rpm (Fig. 5). Three temperatures (380, 400 and 420°C) and different reaction times (0–360 min) under nitrogen flow were studied. The effluent from the reactor was connected to a water-cooled trap in order to condense the Achilias et al. (2007) used a laboratory-scale fixed bed reactor (Fig. 6) to study the thermal and catalytic degradation of polyethylene. The reactor was filled with the FCC catalyst and the piston was filled with the polymer. The time of the experiment was 17 min and the reaction temperature 450 0C. Experimental conditions and product yield from the thermal and catalytic pyrolysis of LDPE, HDPE and PP appear in Table 13.


Table 13. Experimental conditions and product yield from the thermal and catalytic pyrolysis of LDPE, HDPE and PP (Achilias et al., 2007).

Fig. 6. The fixed bed reactor system (Achilias et al., 2007).

Recent Advances in the Chemical Recycling

processes (Slapak et al., 1999).

successful recycling (Wu et al., 2009).

added to obtain high purity hydrogen chloride gas.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 35

Beside all above problems, net energy recovered by incineration of PVC-rich waste is not high enough to make it highly economic. As most hydrocarbon polymers, the calorific value from incineration of PVC in an ideal conditions is about 64 MJ/kg, compared to, for example, 17 MJ/kg for paper, or 16 MJ/kg for wood. Moreover, PVC is inherently difficult to combust, so that complete combustion of PVC-rich waste occurs at such high

Therefore mechanical and/or chemical recycling of PVC plastic wastes seems the logical solution One usual approach for chemical recycling of PVC wastes is currently "thermal cracking" via hydrogenation, pyrolysis or gasification (Ryu et al., 2007; Williams and Williams, 1998; DeMarco et al., 2002; Kaminsky and Kim, 1999; Borgianni et al., 2002).

The main intermediate product of the thermal cracking is a polyene material that continues to degrade by evolution of aromatics and converts to a products which their composition will be strongly determined by processing variable such as type of atmosphere, temperature and residence time. In an inert atmosphere, the degradation products will be hydrochloric acid (HCl), gaseous and liquid hydrocarbons, and char, which among them HCl is a main product and can be reused either in vinyl chloride production, or in other chemical

In the case of manufacturing process of vinyl chloride, a gas purification unit must also be

In a steam atmosphere at high temperatures, the hydrocarbon fraction will be converted into the some other products such as carbon monoxide, carbon dioxide and hydrogen. In a reported process bench-scale bubbling fluidized bed to investigate some processing parameters on the product outcome. The choice of type of bed material is essential for the product outcome, so that the use of catalytic inactive solid quartz as bed material results in the production of large amounts of char and tar, whereas the application of catalytic active material such as porous alumina results in a high conversion of PVC into the syngas. Moreover, according to their results, temperature has a large impact on the composition of the products, so that the carbon to gas conversion improved from about 70% at 1150 K to approximately 100% by increasing the reactor temperature to 1250 K. For chemical recycling of PVC, an increase in efficiency of dehydrochlorination process is usually attributed to the

It has been also reported that the emission of hydrogen chloride changes significantly with the oxides used indicating the chlorine fixing ability of oxides and also that utilization of poly(ethylene glycol) (PEG) can accelerate dehydrochlorination of PVC, so that at 210 °C for 1 h the dehydrochlorination degree was as high as 74% for PVC/PEG, while for PVC only 50%. Moreover, they demonstrated that for PVC/ PEG the decomposition of PVC shifted to lower temperatures compared with that of pure PVC, suggesting some interactions exist between PEG and PVC that caused the faster dehydrochlorination rate. According to their results, during this process, no waste byproducts such as KCl were produced, and

An alternative method to thermal process of dehydrochlorination is the rather easy process of dehydrochlorination under the influence of alkaline media to recover hydrochloric acid

satisfactory recyclability of PEG (10 cycles) can be obtained (Wu et al., 2009).

temperatures (>1700 K), that it is economically prohibitive (Xiong, 2010).
