Current Opinion in Solid State and Materials Science 8 (2004) 419–425

Feedstock recycling of polymer wastes

Arthur A. Garforth a,*, Salmiaton Ali b, Jesus Hernandez-Martınez a, Aaron Akah a

a Environmental Technology Centre, Department of Chemical Engineering, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UKb Department of Chemical and Environmental Engineering, Faculty of Engineering, UNIPUTRA, 43400 UPM Serdang, Selangor, Malaysia

Abstract

Current common polymer waste recycling methods, mechanical recycling and energy recovery, have drawbacks such as labour

intensive sorting and atmospheric pollution. Feedstock recycling has emerged as an environmentally successful alternative for poly-

mer waste management.

� 2005 Published by Elsevier Ltd.

Keywords: Plastics; Polymer recycling; Feedstock recycling; Tertiary recycling; Catalytic cracking

1. Polymer recycling

Polymer waste might be regarded as a potentially

cheap source of chemicals and energy, although its recy-

cling varies widely across Europe [*1]. Disposing of the

waste to landfill is becoming undesirable due to legisla-

tive pressures (where waste to landfill must be reduced

by 35% over the period from 1995 to 2020) [2], risingcosts, the generation of explosive greenhouse gases (such

as methane) and the poor biodegradability of commonly

used packaging polymers.

The two main alternatives for treating municipal and

industrial polymer wastes are energy recycling, where

wastes are incinerated with some energy recovery and

mechanical recycling. The incineration of polymer waste

meets with strong societal opposition [3] and, there is theKyoto Protocol to consider, as the UK moves towards

its domestic goal of reducing carbon dioxide emissions

by 20% by 2010 [2]. Mechanical recycling (the conver-

sion of ‘‘scrap’’ polymer into new products) is a popular

recovery path for manufacturers and is carried out on

single-polymer waste streams as a market for recycled

1359-0286/$ - see front matter � 2005 Published by Elsevier Ltd.

doi:10.1016/j.cossms.2005.04.003

* Corresponding author.

E-mail address: arthur.garforth@manchester.ac.uk (A.A. Gar-

forth).

products can only be found if the quality is close to that

of the original. Unfortunately these products are often

more expensive than virgin plastic [4,5]. In 2002 in the

UK, only 17% of 3.8 million tonnes of polymer waste

was recycled by these methods, the remainder was

land-filled or incinerated (without energy recovery) [*1].

2. Feedstock recycling—current state of the art

Feedstock recycling, also known as chemical recy-

cling or tertiary recycling, aims to convert waste

polymer into original monomers or other valuable

chemicals. These products are useful as feedstock for a

variety of downstream industrial processes or as trans-

portation fuels. There are three main approaches: depo-lymerisation, partial oxidation and cracking (thermal,

catalytic and hydrocracking) [*6].

2.1. Depolymerisation

Polymers are divided into two groups: (i) condensa-

tion polymers and (ii) addition polymers. Condensation

polymers which include materials such as polyamides,polyesters, nylons and polyethylene terephthalate

(PET), can be depolymerised via reversible synthesis

Fig. 1. Typical zeolites used in polymer cracking [57]: (a) H-ZSM-5,

(b) H-Mor (Mordenite), (c) H-Y or HUS-Y and (d) H-Beta.

420 A.A. Garforth et al. / Current Opinion in Solid State and Materials Science 8 (2004) 419–425

reactions to initial diacids and diols or diamines. Typical

depolymerisation reactions such as alcoholysis, glycoly-

sis and hydrolysis yield high conversion to their raw

monomers [7].

In contrast, addition polymers which includematerials

such as polyolefins, typically making up 60–70% of muni-cipal solid waste plastics [*1,3,4], cannot be easily depoly-

merised into the original monomers. However, the results

obtained in the thermal depolymerisation of polymethyl-

methacrylate (PMMA) are noteworthy since at 723 K, a

98% yield to the monomer has been reported [*8].

2.2. Partial oxidation

The direct combustion of polymer waste, which has a

good calorific value, may be detrimental to the environ-

ment because of the production of noxious substances

such as light hydrocarbons, NOx, sulfur oxides and

dioxins. Partial oxidation (using oxygen and/or steam),

however, could generate a mixture of hydrocarbons

and synthesis gas (CO and H2), the quantity and quality

being dependent on the type of polymer used. Borgianniet al. [9] showed the possibilities of recovering energy

from waste containing polyvinyl chloride (PVC) by a

gasification process without additional dechlorination

facilities. A new type of waste gasification and smelting

system using iron-making and steel-making technologies

has been described by Yamamoto et al. [10], reportedly

to produce a dioxin-free and high-calorie purified gas.

Hydrogen production efficiency of 60–70% from poly-mer waste has been reported for a two-stage pyrolysis

and partial oxidation process [*11]. Co-gasification of

biomass with polymer waste has also been shown to in-

crease the amount of hydrogen produced while the CO

content reduced [12]. The production of bulk chemicals,

such as acetic acid, from polyolefins via oxidation using

NO and/or O2, is also possible [13,14].

2.3. Cracking: hydro-, thermal- and catalytic

Cracking processes break down polymer chains into

useful lower molecular weight compounds. This can be

achieved by reaction with hydrogen, known as hydro-

cracking or by reaction in an inert atmosphere (pyrolytic

methods), which can be either thermal or catalytic

cracking.Hydrocracking of polymer waste typically involves

reaction with hydrogen over a catalyst in a stirred batch

autoclave at moderate temperatures and pressures (typ-

ically 423–673 K and 3–10 MPa hydrogen). The work

reported, mainly focuses on obtaining a high quality

gasoline starting from a wide range of feeds. Typical

feeds include polyolefins, PET, polystyrene (PS), polyvi-

nyl chloride (PVC) and mixed polymers [15,*16,17–21],polymer waste from municipal solid waste and other

sources [17,18,22–26], co-mixing of polymers with coal

[24,25,27–31], co-mixing of polymers with different refin-

ery oils such as vacuum gas–oil [32–36] and scrap tyres

alone or co-processed with coal [37–41]. To aid mixing

and reaction, solvents such as 1-methyl naphthalene,

tetralin and decalin have been used with some success

[25,28,41]. Several catalysts, classically used in refineryhydrocracking reactions, have been evaluated and in-

clude transition metals (e.g., Pt, Ni, Mo, Fe) supported

on acid solids (such as alumina, amorphous silica–alu-

mina, zeolites and sulphated zirconia). These catalysts

incorporate both cracking and hydrogenation activities

and although gasoline product range streams have been

obtained, little information on metal and catalyst sur-

face areas, Si/Al ratio or sensitivity to deactivation isquoted.

In thermal degradation, the process produces a broad

product range and requires high operating temperatures,

typically more than 773 K and even up to 1173 K

[*8,42–49]. On the other hand, catalytic degradation

might provide a solution to these problems by allowing

control of the product distribution and reducing the

reaction temperature [48,50–53].Catalytic cracking studies have been mainly limited

to pure polymers (predominantly using polyolefins and

PS) and fresh, pure acid catalysts (zeolites predominat-

ing). Zeolites are crystalline, porous aluminosilicates

[54–56] characterised by channel networks and pore

openings of molecular dimensions (see Fig. 1, [57])

leading to increased shape selectivity in petrochemical

Fig. 2. Mixture of HDPE/ZSM-5 at 478 K, 200 times magnification.

Reprinted from Garforth and co-workers [*85].

A.A. Garforth et al. / Current Opinion in Solid State and Materials Science 8 (2004) 419–425 421

reactions [54–56,58]. PVC is problematic because HCl

strips from the polymer at relatively low temperatures

[59,60]. In Japan, a low temperature thermal cracking

stage is employed prior to catalytic cracking, however,

co-mingling with other polymer waste is required to

boost the H content of the residual partially crackedpolymer waste. Typically iron-based catalysts have been

employed to dechlorinate the PVC/mixed polymer-de-

rived oil [52,61–63]. The catalytic cracking of PS to ben-

zene, toluene and xylene (BTX), as well as styrene

monomer, has been carried out by a number of research-

ers at operating temperatures from 623–823 K over acid

catalysts such as zeolites (HMOR HZSM-5, HY), amor-

phous SiO2–Al2O3, BaO powder and a sulfur-promotedzirconia [64–67]. Predominantly, catalytic cracking re-

search has focussed on the degradation of polyolefins

to gas, liquid and waxy products using a range of acid

catalysts (typically, amorphous silica alumina and zeo-

lites). For example, in Japan, legislative pressures have

resulted in research targeting a stable liquid product.

Other researchers have targeted an end use, such as, gas-

oline-range hydrocarbons and others the production ofethene and propene [15,48,68–74].

A variety of reactor types has been used including

batch [53,70,75] and fixed bed [52,68,69,71,76–79],

or non-catalytically using thermal degradation in a

fluidised bed reactor or kiln [59,80,*81]. With batch

reactors, secondary cracking reactions predominate,

yielding a broad range of products including heavy aro-

matics, coke and saturated hydrocarbons. Fixed bedreactors are prone to blocking due to the viscous nature

of melted polymer presenting problems when scaling-up

[82]. Non-catalytic thermal cracking using a fluidised

bed reactor with sand as a fluidising agent or kiln

requires a higher operating temperature and produces

products in a very broad range [*81].

On the other hand, the use of a fluidised bed reactor

has advantages in terms of heat and mass transfer, aswell as constant temperatures throughout the reactor

[48,82,83]. Recent work has logically extended studies

to fluid catalytic cracking (FCC) catalysts with compar-

isons to pure zeolites and silica alumina [74,84,*85].

Before design predictions can be made for a pyrolysis

process on an industrial scale, an understanding of the

interface between the polymer and the catalyst must be

developed. The mechanism of interaction is highly com-plex, with three phases (liquid polymer, solid catalyst

and gaseous products), mass transfer by diffusion, con-

vection and bulk flow as well as cracking-type reactions

with a large number of products. Fig. 2 shows a scan-

ning electron micrograph (SEM) of a finely blended

mixture of high density polyethylene (HDPE) and

ZSM-5 after heating from room temperature to 478 K.

The polymer particles have melted and flowed but indi-vidual catalyst and polymer particles were still notice-

able. On increasing the temperature to 573 K, the

melted polymer has completely ‘‘wetted’’ the zeolite par-

ticles (Fig. 3).The catalytic degradation of HDPE has been carried

out in a laboratory fluidised bed reactor using pure zeo-

lites and fresh, steam deactivated and ‘‘equilibrium’’ cat-

alysts (E-Cats) with different rare earth oxides and Ni

and V loadings (listed in Table 1) [48,83,86–88].

At 723 K, the products from polymer cracking were

mostly gases in the range C1–C9 (determined by GC

analysis), and coke and unreacted polymer (determinedby thermogravimetric analysis) [48,89]. As expected,

trends in polymer cracking (Table 3) reflected the differ-

ent nature of catalysts, with fresh commercial FCC cat-

alysts and pure ZSM-5 catalyst converting 85–90% of

their feeds to gaseous, liquid and carbonaceous prod-

ucts. The lower activity of pure US-Y (ex Crosfield

Chemical) was expected due to its rapid deactivation.

On the other hand, the less active steamed and equilib-rium catalysts showed only 60–70% conversion to the

volatile products. The E-Cats showed negligible loss in

overall conversion of HDPE due to metal contamina-

tion, although the products of polymer degradation

were olefin-rich compared with steam deactivated

Cat-1S and -7S.

Figs. 4 and 5 show selected olefin and paraffin prod-

ucts in the carbon range of C3–C6, respectively, for thecatalysts. US-Y is the major active component in com-

mercial FCC catalysts and therefore the product yields

compared favourably. The level of activity of the vari-

ous catalysts was reflected in the amount of primary

(olefin) versus secondary (paraffin) products observed.

With high acidities of both fresh catalysts (Cat-1 and -7,

Table 1

Catalyst details (supplied by Engelhard Corporation, USA) [88]

Catalyst Commercial name REO (wt%) UCS (A) MSA (m2/g) ZSA (m2/g) Ni (ppm) V (ppm)

Cat-1 Fresh commercial FCC catalyst 0.8 24.4 112 264 – –

Cat-7 Fresh commercial FCC catalyst 9.6 24.7 90 331 – –

Cat-1S a Steam deactivated FCC catalyst 0.8 24.3 90 198 – –

Cat-7S a Steam deactivated FCC catalyst 9.6 24.5 72 241 – –

E-Cat 1 Equilibrium FCC catalysts 1.3 24.3 76 99 171 217

E-Cat 2 Equilibrium FCC catalysts 1.6 24.3 32 95 5400 6580

a Steaming conditions: 4 h/1061 K/100% steam.

Fig. 3. Mixture of HDPE/ZSM-5 at 573 K, 200 times magnification. Reprinted from Garforth and co-workers [*85].

Table 2

Weight% of product distributions at T = 723 K; C/P = 6:1 [88]

ZSM-5 US-Y Cat-1 Cat-7 Cat-1S Cat-7S E-Cat1 E-Cat2

Gaseous 83.7 55.9 75.0 71.8 50.4 55.8 64.5 65.8

Liquid 2.0 0.5 9.0 6.8 7.2 7.8 1.4 1.4

Coke 2.4 4.5 6.5 7.2 3.0 4.9 1.5 1.2

Involatile 11.9 39.1 9.5 14.2 39.4 31.5 32.6 31.6

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Gaseous product distribution

C1–C4 68.6 36.6 44.4 47.4 38.4 44.4 35.2 37.1

C5–C9 23.1 60.2 52.2 48.8 60.2 52.8 63.4 62.6

BTX 8.3 3.2 3.4 3.8 1.4 2.8 1.4 0.3

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Total gaseous product

Paraffins 27.0 48.8 53.7 60.0 31.4 48.7 23.6 23.0

Olefins 64.7 47.8 42.5 35.7 67.1 48.6 74.9 76.6

Yield (wt%) = (P(g)/Polymer feed (g)) · 100.

422 A.A. Garforth et al. / Current Opinion in Solid State and Materials Science 8 (2004) 419–425

Table 2), high reactivity was expected and a high yield of

secondary products, paraffins, was observed.

By contrast, the used catalysts with lower acidities and

poisoned with heavy metals yielded predominantly ole-

finic products mostly in the carbon range of C3–C6. Evi-

dence of high REO stabilisation of steam deactivated

catalyst, Cat-7S, was noted with a yield of balanced pri-

mary and secondary products (Figs. 4 and 5). During the

0

5

10

15

20

25

ZSM5 USY Cat1 Cat7 Cat1S Cat7S ECat1 ECat2

C3= C4=

C5= C6=

Fig. 4. Selected olefin products (wt%) at T = 450 �C; C/P = 6:1.

0

5

10

15

20

25

ZSM5 USY Cat1 Cat7 Cat1S Cat7S ECat1 ECat2

C3 C4

C5 C6

Fig. 5. Selected paraffin products (wt%) at T = 723 K; C/P = 6:1.

Table 3

Total plastic waste generated and recovered in Western Europe (kt) [*1]

1993 1995 1997 1999 2001 2003

Total plastics waste 16,211 16,056 16,975 19,166 19,980 21,150

Total plastics waste recovered 3340 4019 4364 6183 7357 8230

Mechanical recycling 915 1222 1455 1888 2521 3130

Feedstock recycling 0 99 334 346 298 350

Energy recovery 2425 2698 2575 3949 4538 4750

%Total plastics waste recovered 21 26 26 32 37 39

A.A. Garforth et al. / Current Opinion in Solid State and Materials Science 8 (2004) 419–425 423

steaming process in the FCC regenerator, catalysts will

lose some of framework aluminium ions, creating defects

in the crystals and leading to decreased catalyst acidities

[87]. Nevertheless, with the presence of RE in the FCC

catalysts, the steam dealumination is hindered. There-

fore, with RE, the catalyst activities are maintained by

reducing the amount of crystal destruction as seen here.

3. Conclusions

Although feedstock recycling has been heralded as

having great potential, polymer waste recycling levels

have remained virtually unchanged at 350 kt since

1997 (see Table 3, [*1]). High costs associated with col-

lection, sorting and transportation to provide a guaran-teed supply of low chlorine-containing polymer waste to

recycling sites remain significant. Schemes such as

Duales System Deutschland [*90] in Germany (‘‘green

dot’’) have addressed this issue but there remains the

high energy and process costs of the feedstock recycling

technology. Thermal and catalytic cracking although

effective require significant operating temperatures and

are strongly endothermic, leading to large adiabatic tem-

perature falls across reactors. However, improving the

economics of the process itself by using exhaustedzero-cost catalysts to produce a tailored product will

help to make the process viable [83].

Oxidation methods, energetically more favourable,

are at high temperature and have associated difficul-

ties such as dangerous emissions, product quality and

expensive materials of construction. Hydrocracking

studies have been limited to date and merit further study

since the process is exothermic and can be carried out atsignificantly lower temperatures.

Another strategy worth considering is the targeting of

large volume guaranteed waste streams, such as, from

paper recycling plants to reduce collections costs. If this

is linked with careful characterisation of this type of

waste stream, the supply of a quality controlled polymer

waste should be possible.

Acknowledgements

This work was performed with the financial support

of the University Putra Malaysia. Thank you to Dr.

D.H. Harris (Engelhard Corporation) for catalysts and

technical advice. Thanks also to Miss S. Maegaard for

her contribution on thermal analysis and microscopyand also Mr. R.J. Plaisted of the Centre of Micropo-

rous Materials. Special thanks to Dr. D.L. Cresswell

for useful discussion during the preparation of this

review.

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