IJCPE Vol.9 No.1 (March 2008) 23

Iraqi Journal of Chemical and Petroleum Engineering

Vol.9 No.1 (March 2008) 23-29
ISSN: 1997-4884

Pyrolysis of High-density Polyethylene for the Production of Fuel-like
Liquid Hydrocarbon

Ammar S. Abbas* and Sawsan D. A. Shubar

*
Chemical Engineering Department - College of Engineering - University of Baghdad – Iraq

Abstract

Pyrolysis of high density polyethylene (HDPE) was carried out in a 750 cm
3
stainless steel autoclave reactor,

with temperature ranging from 470 to 495° C and reaction times up to 90 minute. The influence of the operating

conditions on the component yields was studied.

It was found that the optimum cracking condition for HDPE that maximized the oil yield to 70 wt. % was 480°C

and 20 minutes.

The results show that for higher cracking temperature, and longer reaction times there was higher production

of gas and coke. Furthermore, higher temperature increases the aromatics and produce lighter oil with lower viscosity.

Keywords: pyrolysis, high density polyethylene, kinetics, activation energy.

Introduction

Polyethylene is the major component of the total

plastic content of the municipal solid waste (MSW). For

example it represents 55 wt. % of total plastics consumed

in Australia in 2003 [1].

Recycling is one of three ways for utilization and

minimization of the huge amount of waste. The others are

landfilling and incineration with or without energy

recovery. Neither landfilling nor incineration can solve

the growing problem of huge amount of waste [2].

Recycling can be classified into the following

categories: primary recycling or re-extrusion, secondary

mechanical recycling and tertiary (chemical or thermal

recycling) [3].

Pyrolysis involves the degradation of the polymers by

heating in an inert atmosphere. The process is usually

conducted at moderate temperatures between 400-800 °C

and results in the formation of volatile fractions that may

be separated into condensable hydrocarbon oil and a non-

condensable high calorific value gas [4-6].

Plastic pyrolysis may serve as a stand alone operation

or preferably as a pretreatment to yield a stream to be

blended into a refinery or petrochemical feed stream [7],

Where the liquid obtained had a low octane number

although the oil has a high cetane number due to its low

aromatics content [8].

Polyolefin resins (contain only carbon and hydrogen

i.e. PE, PP) of various origins are a desirable pyrolysis

feedstock [9]. So, pyrolysis appears to be a technique that

is able to reduce a bulky, high polluting industrial waste

while producing energy and/or valuable chemical

compounds [10].

As an advantage, pyrolysis can treat all the mixtures

consisting of various types of plastics without separation

or treatment [11].

The produced oil is distributed to end users, typically

as a cheaper substitute for heavy oil and it can be used in

industrial boilers, burners, and power generators [12].

University of Baghdad

College of Engineering
Iraqi Journal of Chemical

and Petroleum Engineering

Pyrolysis of High-density Polyethylene for the Production of Fuel-like Liquid Hydrocarbon

IJCPE Vol.9 No.1 (March 2008) 24

Ng et al. [8] held a thermal decomposition for PE in a

batch reactor at a temperature range of 450 to 500° C, and

the reaction was carried out for 10 minute. The final

pressure at the reaction temperature ranged from 1.38 to

16.13 MPa. The gases were analyzed by gas

chromatography, and the other products were separated

into naphtha, gas oil and residue by distillation. The

amount of residue decreased with temperature, and the

amount of gas increased with temperature. The yield of

gas oil was maximized at 470° C, and it consisted

primarily of normal saturates and lesser amounts of α-

olefins. Only small amounts of branched hydrocarbons

were detected in pyrolysis products.

Pinto et al. [13] carried out thermal decomposition for

PE and PP in 1 liter autoclave .They tried to optimize the

polymer pyrolysis on experimental conditions used. It

was suggested that the run temperature of 450° C,

reaction time of 30 min and an initial pressure of 0.14

Mpa is the optimum. The alkanes formed in greater

amounts were those with carbon atoms between 5 and 11.

Branches and cycled alkanes appeared in very low

concentrations with the exception of methylcyclohexane.

Miskolczi et al. [2] applied thermal cracking for

polyolefin's (PE, PP) at 525° C with a horizontal tubular

reactor. It was found that the cracking parameters (the

type of waste polymers and residence time) affected not

only the yields but also the composition of products.

Miller et al., [14] used a thermal atmospheric pressure

pyrolysis that converts high molecular weight molecules

to lower molecular weight in the lube oil range. The

major by product is diesel with little production of C4-

gas. The pyrolysis yield products were in the range of 37

to 57 wt. %, whereas the potential lube yields were 60-70

wt. %.

The aim of this work is to investigate the pyrolysis

behavior of HDPE in a batch process. The effects of the

cracking parameters (reaction temperature and residence

time) on the yield, group composition, distribution of

distillate fractions and final properties of fuel like liquid

products were also investigated.

Experimental Work

Materials

Feedstock
HDPE pellets were obtained from the State Enterprise

of Petrochemicals Industry/Basrah. The main properties

of HDPE polymer are shown in Table 1.

Nitrogen gas

Nitrogen gas was supplied from Dijlah factory with a

purity of 99.9%.

Table 1 Physical and chemical properties of feedstock

Property
HDPE Sepilex

HHM5502

Test

method

Density (g/cm3) 0.9550
ASTMD

1505

Melt flow index

(g/10 min)
0.35

ASTMD

1238

Deflection

temperature (°C)
75-80 -

Melting point (°C) 130-135 -

Average particle

diameter (mm)
3 -

Pyrolysis apparatus and procedure

The pyrolysis was performed in a 750 cm
3
stainless

steel autoclave of 5 mm thick wall, equipped with a

motor driven stirrer with two blades used for agitation.

Thermocouple (type K) with digital temperature recorder

was used. A pressure gauge 1- 250 bar (0.1-25MPa) was

connected to the top of the reactor to read the pressure

inside the reactor.

Heat was supplied to the reactor by 1200 Watt

electrical heater connected to a voltage regulator in order

to adjust the heating temperature. A schematic diagram of

the experiments is shown in Fig. (1).

A tubing system was connected to the top of the

reactor. All tubing had (302 mm OD and 204 mm ID)

thick stainless steel wall. A big metal container

positioned in a cooler was ready to quench the reactor

inside it after each run. The quenching process was done

by using cooled water at 4° C.

The autoclave was first cleaned with sand paper, and

solvents, and then it was loaded with a measured amount

(175 g) of the pellets and closed. The autoclave was then

bolted tightly and evacuated by applying vacuum

0.068MPa then tested with N2 gas by flushing the system

twice to remove any air present. The reactor was closed

at atmospheric pressure and it generated maximum

pressure of approximately 5MPa at 495° C. After the

charged feed reached 200° C, the stirrer was put in

motion.

Reactor temperature was increased to the pre-set

temperature by turning on the electrical heater. The

accuracy of temperature readings was less than 5° C and

the time zero was taken as the time after which the

reaction melt reached the specified reaction temperature.

After reaching the specified time in any temperature

the heating was stopped and the reactor quenched in the

water bath of 4° C. Then, after reaching about 25° C, the

gaseous products were vented and the products were

weighted. The mass balance of reactants and products

was estimated and analyzed.

Ammar S. Abbas and Sawsan D. A. Shubar

IJCPE Vol.9 No.1 (March 2008) 25

Mixer

Gauge pressure

Flange

Electrical heater

Reactor

Impeller

Needle valve

Electrical power

Regulator

To Vacuum

Or N2 inlet

Fig. 1 Schematic diagram of the experimental apparatus

Analysis Methods

Yields calculations
The mass of gas formed was determined by the weight

loss after discharging. The product slurry was then

removed from the reactor. About 10 g of the slurry

product was solvent-extracted with hexane to remove the

oil formed. The hexane-insoluble were extracted with

THF to remove asphaltenes and preasphaltenes. In all

these runs, the amounts of asphaltenes and preasphaltenes

were negligible. The THF-insolubles were then extracted

with decalin at 140 °C [15].

Preliminary distillation (IP 24/55)
50 cm

3
Liquid product was distillated in atmospheric

pressure and constant heating rate (about 5 cm
3
/min) in

the following fractions: (IBP-140° C), (140-200° C),

(200-270° C) and (>270° C).

Other tests
 Method of measuring aromatic plus olefin

fractions (ASTM D 1019-68).

 Olefin content (IP 128/63).

 Liquid density measurements (ASTM D 1505).
The density was taken to both the fuel like product

and the light distillate (<270°C).

 Liquid viscosity (ASTM D 2515).

Results and Discussion

Effect of reaction time and temperature on
product yield

The effect of reaction time on the formation of gas, oil,

coke and un-reacted polymer at different reaction

temperature was studied in time range up to 90 minute.

Results of this investigation were presented in Fig. 2 to

Fig. 4.

For HDPE pyrolysis at 480° C the gas increased to

about 56 wt. % , while at 495° C it increased up to 69.5

wt. %.

It can be deduced that the primary pyrolysis of PE

takes place through a free-radical transfer that leads to

low yield of gases. Nevertheless, the more gases obtained

from secondary pyrolysis of waxes at gas phase can be

interpreted as a consequence of the propagation reaction

[16].

The produce oil at 470° C increased up to a maximum

yield about 71 wt. % at about 30 minute and then

decreased to reach about 50 wt. % at 90 minute as in

Fig.2.

The oil produce from HDPE at 480° C reaches

maximum (about 70) at 20 minute and then decreases

with the reaction time increases, as seen in Fig. 3.

Hence, there exists an optimum reaction time at which

the liquid products are maximized. This optimum time

occurs around at 20 minutes for HDPE.

Ramdoss and Tarrer, [17] found that optimum time was

5-10 minutes for producing oil from post consumer PE

and PP waste at 500°C.

The other observation in Fig.s 2 to 4 was the coke

formation where the coke raised up to 0.5 wt % at 480° C

and 90 minute for HDPE pyrolysis as in Fig. 3. Coke

formation was likely the result of the secondary reactions

of products (coke precursors) formed during the primary

pyrolysis process. These results agree with those of

Horvat and Ng, [18].

The experimental results in Fig.s 2 to 4 show that

practically all the mass of PE is exhausted when the

maximum point for oil production was reached. The oil

yield decreased since the secondary gas and coke were

formed. It is also seen that the mass fraction of the plastic

being decomposed decreases with pyrolysis time.

Park et al. [19] and Walendziewski [20] had concluded

that shorter times favor larger yields of liquid and longer

reaction times favors larger gas yields at the same

temperature.

The main effect of increasing temperature within this

range of temperature 470 to 495° C was increases the rate

of formation of gases and decreases the rate of formation

of solid residue (un-reacted plastic). Fig. 5 demonstrates

the effect of temperature on the yield of gas, oil, coke and

plastic itself (HDPE) at the optimum time.

It was found that the gas yield resulting from

decomposition HDPE increased from about 14 wt. % to

about 25 wt. % when the pyrolysis temperature was

raised from 470° C to 495° C. This increasing in gas

yield was probably due to the increase in the rate of main

chain sigma bond cleavage reactions in the more

thermally energetic high temperature environments. Also

the growing yield of gas could be caused by the

differences in stability of polymer chains. Therefore at

495° C, the C-C bonds cracked more rapidly than at

lower temperature (i.e. 470° C) and these results in higher

gas yield [18].

Pyrolysis of High-density Polyethylene for the Production of Fuel-like Liquid Hydrocarbon

IJCPE Vol.9 No.1 (March 2008) 26

Time, min

0 20 40 60 80 100

Y
ie

ld
,
%

100

HDPE

Temp.= 470
o
C

Fig. 2 Yields of different products from pyrolysis of

HDPE at 470° C reaction temperature

Time, min

0 20 40 60 80 100

Y
ie

ld
,
%

100

HDPE

Temp.= 480
o
C

Fig. 3 Yield of different products from pyrolysis of

HDPE at 480° C reaction temperature

Time, min

0 20 40 60 80 100

Y
ie

ld
,
%

100

HDPE

Temp.= 495
o
C

Fig. 4 Yield of different products from pyrolysis of

HDPE at 495° C reaction temperature

The effect of temperature on the yield of oil was

tracked. There was an increase in the yield of oil from

about 67 wt. % to about 69 wt. %. Then, it could be seen

that there was a slight decrease in oil yield at optimum

cracking time.

It was noticed that the decomposition temperature

influence the amount of volatile products. As the

decomposition temperature increases the minimum length

of the fragment which can evaporate under the prevailing

conditions also increases [10, 21].

The effect of temperature on coke formation at fixed

time was also studied. It can be seen from Fig. 5 that the

coke increased from about 0.05 wt. % at 470° C to about

0.23 wt. % at 495° C, both at 20 minute for HDPE

pyrolysis. The reason might be the mobility of the

thermally generated free radicals.

Temperature,
o
C

465 470 475 480 485 490 495 500

Y
ie

ld
,
%

HDPE

Time= 20 min

Fig. 5 Variation of components yield with pyrolysis

temperature of fuel like liquid produced from HDPE

pyrolysis at 20 minute reaction time

The effect of reaction temperature on the plastic itself

was shown in Fig. 5. It is clear that the decomposition of

HDPE increased with increasing reaction time. These

observations are in good agreement with the results of

Conesa et al. [16] and Wong et al. [22].

The density and viscosity of produced fuel-like liquid

hydrocarbon were 0.9076 g/cm
3
and 2.056 Cst,

respectively, at 480° C and 20 minute.

Effect of reaction time and temperature on
group composition

From Fig. 6 it is obvious that aromatic is formed in a

large extent with increasing the pyrolysis time. It can be

raised up from about 11 to about 15 wt. % for HDPE at

480° C and 90 minutes reaction time.

This indicates that higher aromatic content was found

with higher residence times. The cause may be that in

closed systems volatile products remain in the reaction

zone and are in equilibrium with liquid phase and can

contribute to secondary reactions, such as ring formation

and aromatization. Mosio-Mosiewski et al. [23], Pinto et

al. [13] and Ng, et al. [8] agreed with this trend.

Ammar S. Abbas and Sawsan D. A. Shubar

IJCPE Vol.9 No.1 (March 2008) 27

The effect of pyrolysis time on the alkene content

during HDPE pyrolysis is obvious in Fig. 6. It can be

seen that the alkene content decreased slightly from about

15 % at 10 minute to 11% at 90 minutes and 480° C for

HDPE. The reason of this phenomenon was the

probabilities of secondary reactions (e.g. re-

polymerization of alkenes) become higher with long

cracking time [2].

The effect of reaction temperature in the ranges of 470

to 495° C on the relative amounts of alkanes, alkenes and

aromatics at their optimum reaction time, where shown in

Fig. 7, which illustrated that the aromatic content at the

optimum time for the polymers increases with increasing

the temperature.

Group types

Alkane Alkene Aromatic

G
ro

u
p

C
o

m
p

o
s
it
io

n
,
w

t
%

100

Time = 10 min

Time = 20 min

Time = 30 min

Time = 45 min

Time = 60 min

Time = 90 min

HDPE, reaction temperature = 480
o
C

Fig. 6 Group composition analysis of fuel like liquid

(<270° C) produced from pyrolysis of HDPE at 480° C

reaction temperature

The aromatic content increased from about 4 to about

15 wt. %. The formation of aromatics at high

temperatures occurs due to lighter hydrocarbons such as

ethene and propene reacting to form aromatic compounds

as benzene and toluene. It was reported that at higher

temperatures ethene and propene were unstable [24].

Fig. 7 shows that the increase in this temperature range

started to decrease alkenes from about 17 to about 11 wt.

%. According to the mechanism of alkane and alkene

formation, alkene formation is a uni-molecular reaction

and alkane formation is a bimolecular reaction. So, higher

temperature makes alkane formation (intermolecular

propagation reaction), the more favorable reaction.

Bockhourn et al, [25] pointed out that the contribution of

energies of activation energy to the apparent alkane and

alkene formation was 42 and 112 kJ /mol, respectively.

Group types

Alkane Alkene Aromatic

G
ro

u
p
C

o
m

p
o
s
it
io

n
,
w

t
%

100

Temp. = 470
o
C

Temp. = 480
o
C

Temp. = 495
o
C

LDPE, reaction time = 10 min

Fig. 7 Group composition analysis of fuel like liquid

(<270° C) produced from pyrolysis of HDPE at 20 min

reaction time

Effect of reaction time and temperature on the
distillate fractions

The effect of pyrolysis time on the relative amount of

fractions of oil produced from atmospheric distillation

was shown in Fig. 8. It can be seen that the first two

fractions IBP-140° C and 140-200° C increased with

increasing pyrolysis time up to 90 minute.

The first fraction IBP-140° C raised from 6 wt % to 15

wt % and the second fraction (140-200 °C) raised from

15 wt % to 36 wt %. The third fraction (200-270° C)

increased slightly during the first 45 minute, and then it

began to decrease. While the residue (>270°C) decreased

sharply during the first 45 minute, then it began to

increase slightly.

The results suggest that as pyrolysis time increased, the

heavier products decomposes to lighter ones, because at

higher pyrolysis time the carbon chain was exposed to

more forceful thermal decomposition effects, due to its

being under harsh environment for a long time.

Rangarajan et al. [25] and Walendziewski and Steininger

[9] agreed with these findings.

When distilling the fuel like products from HDPE

pyrolysis, it was noticed that the first two fractions (IBP-

140° C and 140-200° C) increased with increasing the

pyrolysis temperature from 470 to 495° C at fixed

reaction time. The heavier fractions (200-270° C) and the

residue (>270° C) decreased with increasing the reaction

temperature for both polymers, as shown in Fig. 9.

Pyrolysis of High-density Polyethylene for the Production of Fuel-like Liquid Hydrocarbon

IJCPE Vol.9 No.1 (March 2008) 28

Distillation fraction range,
o
C

IBP-140 140-200 200-270 > 270

D
is

ti
ll
a

te
,
w

t.
%

Time = 10 min

Time = 20 min

Time = 45 min

Time = 60 min

Time = 90 min

HDPE, reaction temperature = 480
o
C

Fig. 8 Variation of distillate fractions weight percent with

reaction time of fuel like liquid produced from HDPE

pyrolysis at 480° C reaction temperature

Distillation fraction range,
o
C

IBP-140 140-200 200-270 > 270

D
is

ti
ll
a

te
,
w

t.
%

Temp. = 470
o
C

Temp. = 480
o
C

Temp. = 495
o
C

HDPE, reaction time = 20 min

Fig. 9 Reaction temperature effects on the distribution of

distillate fractions of fuel like liquids and light products

out from HDPE pyrolysis at 20 minute

This may be explained that among the various product

fractions in each of the polymers, higher energy would be

required to produce lighter fractions because on the

average it takes more derivative steps to form the lighter

products. Also high temperature increased the rate of

volatilization of longer chains, where they undergo β-

scission at a rate much higher than that found in light

fractions. This observation fit the trend observed by

Ranjargan et al. [25].

Conclusions

Based on present work, the following points can be

concluded:

1. PE pyrolysis yields a mixture of oil, gas. The higher
the pyrolysis temperature and the longer the reaction

times increase the gas yield.

2. There was an optimum temperature and pyrolysis
time and which the liquid products is maximized. This

optimum temperature occurs around 480 °C and the

optimum reaction time was 20 minute for HDPE with

oil yield 69.2 wt %.

3. It was found that for higher cracking temperature and
longer reaction time, there was higher production of

gases and coke. Gas yield was increase from 14.23 %

at 470° C to 25.78 % at 495° C at 20 minute reaction

time, while gas yield increased up to 55.92 % at 90

minute.

4. Aromatics increased with temperature and time while
alkenes decreased.

5. Distillation of fuel-like liquids show more light
fractions for fuel like liquid produced at higher

temperature and longer time.

Nomenclature

C Mass fraction of coke , wt. %

E Activation energy , J/mol

G Mass fraction of gas , wt. %

O Mass fraction of oil, wt. %

P Mass fraction of un-reacted plastics , wt. %

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