KINETICS STUDY OF POLYETHYLENE DEGRADATION IN BATCH REACTOR
Ajay Kumar1, Manisha Sahai1, H U Khan1, M O Garg1 & Sanat Kumar1,*1CSIR-Indian Institute of Petroleum, Dehradun-248005, Indiae-mail:sanat@iip.res.in, phone +91-135-2525794 fax +91-1352660202

Abstract
A kinetic study on the thermal and catalytic degradation of waste plastics was carried out under atmospheric pressure.
Thermal gravimetric analysis (TGA) technique was used to identify the pyrolysis temperature used in this work. The crackingof waste polyethylene was carried out in vertical batch reactor at atmospheric pressure followed by catalytic conversion inhorizontal fixed bed reactor, both the reactors being made of glass. Waste plastics pellets were fed in the vertical reactor andthe reactor was heated to the desired temperature according to a predetermined ramp. The horizontal reactor was packed
with catalyst and was maintained at a fixed temperature for vapour phase conversion of pyrosylate vapours. These vapours
were subsequently condensed to obtain liquid products and C1-C5 gases. Weight loss curves have been generated usingmaterial balance and an empirical correlation for loss of weight plastics in the pyrolysis reactor has been proposed. In thepresent case the use of catalyst yields more of gaseous products and less of the liquid products as compared to the noncatalytic process. For kinetic study the end products were lumped into three fractions of monoaromatics, polyaromatics andnon-aromatics hydrocarbons. Here one step and two step lumped kinetic models have been proposed for both non-catalyticand catalytic conversion of waste plastics. The one step and two step models contain three and six kinetic constants
respectively and these have been determined from the experimental data.

Keywords: Polyethylene, thermal cracking, catalytic cracking, kinetics, lumped model

1. Introduction

2. Materials and Methods

The consumption of plastics world over as well as inIndia is increasing at a rapid rate. This increased
consumption has simultaneously resulted in generationof enormous amount of wastes, which being nonbiodegradable are environmental hazards. The usual
methods of recycling and reuse like mechanicalrecycling, incineration and land filling have inherentdisadvantages like turning enormous amount of land towaste, evolution of poisonous gases etc. Therefore theirdisposal in an environment friendly way has become atechnological and social subject that has attracted a lot ofattention from researchers, business people,
environmental activists and the public in general. Thereare enough evidences in literature to suggest thatchemical recycling of waste plastics has the potential toovercome the environmental problem concerned with thedisposal of plastic wastes while simultaneously producing
value added products. [1-5].

2.1 Material:

Waste polyethylene has been degraded into
liquid and gaseous hydrocarbons with and without theuse of catalyst by following a single stage two step
process in involving melting, homogenization andvaporization of waste plastics in the first step andsecondary conversion gaseous vapors in the second
step followed by condensation to obtain liquid products.
The kinetic parameters have been obtained for both
catalytic and non-catalytic degradation.

The plastics (polyethylene) used in this study, is ahigh density polyethylene plant waste in pellet form
having milky white colour. This plant waste is a low
molecular weight by product accounting for about 15 % of the plant production.
The catalyst used in this study is a tailor madezeolite catalyst in extrudate form.

2.2 Procedure:
300 grams of waste plastic feed was pyroyzed in an
inert aymosphere The pyrolysis unit was heatedupto a fixed temperature (450 & 500 oC) according
to a pre determined temperature ramp. The vapors
from pyrolysis unit were subsequently passed
through a fixed bed of catalyst and maintained at aconstant temperature. The vapors were condensedin condensers maintained at a temperature of 4 oC
and collected in the receiver.
The weight of the liquid hydrocarbons collected in
the receiver and the residue from pyrolysis unit
collected at the end of the experiment as well as thevolume of uncondensed gases were recorded. The
liquid hydrocarbon was characterized for aromatic
content by super critical chromatography following
ASTM D 5186 method and the composition of gas

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

was determined by GC analysis. The mass balance
was confirmed after each experiment by equating
the weight of feed taken to the sum of the weight of
the liquid hydrocarbon, the residue in pyrolysis
reactor and volume of uncondensed gases which
was converted to corresponding weight based on
the their GC analysis.

3. Results and Discussion
3.1 Effect of temperature on rate of degradation
It has been observed that the rate of degradation ofplastics in the pyrolyzer depends upon thetemperature and the mass remaining at any time in
the pyrolyzer has been correlatedW(t) = Wo *e-t2*2*10-9*(T-373)1.96
Where,
t : time in minutes
Wo : Initial wt of feed
T : Temperature in oC
3.2 Effect of temperature on nature of degradedproducts
The composition of uncondensed gases obtainedwith and without catalyst at different temperatures
has been investigated The net gas evolved
drastically increases on increasing temperature andon addition of catalyst at the cost of liquid yield. The
composition of gas as well liquid also changes withthe addition of catalyst. The average molecularweight of uncondensable gases for the thermal caseis 20.8 whereas molecular weight for the catalytic
case is 11.6 mainly due to the presence ofhydrogen. The aromatic content of liquid increases
with the addition of catalyst.

One step lumped model
A one step kinetic model has been proposed basedon cumulative weight of monoaromatics,
polyaromatics and nonaromatics with time assuming
the rate of formation of these lumps as a nth order
reaction with respect to time the weight of plastics
remaining in the reactor.i.e.
PEMonoaromatics
PE
Polyaromatics
PENonaromatics
The order of reaction and rate constant weredetermined using derivative method.
Two step lumped model
An attempt at a two step model has also beenmade. The basic assumptions are as follows
PEMonoaromatics
PE
Polyaromatics
PE
Nonaromaticshigher
PE
Nonaromaticslower
Nonaromaticshigher
αXMonoaromatics+Nonaromatics lower
β X Monoaromatics
Polyaromatics
The order of reaction and rate constants weredetermined using derivative method.

4. Conclusions
A correlation has been proposed for the rate ofdegradation with the temperature of the pyrolyzer. Inthe present case addition of catalyst increases theproportion of hydrogen in the uncondensed gases.
The proposed one step model with three reaction
rates constants is able to explain the thermal
decomposition well and indicates an order transition
takes place after a certain time for the formation of
non-aromatic hydrocarbons. The two stepmechanism models proposed with six reaction rate
constants has the advantage as being moretheoretically sound and modeling the real situation.

References[1] S. F. Sodero, F. Berruti F and L.A. behie
Chem.Eng.Sci. 51(1996) 2805-10

Comparison of gas composition for the thermal andcatalytic cases
3.3 Kinetic model of waste plastics cracking
One step and two step lumped kinetic models wereproposed including both thermal and catalytic
cracking of waste plastics and rate constants forboth models were studied under assumption that nomass transfer resistance inside and outside ofcatalyst .The rate constants were studied at 450
and 500 OC.

[2] M.I. Mastellone, F.Pergini, M. Ponte and U.
Arena Polym.Degrad.Stab 76 (2002) 479[3]Y.H. Lin and M.H.Yang. Appl Catal B:Environ 69(2007)145[4]N.S. Patel and M.K.D.Desai. Int`l J. Engg.2(2013) 1-5[5] M.R.Jan, J.shah and A. Gulab Fuel 105 (2012)595-602

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013