Production of Liquid Fuels from Recycled Plastics using Acidic HNaY Catalysts

Production of Liquid Fuels from Recycled Plastics using

Acidic HNaY Catalysts

Maja Anna Jaśkiewicz

Dissertation to obtain the Master Degree in

Chemical Engineering

Jury:

President: Prof. Sebastião Manuel Tavares da Silva Alves

Supervisors: Prof. Maria Amélia Nortadas Duarte de Almeida Lemos

Prof. Francisco Manuel da Silva Lemos

Vogal: Prof. Filipe Jose da Cunha Monteiro Gama Freire

Junho 2011

ii

“EXPERIENCE IS THE NAME EVERYONE GIVES TO THEIR MISTAKES”

OSCAR WILDE

iii

ACKNOWLEDGEMENTS

First of all, I would like to express my gratitude to Professor Franscisco Lemos and Professor

Amélia Lemos for the opportunity to work in their team, for all the help and attention that they have

given me. Their scientific guidance, motivation and support were the basics for the preparation of this

work.

I would like to thank also Professor Helena Janik from my home Institution – Gdańsk

University of Technology, without her help and encouragement I wouldn‟t have gone to work on my

Master‟s thesis abroad.

A special thank is dedicated to Anabela Coelho, my co-worker, You have helped me and

patiently explained things that were unclear, despite the language barrier that was sometimes

between us.

I would like to acknowledge Instituto Superior Tecnico -UTL for giving me a chance to study

and perform my thesis there; and also Gdańsk University of Technology, for their financial support

throughout my whole exchange period.

Lastly, I would like to thank my family, especially my Mother, and all of my friends, for

encouragement, support and understanding.

Maja Jaśkiewicz

iv

ABSTRACT

This study investigates thermal and catalytic cracking of high-density polyethylene and low-

density polyethylene ( HDPE and LDPE) using simultaneously thermogravimetry (TG) and

differential scanning calorimetry (DSC) techniques.

The catalysts chosen for this work were Ultra Stable acidic Y zeolites (USHY). For the purpose

of the study, the catalysts were subjected to ion exchange with sodium nitrate in order to decrease

their acidity. The amount of acid sites was calculated by performing the temperature programmed

desorption (TPD) with ammonia. Afterwards the relationship between the acidity and activity of the

zeolites was described.

The TG-DSC analysis allowed to study the conditions of thermal, as well as catalytic

degradations. It was found that the degradation temperatures decreased during the reactions involving

catalysts and also that the modified acidity of zeolites didn‟t influence much the cracking process.

Moreover if was found that the structure of the polymer itself induces changes in cracking conditions.

More branched molecule – LDPE, presented lower melting and pre-cracking temperatures than linear

HDPE.

Signals from TG-DSC were fitted into the developed kinetic model in order to describe the

kinetics of the process by several kinetic and thermodynamic parameters. The estimated values

showed a decrease in activation energy for the catalytic processes.

The degradation products were analyzed using gas chromatography (GC). It was observed

that there is a significant difference in product distribution between thermal and catalytic process.

Presence of the catalyst favored the formation of C4 -C6 hydrocarbons, whereas after thermal

degradation the majority of products were in C1-C4 range. The acidity of the zeolites had no influence

on the product distribution. Overall it was found that the amount of olefins was higher than paraffins for

all of the analyses.

KEYWORDS: Polyethylene, Kinetic modeling, thermal and catalytic degradation, differential

scanning calorimetry (DSC), thermogravimetry (TG), ammonia temperature programmed

desorption(TPD), fuels

v

RESUMO

Este estudo investiga o cracking térmico e catalítico do Polietileno de Alta e Baixa Densidades

(PEAD e PEBD) usando simultaneamente técnicas, de termogravimetria (TG) e calorimetria

diferencial de varrimento (DSC).

O catalisador escolhido para este trabalho foi o zeólito Y Ultra estável (USHY). Para o

propósito do estudo, os catalisadores foram submetidos a permuta iónica com nitrato de sódio, a f im

de diminuir a sua acidez. A quantidade de centros ácidos foi calculada pela realização da dessorção

de amoníaco a temperatura programada (TPD). Posteriormente, a relação entre a acidez e a

actividade dos zeólitos, foi descrita.

A análise TG-DSC permitiu estudar as degradações térmicas e catalíticas. Verificou-se que, a

temperatura de degradação diminuiu quando os catalisadores foram usados nas reacções e também

que os zeólitos modificados por permuta iónica não influenciaram muito o processo de crack ing. Além

disso, constatou-se que a estrutura do polímero induz mudanças nas condições crack ing. O PEBD -

estrutura com moléculas ramificadas, apresentou menores temperaturas de fusão e pré -cracking do

que as do Polietileno linear - PEAD.

Sinais do TG-DSC foram ajustados ao modelo cinético desenvolvido, de forma a descrever a

cinética do processo através dos vários parâmetros cinéticos e termodinâmicos. Os valores estimados

mostraram uma diminuição na energia de activação para os processos catalíticos.

Os produtos da degradação foram analisados através de cromatografia gasosa (GC). Foi

observado que existe uma diferença significativa na distribuição do produto entre o processo térmico

e catalítico. A presença do catalisador favoreceu a formação de hidrocarbonetos C4-C6, enquanto

que na degradação térmica a maioria dos produtos situa-se no intervalo de C1-C4. A acidez dos

zeólitos não teve influência sobre a distribuição do produto. No geral, verificou-se que a quantidade

de olefinas foi superior à quantidade de parafinas, para todas as análises.

PALAVRAS-CHAVE: Polietileno, modelo cinético, degradação térmica e catalítica, termogravimetria (TG) e calorimetria diferencial de varrimento (DSC), dessorção de amoníaco a

temperatura programada (TPD), combustíveis

vi

NOMENCLATURE

PE - Polyethylene

PP - Polypropylene

PVC - Polyvinylchloride

PS - Polystyrene

PET - Polyethylene Terephthalate

HDPE - High Density Polyethylene

LDPE - Low Density Polyethylene

APME - Association of Plastics Manufacturers in Europe

DSC - Differential Scanning Calorimetry

TA - Thermal Analysis

TG - Thermogravimetry

TGA - Thermogravimetric Analysis

GC - Gas Chromatography

FID - Flame Ionization Detector

SEM - Scanning Electron Microscope

TPD – Temperature Programmed Desorption

NH3 – Ammonia

qi – amount of acid sites occupied by ammonia

αi – desorption energy from non acidic sites

β – sensitivity of acidity

N - Number of C-C bonds per unit mass

n - Carbon atoms

α - Average number of bonds lost to the gas phase, per unit mass of evaporated material, at a

given moment

t - Time

k(T) - Corresponding temperature-dependent rate constant

Tref - Reference temperature chosen within the range of significant weight loss (573 K in this

case)

Ea - Apparent activation energy

kref - Kinetic constant at the reference temperature (Tref)

m - Weight of the sample

Cp - Average heat capacity

ΔHC-C - Average C-C bond enthalpy

ΔHv ap - Average vaporization enthalpy per unit mass

vii

H - Hydrogen

C - Carbon

k - Kinetic constant rate

n - Order of the reaction in respect to the polymer

T - Temperature

Tdegrad. - Degradation temperature

k0 - Pre-exponential factor

R - Universal gas constant (0.008314 kJ/mol.K)

ΔHreaction - Reaction enthalpy,

ΔHvap - Vaporization enthalpy

ΔHC-C - Bond enthalpy

Cp - Heat capacity

viii

GENERAL INDEX

ACKNOWLEDGEMENTS......................................................................................................... iii

ABSTRACT ............................................................................................................................... iv

RESUMO .................................................................................................................................... v

NOMENCLATURE .................................................................................................................... vi

GENERAL INDEX ................................................................................................................... viii

INDEX OF FIGURES ................................................................................................................ xi

INDEX OF TABLES ................................................................................................................ xiii

Chapter 1 Introduction.......................................................................................................... 1

1.1. Aim of the thesis .................................................................................................................. 1

1.2. Polymers.............................................................................................................................. 2

1.2.1. Classification of polymers ................................................................................................ 2

1.2.2. Global production and consumption of plastics ............................................................... 2

1.2.2.1. Polymers as Fuels ................................................................................................. 4

1.2.2.2. Environmental concerns ........................................................................................ 5

1.3. Polyethylene ........................................................................................................................ 6

1.3.1. Characterization and basic properties of High Density Polyethylene ............................. 7

1.3.2. Characterization and basic properties of Low Density Polyethylene .............................. 7

1.4. Methods of Polyethylene Degradation ................................................................................ 7

1.4.1. Thermal cracking .............................................................................................................. 8

1.4.1.1. Reaction mechanism ............................................................................................. 8

1.4.2. Catalytic degradation ..................................................................................................... 10

1.4.2.1. Reaction mechanism ........................................................................................... 11

1.5. Zeolite catalysts................................................................................................................. 13

1.5.1. Structure ......................................................................................................................... 14

1.5.2. Acidity of zeolites............................................................................................................ 15

1.5.3. Zeolites deactivation ...................................................................................................... 16

ix

Chapter 2 Material preparation and characterization. Apparatus ................................... 17

2.1. Polyethylene ...................................................................................................................... 17

2.2. Catalysts ............................................................................................................................ 17

2.2.1. Zeolite specifications ...................................................................................................... 17

2.2.2. Catalysts preparation ..................................................................................................... 17

2.2.2.1. Ion exchange........................................................................................................ 17

2.2.2.2. Calcination ........................................................................................................... 18

2.2.2.3. Catalyst storage ................................................................................................... 19

2.2.3. Catalysts characterization .............................................................................................. 19

2.2.3.1. Microscopy ........................................................................................................... 19

2.2.3.2. Chemical analysis ................................................................................................ 20

2.2.3.2. Acidity ................................................................................................................... 20

2.3. TG and DSC analysis........................................................................................................ 27

2.3.1. Equipment ...................................................................................................................... 27

2.3.2. Sample preparation ........................................................................................................ 28

2.3.3. Temperature profile ........................................................................................................ 28

2.3.3.1. Thermal and catalytic degradation ...................................................................... 28

2.3.3.2. Catalyst regeneration........................................................................................... 28

2.4. Gas Chromatography product analysis ............................................................................ 29

2.4.1. Equipment ...................................................................................................................... 29

2.4.2. Product collection ........................................................................................................... 29

2.4.3. Temperature profile ........................................................................................................ 30

Chapter 3 Degradation of polyethylene .............................................................................. 31

3.1. Thermal decomposition ..................................................................................................... 31

3.1.1. HDPE ...................................................................................................................... 31

3.1.1.2. Product distribution .......................................................................................... 32

3.1.2. LDPE ....................................................................................................................... 33


3.1.3. Comparison of the results for HDPE and LDPE..................................................... 35

x

3.1.3.1.Kinetic Modeling.................................................................................................... 38

3.1.3.1.1. Analysis of the estimated kinetic and thermodynamic parameters .............. 39

3.2. Catalytic cracking .............................................................................................................. 41

3.2.1. HDPE ...................................................................................................................... 41

3.2.1.1. Influence of catalyst acidity on cracking conditions ......................................... 41

3.2.1.2. Analysis of obtained kinetic and thermodynamic parameters ......................... 43


3.2.1.4. Coke deposition on catalysts ........................................................................... 46

3.2.2. LDPE ....................................................................................................................... 46

3.2.2.1. Influence of catalyst acidity on cracking conditions ......................................... 46

3.2.2.2. Analysis of the estimated kinetic and thermodynamic parameters ................. 48


3.2.2.4. Coke deposition on catalysts ........................................................................... 51

3.2.3. Evaluation of performance of catalysts with different acidity over the two polymers

........................................................................................................................................... 51

Chapter 4 Conclusions .......................................................................................................... 55

4.1. Future perspectives ........................................................................................................ 56

Bibliography............................................................................................................................ 57

Appendix A Data analysis ..................................................................................................... 60

xi

INDEX OF FIGURES

Figure 1.1. World plastics production by countries ......................................................................... 3

Figure 1.2. European plastic demand by segments ........................................................................ 4

Figure 1.3. Fragment of polyethylene C-C backbone....................................................................... 6

Figure 1.4. Thermal cracking reaction mechanism ....................................................................... 10

Figure 1.5. Structure of FAU zeolite ............................................................................................ 14

Figure 2.1. a) High Density Polyethylene in powder form ............................................................... 17

b) Low Density Polyethylene in pellets .......................................................................................... 17

Figure 2.2. Scheme of ion exchanges ......................................................................................... 18

Figure 2.3. a) Image of Y-82 catalyst obtained by FEG-SEM ........................................................ 19

Figure 2.3. b) Image of Y-82_4 catalyst obtained by FEG-SEM..................................................... 20

Figure 2.4. Temperature profile for TPD ...................................................................................... 22

Figure 2.5. Ammonia TPD curve and blank .................................................................................. 23

Figure 2.6. Comparison between the ammonia TPD model curves obtained for different zeolites ..... 24

Figure 2.7. Ammonia TPD decomposition curves for Y82 zeolite ................................................... 25

Figure 2.8. Distribution of acid sites over various activation energies obtained for different zeolites .. 26

Figure 2.9. TG/DSC installation setup .......................................................................................... 27

Figure 2.10. TG-DSC oven with balance ...................................................................................... 28

Figure 2.11.Temperature profile for thermal and catalytic cracking in TG/DSC ................................ 28

Figure 2.12. Temperature profile for catalyst regeneration ............................................................. 29

Figure 2.13. Gas Chromatography installation scheme................................................................. 29

Figure 2.14. a) Round bottomed flasks with collected gases; b) Injection of the sample into GC using a

microsyringe ............................................................................................................................... 30

Figure 2.15. Temperature program for the analysis in the GC ........................................................ 30

Figure 3.1. Heat flux and weight loss curves obtained for thermal cracking of HDPE ....................... 32

Figure 3.2. Product distribution obtained from thermal cracking of HDPE ....................................... 33

Figure 3.3. Heat flux and weight loss curves obtained for thermal cracking of LDPE ....................... 34

Figure 3.4. Product distribution obtained from thermal cracking of LDPE ........................................ 35

Figure 3.5. Comparison of heat flux curves for thermal cracking of HDPE and LDPE ...................... 36

Figure 3.6. Comparison of product distribution obtained from thermal cracking of HDPE and LDPE . 37

Figure 3.7. Experimental (blue) and calculated (red) heat flow curves for thermal cracking of a) HDPE

and b) LDPE ............................................................................................................................... 40

Figure 3.8.TG-DSC curves obtained for thermal and catalytic degradations of HDPE using catalyst

with different acidity ..................................................................................................................... 42

Figure 3.9.TG-DSC curve for catalytic cracking of HDPE using Y-82 zeolite ................................... 43

Figure 3.10. Product distribution obtained for catalytic cracking of HDPE ....................................... 45

Figure 3.11.TG-DSC curves obtained for thermal and catalytic degradations of LDPE using cataly st

with different acidity ..................................................................................................................... 47

Figure 3.12. TG-DSC curve for catalytic cracking of LDPE using Y-82 zeolite ................................. 48

Figure 3.13 Product distribution obtained for catalytic cracking of LDPE ......................................... 50

Figure 3.14. Comparison of catalytic cracking product distribution for HDPE and LDPE .................. 54

Figure A1.1. Experimental (blue) and calculated (red) heat flow curves for catalytic cracking of HDPE

using Y-82 zeolite........................................................................................................................ 61


using Y-82_1 zeolite .................................................................................................................... 61


using Y-82_2 zeolite .................................................................................................................... 62


using Y-82_3 zeolite .................................................................................................................... 62

xii


using Y-82_4 zeolite .................................................................................................................... 63


using Y-82_4 zeolite .................................................................................................................... 63

Figure A1.7. Experimental (blue) and calculated (red) heat flow curves for catalytic cracking of LDPE

using Y-82_4 zeolite .................................................................................................................... 75


using Y-82_4 zeolite .................................................................................................................... 75


using Y-82_4 zeolite .................................................................................................................... 76


using Y-82_4 zeolite .................................................................................................................... 76

Figure A 3.1.Comparison between blank curves for ammonia TPD experiments ............................. 68

xiii

INDEX OF TABLES

Table 1.1. Calorific values of various fuels ...................................................................................... 4

Table 2.1. Hashimoto parameters ................................................................................................ 21

Table 2.2. Total amount of acid sites ............................................................................................ 25

Table 2.3. Relative number of acid sites (%) as a function of activation energy .............................. 26

Table 3.1. Degradation temperatures obtained in thermal cracking for HDPE and LDPE ................. 36

Table 3.2. Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for thermal degradation

of HDPE and LDPE ..................................................................................................................... 37

Table 3.3. Model parameters obtained by fitting the kinetic model to experimental data for thermal

cracking of HDPE and LDPE........................................................................................................ 40

Table 3.4. Degradation temperatures obtained in catalytic cracking for HDPE ................................ 42

Table 3.5. Model parameters obtained by fitting the kinetic model to experimental data for catalytic

cracking of HDPE with catalyst of different acidity .......................................................................... 44

Table 3.6. Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for catalytic degradation

of HDPE ..................................................................................................................................... 45

Table 3.7. Coke deposition on catalysts....................................................................................... 46

Table 3.8. Degradation temperatures obtained in catalytic cracking for LDPE ................................. 48

Table 3.9. Model parameters obtained by fitting the kinetic model to experimental data for catalytic

cracking of LDPE with catalyst of different acidity .......................................................................... 49

Table 3.10. Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for catalytic degradation

of LDPE...................................................................................................................................... 51

Table 3.11. Coke deposition on catalysts for LDPE catalytic cracking cracking................................ 51

Table A1.1. Experiental runs performed in this work ...................................................................... 60

A 2.1. Gas Chromatography analysis............................................................................................ 66

Table A 2.1. Retention times for products formed during thermal and catalytic cracking................... 66

Table A 2.2. Molar distribution of products for HDPE thermal degradation ...................................... 67

1

CHAPTER 1 INTRODUCTION

1.1. A IM OF THE THESIS

One of the problems of modern society is waste management, and the management of the

plastic wastes became a major concern, because of our consumer li festyle. The amount of plastics on

Earth is enormous, mostly since plastics have replaced materials such as glass, ceramic or wood in

the manufacture of everyday use objects. Another concern of our and future generations is depletion

of natural crude oil resources, and what‟s involved – constant increase of gasoline prices.

Studies presented in this work present the possibility of solving those global problems, by recycling of

plastic wastes by incorporating them in the petrochemical industry, as a feed to FCC process. [1-4]

The main objective of this work is to study the production of liquid fuels by thermal and

catalytic degradation of polyethylene using acidic HNaY zeolites as catalysts.

Furthermore the influence of zeolite acidity on the catalytic degradation will be investigated.

Degradations will be performed on TG/DSC apparatus and the data collected will be evaluated

with the use of a kinetic model, that will help to better understand the differences between degradation

parameters for each catalyst.

Part of this work was used for writing the extended abstract “ Catalytic Cracking of High

Density Polyethylene Using USY zeolites – a TG/DSC Study” that will be presented at CHEMPOR

2011 conference, held in Lisbon, Portugal.

2

1.2. POLY MERS

The word polymer is derived from Greek word polymeres, which means built from many parts. [5]They

are chemical compounds with high molecular mass, and consist of multiple structural units.

Polymers revolutionized our world, we can find them everywhere around us, as well as in the items of

everyday use, in laboratories, industrial plants. Plastics, materials made from polymers, have replaced

wood, glass, metal and ceramics as a raw material for further use.

In the next chapters more information about the classification, production, consumption and recycling

of polymers will be provided.

1.2.1. CLASSIFICATION OF POLYMERS

There are numerous classifications of polymers, depending on their origin (natural or synthetic),

chemical structure (polyolefins, polyesters, polyamides), thermal behavior or polymerization mode.

From the point of view of this work, the two criteria that are of the most interest are the last two

presented above, because of their importance in polymer degradation.

Heat can influence the plastic material in two ways; in one of them the polymer may soften and be

able to form different shapes when a particular temperature is applied, since there are no cross -

linkings between the polymeric chains [6]. This group is called thermoplastics.

The second group are thermosets, those polymers contain in their structure strong covalent bonds,

which create a three-dimensional network structures. Heat applied to those materials doesn‟t change

their shape.

Classification based on polymerization mechanism divides polymers also on two groups: the addition

polymers and the condensation polymers.

Materials that belong to the first group polymerize by addition of monomers in a sequence, so that the

polymer chain is growing, and there is no release of any molecules to the reaction environment.

On the other hand, the condensation polymers evolve by discharging small molecules (like water) from

the chain. [5,6]

1.2.2. GLOBAL PRODUCTION AND CONSUMPTION OF PLASTICS

Plastic market is a fast growing branch of global industry, with the 9% average demand growth rate.

However, in recent years, due to the economic crisis this demand had decreased, especially in

3

Europe. Fortunately plastics future is very promising, they are replacing other materials because of

their wide range of applications.

Apart from that they may have a positive influence on the environment. The use of polymeric

materials can reduce exhaust gases emission and fuel consumption in automobiles – polymer are

lighter than metals so a vehicle constructed with their equivalents have reduced weight, hence burns

less fuel. [7,8]

As it is presented on the fig.1.1 the total world production of plastics in 2009 was 230 Mtonnes. One

third of the global plastics production is made in Asia, this is of course connected with the mass

production for European and American market. In Europe the leader in plastics production is

Germany. [7]

Figure 1.1. World plastics production by countries, 2009 [7]

One of the main groups of plastics are polyolefins. Polyolefins are thermoplastic polymers

made from petroleum. In this group, basic plastic materials are polyethylene

(PE) and polypropylene (PP). Among the plastics they occupy a prominent place in the world under

terms of production volume. They have no smell or taste, are not toxic, not soluble in

water, are chemically resistant, easy processing, light and cheap. These characteristics have

contributed to the huge popularity of polyolefins in our environment. In terms of production volume

polyolefin outperform all other plastics. The high production numbers are associated by the global

market demands. The factories must keep their manufacture in order to provide different sectors with

products they order. [7]

Rest of Asia16,7%

China15,2%

Japan5,6%

Latin America4,0%NAFTA

23,2%

Middle East, Africa8,1%

CIS3,0%

Other EU 27+ N, CH4,0%

Benelux4,5%

France3,0%

Italy2,0%

UK1,5%

Spain1,5%

Germany7,6%

230 Mtonne

4

Figure 1.2 illustrates the demand for plastics by sector, on the European market. The overall demand

in 2009 was 45 Mtonnes.

Packaging is the segment that requires the highest supplies (40%) of polyethylene (PE) ,

polypropylene (PP) and polyethylene teraphtalate (PET) ; second largest segment is building and

construction that demands 20% mostly of polyvinyl chloride (PVC). Sectors that stand for „others‟ are

small industries, such as: sport, leisure, agriculture.

Figure 1.2. European plastic demand by segments, 2009 [7]

The global consumption demand per capita is almost 100 kg per year and it increases with 4% annual

growth rate; 78% of this demand corresponds to thermoplastics. [ 6,7]

1.2.2.1. POLYMERS AS FUELS

Waste PE have a large calorific value and can be used as fuel (Table 1.1) to

production of electricity and heat. One kilogram of polyethylene can produce comparable amount of

heat as heating oil or gasoline. [10]

Table 1.1. Calorific values of various fuels [10]

Fuel Calorific Value (MJ/kg)

Coal 25-28

Wood 7-14

Heating oil/Fuel oil 40-48

Gasoline 46

Polyethylene 44

Packaging40%

Building and Construction

20%

Automotive7%

Electrical and Electronic equipment

6%

Others27%

45 Mtonne

5

The processing of secondary raw materials (i.e. recycling) belongs to a group of technology allowing

management of wastes, accumulated both from production and derived from used products on the

market. The specific case of waste from recycling of certain plastics involves the processing them into

fuel components (petrol, diesel). For this purpose there can be used conventional technologies

present for years in the refining industry as thermal c racking (pyrolysis), catalytic cracking (FCC) or

hydrocracking. The usual method (in the case of polyolefins waste) is thermal cracking. [10]

Companies such as BASF, Fuji Tech. Conrad. and BP have developed py rolysis technologies in the

fluidised bed. The disadvantage of this method on one hand is the application of high temperatures

(usually higher than 500 ° C) and a large number of products changes that require further processing

steps. On the other hand the process of pyrolysis of waste technologically is very complex.

Competitive method for pyrolysis is the catalytic cracking. It allows, within certain limits, to control both

efficiency and product distribution degradation of polymers, as well as lower the temperature of the

process. In a typical cracker using industrial catalysts and with the reactor bed operating in the

temperature range of 450-515 ° C, polyethylene is converted into hydrocarbons: gas fraction (yield 40-

50%) and liquid fraction (yield 30-40%). Both processes, pyrolysis and catalytic cracking proceed to

form hydrocarbon-rich fractions in olefins and aromatics. The amount of olefins obtained can be further

controlled in hydrocracking processes.[11-15]

1.2.2.2. ENVIRONMENTAL CONCERNS

The knowledge of demand and consumption is the key information in order to obtain actual data on

plastic wastes produced and how to deal with them.

The environmental awareness of today‟s society about the problems with waste plastics is increasing

thanks to the education in this field. Recycling of materials is not only trendy, but in some countries

there are penalty fees associated with proper disposing of wastes.

Landfilling of polyethylene is now the dominant way of PE waste management. Lack of segregation of

waste is not the only cause. The popularity of this polymer applications makes it a large dispersion in

the total mass of waste from households, from where it is difficult to release the wastes. For example,

beverage cartons consist of 20% polyethylene glued to the paper and aluminum foil. Additional

obstacle on the road to segregation of waste, even for those who are knowingly concerned for t he

environment, is itself identification of the polyethylene product. Along with the very poor system of

selective waste collection this contributes to a virtually irreversible loss of most of the polyethylene

wastes. During landfilling polyethylene undergoes a slow process of oxidation and biodegradation.

The speed of these processes depends on such factors as: access of air (oxygen), solar radiation,

temperature, humidity and the conditions of development opportunities within the biota, landfill.

6

It is estimated that the period of degradation of polyethylene in typical landfill conditions is over 200

years. [16]

Polyolefin wastes are light and occupy a large volume. This raises the cost of their transportation and

contributes to the rapid filling of landfills. Not protected areas in the dumps landfills are the source of

light fragments of polyethylene foils being distributed by wind and cluttering the neighboring areas.

Nevertheless, the issue of landfilling of plastic wastes is current and urgent. Polymers are mostly

resistant to biological degradation and are highly combustible, this causes serious environmental

threats. Legal policies are being designed and used to deal with this problem. According to the

European Commission Directive 1999/31/EC on the Landfill of Wastes, the membership countries

were obliged to reduce the amount of disposed polymers to 35% by 2010. [6,17]

For the reason of environmental concerns correlated with landfill disposal, the interest in recycling and

recovery of plastic wastes is gradually growing. Research are being done, experiments conducted, in

order to find the best and most efficient way to recover polymers. There are several options available:

chemical depolymerisation, gasification, thermal degradation and catalytic cracking. This work sets its

focus on the latter two.

1.3. POLY ETHYLENE

Polyethylene (PE) is a polyolefin with thermoplastic behavior. It is a very long molecule built by

repeating ethylene monomers (fig.1.3) . The majority of plastic wastes are made up from PE, due to its

wide range applications.

Figure 1.3. Fragment of polyethylene C-C backbone

Polyethylene presents various properties, which depend on the degree of chain branching. There are

two forms of PE to distinguish : High Density Polyethylene ( HDPE) and Low Density Polyethylene (

LDPE). [18,19]

7

1.3.1. CHARACTERIZATION AND BASIC PROPERTIES OF HIGH DENSITY POLYETHYLENE

HDPE is a linear c rystalline material with a molecular weight below 300 000 g/mol. It is harder and

more rigid than low density PE, also it possesses better tensile strength and high compressive

strength. [ 16]

High Density PE is produced in low temperatures (60-200oC) using Phillips or Ziegler-Natta catalysis.

[18]

The main applications of HDPE include water pipes, food and detergents containers and gas tanks.

1.3.2. CHARACTERIZATION AND BASIC PROPERTIES OF LOW DENSITY POLYETHYLENE

Low Density Polyethylene is a translucent, semi-crystalline material, with higher degree of branching

than HDPE.

It is produced at high pressure and temperature, by a free radical initiated polymerization of PE. [18]

Main advantages of LDPE are toughness, resistance to chemicals and low water absorption. The

polymer is mainly used for plastic bags production, dispensing bottles and juice and milk cartons. [20]

1.4. METHODS OF POLY ETHYLENE DEGRADA TION

Polyethylene subjected to high temperatures is degraded. Long carbon chains

break and create shorter products with smaller molecular weight. This process cannot be strictly

controlled and the final product is a mixture of a large number of different hydrocarbons, saturated and

unsaturated, with different numbers of carbon atoms in the molecule from C1 to about C40.

Depending on the chosen technology different proportions of gas, liquid, solids (paraffin) and finally

coke may be obtained. Many technologies used in the treatment of waste PE have their roots in the

petrochemical industry, which seems obvious because of the chemical similarity between the raw

materials.

Degradation of polyethylene by means of thermal and catalytic degradation was presented in many

studies before. [11-16, 21-36, 38-40] In this chapter the differences between the processes will be

presented, according to the general process conditions and reaction mechanisms.

To measure the degradation rate it is necessary to monitor the change in the mass of the sample, as

well as the kinetic parameters that are the variables in the process. For this purpose in this study the

thermogravimetrc (TG) and differential scanning calorimetry (DSC) methods are used. Those two

analysis combined give information regarding the sample weight loss and heat flow of the reaction,

and more importantly, after the kinetic analysis - the rate of breaking the bonds. The signals from

TG/DSC may be incorporated into a kinetic model, which will be described in chapter 3.1.1.1.

8

1.4.1. THERMAL CRACKING

Heating polyolefins at temperatures 380-450° C leads to progressive polymer chain cracking. The

higher the temperature the faster the process proceeds. As the cracking process occurs smaller and

smaller molecules are produced and gradually evaporate from the reaction mixture.

The gas phase includes a variety of gaseous hydrocarbons (C1-C6) such as methane, ethane,

ethylene, propane, isopropylene etc., and higher condensing products.

The products that condense at room temperature can be divided into two fractions : gasoline with a

boiling point of products in range -1oC – 180

oC and oil fraction with a boiling point 200-340°C. With

them - distilled fractions of paraffin, which unless they are separated, they tend to slowly crystallize

and precipitate.

In general, thermal cracking process includes pre-heating of and melting of polyethylene, only then

can begin the process of cracking of the polymer chains in the endothermic

reaction accompanied by evaporation of the formed products.

PE melting is not simple because polymer has low thermal conductivity, and after melting it is initially

formed as a viscous liquid. In some designs PE is pre-melted and then

introduced into the reactor. Techniques of polyethylene thermal cracking can be

divided in terms of differences in the way of supplying energy to waste and

sustaining the direction of endothermic cracking reaction. [22, 28, 30]

Thermal cracking of PE involves applying such amount of energy in order to break down the C-C

bonds. This energy is called the depolymerization energy and can be calculated by the following

expression:

Ed=46,9+0,65ΔEp,

Where Ep is the energy needed for polymerization. [17]

1.4.1.1. REACTION MECHANISM

The mechanism of thermal depolymerization of polyethylene involves random hemolytic cleavage,

meaning that carbon atoms connected by the breakable bond retain one of the bonding electrons after

the bond is broken. [37] As a result, free radicals are formed. The free radicals reduce the bond

energy of neighboring C-C bonds by 67kJ [17]

9

Thermal cracking mechanism proceeds by the following steps:

Initiation

Propagation

Termination

The mechanism is initiated by a random scission of polymer into primary radicals (Rp), subsequently

the primary radicals propagate into β-scission forming ethane and, by means of intramolecular

hydrogen transfer, more stable secondary radicals (Rs).

Following the scheme in fig 1.4 adapted from the work on by Bockhorn [38], it can be noted that due to

propagation, a primary radical is created after each β -scission (reaction 4 and 4‟). Finally the

termination step occurs by fusion of primary radicals to form an inert molecule.

10

Figure 1.4. Thermal cracking reaction mechanism [38]

1.4.2. CATALYTIC DEGRADATION

11

Catalytic cracking of polyethylene is the breaking of the C-C bonds in

the molecule of the raw material to form smaller molecular weight products catalyzed by solid acids.

The presence of the catalyst in the reaction decreases the activation energy needed for the reaction to

occur.

As a result of interaction between the hydrocarbon molecule and the acidic center of the catalyst,

unstable intermediates are formed – carbocations. Olefins incorporate in their structure a proton from

the Broensted centers and paraffins, being less reactive, react only at high temperatures.

Compared to thermal cracking, the products evolved from catalytic cracking contain more branched

paraffins, cycloparaffins and aromatic hydrocarbons. There are also more butanes and butenes and

less methane, ethane and ethylene formed. [23, 25]

Studies [1, 4, 22, 57]have shown the following:

Catalytic cracking enables lower degradation temperatures due to lower activation energy

required for process initiation, hence shorter cracking time

Higher aromatic content – better fuel quality

Higher selectivity to liquid products, within the boiling range of gasoline

1.4.2.1. REACTION MECHANISM

As stated before during catalytic cracking with acid catalyst, some ions are formed on the catalyst

surface, mostly carbenium ions (the most privileged is the tertiary carbon atom). Carbenium ions do

not arise due to breakage of C-C bonds, because the energy needed to break the polymer chain and

form a carbocation and a negative ion requires 1080kJ/mol whereas formation of free radical requires

only 330kJ/mol.[58] However, some authors assume that the protolytic cracking, by which an acid site

directly breaks a C-C bond, may have a role in catalytic cracking at high temperatures. Protolytic

cracking of an alkane will produce another, shorter, alkane, and a carbenium ion. It is also possible

that after the formation of a carbonium ion from an alkane molecule hydrogen is evolved, also

producing a carbenium ion. Carbenium ions are easily formed during the attachment of a proton to an

olefin from the Broensted centers or by hydride abstraction from a parafine by a Lewis acid center .

[58]

Generally it is considered that only the Broensted centers are needed for the cracking reaction to

occur.

At temperatures above above 580°C zeolites undergo dehydration, a process that converts Bronsted

acid sites into Lewis centers and for higher temperatures catalysts may lose their activity.

Carbocations may have different stability and undergo fast changes leading to more stable forms.

The stability of carbocations is presented in the following order:

12

methyl <ethyl < primary< secondary <tertiary

The positive charge in the carbenium ion has a tendency to move towards the center of the chain, due

to the increased stability of secondary and tertiary carbocations; carbon chain isomerization also

occurs therefore forming isomers. [39, 58]

Moreover carbenium ion can undergo scission creating an olefin and a new carbenium ion. It is

important to notice that, the bond which is broken is in the β position realtive to the positive charge on

the carbon atom and, thus, the shortest olefin that is likely to be formed is propene, although some

ethene is also produced in catalytic cracking.

From the above mechanism is clear why the catalytic cracking is preferred to thermal cracking: in the

latter breaking of the bonds is more incidental, and in the catalytic cracking it is more organized and

therefore more selective, leading to more of the preferred products, apart from being carried -out at

temperatures that are much lower than the ones needed for thermal cracking. [39]

According to the mechanism proposed by Corma and Orchilles [40] the reaction occurs in three steps,

just like the thermal cracking : initiation, propagation and termination; however with distinct changes.

The mechanism of catalytic cracking is initiated by the formation of carbenium ion (R-CH+) through

proton addition to olefin or hydrogen removal from paraffin. If we consider olefin, this step would

involve the attack of one of the catalyst Bronsted acid sites to the polymer double bond.

Another reaction resulting from proton attack is breaking of C-C bond located by the carbocation atom

through β-scission.

The propagation step is proceeded by the transfer of an hydride ion from a reactant molecule to the

absorbed carbenium ion, and termination corresponds to desorption of absorbed carbenium ion to

give an olefin, while the acid site returns to its initial state.

Buekens [10], in his work, explains more about the fate of the carbenium ion during the propagation

step. The carbocation may undergo the isomerization by hydrogen or carbon shifts, leading to the

rearrangement of the double bond position and/or of the skeletal carbon chain.

13

Carbenium ion may also undergo aromatization and cyclization reactions. An example is when

hydride-ion abstraction first takes place on an olefin at a position several carbons removed from the

double bond, the result being the formation of an olefinic carbonium ion:

The aromatization occurs because the carbenium ion may experience the intramolecular attack on the

double bond, which is presented below:

1.5. ZEOLITE CATALYSTS

The word “zeolite” has its origins in the Greek language - “zeō” – to boil and “lithos” which means

stone. The person who for the first time used the term “zeolite” was the Swedish mineralogist Axel

Frederik Cronstedt in 1756.

Zeolites are microporous minerals, whose framework is composed of aluminium, silica, oxygen and

auxiliary cations (e.g. sodium, potassium calcium, magnesium), and within the pores they conta in

water or other molecules.

Their origin may also be from natural sources (minerals such as natrolite, analcime) or synthesized in

the laboratory. The main advantage of synthetic zeolites over the natural ones is that a zeolite

structure can be designed in any way, in unlimited variations. [41, 43]

The general chemical formula for zeolites is defined as:

Mx/n(AlO2)x(SiO2)y*zH2O

In this expression, M stands for a corresponding cation with n valency and z is the water content. The

x and y stand for the number of atoms of aluminium and silica. [43]

The application of zeolites has a very wide range, from water purification and detergents, through

nuclear and petrochemical industry, agriculture, and even in everyday pet care as a cat litter.

14

This work focuses on the zeolites used in petrochemical industry, for catalytic cracking of petroleum

fractions, particularly the Ultra Stable Y zeolite (USHY).

USHY zeolite is a form of Y zeolite dealuminated by steam, with greater Si/Al framework ratio and also

some extra-framework aluminum and, therefore, it possesses an enhanced thermal stability. [44]

1.5.1. STRUCTURE

The general structure of zeolites is based on the presence of various cavities, which are formed by the

three dimensional framework generated by the silicon and aluminum tetrahedra. The main property of

the cavities/pores within the zeolite is their ability to let through the molecules. Dimensions of the

molecules that may leave or enter the zeolite is restricted by the pore diameter.

Zeolite framework consists of four networks connecting the atoms, with aluminum or silicon as a

central atom and four oxygens on the sides. This provides with SiO4 molecules and AlO4- ion, which

would indicate that zeolite‟s frameworks are anionic, however the negative charge on aluminum atom

is neutralized by extra-framework species. To balance the electrical charge, exchangeable cations are

present in the inner zeolite structure. [42]

Zeolite Y has a three dimensional structure that consists of sodalite cages, which are connected by

hexagonal faces. Sodalite cages surround the large cavity in the inner framework (supercage) ( fig 1.

35 ) with 13 Å diameter. The outer structure is defined by 12- membered oxygen rings, that surround

pores of 7,4 Å diameter. [42, 46]

Figure 1.5. Structure of FAU zeolite [42]

15

1.5.2. ACIDITY OF ZEOLITES

Zeolites can be catalytically active solids for acid catalysis, due to the presence, in their structure, of

active acid centers. It is very important to define the concentration, density, strength and distribution of

acid sites in a zeolite, and also to differentiate between their chemical origin - Bronsted or Lewis acid

site.

By definition, Bronsted acid is a proton donor and Lewis acid is acceptor of an electron pair.

Bronsted acid sites are generated when one silicium (Si4+

) atom is substituted by a metal with the

valency of three, usually aluminium (Al3+

). This creates a negative charge, that needs to be balanced

with a proton.

The proton is connected to the oxygen that bridges between a Si and the Al in the framework –

creating a bridging hydroxyl group [Si(OH)Al] . This hydroxyl group acts as a proton donor – a

Bronsted acid.

Consequently the maximum number of acid sites is equal to the number of framework aluminum

atoms, however due to dehydroxylation and dealumination, some cations are lost. Hence, the r eal

amount of acid sites depend on Si/Al ratio, according with the rule, the lower the ratio, the higher

number of acid sites (the stronger acidity). The maximum Si/Al ratio is 1, according to the Lowenstein

rule. [44, 45, 47]

Factors that influence the catalytic activity of the acid sites, apart from Si/Al ratio, are:

The Si-O-T ( where T is the tetrahedral atom, e.g. Al) bond angle which affects the partial

charge of hydroxyl group; for Y zeolite it should range between 138o and 147

o

Substitution of Si atoms with trivalent cations other than aluminum, generates the following

sequence of acid strengths:

B(OH)Si < In(OH)Si << Fe(OH)Si < Ga(OH)Si < Al(OH)Si

Accessibility of acid sites – the studies have shown that the acid sites located in the

supercages are more „open‟ towards organic molecules than the acid sites located in

hexagonal prisms. [43]

Also extraframework aluminum atoms (EFAL) are linked to the presence of Lewis acid sites, apart

from influencing the strength of the Bronsted acid sites present in the structure. These EFAL might be,

for instances, the Al atoms that are displaced from the zeolite framework during dealumination. [47]

16

1.5.3. ZEOLITES DEACTIVATION

Catalyst lose their activity mostly because their acid sites become inhibited by impurities or by

deposition of carbonaceous material, referred to as coke.

Coke is deposited on zeolites as a result of cracking side reactions. [49]

The deposition of coke, which blocks the acid sites and produce a deactivation of the catalyst, is a

partially reversible process, the catalyst may still be regenerated by burning the residue with pure

oxygen at high temperatures.

Zeolites are deactivated by coke formation due to blocking of catalyst pores. This results in decreased

rate of polymer transformations or modified selectivity of the zeolite towards some molecules. [ 50 ]

Under the conditions of catalytic cracking, there are many reaction pathways leading to the formation

of coke. Important reactions are the cyclization of alkines and di - and polyalkenes followed by

aromatization and condensation.

Cyclization and aromatization can occur due to inner carbenium ion and hydrogen migrations. It is not

likely that cycloalkanes isomerize prior to cracking, most of the C9 and aromatic compounds are

formed from them. Alkylbenzenes become benzene below 500°C. However, benzene rings substituted

two or more groups of methyl undergo disproportionation and isomerization leading to only small

amounts of benzene. As a result of those reactions polyaromatic compounds are formed – namely

coke. [39].

17

CHAPTER 2 MATERIAL PREPARATION AND CHARACTERIZATION. APPARATUS

2.1. POLY ETHYLENE

Polymer, used for thermal and catalytic decomposition in this studies, was low and high density

polyethylene. The material was supplied by Borealis and it doesn‟t possess any additives.

The low density PE is formed into small pellets with the average molecular weight of, Mw =376000.

High density PE has a powdery form and its average molecular weight Mw=290000.

Figure 2.1. a) High Density Polyethylene in powder form

b) Low Density Polyethylene in pellets

2.2. CATALYSTS

2.2.1. ZEOLITE SPECIFICATIONS

The original catalyst used in this work was a commercial Y-82 zeolite with Si/Al ratio of 4.5 supplied by

UOP.

2.2.2. CATALYSTS PREPARATION

2.2.2.1. ION EXCHANGE

To obtain different acidic strengths of the catalyst ion exchange had to be performed. The original Y -

82 was mixed with 0.5M and 1M solution of sodium nitrate. During this step protons were replaced

a) b) a)

18

with sodium ions and by this modification the total amount of Bronsted acid sites was decreased.

Figure 2.2 illustrates the preparation of four different catalysts.

The ion exchanges were performed by stirring the zeolite with sodium nitrate solution at room

temperature for four hours. The volume of NaNO3 solution used was 4ml per gram of catalyst.

Afterwards the zeolite was filtrated and washed with distilled water.

The catalysts after the exchanges were designated with the names : Y-82 for the original zeolite, Y-

82_1 for zeolite exchanged with 0.5M NaNO3 , Y-82_2 for zeolite exchanged with 1M NaNO3 , Y-82_3

for zeolite two times exchanged with 1M NaNO3 and Y-82_4 for zeolite three times exchanged with

1M NaNO3 .

Figure 2.2. Scheme of ion exchanges

2.2.2.2. CALCINATION

To purify the catalyst material from any trace compounds, it underwent a calcination process. The

calcination was carried-out in a tubular reactor and the zeolite was heated at 10oC/min, up to the

temperature 500ºC; after it stayed at this temperature was kept for 8 hours. The calcination took place

under the dry air flow of 0.5l/h per gram of catalyst.

19

2.2.2.3. CATALYST STORAGE

To guarantee the saturation of catalyst structure at all times, in order to make sure the catalyst

wouldn‟t absorb any contaminants from the air and to ensure that the zeolite always contains the same

amount of water, it was stored in a closed container at room temperature over a solution of calcium

carbonate.

2.2.3. CATALYSTS CHARACTERIZATION

2.2.3.1. MICROSCOPY

Scanning Electron Microscopy (SEM) was performed at MicroLab of Instituto de Ciencia e Engenharia

de Materiais e Superficies of Instituto Superior Tecnico using JEOL JSM-7001F equipment. The

analysis was done in order to determine the shape of the catalyst particles.

Two catalyst samples were analyzed at MicroLab using FEG-SEM, the parent catalyst Y-82 and the

least acidic zeolite, exchanged three times with sodium nitrate solution Y-82_4.

As it is presented in fig.2.3 a) and b) the size of the catalyst particles doesn‟t change after the ion

exchange and can be estimated to be around 1µm.

Figure 2.3. a) Image of Y-82 catalyst obtained by FEG-SEM

20

Figure 2.3. b) Image of Y-82_4 catalyst obtained by FEG-SEM

2.2.3.2. CHEMICAL ANALYSIS

The elemental chemical analysis of USHY zeolites was performed at the Laboratorio de Analises of

Instituto Superior Tecnico, to determine the total amounts of silica, aluminum and sodium in the

catalysts.

2.2.3.2. ACIDITY

The acidity of catalyst used in this work was measured by ammonia Temperature Programmed

Desorption (TPD). This method involves saturating the sample with an adsorbate (in this case –

ammonia) and heating it according to a previously established temperature profile. The procedure

uses ammonia because it is a strong base and a small molecule and, thus, it may access most of the

acid sites in zeolites.

The correlation between activity and acidity in zeolites is very well established. In order to measure the

acid strength, it is important to develop the appropriate acidity scale. [51-55]

Determination of the acid site strength distribution as a function of activation energy for ammonia

adsorption, involves performing a digital deconvulation procedure on NH3-TPD curves into several

components. They represent the sites characterized by the amount of acid sites with uniform acidity.

21

The desorption process is assumed to follow an irreversible first -order kinetic rate, given by the

equation:

where qi is the amount of sites occupied by ammonia molecules at time t, k i is the pre-exponential

factor, is the activation energy for ammonia desorption and R and T are the gas constant and

the absolute temperature.

Zeolites show a diversity of acidities, because the acid sites occur in various locations, and, hence,

there are different energies for desorption of ammonia. For this reason the thermogram of TPD

consists of a range of desorption curves, each one corresponding to different acid site.

Therefore to properly describe the TPD curve, the sum of the desorption curves for each type of acid

sites is needed. To represent the different desorption activation energy values a descrete energy grid

was use.

According to the above, the general TPD curve is described by the equation:

Where

is the overall rate of ammonia desorption.

In order to obtain values of , through multi-linear least-square regression, the pre-exponential factor

must be known. Hashimoto described the relationship between the pre-exponential factor and

activation energy for ammonia desorption, involving two, constant for the same zeolite family,

parameters – α and β. [52]

For USHY type of zeolites the values of α and β were determined previously by Pedro Oliveira [52]

and are presented in the table below.

Table 2.1. Hashimoto parameters

Parameter Value

αi (min-1

) 1.19 x 103

β ( mol/kJ) 7.39 x 10-2

22

Prior to TG-DSC experiments, the catalyst sample (20mg) was placed, together with ammonia in

Schlenk and put into a controlled temperature water bath for 8 hours.

Afterwards TG-DSC experiments were held in TA Instruments SDT 2960 Simultaneous DSC -TGA

apparatus. The analysis consisted of two runs with the same temperature profile (see fig.2.4), first one

was with the ammonia absorbed catalyst and the other was with a fresh zeolite. This latter blank test

was performed to establish a proper baseline and to check the extent of dehydroxylation reactions.

The peak corresponding to the loss of mass due to dehydroxylation occurs at temperatures between

600-800oC and is due to the release of water from the zeolite framework.

Figure 2.5 presents two curves obtained during the experiment: the catalyst with adsorbed ammonia

and fresh catalyst. The curve of catalyst with ammonia shows two peaks at lower temperatures and a

very distinctive peak with the maximum at 700oC. Usually this peak corresponds to the

dehydroxylation peak and is substracted in afterwards simulated model curves. However, the blank

test presents no significant mass loss of the catalyst in the higher temperature range, meaning the

third peak in the ammonia TPD curve may be related to very strong acid sites.

Figure 2.4. Temperature profile for TPD

23

Figure 2.5. Ammonia TPD curve and blank

The model ammonia TPD curves presented below were developed by subtracting the data obtained

from blank test to the one with the ammonia adsorbed on the catalyst and fitting them into the model

equations introduced in previous chapter.

As it can be noted in figure 2.6, the most acidic catalyst, the parent zeolite, presents three very

distinctive peaks with a large area under the curves, corresponding to the amount of adsorbed

ammonia, which should be equivalent to the amount of acid sites present in the catalyst. The first two

peaks refer to less acidic acid sites, whereas the last peak is related to the very strong acid sites,

because the higher activation energy is needed for the desorption of ammonia from those sites.

The behavior of the ion-exchanged catalysts is as expected. The higher the sodium content in the

zeolite the lesser the amount of ammonia absorbed, nevertheless at high activation energies the

strong acid sites peak still occurs, albeit smaller; however it seems to be shifted towards higher

temperatures, meaning the catalyst still possesses strong acid sites.

It might be concluded from the graph that the USHY family of zeolites presents a very strong catalytic

activity due to the amount and strength of the acid sites.

Graph presenting the baseline curves for ammonia TPD may be found in APPENDIX, sec tion A3.1.

150 300 450 600 750 900

Temperature (oC)

Y82 Blank

Y82 with ammonia

24

Figure 2.6. Comparison between the ammonia TPD model curves obtained for different

zeolites

The activation energy grid was chosen after fitting the experimental data into the model and using

“Solver” tool in an Excel ( ©Microsoft Corp.) spreadsheet. This set is very wide, to show the accurate

distribution of acid sites. Energy values are in kJ/mol and are the same for all of the catalysts; the grid

is as follows: 50, 60, 70, 80, 90, 110, 150, 160, 180, 200, 250, 280.

Figure 2.7 presents the decomposition curves for Y-82 zeolite. Each decomposition curve corresponds

to different desorption activation energy and shows the amount of ammonia adsorbed to the acid sites.

Calculating the quantity of ammonia per each decomposition curve for all of the catalysts, led to the

information about the total number of acid sites in catalysts. Those values are s hown in table 2.2 The

most acidic catalyst presents the highest amount of acid sites, as predicted. The zeolites which were

exchanged with sodium ions in different concentrations, show smaller number of acid sites than the

parent catalyst. Yet, it needs to be noted that the values of acid sites do not vary significantly between

the exchanged catalysts, so it might be assumed that the degree of exchange does not change the

amount of available acid sites for this particular catalyst.

0,0E+00

5,0E-06

1,0E-05

1,5E-05

2,0E-05

2,5E-05

3,0E-05

150 400 650 900

dm

/dt(

mm

ol.

s-1

)

Temperature(oC)

Y-82

Y-82_1

Y-82_2

Y-82_3

Y-82_4

25

Figure 2.7. Ammonia TPD decomposition curves for Y82 zeolite

Table 2.2. Total amount of acid sites

catalyst acid sites (mmol/mgzeolite)

Y-82 3.37x10-3

Y-82_1 1.21x10-3

Y-82_2 1.00x10-3

Y-82_3 1.07x10-3

Y-82_4 9.77x10-4

Figure 2.8 represents the amounts of acid sites as a function of their strength as measured by the

activation energy for the desorption of ammonia.

0,0E+00

5,0E-06

1,0E-05

1,5E-05

2,0E-05

2,5E-05

3,0E-05

150 300 450 600 750 900

dm

/dt(

mm

ol.s

-1)

Temperature(ºC)

model curve

50

60

70

80

90

110

150

160

180

200

250

280

26

Figure 2.8. Distribution of acid sites over various activation energies obtained for different

zeolites

The relative number of acid sites as a function of activation energy was calculated by dividing the

amount of acid sites for an individual activation energy over the total amount of acid sites in each of

the catalyst. This shows the percentage of adsorbed ammonia along the energy grid.

Table 2.3. Relative number of acid sites (%) as a function of activation energy

Activation energy (kJ/mol) Relative number of acid sites (%)

Y-82 Y-82_1 Y-82_2 Y-82_3 Y-82_4

50 20.2 0.0 0.9 0.0 5.7

60 8.9 4.2 14.1 14.3 26.9

70 16.7 36.9 23.5 29.3 27.7

80 8.9 14.3 26.6 21.3 25.4

90 14.0 23.6 8.3 14.7 7.6

110 5.8 0.0 0.0 0.1 0.1

150 0.0 0.0 0.0 0.0 0.0

160 15.1 1.9 0.0 0.6 0.0

180 5.8 2.7 5.2 2.4 0.1

200 4.4 16.3 21.4 16.7 6.7

250 3.3 0.0 0.0 1.1 0.0

280 0.0 0.0 0.0 0.1 0.0

0,00E+00

2,00E-04

4,00E-04

6,00E-04

8,00E-04

50 60 70 80 90 110 150 160 180 200 250 280

q (m

mo

l/m

gzeo)

Activation energy (kJ/mol)

Y-82_4

Y-82_3

Y-82_2

Y-82_1

Y-82

27

2.3. TG A ND DSC ANALYSIS

Thermogravimetry and Differencial Scanning Calorimetry analysis were used in this study to obtain

signals referring to the weight change of the sample as well as the change in heat flux, including the

heat that is required for the breaking of the C-C bonds.

The experiment consisted of two runs ( 1st

and 2nd

cycle respectively) and catalyst regeneration ( 3rd

cycle, only when the catalyst was present).

During the first cycle, the polymer sample (pure or mixed with the catalyst) was placed in quartz pan

and heated according to the temperature profile (chapter 2.3.3.1.). After the first run was complete, a

second cycle begun without changing the geometry and conditions inside the furnace, no new polymer

was added. This 2nd

run was performed mainly to obtain the baseline for the DSC signal [57]. Both

cycles were carried out under continuous nitrogen flow rate of 80ml/min.

Catalyst regeneration was performed to observe the coke deposition on the catalyst. Throughout the

3rd

cycle, air, with the flow of 75ml/min, was burning the coke at a given temperature profile (chapter

2.3.3.2.).

2.3.1. EQUIPMENT

Thermogravimetric (TG) and Differential

Scanning Calorimetric (DSC) analyses

were performed in SDT 2960 DSC-TGA

apparatus. The machine is equipped with

furnace, gas flow lines, gas flow measurer

and computer.

Figure 2.9. TG/DSC installation setup

28

2.3.2. SAMPLE PREPARATION

Polyethylene samples, with and without the catalyst, were placed in quartz pans on the balance. Next

the whole system was purged for 30 minutes with nitrogen, to wash out all the oxygen from the heating

chamber.

The amount of PE used for thermal and catalytic degradation was around 10 mg, whereas the amount

of zeolite added equaled to 1mg.

Figure 2.10 presents a close up view on the

TG furnace. The sample holders on which the

pans with the samples are placed can be

viewed in the open furnace chamber.

Figure 2.10. TG-DSC oven with balance

2.3.3. TEMPERATURE PROFILE

2.3.3.1. THERMAL AND CATALYTIC

DEGRADATION

The temperature profile for thermal and catalytic degradation was alike. First there was a constant

increase of temperature, 10oC per min, up to 600

oC. The final temperature was kept constant for 10

minutes, afterwards there was a natural cooling down to the room temperature.

Figure 2.11.Temperature profile for thermal and catalytic crack ing in TG/DSC

2.3.3.2. CATALYST REGENERATION

The catalyst regeneration temperature profile begun with the heating of the TG/DSC apparatus to

210oC at 10

oC/min rate, this temperature was kept constant for 30 minutes. Afterwards, another

increase of temperature was programmed with the same heating rate, up to 700oC. When the

temperature reached its final limit, the system maintained it for 15 minutes before cooling down to the

room temperature.

29

Figure 2.12. Temperature profile for catalyst regeneration

2.4. GAS CHROMA TOGRA PHY PRODUCT A NALYSIS

2.4.1. EQUIPMENT

The gas chromatograph used in the experiments was Shimadazu GC-9A, working under the nitrogen

pressure of 2 bar, equipped with flame ionizing detector (FID) and PLOT column (KCl/Al2O3).

The machine was connected to Shimadazu C-R3A integrator in order to analyze the electrical signal.

Gas Chromatograph is equipped with the column oven, injector, flow control sector, split and FID

detector.

Figure 2.13. Gas Chromatography installation scheme

2.4.2. PRODUCT COLLECTION

30

Gases for the analysis were collected directly from the output of TA Universal Analysis 2000. A Teflon

tube was connecting the apparatus to the opening of volumetric flask filled with water (V=1000ml), that

was set up inverted on water container.

After the collection the gases produced during polymer degradation were sampled using a 100µl

syringe and injected to the GC.

Figure 2.14. a) Round bottomed flasks with collected gases; b) Injection of the sample into

GC using a microsyringe

2.4.3. TEMPERATURE PROFILE

The column oven was heated from its initial temperature ( 50oC) at constant rate 10

oC/min up to the

temperature of 160oC. Afterwards, the oven was heated at different rate, 5

oC/min up to the final

temperature 200oC, which was kept constant for 15 minutes.

Figure 2.15. Temperature program for the analysis in the GC

31

CHAPTER 3 DEGRADATION OF POLYET HYLENE

This chapter is fully devoted to the experimental part of this work . The degradation of HDPE and

LDPE, thermal and catalytic as well, was performed in an inert atmosphere using TG and DSC

methods explained in previous chapter.

In this part the main focus will lay on describing the behavior of the polymers with and without the

presence of catalyst; the degradation temperatures, coke deposition. The products formed during the

cracking reactions were evaluated using gas chromatography and analyzed according to their

retention times.

In addition thermal and catalytic cracking TG-DSC signals will be evaluated using a kinetic model, to

have a better close-up into the chemistry of the process itself and to estimate some of the kinetic and

thermodynamic parameters.

The catalysts used for the experiments were described and characterized in previous chapters.

3.1. THERMAL DECOMPOSITION

3.1.1. HDPE

Figure 3.1 presents the graph consisting of two curves, the corrected heat flux and weight fraction, as

a function of temperature.

The corrected heat flux line is created by subtracting the signal obtained from 1st

degradation run ( 1st

cycle) with the second degradation run ( 2nd

cycle). As it is shown, the curve possesses two peaks,

one around 130oC and the other circa 480

oC. The first peak corresponds to the melting of polymer, ,

the second one is the cracking itself. The cracking peak starts when the first bond in the polymer

structure is broken, without any weight change at that time, and ends with the full degrad ation of the

sample.

The blue curve on fig.3.1 represents the weight fraction change during thermal cracking. The mass of

the polymer starts to change after the first bonds are broken, when the cracking products are light

enough to vaporize to the gas phase. It can be observed that the weight change is very rapid with a

steep slope.

32

Figure 3.1. Heat flux and weight loss curves obtained for thermal cracking of HDPE

3.1.1.2. Product distribution

The gas fraction collected from TG-DSC experiments, was analyzed using gas chromatography. The

products were recognized according to their retentions times. The table with retention times may be

found in APPENDIX.

The results of the HDPE sample analysis are shown in fig.3.2. Graph represents, in molar percent,

the distribution of hydrocarbons obtained during thermal decomposition, divided into three groups:

paraffins, olefins and aromatic compounds.

It is easy to notice that the majority of products formed during thermal cracking are olefins in C2-C5

range, the most favorable compounds are propylene, butene, iso and cisbutene and pentene.

The content of paraffins is very small comparing to olefins. Alkanes formed from HDPE decomposition

are between C1 to C6, however their individual molar fraction is never higher than 5%.

Formation of aromatic compounds is the least favorable, only C6 – benzene and C9 , were found in

the sample.

Thermal cracking of HDPE favours production of shorter hydrocarbons, namely olefins.

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

-2,5

-2

-1,5

-1

-0,5

0

0,5

0 200 400 600 800

Weig

ht

fracti

on

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

heat f lux

weight f raction

33

Figure 3.2. Product distribution obtained from thermal cracking of HDPE

3.1.2. LDPE

Corrected heat flux and weight fraction curves for thermal cracking of LDPE are presented on figure

3.3.

Similarly as in the case of HDPE, the first peak of heat flux line corresponds to the melting of the

sample, around 110oC, and second is cracking, around 480

oC.

The graph shows that the weight change begins after the breakage of first bonds. Weight fraction

curve is also very steep, the sample vaporizes quickly.

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct

dis

trib

uti

on

(%

mo

lar)

Number of carbon atoms

Paraf f ins

Olef ins

Aromatics

34

‟

Figure 3.3. Heat flux and weight loss curves obtained for thermal cracking of LDPE


Thermal cracking of LDPE produces hydrocarbons with C1-C12 range, however the most favorable

are short-chained compounds.

As in the case of HDPE the majority of products obtained from LDPE degradation are alkenes in C2-

C6 range. The dominating compounds are propylene, trans iso and cis -butene isomers, as well as

pentene and hexene.

Paraffins are formed in smaller amounts and mostly consist of methane, ethane, propane, butane,

pentane and hexane.

Thermal cracking of LDPE does not favor c reation of aromatic compounds, their percentage is the

smallest. Aromatic fraction is composed of benzene, meta and p-xylene, trimethylbenzene and C9

aromatic heavy hydrocarbons.

0

0,2

0,4

0,6

0,8

1

1,2

-2

-1,8

-1,6

-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

0 100 200 300 400 500 600 700

Weig

ht

fracti

on

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

heat f lux

weight f raction

35

Figure 3.4. Product distribution obtained from thermal cracking of LDPE

3.1.3. COMPARISON OF THE RESULTS FOR HDPE AND LDPE

Behaviour of HDPE and LDPE in thermal cracking is slightly different due to the differences in their

chain structure, high density polyethylene is more linear than the low density one.

Comparing the heat flux curves (fig 3.5) for both polymers, it might be concluded that LDPE presents

lower degradation temperatures than HDPE. The precise values for those temperatures are presented

in table 3.1 As it is seen the melting temperature for low-density PE is smaller by around 20oC than for

HDPE. This is strongly connected with the presence of side chains, more branched structure.

The temperatures of cracking do not vary by much, they are quite similar. For HPDE the cracking

temperature value is 483oC; and on the other hand LDPE cracks at 476

oC.

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct

dis

trib

uti

on

(%

mo

lar)


Paraf f ins

Olef ins

Aromatics

36

Figure 3.5. Comparison of heat flux curves for thermal cracking of HDPE and LDPE

Table 3.1. Degradation temperatures obtained in thermal cracking for HDPE and LDPE

Polymer type Melting temperature Cracking temperature

HDPE 134 483

LDPE 113 476

Figure 3.6 presents the overall product distribution, lumped by the number of carbon atoms.

For both reactions the favored products are smaller, shorter molecules in C1-C6 range, the heavier

fractions are less important. Thermal cracking occurs in higher temperatures, and for this reason the

majority of formed compounds are light fractions.

Thermal cracking of HDPE produces more hydrocarbons is C3-C5 range, whereas degradation of

LDPE creates significantly more products with six carbon atoms.

-2,5

-2

-1,5

-1

-0,5

0

0,5

0 100 200 300 400 500 600 700

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

HDPE

LDPE

37

Figure 3.6. Comparison of product distribution obtained from thermal cracking of HDPE and LDPE

Analyzing the product distribution with closer look and taking into considerations figures 3.4 and 3.2

and additional calculations found in APPENDIX the overall olefin to paraffin ratio, hydrogen to carbon

ratio and aromatic content (%) may be computed.

Table 3.2 presents the more relevant values. Thermal c racking of HDPE produces less paraffins than

LDPE, because the O/P ratio is 6.1, which is almost twice higher than for LDPE (O/P=3.6) and

additionally the hydrogen to carbon ratio is lower for high -density PE, meaning there are more

compounds with double bonds formed.

Content of aromatic compounds in HDPE and LDPE thermal cracking is 1% and 3%, respectively.

According to these values, it seems that during thermal degradation of more branched polymers, the

aromatization reactions are slightly more favored. This might be explained according to the

observation mentioned above, LDPE produces more hydrocarbons with six carbon atoms, and they

are the base compounds for cyclization reactions.

Table 3.2. Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for thermal degradation of HDPE

and LDPE

O/P ratio H/C ratio % aromatic

HDPE 6.1 2.1 1

LDPE 3.6 2.5 3

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct

dis

trib

uti

on

(%

mo

lar)


HDPE

LDPE

38

3.1.3.1.K INETIC MODELING

Degradation experiments occur through a successive breakage of bonds and to describe this process

a simple kinetic model was used in this study. The model includes all the processes taking place

during the pyrolysis by accounting for the heat involved in each of these processes – for each broken

bond, some energy is required, and this energy should be seen in the signal of heat flow. Using only

TG analysis there is only possibility of seeing the mass loss of a sample, and this mass loss, although

arising from the production of lighter products, only occurs when the products are small enough to

vaporize. Smaller, shorter compounds produced during degradation are volatile, so they will be

detected when they leave the system. Breakages leading to heavier, non-volatile molecules, can be

seen only using DSC analysis. Only when combining the two signals it is possibile to obtain a more

complete information about the degradation process.

Hence, the model used was constructed by considering both material, to the number of bonds of the

sample, and energy balances as a function of time. If we assume that the polymer is a long alkane

chain, that has n carbon atoms, the number of C-C bonds per unit mass, N, will be given by the

following expression:

N=

Taking into account that n is a very large number, the amount of C-C bonds at the beginning of the run

will be close to 1/14, and this value is assumed to be the bond density of the sample.

However, the number of the actual bonds that may be broken is only half of this value, supposing that

our process is very effective. According to this argument, the total breakable bond density used for

polyethylene in this study was 1/28.

To balance the number of breakable bonds we will have to take und er considerations all the

processes that lead to a loss of breakable bonds, that is, cracking and evaporation; during the latter

small molecules will carry into the gas phase a certain amount of unbreakable bonds. Additionally to

properly balance the number of bonds, it was assumed that the process of breaking the bonds is a

first-order reaction in relation to bond concentration.

Therefore the balance equation for the number of breakable bonds is expressed as:

where dm/dt is rate of weight loss and α is the average number of bonds lost to the gas phase per unit

mass evaporated.

The k(T), temperature dependent rate constant, is described by the Arrhenius law:

(1)

(3)

(2)

39

where Tref is a reference temperature ( 573 K in this case), Ea is the reaction activation energy and k ref

is the kinetic constant at the reference temperature. The use of Arrhenius expression in this formis

intended to reduce the parameter correlation during fitting.

To estimate the heat flow that is measured in the experiment we have to write the energy balance to

the pan, with the assumption that the apparatus is capable of correctly compensating the required

flows.

Following this, the heat flow is given as:

Heat Flow =

where m is the weight of the sample at any given time, obtained experimentally, Cp is the average

heat capacity of the mixture in the pan, ΔHC-C is the average C-C bond enthalpy and ΔHv ap the average

vaporization enthalpy per unit mass.

The model was fitted within the range of the degradation temperatures obtained from the experiments.

The fitting parameters were k ref , Ea, α, Cp, ΔHC-C, and ΔHv ap. The last three parameters initially were

guessed from published data.

Equations 2 and 4 were solved numerically, using the Euler method, for each run, and the model

parameters were estimated by least-squares procedure, using the sum of the squares of the residues

on the heat flow as the objective function (O.F) to be minimized:

O.F =

The optimization was carried out using the “Solver” tool in an Excel (©Microsoft Corp.) spreadsheet.

[57]

3.1.3.1.1. Analysis of the estimated kinetic and thermodynamic parameters

The experimental data obtained from TG-DSC experiments was fitted to the model described above in

order to estimate the kinetic and thermodynamic parameters, which will help to better understand the

kinetics of the process. In this section the comparison between thermal cracking of high -density

polyethylene and low-density polyethylene will be made.

The parameters obtained can be seen in table 3.3. It is seen that values for both polymers differ, which

means that every process has its own kinetics and should be described separately. From the data

presented below, the information of main interest is the activation energy for the thermal cracking of

two polymers. The lower value is observed for reaction with LDPE – 89kJ/mol. Higher activation

energy for thermal degradation of HDPE was expected, because this polymer presents much more

(4)

(5)

40

-20

-15

-10

-5

0

0 200 400 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (oC)

Experimental data

Model

-25

-20

-15

-10

-5

0

0 200 400 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (oC)

Experimental data

Model

linear structure with few or no branching. Polymers with lots of side chains attached to the main

carbon backbone , such as LDPE, are more easily cracked and, hence, they need a lower activation

energy.

Table 3.3. Model parameters obtained by fitting the kinetic model to experimental data for thermal cracking of

HDPE and LDPE

HDPE Thermal Degradation LDPE Thermal Degradation

k ref(min-1

) 2.99 x 10-5

6.27 x 10-5

Ea (kJ/mol) 111 89

ΔH c-c (kJ/mol) 145 196

Alfa (Bond mole/g) 1,35 x 10-4

2,25 x 10-4

Eaα (kJ/mol) 114 108

ΔH vap (J/g) 276 302

Cp (J/g) 2.99 2.50

Figure 3.7. Experimental (b lue) and calculated (red) heat flow curves for thermal cracking of a) HDPE

and b) LDPE

To sum up the conclusions for the thermal cracking of HDPE and LDPE, they present slightly different

results due to the structure of the polymers. They vary in degradation temperatures, present different

kinetic and thermodynamic parameters as well as different cracking products; despite the fact that they

are similar in the range of the number of carbon atoms of the products they differ in the distribution.

41

3.2. CATALYTIC CRACKING

3.2.1. HDPE

3.2.1.1. Influence of catalyst acidity on cracking conditions

The graph presented in figure 3.8 shows the heat flux curves obtained from DSC signals for catalytic

degradation of HDPE. There are six curves on the graph, each corresponding to an individual run for

the cracking of a polymer sample. For comparison the catalysts results are shown together with

thermal process.

The first noticeable difference between catalytic and thermal reactions is the presence of c racking

peak at lower temperatures for processes involving a catalyst. Such situation was expected, because

the main reason for the use of catalyst is to decrease the activation energy, therefore enabling the

reaction to run at lower temperatures.

The other, a rather significant difference is the occurrence of two endothermic peaks for catalytic

cracking process. The first peak might correspond to pre-cracking of the polymer which might take

place at the surface of the catalyst. The second peak is the cracking of the species formed during the

pre-cracking, that will probably take place in the acid sites inside the zeolite pores. [ 28]

Table 3.4 presents the values of the degradation temperatures. The melting of the polymer occurs at

similar temperatures regardless of the catalyst used (~134oC), and is the same for the thermal

process, meaning the presence of catalyst does not influence the heat flow due to melting.

Taking into account the curves of heat flux signals, it may be observed that the endothermic cracking

peaks occur at different temperatures. Figure 3.9 shows the example of combined TG-DSC signals for

catalytic cracking of HDPE using Y-82 zeolite.

Analyzing the data presented in table 3.4 it may be observed that there is only a small influence of the

degree of exchange and cracking temperatures of HDPE, because for all of the zeolites the

degradation temperatures are similar.

Giving for example the temperatures of 1st

cracking peak, the lowest temperature was observed for

HDPE + Y-82 and HDPE + Y-82_2 samples, so for the parent zeolite and catalyst exchanged with 1M

NaNO3 solution, 407oC and 406

oC respectively. However the highest temperature was obtained for

HDPE + Y-82_3 sample – 418oC, a relatively small difference when compared to the parent zeolite.

42

Table 3.4. Degradation temperatures obtained in catalytic cracking for HDPE

Melting

temperature (oC) 1

st cracking temperature(

oC)

2nd

cracking temperature (

oC)

HDPE+ Y-82 133 407 430

HDPE + Y-82_1 133 411 433

HDPE + Y-82_2 134 406 433

HDPE + Y-82_3 134 418 427

HDPE + Y-82_4 133 410 435

Figure 3.8.TG-DSC curves obtained for thermal and catalytic degradations of HDPE using catalyst with

different acidity

-2,5

-2

-1,5

-1

-0,5

0

0,5

0 100 200 300 400 500 600 700

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

HDPE

HDPE + Y-82

HDPE+ Y-82_1

HDPE+ Y-82_2

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500 600 700

Weig

ht

fracti

on

Temperature (oC)

HDPE

HDPE + Y-82

HDPE + Y-82_1

HDPE + Y-82_2

HDPE + Y-82_3

HDPE + Y-82_4

43

Figure 3.9.TG-DSC curve for catalytic cracking of HDPE using Y-82 zeolite

3.2.1.2. Analysis of obtained kinetic and thermodynamic parameters

The comparison of the kinetic parameters obtained for thermal degradation and catalytic degradation

of HDPE is presented in table 3.5. The data shows the relationship between the acidity of the catalyst

with the kinetics of the reaction. However it has to be indicated that there were several problems

during the data fitting. First of all, the linear correction of the slope had to be made to the heat flux

curves, because the heat flux curve should have appear as a straight line between the melting and

cracking peaks, not inclined.

When comparing the activation energies, it may be noticed that the presence of catalyst makes a

significant difference in lowering its value. For thermal cracking of HDPE the calculated activation

energy was 111 kJ/mol , whereas for the most acidic catalyst only 74 kJ/mol. Those values may be

compared to the parameters presented by A. Coelho (which used the same kinetic model but for a

different catalyst) [57], which were 111 kJ/mol and 91 kJ/mol for thermal and catalytic process using

an HZSM-5 catalyst, respectively. For the ion-exchanged catalysts the activation energy values were

also lower than for thermal cracking, but slightly higher than the parent catalyst, this is connected with

the number of acid sites present in the zeolites, which was presented in previous chapters. The

activation energy values may be compared with the degradation temperatures presented in table 3.4.

The lowest pre-cracking temperatures were observed for Y-82 and Y-82_2 zeolites, which happen to

have the lowest activation energies as well.

Values obtained for bonding enthalpy are very low comparing with the data presented by A. Coelho –

173 kJ/mol and 369 kJ/mol for thermal and cracking processes, respecti vely, while in this work the

calculated ΔHC-C is 145 and 41, respectively. The values for catalytic processes are extremely low

but no explanation for this fact has been found yet.

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

-2

-1,8

-1,6

-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

0 100 200 300 400 500 600 700

Weig

ht

fracti

on

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

44

Table 3.5. Model parameters obtained by fitting the kinetic model to experimental data for catalytic cracking of

HDPE with catalyst of different acidity

HDPE Thermal

Degradation HDPE+ Y-82

HDPE + Y-82_1

HDPE + Y-82_2

HDPE + Y-82_3

HDPE + Y-82_4

k ref(min-1

) 3x10-5

3.72x10-7

5.40x10-6

2.92x10-7

5.83x10-6

5.19x10-6

Ea (kJ/mol) 111 74 81 77 82 80

ΔH c-c (kJ/mol) 145 41 51 46 48 61

alfa (Bond mole/g) 1,35x10

-4 9.53x10

-4 1.40x10

-4 2.22x10

-4 1.34x10

-4 3.21x10

-4

Eaα (kJ/mol) 114 89 236 311 153 166

ΔH vap (J/g) 276 431 485 477 399 416

Cp (J/g) 2.99 2.02 4.23 4.30 3.92 3.76


Figure 3.10 presents thermal (for comparison) and catalytic cracking product distribution, obtained for

HDPE sample degradation over Y-82, Y-82_1, Y-82_2, Y-82_3, Y-82_4 zeolites.

As it was previously described in chapter 3.1.1.2. thermal degradation process forms the majority of

hydrocarbons with the range between C1-C4. Longer-chained compounds occur only in very small

amounts. Whereas, when catalyst is present during the reaction, a shift towards C4-C8 products is

observed.

The highest yield for all of the catalysts was obtained for C4 products. Among them there can be

distinguished paraffin compounds such as butane and isobutane, as well as olefins with butylene and

its isomers ( trans-, cis- and iso-). The catalyst that produces the most C4 hydrocarbons is Y-82_4,

almost 40% molar of all products.

Other products that were the most common for catalytic processes were C3 (propane, propene), C5 (

pentane and isopentane, pentylene), C6 ( hexane and hexane), C7 ( heptane and heptene) and C8 (

octane and octene). Hydrocarbons with more than 8 carbon atoms constituted to less than 5% molar

of total obtained products.

45

Figure 3.10. Product distribution obtained for catalytic cracking of HDPE

Ratios of olefin/paraffin as well as hydrogen/carbon and aromatic content in formed products are

presented in table 3.6. The highest production of olefins is observed for Y-82_4 zeolite, due to high

concentrations of linear butenes. The least proportion of olefins are produced for Y-82_2 zeolite, O/P

ratio equals 1.4, meaning that there were almost as much alkanes as alkenes analyzed. For the rest of

the catalyst the ratios are quite similar; there is no pattern regarding the degree of exchange and olefin

to paraffin ratio.

The hydrogen to carbon proportion for all of the exchanged catalyst was the same – 2.1; the only

different value was obtained for the parent Y-82 zeolite – 2.3.

Analysis of the aromatic compounds content leads to the conclusio n that there is no significant

influence of the acidity on aromatic compounds formation. All of the catalysts present rather poor

aromatic yields, with no more than 2% of overall products.

Table 3.6. Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for catalytic degradation of HDPE

O/P ratio H/C ratio % aromatic

HDPE + Y-82 1.8 2.3 1.7

HDPE + Y-82_1 1.9 2.1 0.6

HDPE + Y-82_2 1.4 2.1 0.4

HDPE + Y-82_3 1.6 2.1 1.5

HDPE + Y-82_4 2.6 2.1 0.1

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct

dis

trib

uti

on

(%

mo

lar)


HDPE + Y-82

HDPE + Y-82_1

HDPE + Y-82_2

HDPE + Y-82_3

HDPE + Y-82_4

HDPE Thermal Degradation

46

3.2.1.4. Coke deposition on catalysts

After completion of 1st

and 2nd

cycle, in the degradation pan in TG-DSC apparatus there is no polymer

present, only the catalyst. However, when analyzing the weight loss graphs it is noticed that the final

mass is slightly more than the initial catalyst mass; this situation happens before during the catalytic

cracking of polymers, some residue is formed, which is called coke.

Table 3.7 presents the weight and percent coke deposition on used catalysts. The amount of coke

was computed by burning the remained catalyst under air. The highest amount of coke was deposited

on Y-82 catalyst, almost twice as much as for the rest of the catalysts, which is due to its highest

acidity, it might be concluded that the higher amount of acid sites the more probable it is the

deactivation by coke.

Table 3.7. Coke deposition on catalysts

Catalyst Coke weight (mg) Coke %

Y-82 0.250 34

Y-82_1 0.126 14

Y-82_2 0.163 17

Y-82_3 0.136 15

Y-82_4 0.109 11

3.2.2. LDPE

3.2.2.1. Influence of catalyst acidity on cracking conditions

Similar like in the HDPE case, in this study the influence of catalyst acidity on cracking conditions was

investigated.

Fig.3.11 illustrates TG and DSC curves for catalytic cracking of LDPE, with the comparison with

thermal degradation of the same polymer. The presence of catalyst lowers the amount of heat needed

to crack LDPE, the peaks for catalytic cracking occur at lower temperatures than for thermal

degradation.

The DSC curves obtained for LDPE catalytic processes are similar to the ones corresponding to

HDPE; three peaks are present. The first one is related with the heat change during melting of the

polymer and the other two refer to catalytic cracking. The firs t of the endothermic peaks corresponding

to the cracking process corresponds to the pre-cracking on the surface of the zeolite and the other to

the cracking inside zeolite pores. For LDPE the cracking peaks are broader and it might be concluded

47

that the cracking process itself took more time than in the HDPE case, probablydue to the ramification

of the polymer chains which limits their diffusion inside the catalyst pores.

Figure 3.11.TG-DSC curves obtained for thermal and catalytic degradations of LDPE using catalyst with

different acidity

The fig.3.12 presents the combined TG-DSC signal on one graph for catalytic degradation of LDPE

with Y-82 catalyst. From this graph it is observed that change in heat flux corresponding to breaking of

bonds in polymer structure occurs before the weight change.

The actual temperatures for melting and cracking of LDPE are presented in table 3.8. All of the

catalysts melted the polymer at similar temperature(112-113oC), which was also alike for the thermal

degradation process. Among the temperatures of 1st

and 2nd

cracking there is noticeable tendency,

with one exception, of increasing the temperature needed for the cracking process to occur with

-1,8

-1,6

-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

0 100 200 300 400 500 600

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

LDPE

LDPE+ Y-82

LDPE+ Y-82_1

LDPE + Y-82_2

0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500 600

Weig

ht

fracti

on

Temperature (oC)

LDPE

LDPE + Y-82

LDPE + Y-82_1

48

decreasing acidity. The Y -82 zeolite cracks LDPE at 402oC and then at 432

oC, other catalysts present

higher 1st

as well as 2nd

cracking temperatures.

Degradation of LDPE with Y-82_4 zeolite does not follow the increased temperature for decreased

acidity pattern. Although this catalyst presents the least acidic sites and is the most exchanged, it

cracks LDPE at temperatures comparable with Y-82_1 zeolite.

Table 3.8. Degradation temperatures obtained in catalytic cracking for LDPE

Melting

temperature (oC)

1st

cracking

temperature (oC)

2nd

cracking

temperature (oC)

LDPE + Y-82 113 402 432

LDPE + Y-82_1 112 415 435

LDPE + Y-82_2 112 418 447

LDPE + Y-82_3 113 418 452

LDPE + Y-82_4 113 412 439

Figure 3.12. TG-DSC curve for catalytic cracking of LDPE using Y-82 zeolite

3.2.2.2. Analysis of the estimated kinetic and thermodynamic parameters

Table 3.9 presents the parameters calculated by the model and as in the case of HDPE, the activation

energy for the process decreased when the catalyst was present.

Values obtained for the k ref may not be valid, because it was expected to see the increase of reaction

order in accordance with the data for LDPE presented by A. Coelho. Furthermore, the other

parameters show inconsistency that draw a conclusion that despite fitting the model curves t o the

experimental curves ( see APPENDIX) , the values obtained are partially incorrect.

0

0,2

0,4

0,6

0,8

1

1,2

-1,6

-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

0 100 200 300 400 500 600 700

Weig

ht

fracti

on

Heat

Flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

49

Table 3.9. Model parameters obtained by fitting the kinetic model to experimental data for catalytic cracking of

LDPE with catalyst of different acidity

LDPE Thermal

Degradation LDPE +

Y-82 LDPE + Y-82_1

LDPE + Y-82_2

LDPE + Y-82_3

LDPE + Y-82_4

k ref(min-1

) 6.27x10-5

6.27 x10-5

3.41 x10-7

5.69 x10-5

9.82 x10-5

6.18 x10-5

Ea (kJ/mol) 89 67 68 67 70 83

ΔH c-c (kJ/mol) 196 19 20 176 315 192

alfa (Bond

mole/g) 2.25 x10-4

1.19 x10-4

1.28 x10-4

2.41 x10-4

1.47 x10-4

2.37 x10-4

Eaα (kJ/mol) 108 89 111 141 148 139

ΔH vap (J/g) 302 431 471 463 392 409

Cp (J/g) 2.50 3.26 3.37 3.62 3.39 2.90


LDPE catalytic cracking product distribution with comparison to thermal degradation is shown on

fig.3.13. Similar to catalytic cracking of HDPE the presence of catalyst shifted the formation of

products towards longer and more branched compounds with C4-C6. LDPE samples with different

zeolites produce significantly lower amounts of C2 and C3 compounds, whereas during thermal

degradation they are the main constituents of analyzed gas.

The highest amounts of hydrocarbons were obtained for alkanes and alkenes with four carbon atoms;

they constituted to around 30% molar of all formed products for each polymer plus catalyst run. The

most common C4 hydrocarbons analyzed were: butane and butene with their isomers.

Second more abundant product are C5 compounds(pentane, isopentane pentene), and again for all of

the catalysts the amounts of obtained products are similar, around 25% molar.

Hydrocarbons with three and six carbon atoms were obtained in lower concentrations, but still in such

amounts that the results between the catalysts can be compared.

The highest variations in molar product distribution are observed for C7-C12 hydrocarbons. For

example Y-82_2 and Y-82_4 when cracking LDPE each produce around 8% molar compounds

containing 7 carbon atoms in the structure.

Zeolite Y-82_1 cracks low density PE to produce a whole range of hydrocarbons, from C1 to C12.

Comparing to other catalysts it forms high amounts of C9, mostly nonane, nonene and aromatics, and

C12, dodecane and dodecene in almost equal concentrations.

50

Figure 3.13 Product distribution obtained for catalytic cracking of LDPE

Table 3.10 presents olefin to paraffin and hydrogen to carbon ratio as well as the molar % of

aromatics. There is no significant relationship between the acidity of the catalyst and formation of

alkanes or alkenes. The highest production of olefins is however observed for Y-82 zeolite, 2.4 ratio,

this is connected with high concentrations of propene, isobutene and pentene ; and the lowest for Y -

82_3 zeolite, with the ratio of 1.3 – overall similar amounts of olefins and paraffins were formed.

Most of the catalyst produced only small fraction of aromatics form LDPE during cracking, except for

Y-82_1 zeolite. The molar % of cyclic aromatic compounds obtained is 8%. The reason for this high

content is formation of high concentrations of t rimethylbenzene and a compound called aromatic 9 (

this explains also higher than for other catalysts product distribution of C9 hydrocarbon seen on

fig.5.1).

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12

Pro

du

ct

dis

trib

uti

on

(%

mo

lar)


LDPE + Y-82

LDPE + Y-82_1

LDPE + Y-82_2

LDPE + Y-82_3

LDPE + Y-82_4

LDPE

51

Table 3.10. Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for catalytic degradation of LDPE

O/P ratio H/C ratio % Aromatic

LDPE + Y-82 2.4 2.1 0.8

LDPE + Y-82_1 2.3 3.1 8

LDPE + Y-82_2 1.5 2.2 1

LDPE + Y-82_3 1.3 2.2 1

LDPE + Y-82_4 1.9 2.1 1.2

3.2.2.4. Coke deposition on catalysts

The amounts of coke deposited on the catalysts after the degradation runs are similar, the highest

amounts of coke were observed for Y-82_3 and the lowest for Y-82_1. There is no significant

connection between the amount of coke deposited and the acidity of the catalyst.

Table 3.11. Coke deposition on catalysts for LDPE catalytic cracking cracking

Catalyst Coke weight (mg) Coke %

Y-82 0.160 18

Y-82_1 0.113 12

Y-82_2 0.144 16

Y-82_3 0.176 19

Y-82_4 0.167 18

3.2.3. EVALUATION OF PERFORMANCE OF CATALYSTS WITH DIFFERENT ACIDITY OVER THE TWO

POLYMERS

The evaluation made in this chapter is based on information provided in earlier sections of this work

and is focused on comparing the performance of all of the catalyst during HDPE and LDPE cracking.

Crack ing conditions

The DSC curves for degradation both HDPE and LDPE are alike, they consist of one melting peak and

two cracking peaks. The temperature of melting of LDPE for both catalytic and thermal processes is

lower than for HDPE. As it was previously described, the low density PE has more branched structure,

so the side chains will melt down more easily with lower temperatures but they will also diffuse more

difficultly inside the zeolite pores.

The temperature needed for the pre –cracking of LDPE is slightly lower than for HDPE, the 1st

cracking occurs at 402oC and 407

oC for Y-82 zeolite for LDPE and HDPE respectively. Nevertheless,

the temperatures of pre-cracking for both polymers are more or less in the same range 402 -418oC.

52

The second catalytic cracking peak occurs at wider distributions of temperatures, especially for the

degradation of low density PE, and the values obtained for particular catalyst are hard to compare. In

general 2nd

cracking occurs at lower temperatures for HDPE.

Moreover, in both situations, the acidity of the catalyst does not influence very much the temperature

for polymer cracking.

Kinetic parameters

It is possible to conclude that presence of catalyst decreases the activation energy as it was expected

and the values for the less acidic catalysts are lower, but within the small range.

The second important conclusion is the relationship between the branching of polymer and the

activation barrier needed for the cracking to occur. HDPE being more linear polymer presents higher

activation energy than LDPE.

Product distribution

In previous sections it was already mentioned that the acidity of the catalyst does not influence the

cracking product distribution, nor the olefin to paraffin ratio or the content of aromatic compounds.

In this part the product distribution for HDPE and LDPE sample with each catalyst will be analyzed.

Y-82 zeolite

The graphs presenting the product distribution for HDPE and LDPE separately are shown on fig 3.14.

It is seen that this catalyst produces similar amounts of olefins and paraffins, with a slightly increased

molar fraction of C4 and C5 alkenes for LDPE, which results in higher olefin to paraffin ratio than for

HDPE cracking.

Y-82_1

In catalytic degradation of HDPE this zeolite presents a higher formation of C6, C7 and C8 paraffins

than for LDPE cracking. However, it may be observed that for LDPE cracking there were more C8, C9

aromatic compounds, as well as C12 alkanes and alkenes formed. .

Y-82_2

Both polymers combined with this zeolite present similar product distribution, they have the same

olefin to paraffin ratio and the percent of formed aromatic compounds is very low. Only during the

cracking of LDPE this catalyst shows higher methane production.

53

Y-82_3

Product distribution for both polymers using the same catalyst is alike. Olefin to paraffin ratio is very

close and aromatic content is low. Longer-chained compounds are either not formed at all or formed in

trace amounts.

Y-82_4

This zeolite seems to favor the formation of olefins in catalytic cracking of HDPE, levels of C2 and C2

alkanes are very low compared with LDPE product distribution. The content of aromatic products is

slightly higher after LDPE cracking.

Coke deposition

The amount of coke deposited oscillates between 0.113mg to 0.176mg for most of the catalysts,

though Y-82 zeolite presents very high coke residue after HDPE cracking – 0.250mg. There have

been no noticeable relationship between the amount of coke deposited by each catalyst and the type

of the polymer.

54

Figure 3.14. Comparison of catalytic cracking product distribution for HDPE and LDPE

55

CHAPTER 4 CONCLUSIONS

The experimental part of this work involved preparation of Y-82 zeolites with decreasing acidity and

then evaluation of their performance in catalytic cracking over two polymers – HDPE and LDPE using

TG and DSC analysis as well as gas chromatography with comparison to thermal degradation

process.

A kinetic model was used to fit the TG-DSC signals, which would calculate the kinetic and

thermodynamic parameters . This step was done to better understand the kinetics of the processes

itself and to distinguish the major differences between catalytic and thermal cracking.

During the thermogravimetric and differential scanning calorimetric experiments it was possible to

observe the influence of the polymer type and presence or absence of catalyst on the cracking

temperatures. First of all it was noticed that the degradation temperatures for thermal c racking were

different for HDPE and LDPE. This difference is likely to originate in the structure of the polymers.

Compounds with higher degree of branching present lower cracking temperatures and, connected with

this, lower activation energies.

For the catalytic cracking reactions the most important observation was the occurrence of two

endothermic peaks relating to polymer cracking. The first cracking peak is assumed to be the pre-

cracking happening on the catalyst surface, whereas the second cracking occurs inside the zeolite

pores and it transforms the products evolved during the pre-cracking. Another observation was the

expected decrease in degradation temperatures during the reactions involving catalysts.

The acidity of the catalyst had somehow influence on the cracking temperatures, the zeolite with the

least amount of sodium needed the lowest activation energy to crack the polymer. The temperatures

for catalysts with less active sites were slightly higher, but still comparable with each other. There were

also no significant differences between catalytic cracking conditions of HDPE and LDPE.

Gas fraction formed during TG-DSC experiments was analyzed using gas chromatography to obtain

the distribution of products. The presence of the catalyst during the process had major influence on

product formation. In thermal cracking the most comon compounds were in C1-C4 range with the

highest distribution of C3 hydrocarbons. As for the catalytic process, there was a shift to C4-C6

products. However, no significant relationship was found between the acidity of the catalyst and the

product distribution.

The results for the two polymers were different only to some extent, some catalysts, namely Y -82_2

presented increased aromatic product formation during the catalytic cracking of LDPE.

From the calculated olefin to paraffin ratio it might be concluded that the majority of formed products

were alkenes.

56

All of the catalytic cracking experiments resulted in zeolite deactivation by the deposition of coke on its

surface; no relationship between the number of acid sites and the coke percent in the catalyst was

detected.

4.1. FUTURE PERSPECTIV ES

The approach taken up in this study was innovative in some extent. The decreasing of zeolite acidity

and observing the influence on the cracking conditions were investigated. The conclusions drawn from

the experiments rise questions and doubts.

As for the future recommendations, the further work should focus on improving the model, so as to

allow a better description of the processes involved. Without it is difficult to get deeper into the kinetics

of the process.

Moreover, other polymers should be studied or mixtures of polymers, using the catalyst presented in

this work.

57

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60

APPENDIX A DATA ANALYSIS

Table A1.1. Experiental runs performed in this work

sample total initial mass final mass dmass Tdegr/1st cracking T degr/2nd cracking initial mass polymer cat mass

HDPE 10.024 0.024 10.000 483.16 - 10.024 -

HDPE + Y-82 11.242 1.046 10.196 407.17 430.14 10.196 1.046

HDPE + Y-82_1 11.141 0.921 10.220 411.85 433.97 10.220 0.921

HDPE + Y-82_2 11.439 1.230 10.209 406.21 433.72 10.209 1.230

HDPE + Y-82_3 11.573 1.050 10.523 418.05 427.83 10.523 1.050

HDPE + Y-82_4 11.898 1.380 10.518 410.82 435.55 10.518 1.380

LDPE 11.004 0.084 10.920 476.03 - 10.920 -

LDPE + Y-82 11.682 1.014 10.668 402.41 432.35 10.668 1.014

LDPE + Y-82_1 11.416 1.061 10.355 415.01 435.22 10.355 1.061

LDPE + Y-82_2 11.119 0.952 10.167 417.82 446.96 10.167 0.952

LDPE + Y-82_3 12.196 1.099 11.097 417.68 452.15 11.097 1.099

LDPE + Y-82_4 11.434 0.897 10.537 412.33 438.84 10.537 0.897

61

A.1.2. Kinetic modeling

Figure A1.1. Experimental (b lue) and calculated (red) heat flow curves for catalytic cracking of HDPE

using Y-82 zeolite


using Y-82_1 zeolite

-20

-15

-10

-5

0

0 200 400 600Heat

flu

x c

orr

ecte

d(m

W/m

g)

Temperature (ºC)

Experimental data

Model

-25

-20

-15

-10

-5

0

0 200 400 600

Heat

flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

Experimental dataModel

62





-25

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d (

mW

/mg

)

Temperature (oC)

Experimental data

Model

-25

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d(m

W/m

g)

Temperature (ºC)

Experimental data

Model

63




using Y-82

-25

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Th

erm

al

flu

x c

orr

ecte

d(m

W/m

g)

Temperature (ºC)

Experimental data

Model

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (oC)

Experimental data

Model

64





-25

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (ºC)

Experimental data

Model

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (ºC)

Experimental data

Model

65





-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (ºC)

Experimental data

Model

-20

-15

-10

-5

0

0 100 200 300 400 500 600

Heat

flu

x c

orr

ecte

d (

mW

)

Temperature (ºC)

Experimental data

Model

66

A 2.1. Gas Chromatography analysis

Table A 2.1. Retention times for products formed during thermal and catalytic cracking

Retention time [min] Compound Number of carbon

atoms 1.60 Methane 1

1.69 Ethane 2

1.72 Ethylene 2

1.90 Propane 3

2.10 Propylene 3

2.50 Isobutane 4

2.90 Butane 4

3.70 Transbutylene 4

3.94 Isobutylene 4

4.40 Cisbutylene 4

5.20 Isopentane 5

5.50 n-pentane 5

6.00 Pentylene 5

7.80 Hexane 6

9.30 Hexene 6

11.00 Heptane 7

12.10 Heptene 7

12.90 Benzene 6

13.30 Octane 8

14.50 Octene 8

15.10 Toluene 7

15.40 Nonane 9

16.80 Nonene 9

17.00 Ethylbenzene 8

17.20 m-Xylene 8

17.30 p-Xylene 8

17.90 o-Xylene 8

18.50 Decane 10

19.00 Decene 10

19.35 Trimethylbenzene 9

20.00 Aromatic 9 9

21.50 Undecane 11

22.00 Undecene 11

23.00 Dodecane 12

25.50 Dodecene 12

67

The olefin to paraffin ratio was calculated using the data provided by GC. An example of such

calculations is shown in tables below

Table A 2.2. Molar distribution of products for HDPE thermal degradation

Number of

carbon atoms

Paraffins (mol)

Olefins (mol)

Aromatics (mol) Total

Average Number of

Carbons (mol)

Average Number of

Hydrogens (mol)

1 0.0316 0 0 0.031 0.031 0.126

2 0.025 0.069 0 0.094 0.189 0.429

3 0.0374 0.268 0 0.305 0.917 1.910

4 0.0235 0.276 0 0.299 1.198 2.443

5 0.0112 0.170 0 0.182 0.910 1.842

6 0.0097 0.028 0.010 0.048 0.293 0.542

7 0 0.006 0 0.006 0.046 0.093

8 0 0 0 0 0.006 0.013

9 0 0 0,000 0 0.001 0.012

10 0 0.019 0 0.01 0.192 0.384

11 0 0 0 0 0 0

12 0 0.010 0 0.01 0.121 0.242

Total 0.138 0.850 0.0108

Total 3.909 8.042

1

68

A 3.1. Acidity of zeolites

Figure A 3.1.Comparison between blank curves for ammonia TPD experiments

0,00E+00

5,00E-03

1,00E-02

1,50E-02

2,00E-02

2,50E-02

3,00E-02

3,50E-02

4,00E-02

4,50E-02

5,00E-02

150 300 450 600 750 900

dq

/dt(

mm

ol.

s-1

)

Temperature (oC)

Y-82

Y-82_1

Y-82_2

Y-82_3

Y-82_4

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