Joao B. P. Soares and

Timothy F. L. McKenna

Polyolefin Reaction Engineering

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Joao B. P. Soares and Timothy F. L. McKenna

Polyolefin Reaction Engineering

The Authors

Prof. Dr. Joao B. P. SoaresUniversity of WaterlooDepartment of Chemical EngineeringUniversity Avenue West 200Waterloo, ON N2L 3G1Canada

Prof. Dr. Timothy F. L. McKennaC2P2 UMR 5265ESCPE Lyon, Bat 308F43 Blvd du 11 Novembre 191869616 Villeurbanne CedexFrance

All books published by Wiley-VCH arecarefully produced. Nevertheless, authors,editors, and publisher do not warrant theinformation contained in these books,including this book, to be free of errors.Readers are advised to keep in mind thatstatements, data, illustrations, proceduraldetails or other items may inadvertently beinaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing-in-PublicationDataA catalogue record for this book is availablefrom the British Library.

Bibliographic information published by theDeutsche NationalbibliothekThe Deutsche Nationalbibliotheklists this publication in the DeutscheNationalbibliografie; detailed bibliographicdata are available on the Internet at<http://dnb.d-nb.de>.

© 2012 Wiley-VCH Verlag GmbH & Co.KGaA, Boschstr. 12, 69469 Weinheim,Germany

All rights reserved (including those oftranslation into other languages). No partof this book may be reproduced in anyform – by photoprinting, microfilm, or anyother means – nor transmitted or translatedinto a machine language without writtenpermission from the publishers. Registerednames, trademarks, etc. used in this book,even when not specifically marked as such,are not to be considered unprotected by law.

Print ISBN: 978-3-527-31710-3ePDF ISBN: 978-3-527-64697-5ePub ISBN: 978-3-527-64696-8Mobi ISBN: 978-3-527-64695-1oBook ISBN: 978-3-527-64694-4

Cover Design Adam-Design, WeinheimTypesetting Laserwords Private Limited,Chennai, IndiaPrinting and Binding Markono Print MediaPte Ltd, Singapore

V

To our wives, Maria Soares and SalimaBoutti-McKenna, for their love, dedication, andpatience while we wrote this book, not to mention the interminably long hours we spentdiscussing polyolefins in their presence. This book belongs to both of you, but you don’tneed to read it – you have heard all about it already.

Joao Soares and Timothy McKenna

VII

Contents

Acknowledgments XI

Preface XIII

Nomenclature XVII

1 Introduction to Polyolefins 11.1 Introduction 11.2 Polyethylene Resins 41.3 Polypropylene Resins 10

Further Reading 13

2 Polyolefin Microstructural Characterization 152.1 Introduction 152.2 Molecular Weight Distribution 172.2.1 Size Exclusion Chromatography 172.2.2 Field Flow Fractionation 272.3 Chemical Composition Distribution 292.3.1 Crystallizability-Based Techniques 292.3.2 High-Performance Liquid Chromatography 402.4 Cross-Fractionation Techniques 432.5 Long-Chain Branching 46

Further Reading 51

3 Polymerization Catalysis and Mechanism 533.1 Introduction 533.2 Catalyst Types 563.2.1 Ziegler–Natta Catalysts 563.2.2 Phillips Catalysts 613.2.3 Metallocenes 623.2.4 Late Transition Metal Catalysts 673.3 Supporting Single-Site Catalysts 70

VIII Contents

3.4 Polymerization Mechanism with Coordination Catalysts 76Further Reading 86

4 Polyolefin Reactors and Processes 874.1 Introduction 874.2 Reactor Configurations and Design 894.2.1 Gas-Phase Reactors 904.2.1.1 Fluidized Bed Gas-Phase Reactors 914.2.1.2 Vertical Stirred Bed Reactor 974.2.1.3 Horizontal Stirred Gas-Phase Reactor 994.2.1.4 Multizone Circulating Reactor 1024.2.2 Slurry-Phase Reactors 1044.2.2.1 Autoclaves 1054.2.2.2 Slurry Loop Reactors 1064.2.3 Solution Reactors 1074.2.4 Summary of Reactor Types for Olefin Polymerization 1084.3 Olefin Polymerization Processes 1094.3.1 Polyethylene Manufacturing Processes 1124.3.1.1 Slurry (Inert Diluent) Processes 1124.3.1.2 Gas-Phase Processes 1154.3.1.3 Mixed-Phase Processes 1184.3.1.4 Solution Processes 1194.3.2 Polypropylene Manufacturing Processes 1214.3.2.1 Slurry (Inert Diluent) Processes 1224.3.2.2 Gas-Phase Processes 1224.3.2.3 Mixed-Phase Processes 1254.4 Conclusion 128

References 128Further Reading 129

5 Polymerization Kinetics 1315.1 Introduction 1315.2 Fundamental Model for Polymerization Kinetics 1345.2.1 Single-Site Catalysts 1345.2.1.1 Homopolymerization 1345.2.1.2 Copolymerization 1455.2.2 Multiple-Site Catalysts 1495.2.3 Temperature Dependence of Kinetic Constants 1525.2.4 Number of Moles of Active Sites 1545.3 Nonstandard Polymerization Kinetics Models 1565.3.1 Polymerization Orders Greater than One 1565.3.2 Hydrogen Effect on the Polymerization Rate 1615.3.3 Comonomer Effect on the Polymerization Rate 1735.3.4 Negative Polymerization Orders with Late Transition Metal

Catalysts 179

Contents IX

5.4 Vapor-Liquid-Solid Equilibrium Considerations 181Further Reading 184

6 Polyolefin Microstructural Modeling 1876.1 Introduction 1876.2 Instantaneous Distributions 1886.2.1 Molecular Weight Distribution 1886.2.1.1 Single-Site Catalysts 1886.2.1.2 Multiple-Site Catalysts 1996.2.2 Chemical Composition Distribution 2126.2.2.1 Single-Site Catalysts 2126.2.2.2 Multiple-Site Catalysts 2226.2.3 Comonomer Sequence Length Distribution 2326.2.4 Long-Chain Branching Distribution 2376.2.5 Polypropylene: Regio- and Stereoregularity 2506.3 Monte Carlo Simulation 2516.3.1 Steady-State Monte Carlo Models 2526.3.2 Dynamic Monte Carlo Models 262

Further Reading 268

7 Particle Growth and Single Particle Modeling 2717.1 Introduction 2717.2 Particle Fragmentation and Growth 2747.2.1 The Fragmentation Step 2757.2.2 Particle Growth 2847.3 Single Particle Models 2867.3.1 Particle Mass and Energy Balances: the Multigrain Model

(MGM) 2877.3.2 The Polymer Flow Model (PFM) 2927.3.3 An Analysis of Particle Growth with the MGM/PFM Approach 2957.3.4 Convection in the Particles – High Mass Transfer Rates at Short

Times 3017.4 Limitations of the PFM/MGM Approach: Particle Morphology 304

References 307Further Reading 307

8 Developing Models for Industrial Reactors 3118.1 Introduction 311

References 321Further Reading 322

Index 325

XI

Acknowledgments

Personally I’m always ready to learn, although I do not always like being taught.

Sir Winston Churchill (1874–1965)

Several of the concepts covered in this book arose from our daily interactionswith students and colleagues in academia and industry. They are too many to benamed individually here, but we would like to express our sincere gratitude to theiroutstanding contributions that are summarized in this work. We did like beingthought by all of you.

First, we would like to thank our former mentors, who trusted and guided uswhen we were starting our careers, and kept encouraging us throughout theseyears. Their mentoring, support, and friendship are greatly appreciated.

This book could not have been written without the dedication of our graduatestudents, post-doctoral fellows, and research assistants, who toiled day after dayin our laboratories to propose and test hypotheses, challenge us with unexpectednew results, and in the process advance our understanding of polyolefin reactionengineering. Several of their results are interspersed throughout this book andconstitute main contributions to the field of olefin polymerization science andengineering. We are very thankful to their hard work, perseverance, and confidencein us as their supervisors.

We would also like to thank our academic and industrial collaborators whoover the years helped us better understand olefin polymerization and polyolefincharacterization, often kindly allowing us to use their laboratory facilities (for free!)to complement the work done in our institutions. We are indeed indebted to theseextraordinary colleagues and look forward to continue working with them in thefuture.

Finally, we would like to thank the polyolefin companies all over the worldthat have hired us as consultants and instructors of our industrial short courseon Polyolefin Reaction Engineering. This book is a result, in large part, from thestimulating discussions we had with the scientists and engineers who took thesecourses. If it is true, as said by Scott Adams, the creator of the comic stripDilbert, that ‘‘Give a man a fish, and you’ll feed him for a day. Teach a man tofish, and he’ll buy a funny hat. Talk to a hungry man about fish, and you’re a

XII Acknowledgments

consultant’’, then we hope that talking to the course participants over these yearshas at least stimulated them to look deeper into the vast sea of polyolefin reactionengineering.

XIII

Preface

It is the mark of an instructed mind to rest satisfied with the degree of precisionwhich the nature of the subject permits and not to seek exactness where onlyan approximation of the truth is possible.

Aristotle (384–322 BC)The art of being wise is the art of knowing what to overlook.

William James (1842–1910)

The manufacture of polyolefins with coordination catalysts has been a leadingforce in the synthetic plastic industry since the early 1960s. Owing to the constantdevelopments in catalysis, polymerization processes, and polyolefin characteriza-tion instruments, it continues to be a vibrant area of research and developmenttoday.

We have been working in this area for over 15 years, always feeling that therewas a need for a book that summarized the most important aspects of polyolefinreaction engineering. This book reflects our views on this important industry. Itgrew out of interactions with the polyolefin industry through consulting activitiesand short courses, where we first detected a clear need to summarize, in one singlesource, the most generally accepted theories in olefin polymerization kinetics,catalysis, particle growth, and polyolefin characterization.

As quoted from Aristotle above, we will rest satisfied with the degree of precisionwhich the nature of the subject permits and hope that our readers agree with us thatthis is indeed the mark of an instructed mind. It was not our intention to performan extensive scholarly review of the literature for each of the topics covered in thisbook. We felt that this approach would lead to a long and tedious text that wouldbecome quickly outdated; several excellent reviews summarizing the most recentfindings on polyolefin manufacturing and characterization are published regularlyand are more adequate for this purpose. Instead, we present our interpretation ofthe field of polyolefin reaction engineering. Since any selection process is alwayssubjective, we may have left out some approaches considered to be relevant byothers, but we tried to be as encompassing as possible, considering the limitationsof a book of this type. We have also sparsely used references in the main body ofthe chapters but added reference sections at their end where we discussed some

XIV Preface

alternative theories, presented exceptions to the general approach followed in thechapters, and suggested additional readings. The reference sections are not meantto be exhaustive compilations of the literature but sources of supplemental readingsand a door to the vast literature in the area. We hope this approach will makethis book a pleasant reading and also provide the reader with additional sources ofreference.

Chapter 1 introduces the field of polyolefins, with an overview on polyolefintypes, catalyst systems, and reactor configurations. We also introduce our generalphilosophy of using mathematical models to link polymerization kinetics, massand heat transfer processes at several length scales, and polymer microstructurecharacterization for a complete understanding of olefin polymerization processes.

We discuss polyolefin microstructure, as defined by their distributions of molec-ular weight, chemical composition, stereo- and regioregularity, and long-chainbranching, in Chapter 2. It is not an overstatement to say that among all syntheticpolymers, polyolefins are the ones where microstructure control is the most im-portant concern. Polyolefin microstructure is a constant theme in all chapters ofthis book and is our best guide to understanding catalysis, kinetics, mass and heattransfer resistances, and reactor behavior.

Chapter 3 is dedicated to polymerization catalysis and mechanisms. The fieldof coordination catalysis is huge and, undoubtedly, the main driving force behindinnovation in the polyolefin manufacturing industry; to give it proper treatment, aseparate book would be necessary. Rather, we decided to focus on the most salientaspects of the several classes of olefin catalysts, their general behavior patterns andmechanisms, and how they can be related to polymerization kinetics and polyolefinmicrostructural properties.

The subject of Chapter 4, polymerization reactors, is particularly dear to us,polymer reactor engineers. In fact, polyolefin manufacturing is a ‘‘dream cometrue’’ for polymer reactor engineers because practically all possible configurationsof chemical reactors can be encountered. A great deal of creativity went into reactordesign, heat removal strategies, series and parallel reactor arrangements, and costreduction schemes of polyolefin reactors. We start the chapter by discussing reactorconfigurations used in olefin polymerization and then continue with a descriptionof the leading processes for polyethylene and polypropylene production.

Chapter 5 is the first chapter dedicated to the mathematical modeling of olefinpolymerization. We start our derivations with what we like to call the fundamentalmodel for olefin polymerization kinetics and develop, from basic principles, itsmost general expressions for the rates of catalyst activation, polymerization, andcatalyst deactivation. The fundamental model, albeit widely used, does not accountfor several phenomena encountered in olefin polymerization; therefore, somealternative polymerization kinetic schemes are discussed at the end of this chapter.

In Chapter 6, we develop mathematical models to describe the microstructureof polyolefins. This is one of the core chapters of the book and helps connectpolymerization kinetics, catalysis, and mass and heat transfer resistances to finalpolymer performance. We opted to keep the mathematical treatment as simple

Preface XV

as possible, without compromising the most relevant aspects of this importantsubject.

Particle fragmentation and growth are covered in Chapter 7. These models arecollectively called single particle models and can be subdivided into polymer growthmodels and morphology development models. The two most well-establishedparticle growth models are the polymeric flow model and the multigrain model.These models are used to describe heat and mass transfer in the polymericparticle after fragmentation takes place. The fragmentation of the catalyst particlesthemselves (described with morphology development models) is much harder tomodel, and there is still no well-accepted quantitative model to tackle this importantsubject. We review the main modeling alternatives in this field.

Finally, Chapter 8 is dedicated to macroscopic reactor modeling. This chapter is,in a way, the most conventional chapter from the chemical engineering point ofview, since it involves well-known concepts of reactor residence time distribution,micromixing and macromixing, and reactor heat removal issues. The combinationof macroscopic reactor models, single particle models, detailed polymerizationkinetics, and polymer microstructural distributions, however, is very challengingand represents the ultimate goal of polyolefin reactor engineers.

XVII

Nomenclature

What’s in a name? William Shakespeare (1564–1616)

Acronyms

CCD chemical composition distributionCEF crystallization elution fractionationCFC cross-fractionationCGC constrained geometry catalystCLD chain length distributionCRYSTAF crystallization analysis fractionationCSLD comonomer sequence length distributionCSTR continuous stirred tank reactorCXRT computed X-ray tomographyDEAC diethyl aluminum chlorideDIBP di-iso-butylphthalateDSC differential scanning calorimetryEAO ethylaluminoxaneEB ethyl benzoateEDX energy dispersive X-ray spectroscopyEGMBE ethylene glycol monobutyletherELSD evaporative light scattering detectorEPDM ethylene-propylene-diene monomer rubberEPR ethylene–propylene rubberFBR fluidized bed reactorFFF field flow fractionationFTIR Fourier-transform infraredGPC gel permeation chromatographyHDPE high-density polyethyleneHMDS hexamethyldisilazineHPLC high-performance liquid chromatographyHSBR horizontal stirred bed reactor

XVIII Nomenclature

IR infraredLALLS low-angle laser light scatteringLCB long-chain branchLDPE low-density polyethyleneLLDPE linear low-density polyethyleneLS light scatteringMALLS multiangle laser light scatteringMAO methylaluminoxaneMDPE medium-density polyethyleneMFI melt flow indexMFR melt flow rateMGM multigrain modelMI melt indexMWD molecular weight distributionMZCR multizone circulating reactorNMR nuclear magnetic resonanceNPTMS n-propyltrimethoxysilaneODCB orthodichlorobenzenePDI polydispersity indexPFM polymer flow modelPFR plug flow reactorPP polypropylenePSD particle size distributionRND random number generated in the interval [0,1]RTD residence time distributionSCB short-chain branchSEC size exclusion chromatographySEM scanning electron microscopySLD sequence length distributionSPM single particle modeltBAO t-butylaluminoxaneTCB tricholorobenzeneTEA triethyl aluminumTEM transmission electron microscopyTGIC temperature gradient interaction chromatographyTMA trimethyl aluminumTOF turnover frequencyTREF temperature rising elution fractionationUHMWPE ultrahigh-molecular weight polyethyleneULDPE ultralow-density polyethyleneVLDPE very low-density polyethyleneVISC viscometerVSBR vertical stirred bed reactor

Nomenclature XIX

Symbols

a Mark–Houwink equation constant, Eq. (2.7)as specific surface area of the supportA monomer type AA total reactor heat transfer areaAi Arrhenius law preexponential factor for reaction of type iAS support specific surface areaAl cocatalysts[AS∗] concentration of active sites per unit surface area in the microparticleB monomer type BBn average number of long-chain branches per polymer chainC catalyst precursor or active siteC∗ active site[C0] initial concentration of active sitesCd deactivated catalytic siteCp heat capacityDb bulk diffusivityDeff effective diffusivity in the macroparticledp polymer (or catalyst) particle diameterDp diffusivity in the primary particleDr dead polymer chainDr,i dead polymer chain of length r having i long-chain branchesD=

r,i dead polymer chain of length r having i long-chain branchesand a terminal unsaturation (macromonomer)

E(t) reactor residence time distributionEi Arrhenius law activation energy for reaction of type if = molar fraction of macromonomers in the reactorfi molar fraction of monomer type i in the polymerization mediumfr frequency Flory chain length distribution, Eq. (6.13)fr overall frequency chain length distribution for chain having long-chain

branches, Eq. (6.101)frk frequency chain length distribution for chains with k long-chain

branches per chain, Eq. (6.86)flogr k frequency chain length distribution for chains with k long-chain

branches per chain, log scale, Eq. (6.88)F monomer molar flow rate to the reactorFA comonomer molar fraction in the copolymerFA average comonomer molar fraction in the copolymerFBr molar fraction of comonomer B as a function of chain lengthFM,in molar flow rate of the monomer feed to the reactorFM,out molar flow rate of the monomer exiting the reactorg branching index, Eq. (2.18)g ′ viscosity branching index, Eq. (2.17)�G Gibbs free energy change

XX Nomenclature

h average convective heat transfer coefficient between themacroparticle and surroundings

�H enthalpy change�Hp average enthalpy of polymerization�Hr enthalpy of reaction�Hu enthalpy of melting for a crystallizable repeating unit,

Eq. (2.26)�Hvap enthalpy of vaporizationI1 Bessel function of the first kind and order 1ka site activation rate constantkc, k−

c forward and reverse rate constants, respectively, for theformation of dormant site with Ni-diimine catalysts, Table 5.8

kd first-order deactivation rate constantk∗

d second-order deactivation rate constantkf forward rate constant for reversible monomer coordination or

β-agostic interaction; thermal conductivitykfL effective thermal diffusivity in the macroparticlekfp thermal conductivity of the polymer layer around the catalyst

fragment in the microparticlekiH rate constant for initiation of metal hydride active siteskp propagation rate constantk′

p apparent propagation rate constant, Eq. (5.115)

kp pseudo-propagation rate constantkp apparent propagation rate constantkpi propagation rate constant for monomer type

i (Bernoullian model)kpij propagation rate constant for chain terminated in monomer

type i coordinating with monomer type j (terminal model)kpijk propagation rate constant for chain terminated in monomer

types i and j coordinating with monomer type k(penultimate model)

kpm propagation rate constant for meso insertion (propylene)kpr propagation rate constant for racemic insertion (propylene)kr reverse rate constant for reversible monomer coordination or

β-agostic interactionktAl rate constant for transfer to cocatalystktβ rate constant for β-hydride eliminationktH rate constant for transfer to hydrogenktM rate constant for transfer to monomerK Mark–Houwink equation constant, Eq. (2.7)Ka initiation frequency, ka[Al]Keq equilibrium constant for dormant sites, Eq. (5.67)Kg–l, K∗

g–l gas–liquid partition coefficients, Eq. (5.113)

Nomenclature XXI

Kg–s, K∗g–s, K

′g–s gas–solid partition coefficients, Eqs (5.111) and (5.114)

Kl–s liquid–solid partition coefficient, Eq. (5.112)KH Henry law constantKT lumped chain-transfer constant, Eq. (5.74)K1

T lumped chain-transfer constant, Eq. (5.70)KH

T lumped chain-transfer constant, Eq. (5.71)mi mass fraction of polymer made on site type imk mass fraction of chains with k long-chain branchesmp mass of polymer, polymer yieldmvap vaporization ratemw molecular weight of repeating unit; in the case of copolymers,

the average molecular weight of the repeating unitsM molecular weightM monomerMC molar mass of catalystMn number average molecular weightMv viscosity average molecular weightMw weight average molecular weightMW polymer molecular weightn number of long-chain branches per chain; number of active

site typesnc(v) polymer particle size distributionnC0

, nC0 number of moles of catalyst

nLCB average number of long-chain branches in a polymer samplenM number of moles of monomernw weight average number of long-chain branches per chainNA Avogadro numberNi flux of species iNs number of macroparticles per unit volume of the reactorNu Nusselt numberPA, PB probability of propagation of monomers A and B, respectivelyP∗

H metal hydride active sitePM partial pressure of monomerPp propagation probabilityPr living chain with length rPr

i living polymer chain with length r terminated in monomertype i(A or B for binary copolymers) or 1-2 or 2-1 insertionsfor polypropylene

P∗r,i living polymer chain of length r having i long-chain branches

P1 dormant site due to β-agostic interaction, Table 5.5Pr dormant site for Ni-diimine catalysts, Table 5.8Pt termination probabilityPDI polydispersity index

XXII Nomenclature

PDI polydispersity index for chains containing long-chain branchesPDIFA polydispersity index as a function of copolymer compositionPDIk polydispersity index for chain with k long-chain branches, Eq. (6.106)Pr Prandtl numberQ heat generation rater polymer chain lengthri comonomer reactivity ratiorL radial position in macroparticle (multigrain model)rn number average chain lengthrn number average chain length for chains containing long-chain

branches, Eq. (6.107)rn number average molecular weight that would result

in the absence of long-chain branch formationreactions, see footnote 13 in Chapter 6

rnFA number average chain length as a function of copolymer compositionrnk number average chain length for chains with k long-chain branches,

Eq. (6.103)rs radial position in the microparticle (multigrain model)rw weight average chain lengthrw weight average chain length for chains containing long-chain

branches, Eq. (6.108)rwFA weight average chain length as a function of copolymer compositionrwk weight average chain length for chains with k long-chain branches,

Eq. (6.104)rzk

z-average chain length for chains with k long-chain branches,Eq. (6.105)

r20 root-mean-square end-to-end distance of a polymer chain

R gas constantRc catalyst fragment radius (multigrain model)Ri reaction rate of species i⟨R2

g

⟩b

squared radius of gyration of branched chains⟨R2

g

⟩l

squared radius of gyration of linear chains

RL macroparticle radius (multigrain model)Rp polymerization rateRp average polymerization rate per unit volume of the reactorRp

′average polymerization rate per polymer particle

RS microparticle radius (multigrain model)Rt chain-transfer rateRe Reynolds number�S entropy changeSc Schmidt numberSh Sherwood numbert time

Nomenclature XXIII

t average reactor residence timetR reactor residence timet 1

2catalyst half-time

T temperatureTc crystallization temperatureTi0 reactor inlet temperatureTm melting temperatureT0

m melting temperature of an infinitely long polyethylene chainTS temperature in the microparticleTw reactor coolant temperatureU global heat-transfer coefficientV Monte Carlo control volume; reactor volumeVe elution volumeVi interstitial volumeVp pore volumeVR reactor volumewlog r weight Flory chain length distribution, log scale, Eq. (6.24)wr weight Flory chain length distribution, Eq. (6.17)wr cumulative weight Flory chain length distributionwr overall weight chain length distribution for chain having long-chain

branches, Eq. (6.102)wr,FA Stockmayer bivariate distribution, Eq. (6.60)wrk weight chain length distribution for chains with k long-chain

branches per chain, Eq. (6.87)wlogrk weight chain length distribution for chains with k long-chain

branches per chain, log scale, Eq. (6.89)wlog MW,FA Stockmayer bivariate distribution, log scale, Eq. (6.61)wr,FAk trivatiate distribution of chain length, chemical composition, and

long-chain branching, Eq. (6.117)wry Stockmayer bivariate distribution, Eq. (6.56)wlog MW weight Flory molecular weight distribution, log scale, Eq. (6.32)wMW weight Flory molecular weight distribution, Eq. (6.30)WC mass of catalystxc mass fraction of catalyst in a supported catalystxi molar fraction of comonomer i in the copolymery deviation from average comonomer molar fraction in the

copolymer, Eq. (6.57)yk molar fraction of chains with k long-chain branchesYi ith moment of living polymer[Y0] total concentration of active sites or living polymer chainsZ compressibility factor

XXIV Nomenclature

Greek Letters

α polymer chain hydrodynamic volume constant, Eq. (2.4);long-chain branching parameter, Eq. (6.93)

β, β Stockmayer bivariate distribution parameters, Eqs. (6.58) and(6.62), respectively

ε exponent relating g ′ to g, Eq. (2.19);macroparticle void fraction

η catalyst site efficiency, Eq. (5.45)[η] intrinsic viscosityφ fraction of actives sites with growing polymer chains, Eq. (5.55)φi fraction of living chains terminated in monomer type iφk Catalan numbers, Eq. (6.95)� polymer chain hydrodynamic volume constant defined in Eq. (2.4)κ size exclusion partition coefficientλ, λn number of long-chain branches per 1000 C atomsμ long-chain branching parameter, Eq. (6.94); viscosityρC support (catalyst) densityρCp average value of the heat capacity per unit volume

of the macroparticleτ Flory most probable chain length distribution parameter; ratio

of all chain-transfer rates to the propagation rate, Eq. (6.29);g macroparticle tortuosity

τ Flory most probable molecular weight distribution parameter,Eq. (6.31)

τB chain length distribution parameter for polymers containinglong-chain branches, Eq. (6.90)

τ d characteristic diffusion time in the macroparticle

Superscripts and Subscripts

ˆ pseudokinetic constant– average12, 21 1-2 or 2-1 propylene insertionsA, B monomer typesbulk bulk conditionsC catalyst, monomer type C in the case of

terpolymerizationl liquid phaseM monomerMC Monte Carlo simulation rates and constantsP polymers solid polymer phase

1

1Introduction to Polyolefins

It is a near perfect molecule [ . . . ].Jim Pritchard, Phillips Petroleum Company

1.1Introduction

Polyolefins are used in a wide variety of applications, including grocery bags,containers, toys, adhesives, home appliances, engineering plastics, automotiveparts, medical applications, and prosthetic implants. They can be either amorphousor highly crystalline, and they behave as thermoplastics, thermoplastic elastomers,or thermosets.

Despite their usefulness, polyolefins are made of monomers composed of onlycarbon and hydrogen atoms. We are so used to these remarkable polymers thatwe do not stop and ask how materials made out of such simple units achieve thisextraordinary range of properties and applications. The answer to this questionlies in how the monomer molecules are connected in the polymer chain to definethe molecular architecture of polyolefins. By simply manipulating how ethylene,propylene, and higher α-olefins are bound in the polymer chain, polyolefins withentirely new properties can be produced.

Polyolefins can be divided into two main types, polyethylene and polypropylene,which are subdivided into several grades for different applications, as discussedlater in this chapter.1) Taking a somewhat simplistic view, three components areneeded to make a polyolefin: monomer/comonomer, catalyst/initiator system, andpolymerization reactor. We will start our discussion by taking a brief look at eachof these three components.

Commercial polyethylene resins, despite their name, are most often copoly-mers of ethylene, with varying fractions of an α-olefin comonomer. The most

1) Ethylene-propylene-diene(EPDM) terpoly-mers, another important polyolefin type,are elastomers with a wide range of ap-plications. They are made using catalystsand processes similar to those used to

produce polyethylene and polypropylene.Even though they are not discussed explic-itly in this book, several of the methodsexplained herein can be easily adopted toEPDM processes.

Polyolefin Reaction Engineering, First Edition. Joao B. P. Soares and Timothy F. L. McKenna.© 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 1 Introduction to Polyolefins

commonly used α-olefins are 1-butene, 1-hexene, and 1-octene. They are usedto decrease the density and crystallinity of the polyolefin, changing its physicalproperties and applications. Industrial polypropylene resins are mostly isotacticmaterials, but a few syndiotactic grades are also available. There are two main typesof propylene copolymers: random propylene/ethylene copolymers2) and impactpropylene/ethylene copolymers.

At the heart of all polyolefin manufacturing processes is the system used topromote polymer chain growth. For industrial applications, polyethylene is madewith either free radical initiators or coordination catalysts, while polypropyleneis produced only with coordination catalysts. Low-density polyethylene (LDPE)is made using free radical processes and contains short chain branches (SCB)and long chain branches (LCB). Its microstructure is very different from that ofpolyethylenes made with coordination catalysts. Coordination catalysts can controlpolymer microstructure much more efficiently than free radical initiators andare used to make polyolefins with a range of properties unimaginable beforetheir discovery. The quotation at the beginning of this chapter was motivated bythe revolutionary synthesis of unbranched, linear polyethylene, the ‘‘near perfectmolecule,’’ using a Phillips catalyst.

Catalyst design is behind the success of modern industrial olefin polymerizationprocesses because the catalyst determines how the monomers will be linked inthe polymer chain, effectively defining the polymer microstructure and properties.Industrial and academic research on olefin polymerization catalysis have beenvery dynamic since the original discoveries of Ziegler and Natta (Ziegler–Nattacatalysts) and Hogan and Banks (Phillips catalysts), with many catalyst familiesbeing developed and optimized at a rapid pace. There are basically four main typesof olefin polymerization catalysts: (i) Ziegler–Natta catalysts, (ii) Phillips catalysts,(iii) metallocene catalysts, and (iv) late transition metal catalysts. Ziegler–Nattaand Phillips catalyst were discovered in the early 1950s, initiating a paradigmshift in olefin polymerization processes, while metallocene and late transitionmetal catalysts (sometimes called post-metallocenes) were developed in the 1980sand 1990s, respectively. In Chapter 3, olefin polymerization catalysts and mecha-nisms are discussed in detail, while polymerization kinetic models are developedin Chapter 5. For our purposes in this chapter, it suffices to say that polymer-ization with all coordination catalyst types involves monomer coordination to thetransition metal active site before insertion in the metal–carbon bond at oneend of the polymer chain. The coordination step is responsible for the versatil-ity of these catalysts: since the incoming monomer needs to coordinate to theactive site before propagation may occur, the electronic and steric environmentaround it can be changed to alter polymerization parameters that control thechain microstructure, such as propagation and chain transfer rates, comonomerreactivity ratios, stereoselectivity, and regioselectivity. This concept is illustrated inFigure 1.1.

2) Propylene/1-butene copolymers andpropylene/ethylene/α-olefin terpolymers

are also manufactured, but in a smallerscale.

1.1 Introduction 3

Ti CH2–CH2

CH2 CH2

Figure 1.1 Coordination step before ethy-lene insertion on a polyethylene chain(active site control).

R* + M RM*

RM* + M RMM*

R(M)n* + M R(M)n+1*

Figure 1.2 Generic free radical polymerization(chain-end control). R∗, free radical initiator;M, monomer.

This mechanism controls the polymer microstructure better than free radicalpolymerization, where the chemical nature of the free radical initiator stops beingimportant after a few propagation steps since the polymerization locus moves awayfrom the initiator molecule, as illustrated in Figure 1.2. Free radical processes forLDPE production are not the main topic of this book and are only discussed brieflyas comparative examples.

Even though a few processes for ethylene polymerization use homogeneouscatalysts in solution reactors, most olefin polymerization processes operate withheterogeneous catalysts in two-phase or three-phase reactors. This adds an ad-ditional level of complexity to these systems since inter- and intraparticle massand heat transfer resistances during polymerization may affect the polymerizationrate and polymer microstructure. If significant, mass and energy transport limita-tions create nonuniform polymerization conditions within the catalyst particles thatlead to nonuniform polymer microstructures. Many other challenging problems areassociated with the use of heterogeneous catalysts for olefin polymerization, such ascatalyst particle breakup, agglomeration, growth, and morphological development,all of which are discussed in Chapter 7.

All the phenomena mentioned above take place in the polymerization reactor.The variety of polyolefin reactors can be surprising for someone new to the field:polyolefins are made in autoclave reactors, single- and double-loop reactors, tubularreactors, and fluidized-bed reactors; these processes may be run in solution, slurry,bulk, or gas phase. Each reactor configuration brings with it certain advantages,but it also has some disadvantages; the ability to select the proper process for agiven application is an important requirement for a polyolefin reaction engineer.Chapter 4 discusses these different reactor configurations and highlights someimportant polyolefin manufacturing processes. Reactor models that take intoaccount micromixing and macromixing effects, residence time distributions, andmass and heat transfer phenomena at the reactor level are needed to simulate theseprocesses, as explained in Chapter 8.

Catalyst type, polymerization mechanism and kinetics, inter- and intraparticlemass and heat transfer phenomena, and macroscale reactor modeling are essentialfor the design, operation, optimization, and control of polyolefin reactors. Figure 1.3

4 1 Introduction to Polyolefins

Multiple length scales High complexity

RM

Figure 1.3 Modeling scales in polyolefin reaction engineering.

shows how these different length scales are needed to help us understand olefinpolymerization processes.

In summary, much science and technology is hidden behind the apparentsimplicity of everyday polyethylene and polypropylene consumer goods. No othersynthetic polymer is made with such a variety of catalyst types, reactor configura-tions, and microstructural complexity. In this book, we will explain how, from suchsimple monomers, polyolefins have become the dominant commodity plastic inthe twenty-first century.

1.2Polyethylene Resins

Polyethylene resins are classified into three main types: LDPE, linear low-densitypolyethylene (LLDPE), and high-density polyethylene (HDPE). This traditionalclassification distinguishes each polyethylene type according to its densityrange: 0.915–0.940 g cm−3 for LDPE, 0.915–0.94 g cm−3 for LLDPE,3) and0.945–0.97 g cm−3 for HDPE, although these limits may vary slightly amongdifferent sources. Lower-density polyethylene resins (<0.915 g cm−3) aresometimes called ultra low-density polyethylene (ULDPE) or very low-densitypolyethylene (VLDPE). HDPE with molecular weight averages of several millionsis called ultrahigh molecular weight polyethylene (UHMWPE). To reduce the

3) Sometimes polyethylene resins in therange 0.926–0.940 g cm−3 are calledmedium-density polyethylene (MDPE).