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Some Engineering Opportunities and Challenges when Producing Polymer
Nigerian Academy of Engineering June 2013 Some Engineering Opportunities and Challenges when Producing Polymer Materials from Oil and Gas by W. Harmon Ray This talk will discuss the opportunities and technological challenges when producing needed quantities of high value polymers from oil and gas. University of Wisconsin - Madison Dept. of Chemical Engineering
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W. Harmon Ray Vilas Research Professor Emeritus
US National Academy of Engineering (Class of 1991)
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OUTLINE INTRODUCTION FUNDAMENTALS PROCESS DESIGN AND OPERATIONS
General Considerations Production in Nigeria LOW VOLUME PRODUCTION CONCLUSIONS
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Background; Polymer Production Basics (Worldwide & Nigeria)
INTRODUCTION
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Polymers in our Automobiles
Body Exterior Impact PP Body Interior First let us consider the polymer products we use in our daily lives. (mention specific polymers on slide) Include Butacite (laminate for windshield) Coatings Safety Glass Polyvinylbutyral Electrical/Electronics Polyacetal Fuel Tanks HDPE Powertrain
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Polymers in our Clothing
The polymer products we use in our daily lives. (mention specific polymers on slide) • Dacron Polyester (PET) (Polyethylene Terephthalate) • Lycra Spandex • Gore-Tex (Polytetrafluoroethylene) • Nylon 66 (HMD/AA) (Hexamethylene diamine/Adipic acid) • Polypropylene (PP)
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Polymers in Food Packaging
Polyethylene & ethylene copolymers (LDPE, LLDPE, EVA) Polystyrene (PS) Discuss the polymer products we use in our daily lives. (mention specific polymers on slide) Polyethylene Terephthalate (PET) & High-density Polyethylene (HDPE)
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Polymers in Everyday Life
Nylon/Polyesters PolyTFE Nylon/ABS Polyformaldehyde ABS/HIPS/PP PS/PC HDPE/PP/PVC ABS/HIPS/PC Discuss the polymer products we use in our daily lives. (mention specific polymers on slide) Copolymers of Styrene, Methacrylates, Acrylates, Epoxies. Solvent borne, Latex emulsion, Powder coat. (Particle size distribution and particle morphology are critical)
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Polymers in Protective Apparel
No melting point; stable beyond 350 C Nomex (Poly-metaphenylene isophthalamide) Liquid Crystal Polymers Discuss the polymer products we use in our daily lives. (mention specific polymers on slide) Kevlar (aromatic nylon) (Poly-paraphenylene terephthalamide)
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Worldwide Scale of Polymers: ~ 300 Million tons/yr
Polymer production is a huge world-wide industry with a large annual growth rate
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5 types of polymers make up 85% of the polymers produced in the world:
• Various types of Polyethylene- - Low-density Polyethylene (LDPE) – Linear Low-density Polyethylene (LLDPE) - High-density Polyethylene (HDPE) • Polypropylene (PP) • Polyvinyl Chloride (PVC) • Polystyrene (PS) and Expanded Polystyrene (EPS) • Polyethylene Terephthalate (PET)
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Some Polymer Production in Nigeria
High Density and Linear Low Density Polyethylene: • Sclairtech (Nova Chem) HDPE/LLPDE (1-butene) 250,000 tons/yr (Port Harcourt) • Methanol to Olefins Project underway to add 400,000 tons/yr of HDPE (Port Harcourt) Polypropylene: • Spheripol (Basell) PP homopolymer/High Impact PP 95,000 tons/yr (Port Harcourt) • Older PP Plant 35,000 tons/yr (Warri) 400,000 tons/yr of PP (Port Harcourt) Bottle Grade Polyethylene Terephthalate (PET): • Buehler Two Stage Solid State Process 75,000 tons/yr (Port Harcourt) _________________________ Business Monitor International (Jan 2013) Special Chem Industry News (July 2012) The principal polymers produced in Nigeria today.
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Annual Cost of Polymer Imports to Nigeria in 2010
High Density Polyethylene (HDPE)*: $85 Million Low Density Polyethylenes (LDPE/LLDPE)*: $376 Million Other Polyethylene*: $38 Million Polypropylene (PP, all types)*: $150 Million Polyethylene Terephthalate (PET)*: $45 Million Polyvinyl Chloride (PVC)*: $125 Million Polystyrene (PS, EPS)*: $13 Million All Imports of Synthetic Polymers ~ $1 Billion/yr _________________________ *United Nations Commodity Trade Statistics In spite of domestic production, there are large volumes of the principal polymers imported into Nigeria today with a cost of ~ 1 Billion US dollars a year against the balance of payments of Nigeria. There seem to be large incentives to expand polymer production to take advantage of the crude oil and natural gas resources in Nigeria. This would eliminate the large volume of imports and provide materials for export to the region and beyond.
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We will discuss both of these approaches. Purification
Separation Purification Cracking Catalytic Conversion What are the steps in producing petrochemicals (including polymers) from oil and gas? • The traditional approach begins with fractionation of crude oil to various boiling point fractions, and then feeding these to cracking furnaces which convert these to olefins and other chemicals. • However, more recently a second route to polymers, the Methanol to Olefins (MTO) Process, involves converting raw materials to synthesis gas (CO and H2), reacting this to methanol, and then converting methanol to olefins such as ethylene and propylene. We will discuss both of these approaches. Purification Methanol to Olefins Dr.K.R.Krishnamurthy (2009)
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Ethylene and Propylene Production
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The MTO process can also be used with other hydrocarbon sources such as Coal and Biomass.
The first step is to produce synthesis gas (CO, H2) and then use the zeolites with controlled pore size to make ethylene and propylene.
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Project Currently underway in Nigeria
The ethylene and propylene required for polyethylene and polypropylene processes can be produced from the MTO process. Project Currently underway in Nigeria Dr.K.R.Krishnamurthy (2009)
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Zeolite Catalysts for Olefins from Methanol For High Propylene yield
The enabling catalysts are zeolites with controlled pore structures. Smaller pores will produce mostly ethylene and propylene while larger pores will allow larger molecules such as propylene, butylene, and aromatics to be produced. SAPO-34 produces mostly ethylene, while ZSM-5 yields both ethylene and propylene as the principal products. For Ethylene For High Propylene yield
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Polymerization Kinetics and Mechanisms
FUNDAMENTALS
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Polymerization Kinetics
Fundamental Kinetic Mechanisms Polycondensation Catalyzed Uncatalyzed Ionic/Group Transfer Living Free Radical Trans Metal Catalysis Ionic Free Radical Trans Metal Catal Addition Polymerization Living Chain Terminated Let us now consider the chemical reactions and processes leading to the major polymers of interest. There are two principal Chemical Mechanisms for making polymers: Polycondensation and Addition Polymerization. We will consider each mechanism in turn as we discuss the important polymers made. • Polycondensation involves the reaction between two growing chains, usually stripping out a small molecule (hence the name “polycondensation”) . • Addition Polymerization involves adding a small molecule (the monomer) to a growing polymer chain one unit at a time. The growing chain can be “living” so that it continues to grow until it exits the reactor. Alternatively the growing chain can be “ chain terminated” by reaction with another molecule or even another polymer chain after a short lifetime.
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Transfer to dormant state
Addition Polymerization Kinetics Initiation I ßR • k d R + M P 1 i Propagation Pj n +1 Pi Mj pij Transfer to dormant state D A C k / k r f m tc n+m Termination (incl. chain transfer) tf T / Dn+Dm C H Y C H X C H Y Almost all of the high-volume commodity polymers are made by the Addition Polymerization Mechanism. The reactants are monomers containing double bonds that can react with a free-radical or a catalytic site to produce long chain molecules. For the free-radical mechanism, there is an initiation step where an initiator such as a peroxide can spontaneous decompose to form free radicals. In a fraction of a second these react with monomers to form long polymer chains of various lengths. The free radical on the chain is removed by reaction with another growing chain or with another species that consumes the free radical. Most polyethylenes and all polyproplene products are produced using a transition metal catalyst supported on a solid particle. The active sites on the catalyst particle are activated by a cocatalyst, and then polymerization begins by adding the ethylene or propylene monomer to the active site by a heterogeneous catalytic mechanism. • X & Y are functional groups with specific properties • Monomer addition at each step is statistical based on kpij and M1/M2 ratio • The chain length is also statistical depending on relative rates of initiation, propagation, and termination or CH 2 X Y •
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Molecular Weight Distributions
Polymer mass Viscosity average Weight average= Mw Number average = Mn Polydispersity: Mw/Mn=2 - 20 for commodities; as low as 1.1 for some specialities For “narrow” distribution of Mw/Mn=2, σ= Mn !! Because each chain reacts with monomer units (in addition polymerization) for a different length of time before the chain is terminated or exits the reactor, each polymer molecule is a different length and molecular weight. Similarly for condensation polymerization, each chain reacts with chains of different lengths before exiting the reactor or the reaction is terminated. Thus the product has a distribution of chain lengths (and molecular weights) and these molecular weight distributions are usually very broad. Chain length Molecular weight MWD’s are usually very Broad!
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Distributions with Multiple Monomers
Random (most common) Alternating Block Graft Sequence Length Distributions (% 1’s, 2’s, 3’s, ... in a row for each monomer): • controls crystal structure and total crystallinity • controls chain stiffness • related to material strength, elasticity, etc. • controls side chain functionality for surface properties, cross-linking, etc. Composition Distribution (% of each Monomer in each Polymer chain) Syndiotactic, Isotactic, or Atactic addition is equivalent to having different comonomers! When multiple monomers are in the reactor, each polymer chain is a mixture of these different monomers. Thus each chain could have a different monomer composition; i.e.; a different fraction of one monomer versus another. Also depending on the order in which the different monomers enter the chain, there can be different sequence lengths of each monomer. For example, the order can be completely random or the catalyst could force the order to be perfectly alternating or the catalyst could favor the addition of the previous monomer over inserting another type – leading to block copolymers. Finally, if the polymer chain has reactive groups in the chain, one can graft other chains to the molecular in a second reaction step.
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Key to control of Polyethylene properties --> Branching
Unbranched Polyethylene (rare) Short-chain Branched PE Short and Long-chain Branched PE HDPE & LLDPE LDPE Polyethylene is the largest volume polymer produced in the world, and the oldest polyolefin. It was first produced from free-radical initiators in the 1940’s by using very high pressure reactors ( bar) so that ethylene becomes a supercritical fluid (much like a liquid) instead of a gas. The product is call Low-Density Polyethylene (LDPE). In the 1950’s Ziegler (in Germany) discovered transition metal catalysts that could effectively produce polyethylene under very mild conditions (< 100C and ~ 20 – 40 bar). The polyethylene produced could have a wide range of densities (i.e.; crystallinity) by copolymerization with alpha-olefins such as butene, hexene, or octene. so that the principal products are High-Density Polyethylene (HDPE) and Linear Low-Density Polyethylene (LLDPE). • For high pressure LDPE, short and long-chain branches are formed naturally and are controlled by reaction conditions (temperature and pressure). • For HDPE and LLDPE, short-chain branches are formed by copolymerization with α-olefins such as butene, hexene, or octene. No long-chain branching occurs except with special catalysts allowing polymer chains to be inserted.
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Branching Distributions
Linear Low-Density Polyethylene - LLDPE Examples: Distributions: • Short chains per 1000 C • Branch sequence - long runs vs short runs - concentrated in long chains vs short chains • Long chains • Morphology of Long chains: - generations - gel formation Linear molecule ca. 10 to 35 short side chains per 1000 C – atoms Both short and long-chain branching ca. 4 to 10 short side chains per 1000 C - atoms With Special Catalysts High-Density Polyethylene HDPE Very High Pressure Low-Density Polyethylene LDPE The types of short-chain and long-chain branching for HDPE, LDPE, and LLDPE are illustrated here. The use of transition metal catalysts normally does not produce long-chain branching. However, with special catalysts, long chain branches can be added to the polymer backbone. This is seldom done in industrial practice.
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Short Chain Branching influences degree of Crystallinity in Semi-Crystalline Polymers
For many polymers, segments of the polymer chain can fold and form crystals at temperatures below the melting point, thus increasing the density and strength of the final product. Having short chain branches in the polymer chain will disrupt this folding and hinder crystallization. Thus polymer chains with fewer short chain branches will have a larger fraction of the polymer chain in crystals (higher crystallinity) and the resulting polymer will have higher density. Why be interested in crystallinity effects? Monomer swells amorphous polymer, not crystals Crystallites act like “tie” segments in polymer networks Polymer properties depend on crystal and tie molecule size distributions S
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Effect of Long-Chain Branches on Polymer Viscosity
10-2 102 100 104 106 Viscosity (Poise) Shear Rate γ (Sec-1) Branched Linear Long-chain branching influences the flow properties of the heated polymer when making products such as plastic bags, film, bottles, etc. The viscosity of the long-chain branched polymer decreases much more with increased flow-rate (i.e.; shear rate) through nozzles, dies, etc. than polymer without long-chain branching. Thus polymers with long-chain branching can be converted to film, molded products, etc. at higher production rates because of the higher flow rates through complex film blowing, molding, etc. operations. Alteration of viscosity-shear rate behavior due to the presence of long branches.
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PROCESS DESIGN & OPERATIONS
General Considerations; Polymer Production in Nigeria PROCESS DESIGN & OPERATIONS
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Making polymers at low cost and in quantity – polymerization process issues
• Batch, semi-batch or continuous process? • In solution/melt or multiphase suspension, emulsion, or dispersion? • Can we scale-up (heat removal, mixing, mass transfer) to produce the polymer we made in the lab? • Can we control the process in order to obtain reproducible quality? • Will the costs of production be low enough? • More than 85% of all polymer production is by Exothermic Addition Polymerization! Can we control the reactor temperature and prevent process runaway? • Will the process be safe and friendly to the environment? Let us now consider the process design and operation issues of polymerization reactors for producing commercial quantities of polymers. Most processes for commodity polymers have been operated for decades, and are available to be licensed and constructed for new operations around the world. We will begin with the processes currently operating in Nigeria, and then consider other widely used processes that have been shown to be effective and are available to be licensed.
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Thermal Parameters for Addition Polymerization Dynamics
Heat of Poly Pure Mon Conc Adiab. Temp Rise Heat Removal Duty Monomer Kcal /Mol mol/lit C Kcal /Kg Ethylene 25.9 14.2 1609 922 Propylene 20.1 12.3 850 476 1-Butene 19.9 10.6 647 355 Isobutylene 11.5 395 204 1,3-Butadiene 17.4 589 322 Isoprene 17.9 10.0 496 263 Styrene 8.7 398 167 alpha- Methylstyrene 8.4 7.7 173 71 Vinyl chloride 17.2 14.6 803 275 Vinylidene chloride 12.5 655 180 Tetrafluoroethylene 38.9 15.2 1447 390 Acrylic Acid 16.0 464 222 Acrylonitrile 18.3 774 344 Maleic Anhydride 14.1 15.3 491 144 Vinyl acetate 21.0 10.8 519 244 Methyl acrylate 18.6 11.1 452 216 methacrylate 13.4 9.4 260 134 Methacrylic acid 11.8 488 Butyl 7.0 310 145 The principal design, control, and safety problem with most polymer production processes is the difficulty in controlling the temperature of the chemical reactor. A measure of the potential difficulties for a reaction is the Adiabatic Temperature Rise when all reactants have been consumed. The adiabatic temperature rise of a reaction is the maximum reactor temperature increase after all the monomer has been reacted and when there is no cooling of the reactor or if the reaction is so fast, that there is no time for cooling. The two largest volume polymers. polyethylene and polypropylene have especially difficult problems, with very high values of adiabatic temperature rise for polymerization. The difficulties for ethylene are greatly increased by the fact that above ~ 300 C, an ethylene decomposition reaction kicks off with an Adiabatic Temperature rise of ~ 3000 C!! A Major Design and Control Problem!
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Polymerization Reactors
Features Single phase Solution (liquid or supercritical fluid) Multiphase with particle in liquid or gas dispersion Heat removal by wall cooling, recycle cooling or evaporative cooling TC To recycle Product Catalyst The major types of reactors used in the polymer industry are shown: Fluidized Bed, liquid tube or loop, liquid stirred tank, or gas/particle stirred bed. For solution processes, single phase batch or continuous stirred tank reactors, tubular reactors or continuous loop reactors are usually the reactors of choice. For reactions involving particles such as suspension, emulsion, or particle dispersion processes, stirred tank and loop liquid/particle processes, gas/particle fluidized beds, and stirred particle beds are used. These reactors can be operated in an adiabatic mode, or have cooling from wall cooling, rapid recycle of gas or liquid with external cooling, or evaporative cooling where reactor gas is condensed and recycled to the reactor where it evaporates. CW Monomer Hydrogen Polymer
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Some Polymer Production in Nigeria
High Density and Linear Low Density Polyethylene: • Sclairtech (Nova Chem) HDPE/LLPDE (1-butene) 250,000 tons/yr (Port Harcourt) • Methanol to Olefins Project underway to add 400,000 tons/yr of HDPE (Port Harcourt) Polypropylene: • Spheripol (Basell) PP homopolymer/High Impact PP 95,000 tons/yr (Port Harcourt) • Older PP Plant 35,000 tons/yr (Warri) 400,000 tons/yr of PP (Port Harcourt) Bottle Grade Polyethylene Terephthalate (PET): • Buehler Two Stage Solid State Process 75,000 tons/yr (Port Harcourt) _________________________ Business Monitor International (Jan 2013) Special Chem Industry News (July 2012) Let us consider the principal polymers produced in Nigeria today. We will begin with the current process for producing Polyethylene.
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Meyers, R.A. “Handbook of Petrochemical Production Processes”
Dupont/Nova Sclairtech HDPE/LLDPE Solution Process (Polyethylene Process used at Port Harcourt Plant) (Cyclohexane) (1-Butene or 1-Octene) Flexible process: products range from clear film (ρ=0.91) to bottles (ρ=0.96) 1 or 2 Reactors (Transition Metal) Reactor Adiabatic to 300 C, 140 bar. Res time ~ 30 min Meyers, R.A. “Handbook of Petrochemical Production Processes” Soares, J.B.P. & T.F.L. McKenna “Polyolefin Reaction Engineering” The Dupont/Nova Sclairtech process was invented by Dupont and later acquired by Nova Chemicals. It is a solution process for producing HDPE and LLDPE from ethylene and comonomers such as 1-butene, or 1-octene. Only a small amount of comonomer is used to create short chain branches in the polyethylene chain. By varying the amount of comonomer, one can control the polymer density (i.e.; crystallinity). The advantage of this very flexible process is that it can produce a broad range of polyethylene products, from clear film to bottles, by adjusting the comonomer content and molecular weight of the polymer. The feed material is ethylene and comonomer dissolved in a solvent together with a catalyst. The reactor operates adiabatically, but the cold solvent dilutes the ethylene, and there is incomplete conversion of ethylene so that the observed reactor temperature rise is controlled. The mean residence time in the reactor is 30 min. The solvent and unreacted monomers are separated from the polymer and recycled. The polymer is chopped into pellets for shipment.
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Mol Wt (chain length)->
Dupont/Nova Sclairtech HDPE/LLDPE Solution Process Some Possible Products Mol Wt (chain length)-> A wide range of products can be made from adjusting the operating conditions and feed composition to the reactor. The polymer molecular weight (as monitored by Melt Index measurement) and the density (crystallinity) can be adjusted over a wide range as shown. The Melt Index measurement is an easy quality test by measuring the rate that polymer is extruded by a weighted plunger from a hole in a precisely heated metal block. It is an empirical, but reproducible measure of average molecular weight. VLDPE LLDPE HDPE (Crystallinity)
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OTHER POLYETHYLENE PROCESSES
As polymer production capacity is expanded, there are other widely used polyethylene processes that can be licensed and constructed. Let us consider a few.
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LDPE AUTOCLAVE REACTOR
Monomer Adiabatic reactor 190 to °C 2000 to 3000 bar (supercritical fluid) 10 to 20 % conversion DuPont type (L/D = 2 - 4) 10 to sec residence time Reactor temperature is controlled by the amount of initiator in the feed Initiator TC T Low-density Polyethylene (LDPE) is produced worldwide using high pressure autoclaves (continuous stirred tank reactors) sometimes having multiple zones. Also high pressure tubular reactors are used to produce LDPE. Here we will focus on a single stage CSTR to illustrate the behavior of these high pressure processes. The process was the first to make polyethylene and has the advantage that the LDPE product naturally has long-chain branching, and is easier to process in polymer processing equipment because of the lower viscosity at high throughputs. By contrast, the transition catalyzed product, LLDPE has no long-chain branching except with very special catalysts. (Read from the slide and go through the operating conditions with special emphasis on adiabatic operation (with temperature range), short residence time, low ethylene conversion (thus high recycle rate of ethylene), high pressure range, and limiting reaction temperature through initiator feed.) Products
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POLYMERIZATION vs DECOMPOSITION
10+06 Decomposition vs Polymerization • Heat of Rxn 40% larger • Activation Energy 900% larger • Runaway starts at T ~ 300C with fast reaction w/o initiator 10+04 Decomposition of Ethylene 10+02 1 Reaction Rate, mol/lt-sec 10-02 Polymerization 10-04 This is an example of the polymerization reaction rate and the ethylene decomposition reaction rate with temperature. Point out that ethylene decomposition increases very rapidly with temperature, has a heat of reaction 40% higher than converting ethylene to polymer, and has an adiabatic temperature rise of ~ 3000 C ! Thus it is very important that the reaction temperature stay safely below ~ 300 C to avoid a decomposition event (called a “decomp” in industry) 10-06 10-08 Temperature, °C
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CONTINUATION AND STABILITY DIAGRAM EFFECT OF RESIDENCE TIME ON TEMPERATURE - PERFECT MIXING MODEL
3000 STABLE It would be practically impossible to operate a reactor with this small operating window, and yet these reactors actually operate! ?? 1000 Temperature, °C UNSTABLE If we assume perfect mixing in the LDPE reactor, there are 3 stable steady states, but the low temperature steady state produces little polymer and the high temperature steady state corresponds to a decomp. The moderate stable steady state is in the desired temperature range for making polymer, but the very narrow range of reactor residence times would not allow safe operation. So why can these reactors operate and produce polymer? 300 STABLE UNSTABLE STABLE 100 100 200 300 400 500 Residence Time, sec
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Evidence of Imperfect Feed Mixing
Simple compartment models are adequate to explain curves of initiator consumption in LDPE autoclave reactors (Marini and Georgakis, 1984) IMPERFECT MIXING Experiments show that changing feed location, velocity, or agitator design will strongly affect initiator efficiency! 200 220 240 260 280 300 Temperature, °C Initiator Consumption, g/kg polymer 0.10 0.09 0.08 0.07 0.06 0.05 Perfect Mixing Imperfect van der Molen et al. (1982) Experimental evidence shows that there is imperfect mixing of the feed with the rest of the reactor. This can be modeled by using compartment models and Computational Fluid Dynamics (CFD) models to represent various degrees of mixing in the reactor. Carlos Villa
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MIXING INTENSITY = RECYCLE RATIO EFFECT OF RESIDENCE TIME ON TEMPERATURE - IMPERFECT MIXING MODEL
300 100 500 50 R = 10 250 I II III QR QO QF QR = RQO Good reactor design involves understanding and controlling the degree of imperfect mixing 200 Temperature, °C A simple 3-compartment model can show the effects of imperfect mixing of the feed material on the reactor steady states. With this model one can simulate the varying degrees of feed mixing by the variable R, which is the fraction of the feed material that is recycled through the feed plume compartments. Very large values of R represent perfect mixing and low values represent poor mixing of the feed. Note that for poor feed mixing there is a broad range of residence times over which stable operation at the desired temperature is possible. Thus this is an example where perfect mixing of a reactor is not good, and some imperfect mixing is optimal. 150 100 50 150 250 350 450 Residence Time, sec
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Computational Fluid Mechanics (CFD) combined with
Compartment Models allow detailed information decomp active (shaded region) Contours of Radical Conc. x 107 mol/L 580 A C Temp. rise: K/ppm 55.4 27.6 0.00 B out 540 in Exit Temperature (K) 500 300 ppm TBPOA feed Temp. rise: K/ppm 3.41 1.71 0.00 Ncompart 460 CFD results Limit point Stable Unstable out A: 1 B: 3 C: 100 These approximate compartment model results can be confirmed through detailed CFD modeling. These methods combined with reactor data can be used to determine rules for safe operation. 420 in 200 400 600 30 ppm TBPOA feed Initiator Feed Fraction (ppm) Decomp active at temperatures above about 560 K; modeled using approach of Zhang et al. (1996) Tfeed = 360 K (87 °C); 200 rpm stirring rate; t = 32 sec
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Title Decomp Safety Equipment LDPE Reactor
Vented to the Neighborhood Ethylene & Molten Polymer In industrial practice, reactor decomps happen about every 1-2 years. A major LDPE polymer producer in the USA stated that they consider good operating practice is when these decomps occur only twice in 5 years. Thus safety equipment is necessary to handle these decomps. One such arrangement is shown in the figure. LDPE Reactor Emergency Relief Valves Emergency Relief Valves
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Consequences of LDPE Decomp
• Burnt and molten polymer rains down on the surrounding area, leading to possible difficulties inside the plant, but also coats the employees cars in the parking lot, and dumps material on neighbors • A large ethylene cloud on a windless day could be ignited by flames from other equipment inside the plant or on neighboring properties. In this case the explosion can damage other equipment leading to leaks that can ignite and cause a fire or the release of toxic chemicals. • If the ethylene cloud moves safely away from the plant to an open area, it can be ignited with a flare. See the slide itself
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Heterogeneous Catalytic Processes
for Polyolefins Today most polyolefin processes involve transition metal heterogeneous catalysts supported on inert particles. These particles are designed to be used with either gas phase or liquid slurry phase reactors.
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Fluidized Bed Reactors
(Union Carbide (now Univation), BP, and in-house technology) - the most widely used process for making catalytic polyethylene products (high density (HDPE) and linear low density (LLDPE) Polyethylene) Often there are 2 fluidized beds in series with the polymer particles flowing between them. This allows bimodal MWD or layered products by making a different polymer grade in each reactor, but growing together on the same catalyst particle.
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Gas Phase Fluidized Bed Polyethylene Process
Microscale kpij Pkn,i* Mj Pkn+1,j* (Growth of a polymer chain) Mesoscale C2H4 The particle size distribution (PSD) depends on: • the PSD of the original catalyst support, • the catalyst activity kinetics, • the residence time distribution of the reactor The particle is the micro-reactor inside the fluidized bed which is the macro-environment. Catalyst Polymer Particle Macroscale Bubble Phase Emulsion Phase Catalyst Cocatalyst Poison Cooler Purge Hydrogen Monomers Product Fluidization of the growing polymer particles in a Fluidized Bed Reactor is a very popular process developed by Union Carbide, (now Univation) and BP for polyethylene and polypropylene. The process uses a supported catalyst particle which grows with time as monomer diffuses into the pores, fractures the catalyst support into microparticles, and these microparticles grow as monomer is converted to polymer. The assemblage of microparticles forms a macroparticle. The unreacted monomer ethylene together with an inert cooling gas is recycled and cooled by a heat exchanger. The inert gas is often selected to be condensed in the recycle cooler and re-injected into the reactor as a liquid which flashes while in the reactor to provide extra heat removal. Inlet Stream
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Heterogeneous Catalytic Olefin Polymerization
12 g/g-cat 108 g/g-cat 880 g/g-cat Kakugo, M., H. Sadatoshi, & J. Sakai, Catalytic Olefin Polymerization, T. Keii & K. Soga,Editors; Elsevier (1990), pp Breakup of catalyst and growth of polymer particle (TEM showing catalyst fragments inside polymer microparticles) Metal catalyst (TiCl4, Cr, Zr, etc) supported on SiO2 Chakraborti, S.,A.K. Datye, & N. J. Long, J. Catalysis, 108, p 444 (1987) 30 nm The career of the heterogenous catalyst whether in a gas phase or liquid slurry reactor is shown here. The catalyst is supported on a porous particle (e.g.; silica) as shown. When exposed to an olefin such as ethylene or propylene, the polymerization begins producing polymer which fractures the catalyst particle into small pieces glued together by the polymer. The assemblage of polymer coated microparticles grows until it exits the reactor. The active catalyst on the tiny fragment of the original catalyst particle is fully encapsulated by polymer and the monomer must diffuse through the gas or liquid filled pores of the macroparticle and then diffuse through the polymer film surrounding the microparticle to reach the active site of the catalyst and be inserted into the polymer chain.
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Multistage Fluidized Bed Polyethylene Process
Comonomer Sometimes it is desired to make bimodal MWD or “layered” products with a different monomer mix or operating conditions in the second reactor. The polymer particles from the first reactor can flow into the second reactor and the second layer reacted and be combined with the polymer from the first reactor.
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Particle Growth Behavior
1 4 9 Surface area The principal problem with gas phase polyethylene processes is the thermal stability of the individual catalyst particles which quickly grow into polymer particles. Each particle will begin as a small catalyst particle (~ 50 micrometers in diameter), which grows as polymer is formed around the catalyst fragments inside the particle. These polymer particles generate large amounts of heat from polymerization, but are so small initially that there is little heat removed from the small surface area of the particle. Thus initially the particle temperature can rise rapidly, further increasing the reaction rate and heat produced. If the initial reaction rate of the particle is small and does not increase too rapidly with time, then as the polymer particle grows, the surface area for heat transfer increases dramatically, but the heat generated remains nearly constant or even declines as the catalyst decays in activity. This is illustrated on the next slide. (g pol/g cat)
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Fluidized Bed Particle Temperatures
Temperature difference DT [C] Particle diameter dP [mm] Melted 15C temp rise dcat dpmin Quasi-steady state particle temperature, fully activated Dynamic particle temperature Prepolymerization or slow activation Direct injection, fast activation The Sigmoidal curve is the polymer particle temperature rise after injection into the reactor vs the particle diameter increase after injection for a catalyst of fixed activity. The two particle trajectories of temperature vs particle diameter are for two cases: For a catalyst particle that achieves full activity immediately upon injection into the reactor. This particle quickly rises in temperature to the upper steady state and melts without growing very large – only 2x the catalyst diameter. For a catalyst particle that has slow activation after injection into the reactor, either through the kinetics of catalyst activation or through having a small prepolymerization reactor before the fluidized bed that has mild polymerization conditions and a short residence time so that the catalyst particle has increased in size before injection into the fluidized bed. This particle grows slowly at first and has adequate heat transfer area for cooling before the catalyst achieves full activity. It produces a lot of polymer and grows 7x the initial catalyst diameter. Read the yellow note on the slide. Sticky particles adhere to the reactor wall and attract more particles. After some buildup, the polymer sheets on the wall fall down and disrupt the fluidization, causing the entire polymer bed to melt.
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Polymer Product Images
Some good product with free flowing powder compared with powder with a small amount of particle melting. In fluidized beds, the chunks and sheets of melted polymer are much larger than shown here. Particles overheating and sticking to reactor walls or agglomerating: chunks observed Good operation: free-flowing powder; no chunks observed
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Consequences of Particle Melting
• Overheated polymer particles melt and stick to the reactor walls forming sheets of solidified PE. • Eventually these sheets fall down and block the gas circulation so that remaining particles are no longer fluidized, and melt together. • The inside of the reactor becomes one large mass of solidified Polyethylene! • There is no reactor runaway danger because the reaction essentially stops when the polymer mass limits monomer diffusion to the encapsulated catalyst. • The process must be shut down for weeks to allow the reactor to be cleaned out; this involves small charges of dynamite and the use of chain saws to break apart and then remove many tons of polyethylene from the reactor. Read the text on the slide then use the following comments: Although overall reactor instability problems are usually avoided by having good process understanding, the problems of individual particle melting leading to reactor fouling and shutdown does happen occasionally. Simple events such as having a few particles with higher than normal catalyst activity or outside the normal size range which melt and agglomerate or electrostatic forces that cause the particles to be attracted to each other or to the reactor wall can cause reactor fouling and eventual meltdown.
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Liquid Slurry Polyethylene Reactors
Hexene, Qm2 F Qcat, (Qcoc) Qm1 Isobutane Solvent QS CW Stirred Tank CW Inputs Outputs Catalyst (Cocat.) Loop RTD & CSTR RTD at high recirculation rates; only need CSTR model to represent both types of reactors Heat transfer area per unit volume (A/V) is larger in loops. Heat transfer coefficient (h) is also larger in loops (but varies with recirculation rate) Tanks can remove less heat per unit volume and thus have lower productivity per unit volume Another popular process for making polyethylene is the Liquid Slurry Process, in Continuous Stirred Tank Reactors or Loop Reactors. The Loop Reactors are most popular because of the high heat transfer area per unit volume of the reactor, allowing higher productivity and stability for the same production rates.
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Formosa Plastics 550 Million lb/yr ChevronPhillips HDPE/LLDPE loop
reactors, Point Comfort, TX (photo from Formosa Plastics) An example of Loop Reactors. Often there are two loops in tandem to allow the production of a wider range of products.
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Titanium Catalyst: Effect of Residence Time on Loop Reactor Stability
40 60 80 100 120 140 160 180 2000 4000 6000 8000 10000 Residence time of catalyst (sec) A B Reactor Temperature, °C PE melting pt. Liquid boiling pt. Hopf points Limit Points Stable States Unstable States h/h0*(A/V) = 65 cm2/L, fcatfeed = 3x10-5 h/h0*(A/V) = 65 cm2/L, fcatfeed = 3x10-5 0.2 0.4 0.6 0.8 1 2000 4000 6000 8000 10000 Residence time of catalyst (sec) A B Ethylene Conversion Depending on the loop reactor design, the type of catalyst, and reactor residence time, there are regions of reactor operation which can lead to process instability. Here for a typical Titanium catalyst, we show the unstable operating regime vs reactor residence time. A good choice of operating conditions would be point A, corresponding to a 2 hr residence time. However, if the catalyst feed were increased to lower the residence time of the catalyst to point B, the steady state would be unstable. The dynamic behaviour at point B is shown on the next slide. 2 4 6 8 10 12 2000 4000 6000 time, sec Half-life = 30 min Rp, kg poly/(g cat-hr) • Unstable steady states can exist for a wide range of residence times! • Reactor design and operation procedures must avoid these unstable operating conditions!
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Titanium Catalyst: Example of Sustained Oscillations Possible in the Loop Reactor
PE Melting Pt. Liquid boiling pt. New Steady-State fcatfeed = 3.0x10-5, h/h0*(A/V) = 65 cm2/L Disturbance: change from t=7200 sec to t=3600 sec after 5000 sec. 20 40 60 80 100 120 140 160 180 200 9000 18000 27000 36000 time, sec Reactor Temperature, °C Under these operating conditions, the process would become unstable and will oscillate with long periods (~ 40 minutes). However, the polymer particles would melt at the temperature peaks and reaction in those particles would stop because the monomer dissolved in the slurry liquid could not diffuse through melted polymer. The engineering challenge is to choose the reactor design details and maintain operating conditions that avoid regions of oscillation. Large oscillations in temperature cause both solvent boiling point and polymer melting point to be exceeded. Period of oscillation is ~2500 sec (~40 min).
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Consequences of Oscillatory Operation
• Melting polymer particles would coagulate and create large chunks of polymer which could plug the tubes and foul the impeller – leading to a stagnant mass. In the melted particles and chunks, the catalyst would no longer have access to diffusing monomer so the polymerization would stop. • Removing the mass of polymer and cleaning the reactor would require a major shutdown and cleanup effort. Although these events are possible they are quite rare because of good process understanding.
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Some Polymer Production in Nigeria
High Density and Linear Low Density Polyethylene: • Sclairtech (Nova Chem) HDPE/LLPDE (1-butene) 250,000 tons/yr (Port Harcourt) • Methanol to Olefins Project underway to add 400,000 tons/yr of HDPE (Port Harcourt) Polypropylene: • Spheripol (Basell) PP Homopolymer/High Impact PP 95,000 tons/yr (Port Harcourt) • Older PP Plant 35,000 tons/yr (Warri) 400,000 tons/yr of PP (Port Harcourt) Bottle Grade Polyethylene Terephthalate (PET): • Buehler Two Stage Solid State Process 75,000 tons/yr (Port Harcourt) _________________________ Business Monitor International (Jan 2013) Special Chem Industry News (July 2012) Currently Polypropylene is produced in Nigeria with the popular Basell Spheripol Process at Port Harcourt and a small production with older technology at Warri. We will discuss the process technology used at the Port Harcourt plant.
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Atactic PP: random methyl groups and no crystallinity
Polypropylene * * * * * * * * * * * * Isotactic PP (crystalline) Syndiotactic PP (crystalline) Atactic PP: random methyl groups and no crystallinity • Commercial homopolymer Polypropylene is mostly isotactic with some small sections of the chain atactic (from insertion errors). The material density lies between that of LDPE and HDPE. • Other Polypropylene products can be copolymers with ethylene to produce lower density (less rigid) materials or even an ethylene - propylene amorphous rubber material. • “Impact Polypropylene” is a well-mixed physical mixture of isotactic polypropylene and ethylene-propylene rubber. Polypropylene was first synthesized using transition metal catalysts by Natta in Italy in the 1950’s, and was a totally new molecule. Today polypropylene is the second largest volume polymer produced in the world, and the process issues are similar to polyethylene, but usually less troublesome. Depending on the position of the pendant CH3 on the polymer chain, the properties of polypropylene can be greatly altered. Like polyethylene, the crystallinity and related density of the product depends on the stereoregularity of the chain. As shown in the slide, and depending on the catalyst used, the pendant CH3 group can be regularly on one side of the chain (Isotactic) or perfectly alternating along the chain (Syndiotactic). Both of these forms will have a high degree of crystallinity and strength properties. If the catalyst does not regulate the position of the CH3 along the chain, then the polymer will be atactic, being a viscous liquid with no crystallinity.
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Thermal Parameters for Addition Polymerization Dynamics
Heat of Poly Pure Mon Conc Adiab. Temp Rise Heat Removal Duty Monomer Kcal /Mol mol/lit C Kcal /Kg Ethylene 25.9 14.2 1609 922 Propylene 20.1 12.3 850 476 1-Butene 19.9 10.6 647 355 Isobutylene 11.5 395 204 1,3-Butadiene 17.4 589 322 Isoprene 17.9 10.0 496 263 Styrene 8.7 398 167 alpha- Methylstyrene 8.4 7.7 173 71 Vinyl chloride 17.2 14.6 803 275 Vinylidene chloride 12.5 655 180 Tetrafluoroethylene 38.9 15.2 1447 390 Acrylic Acid 16.0 464 222 Acrylonitrile 18.3 774 344 Maleic Anhydride 14.1 15.3 491 144 Vinyl acetate 21.0 10.8 519 244 Methyl acrylate 18.6 11.1 452 216 methacrylate 13.4 9.4 260 134 Methacrylic acid 11.8 488 Butyl 7.0 310 145 Although propylene polymerization is highly exothermic, it has many fewer problems than for ethylene polymerization. The adiabatic temperature rise is about half that of ethylene, the rates of reaction are lower, and there is no danger of the spontaneous decomposition of propylene under process conditions. The melting point of polypropylene is higher than that of polyethylene so polymer particle melting is much less of an issue. Also, crystalline polypropylene cannot be made with a free-radical process, so transition metal catalysts are used to produce the various types of polypropylene. A Major Design and Control Problem!
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Basell Spheripol Loop-FBR Polypropylene/Impact PP Process
The Port Harcourt Process (TiCl4 - MgCl2) Propylene Ethylene Hydrogen Catalyst Cocatalyst ( Al - Alkyl ) CW STEAM Finishing N 2 Fluidized Bed Reactor 80 o C , 20 atm Loop Reactors (liquid propylene) 70 C , atm Prepolymerization PURGE E/P Rubber produced in PP particles Currently polypropylene is being produced at Port Harcourt using the Basell Spheripol Loop-Fluidized Bed process. Propylene is fed as a liquid. This process employs two loop reactors in series followed by a fluidized bed. There is a small prepolymerization reactor ahead of the loops to allow the catalyst particles to have a gentle initial breakup and growth to eliminate fines and to produce very regular spherical particles that will retain their spherical shape throughout the process (hence the name Spheripol). The product from the loop reactors can be sold as Propylene Homopolymer or some of the polymer particles can proceed to the fluidized bed where a copolymer of ethylene and propylene (E/P Rubber) is formed in the particles. This product is Impact Polypropylene, which has good impact resistance properties.
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Liquid Propylene Slurry Loop Reactor - the most
widely used process for making Polypropylene A two-loop reactor process is shown.
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OTHER POLYPROPYLENE PROCESSES
There are many other processes that have been developed for making polypropylene.
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Some Polypropylene Processes
Features PP/catalyst particles Multiphase liquid or gas dispersions Heat removal by surface, recycle or evaporation • multiple reactors for complex products Horizontal Stirred Bed Catalyst 1st REACTOR Particles Product FBR Gas + particles TC To recycle Product Catalyst Vertical Stirred Bed Here are some processes that have become popular over the years. The loop reactors have used liquid propylene as a feedstock, while the Fluidized Bed and the two types of stirred gas/particle beds have used gaseous propylene. Loop CW Monomer Hydrogen Polymer Gas + particles Liquid + particles
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Basell Spherizone Polypropylene Reactor 2002
A relatively new process from Basell has multiple zones in one reactor, which allows production of bimodal and other complex products in a single reactor. It has great versatility, but is not the choice for high production rates of a few products. • More Uniform Product for layered copolymer • Bimodal MWD in a single reactor • blocky copolymer for catalysts with long chain lifetimes • single reactor rather than two for complex products
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Some Polymer Production in Nigeria
High Density and Linear Low Density Polyethylene: • Sclairtech (Nova Chem) HDPE/LLPDE (1-butene) 250,000 tons/yr (Port Harcourt) • Methanol to Olefins Project underway to add 400,000 tons/yr of HDPE (Port Harcourt) Polypropylene: • Spheripol (Basell) PP Homopolymer/High Impact PP 95,000 tons/yr (Port Harcourt) • Older PP Plant 35,000 tons/yr (Warri) 400,000 tons/yr of PP (Port Harcourt) Bottle Grade Polyethylene Terephthalate (PET): • Buehler Two Stage Solid State Process 75,000 tons/yr (Port Harcourt) _________________________ Business Monitor International (Jan 2013) Special Chem Industry News (July 2012) Currently the Port Harcourt PET Solid State Process is designed to take PET chips as a feed and reprocess them to make other products. The chips could be made from the melt process and crystallized or could be recycled PET. This is possible because PET polymer formation is reversible, so the PET chain can be unzipped and reformed to the specifications of other PET products. Let us discuss PET production issues.
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POLYETHYLENE TEREPHTHALATE (POLYESTER) PROCESSES
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Polycondensation (Chain Addition) Kinetics
Primary Polycondensation Polymers: Polyesters (e.g., Polyethylene Terephthalate (PET)) • Polyamides (e.g. Nylon 66) --COOCH2CH2OH 2 ~~-OOC-- --COOCH2CH2 -~~ ~-OOC-- + HOCH 2 CH OH ~~~~NH2 + HOOC~~~~ ~~~~NHOC~~~~ + H2 O kf Pn + Pm Pn+m + Condensate Chain building kr Polyesters such as PET and also polyamides such as Nylon are made by chain-addition mechanisms in which a small molecule called a condensate is stripped out. This chain-addition mechanism is strongly reversible so that the condensate must be removed in order to allow the reaction to proceed to high polymer. Also there is no large heat of reaction for these chain-addition reactions. In addition, there are redistribution reactions between the polymer species which do not increase the average chain length, but narrow the chain length distribution. Finally at very high temperatures there are chain degradation reactions that will break the molecules into smaller pieces. Thus the operating temperature is limited to avoid these degradation reactions. kredis Pn + Pm Pn+m-r + Pr Redistribution (Does not build polymer but returns CLD to the Flory distribution)
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Polyethylene Terephthalate (PET) & Reagents
Terephthalic Acid Ethylene Glycol Polyethylene Terephthlate (PET) PET is made from an organic acid and glycol which forms the polyester. The most common reactants are Terephthalic Acid and Ethylene Glycol. These combine quickly to form small polyester chains which then react with each other to make longer chains and strip out the condensate, Ethylene Glycol.
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Molecular Weight & Chain Length Distributions - Polycondensation
For many condensation polymers equilibrium limitations allow only very short chains in a closed system without condensate removal Polycondensation (polyester, polyamide, epoxy, polycarbonate, etc.) Polycondensation (growth) Redistribution (CLD adjustment) If the condensate is not removed, PET can only grow to a chain length of 2 because of the equilibrium limitation. Polyamide 6,6 can grow larger, to a chain length of 21, without condensate removal. However, neither of these are useful polymers. Thus procedures for removal of condensate are the most important part of the PET process. • Reactions occur only between polymer molecules; a “living” polymerization. • Strong equilibrium limitation, so condensate is removed to allow continued chain growth. Degree of Polymerization Usually measured by GPC or Intrinsic Viscosity (IV)
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PET Resin Specifications for Different Products
PET Product Intrinsic Viscosity Approx. DPn Polyester Fibers dl/g 100 Soft Drink Bottles dl/g 150 Auto Tire Cords dl/g 200+ The most important products from PET require an average chain length (measured as degree of polymerization, DPn) greater than Polyester fibers can be made much more easily than molded products or bottles, and for automobile tires. For molded products and tire cords, the chains must be very long, requiring special reactor designs.
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PET Direct Esterification Process Flowsheet
(Terephthalic Acid (TPA) and Ethylene Glycol (EG) Feedstock) Fiber Spinning PET EG TPA Water EG EG DP = 100 EG PET DP = 20-30 Prepolymer DP = 150 DP = 2-4 Molding 1 esterification pre-polycondensation finishing stage solid state stage A typical process for Fiber grade and Bottle grade PET. The boxes show operating temperatures, the pressures or degree of vacuum required, and the residence time in each stage. Note that the residence times are very long because large amounts of condensate has to be removed from highly viscous materials and even from solid particles by evaporation for molded products such as bottles. C 3 - 5 bar 7,000-10,000s C 0.03 bar 5,000-7,000s C 0.001 bar 10,000-20,000s C 1 bar 20,000-90,000s
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The third stage of the process requires high temperatures (~ 290 C), high vacuum (0.001 bar) and high surface areas to evaporate Ethylene Glycol from very viscous polymer. The Disk-Ring Reactor produces large surface areas of thin films of polymer attached to the slowly rotating wheels. Other designs which create high surface areas from viscous melts are also used.
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PET Direct Esterification Process Flowsheet
(Terephthalic Acid (TPA) and Ethylene Glycol (EG) Feedstock) Fiber Spinning PET EG TPA Water EG EG DP = 100 EG PET DP = 20-30 Prepolymer DP = 150 DP = 2-4 Molding 1 esterification pre-polycondensation finishing stage solid state stage The material exiting the third stage has sufficient DP for Fiber Spinning to polyester clothing and other uses. However, for bottles and other molded products, the DP must be much higher, so a Solid State Polycondensation stage is used. To accomplish this, the product from the third stage is cooled to crystalize the melt, and converted to very small particles that are fed to a moving packed bed reactor, where an inert gas is passed though the reactor to remove the Ethylene Glycol condensate to the required low level to allow the chain length to reach DP~150 for bottles. C 3 - 5 bar 7,000-10,000s C 0.03 bar 5,000-7,000s C 0.001 bar 10,000-20,000s C 1 bar 20,000-90,000s
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THE MOVING PACKED BED Polymer Feed Purge Exhaust
Purge Gas Product Polymer Standard Conditions for PET: -Temperature about 230oC -Purge gas is N2 -Residence Time about 3hrs. -Particle size about 2 mm Some conditions for making bottles using Solid State Polycondensation with a moving packed bed reactor. Nitrogen flows countercurrent to the particle flow and is used to carry away the condensate. The operating conditions are for producing polymer for bottles are noted. Other products would require longer residence times.
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LOW VOLUME PRODUCTION PROCESSES
Multiphase Polymerization Processes LOW VOLUME PRODUCTION PROCESSES
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Polymerization in Multiple Phases
Multiple Phase Polymerization Processes Gaseous Media Fluidized Agitated Emulsion Dispersion Suspension Inverse Emulsion Inverse Suspension Precipitation Liquid Media Aqueous Organic There are many other polymers that are made in somewhat lower volumes than the polymers discussed so far. Most of these are made in stirred tank reactors in either a single phase or multiple phases. Some of the higher volume ones such as Polyvinyl Chloride (PVC), Polystyrene and Expandable Polystyrene (PS, EPS), are made in multiphase reactors such as Emulsion, Suspension, or Precipitation processes.
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Suspension Polymerization
monomer droplets d=0.1-5 mm Monomer : Water immiscible Initiator : Oil soluble Suspending Agents : 1)organic polymers 2) inorganic powders M / W = 50: :75 Each Particle Behaves as Bulk Reactor water initiator suspending agent Suspension Polymerization reactors are usually stirred tanks in which an water-immiscible monomer and an oil soluble initiator appear as droplets created by the shear field of an agitator and stabilized with suspending agents. The suspending agents keep the particles from agglomerating so that the particles have a predictable size throughout the polymerization. The polymerization proceeds until all the monomer has been converted to polymer inside the particles. The process has the advantage that close temperature control of the polymerizing droplets is provided by the water phase. Precipitation Polymerization is an alternate batch process for Polyvinyl Chloride (PVC) because the polymer is insoluble in the monomer. Thus a stirred reactor, initially filled with liquid monomer and initiator, will begin with PVC particles precipitating into the monomer liquid, until there is a phase inversion with monomer sorbed into the PVC particles and finally only PVC powder in the reactor. Advantages: Low Viscosity, Excellent Temperature Control. Easy Product Separation and Little Contamination. Examples: PVC, ABS, Polystyrene, Poly(vinyl acetate) Particle size distribution is determined by the shear field early in the batch time - little change thereafter.
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MULTICOMPONENT EMULSION POLYMERIZATION
(for convenient polymer synthesis, rubber, latex paints, etc.) MONOMER DROPLET PHASE (5µ) Particle size dist’n determined by the kinetics of particle nucleation and the residence time distribution in continuous reactors monomers MICELLE PHASE Emulsion Polymerization is an even more complex polymerization in which monomer droplets are suspended in the aqueous phase of the reactor. The aqueous phase contains a water soluble free-radical initiator, surfactant, and very small amounts of dissolved monomer. Polymerization begins in the aqueous phase until the polymer chains are large enough to precipitate out to form polymer particles stabilized by the surfactant. The polymer particles also contain monomer transported from the monomer droplets through the water phase to the particle phase. Thus the polymer particles grow larger and become more numerous. When all of the monomer in the monomer droplets has been polymerized, the reactor contains a surfactant stabilized emulsion of polymer particles in water. Although this sequence of events describes batch emulsion polymerization, the process also operates in a continuous mode where the initiator, monomer, and surfactant in water are fed continuously, and the reactor contents are withdrawn continuously. Polymers of styrene, methyl methacrylate, Vinyl Chloride, and others are made this way and used as paints or coatings (e.g.; latex paint) or recovered as bulk polymer (e.g.; synthetic rubber) AQUEOUS PHASE POLYMER PARTICLE PHASE (5-500nm) monomer copolymer live polymer initiator initiator monomers surfactant oligomers
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Final Remarks; Going on From Here
CONCLUSIONS
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Some Final Remarks • There appear to be great opportunities to produce large volumes of a broader range of valuable polymers in Nigeria that would expand the domestic market, significantly reduce imports, and offer materials for the export market. • There are well-regarded and successful polymer production processes available for license that could be installed in Nigeria to take advantage of the large volumes of inexpensive oil, gas, and refined products available locally. • Creating and expanding the production of polymers would allow the expansion of the domestic polymer processing industry to provide more variety and larger volumes of polymer consumer products for both domestic use and export.
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