Green Chemistry

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A Green Chemistry Module

Suggested Use:An industrial chemistry course in discussions of the petrochemical industry and/or industrial polymerizations or in sections of a polymer course dealing with petrochemical feedstocks.

Petretec – Dupont’s Technology for Polyester Regeneration

 

Trudy A. Dickneider, Ph.D., Department of ChemistryUniversity of Scranton

trudy.dickneider@scranton.edu

 

Introduction

Polyethylene terephthalate (PET) is the most important commercial polyester polymer. In the United States and Canada over 4 billion pounds per year are produced from petrochemical feedstocks.1Its characteristic properties are the reason for its many uses. Since it is stronger than cotton and cellulose, yet mixes well with cotton fibers, it is used in fibers. Cloth made from these fibers is resistant to creasing. These fibers are known under the familiar trade names Dacron and Fortrel, and are widely used in consumer goods such as clothing and linens.PET also forms a clear polymer so it is also used in films such as Mylar, as well as in photographic films and transparencies.Polyester fibers are also used industrially in ropes and filter construction and are mixed with steel cord in the manufacture of tires.Polyesters also have medical applications since their strong fibers can be used to surgically repair damaged tissues.
An excellent survey of the history of the development of PET is given in Real-World Cases in Green Chemistry 2. The uses of PET mushroomed with the improvement in processes for PET production that involved fast crystallization of the polymer. This allows it to be molded into containers, especially those for soft drinks – the ubiquitous 2 liter soda bottle. In fact, the production of soft drink bottles accounts for over half of the yearly production of PET 1
The very large amounts of PET produced each year pose two potential problems. One has to do with the raw materials for its manufacture and the other involves the disposal of products made with PET, especially beverage containers. We will begin with a consideration of their manufacture and then cover their end fate.
Polyesters, as are all polymers, are made from materials that are derived from the refining and reforming of petroleum so our consideration of the industrial chemistry of PET must begin at the source.

Crude Oil – The Beginning of the Story

Image 004The road to any plastic produced today begins with a fossil fuel – oil, natural gas, or coal.Most polymers are produced from petrochemical feedstocks that are the end product of the refining and reforming of crude oil. Crude oil, more properly called petroleum, is a complex mixture of thousands (maybe millions!) of compounds.While most of these compounds are hydrocarbons, some contain oxygen, nitrogen, or sulfur, and there are trace amounts of metals, usually present in large molecules called porphyrins.The scheme below shows the average composition of crude oil along with some representatives of each class of compounds.3
Image 006
Petroleum has formed over millions of years as the organic material derived from marine and terrestrial organisms and deposited in sedimentary environments has been matured by the action of microbes, heat, and pressure.The types of compounds present, therefore, reflect the source input.As a result the nature of petroleum varies with it’s geographical location, input source, and geological history. The scheme below summarizes these compounds as they are described in the petroleum industry.

Image 008

The American Petroleum Institute (API) has developed a universal system for classifying crude oils based on their density.Rather than expressing the density in the traditional terms of weight per unit volume, the API gravity is described in degrees of the API scale.Petroleum characteristics as a function of API gravity are summarized below.

API Range
Description
Viscosity
Color
Components
0o – 22.3o
HEAVY
Extreme
Dark
Asphalt
22.3o – 31.3o
MEDIUM
Moderate
brown
Gasoline & diesel
31.3o – 47o
LIGHT
Fluid
Light yellow
Condensate/gasoline

An average crude oil with an API gravity of 35o would have the following composition:

Image 010

It is estimated that the world reserves of petroleum may total 2 trillion barrels.However, the known and proven, in the ground, reserves total some 1034 thousand million barrels.That’s just over 1 billion barrels.While this seems like a huge number, all fossil fuels are non-renewable resources so conservation and wise use of the petroleum produced are imperative.

The products that come to use from crude oil are almost too numerous to mention.Petroluem products are involved in the manufacture of goods used in home and commercial construction, automobiles, fibers for clothing, holiday decorations, food processing and packaging, medical devices, and the synthesis of pharmaceuticals. The list goes on and on as can be seen at http://www.api.org/edu/petprods.htm. This site, sponsored by the American Petroleum Institute, summarizes many petroleum products in everyday life.The route from crude oil to sweaters, CD’s, car bumpers, roofing shingles, etc., is a long one involving a great deal of chemistry called refining and reforming.The products which can be derived from an average barrel of crude oil, which contains 42 gallons, are shown below.5 

Image 012

 


  
  
 

In this module we are most concerned with the petrochemical feedstocks which account for 2.7% (by volume) of every barrel of crude. In general, petrochemical feedstocks can be categorized by source. Shown below are the feedstock chemicals derived from each major fossil fuel source. 
  
  
  
 Image 014

Some 40 million barrels of these feedstocks are used daily in the world’s refineries to produce petrochemicals which are then used by other aspects of the chemical industry to produce intermediates for further production or consumer goods.4We can see, then, that crude oil produces two major categories of products – fuels and feedstocks.

While petroleum has been found in some amount on every continent, the major commercial reserves are summarized below. 6

Image 016

The composition of crude oils from each of these areas differs, therefore, its commercial value is also different.High grade crudes which directly produce large amounts of gasoline have the most commercial value.Those which need considerable reforming to produce significant amounts of gasoline or contain larger than usual amounts of metals such as vanadium (which poisons or shortens the life of the catalysts used in reforming) have the lowest dollar value. The average crude we discussed above with an API Gravity of 35o, after refining would yield the product mixture shown below.3

Refinery Product
Hydrocarbon Range
Percent
Gasoline 
C5 – C10
27
Kerosene
C11 – C18
15
Diesel 
C14 – C18
11
Heavy Gas Oil
C12 – C25
10
Lubricating Oil
C20 – C40
20
Residuum
> C40
17

While there are direct markets for the fuels (gasoline, kerosene, and diesel) in order to be profitable the other components of the crude oil, especially the gas oil and residuum need to be converted into marketable products. This is the role of reforming.

Producing the Para-xylene – Refining and Reforming 

Image 020The dimethylbenzenes, commonly known as the xylenes, are important industrial chemicals. They are used in the manufacture of dyes, and in the production of benzoic acid, phthalic anhydride, and the iso- and terephthalic acids. The dimethyl esters of these acids are used in polymerization reactions producing a large family of polyesters.It is one of these, polyethylene terephthalate that is the subject of this module. The production of polyethylene terephthalate begins with the para isomer of xylene.The three isomeric xylenes can be produced from coal tar distillate, but the major source is crude oil.A mixture of isomers is recovered from the fossil fuel source. It contains 50-60% of meta-xylene and 20-25% each of the ortho and para isomers. The first step in recovering the p-xylene used for polymer production is its separation from the other isomers.As can be seen from the xylene family data shown below, the boiling points of the meta and para isomers are very similar. The difference between their boiling points and that of the ortho isomer allow them to be fractionally distilled from the ortho isomer. When the distillate cools, the para isomer crystallizes allowing it to be separated from the meta isomer by fractional crystallization.

Image 022

The XYLENES

ortho
meta
para
Boiling point
144oC
139.3oC
137-138oC
Melting point
-25oC
-47.4oC
13-14oC

In 1999 some 8,802 million pounds of para-xylene were produced. 7This was an increase of 5 percent over the previous year and represents nearly 20% growth over the last two years. This make p-xylene one of the fastest growing of the industrial petrochemicals. The total capacity for p-xylene production is 10,080 million pounds so yearly production is running at nearly 90% of capacity to meet industrial demand.Several of the large oil companies are major suppliers of p-xylene, including Mobil, Exxon, and Chevron. The most important producer is Amoco, with its plants in Decatur, Alabama, and Texas City, Texas, supplying nearly half of the total yearly market. In addition, the Amoco plants are the only suppliers to directly produce the purified terephthalic acid needed for polymerization. 8

Although there are other uses for the xylenes in the dye and perfume industry, and p-xylene is used as a solvent and is important in the manufacture of some herbicides, almost all of the p-xylene produced is used in the manufacture of purified terephthalic acid (PTA). The PTA is converted to polyester fibers, resins, and films, and into dimethyl terephthalate (DMT).

The xylenes, along with countless other industrially important petrochemicals are produced from crude oil by the processes of refining and reforming. The umbrella term refining is often used to describe the process of conversion of crude oil into useful products. However, there are at least five distinct processes involved. 

Image 024The process really begins in the oil field where the crude oil is recovered from the reservoir in which it is stored by drilling. The reservoir rock is usually several thousand feet underground. The oil is produced from the well, collected and transported by tanker or pipeline to the refinery.

The first step in the treatment of the crude oil is DESALTING in which the crude oil is washed to remove suspended water, salt, dirt, and other non-organic impurities which may be part of the oil or may have contaminated it during production.The desalted oil then enters the REFINING process where it is distilled under both atmospheric pressure and vacuum conditions. This distillation produces some marketable products but many components require conversion in which the composition of the products, their molecular structures, are modified. This is known as REFORMING.

The reformed products then undergo BLENDING, a process which combines some of the straight run distillation products with portions of the converted products to produce gasolines and other products whose compositons have been designed to provide needed commercial properties.These operations are conducted in a refinery, usually a huge complex covering many acres, containing distillation towers, processing and storage tanks, and large-scale reactors all connected by miles of pipelines. While desalting and blending are straightforward processes, the refining and reforming, the place where the chemistry happens, need further examination. A detailed diagram of a refinery can be seen athttp://www.chevron.com/explore/science/refinery/chart.html.

A refining tower is a huge (sometimes > 100 feet tall) distillation column. The crude oil is continuously feed into the tower and the distilled products are continuously removed. In this way a refinery can distill thousands of barrels of oil a day. The oil, which has an average boiling point of around 420oC is heated in a furnace which vaporizes most of the oil and the gases and hot liquids are fed into the base of the tower.9 The interior of the tower consists of a series of plates, known as bubble plates, on which the gases condense and then are revaporized to condense on the next higher plate, where they are again vaporized to condense farther up the tower. The components with the lowest boiling point exit the top of the tower, and the materials with the highest boiling point remain at the bottom of the tower. In this way the components of the petroleum are separated by boiling point according to their weight. These products are known as straight run products. Certainly, the gasoline produced has a market at this point, but since usually less than 30% of an average crude oil will distill straight run gasoline, conversion of the remainder, especially the residuum, is needed to maximize the value of the crude oil. A schematic of the refining process is shown below.

Image 026

Reforming involves the chemical transformation of the products from the distillation. The reactions which take place change the size and structure of the molecules in the distillation fractions, producing usable materials from the residuum and converting other products to ones with a higher commercial value.9,10These reactions involve the actions of heat and pressure, often in the presence of a catalyst and are often designated as PROCESSING REACTIONS.The schematic below shows the flow of straight run products from the refining tower through the processes of reforming to finished products.

 Image 028
The processing or conversion reactions are designed to crack large molecules into smaller ones, combine small molecules into larger ones by alkylation and polymerization reactions, and rearrange the structure of molelcules by reforming reactions and isomerizations. The mechanisms for these reactions involve both free radical and ionic processes with the temperature and pressure conditions often determining the mechanistic pathway. The significance of these reactions is seen in the fact that 70% of crude oil in the United States undergoes some conversion process. 10
The cracking reactions include both thermal and catalytic processes, however, thermal cracking has been almost totally replaced by catalytic processes.11 Catalytic cracking is basically a controlled pyrolysis reaction in which long chain hydrocarbons are broken into smaller, gasoline range hydrocarbons through cleavage of carbon-carbon bonds in the presence of a catalyst. An example is shown below.
Image 030

Reactions to combine molecules include alkylation and polymerization reactions.11 In an alkylation reaction alkenes are bonded to an alkane or an aromatic compound.These reactions also allow production of brached alkanes by combination of a straight chain alkene with an isoalkane.  Both types of reactions are shown below.

Image 032

Molecules can also be combined through polymerization reactions in which alkenes are linked together under the action of heat, pressure, and a catalyst.  A sample reaction is shown below.

Image 034

Often it is necessary in conversion reactions to change the molecular structure, not by adding alkyl fragments or breaking chains, but rather by rearranging the original structure. This can be accomplished by reforming reactions. Catalytic reforming and isomerization reactions are used to improve the octane number of distillation products and to produce aromatic compounds for chemical manufacturing.In these processes, naphtha from the refining process is converted to a mixture of compounds known as reformate. The C6 to Ccompounds in the naphtha are converted to alkanes, cycloalkanes, and aromatics. The composition of the aromatics is of particular importance for the subject of this module. The aromatics are a mix of benzene (16%), toluene (47%), and the xylenes (37%).Examples of these reactions are shown below.11

Image 036

The operations conducted in a refinery also include many mechanical techniques and processes involving improving the physical characteristics of the refined and reformed products. These include dewaxing, and extracting. Some of these are shown in an excellent refinery diagram on the Amoco web site (http//www.Amoco.com/resource_pool/design/refinery_flow.jpeg)

The process of refining and reforming crude oil produces the fuels for heating and transportation needs, lubricating oils, greases, and asphalts for mechanical devices and road construction as well as the petrochemical feedstocks for chemical manufacturing.One of the most important areas of chemical manufacturing involves polymerization reactions.

 

Polymerization – Reactions for Linking the Pieces

The p-xylene recovered from crude oil and produced through conversion reactions is just one of the starting materials for the synthesis of PET. The other compound needed is ethylene which is recovered from the refining of crude oil. However, it is necessary to derivatize both the p-xylene and the ethylene to produce the monomers needed for the polymerization reaction that will yield the PET.
Ethylene, produced from thermal cracking is treated with oxygen in the presence of a silver catalyst to produce ethylene oxide, which is then reacted with water in the presence of an acid to produce ethylene glycol, one of the needed monomers.  The reaction is shown below.
Image 038
The reactions needed to convert the p-xylene into the needed substrate are more complex.
The p-xylene is first oxidized to produce terephthalic acid (TA) which is then esterified to dimethylterephthalate (DMT). This can be accomplished by a two step sequence or in one step reaction in which the oxidation is conducted using a cobalt catalyst in the presence of methanol.  Both reactions are shown below.

Image 040

The DMT produced from the one step reaction must undergo a five column distillation procedure to yield material pure enough to be used in polymerization reactions.

Every year C&E News surveys the quantities of chemicals produced and lists the Top 50 Chemicals. Both TA and DMT are listed and they are the only chemicals in the top 50 that are derived from p-xylene.

The PET is produced by means of a polymerization reaction of these two monomers – the ethylene glycol and the dimethyl terephthalate. Polymers are compounds composed of repeating units. The monomeric units are linked together in a polymerization reaction to form the oligomer, consisting of many units. The polymer industry produces over 60 billion pounds of polymers per year.The industry consumes about 130 billion pounds of chemical feedstocks per year.11 This accounts for nearly half of the organic compounds produced each year in the United States. Furthermore, nearly half of the chemists employed in the chemical industries work with polymers either in synthesis or manufacturing.

There are basically two types of polymerization reactions. In chain growth polymerization the monomers continue to add to a growing chain once the reaction has begun. This produces polymers of high molecular weight and involves reactions with ionic or free radical intermediates. The other process, known as step growth polymerization involves a condensation reaction in which the two functional groups react with each other to eliminate a small neutral molecule, usually water. This polymerization can be controlled to limit the length of the chain and to give a low molecular weight polymer.

Polyethylene terephthalate (PET) is a polyester polymer. Polyesters can be synthesized two ways. The first method is a direct reaction of a diacid with a diol. To produce PET, terephthalic acid is reacted with ethylene glycol as shown below.

Image 042

This reaction is a typical Fisher type esterification in which an acid is reacted with an alcohol and follows the usual mechanism for that reaction. The fact that each molecule is difunctional produces a polymer by the reaction.

The other synthesis of PET involves an ester interchange of a diester and a diol.This is a transesterification reaction in which one ester is transformed into another.The synthesis of PET by this method reacts dimethylterephthalate with ethylene glycol as below.

Image 044

The original synthesis of PET was performed by Whinfield and Dixon.11 They used a transesterfication of DMT and glycol in a 1 : 2.4 ratio, distilling methanol out of the reaction mixture as the synthesis progressed.Their polymerization was conducted at 200-290oC in the presence of an SbO3 catalyst.Later techniques employed in the polymer industry used a step growth polymerization of terephthalic acid with an excess of ethylene glycol at 250oC at a reaction pressure of 60psi.This formed a polymer of from 1 to 6 repeating units. During the 1970’s syntheses of PET used three times as much DMT as TA. By the 1980’s the amounts of DMT and TA being used were nearly equal. In the United States today the ratio of TA to DMT used in polyester synthesis is TA 46 : DMT 54.

The initial problem with using TA was the purity of the acid.The esterification to DMT allowed separation of a pure product.The use of TA increased as techniques became available for producing pure terephthalic acid, known as PTA. The PTA is 99% pure. It is produced from the oxidation of p-xylene in the presence of cobalt and manganese salts of heavy metal bromides. 10,11

As noted earlier, Amoco is the major supplier of PTA. The Amoco process for the production of PTA crystallizes the crude TA (90% yield of 99.6% pure TA). When the acetic acid by product and the unreacted p-xylene are removed by evaporation, the TA is further purified by washing in hot water.The major impurity remaining is p-formylbenzoic acid which is hydrogenated to p-methoxybenzoic acid. The TA can then be separated by fractional crystallization which yields PTA which is 99.9% pure terephthalic acid.The uses of these two monomers, TA and DMT, are shown below. 11

Percentage of Total
Polyester fiber
55 
Exports
21 
Polyester resin
10 
Polyester film
10 
Miscellaneous

The Brown Side of the Story – Uses and Abuses of PET

There are several factors involved in the production and uses of PET that make it an environmentally unfriendly material. The brown side of PET is significant because of the very large amounts produced and used today. This production uses large amounts of petroleum, a valuable and non-renewable resource. And this PET ends up in goods that eventually need proper disposal.Of the more than 4 billion pounds produced in 1998, only 745 million pounds were recycled.12The remaining 81%, some 325.5 billion pounds, were either landfilled or incinerated. Part of this low recycling percentage results from a lack of recycling programs in many areas. However, the brown side of PET is made browner by the fact that under the existing recycling technology, known as mechanical recycling, much of these 325.5B pounds cannot be recycled because they contain significant amounts of impurities such as dyes and metals which interfere with reprocessing.So materials, such as metallized polyester films, such as some Mylars, which are used in solar window coatings, electronics, and photographic materials, are excluded from mechanical recycling.13
Another brown side of PET results from the possible uses of the recycled PET.The major use of PET is the production of soft drink containers. Recylced PET (RPET) cannot be used for production of these containers since the temperatures involved in processing are not high enough to ensure sterilization of the product.2 So, the use of RPET does not reduce the amounts of new or virgin PET needed for food containers. During 1998, there were 20 mechanical recycling plants operating in the United States. They produced 588 million pounds of PET flake. The end products of RPET include products such as non-food bottles, fibers for clothes and carpets, films, sheets, and other commercial products. 13
These two factors, consumption of petrochemicals and high landfill amounts, combine to make PET a brown material.Recognizing this, Dupont, among others, focused their efforts on developing recycling technology able to use all types of PET and produce RPET that is equivalent in its purity and quality to virgin PET. The process that has resulted is known as Dupont’s Petretec.
Green Chemistry – The Petretec Process
Dupont has a long history of involvement in the recycling of plastics. They were the first to mechanically reprocess PET starting in 1988. The story of Dupont’s history in the production, uses, and recycling of polyesters is told athttp://www.dupont.com/polyester/story.html a site with many useful links to other aspects of the polyester industry.

Focusing on the recycling problems of PET, Dupont developed four goals, according to Mike Harnagel, the vice president and general manager of Dupont Films.13 These goals were to:

  • avoid landfilling polyester waste materials
  • increase public awareness of the greenness of polyester materials
  • reduce overall consumption of oil derivatives
  • retain the chemical value of polyester
To accomplish these goals Dupont developed an entirely new recycling technology. This green process is known as Polyester Regeneration Technology (Petretec).14The company then spent $12 million to convert an existing plant for the operation of their new regeneration technology. The Petretec process can accept polyester films, fibers, and plastics with much higher levels of contaminants than workable for mechanical recycling as recyclable material.The Petretec process uses chemical reactions to essentially “unzip” the polyester molecules.
The Petretec process15-19 begins with a determination of the contaminant levels in the scrap PET.Any PET which contains metals, dyes, or other materials which would interfere with the recycling are separated and destined for mechanical recycling, landfilling, or incineration. The remaining PET is dissolved in DMT at temperatures above 220oC. This forms a solution of the scrap PET in DMT. In a depolymerizing transesterification reaction the dissolved PET is reacted with methanol to produce the original monomers of the polymer as shown below. The reaction is conducted on an industrial scale in a methanolysis reactor at 260-300oC, at a pressure of 340-650kPa.
Image 046
From the methanolysis reactor the DMT and ethylene glycol (EG), mixed with excess methanol are passed through a methanol removal column. The methanol removed in this way is recycled in the process. The DMT and EG form an azeotrope that prevents their separation by distillation. To overcome this the Dupont chemists add methyl p-toluate (MPT) at this point in the Petretec process. This forms an azeotrope of MPT and EG, allowing the separation of the DMT from the other two components. The MPT/EG distillate forms a two-layer solution. The top layer is enriched in MPT and can be recycled in the process. The DMT is then fractionally distilled to increase its purity. This process is outlined in the schematic below.
Image 048

The depolymerization reaction that is the heart of the Petretec technology produces DMT and ethylene glycol, the molecules from which the PET was formed, not polyester flake, the product of mechanical recycling. The tremendous advantage of this is that these reproduced monomers are identical to those used as raw materials for the polymerization reaction. Therefore, there are no limits on the uses of the PET made from them. This results in a reduction of the dependence on petrochemicals for production. The brown side of PET is largely overcome by the green Petretec process.Dupont operates the Petretec process at their Cape Fear facility in North Carolina.The plant has a reprocessing capacity of 100 million pounds per year.2

Study Questions:

  1. What is the molecular composition range for each of the categories listed in the “Uses of Crude Oil”? What is meant by terms such as still gas, residual fuel oil, coke, etc.?
  1. What is octane number? What types of compounds have the highest octane numbers, what compound class has the lowest octane numbers? Why is this so?
  1. Draw a reasonable mechanism for the alkylation reaction that is part of the conversion processes involved in reforming of petroleum. What factors would determine whether the reaction involves free radicals or ionic intermediates?
  1. Outline a mechanism for the reactions between TA and ethylene glycol and DMT and ethylene glycol to produce PET.
  1. What would be the economic advantages of using TA (as PTA) in the production of PET, rather than DMT?
  1. Outline a synthesis of poly(butylenes)terephthalate from both TA and DMT. What are the major uses of PBT?
  1. Outline a reasonable mechanism for the transesterification reaction that is the heart of the Petretec process.
  1. What is an azeotrope? What is the boiling point range of an azeotrope ?

 

Suggestions for Further Study

1.Visit the web site for the American Petroleum Institute and research the basis of the API Gravity scale.What are the standards for the high and low ends of the scale? How was the scale developed? What do you think are the advantages of classifying crude oils in this way?
2.Visit the web site of the British Petroleum Company (http://www.bp.com) and using their World Energy Report determine the distribution of the proven petroleum reserves on each continent by country.

References

  1. Morse, P.M., PET Producers Face Rough Transition Maket. Chem. Eng. News, July 22, 1998, pp 33-35.
  1. Cann, M.C., Connelly, M.E., “Dupont Petretec Polyester Regeneration Technology” in Real-World Cases in Green Chemistry. American Chemical Society (2000).
  1. Hunt, J. Petroleum Geochemistry and Geology, 2nd Edition, W. H. Freeman and Company (1996) in Chapter 3, Petroelum and its Products.
  1. Facts and Figures on Oil, American Petroleum Institute, www.api.org/faqs, accessed August 2000.
  1. What a Barrel of Oil Makes. American Petroleum Institute, www.api.org/edu/factsoil.htm#barrel, accessed August 2000.
  1. Statistical Review of World Energy, British Petroleum Company, www.bp.com/world energy/, accessed July 2000.
  1. Facts & Figures for the Chemical Industry, Chem. Eng. News, June 26, 2000.
  1. Chemical Profile of p-xylene. Fobchemicals.com at www.chemexpo.com/news/PROFILE980515.cfm, accessed July 2000.
  1. Kent, James A., ed., Riegel’s Handbook of Industrial Chemistry, 8th Edition. Van Nostrand Reinhold Company (1983).
  1. Austin, George T. Shreve’s Chemical Process Industries, 5th Edition. McGraw-Hill Book Company (1984).
  1. Chenier, Philip J., Survey of Industrial Chemistry, 2nd Revised Edition. VCH Publishers (1992).
  1. PET Facts, National Association for PET Container Resources. www.napcor.com/toolbox/funfacts.html, accessed July 2000.
  1. “It Starts with a Little Imagination”, Dupont Magazine, November/December 1996 www.Dupont.com/corp/products/dupontmag/novdec96/petretec.html, accessed July 2000.
  1. The Petretec Process of Dupont was a nominee for a 1997 Presidential Green Chemistry Challenge Award. More information about the program is available at the program web site at www.epa.gov/greenchemistry, accessed July 2000.
  1. Michel, R. Methanolysis of PET. ARC’96 Technol. Spark Recycl. Conf. Proc., 3rd, 349-356. Walling, J., Walling, R. L., eds., Society of Plastics Engineers, Brookfield, CT (1996).
  1. Michel, R., Jones, P., Everhart, D.. Dupont Polyester Regeneration Technology, a proposal submitted to the Presidential Green Chemistry Challenge Awards Program, 1997.
  1. Michel, R. E., Recovery of Methyl Esters of Aromatic Acids and Glycols from Thermoplastic Polyester Scrap Using Methanol Vapor. Eur. Patent 484,963, May 13, 1992.
  1. Hepner, R.R., Michel, R.E.. Process for the Separation of Glycols from Dimethyl Terephtahlate. U.S. Patent 5,391,263, Feb. 21, 1995.
  1. Michel, R.E., Recovery of Dimethyl Terephthalate from Polymer Wastes. U.S. Patent 5,504,122, April 2, 1996.