Petrochemical products

Petrochemicals are organic chemicals derived from petroleum or natural gas. They are of extreme importance in contemporary life, accounting for the production of almost all plastics, other synthetic materials, and organic chemicals. Although an enormous variety of organic chemicals can be (and are) made from petroleum or natural gas, usually the term “petrochemicals” is restricted to those substances produced in very large amounts.

Background

The origin of the petrochemical industry may be traced to the first production of isopropyl alcohol from propylene in 1920. This effort was originated by the Standard Oil Company in New Jersey. The industry grew slowly but steadily during the 1920s and 1930s and then received an enormous boost from World War II, with its tremendous demand for synthetic materials. By about 1950 the industry was firmly established in the United States.

Ethylene and Polyethylene

The most important petrochemical is ethylene. It is manufactured in greater quantity than any other organic chemical. Various raw materials can be used to manufacture ethylene, including ethane, propane, and distillates such as naphtha. Regardless of the raw material, the ethylene production process involves thermally driven reactions (so-called cracking) in a temperature range between 750° and 900° Celsius. Steam is used to dilute the feed to the ethylene production furnace. The amount of steam used varies, depending on the specific material used to make the ethylene. The annual worldwide production of ethylene exceeds 100 million metric tons.

About half of the ethylene produced is converted to polyethylene. The two major types of polyethylene are known as low-density polyethylene (often abbreviated as LDPE) and high-density polyethylene (HDPE). One of the most important applications of LDPE is in clear plastic wrapping film. HDPE has a wider range of uses by virtue of its superior mechanical properties. Familiar applications of HDPE include bottles, such as those used for laundry detergents, and housewares, such as storage crates and home cleaning accessories such as buckets, pans, and pails.

Vinyl Chloride and Ethylene Glycol

A second major use of ethylene is its conversion to vinyl chloride. This conversion is effected by a process called oxychlorination: reaction of ethylene with hydrogen chloride and oxygen. Vinyl chloride is used in the manufacture of polyvinyl chloride, or poly, most commonly known as PVC. Depending on how the PVC is produced (specifically, through the addition of “plasticizers” that alter its physical or mechanical properties), it can have a range of and flexibility. Consequently, PVC is a versatile material with many common uses that include floor tile, garden hose, artificial leather, house siding, plastic films, pipe, and toys. In the days when music was recorded on phonograph records, they were usually made of PVC—hence the slang term “vinyl.”

The of ethylene produces ethylene oxide, a chemical that easily reacts with water to form ethylene glycol, a useful component of antifreeze. Ethylene glycol is also used in the manufacture of polyethylene terephthalate, commonly known as PET. This polymer is an example of the largest class of synthetic textile fibers, the polyesters. PET is also used for both audio and video magnetic recording tapes, in soft drink bottles, and in “microwave-in-a-pouch” food containers.

Propylene, Polypropylene, and Propylene glycol

Propylene is the second most important of the petrochemicals. Although ethylene superseded it in importance (in terms of tonnage production), propylene was the first significant petrochemical. In the 1920s and 1930s, propylene was a by-product of gasoline manufacture. To increase the yield of gasoline from a refinery, other petroleum products of lower value were subjected to intense heating (thermal cracking), which broke the molecules into new, smaller compounds, many of which could be used in gasoline. In addition, however, thermal cracking led to some by-products, such as propylene, of molecular size even smaller than gasoline. The beginning of the petrochemical industry was the use of this by-product propylene for producing isopropyl alcohol. Most people encounter isopropyl alcohol primarily as the active ingredient in “rubbing alcohol,” but it has more important uses as an industrial solvent and as raw material for making acetone, another useful solvent.

Today the propylene situation is greatly changed. The thermal cracking process for gasoline is obsolete, so there is no by-product propylene. Instead, propylene is made in much the same way as ethylene, using either propane or naphtha as the raw material. The raw material, mixed with steam, is cracked at temperatures of 800° to 900° Celsius. The dominant use of propylene is in the production of polypropylene.

The properties of polypropylene—and consequently its uses—depend heavily on the way the propylene molecules are connected. Special catalysts to control the outcome of the polymerization of polypropylene were discovered by Karl Ziegler and Giulio Natta, for which achievement they were awarded the 1963 Nobel Prize in Chemistry. A common application for high-quality polypropylene is in microwave-safe dishes and food containers. Some of the lower-strength grades of polypropylene are useful as flexible, clear plastic films—for example, as food wrap and as the plastic coverings on disposable diapers. Polypropylene and “copolymers” of polypropylene and polyethylene are widely used as materials in automobiles. Examples of automotive applications include bumper covers, air ducts, body trim panels, interior trim and seat covers, and battery casings.

Propylene can also be converted to propylene and then to propylene glycol. This material is used directly in antifreeze, brake fluid, and hydraulic fluid. It is also used as a moisturizer in pet foods and tobacco products. Propylene glycol is converted to a special family of compounds called urethanes, the basic materials for the production for polyurethane products. Many kinds of urethanes can be made from propylene glycol, depending on the chemicals chosen for the process. Consequently, the eventual polyurethanes have, as a family, a wide range of properties. Common applications of polyurethanes include sound and heat insulation, furniture cushions, automobile bumpers, and plastic flooring and roofing.

Acrylics, Polyacrylates, and Polyacrylonitrile

A more severe oxidation of propylene leads to acrylic acid, the starting material for acrylic paints. Sodium or ammonium salts of acrylic acid polymerize to the polyacrylates. When polyacrylates are mixed with small amounts of other copolymers, they form polyacrylate “super-absorbing” polymers that have an exceptional capacity for absorbing water or water solutions. The major use of these remarkable materials is in the lining of disposable diapers.

The reaction of propylene with ammonia in the presence of oxygen (“ammoxidation”) forms acrylonitrile. This is the starting material for polyacrylonitrile, or PAN. Acrylic textiles, such as Acrilan and Orlon, amount to about 20 percent of all synthetic fibers produced. PAN is also used to make carbon fibers. Initially, PAN-based carbon fibers were extremely expensive (about $100 per kilogram), so they were limited to military and aerospace applications. As an example, about 10 percent of the weight of an F-18 fighter aircraft is PAN-based materials. Other applications include use in the space shuttle’s cargo bay doors and in nozzles in the shuttle’s rockets. Improved manufacturing know-how reduced the cost of carbon fibers significantly, and carbon- or graphite-fiber items are increasingly available to consumers; among them are graphite tennis rackets and golf clubs.

The BTX Compounds and Styrene

Catalytic reforming of petroleum, a process used to enhance the octane number of gasoline, produces as by-products the family of compounds benzene, toluene, and xylene, sometimes lumped together and called BTX. They are high-tonnage materials, but not as important as ethylene and propylene.

Benzene and ethylene react to produce ethylbenzene, which is converted to styrene. Styrene is the raw material for making polystyrene. Polystyrene is another example of the petrochemical products that seem ubiquitous in modern life. Applications of polystyrene include Styrofoam food cartons (such as those used for eggs in supermarkets), cups and food packaging at fast food restaurants, plastic utensils, toys, and the foam “peanuts” used as packaging material. Polystyrene and other uses of benzene are so important that the major use of toluene is conversion to benzene. Xylenes are used as solvents. One particular xylene, para-xylene, is converted to terephthalic acid. This compound, reacted with ethylene glycol (described above), produces PET.

The petrochemical industry has an immense economic impact, both in the United States and worldwide. In the United States, twenty-nine of the top fifty industrial chemicals are organic (though not all are petrochemicals). In addition to its economic impact, the petrochemical impact experiences continued criticism from activists and some scientists. They argue that the exposure to the chemicals used in the manufacturing of petrochemicals significantly increases the risk of developing cancer. Additionally, they argue that the manufacturing process releases significant quantities of greenhouse gases, accelerating global climate change.

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