Energy for Plastic

Curtis W. Hamman
October 24, 2010

Submitted as coursework for Physics 240, Stanford University, Fall 2010

Plastics in Modern Society

At the dawn of the 21st century, electrification, the automobile and the airplane were named the top 3 greatest engineering achievements of the 20th century by the National Academy of Engineering (NAE) [1]. High-performance materials, including metals, polymers, ceramics and composites, rounded out the list at #20. The availability of high-performance synthetic polymers, known as plastics, shaped the development of electrification, automobiles and airplanes. Electrification of common household appliances and the tightly-packed wiring inside computers often relies upon flexible plastic material for insulation. Similarly, plastic tubing is used in electrical conduits and buildings to protect electrical cables from moisture and corrosive substances. Light-weight plastics are also widely used in automotive fuel system components including pumps, tubing, fans and gas tanks, in addition, to the familiar feel of plastics in steering wheels, tires, and fiberglass windshields. Orville and Wilbur Wright built the first successful airplane using little more than Spruce wood, twine, and cotton fabric. Like the Wright Brothers, the success of the airplane industry has relied upon the availability of materials that are light-weight, strong and easy to machine, assemble and repair. By the early 1940's, plastic planes provided many advantages to the metal-riveted airframes of World War II (which had already seen the use of acrylic plastic in cockpit windows) that engineers invested heavily in their development. [2] Today, commercial aircraft routinely fly on wings made of plastic, including the new Boeing 787 Dreamliner whose airframe is nearly 50% carbon-fiber reinforced plastic composite by weight (up from 12% for the Boeing 777). [3]

Hydrocarbon Feedstocks

Five of the most widely produced plastics by mass are polyethylene, polypropylene, polyvinyl chloride (PVC), polystyrene, and polyethylene-terephthalate (PET). [4,5] Together, they account for roughly 80% of North American production with other thermoplastics and thermosets, such as nylon, epoxies, latex, and other resins accounting for the remainder. Polyethylene alone (including LDPE, LLDPE and HDPE) accounts for nearly 40% of plastic production in North America with an annual production of roughly 16 million metric tons. [4,5] As its name suggests, the basic building block of polyethylene is ethylene, which is also used to synthesize many other plastics and industrial chemicals such as ethylbenzene, a precursor to polystyrene, and ethylene dichloride, a precursor to PVC. [6-8] As a result, the means by which ethylene is produced are central to the production of plastics today.

Despite the widespread use of plastic in modern society, plastics are a relatively new engineering development. In 1940, annual U.S. ethylene production was about 140,000 metric tons; by 1976, production had risen to 10 million metric tons. [9] Readily available raw materials, known as feedstocks, were needed to meet this demand. Since hydrogen and carbon are the key elements in plastics such as polyethylene (whose monomer, ethylene, has a chemical formula of C2H4), their rapid production required the availability of hydrocarbon feedstocks. [6-8] Possible hydrocarbon feedstocks include recently dead biological material known as bio-mass and long dead biological sediments known as fossil fuels, which includes natural gas, petroleum and coal. A few specialty plastics, such as cellophane, are made from bio-mass, but most plastics are not made from bio-feedstock sources today. [10] Today, most plastics are made from feedstock derived from crude oil and natural gas. [6,8,9] Ethylene and other similar building blocks of plastics such as propylene, however, are not found in significant concentrations in modern geologic sediments. [11] To address this, new technologies were developed to synthesize and separate ethylene (and other monomers) from petroleum. Development of petrochemical technologies in the mid-twentieth century, which is #18 on the NAE list of greatest engineering achievements, provided access to cheap and abundant hydrocarbon feedstock resources and the means to synthesize the basic building blocks that fueled the rise of a plastic society. [1,8-10]

Today, the chief feedstock for ethylene is still found in the byproducts of oil and gas refineries, namely petroleum fractions and natural gas liquids such as propane and ethane. [6,8,9] Most byproducts of oil and gas refineries are heavy hydrocarbons with many carbon atoms per molecule, e.g. kerosene and naphtha, or saturated hydrocarbons with no carbon-carbon double bonds, e.g. ethane and propane. These byproducts are typically of little commercial value in their unprocessed form. Lightweight hydrocarbons such as gasoline for internal combustion engines and simple, unsaturated hydrocarbons such as ethylene for plastics are preferred. As a result, these complex, high-molecular weight byproducts must be separated and processed in order to make use of these simpler, more useful hydrocarbons. [1,8] For example, this may be accomplished by steam cracking of ethane and propane as well as fluid catalytic cracking of petroleum fractions. Low-temperature distillation and dehydrogenation of ethane and propane is also used to synthesize ethylene and propylene. The abundance of domestic supplies of natural gas and crude oil often dictate the preferred feedstock and method for ethylene production in a given region. For example, in North America and the Middle East, ethylene is typically produced from readily available natural gas resources while, in Europe and Asia, cracking of crude oil is more common. [6,8,9] Technological investment provided the means to synthesize ethylene from petroleum-based hydrocarbon feedstocks. Such methods were an instrumental development in the petrochemical industry that largely enabled the production of plastics almost exclusively from existing hydrocarbon feedstocks. [1,8,9]

Estimating Feedstock Energy

To estimate the hydrocarbon feedstock energy used to make all plastics, the energy content of polyethylene plastic, which alone accounts for nearly 40% of plastics production by mass, is a key factor. [4,5] Calorimetry experiments have measured the net heat of combustion (LHV) for polyethylene to be about 4.5 x 107 J/kg, which is higher than other major plastics. [12] No distinction is made between different grades of polyethylene in the following analysis. Recall that ethylene, the key building block for polyethylene, is derived from primarily two hydrocarbon feedstocks: natural gas and crude oil, of which ethane is a key product for ethylene production. The actual composition and refining of these hydrocarbon feedstocks to extract ethane and ethylene varies widely, but the LHV for ethane is 4.8 x 107 J/kg while the LHV for ethylene is 4.7 x 107 J/kg, which are both about the same as for polyethylene. [13] This indicates that the energy of the feedstock is largely sequestered in the polyethylene polymer. Therefore, for simplicity, we will work backwards from reported figures for the total mass of polyethylene plastic (in kg) produced in a given year and multiply by the net heat of combustion for polyethylene (in J/kg) to estimate the total hydrocarbon feedstock energy (in J) diverted to produce polyethylene plastic that year. To provide an upper estimate for all plastics, this figure for polyethylene is divided by 0.4 since about 4 kg of polyethylene plastic were made for every 10 kg of plastic in 2008. [4,5]

This upper estimate makes many assumptions. To begin, not all plastics are chiefly composed of hydrogen and carbon like polyethylene. For example, PET is about 33% oxygen by mass while PVC is about 57% chlorine by mass. [13] These additives come from non-hydrocarbon feedstocks. To account for the hydrocarbon feedstock energy, observe that three of the five most widely produced plastics, namely polyethylene, polypropylene and polystyrene, together account for nearly 60% of all plastics production. [4,5] Both polypropylene and polystyrene are pure hydrocarbon polymers with similar heats of combustion as their feedstock counterparts and polyethylene. [12,13] A lower estimate for the fraction of annual hydrocarbon resources diverted to plastics production is then found by multiplying the upper estimate by 0.6. Both the upper and lower estimates are expected to be of the same order of magnitude as the actual figure. Note that non-hydrocarbon materials and inorganic chemical additives are not accounted for in this feedstock energy analysis. Furthermore, any losses of feedstock in the refining process are not directly accounted for in this analysis (e.g. conversion of ethane into coke via pyrolysis and other refinery byproducts). [6,8] Once these estimates for hydrocarbon feedstock energy used to make plastic are added to the reported figures for the non-feedstock energy used in the plastics industry, upper and lower estimates of the energy for plastic are found. The energy for plastic is then compared to reported numbers for global and U.S. primary energy consumption, which is the net sum of energy from petroleum, natural gas, coal, renewable, and nuclear resources consumed in the transportation, industrial, residential and commercial, and electric power sectors. Note that much of the production and process information is based on census sources and sparse industry data, which may be incomplete owing to non-disclosure of trade secrets, lack of non-proprietary information, regional variations, inconsistent use of unit conversion factors, lack of information about the internal algorithms and sources used in most estimates, and the overall complexity of the chemical industry.

Total Energy Estimates

Estimates for the amount of polyethylene produced world-wide in 2008 range from about 50 to 110 million metrics tons depending on the source. [5,14] We will use 80 million metrics tons for global production of polyethylene in the following analysis, which is five times that used for U.S. production in the next paragraph. Therefore, about (8.0 x 1010 kg)(4.5 x 107 J/kg) = 3.6 x 1018 J of energy were diverted to polyethylene-based plastics in 2008. This translates to an upper estimate of (3.6 x 1018 J)/0.4 = 9.0 x 1018 J per year for all plastics assuming world-wide production of plastics was 40% polyethylene by mass. The lower estimate predicts 0.6(3.6 x 1018 J)/0.4 = 5.4 x 1018 J per year. The world-wide consumption of hydrocarbons (i.e. natural gas, coal, and oil) amounted to about 4.2 x 1020 J in 2008. [15] Thus, between 1.3% and 2.1% of primary hydrocarbon resources consumed each year are diverted to hydrocarbon feedstocks for the production of plastics world-wide. For economic reasons, coal is not currently used as a feedstock for plastics production today. [6,8,9,16] If we neglect coal, then about 2.8 x 1020 J of oil and natural gas resources were consumed in 2008. [15] From this, between 1.9% and 3.2% of the oil and natural gas resources consumed in 2008 were diverted for use as feedstock for plastics world-wide.

In 2008, U.S. production of oil and gas resources amounted to about 3.5 x 1019 J while consumption reached 6.3 x 1019 J owing to the influx of oil imports. [15] In 2008, U.S. production of ethylene was about 23 million metric tons and production of polyethylene was about 16 million metric tons. [4,5] From these figures, an upper estimate for the hydrocarbon feedstock energy used to produce all plastics is (1.6 x 1010 kg)(4.5 x 107 J/kg)/0.4 = 1.8 x 1018 J. The lower estimate is then 0.6(1.8 x 1018 J) = 1.1 x 1018 J. Thus, based on the chemical potential energy for plastic materials and reported plastic production numbers, between 3.1% and 5.1% of domestic U.S. oil and gas production and between 1.7% and 2.9% of U.S. oil and gas consumed each year is used as hydrocarbon feedstock energy for plastics. To check this estimate, reported figures for the feedstock energy used by the "Plastic Materials and Resins" industry (NAICS 325211) in 2006 was 1198 trillion Btu or 1.26 x 1018 J, which is within the upper and lower estimates using 2008 figures. [17]

In 2006, approximately 349 trillion Btu, or about 3.7 x 1017 J, were used to manufacture plastics (NAICS 325211) not including the energy content of the hydrocarbon feedstocks. [17] Additionally, 336 trillion Btu, or about 3.5 x 1017 J, were used to manufacture plastic products (NAICS 326). [17] Note that the sum of these two industry categories likely does not include the non-feedstock energy used to synthesize the base petrochemicals, such as ethylene. The relevant non-feedstock figures for the petrochemical industry (NAICS 325110) were withheld from to avoid disclosure of trade secret information [17]. Additionally, these numbers may also not account for the energy used to manufacture plastic materials that originate as secondary products of other industries. As a result, the estimate of 7.2 x 1017 J for non-feedstock energy of both NAICS 325211 and NAICS 326 may significantly underestimate the actual figure used to make plastics. Using a U.S. plastics production figure of 40 million metric tons in 2008 [4,5], this translates to (7.2 x 1017 J)/(4.0 x 1010 kg) = 1.8 x 107 J/kg of plastic, which is between about one-half and one-third that suggested for the process energy requirements for most plastics. [6,8,16] We will then assume an average U.S. process energy requirement (i.e. non-feedstock energy) of between 3.6 x 107 J/kg and 5.4 x 107 J/kg of plastic. Using an estimate of 40 million metric tons for U.S. plastics production in 2008 [4,5], the non-feedstock energy used to make plastic has a low estimate of (4.0 x 1010 kg)(3.6 x 107 J/kg) = 1.4 x 1018 J and a high estimate of (4.0 x 1010 kg)(5.4 x 107 J/kg) = 2.2 x 1018 J. If we assume that the non-feedstock energy and production numbers in 2006 are comparable to that in 2008, then the total energy for plastics production in the U.S. in 2008 is then the sum of the feedstock energy (between 1.1 x 1018 J and 1.8 x 1018 J) and non-feedstock energy (between 1.4 x 1018 J and 2.2 x 1018 J), which gives a low estimate of 2.5 x 1018 J and a high estimate of 4.0 x 1018 J. Primary energy consumption in the U.S. in 2008 was about 1.0 x 1020 J. [15,18] From these estimates, between 2.5% and 4.0% of total U.S. primary energy consumption in 2008 was due to the energy for plastic.


By multiplying the net heat of combustion for plastic (in J/kg) by the mass of plastic produced (in kg/year), upper and lower estimates for the total hydrocarbon feedstock energy (in J/year) used in the plastics industry were found. When added to the reported estimates for the non-feedstock energy used in the production of plastics, estimates of the energy for plastic were found and compared with the global and U.S. energy consumption. World-wide between 2.5% and 3.6% of oil and natural gas consumption was used as feedstock for plastics in 2008. The amount of hydrocarbon feedstock energy diverted to chemically produce plastic materials in the U.S. was between 1.1 x 1018 J and 1.8 x 1018 J in 2008, which is between 3.1% and 5.1% of the combustible energy content of U.S. oil and gas production. The energy-intensive processes used to synthesize key petrochemicals, produce plastics, and manufacture plastic products consumed between 1.4 x 1018 J and 2.2 x 1018 J in 2008. Total U.S. primary energy consumption was about 1.0 x 1020 J in 2008. Based on these estimates, the energy for plastic, including both hydrocarbon feedstock energy, energy used to refine feedstocks into base plastics, and the energy to manufacture plastic products, was between 2.5% and 4.0% of the total U.S. primary energy consumption in 2008.

© Curtis W. Hamman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


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