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Home Mono Ethylene Glycol (MEG)

Ethylene glycol

Ethylene glycol (IUPAC name: ethan-1,2-diol) is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid. Ethylene glycol is toxic, and ingestion can result in death.

Ethylene glycol is not to be confused with diethylene glycol, a heavier ether diol, or with polyethylene glycol, a nontoxic polyether polymer.

Production

Historical aspects and natural occurrence

Ethylene glycol was first prepared in 1859 by the French chemist Charles-Adolphe Wurtz from ethylene glycol diacetate via saponification with potassium hydroxide and, in 1860, from the hydration of ethylene oxide. There appears to have been no commercial manufacture or application of ethylene glycol prior to World War I, when it was synthesized from ethylene dichloride in Germany and used as a substitute for glycerol in the explosives industry.

In the United States, semicommercial production of ethylene glycol via ethylene chlorohydrin started in 1917. The first large-scale commercial glycol plant was erected in 1925 at South Charleston, West Virginia, by Carbide and Carbon Chemicals Co. (now Union Carbide Corp.). By 1929, ethylene glycol was being used by almost all dynamite manufacturers.

In 1937, Carbide started up the first plant based on Lefort's process for vapor-phase oxidation of ethylene to ethylene oxide. Carbide maintained a monopoly on the direct oxidation process until 1953, when the Scientific Design process was commercialized and offered for licenses.

This molecule has been observed in outer space.

Current methods

Ethylene glycol is produced from ethylene (ethene), via the intermediate ethylene oxide. Ethylene oxide reacts with water to produce ethylene glycol according to the chemical equation:

C2H4O + H2O → HO–CH2CH2–OH

This reaction can be catalyzed by either acids or bases, or can occur at neutral pH under elevated temperatures. The highest yields of ethylene glycol occur at acidic or neutral pH with a large excess of water. Under these conditions, ethylene glycol yields of 90% can be achieved. The major byproducts are the ethylene glycol oligomers diethylene glycol, triethylene glycol, and tetraethylene glycol. About 6.7 billion kilograms are produced annually.

A higher selectivity is achieved by use of the OMEGA-Process. In the OMEGA process, the ethylene oxide is first converted with carbon dioxide (CO2) to ethylene carbonate to then react with water in a second step to selectively produce mono-ethylene glycol. The carbon dioxide is released in this step again and can be fed back into the process circuit. The carbon dioxide comes in part from the ethylene oxide production, where a part of the ethylene is completely oxidized.

Uses

The major end uses of ethylene glycol are to make PET melt, which accounted for 86% of total ethylene glycol consumption in 2009, and antifreeze, at around 7% of total consumption. Because this material is cheaply available, it finds many niche applications.

Coolant and heat transfer agent

The major use of ethylene glycol is as a medium for convective heat transfer in, for example, automobiles and liquid cooled computers. Ethylene glycol is also commonly used in chilled water air conditioning systems that place either the chiller or air handlers outside, or systems that must cool below the freezing temperature of water. In geothermal heating/cooling systems, ethylene glycol is the fluid that transports heat through the use of a geothermal heat pump. The ethylene glycol either gains energy from the source (lake, ocean, water well) or dissipates heat to the source, depending if the system is being used for heating or cooling.

Pure ethylene glycol has a specific heat capacity about one half that of water. So, while providing freeze protection and an increased boiling point, ethylene glycol lowers the specific heat capacity of water mixtures relative to pure water. A 50/50 mix by mass has a specific heat capacity of about 0.75 BTU/lb F, thus requiring increased flow rates in same system comparisons with water. Additionally, the increase in boiling point over pure water inhibits nucleate boiling on heat transfer surfaces thus reducing heat transfer efficiency in some cases, such as gasoline engine cylinder walls. Therefore, pure ethylene glycol should not be used as an engine coolant in most cases.

Antifreeze

Due to its low freezing point and tendency to form glasses, ethylene glycol resists freezing. A mixture of 60% ethylene glycol and 40% water does not freeze until temperatures below −45 °C (−49°F). Diethyleneglycol behaves similarly. It is used as a deicing fluid for windshields and aircraft. The antifreeze capabilities of ethylene glycol have made it an important component of vitrification (anticrystallization) mixtures for low-temperature preservation of biological tissues and organs.

Ethylene glycol disrupts hydrogen bonding when dissolved in water. Pure ethylene glycol freezes at about −12°C (10.4°F), but when mixed with water molecules, neither can readily form a solid crystal structure, and therefore the freezing point of the mixture is depressed significantly. The minimum freezing point is observed when the ethylene glycol percent in water is about 70%, as shown below. This is the reason pure ethylene glycol is not used as an antifreeze—water is a necessary component as well.

Ethylene glycol freezing point vs. concentration in water
Weight Percent EG (%) Freezing Point (deg F) Freezing Point (deg C)
0 32 0
10 25 -4
20 20 -7
30 5 -15
40 -10 -23
50 -30 -34
60 -55 -48
70 -60 -51
80 -50 -45
90 -20 -29
100 -10 -12

However, the boiling point for aqueous ethylene glycol increases monotonically with increasing ethylene glycol percentage. Thus, the use of ethylene glycol not only depresses the freezing point, but also elevates the boiling point such that the operating range for the heat transfer fluid is broadened on both ends of the temperature scale. The increase in boiling temperature is due to pure ethylene glycol having a much higher boiling point and lower vapor pressure than pure water; there is no chemical stabilization against boiling of the liquid phase at intermediate compositions, as there is against freezing.

Ethylene glycol boiling point vs. concentration in water
Weight Percent EG (%) Boiling Point (deg F) Boiling Point (deg C)
0 212 100
10 215 102
20 215 102
30 220 104
40 220 104
50 225 107
60 230 110
70 240 116
80 255 124
90 285 140
100 387 197

Precursor to polymers

In the plastics industry, ethylene glycol is important precursor to polyester fibers and resins. Polyethylene terephthalate, used to make plastic bottles for soft drinks, is prepared from ethylene glycol.

Hydrate inhibition

Because of its high boiling point and affinity for water, ethylene glycol is a useful desiccant. Ethylene glycol is widely used to inhibit the formation of natural gas clathrates (hydrates) in long multiphase pipelines that convey natural gas from remote gas fields to an onshore processing facility. Ethylene glycol can be recovered from the natural gas and reused as an inhibitor after purification treatment that removes water and inorganic salts.

Natural gas is dehydrated by ethylene glycol. In this application, ethylene glycol flows down from the top of a tower and meets a rising mixture of water vapor and hydrocarbon gases. Dry gas exits from the top of the tower. The glycol and water are separated, and the glycol recycled. Instead of removing water, ethylene glycol can also be used to depress the temperature at which hydrates are formed. The purity of glycol used for hydrate suppression (monoethylene glycol) is typically around 80%, whereas the purity of glycol used for dehydration (triethylene glycol) is typically 95 to more than 99%. Moreover, the injection rate for hydrate suppression is much lower than the circulation rate in a glycol dehydration tower.

Niche applications

Minor uses of ethylene glycol include the manufacture of capacitors, as a chemical intermediate in the manufacture of 1,4-dioxane, and as an additive to prevent corrosion in liquid cooling systems for personal computers. Ethylene glycol is also used in the manufacture of some vaccines, but it is not itself present in these injections. It is used as a minor (1–2%) ingredient in shoe polish and also in some inks and dyes. Ethylene glycol has seen some use as a rot and fungal treatment for wood, both as a preventative and a treatment after the fact. It has been used in a few cases to treat partially rotted wooden objects to be displayed in museums. It is one of only a few treatments that are successful in dealing with rot in wooden boats, and is relatively cheap. Ethylene glycol may also be one of the minor ingredients in screen cleaning solutions, along with the main ingredient isopropyl alcohol. Ethylene glycol is commonly used as a preservative for biological specimens, especially in secondary schools during dissection as a safer alternative to formaldehyde. It can also be used in killing jars. It is also used as part of the water-based fluid used to control subsea oil and gas production equipment.

Chemical reactions

Ethylene glycol is used as a protecting group for carbonyl groups in organic synthesis. Treating a ketone or aldehyde with ethylene glycol in the presence of an acid catalyst (e.g., p-toluenesulfonic acid; BF3•Et2O) gives the corresponding a 1,3-dioxolane, which is resistant to bases and other nucleophiles. The 1,3-dioxolane protecting group can thereafter be removed by further acid hydrolysis. In this example, isophorone was protected using ethylene glycol with p-toluenesulfonic acid in moderate yield. Water was removed by azeotropic distillation to shift the equilibrium to the right.

Toxicity

Ethylene glycol is moderately toxic with an oral LDLO = 786 mg/kg for humans. The major danger is due to its sweet taste. Because of that, children and animals are more inclined to consume large quantities of it than of other poisons. Upon ingestion, ethylene glycol is oxidized to glycolic acid which is, in turn, oxidized to oxalic acid, which is toxic. It and its toxic byproducts first affect the central nervous system, then the heart, and finally the kidneys. Ingestion of sufficient amounts can be fatal if untreated.

According to the annual report of the American Association of Poison Control Centers' National Poison Data System in 2007, there were about 1000 total cases resulting in 16 deaths. The 2008 American Association of Poison Control Centers' National Poison Data System annual report lists 7 deaths.(Toxicity, Ethylene Glycol)

In the environment

  • The primary source of ethylene glycol in the environment is from run-off at airports where it is used in deicing agents for runways and airplanes. Ethylene glycol can also enter the environment through the disposal of products that contain it.
  • Ethylene glycol in air will break down in about 10 days.
  • Ethylene glycol in water and in soil will break down within several days to a few weeks.