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Ethylene glycol (IUPAC name: ethane-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 (although it should be noted that ethylene glycol is toxic, and ingestion can result in death).

Production

Historical aspects and natural occurrence

Ethylene glycol was first prepared in 1859 by the Frenchmarker 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 was 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, via the intermediate ethylene oxide. Ethylene oxide reacts with water to produce ethylene glycol according to the chemical equation
C2H4O + H2O → HOCH2CH2OH


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.

Uses

Approximately 60% of ethylene glycol is consumed for antifreeze, and the remainder is mainly used as a precursor to polymers. 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.

Due to its low freezing point and tendency to form glasses, ethylene glycol resists freezing. The freezing point of a mixture of 60% ethylene glycol and 40% water freezes below -45 °C. 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 (anti-crystallization) 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 9 deg F, but when intermixed with water molecules, neither can form an efficient 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. 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.

This latter quality is in accordance with Raoult's Law, which predicts the decreased vapour pressure of the solvent (water) proportional to a decrease in its concentration. The numerical value of vapour pressure is a measure of the (temperature-specific) pressure at which there exists a thermodynamic equilibrium between the liquid and gaseous phases; that is, an equal rate of vaporisation and condensation.

For example, if the vapour pressure is 100 torr for any substance at a given temperature, that substance will predominantly vapourise if the true pressure of the environment is equal to or less than 100 torr. Conversely, if the environmental pressure is higher than the vapour pressure, condensation will be predominant.

Notwithstanding that the total vapour pressure for the system is the sum of individual vapour pressures, at 373K (100 deg C or 212 deg F) the vapour pressure gained by the addition of 1/10ths ethylene glycol is negligible (+4mmHg) in comparison to the loss of 1/10th water (-76mmHg) and thus does little to offset the aforementioned.

Vapor pressure is exponentially proportional to temperature. When temperature is reduced, vapour pressure is lowered for all substances; water itself has little appreciable vapour pressure at 0C. Thus, it is important to note that the mechanism which precludes boiling is NOT responsible for the anti-freeze properties observed at low temperatures. Appreciating that the mechanisms responsible are different will help one understand the apparently paradoxical, dualistic effects of ethylene glycol on water thermodynamics.

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 (mono-ethylene glycol) is typically around 80%, whereas the purity of glycol used for dehydration (tri-ethylene glycol) is typically 95-99+%. 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 specimens in schools, frequently during dissection. It is said to be safer than formaldehyde, but the safety is questionable.

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

The major danger from ethylene glycol is ingestion as it is somewhat toxic with LD50 = 1.4 g/kg for humans. Due to its sweet taste, children and animals will sometimes consume large quantities of it if given access to antifreeze. 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.

Industrial hazards

Ethylene glycol can begin to break down at 230° – 250°F (110° – 121°C). Note that breakdown can occur when the system bulk (average) temperature is below these limits, because surface temperatures in heat exchangers and boilers can be locally well above these temperatures.

The electrolysis of ethylene glycol solutions with a silver anode results in an exothermic reaction. In the Apollo 1 fire catastrophe a coolant consisting of ethylene glycol and water was implicated as a possible cause via this reaction.

References

  1. Siegfried Rebsdat1 and Dieter Mayer "Ethylene Glycol” in Ullmann’s Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. .
  2. Ethylene glycol. National Institute for Occupational Safety and Health. Emergency Response Database. August 22, 2008. Retrieved December 31, 2008.


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