ETHYLENE GLYCOL ( HOCH2CH2OH)
Ethylene glycol was first prepared by Wurtz in 1859 by hydrolysis of ethylene glycol diacetate. It was used in world war I, when it was used as a substitute for glycerol in explosives manufacture. It is a colorless, odorless, hygroscopic liquid completely miscible with water and many organic liquids. Ethylene glycol markedly reduce the freezing point of water.
Its unique chemical behavior is attributed to the presence of hydroxyl groups in adjacent carbon atoms. Ethylene glycol is esterified with terephthalic acid or transesterfied with dimethyl terephthalate to form bis (hydroxyethyl) terephthalate (BHET)
USES
1)About 40% of the ethylene glycol produced domestically is used as a non-volatile antifreeze for liquid cooled motor vehicles.
2) Approximately 35% of the total production is consumed in the manufacture of
polyester fibre and film.
3)The remaining 25% of ethylene glycol is used in a variety of applications. It
provides freeze-thaw stabilization to latex coatings and improves flexibility and drying time of oil-based paints containing alkyd resins.
4)It is also used as a heat transfer fluid, in aircraft and runaway deicing mixtures, as
a dehydrating agent for natural gas, in motor oil additives, and as an additive in the formulation of inks, pesticides, wood stains, adhesives and many other products.
5)In explosives water gels and slurries, it lowers the freezing point and acts as an
coupling agent between water and other components.
6)A minor but important use for high purity ethylene glycol is a solvent and
suspending medium for ammonium perborate, the conductor in practically all
electrolyte capacitors.
MANUFACTURE
The first commercial application of the lefort direct ethylene oxidation to ethylene oxide followed in 1937, and from then on hydrolysis of ethylene oxide became and remains the main commercial source of ethylene glycol.
FLOWSHEET
water recycle Refined ethylene
1)
glycol
2)
a) b) c) d
Higher
glycols
1- ethylene oxide
2- water
a - reactor
b - evaporator series
c - drying still
d - refining still
Ethylene oxide-water mixture is preheated to 200 C whereby ethylene oxide is converted to ethylene glycol. Di, tri, tetra and polyethylene glycols are also produced, but with respective decreasing yields. The formation of these higher homologus is inevitable because ethylene oxide reacts with ethylene glycols more quickly than with water; their yields can be minimized if an excess of water is used.
After leaving the reactor, the product mixture is purified by passing it through successive distillation columns with decreasing pressure. Water is first removed and returned to the reactor, the mono, di and triethylene glycols are then separated by vacuum distillation. The evaporator in series has a function of removing water and recycling back to the reactor. In the distillation column, ethylene glycol is separated at the top from higher glycol.
As shown in the flowsheet, there is one distillation column where the separation of ethylene glycol takes place from higher glycols. If two or three distillation columns are used, then the mono, di and tri glycols can be separated easily according to there boiling point.
Ethylene oxide hydrolysis proceeds with either acid or base catalysis or uncatalyzed in neutral medium. In acid catalyzed hydrolysis, protonation of oxide activates it for reaction with water. The reaction is conducted with large excess of water at moderate temperatures to obtain 85-90% monoethylene glycol selectivity.
Base catalysis results in considerably lower monoethylene glycol selectivity. Neutral hydrolysis (pH 6-10), conducted in the presence of a large excess of water at high temperature and pressure, increases glycol selectivity to 89-91%.
ALTERNATIVE METHODS
Much research has been carried out to improve this process. Many catalysts have been described in literature that are able to optomize selectivity or lower the reaction temperature and excess of water.
Catalyst that improve selectivity include molybdates, vanadates, ion exchangers and organic antimony compounds.
Synthesis of ethylene glycol via the intermediate ethylene carbonate seems to be a promising alternative. This compound is obtained in higher yield (98%) by reacting ethylene oxide with carbon dioxide. Only double the molar quantity of water is required for this reaction.
According to a halcon patent, ethylene oxide can be extracted from aqueous solution, formed during its production with supercritical carbondioxide. An ethylene oxide- carbon dioxide solution is obtained which reacts to form ethylene carbonate. Hydrolysis of ethylene carbonate yields ethylene glycol.
Possible catalyst for this reaction are quaternary ammonium and phosphonium salts. A more recently developed catalyst system is based on use of Pd(II) complexes. A mixture of PdCl2, LiCl and NaNO3 in acetic acid and acetic anhydride has been shown to give a 95% selectivity
During this process, Pd(II) is reduced to Pd(0). If PdCl2-CuCl2-CuOCOCH3 is used, rteaction proceeds under mild conditions without formation of a precipitate. A yield of over 90% is obtained.
Other alternatives are as discussed below:
1)ethylene glycol can be produced by reaction of formaldehyde with carbon monoxide. This route first produces glycolic acid which is converted by esterification and hydrogenolysis to ethylene glycol.
HCHO +CO + H20 HOCH2COOH
HOCH2COOH +ROH HOCH2COOR + H20
HOCH2COOR + 2H2 HOCH2CH2OH + ROH
2)More recently Halcon international developed an efficient, direct ethylene oxidation process to produce ethylene glycol via hydrolysis of its acetates. This process employs a tellurium oxide-bromide catalyst in acetic acid to yield a mixture of ethylene glycol, mono and diacetates.
3)Oxidation of ethylene to ethylene glycol in an aqueous medium using an iron copper catalyst.
Fe-Cu
CH2=CH2 + ½ O2 + H20 HOCH2CH2OH
4)rhodium catalyzed production of ethylene glycol from synthesis gas( mixture of
carbon monoxide and hydrogen)
rhodium
2CO + 3H2 HOCH2CH2OH
REFERENCES:
1)KIRK OTHMER ENCYCLOPEDIA OF CHEMICAL ENGINEERING
2)ULLMANN,S ENCYCLOPEDIA