Korean J. Chem. Eng., 18(5), 571-579 (2001) FEATURED REVIEW Ethylene Glycols Technology Robert Fulton Dye 3011 Fairway Drive, Sugar Land, TX 77478, USA Abstract−Ethylene glycol (EG) or monoethylene glycol (MEG), the adduct of ethylene oxide (EO) and water, is the simplest glycol. It is the first of a homologous series of three dihydroxy alcohols discussed in this article. Diethylene and triethylene glycols (DEG, TEG) are the other two. These glycols are composed solely of carbon, hydrogen and oxygen. Although they have similar chemical properties, their applications vary mainly with physical properties such as viscosity, hygroscopicity and boiling point. The commercial route to ethylene glycols in use today involves the non- catalyzed thermal hydrolysis of ethylene oxide in water. This process produces chiefly mono-, di- and triethylene glycols and a small amount of tetraethylene and heavier glycols. The yield of monoethylene glycol via hydrolysis is controlled by the water-to-ethylene oxide ratio in the feed to the reactor system. Removal of excess water following the glycols-forming hydrolysis is energy intensive and requires capital investment in evaporators. Such costs limit the amount of excess water which is used. In practice, reactor feed water content is such that the selectivity to monoethylene glycol achieved ranges from 89-91%. The equipment elements in a simplified process flow diagram are discussed along with recommended materials of construction. Among other items discussed are a) a brief review of economic factors; b) health, safety and environmental issues; and c) commercial applications of the three glycols, MEG, DEG and TEG. Finally, recommendations for shipping, handling and storage are discussed. Key words: Monoethylene Glycol, Thermal Hydrolysis, Properties, Toxicology/Environmental, Applications INTRODUCTION tached to separate carbon atoms in an aliphatic chain. Diethylene and triethylene glycols, and tetraethylene glycols, which some authors 1. General also discuss, are oligomers of ethylene glycol. Not discussed exten- Ethylene glycol (EG), the principal topic discussed in this article sively are polyglycols. Polyglycols are higher molecular weight ad- is also interchangeably referred to as monoethylene glycol (MEG). ducts of ethylene oxide distinguished by intervening ether linkages Ethylene glycol is the adduct of ethylene oxide (EO) and water and in the hydrocarbon chain. Polyglycols are commercially important is the simplest glycol, the first of a homologous series of three dihy- and have properties that vary with molecular weight, are water solu- droxy alcohols, namely, mono-, di- and triethylene glycols (MEG, ble, hygroscopic, and undergo reactions common to the lower mole- DEG and TEG). These glycols contain two hydroxyl groups at- cular weight glycols. Table 1. Properties of glycols Property Ethylene glycol Diethylene glycol Triethylene glycol Molecular weight 62.07 106.12 150.17 Boiling point@760 mmHg, oC 197.6 245.0 287.4 Specific gravity@20/20 oC 1.1155 1.1184 1.1254 Density, kg/ m3@40 oC 1099 1103 1109 Coefficient of expansion@20 oCper oC 0.00059 0.00063 0.00067 Flash point, cleveland open cup, oC 115 143 165 Freezing point at 760 mmHg, oC −13.0 −8.0 −7.2 o Refractive index, nD@20 C 1.4318 1.4472 1.4559 Viscosity, @20 oC, cp 20.9 35.7 47.8 Vapor pressure, mmHg@20 oC 0.06 <0.01 <0.01 Heat of vaporization@760 mmHg, cal/gm 191 83 99 Thermal conductivity, cal/(sec)(cm2) (oC/cm)@60 oC 0.000620 0.000398 0.000382 Surface tension, dynes/cm@20 oC 50.5 47.0 47.5 Specific heat, cal/gm/oC@20 oC 0.561 0.539 0.520 Water solubility complete complete complete Source: Shell Bull., 1993. †The author is Principal Engineer with Dye Engineering and Technology Company at the address shown above. USA phone: 281-494-5617; fax: 281-494-0210; no e-mail. 571 572 R. F. Dye The three glycols (mono-, di- and triethylene glycol) discussed are composed solely of carbon, hydrogen and oxygen and have similar chemical properties. They have applications that vary mainly with physical properties such as viscosity, hygroscopicity and boiling point. 2. Structures The three glycols have the following structures and synonyms: Ethylene Glycol HOCH2 CH2OH (Monoethylene Glycol, EG or MEG) Diethylene Glycol HOCH2 CH2 OCH2CH2OH (DEG) Triethylene Glycol HOCH2 CH2 OCH2CH2OCH2CH2OH (TEG) PHYSICAL PROPERTIES Table 1 lists various physical properties and constants for the ethyl- ene glycols. Mono-, di- and triethylene glycols are colorless, odorless, high boiling, hygroscopic liquids that are completely miscible in water and a number of organic liquids. Vapor pressure curves for the ethylene glycols at various tem- peratures are illustrated in Fig. 1. Another important property, ethyl- ene glycols can reduce the freezing point of water markedly as shown in Fig. 2. Fig. 2. Freezing points of monoethylene glycol, diethylene glycol and triethylene glycol aqueous solutions. CHEMICAL PROPERTIES Source: Shell Bull., 1993. The presence of two hydroxyl groups in the glycol molecule de- termines its chemistry and permits reactions such as esterification, dehydration, oxidation, halogenation and formation of alcoholates and acetals to take place. In the following examples, ethylene glycol is used to illustrate some of its reaction characteristics. 1. Esters of Organic Acids Organic acids, anhydrides, and acid chlorides react with the gly- cols to yield mono- and diesters. The molar ratio of the acid to the glycol determines which product will predominate. Fig. 1. Vapor pressures of glycols. Source: Shell Bull., 1993. September, 2001 Ethylene Glycols Technology 573 2. Polyesters of Polybasic Acids terest until World War I, when it was used in Germany as a sub- Polybasic acids or their derivatives will react with the glycols to stitute for glycerol in explosives production [Curme and Johnston, form polyesters. Depending on whether an excess of acid or glycol 1952]. Later, in the 1930’s, when the commercial use of the Lefort is used, the polymer may be terminated with either alcohol or acid direct oxidation route to ethylene oxide began to displace the chlo- groups. rohydrin process, hydrolysis of the oxide to ethylene glycol came into favor. Ethylene glycol was originally produced commercially in the United States from ethylene chlorohydrin. Although hydrolysis with both acid and base catalysts are proven methods for ethylene glycol production, neutral non-catalyzed hy- drolysis has been shown to be equally as popular. In time, the latter gained acceptance as the favored choice used for commercial pro- duction. Today there are no known industrial producers of ethylene 3. Dehydration glycol via catalytic hydrolysis of ethylene oxide. Depending on the catalyst and conditions used the dehydration Neutral hydrolysis (pH 6-10) carried out in the presence of a large of ethylene glycol yields aldehydes, alkyl ethers and cyclic ethers, excess of water at high temperature and pressure, gives selectivities such as dioxane. to monoethylene glycol in the 89-91% range. The principal by- product is diethylene glycol with minor amounts of triethylene, tetra- ethylene and higher molecular weight glycols accounting for the remainder [Kirk-Othmer, 1994]. Selectivity to the different glycols is in practice controlled by vary- ing the ratio of water-to-ethylene oxide with a large excess of water favoring the selectivity to monoethylene glycol. Removing the ex- cess water is energy intensive and requires capital investment in evaporators. Accordingly, this cost factor limits the amount of ex- cess water which can be economically used for control of the non- catalyzed route to ethylene glycol. 2. Hydrolysis Chemistry We cite here the three principal chemical reactions that occur in 4. Oxidation the liquid-phase, non-catalytic thermal hydrolysis of ethylene oxide The oxidation of ethylene glycol by various methods yields ace- (EO) in the presence of excess water. taldehyde, formaldehyde, glycolic acid, oxalic acid, and glyoxal. Monoethylene glycol (MEG) is the principal product of the hy- Glyoxal is the most important oxidation product. The course of the drolysis as follows: reaction is dependent on the reaction conditions. C2H4O+H2O HOCH2CH2OH (MEG) (1) + 22 kcal/gmole The reactions for the only significant by-products, to diethylene gly- col (DEG) and triethylene glycol (TEG) follow: HOCH2CH2OH+C2H4O HOCH2CH2OCH2CH2OH (DEG) (2) +25 kcal/gmole 5. Other Reactions DEG+C2H4O HOCH2CH2OCH2CH2OCH2CH2OH (TEG) (3) The reaction of the ethylene glycols with ethylene oxide leads to +24 kcal/gmole the formation of ether-alcohols. The product composition is gov- erned by the ratio of the reactants. It is interesting to note that the yield of monoethylene glycol (MEG) depends only on the ethylene oxide concentration in the EO: water feed, and is not affected by temperature, pressure or contact time provided that substantially complete reaction (>99%) of the ethyl- ene oxide occurs [Dye, 1966]. 3. Commercial Production Fig. 3 is a simplified process flow diagram for an ethylene gly- cols manufacturing unit. Finished (purified) ethylene oxide (EO) or an EO: water mix- MANUFACTURING ture is the principal feed to the process. This feed is mixed with re- cycled water, e.g., condensates from reboilers on evaporators (3) 1. Process History and (4). Subsequently, the pressure is boosted on this feed via a high- Ethylene glycol was first prepared by Wurtz in 1859 by the hy- head pump, and it is then heated to reaction conditions. The booster drolysis of ethylene glycol diacetate but was not of commercial in- pump and feed preheater are not shown in Fig. 3. Korean J. Chem. Eng.(Vol. 18, No. 5) 574 R. F. Dye 4. Some Feed Quality Requirements The ethylene oxide, or ethylene oxide-water, feed to this process must be substantially free of aldehydes and carbon dioxide (CO2). High levels of these components can recycle and build up to con- centrations that can jeopardize finished product quality, particularly that of the MEG product. 5. Some Equipment Engineering Features Evaporators (2) and (3) are refluxed tray columns that are included in designs to limit recycle of MEG to the glycol reactor.