| Properties
of Ethylene Oxide
2.1 Introduction EO (oxirane) is the simplest cyclic ether. It is a colorless gas or liquid and has a sweet, etheric odor. The structure of an EO molecule is shown in Figure 2.1. The C-C bond is short and the bond angles strained [1]. Note that the atomic distances are given in angstroms. Figure 2.1 The Ethylene Oxide
Molecule EO is very reactive, because its highly strained ring can be opened easily, and it is one of the most versatile chemical intermediates. EO was first prepared in 1859 by Wurtz [2] using potassium hydroxide solution to eliminate hydrochloric acid from ethylene chlorohydrin. The chlorohydrin process developed from Wurtz’s discovery and industrial production began in 1914. The importance and commercial production of EO have steadily grown since then. The direct catalytic oxidation of ethylene, discovered in 1931 by Lefort [3], has gradually superseded the chlorohydrin process. Currently, EO is produced by direct oxidation of ethylene with air or oxygen. Annual worldwide production capacity exceeds 11 million tons, making it an important industrial chemical. Virtually all EO produced is further reacted (section 2.4). Its most important derivative is ethylene glycol, which is used for the manufacture of polyester and in automotive antifreeze. Other EO derivatives include surfactants, solvents, amines, and poly(ethylene) glycols. In addition to being a versatile and commercially important compound, EO has been involved in a number of serious incidents. It is necessary to understand the properties of EO to manage the risks of its use. 2.2 Physical Properties Important physical properties of EO are summarized in Table 2.1.
Graphs and tables of selected temperature dependent properties of EO are provided in Appendix A. Ethylene Oxide Water Mixtures Table 2.2 shows some of the properties of aqueous EO solutions. Of particular
note are the relatively high melting points, which are due to hydrate
formation [4]. Hydrates consist of organic molecules enclosed in a cage
structure. The highest melting point observed is 52°F (11.1°C) and corresponds
to a hydrate composition of
Liquid EO and water are completely miscible in each other in all proportions. EO/water mixtures are highly non-ideal and do not follow Raoult’s Law. Raoult’s Law deviation factors for EO/water mixtures are shown in figures 14 and 15 in Appendix A. Solubility of Ethylene Oxide Gas The solubility of ethylene oxide gas in various compounds has been measured and reported at atmospheric pressure and 22-23°C by Chaigneau [41]. These compounds include water, alcohols, hydrocarbons, oils, chlorocompounds, esters, and waxes. Solubility of Gases in Ethylene Oxide The solubilities of gases in liquid EO vary, increasing in the order nitrogen, argon, methane, ethane. Earlier data [6] have been revised [7]. Increasing temperature tends to increase the solubility. The Henry’s Law Constants for these gases in EO at different temperatures are given in Appendix A. 2.3 Reactive and Combustive Properties Understanding the reactivity and combustion properties of EO is important
in managing the risks of its use. As outlined in chapter 5, it has been
involved in serious incidents.
Combustion
EO is a flammable, explosible chemical. Its fire and explosion characteristics are system dependent. Some of these characteristics for EO/air mixtures are as follows:
Figures 2.2 and 2.3 illustrate the flammable limits for the EO, air and either nitrogen or carbon dioxide ternary mixtures at atmospheric pressure, i.e., 14.7 psia (101.325kPa) [39]. The literature also indicates some variability in the boundary concentration demarcation separating the flammable and non-flammable regions; e.g. [38, 39]. Also, it is important to recognize that mixture pressure affects the flammability characteristics too. Figure 2.4 illustrates the effects of pressure on the flammability region for EO/Nitrogen/Air. Thus, more or less inert dilution may be required depending on whether the pressure is greater or less than atmospheric. The flammable limits of other mixtures of EO with inert gases and air can be found in the literature, e.g., EO with H2O [22]; N2 [23], [22]; N2-H2O [24]; CO2-H2O [24]; CH4 [15]; CO2 [6], [22], [25]; C3H6 [26]; C4H9 [26]; N2-air [6]; CH4-air [6]; CO2-air [20]; CF2Cl2-air [27], [28]; CO2-air, N2-air, R12-air, R134a-air [39]; CO2-air, N2-air, Steam-air [24]; MeBr-air [24].
Flammability of Ethylene Oxide & Water Mixtures In closed systems such as sewers, 100 to 1 water to EO dilution ratios (vol/vol) may be required to produce a mixture that will not support combustion. In open systems, such as around an atmospheric spill, water/EO mixtures can support combustion if the water/EO ratio is less than 22 to 1. Decomposition Pure EO vapor or EO vapor mixed with air or inert gases can decompose explosively. The decomposition reaction is expressed by the following equation:
The reaction can also produce ethane, ethylene, hydrogen, carbon and acetaldehyde [10,17]. At atmospheric pressure, thermal decomposition of pure EO vapor occurs at 1040°F (560°C). This is the number most frequently cited as the decomposition temperature. However, lower gaseous EO decomposition temperatures have been reported — indicating that decomposition temperature is affected by pressure, surface characteristics, volume, and geometry. EO can also ignite and decompose explosively at pressures below atmospheric, down to a pressure of around 4.8-5.8 psia, but at greater than 1040°F (560°C). Once the decomposition reaction has been initiated, it can be propagated from the ignition source through the gas phase as a flame and, under certain conditions, may be explosive. It is important to understand that this reaction can occur in the absence of air or oxygen. High pressure can be generated by decomposition of EO. The maximum theoretical explosive pressure is about 10 times the initial pressure, but can increase to 20 times the initial pressure if liquid EO is present. This phenomenon occurs because liquid EO evaporates and participates in the decomposition reactions which take place in the vapor phase [15]. Mixtures of EO with nitrogen, carbon dioxide and methane will not decompose over certain concentration ranges. Thus, vapor decomposition can be prevented by dilution with a suitable inert gas. Nitrogen is usually the gas of choice, but methane and other diluents have been used. The dilution quantity depends on temperature, pressure, and the expected ignition source and duration [9]. The most thorough discussion of the EO decomposition process is presented in [17]. The minimum total pressure for inert blanketing is important [9, 10], and section 6.5 presents information relevant to the inerting of EO in storage and handling systems. For inerting of vapor spaces of reactors using EO as feeds or reagents, the system is quite complex and beyond the scope of this publication. EO liquid mists will decompose explosively similarly to the vapor. The decomposition of this two-phase mixture yields greater pressures and rates of pressure rise than the vapor alone [17]. Liquid EO can participate in a decomposition that starts in the vapor phase. Explosion of liquid EO, initiated by a strong ignition source within the liquid, was first described in 1980. It is thought that the ignition source vaporizes liquid EO, and the decomposition reaction takes place in the gas phase. Exothermic Reactions with Rust Following a major industrial incident, it was discovered that EO vapor in contact with high surface area metal oxides, such as the gamma form of iron oxide, can undergo exothermic reactions ("disproportionation") that can raise local temperatures above the decomposition temperature of EO [42]. The disproportionation reaction has been represented by the following equation:
But depending on the reaction conditions, the ratio of C2H4 to CO2 has been found to vary from 1.5 to 2.5. This reaction can be initiated at significantly lower temperatures than thermal decomposition. In the case of the industrial incident noted above, the reaction occurred:
It was concluded that loss of reboiler circulation allowed for rapid heat buildup in the vicinity of the iron oxide/polymer deposit, resulting in localized temperatures reaching the EO thermal decomposition temperature. The result was an explosion that destroyed the distillation column. Britton [17] has indicated that rust with very high surface areas can also initiate EO ignition at 284°F (140°C) or below with or without air present. Polymerization EO has a tendency to polymerize. For pure EO, the reaction is slow at ambient temperatures. The reaction is exothermic, releasing 900 BTU per pound of EO reacted [13]. Britton [17] has reported a rust catalyzed heat of polymerization of 1102 ± 121 BTU/lb. The usual catalysts for EO reactions, such as strong alkali [18], iron oxide (rust) [19], and other metal oxides catalyze the reaction. When catalyzed by rust, it is most often a nuisance, causing line and equipment plugging and off-specification product. However, the presence of large quantities of loose rust could pose a significant safety hazard (see discussion above). Britton [17] also indicated self-polymerization at 392°F (200°C) in a closed, near-adiabatic system, and non-catalytic conditions. The condition of metal surfaces is extremely important in determining the rate of EO polymer formation. It has been reported [19] that even clean carbon steel catalyzes polymerization, although at a much slower rate than rusty steel. Other factors that affect rate of polymerization:
Stainless steel is often the best choice for materials of construction, especially when the surface to volume ratio is high. The polymerization reaction has not been found to be auto-catalytic [43]. That is, the presence of polymer does not accelerate the polymerization process. Contamination of EO with catalysts such as KOH or overheating can lead
to runaway polymerization. Reference [18] has a discussion of an EO polymerization
(or "Polycondensation") incident brought about by contamination
of an EO-containing cylinder with chlorine and alkali. The result was
an accelerating or "runaway" reaction that ended with an explosion
after about eight hours. Pure EO polymers have been characterized [44] as clear viscous liquids (molecular weight less than 600) and as opaque white solids (higher molecular weight). However, in industrial settings EO polymer is often dark brown or black, due to the presence of magnetite iron oxide inside the polymer matrix. Note that the density is significantly higher than that of EO. This means that polymer that precipitates inside a storage container will tend to collect on the bottom. Solubility of EO polymer in various solvents, including EO, is a function of molecular weight of the polymer and temperature. In general, higher molecular weight polymer is harder to dissolve. Solubilities of low molecular weight EO polymer in various solvents are given below.
Of note is the extremely low solubility in the non-polar solvent heptane. Polymer samples from EO processing and storage equipment have had molecular weights ranging from a few thousand to over one million. At the upper end of this range, the polymer is quite insoluble in solvents and hot water, and must be removed by physical means. Static Electricity Liquid EO is an electrically conductive fluid and static electricity charges cannot accumulate in metal containers using proper bonding and grounding techniques (see NFPA 77 Static Electricity Guide). Bottom filling is not required unless there are isolated internal areas that might accumulate a charge. Static charge can accumulate in liquid mist produced as a result of splashing and spraying, and excessive fill velocities should be avoided to minimize this effect. {See Eichel [32] for a discussion of electrostatic calculations.} Pure EO vapor minimum ignition energy (MIE) is about 1000 mJ [9]. A typical individual can initiate static discharge energy in the range of 1-50 mJ, and the energy from ordinary spark plugs is ca. 20-30 mJ [31]. Thus, static energy discharge, in the absence of air, is not a significant hazard under normal handling and storage conditions. The presence of even small amounts of air with EO vapor makes it more sensitive to ignition. The MIE for EO with air is reduced to a much lower level (0.06 mJ [9]) and is subject to ignition by static sparking ignition or other common ignition sources. {For comparison the MIE for hydrogen is 0.01-0.02 mJ and that for ethylene is 0.07-0.12 mJ [33].} 2.4 Commercial Chemistry EO is a very versatile compound, storing considerable energy in its ring structure. Its reactions proceed mainly via ring opening and are highly exothermic. Only a few of the large number of possible reactions are briefly discussed here. More detailed information can be found in [8]-[14]. Addition to Compounds with a Reactive Hydrogen Atom EO reacts with compounds containing a reactive hydrogen atom to form a product containing a hydroxyethyl group:
Since the end product of the above reaction contains at least one hydroxyl group, it can react successively with additional EO to produce long chain polyether polymers which are sometimes called poly-oxyethylene-glycols. The molecular weight distribution of the polymers depends on the reaction conditions, catalysts employed and the ratio of reactants. Commercially, the most important of this type of reaction is the reaction with water to produce ethylene glycols. Over half the total EO production is used in ethylene glycol production. The production of poly(ethylene) glycols by this route is also of commercial importance. When used with starting materials other than water (e.g., phenols, ammonia, fatty amines, fatty alcohols, and fatty acids), this reaction, often referred to as ethoxylation, is used to produce the bulk of the other commercially important EO derivatives. Addition to Double Bonds EO can add to compounds with double bonds, e.g., carbon dioxide, to form cyclic products:
EO also adds to other double bond systems, e.g., to R2C=O, SC=S, O2S=O, RN=CO, and OS=O. Catalytic Isomerization to Acetaldehyde Aluminum oxide (Al2O3), phosphoric acid and phosphates, iron oxides, and, under certain conditions, silver, catalyze the isomerization of EO to acetaldehyde.
Other Reactions A highly reactive compound, EO reacts with many other compounds including: hydrogen (catalytic reduction to ethanol); hydrogen sulfide and mercaptans; Grignard reagents; halides; hydrogen cyanide; dimethyl ether; compounds with active methylene or methyne; etc. 2.5 Uses Products derived from EO have many different uses. They include:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
