Production ethylene oxide

In the industrially important epoxidation of ethylene, the main byproducts are carbon dioxide and water. These are formed by parallel combustion of ethylene as well as of ethylene oxide according to the reaction 

Yields of 65—75 mol% at 230—270°C and 10—20 atm total pressure are reported with commercial catalysts. These consist of supported silver doped with alkali and earth alkali metals. High selectivities are only obtained if very small amounts (ppm range) of moderator (e.g. chloroalkanes) are added to the reactants. 

The kinetics of the ethylene oxidation are rather complicated as they depend not only on ethylene and oxygen pressure but also on the concentration of the reaction products. These influence the rate by adsorption competition with the reactants. Moreover, different forms of adsorbed oxygen may occur on the catalyst surface. Consequently, the rate equations proposed in the literature consist of either Langmuir—Hinshelwood and Eley—Rideal types or power rate models with non-integer coefficients. Power rate models are less appropriate as their coefficients inevitably depend on the reaction conditions. 

According to the oxidation scheme, three reactions have to be accounted for. Although the combustion of ethylene oxide is less important than the direct combustion of ethylene, it cannot be neglected. A summary of recently reported rate equations is given in Table 1. Several of them will be discussed in more detail. 

Ayame et al. [32] studied the effect of higher total pressures, up to 11 atm. In the range 200—265°C, data are given for conversions to a maximum of 46%. Maximum selectivity is 71.7%. Neglecting some inhibition effects, the authors find the power rate equations, which include the total pressure as an individual parameter, as presented in Table 1. Numerical values of the rate coefficients in these equations as a function of temperature are collected in Table 2. 

---

Reaction and Heat-Transfer Solvents. Many industrial production processes use solvents as reaction media. Ethylene and propylene are polymerized in hydrocarbon solvents, which dissolves the gaseous reactant and also removes the heat of reaction. Because the polymer is not soluble in the hydrocarbon solvent, polymer recovery is a simple physical operation. Ethylene oxide production is exothermic and the catalyst-filled reaction tubes are surrounded by hydrocarbon heat-transfer duid. 

Catalyst lifetime for contemporary ethylene oxide catalysts is 1—2 years, depending on the severity of service, ie, ethylene oxide production rate and absence of feed poisons, primarily sulfur compounds. A large percentage (>95%) of the silver in spent catalysts can be recovered and recycled the other components are usually discarded because of thek low values. 

Three reactions dominate ethylene oxide production ... 

Equation 1 is referred to as the selective reaction, equation 2 is called the nonselective reaction, and equation 3 is termed the consecutive reaction and is considered to proceed via isomerization of ethylene oxide to acetaldehyde, which undergoes rapid total combustion under the conditions present in the reactor. Only silver has been found to effect the selective partial oxidation of ethylene to ethylene oxide. The maximum selectivity for this reaction is considered to be 85.7%, based on mechanistic considerations. The best catalysts used in ethylene oxide production achieve 80—84% selectivity at commercially useful ethylene—oxygen conversion levels (68,69). 

Ethylene oxide has been produced commercially by two basic routes the ethylene chlorohydrin and direct oxidation processes. The chlorohydrin process was first iatroduced dufing World War I ia Germany by Badische Anilin-und Soda-Eabfik (BASE) and others (95). The process iavolves the reaction of ethylene with hypochlorous acid followed by dehydrochlofination of the resulting chlorohydrin with lime to produce ethylene oxide and calcium chloride. Union Carbide Corp. was the first to commercialize this process ia the United States ia 1925. The chlorohydrin process is not economically competitive, and was quickly replaced by the direct oxidation process as the dominant technology. At the present time, all the ethylene oxide production ia the world is achieved by the direct oxidation process. 

Silver-containing catalysts are used exclusively in all commercial ethylene oxide units, although the catalyst composition may vary considerably (129). Nonsdver-based catalysts such as platinum, palladium, chromium, nickel, cobalt, copper ketenide, gold, thorium, and antimony have been investigated, but are only of academic interest (98,130—135). Catalysts using any of the above metals either have very poor selectivities for ethylene oxide production at the conversion levels required for commercial operation, or combust ethylene completely at useful operating temperatures. 

Process Safety Considerations. Unit optimization studies combined with dynamic simulations of the process may identify operating conditions that are unsafe regarding fire safety, equipment damage potential, and operating sensitivity. Several instances of fires and deflagrations in ethylene oxide production units have been reported in the past (160). These incidents have occurred in both the reaction cycle and ethylene oxide refining areas. Therefore, ethylene oxide units should always be designed to prevent the formation of explosive gas mixtures. 

An appreciation of statistical results can be gained from a study conducted to support the first application of computer control for an ethylene oxide production unit at Union Carbide Corporation in 1958. For the above purpose, twenty years of production experience with many units was correlated by excellent statisticians who had no regard for kinetics or chemistry. In spite of this, they did excellent, although entirely empirical work. One statement they made was ... [ethane has a significant effect on ethylene oxide production.] This was rejected by most technical people because it did not appear to make any sense ethane did not react, did not chemisorb, and went through the reactor unchanged. 

In 1960 the author was charged with the review and improvement of the ethylene oxide technology of Union Carbide Corporation (UCC). A historic overv iew revealed some interesting facts. The basic French patent of Lefort (1931,1935) for ethylene oxide production was purchased by UCC in 1936. In 1937, a pilot-plant was operated and commercial production started in 1938. By 1960, UCC s production experience was several hundred reactor-years. This was expressed as the sum of the number of production reactors, each multiplied by the number of years it had been in operation. Research and development had continued since the purchase of the original patent and the total number of people involved in ethylene oxide related research at one time reached one hundred. 

Two epidemiological studies of workers exposed to ethylene oxide revealed increased rates of leukemia. In one smdy, two cases of leukemia (0.14 expected) and three stomach cancers (0.4 expected) were observed. The other study found three cases of leukemia (0.2 expected). Because these workers had exposures to other potential carcinogens, the findings cannot be linked with certainty to ethylene oxide. The small cohort size, the small number of deaths, and uncertainties about exposure level have also been noted." A number of other studies have not found an increased rate of cancer mortality from ethylene oxide exposure. A mortality study of over 18,000 ethylene oxide workers from 14 plants producing medical supplies and foodstuffs did not find an excess of leukemia or brain, stomach, or pancreatic cancers. There was, however, an increase in non-Hodgkin lymphoma in male workers. A follow-up of 1896 ethylene oxide production workers did not find an increase in mortality from leukemia, non-Hodgkin lymphoma, or brain, pancreatic, or stomach cancers. ... 

Several studies have examined mortality or cancer incidence among chemical workers potentially exposed to 1,2-dichlorocthanc. Hogstedt et al. (1979) performed a cohort mortality study of 175 Swedish ethylene oxide production workers followed from 1961 through 1977. The workers had been employed for at least one year and were potentially exposed to... 

Hogstedt, C., Rohlen, O., Berndtsson, B.S., Axelson, O. Ehrenberg, L. (1979) A cohort study of mortality and cancer incidence in ethylene oxide production workers. Br. J. ind. Med., 36, 276-280... 

Ethylene oxide production from ethylene with supported... 

The following two reactions occur in an ethylene oxide production process ... 

Greenberg HL, Ott MG, Shore RE. Men assigned to ethylene oxide production or other ethylene oxide related chemical manufacturing a mortahty study. Br J Ind Med 1990 47(4) 221-30. 

---