Process analytics in ethylene oxide and ethylene glycol plants – Case Study

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Courtesy of Siemens Industry, Inc. - Process Analytics

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Ethylene Oxide (EO)

Ethylene oxide, C2H4O, under normal conditions, is a colorless, flammable gas. It is very toxic and carcinogen. Industrial production started in 1925 using the chlorohydrin process and was improved in 1931 by introducing the much more economic direct catalytic oxidation method. Currently, almost all ethylene oxide production plants are based on the direct oxidation process with air or oxygen using a silver based catalyst. CO2 and water are produced as by-products of the reaction.

Because EO reacts readily with many chemicals, it is one of the most versatile intermediates in the production of several industrial chemicals, the most notable of which is ethylene glycol. It is also used as a s terilant for medical equipment. However, most of it is converted into products such as fibers, foils, bottles, plasiticizers, solvents, antifreezes, cosmetics, sport articles, CDs etc.

Ethylene Glycol (EG)

Ethylene glycol (C2H4(OH)2) is produced from ethylene oxide through a catalytic reaction with water at higher temperature resulting in a yield of mono-ethylene glycol (MEG), known as glycol, and the by-products di-ethylene glycol (DEG) and tri-ethylene glycol (TEG).

Ethylene glycol is commonly used as an antifreeze agent in automobile cooling systems. It is also used in deicing solutions f or aircraft and boats. Other uses include solvents for the paint and plastic industry, and hydraulic brake fluids. In pure form it is a colorless clear liquid with a sweet taste and a slightly syrupy texture. If ingested, ethylene glycol can damage the kidneys, heart and nervous system.

Integrated EO/EG plants

Modern EO/EG plants are highly integrated units where part or all of the EO produced in the EO section can be recovered as glycols, if desired, in the glycol section. Plant integration allows for significant savings in utilities as well for the recovery of all bleed streams as high-grade-products instead of lower grade products in the case of non-integrated plants.

Ethylene oxide production

The major production steps of the oxygenbased EO process are:

Ethylene (acc. to IUPAC: Ethene) and oxygen are mixed with recycle gas and, after adding a moderating substance such as chloro-ethane, fed into a multitubular reactor. There, ethylene oxide (EO) is selectively produced utilizing a silver-based catalyst at 200 to 300 °C and 10-20 bar. Along with ethylene oxide (80-85 %), CO2, H2O and heat are generated. Reaction heat is recovered by boiling water at elevated pressure on the reactors shellside. It is used at dif ferent locations of the plant.
EO contained in the reactor product gas enters the EO absorber section where EO is scrubbed from the gas by water. The EO-containing water is concentrated by stripping producing crude EO which is suitable for feeding directly to a glycol producing plant. When pure EO is the desired final product, the crude EO is fed to a purification column.
The cycle gas leaving the absorber is fed to the CO2 removal section, where CO2 (a by-product of the EO reaction) is recovered. Some of the CO2 remains in the cycle gas and returns to the EO reactor.

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