This column is based on “Ethylene Oxide Production from Ethylene – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/ethylene-oxide-production-cost.
Ethylene oxide (also known as EO, oxirane and epoxyethane) is the simplest cyclic ether. Because it is highly reactive, ethylene oxide is one of the most versatile chemical intermediates. It is converted into a wide range of products (for example, monoethylene glycol (MEG), surfactants and glycol ethers).
The process examined here (Figure 1) is a typical direct oxidation process in which pure oxygen is used as the oxidizing agent. The process consists of three major sections: (1) oxidation; (2) reagents recovery; and (3) product separation.
Oxidation. Initially, fresh ethylene, methane make-up and oxygen are mixed with recycle gas (recovered downstream). Methane is used as ballast gas to control flammable limits in the process. The mixture is heated by heat exchange with reactor effluent and fed to a multi-tubular catalytic reactor. There, ethylene oxide is selectively produced over a silver catalyst supported on alumina. The heat from this exothermic reaction generates steam on the reactor shell side, and is used for heating purposes throughout the process. Also, part of the ethylene is combusted, generating carbon dioxide and water.
Reagents recovery. The gaseous stream obtained from the reactor outlet is counter-currently contacted with cold water in an absorber for the separation of light gases (mainly CO2, unreacted ethylene, oxygen and methane). The bulk of the gaseous overhead stream is directly cycled back to the EO reaction by a compressor. The remainder of the gaseous overhead stream is treated for CO2 removal before being sent back to the reactor. A liquid solution of EO dissolved in water is withdrawn from the absorber as a side stream.
Products separation. The EO/water stream from the absorber is fed to the top of a stripper, where EO is separated from water. The stripper bottoms — a water/ethylene glycol mixture — is routed to a column for recovering antifreeze-grade monoethylene glycol (MEG), which is sold as a byproduct. The overhead gaseous stream, relatively concentrated in EO, is condensed and sent to a light-ends removal unit. Crude EO withdrawn from the bottom of this unit is fed to a final purification column for the removal of water and heavy impurities. High-purity ethylene oxide (99.9 wt.%) is obtained from the column overhead and is condensed and sent to storage.
Ethylene oxide production was initially based on the chlorohydrin process, where ethylene reacts with hypochlorous acid and the resulting chlorohydrin is dehydrochlorinated with lime, yielding ethylene oxide and calcium chloride. Currently, EO is primarily produced via the more efficient direct oxidation of ethylene, which may be air-based or oxygen-based. Different pathways for EO production are presented in Figure 2.
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce ethylene oxide was about $520 per ton of ethylene oxide in the fourth quarter of 2015. The analysis was based on a plant constructed in the U.S. with capacity to produce 550,000 metric tons per year of EO.
Edited by Scott Jenkins
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