Inside the Reactor: How Ethylene Oxide is Produced

Introduction

Ethylene oxide (EO) is an important chemical intermediate that is most utilized to produce ethylene glycols, surfactants, ethanolamine, and glycol ethers, and as a sterilant for medical equipment. While EO volume is small relative to base petrochemicals, its economic importance comes from its numerous downstream applications in other industries including textiles, automotive, construction, and pharmaceuticals. We must understand EO supply and production mechanisms, not just from an industrial perspective, but also due to potential health and safety issues related to EO hazards and safety management.

What is Ethylene Oxide and Why is it Important?

Ethylene oxide is commercially majorly obtained via the direct oxidation of ethylene (oxidative route) using a silver catalytic process at high temperatures. It is a hazardous chemical such as flammability and toxicity, yet its usefulness in industry makes it difficult to substitute.

Supply Chain Overview of Ethylene Oxide

Different from most petrochemical derivatives, usually, ethylene oxide isn't traded at a global scale because it is so reactive and explosive. Most production facilities are sited near end use or downstream processing facilities to reduce transportation risks. As a result, the EO supply chain is largely domestic or localized, set up around integrated production complexes where feedstock ethylene is produced into EO and then transformed into derivatives such as monoethylene glycol (MEG) on site.

Raw Materials and Sourcing

Ethylene, the primary feedstock of ethylene oxide, is produced by steam cracking hydrocarbons, usually naphtha, ethane or propane. The availability of ethylene is the principal driver of the EO production capacity. The USA, China and the Middle East are the predominant producers and have developed domestic supply chains for ethylene. Availability of ethylene, however, can be challenged by fluctuating naphtha prices, interruption of natural gas delivery, or geopolitical instability such as sanctions or trade tensions.

Since EO is produced in a relatively localized commodity business, companies can heavily invest in site safety systems and leak detection and dedicate captive downstream units to mitigate the transport and handling risks.

Overview of the Production Process

Ethylene oxide (EO) is synthesized via the direct catalytic oxidation of high-purity ethylene gas (≥99.9%) employing a silver catalyst. The synthesis of EO starts with the purification of ethylene to remove the impurity of acetylene and/or sulphur-containing compounds that may deactivate the catalyst. At the same time, oxygen, sourced from either an air separation unit (ASU) via cryogenic or membrane separation, is supplied to conduct the oxidation moderately to ensure some safety for the oxidant. This oxidation reaction occurs in Multitubular fixed-bed reactors at 200-300 °C, (1-3 MPa), during which ethylene is oxidized with oxygen to produce EO as the target product, while some of the ethylene may completely oxidize to carbon dioxide and water as side products. The contact between the reactant gases produces a hot mixture of gases that will need to be quenched quickly and passed through absorbers (or wash towers) to absorb EO. The crude EO product is purified from the absorbers via distillation to yield a high-purity product, while unreacted gases are compressed, purified, and recycled to improve reaction efficiency. The main by-product from the synthesis is carbon dioxide, which can be released up to regulatory limits or captured, as well as small amounts of ethylene glycol present that are isolated downstream during the purification of EO.

Raw Materials and Input Requirements

The principal raw materials for EO production are:

•             Ethylene (purity of ≥ 99.9)

•             Oxygen or air (industrial grade)

•             Silver catalyst, often doped with cesium for selectivity

The reaction is sensitive to impurities, especially acetylene or sulphur species in ethylene, which cause catalyst poisoning. Therefore, there are feedstock purification units. Oxygen is used from air separation units (ASUs) where care is taken during metering to ensure that a flammable mixture is avoided in the reactors.

The catalysts are replaced often, and their performance affects both conversion and selectivity toward EO versus CO2 by-product.

Major Production Routes

Direct Catalytic Oxidation (Main Route)

Ethylene is reacted with oxygen over silver catalysts in the gas-phase. This route makes up greater than 95% of global EO production due to its efficiency and scalability. The bare gas-phase reaction takes place between 200 - 300 °C and 1 - 3 MPa.

2C2H4 + O2 → 2C2H4O (Desired reaction)

Key Producing Regions:

As of 2024, below is the market share of key Ethylene Oxide producing countries

Equipment and Technology Used

The EO manufacturing method employs the following: 

•             Multitubular fixed-bed reactors, commonly loaded with silver catalyst

•             Air separation units (ASU) for the provision of oxygen

•             Quenching and absorption columns to recover EO from gaseous reactants

•             Gas recirculation compressors, heat exchangers, and control systems with significant precision and stability. 

Advanced units incorporate distributed control systems (DCS) and safety interlocks to prevent runaway reactions. Processes and equipment (e.g., heat exchangers) are designed to contain and recover heat from exothermic processes in order to improve the overall thermal balance of the plant.

Environmental and Safety Considerations

Emissions and waste

The primary reaction produces EO but produces a large volume of carbon dioxide as well. This is handled with high-selectivity catalysts and optimizing the reaction conditions. Wastewater from the scrubbing units will have ethylene glycol and other organics present and it will need to be treated with either biological treatment or advanced oxidation.

Safety protocols

EO is flammable, toxic, and categorized as a Group 1 human carcinogen by IARC. For these reasons, EO production is governed by strict safety procedures on site such as the following:

•             Gas leak detection

•             Inerting systems with nitrogen

•             Explosion-proof electrical equipment

•             Regular safety audits and OSHA Process Safety Management (PSM) compliance (including HAZOP, HAZWOP, PLHRA, and other safety risk assessment tools).

Regulations

In the U.S., EO production includes environmental regulations put forth by the Environmental Protection Agency (EPA) for compliance with the Clean Air Act. In the EU, EO production is regulated under both REACH and ETS (Emission Trading Scheme). Countries also impose strict permitting for EO production and transport, limiting its global supply chain movement.

Conclusion and Future Innovations

With the rising need for regulatory scrutiny and the focus on sustainability, producers are investing in:

•             Next-generation catalysts that increase selectivity and minimize by-product CO2

•             Bio-based ethylene routes, although cost and scalability remain an issue

•             Digital twins and AI-driven process optimization to improve safety and efficiency

Future innovations intend to decouple EO from fossil-derived ethylene to make way for green EO with a smaller carbon footprint; however, for now, the industry continues to pursue integrated land-based production and the co-production of EO with a cautious eye on safety risks associated with the chemical.

FAQs

Q1: Why is ethylene oxide not normally shipped throughout the world?

Ethylene oxide is highly flammable and toxic, and it makes explosive mixtures with air, which makes shipping across a distance dangerous. For safety reasons, EO is produced and consumed in an integrated plant, so they are rarely shipped internationally.

Q2: What is the main method of producing ethylene oxide?

The principal route of production is the direct catalytic oxidation of ethylene with a silver-based catalyst. This is a continuous process that occurs in the gas phase and is effective but can only be done under very controlled conditions of temperature and controlled oxygen concentrations in order to avoid process safety hazards.

Q3: How is EO production regulated for environmental and safety hazards?

EO production facilities utilize advanced safety systems such as gas leak detection, Inerting systems, automated emergency shutdowns and other advanced safety systems. Some environmental hazards include CO2 emissions, and wastewater, which can be reduced using current catalyst process improvements, simple emission controls, and waste treatment technologies.