Enhancements in Ethylene Oxide/Ethylene Glycol manufacturing technology
The markets for ethylene oxide (EO) and ethylene glycol (EG) continue to see attractive growth. This means that at least one or two new plants will need to be constructed every year if the industry is to meet the forecast demand.
Han van Milligen, Shell Global Solutions
Brian VanderWilp and Gary J. (Jimmy) Wells, CRI Catalyst Company
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To unlock capacity increases economically, new capital-efficient processes, higher-performance catalysts and continuous process optimisation will be paramount. We review these aspects in this paper, beginning with a look at the innovations and technological achievements that got the industry to where it is today.
Background and early history of EO catalysts
EO is produced by reacting ethylene with oxygen over a silver-based catalyst. These EO catalysts are characterised by several performance factors, including selectivity, activity, productivity and stability. One of the most important measures of an EO catalyst’s performance is its selectivity, which is the ratio of ethylene converted to EO to the total amount of ethylene reacted.
In the early days of EO production, the typical start-of-cycle selectivity for EO catalysts ranged from 68 to 70%, i.e., 30% or more of the ethylene feed to the process was lost to the complete combustion side reaction. Then, in 1971, Shell identified an improvement to the catalyst formulation that helped to boost catalyst selectivity to over 80%. Over the next 15 years, the selectivity of EO catalysts seemingly plateaued, with only minor improvements to selectivity performance being realised. There were those in the industry who believed that EO catalyst advances had reached the theoretical limits of what was achievable. Since then, however, there has been an incredible journey of continuous improvement for EO catalyst performance, including substantial increases in catalyst selectivity, stability and productivity.
The introduction of high-selectivity
In 1986, Shell made a discovery in EO catalyst technology that significantly changed the industry. As a result, it was able to offer new catalysts to the market: HS catalysts. This discovery increased initial selectivity values by more than six percentage points to give start-of-cycle selectivity values of 86% or greater. The impact of this selectivity increase was huge, as it could save customers millions of dollars in ethylene feedstock costs.
Although HS catalysts provided great value with this selectivity boost, their activity and stability were lower compared with traditional catalysts, so they had to be changed out more frequently. Traditional EO catalysts were changed out after three to four years of service. HS catalysts had a life approximately half this.
Over time, both process and catalyst improvements extended the life and selectivity of HS catalysts. On the process side, new plants were designed with less-severe operating conditions in order to cope with HS catalysts’ lower inherent activity. Most notably, the new designs were based on lower volumetric production rates (referred to as the catalyst work rate) and lower reactor inlet carbon dioxide (CO2) concentrations. On the catalyst side, incremental advances in HS catalyst development led to initial selectivities of 90% or more, and greater stability.
Consequently, by the turn of the century, new plants were benefiting from excellent selectivity and achieving a catalyst life of three years or more. However, these benefits came with a compromise: increased capital costs. Larger reactors were necessary to compensate for the lower catalyst work rate and larger CO2 removal units were installed to achieve low inlet CO2 concentrations, both of which resulted in increased capital requirements. As the size of the new world-scale EO production plants increased, the associated reactors became a significant capital (equipment) cost.
The introduction of high-performance
Fast forward to 2010, when yet another breakthrough in catalyst development provided an opportunity to address these capital costs. CRI Catalyst Company (CRI), the global catalyst technology company of the Shell Group and part of CRI/Criterion, Inc., developed a fundamental understanding of the catalyst aging process through focused research and development. This discovery led to new technologies that significantly inhibit catalyst aging and resulted in the HP catalyst family. HP catalysts are characterised by a high initial selectivity (comparable to that of HS catalysts), but with a significantly slower performance decline (see Figure 1). In addition, the HP catalyst family proved able to operate at significantly higher work rates and to be tolerant of higher CO2 concentrations. These performance characteristics provided valuable opportunities for cost-saving changes to the process design.
The EO/EG manufacturing process
Producing EO over a catalyst is the first step in the overall EO/EG manufacturing process. In the reaction section, EO is produced by catalysed, direct partial oxidation of ethylene. Additionally, a portion of the ethylene fully oxidises to form CO2 and water. These reactions take place in an isothermal (tubular) reactor at temperatures of 230–270°C. The reaction is moderated/optimised using an organic chloride. EO is recovered from the reactor product gas by absorption in water. Co-produced CO2 and water are removed, and, after the addition of fresh ethylene and oxygen, the gas mixture is returned to the EO reactor as feed. The EO–water mixture can be routed to a purification section for recovery of high-purity EO and/or to a reaction section where EO and water are converted into glycols.
In the standard thermal glycol reaction process, EO and water are reacted at an elevated temperature (about 200°C) and pressure without catalyst. This process typically yields about 90–92% monoethylene glycol (MEG) and 8–10% heavier glycol products, mainly diethylene glycol (DEG) and triethylene glycol (TEG). The proportion of the higher glycols is limited by using excess water to minimise the reaction between the EO and glycols. The resultant water–glycol mixture from the reactor is then fed to multiple evaporators where the excess water is recovered and largely recycled. Finally, the water-free glycol mixture is separated by distillation into MEG and the higher glycols.
A more modern technology is to react EO with CO2 to form ethylene carbonate (EC) and subsequently react the EC with water to form MEG, both reactions being catalysed. In this two-step process, most of the MEG forms in an EO-free environment, which minimises the co-production of heavier glycols and results in a MEG yield of more than 99%.
Figure 2 shows a basic overview of the EO/EG process, which includes the following major sections:
• EO reaction
• EO recovery
• CO2 removal
• Light ends (LE) removal with optional EO purification
• Glycol reaction and recovery
• Glycol purification.
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