Decarbonisation Technology - May 2024 Issue

streams. The reaction takes place at around 20 bar pressure and 250°C. In the presence of a catalyst, oxygen reacts with ethylene to produce EO, and some CO2 is also generated. However, the conversion rate of ethylene to EO over the catalyst is not very high, and a gas recycle is used to ensure high overall conversion rates. CO2 capture is essential in EO production to avoid an accumulation of CO2 in the reactor gas recycle loop. This CO2 must be captured and removed from the EO production process, and the associated capital and operating costs are absorbed in the EO production cost. CO 2 capture in the ethylene oxide process Gases leaving the EO reactor are generally at around 20 bar and contain around 4% oxygen and 8% CO2. Hot potassium carbonate (HPC) is ideal for selectively removing CO2 from high-pressure gas mixtures such as this. Unlike amine-based CO2 capture solvents, HPC is not degraded by the oxygen present in this stream. The equipment used to capture CO2 is generally a twin-tower absorber and stripper system (see Figure 4 ). The gas leaving the CO2 stripper

column is rich in CO2, with water vapour being the main additional component. Much of the water vapour can be removed simply by cooling, followed by gas/liquid phase separation in a knock-out drum. This stream is ideal for feeding directly to a CO2 liquefier or compressor and pipeline scheme for CO2 transmission. As carbon capture is an integral part of the EO production process, the incremental cost of CO2 transportation for injection at a sequestration site or for utilisation is low in comparison to processes in which a post-combustion CCS scheme is required, with the consequential additional energy inputs to capture the CO2 from the flue gas. To achieve rapid, cost-effective decarbonisation within the petrochemical sector, sequestration of captured CO2 from EO production must be a priority. Captured CO 2 utilisation The Dow Chemical Company is the world’s second largest producer of EO. At the Terneuzen chemicals complex in the Netherlands, Dow can produce up to 195,000 t/a of EO. Dow also produces the Meteor EO-Retro 2000 (MR2000) catalyst for EO production.

CO vent

Lights vent

Purge

Condensates

Lights stripping

Recom- pression

Ethylene oxide product

Methane or other diluent gas

Oxygen

Ethylene

Steam

Water/ glycol/ heavies

2CO + 2HO

A: Reactor feed B: Reactor outlet

C: CO vent

D: Lights vent

E: EO product F: Water

Ethylene

34.6 %

25.4 % 2.1 % 3.9 % 57.2 %

0.4 %

64.6 %

Ethylene oxide (EO) Oxygen

99.7 %

5.8 %

23.4 % 34.6 %

0.8 %

Methane

1.7 % 0.4 %

11.0 %

Water Carbon dioxide (CO)

4.2 % 3.2 %

3.7 % 7.7 %

0.2 % 0.1 %

94.2 %

97.5 %

23.6 %

Figure 4 Oxygen-fed ethylene oxide production with integrated CO2 capture