Carbon capture application to ethylene plants
Operationally proven carbon capture applied to ethylene plants, with examples of deployment in new and existing plants
Myrian Schenk and Jim Middleton
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The production of ethylene originates mainly from steam cracking. This thermal process uses a large amount of energy and, as such, is a significant emitter of CO2 in the industrial sector (Middleton, 2021). The reduction of CO2 emissions from the cracker can be achieved by replacing the methane fuel from the process with alternative lower carbon fuels, such as hydrogen, or installing electrical heating. However, a third option to reduce CO2 emissions is to add a post-combustion carbon capture plant.
Post-combustion carbon capture (CC) has been proven in the power industry and can be applied to steam crackers. Apart from the use of high hydrogen content fuels, CC is the only currently commercially proven technology that can achieve very high levels of reduction of CO2 emissions from steam crackers (Middleton, 2021).
In this article, we will highlight some studies carried out by Technip Energies, in which we show the feasibility of installing a CC plant on a steam cracker and that the operation of the steam cracker should not be affected by the addition of a CC plant.
The installation of CC, both on a new cracker design or as a retrofit to an existing cracker, is relatively straightforward, provided the designer of the CC plant understands how the cracker operates to allow for smooth integration. The right decisions must be made for the proper functioning of the two plants together. The main requirements are:
- A destination for the captured CO2
- A plot area, if possible close to the furnaces (and auxiliary boilers if the capture is planned for these units too)
- The utilities required to run the CC plant
The modifications to ethylene plants to accommodate the addition of a CC plant are principally in the flue gas and utility areas. Therefore, it is relatively straightforward to design new ethylene plants to allow for the future installation of a CC plant; indeed, Technip Energies is currently designing two such plants.
The use of CC on ethylene plants can be complementary to other methods of CO2 reduction, such as partial hydrogen firing and reduced conventional firing in furnaces, both of which reduce flue gas flow rates and lower the operating and capital costs of the CC plant.
The 'conventional' routes for captured CO2 are for enhanced oil recovery (EOR) or sequestration; however, more and more alternative uses of captured CO2 are starting to emerge.
Post-combustion carbon capture technology
Continuous improvement in the affordability of CC is key to enabling the technology to play its part in worldwide CO2 reduction. The integration between CC technology and CC and cracker engineering represents a significant step towards achieving this goal.
At Technip Energies, we maximise the benefits of our alliance with Shell Catalysts & Technologies, licensor of the Cansolv CO2 capture technology, to provide a single point delivery of projects.
Since 2012, we have had an exclusive alliance with Shell Catalysts & Technologies for the power industry. However, in recent years, we have extended our cooperation across numerous projects and sectors in the carbon capture, utilisation and storage industry.
Together, we work on technology and engineering improvements and drive integration to continuously enhance CC solutions to make projects affordable.
Technip Energies and Shell have a team for project delivery and integration of a joint axis of R&D efforts, enabling us to deliver both capital and operating cost reductions in our combined offerings for CC plants.
The Cansolv CO2 capture technology, integrated into the Shell technology portfolio through the acquisition by Shell of Cansolv Technologies Inc. in 2008, is positioned among the leading technologies for CO2 removal (Shell, 2022).
Through projects like SaskPower’s Boundary Dam, the technology has been deployed at the large scale (1 MT/y and above) that characterises many CCUS projects and also at the much smaller scale expected to characterise many CO2 capture applications in the future.
The Cansolv CO2 capture system is an amine-based technology using Shell’s proprietary Cansolv DC-103 absorbent. The process line-up is shown in Figure 1, and relies on standard equipment (vessels, pumps, exchangers) and mass transfer internals (structured packing).
The CO2 is captured from the cooled gas by contact with the aqueous (lean) amine absorbent in the absorber, where multiple structured packing beds are used to promote mass transfer while keeping pressure drop low. Capture efficiencies of up to 99% can be achieved (depending on the application), but the economic optimum is around 95% in most cases.
CO2 absorption is exothermic, which results in a temperature bulge in the absorption column. As high temperatures are detrimental to efficient absorption, at high inlet CO2 concentrations, an intercooler is used to remove heat from the system and maintain an operating temperature profile favourable to absorption: a draw-off tray collects absorbent from the upper packing beds, which is pumped through the intercooler before returning to the lower packing bed.
The upper section of the absorber is a water-wash system that ensures the emissions of solvent and degradation products to the atmosphere are minimised. The water-wash system is usually sufficient to meet the most stringent emissions specifications. However, depending on the project requirements and feed gas characteristics, an aerosol mitigation device can be added downstream of the absorber.
The rich absorbent, loaded with CO2, is regenerated in a stripping column using structured packing to promote mass transfer and reboilers to generate the stripping steam. A condenser is used to condense the stripping steam from the overhead vapours, and the pure (water-saturated) CO2 is released for downstream treatment. A lean-rich heat exchanger is used to recover heat from the hot lean absorbent exiting the stripper to preheat the rich absorbent before it enters the column.
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