Olefins production pathways with reduced CO2 emissions
Regulations call for use of renewable feedstocks and electrification via unconventional, sustainable, and circular routes to ethylene, propylene, and other petrochemicals.
Christopher R Dziedziak and John J Murphy
The Catalyst Group (TCG)
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The focus of recent R&D and commercial developments for novel processes and catalysts for olefins production goes well beyond traditional thermal steam cracking, fluid catalytic cracking (FCC), and propane dehydrogenation (PDH) routes, to include ‘green’ and circular approaches. These routes include, but are not limited to, the utilisation of biomass or waste plastic, renewably produced methane and syngas, direct CO2 conversion, and the electrification of reactors. All of these approaches must address certain critical factors affecting technology viability, notably the CO2 footprint, lifecycle analysis, and overall sustainability in a move towards ‘Net Zero 2050’ for the chemical and polymer industries.
The ‘energy transition’ is impacting and changing the priorities and thinking on conventional olefins production, compelling a closer examination of the shifts toward:
υ Biomass and recycled waste feedstocks to the cracker and FCC units, trending toward the higher production of bioethylene and biopropylene for bio-PE and bio-PP
ϖ The significant investment and progress toward electrification, highlighted by The Cracker of the Future Consortium
ω The emphasis on ESG, CO₂ emissions reduction, and improved energy efficiency.
Historical context and drivers
Refiners and petrochemical companies have seen demand for fuels, petrochemical intermediates, plastics/rubbers, and other products change, with calls for increased circularity and environmental consideration increasing. While they contain favourable demand growth projections above GDP levels, when combined, ethylene and propylene account for the second highest greenhouse gas (GHG) emissions (~250 Mt CO₂e).
For decades, steam cracking has been the dominant method to make olefins. However, the same pitfalls that existed in the past will continue going forward, such as high energy requirements, large quantities of produced GHG emissions (mainly in the form of CO₂), the ethylene/propylene ratio and propylene deficit, and feedstock inflexibility. This has led to a flurry of R&D interest in developing novel processes and catalysts that go beyond traditional thermal steam cracking, FCC, and PDH.
Over the last several years, nations and oil/petrochemical companies have been increasing their pledges towards lowering GHG emissions, in some cases to zero. Consumer goods companies are ramping up efforts to produce less, reuse more waste, and lower their carbon footprints, while consumers and investors are demanding that companies do more to address these environmental issues. How producers and process licensors respond to these changes moving forward will largely determine which chemical and plastics producers will remain leaders.
CO₂ reduction pathways
In 2021, TCGR completed a multi-client study on the topic of unconventional catalytic olefins technologies, addressing topics like the propylene deficit, feedstock availability and flexibility, world-scale production vs stranded facilities, modular or small-scale production, and environmental issues pertaining to resource utilisation, life cycle analysis, and GHG emissions.¹
Highlighting the study’s findings, focusing on pathways towards reduced CO₂ emissions in olefins production, should provide the readers with a better understanding of where their own technology fits in this landscape or possibly which solutions are right for their own operations. They can identify technological gaps and hurdles to overcome and how to plan their strategic and/or commercial objectives in the coming years. Lastly, they should also comprehend the important role that catalysis will play in addressing the challenges for olefins production and, more broadly, the petrochemical/chemical industry.
Fluid catalytic cracking
Recovery of ethylene and propylene from FCC off-gas has gained importance. FCC units have long been a source of propylene as a valued by-product of gasoline production.
Specialised process designs and catalysts have been developed to increase FCC-derived propylene production. Technology licensors have developed and offer FCC technologies that span the propylene production range from 8 to 20+ wt% propylene yield on fresh feed. Several new catalysts with substantially larger propylene yields have been invented, as demonstrated in a recent review on light olefins production via FCC.²
The selective production of light olefins from waste polyolefins in a single step could be a fundamental and economically viable solution to deal with a waste stream that has proven notoriously difficult to recycle. At present, under ideal lab-scale conditions, maximally 75% of C2-C4 olefins can be produced via thermal or catalytic pyrolysis if pure polyolefin feeds are used. For industrial reactors, yields are typically lower than 60%.
Zeolite and zeo-type catalysts with micropores in the range between 4 and 5.6 Å (8 or 10 MR) are favoured due to their high activity and favourable selectivity for light olefins. In line with the tuning of zeolite catalysts for high olefin selectivity, modifications such as bimodal microporous- mesoporous matrices and promoters with, for example, phosphorus are beneficial for improving the selectivity in polyolefins’ catalytic cracking and to reach targets of 90%.
PDH is a growing catalytic technology utilised for propane- to-propylene conversion. On-purpose propane technologies are today responsible for approximately 20% of propylene production. PDH has been an invaluable technology for providing additional propylene supply at economical prices, and as with many other petrochemical processes, carbon intensity remains an issue. The scaling of renewable propane feedstock will be key to addressing the lifecycle footprint of PDH as well as the use of renewable utilities and off-gas CO₂ capture and utilisation. If these improvements are not introduced, PDH technologies could be disrupted by newer, up-and-coming unconventional olefins technologies.
Several industrial and academic research groups are collaborating to further develop, scale up, and demonstrate a toolbox of novel, efficient, and flexible PDH technologies. Specifically, two electrically heated catalytic reactor concepts (EHCR) are under investigation. One system is based on ohmic heating rods inserted in optimally designed 3D catalytic structures, and the second one is an intensified catalytic membrane reactor (CMR) with electrically conductive catalyst supports.³
Dehydrogenation of ethane over Cr or Pt catalysts is limited by equilibrium and allows only very poor yields of ethylene. This route is not competitive with conventional routes. Two new entrants in PDH, Dow’s FCDh and KBR’s K-PRO are challenging the status quo, UOP’s Oleflex and Lummus’ Catofin technologies. Both Dow and KBR have developed fluidised catalyst reactors and regenerators and claim propane consumption on par with UOP and Lummus. KBR’s process is interesting because their proprietary catalyst uses nonprecious metals and no chromium.
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