Fluid Catalytic Cracking (FCC) of waste polyolefins

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 favored due to their high activity and favorable 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%.

Dehydrogenation of renewable propane

Propane dehydrogenation (PDH) is a growing catalytic technology utilized 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, but 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 utilization. If these improvements are not introduced, PDH technologies could be disrupted by newer, up-and-coming unconventional olefins technologies.

Methanol-to-olefins (MTO)

MTO is a technology where methanol is catalytically dehydrated and partially converted to ethylene over alumina and zeolite catalysts. To be competitive vs mega-scale ethane crackers with feedstock cost advantages, methanol has to be produced in huge quantities of approximately 5 × 10⁶ t/y, leading to an olefins output of approximately 2 × 10⁶ t/y of ethylene and propylene. MTO, even based on methane, is not a sustainable option based on the carbon footprint.

Steam cracking (SC) is still the best-performing technology, even from a CO₂ point of view. If the process were to be electrified, the emissions would fall 80-90%. Oxidative coupling of methane (OCM) also looks very promising.

Total CO2 emissions [tCO2/tHVC] for different technologies. Source: https://shorturl.at/twPQ5.

Oxidative Coupling of Methane (OCM)

Several companies have been developing bespoke light olefins processes via OCM. Examples include Siluria Technologies (now part of Lummus), Sulzer Chemtech GTC Technology, Grillo AG, and Sinopec. Another prospective light olefins process is via non-oxidative coupling of methane (NOCM). Examples include SABIC/Dalian Institute of Chemical Physics (DICP)/China National Petroleum Corporation (CNPC).

OCM is considered one of the most promising routes to convert methane into ethylene directly, but it suffers from the conversion-selectivity challenge typical for many selective oxidation processes due to oxidation of the C₂ products in secondary reactions, high methane conversions corresponding to poor C₂ selectivities and a high yield of undesired COx products.

Electrification and novel reactor concepts

The Cracker of the Future is a consortium of chemical majors based in Flanders, Belgium, North Rhine-Westphalia, Germany, and the Netherlands. The consortium came together in 2019, chaired by the Brightlands Chemelot Campus, to investigate the operation of naphtha and gas steam crackers using renewable electricity instead of fossil fuels. Two consortium members, BASF and SABIC, plan to develop an electrically heated cracker supported by Linde Engineering. The project partners have made a funding application to create a multi-megawatt demonstration plant at BASF’s Ludwigshafen site.

CO₂ to olefins

The discovery and development of efficient technologies enabling the use of CO₂ as a starting material for chemical synthesis (at scale) is probably one of the biggest scientific challenges of our time. Two approaches to convert CO₂ to olefins (and other valuable chemicals) being taken by Avantium (via its VOLTA technology) and the Stanford spin-out Twelve are noteworthy. While each has expressed goals to convert CO₂ directly to ethylene, both seem to have shifted focus to other routes – oxalic acid for Avantium and CO for Twelve.

Although significant progress has been made over the last few years, the performance of state-of-the-art technologies seems not yet at the level required for an economically viable large-scale process.

Bio-based routes to olefins

Braskem’s bio-based ethanol production technology called ‘I’M GREEN’ uses sugar cane, a renewable source, as feedstock to produce ethanol. This is then converted into bio-based polyethylene (bio-PE), bio-based EVA (a resin used in the automotive and footwear sectors), and bio-PE wax.

Bio-PE is a renewable drop-in replacement to conventional fossil-based PE. Braskem operates a 200,000 tpy bio-ethylene plant in Brazil, followed by the addition of 60,000 tpy at the time of this writing. Biomass availability and the price gap with petrochemical ethylene are the two most important determinants for the future of bio-ethylene. However, bio-ethylene can also contribute to chemical feedstock security in oil-importing countries.

Waste to olefins

Plastic Energy’s process uses proprietary patented thermal anaerobic conversion (TAC) technology to convert end-of-life (EOL) mixed plastics waste into a type of pyrolysis oil called TACOIL. The process complements traditional mechanical recycling efforts and energy recovery activities to help build a circular economy for used plastic. TACOIL is used as feedstock for the production of chemical naphtha and diesel, which in turn can be used as feedstocks for polymer production. SABIC and partner Plastic Energy formed a 50-50 joint venture (JV) partnership and, in 2020, started the construction of a circular polymer production unit in Geleen, the Netherlands.

Bioprocesses

BASF has developed high-performance industrial enzymes, alpha-amylases, uniquely suited for grain processing and bioethanol production. BASF states that its proprietary enzymes improve fermentation by enabling higher yields, which in turn improves efficiency thus lowering CO₂ emissions and creates more flexible process parameters. LanzaTech has developed a novel gas fermentation technology that captures CO-rich gases and converts the carbon to fuels and chemicals. The company has developed proprietary microbes that ferment the CO gases in the bioreactor. In essence, the process recycles waste carbon into fuels and chemicals. According to the company, the gas fermentation process is an alternative to the Fischer-Tropsch process.

LanzaTech – Waste carbon fermentation for bioethanol production. Source: https://shorturl.at/gCEMY.

Conclusions

Ethylene is a challenging molecule to make directly from CO₂. Many hurdles must be overcome, including purification of CO₂, low conversions and efficiencies, and a problem of ‘CO₂ crossover.’ It is likely to remain further behind other unconventional olefin technologies. The electrification of steam crackers is the most near-term technology that can make the biggest impact on lowering emissions.

“Unconventional Catalytic Olefins Production II: Technological Evaluation and Commercial Assessment – 2021” provides readers 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. 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.

The article was presented by the authors at the 16^th International Conference on Greenhouse Gas Control Technologies (GHGT-16) in France in October 2022.