Die Chemie spielt in unserem Alltag eine große Rolle. In der einen oder anderen Form finden chemische Produkte Anwendung in den Bereichen Reinigung, Kommunikation, Transport, medizinische Versorgung, Pharmazeutika und Geräte. Selbst neue Innovationen im Bereich Energie und energieeffiziente Technologien, wie beispielsweise Kunststoff-Solarzellen, stammen aus dieser Branche. Vor kurzem erreichte der Umsatz der chemischen Industrie 5,2 Billionen Euro, was etwa 7 % des globalen BIP entspricht (Mulder, 2019).

Da die Branche außerordentlich nützliche Produkte liefert, ist es nicht verwunderlich, dass sie den strengsten Vorschriften unterliegt. Auch die Dekarbonisierung der Produktion wird eine Herausforderung sein, der sich die Branche stellen muss. Dennoch hat die chemische Industrie das größte Potenzial für mehr Nachhaltigkeit in der globalen Fertigung, da viele Produktionsprozesse energieintensiv sind.

Tatsächlich entfallen allein in den Vereinigten Staaten 28 % des gesamten industriellen Energiebedarfs auf die chemische Industrie (Leow et al., 2020). Ein Großteil dieses Bedarfs entsteht durch die Verbrennung fossiler Brennstoffe, um die für Dampfcracking-Öfen erforderliche Wärme zu erzeugen. Diese Öfen werden in der Regel zur Herstellung von zwei leichten Kohlenwasserstoffen, Ethylen und Propylen, eingesetzt. Zusammen bilden diese beiden Gase die chemische Grundkomponente für 60 % aller kommerziellen organischen Chemikalien (Gholami et al., 2021).

Warum Nachhaltigkeit für die Chemieindustrie alternativlos ist

Der Bedarf an chemischen Produkten dürfte steigen, da die weltweite Fertigung auf erneuerbare Energien umgestellt wird, darunter auch Technologien, die beispielsweise auf Kunststoffen basieren. Es liegt nahe, dass die chemische Industrie nach Möglichkeiten sucht, ihre Produktionsprozesse zu dekarbonisieren. Andernfalls würden die Dekarbonisierungsbemühungen anderer Branchen und Regierungen sinnlos werden.

Olefine können eine Lösung zur Erreichung dieses Ziels sein. Diese leichten Kohlenwasserstoffgase werden in größeren Mengen produziert als jede andere Klasse von Chemikalien. Von diesen Olefinen machen Ethylen und Propylen den größten Anteil aus, und Dampfcracking ist eine der gängigsten Methoden zu ihrer Herstellung. Wenn Chemieproduzenten also nach einem nachhaltigeren Produktionsprozess suchen, könnte Dampfcracking ein sinnvoller Ansatzpunkt sein.

Fakten zum Kohlenstoffausstoß beim Dampfcracking

Dampfcracking ist zwar das ausgereifteste und kostengünstigste Verfahren zur Herstellung von Chemikalien, aber auch das energieintensivste und umweltschädlichste. Dampfcracking verursacht jährlich mehr als 300 Millionen Tonnen CO₂-Emissionen (Amghizar et al., 2020). Das sind fast 1,2 % der weltweiten jährlichen CO₂-Emissionen, die die chemische Industrie allein angehen kann (Tiseo, 2024).

Das bedeutet, dass die Suche nach Möglichkeiten zur Reduzierung der CO₂-Emissionen durch alternative Energiequellen die Umweltauswirkungen des Dampfcrackings mindern würde.

Wie werden Ethylen und Propylen hergestellt?

Olefins like ethylene and propylene are produced by breaking down natural gas liquids, known as feedstocks, into their constituent parts. There are many different processes by which olefins are produced from natural gases (Phung et al., 2021). Currently, the most established and widely used processes are energy-intensive and emit generous amounts of greenhouse gasses to be sustainable, they include (Ren et al., 2006):

• olefin catalytic cracking
• propane dehydrogenation and oxidative dehydrogenation
• metathesis propylene production
• steam-enhanced catalytic cracking
• steam cracking

Steam cracking is the most widely adopted process for producing olefins from natural gas liquids. This is primarily because it’s more profitable for chemical producers. For example, steam cracking has the lowest total production cost (TPC) of any current olefin-producing process (Gholami et al., 2021).

Heat characteristics of steam cracking

Steam cracking is the process of feeding natural gas and steam inside an exceptionally hot furnace under pressure without the presence of oxygen. Depending on the natural gas being used and the desired olefin to produce, furnaces must first heat up to 800-850 °C. Then there is a rapid drop to 400-500 °C to prevent downstream reactions after the initial splitting (Gholami et al., 2021).

The reaction time needed to split natural gas into olefins like ethylene and propylene is around 0.01-0.02 seconds. In other words, the temperature must drop 300-400 °C faster than the blink of an eye (Amghizar et al., 2020). This “quick cooling” can be particularly challenging because the process creates an endothermic reaction, whereby the chemicals splitting apart in the furnace absorb the surrounding energy and become intensely hot.

To achieve the rapid drop in temperature, the separated gas mixture is forced into a series of cooling tubes called transfer line exchanges (TLE). From the furnace, the now separate chemicals transition through several stages of distillation, chemical treatment, and separations to lastly reach the desired gaseous and liquid olefins.

A green solution for ethylene and propylene production

Several “green” processes for lessening olefin production's environmental impact have been developed recently. For example, methanol-to-olefins (MTO), ethanol-to-propylene (ETP), and bioethanol dehydration, dimerization and metathesis (Phung et al., 2021). While the horizon is promising, these processes are immature, need greater industry adoption and require further optimization to maintain profitability before they can serve as a decarbonizing alternative. Industry experts must seek ready innovations that are deployable today, and steam cracking is just one established process suitable for existing green solutions (Amghizar et al., 2020).

CO₂ output of steam cracking can be lowered by:

• substituting the energy derived from fossil fuels to heat furnaces, e.g. with electric furnaces
• improving heat transfer
• reducin coke formation on the furnace walls

The first reflex would be to think of replacing natural gas with green hydrogen, but apart from the cost of availability questions, the issue of more volume needed with hydrogen needs to be considered. Thus, retrofitting furnaces need to be researched.

Changing the furnace to an electric furnace also brings some changes with it, but naturally is more cost-efficient than hydrogen. In a first of a kind installation, BASF, SABIC and Linde installed two e-furnaces to steamcrack gases. One uses electrical current directly to heat up the pipes, steam flows through, one uses resistance heating elements and their radiative heat wrapped around the pipes (BASF.com, 2024). The systems together have 6 MW, so the tests run on industrial but not full-scale.

As this is a first of its kind project, supported with nearly €15 million by the German government, one might also think about testing the retrofit of traditional furnaces for hot air driven processes, which can be flexible through Kraftblock. What in theory is possible with temperatures reaching up to 1150 °C would need to be tested regarding the integration of hot air instead of burning gas, which has different volume flows and energy density.

Capture and redistribute heat loss from stream cracking

With every rapid drop in temperature needed to split natural gas into olefins like ethylene and propylene, excess heat is lost and could be recycled. Some heat is effectively recycled via water-cooled TLE tubes, which can be reused via steam at the start. However, much of the excess heat from the burning of the furnace is released via flue gases. A lot of waste heat recovery already is done in steam cracking. When there are losses, the capture and reuse of hot flue gas could improve the overall efficiency.

References

Amghizar, I., Dedeyne, J. N., Brown, D. J., Marin, G. B., & Van Geem, K. M. (2020). Sustainable innovations in steam cracking: CO 2 neutral olefin production. Reaction Chemistry & Engineering, 5(2), 239-257.

‍basf.com (2024): BASF, SABIC, and Linde celebrate the start-up of the world's first large-scale electrically heated steam cracking furnace. Online: https://www.basf.com/global/en/media/news-releases/2024/04/p-24-177.html

‍Gholami, Z., Gholami, F., Tišler, Z., & Vakili, M. (2021). A review on the production of light olefins using steam cracking of hydrocarbons. Energies, 14(23), 8190.

Leow, W. R., Lum, Y., Ozden, A., Wang, Y., Nam, D. H., Chen, B., ... & Sargent, E. H. (2020). Chloride-mediated selective electrosynthesis of ethylene and propylene oxides at high current density. Science, 368(6496), 1228-1233.

Mulder, M. (2019). Chemical Industry Contributes $5.7 Trillion to Global GDP and Supports 120 Million Jobs, New Report Shows. The European Chemical Industry Council. https://cefic.org/media-corner/newsroom/chemical-industry-contributes-5-7-trillion-to-global-gdp-and-supports-120-million-jobs-new-report-shows/‍

Phung, T. K., Pham, T. L. M., Vu, K. B., & Busca, G. (2021). (Bio) Propylene production processes: A critical review. Journal of Environmental Chemical Engineering, 9(4), 105673.

Ren, T., Patel, M., & Blok, K. (2006). Olefins from conventional and heavy feedstocks: Energy use in steam cracking and alternative processes. Energy, 31(4), 425-451.

Tiseo, I. (2024). Annual Global emissions of carbon dioxide 1940-2023. Statista. https://www.statista.com/statistics/276629/global-co2-emissions/.