Enhancing the Overall Efficiency of Steam Cracking Furnaces

Since radiant coils are an essential part of steam cracking furnaces, they have a huge impact on the energy efficiency, while operating under severe, alternating reducing and oxidizing conditions at temperatures even beyond 1100°C. Therefore, cast alloys installed in the radiant section need to provide not only high creep strength but also good resistance to corrosion attack, as well as excellent catalytic coking resistance. Coking is not only a limiting factor when it comes to cracking cycle lengths; it also increases the need for higher firing rates, thus emitting more CO₂. Hence, the coil components (centrifugally cast tubes, cast fittings) must be able to form a dense and continuous oxide layer on the inner surface, which should remain stable when exposed to steam cracking conditions, thus restricting the development of catalytically active sites where the unfavorable formation of coke filaments takes place.

Over several decades, iron and nickel based chromia-forming alloys were installed in steam cracking furnaces. Already 17 years ago, alumina-forming alloys were introduced to the industry. New insights on the interactions between protective oxide scales formed on such chromia and alumina-forming alloys and the highly reactive cracking atmospheres provide enlightenment about the degradation mechanism, as well as the coking behavior. Sulfur present in the feedstock as natural constituent or added deliberately also strongly influences the coking rate and material deterioration. As a conclusion, chromia-forming high temperature alloys have a limited resistance to corrosion and coking in steam cracking conditions. Due to a far better thermodynamic stability, alumina scales like those formed on the Centralloy^® HT E tube material are more favorable as a means of protection in such severe conditions.

Pushing further the energy efficiency of steam cracking furnaces means to reduce specific heat input per unit of Ethylene, either decreasing the energy consumption or increasing the production capacity of the plant. To achieve this, enhanced heat transfer tubes are becoming more and more common in the industry. Some years ago S+C developed the internally profiled tube design “Scope^®”, which leads to a uniform, circumferential flow pattern, which improves the heat transfer and as thus reduces the fuel consumption. In addition, the special profile of Scope^® balances the heat distribution in the gas phase as well as in the tube shell, leading to reduced tube metal temperatures – all of these properties decrease the coking rate. Operators therefore gain a higher degree of operational freedom to increase either the conversion and ethylene yield or capacity or cracking cycle length, prolong maintenance intervals and lower greenhouse gas emissions.

In order to maximize the operational benefit of the plant, the innovative Scope^® profile can be combined with the proven Centralloy^® HT E tube material.

Performance studies and results from metallurgical investigations carried out on the alumina-forming Centralloy^® HT E after exposure to service conditions in steam cracking furnaces for several years will be presented as well as results from field applications of the Scope^® profiled tube design.

Furthermore, in 2016 Centralloy^® HT E and Scope^® have been selected within the EU-funded IMPROOF project to create the most efficient steam cracker of this time. Now that IMPROOF is completed successfully, detailed results both from the technology evaluation stages and from the industrial demonstration furnace will be shown.