(94e) Catalytic Coating For Reduced Coke Formation: Experimental and Model Validation

Coke formation on the inner wall of the tubular cracking reactors of steam cracking units has a major influence on the process’ energy efficiency and economic viability. Hence, many efforts are continuously made towards the development of technologies to reduce coke formation. One such technology is the application of a coating on the reactor inner wall. Distinction can be made between the coatings that passivate the inner coil wall [1, 2] and catalytic coatings [3] that convert cokes to carbon oxides.

In this paper we describe our four-year research effort that has discovered a robust, novel coating technology for the inner wall of the furnace tube that prevents this deposition of coke. This coating is based upon a family of ceramic catalysts having a doped perovskite structure that are capable of converting coke to carbon oxides on contact. Thus, when coke forms during cracking it is instantaneously gasified on contact with the wall. We will describe the experiments that led to the discovery of this family of catalysts with tunable activities, as well as methods of applying this catalyst as a coating to the IDs furnace tubes. We have established that the coating is robust to thermal shocks, exposure to a range of common impurities, to multiple, severe decoking cycles and to a 50% decrease in steam dilution. The performance of the coating was further tested in a Jet Stirred Reactor (JSR) set-up  and at the UGent pilot plant for steam cracking [4].

The JSR was used to assess the effect of different coating formulations on both coke formation and product yields, including CO and CO₂. Via thermogravimetric analysis, the amount of coke deposited is measured over time, allowing calculation of both initial and asymptotic coking rates. These rates are compared to those of a reference uncoated alloy. The experiments allowed the optimization of the catalyst activity to reduce coke formation while minimizing the production of carbon oxides. The coating activity after several coking/decoking cycles remained stable.

Several different coatings were also tested on a larger scale in a pilot plant.  This allowed assessing the coating’s performance under typical industrial conditions in a well-controlled and monitored environment. The influence of several process conditions (coil-outlet-temperature, continuous sulfur addition, presulfidization and dilution) and feeds (both ethane and naphtha) was evaluated. Drastically reduced amounts of coke were consistently measured compared to an uncoated reference coil. These pilot experiments showed that the catalyst is robust and maintains anti-coking activity for the equivalent of 10+ years in accelerated life testing.

[1] P. Broutin, F. Ropital, M.F. Reyniers, G.F. Froment, Anticoking coatings for high temperature petrochemical reactors, Oil & Gas Science and Technology - Revue d'IFP Energies Nouvelles, 54 (1999) 375-385.

[2] A. Ganser, K.A. Wynns, A. Kurlekar, Operational experience with diffusion coatings on steam cracker tubes, Materials and Corrosion-Werkstoffe Und Korrosion, 50 (1999) 700-705.

[3] S. Petrone, Y. Chen, R. Deuis, L. Benum, D. Gent, R. Saunders, C. Wong, Catalyzed-assisted Manufacture of Olefins (CAMOL): Realizing Novel Operational Benefits from Furnace Coil Surfaces, in:  AIChE 2008 Spring National Meeting, New Orleans, Louisiana, 2008.

[4] S.P. Pyl, C.M. Schietekat, M.-F. Reyniers, R. Abhari, G.B. Marin, K.M. Van Geem, Biomass to olefins: Cracking of renewable naphtha, Chemical Engineering Journal, 176 (2011) 178-187.