Optimization of the in Situ Pretreatment of High Temperature Ni–Cr Alloys for Ethane Steam Cracking

• Published in: Industrial & engineering chemistry research 2017-02, Vol.56 (6), p.1424-1438
• Main Authors: Sarris, Stamatis A, Olahova, Natalia, Verbeken, Kim, Reyniers, Marie-Françoise, Marin, Guy B, Van Geem, Kevin M
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Coking; Cracking (chemical engineering); Ethane; Intermetallic compounds; Optimization; Pretreatment; Reactor materials
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/acs.iecr.6b04537

Coke inhibition of reactor materials is one of the major research areas in the field of steam cracking. Selecting the optimal in situ pretreatment of a steam cracking coil depends on many different aspects such as the reactor material composition, the process conditions, the pretreatment duration, the atmosphere, and the used additives. Therefore, the effect of eight different pretreatments on the coking resistance of a classical Ni/Cr 35/25 high temperature alloy is evaluated in a thermogravimetric setup with a jet stirred reactor under industrially relevant ethane steam cracking conditions (dilution 0.33 kg H2O/kg C2H6, continuous addition of 41 ppmw S/HC at T = 1160 K, equivalent ethane conversion 68%). Next to the sequence of the preoxidation and steam pretreatment, also presulfiding was evaluated. The coking results proved that a high temperature preoxidation, followed by a steam/air pretreatment at 1173 K for a duration of 15 min, has the best coking performance under ethane cracking conditions. This pretreatment results in a factor of 5 reduction of the coking rate compared to the standard pretreatment used as a reference case. SEM and EDX cross section and surface analyses show that the increased homogeneity of the oxide layer formed together with the Cr and Mn layer passivates the catalytic behavior of the alloy, while the presence of Fe and Ni on the surface leads to increased catalytic and pyrolytic coke formation, which was the case when presulfiding was applied. Optimization of the pretreatment clearly pays off; however, the optimum will be different depending on the starting material.