Abstract
Low-coking reactor material technologies are key for improving the performance and sustainability of steam crackers. In an attempt to appraise the coking performance of an alternative Ti–base alloy during ethane steam cracking, an experimental study was performed in a jet stirred reactor under industrially relevant conditions using thermogravimetry (Tgasphase = 1173 K, Ptot = 0.1 MPa, XC2H6 = 70%, and dilution δ = 0.33 kgH2O/kgHC). Initially, a typical pretreatment used for Fe–Ni–Cr alloys was utilized and compared with a pretreatment at increased temperature, aiming at better surface oxidation and thus suppressing coke formation. The results revealed a decrease in coking rates upon high temperature pretreatment of the Ti–base alloy, however, its coking performance was significantly worse compared to the typically used Fe–Ni–Cr alloys, and carbon oxides formation increased by a factor of 30 or more. Moreover, the analyzed coupons showed crack propagation after coking/decoking and cooling down to ambient temperature. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy indicated that the prompt and unsystematic oxidation of the surface and bulk caused observable crack initiation and propagation due to alloy brittleness. Hence, the tested Ti–base alloy cannot be considered an industrially noteworthy steam cracking reactor alloy.
Highlights
The predominant process for manufacturing light olefins is steam cracking of hydrocarbons [1].Coking of the used materials in different parts of the cracker is one of the key problems of this process.The generated coke is formed on the inner surface and reduces the available cross-sectional area causing an elevated pressure drop along the reactor, convection section, and Transfer Line Exchanger (TLE).Higher average pressures decrease the light olefin’s selectivity because they promote bimolecular over monomolecular reactions
The increased coking upon cyclic aging and the increased values of carbon oxides and H2 formation observed for the Ti–base alloy suggest that oxide spalling and/or crack formation result in exposure of the fresh catalytic sites
Ti–6Al–4V alloy alloy has has been been evaluated evaluated as potential material
Summary
The predominant process for manufacturing light olefins is steam cracking of hydrocarbons [1]. The first technology consists of, among others, the use of finned [5,8], ribbed [6], or partially ribbed tubes and swirl flow [7] tubes All these technologies lead to an increased pressure drop compared to conventional bare tubes, and affect the ethene selectivity. Steam cracking reactors are typically made out of heat-resistant Fe–Ni–Cr alloys (25/35 Cr/Ni) [22,23]. Steam cracking of hydrocarbons—a noncatalytic process—coke forms on the pretreated reactor material, which is traditionally [34] oxidized to inhibit carbon formation. In this work the coking performance of a Ti–base alloy (Ti–6Al–4V), under ethane steam cracking conditions, is studied using a thermogravimetric setup. Scanning electron microscope and energy-dispersive X-ray (SEM and EDX) results help with the interpretation of the coking and product distribution
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