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.
A novel catalytic coating that converts coke to carbon oxides through a reaction with steam has been developed. Several coating formulations were tested in a jet-stirred reactor setup, and the best performing formulation was further evaluated in a pilot plant setup. Application of the coating during steam cracking of ethane at industrially relevant conditions resulted in a reduction of the asymptotic coking rate by 76%. The coating activity remained constant over several coking/decoking cycles. Coupled furnace-reactor run length simulations of an industrial ethane cracking unit were performed and resulted in an increase of the run length by a factor of 6. However, the simulated CO2 yield is higher than the design value of a typical caustic tower.
25Cr-35Ni base alloys are the most frequently used materials for steam cracking reactors. The influence of cyclic aging, reactor temperature, and adding sulfur containing compounds before or during cracking on the rate of coke deposition on a classical 25Cr-35Ni alloy is evaluated using a jet stirred reactor equipped with an electrobalance. As expected, the initial and asymptotic coking rate increased with increasing reactor temperature. Scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) analysis indicated that more Ni and Fe is present on the surface at higher cracking temperatures. Presulfidation led to increased coke deposition and decreased CO yields compared to the reference. When a sulfur containing compound was added continuously, coke deposition increased significantly but carbon oxide formation was suppressed. A pronounced amount of coke was measured in the reactor, followed by suppressed generated amounts of carbon oxides downstream. When combined with the continuous addition of sulfur containing compounds, presulfidation has little effect. Depending on the conditions, the effect of aging of the material is different: during the reference run and when only presulfidation was applied, coking rates increased as the material aged. When sulfur containing compounds were added continuously, with our without presulfidation, coking rates decreased as the material aged. This can be related with increased amounts of MnCr 2 O 4 and Cr 2 O 3 observed by SEM and EDX analysis.
Alloy composition and morphology of the inner wall of steam cracking reactors are well-known key factors that affect their coking tendency. The effect of surface roughness on the coking tendency remains uncharted to date and has been studied here for a 35/25 Ni/Cr wt % alloy in a quartz jet stirred reactor equipped with an electro-balance under coil outlet industrially relevant ethane steam cracking conditions: T gas phase = 1173 K, P tot = 0.1 MPa, and X C2H6 = 70%. Up to 6 times higher initial coking rates have been observed during cyclic aging in an R α surface roughness range of 0.15–7 μm, and cyclic aging proved to have an effect mainly on the catalytic coking behavior. No effect was observed on the asymptotic coking rates. Scanning electron microscopy, energy diffractive X-ray surface analysis, and cross section elemental mappings suggest that the effect of surface roughness and aging on the catalytic coking rate derives mainly from changes in the metal surface composition. The amounts of metallic Ni and Fe show an increasing tendency with increasing surface roughness, explaining the pronounced coke deposition. Using Ekvicalc, thermodynamic calculations were performed proposing that the amount of Cr2O3 gradually decreases followed by an increase of manganese chromite, MnCr2O4.
The coking tendency under steam cracking conditions of CoatAlloy, a newly developed multilayered Al barrier coating deposited on a commercial 25/35 Cr–Ni base alloy and aimed at reducing the coke formation under hydrocarbon atmosphere at >1100 K temperatures was investigated. It was benchmarked to the uncoated commercial 25/35 Cr–Ni base alloy with a known low coking tendency in ethane steam cracking in a pilot plant. The influence of process conditions, such as coil outlet temperature, presulfidation, continuous sulfur addition and aging was evaluated. The applied coating resulted in a reduced coking tendency as well as reduced yields of both CO and CO2 compared to the uncoated coil. The surface of both tested reactor materials was studied by means of SEM and EDX analysis. Further scale up was assessed by simulations of an industrial ethane cracker. All the findings show that the CoatAlloy barrier coating is capable of reducing coke formation and maintains its anticoking activity over multiple cracking–decoking cycles.
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