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.
The service time of an industrial cracker is strongly dependent on the long-term coking behavior and microstructure stability of the reactor coil alloy. Super alloys are known to withstand temperatures up to even 1400 K. In this work, several commercially available alloys have been first exposed to a long term oxidation at 1423 K for 500 h, so-called metallurgic aging. Subsequently, their coking behavior was evaluated in situ in a thermogravimetric setup under ethane steam cracking conditions (Tgasphase = 1173 K, Ptot = 0.1 MPa, XC2H6 = 70%, continuous addition of 41 ppmw S/HC of DMDS, dilution δ = 0.33 kgH2O/kgHC) and compared with their unaged coking behavior. The tested samples were also examined using scanning electron microscopy and energy diffractive X-ray for surface and cross-section analysis. The alloys characterized by increased Cr-Ni content or the addition of Al showed improved stability against bulk oxidation and anti-coking behavior after application of metallurgic aging due to the formation of more stable oxides on the top surface.
The selection of the reactor material with the lowest coking tendency can result in substantial economic benefits for the steam cracking process. One of the remaining unresolved points of discussion is what the influence is of sulfur addition, in particular dimethyl disulfide (DMDS) on various steam cracking reactor alloys with varying Ni-Cr content. To shine some new light on this topic, an extensive thermogravimetric study was performed in a jet stirred reactor set up (JSR) evaluating on-line the coking behavior of four Ni-Cr-Fe alloys under industrially relevant ethane cracking conditions. For each material, the effect of pre-oxidation/pretreatment with and without the presence of DMDS was evaluated, with as objective to minimize the materials coking tendency. The coking rates show that an increased Ni-Cr content of the material improves the coking rates by a factor 2 or more under the studied process conditions. By continuously feeding DMDS, all non Al-containing alloys indicate 7 times higher coking rates than the Blank runs, while the carbon oxide(s) formation is suppressed by a factor 5. In comparison to continuous addition and presulfiding with DMDS, labelled as 'CA+PreS' experiments, the Al-containing alloy outperforms itself significantly when pre-oxidized at 1223 2 K by 50 % in terms of coking rates. The results indicate that Al addition to Ni-Cr-Fe alloys improves their anti-coking performance, provided that the pre-oxidation temperature is higher than for materials without Al. The overall results from coking rates and off-line SEM and EDX analysis for the coked coupons showed an outstanding oxidation homogeneity for the 40/48 Cr-Ni alloy, which was better than the Al-containing alloy at lower pre-oxidation temperatures.
Alumina-based coatings have been claimed as being an advantageous modification in industrial ethylene furnaces. In this work, on-line experimentally measured coking rates of a commercial coating (CoatAlloy™) have pointed out its superiority compared to an uncoated reference material in an electrobalance set-up. Additionally, the effects of presulfiding with 500 ppmw DMDS per H2O, continuous addition of 41 ppmw S per HC of DMDS, and a combination thereof were evaluated during ethane steam cracking under industrially relevant conditions (Tgasphase = 1173 K, Ptot = 0.1 MPa, XC2H6 = 70%, dilution δ = 0.33 kgH2O/kgHC). The examined samples were further evaluated using online thermogravimetry, scanning electron microscopy and energy diffractive X-ray for surface and cross-section analysis together with X-ray photoelectron spectroscopy and wavelength-dispersive X-ray spectroscopy for surface analysis. The passivating coating illustrated a better performance than the reference Ni-Cr Fe-base alloy after application of an improved pretreatment, followed by piddling changes on the product distribution. Presulfiding of the coating affected negatively the observed coking rates in comparison with the reference alloy, so alternative presulfiding and sulfur addition strategies are recommended when using this barrier coating.
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