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

Abstract

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.

Effect of Long-Term High Temperature Oxidation on the Coking Behavior of Ni-Cr Superalloys

“…Unfortunately, the improved performance of the Al-containing alloy is not maintained during the pyrolytic coking stage. Here, the ranking is identical to the original one, with most of the alloys performing similarly to the cyclic aged ones, supporting the idea that the strongest factors influencing the radical pyrolytic coke formation are the cracking conditions, the applied pretreatment and the elemental composition, in agreement with previous observations [ 5 ].…”

“…The coke formation observations are in line with the top surface elemental compositions measured by means of EDX, see Table 4 . In Figure 6 Ni and Fe are chosen as the representative catalytic coking elements, while in Figure 7 Cr is picked for all alloys and Al is picked for the Al-containing, as representative for the passivating behavior [ 5 , 47 ]. The best anti-coking performance is measured for the 40/48 Cr/Ni alloy that has the lowest cumulative concentration of Fe and Ni on its surface, but also the highest amounts of Cr.…”

“…The experimental unit is described in detail in earlier work [ 5 , 41 , 51 ]; however, for practical considerations a short description with the adaptation of some minor parts is included. Figure 2 gives a simplified diagram of the feed section of the unit.…”

“…To keep the cracking severity stable over time and to compensate for the additional conductive heat resistance, the fuel flow rate to the furnace burners has to be increased, which makes that the reactor tube metal temperature rises over time. When the coke or the temperature of the coil is approaching the maximum allowable value, a decoking procedure with a steam/air mixture is necessary [ 3 , 4 , 5 ]. This cyclic cracking-decoking operation affects the annual production capacity if the plant is furnace limited, resulting in higher operating costs.…”

Modelling coke formation in an industrial ethane‐cracking furnace for ethylene production

“…Coke formation is a complex phenomenon. The major mechanisms for coke formation are catalytic (also referred to as heterogeneous catalytic), pyrolytic (also referred to as heterogeneous non‐catalytic), and condensation (also referred to as homogeneous non‐catalytic). Catalytic coke is formed by a series of reactions catalyzed by nickel and iron present on the surface of the coils .…”