(35d) Effect of Contaminants on Coke Formation during Steam Cracking of Propane

The production of light olefins through steam cracking is considered to be a mature technology [1]. However, a lot of room for optimization is still available, especially in the energy intensive unavoidable decoking steps, with fewer decoking steps leading to higher thermal efficiency of the ethylene furnace [2]. The aim of decoking is to rapidly, safely and completely remove the coke formed in the coil, and subsequently reform a continuous oxide layer on the inner side of the reactor wall [3].

In conventional steam cracking feedstocks, contaminants such as sulfur, phosphine, and other heavy metal components are generally present in ultra-trace (ppb) levels which have a direct effect on rate of coke formation in the furnace coils. It has been a normal practice to treat the feedstock to adjust the contaminant levels within close limits to achieve beneficial results in steam cracking.

The purpose of this study was to mimic industrial conditions on rate of coke formation from hydrocarbons in steam cracking with contaminants such as sulfur and phosphorus compounds in ppm and even ppb level. To assist with this research, a thermogravimetric study was performed in a plug flow reactor set up (PFR) evaluating on-line the coking behavior on 35/45 Cr/Ni and Al-content reactor materials (coupons) under the influence of aforementioned contaminants. The coupons were initially pretreated under the presence of steam from 300 °C to 900 °C with 50 °C/h to create a sufficient oxide barrier. The cracking experiments consisted of two cracking cycles per contaminant with four-hour propane cracking at 950°C followed by ramping-up to 1050°C for thirty-minutes per cycle, latter resembling the end-of-run industrial conditions. The first glance results indicate that the presence of phosphine has a positive influence on coke formation by approximately a factor 2. Subsequently, the reactor effluent was quenched to prevent further cracking and its composition was measured online with two gas chromatographs (GC) using nitrogen as internal standard i.e. a refinery gas analyzer dedicated to the analysis of components with less than 5 carbon atoms and a TRACEâ„¢ Ultra GC detecting hydrocarbons ranging from methane to naphthalene. The coupons were subjected to helium atmosphere during stabilization modes (overnight). The morphology of the coked reactor materials were studied using an off-line Scanning Electron Microscope with Energy Dispersive X-ray (SEM with EDX) images of coked coupons.

References

1. ExxonMobil, The Outlook for Energy: A view to 2040. 2012.
2. Ren, T., M.K. Patel, and K. Blok, Steam cracking and methane to olefins: Energy use, CO2 emissions and production costs. Energy. 33(5): p. 817-833. 2008
3. Muñoz Gandarillas, A.E., et al., Coking Resistance of Specialized Coil Materials during Steam Cracking of Sulfur-Free Naphtha. Industrial & Engineering Chemistry Research. 53(35): p. 13644-13655. 2014