Inductively Heated Electro-Balance Unit for Studying Coke Formation and Carburization for High-Temperature Alloys during Steam Cracking

Steam cracking in a semi-industrial dual fluidized bed reactor: Tackling the challenges in thermochemical recycling of plastic waste

Maximizing olefin production via steam cracking of distilled pyrolysis oils from difficult-to-recycle municipal plastic waste and marine litter

Assessing the feasibility of chemical recycling via steam cracking of untreated plastic waste pyrolysis oils: Feedstock impurities, product yields and coke formation

Radiant coils in steam-cracking furnaces operate under severe reducing and oxidizing conditions at temperatures even beyond 1100 °C. Sulphur is present in feedstocks as natural constituent (naphtha cracking) or as deliberate addition (ethane cracking), and can influence the coke formation and also the stability of tube materials. The influence of sulfur as H2S-addition on the stability or degradation of tube materials and on the coke formation was studied. The investigated samples were alumina-forming- as well as chromia-forming alloys taken from conventional radiant tubes. In a laboratory-scale reactor comparative cracking-decoking tests were applied to the samples. Testing conditions were relevant to industrial steam cracking. Sulfur was continuously added as H2S to the synthetic feed. Sample temperature could be varied during cracking up to 1100 °C. The influence of continuous H2S addition to the feed on coke formation and on the tube material deterioration was studied. The investigations reflect the impact of sulfur on the rate of coke formation and on the coke morphology. Particular focus is on how sulfur influences the physical and chemical nature of catalytically active sites. Sulphur stimulates a process of carbon-induced corrosion, which particularly can deteriorate chromia-forming alloys, whereas alumina scales are resistant.

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. Thi...

Contaminant removal from plastic waste pyrolysis oil via depth filtration and the impact on chemical recycling: A simple solution with significant impact

Sulfur‐containing compounds play a key role in many industrial processes. Particularly for the steam cracking process, they have been linked with increased olefin selectivity, CO formation, and coke inhibition. The influence of four different sulfur‐containing additives, methanedithione, (methyldisulfanyl)methane, (methylsulfanyl)methane, and dimethyl sulfoxide, on product selectivity, coke deposition, and CO production during steam cracking of a surrogate light naphtha feed is investigated. The use of online comprehensive 2D gas chromatography with sulfur chemiluminescence detection (GC×GC‐SCD) is the key enabling technology to characterize the sulfur compounds. Steam cracking in a pilot‐plant unit revealed that all studied sulfur compounds are efficient in reducing the CO yield. Simultaneously, they strongly promote coke.

In conventional steam cracking feedstocks, contaminants such as sulfur, phosphine, and heavy metal components, present in trace levels, are believed to affect coke formation on high temperature alloys. To gain an understanding of the role of phosphine coking rates on 25/35, CrNi and Al-containing reactor materials were determined in a plug flow reactor during cracking of a propane feedstock doped with ppb levels of PH3 in the presence of DMDS. The presence of phosphine decreased the asymptotic coking rates by more than 20%, while it had a smaller influence on the catalytic coking rate. The coking rate was more severely reduced for the 25/35 CrNi alloy in comparison to the Al-containing alloy. The ppm levels of phosphine did not affect the olefin yields nor the production of undesired carbon monoxide. The morphology of the coked alloys were studied using an off-line Scanning Electron Microscope with Energy Dispersive X-ray detector (SEM with EDX) images of coked coupons. Two types of coke morphology are observed, i.e., filamentous coke with DMDS as an additive and globular coke in the presence of phosphine. The effect of phosphine on the material has a positive impact on the oxide scale homogeneity of 25/35 CrNi alloy, whereas the Al-containing alloy remained unchanged.

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–d...

Although steam cracking is a mature technology, mitigation of coke formation remains one of the main challenges in the petrochemical industry. To increase the olefin output of existing plants, coil materials that can withstand higher temperatures are desired. This work reviews material technologies that were developed and tested in the past three decades to minimize the rate of coke deposition and extend the furnace run length. The material not only determines the mechanical properties of the coil but also affects the coking rate substantially. In some cases, differences in coking rates by more than a factor 10 have been observed. SiC materials could be operated at significantly higher temperatures, and this leads to higher olefin selectivity if one includes acetylene hydrogenation; however, the mechanical joints make it currently impossible to take advantage of their superior temperature resistance. On the industrial scale, operational improvements have been reported with advanced reactor surface technol...

The influence of phosphorus containing compounds on steam cracking of n-hexane

Combined characterization using HT-GC × GC-FID and FT-ICR MS: A pyrolysis fuel oil case study

Coke formation by steam cracking of propane over preoxidised and prereduced alloy foils has been studied in a tubular reactor at 810–850°C. Coke formation on preoxidised steel involves coke formation on the oxide scale which is low and coke formation on metal exposed by spalling of the scale which is high. Surface analysis by Auger spectroscopy (AES) reveals that the surface concentration of iron and nickel is low on the oxide scale and high on the exposed metal. Coke formation on the surface of the prereduced steel is low and the surface contains chromium and manganese as the predominant metal components. Differences in the product spectra obtained over preoxidised and prereduced steels is suggested to be due to the presence of surface iron and nickel which increases the production of coke and hydrogen and decreases the production of olefins in steam cracking.

The influence of the combination of two Si-containing additives, BTMS and TEOS, with DMDS on coke formation during steam cracking has been evaluated both on a laboratory scale and in a pilot plant unit. Under the optimal presulfidation conditions (T = 1023 K, H2O = 20 g h-1, DMDS in H2O = 750 ppm wt, duration = 1 h), the combination of Si pretreatment + presulfidation + continuous addition of 2 ppm wt DMDS results in a decrease in the rate of coke formation up to 40% when hexane is cracked in the lab-scale unit. Under similar conditions in the pilot plant the coke formation is decreased by 70%, while the CO production decreases by more than 90%. Moreover, the suppressing effect on coke formation remains significant even after several coking/decoking cycles. Simulations of an industrial ethane cracker indicate that the application of Si- and S-containing compounds as additives for the suppression of coke formation can potentially double the run length of industrial steam crackers.

Thermal cracking is always accompanied by coke formation, which becomes deposited on the wall and limits heat transfers in the reactor while increasing pressure drops and possibly even plugging up the reactor. This review article covers undesirable coking operations in steam craking reactors. These coking reactions may take place in the gas phase and/or on the surface of the reactor, with coke being produced during pyrolysis by a complex mechanism that breaks down into a catalytic sequence and a noncatalytic sequence. After a brief description of different experimental set-ups used to measure the coke deposition, on the basis of research described in the literature, the different factors and their importance for coke formation are listed. In particular, we describe the effects of surface properties of stainless-steel and quartz reactors as well as the influence of the cracked feedstock, of temperature, of dilution, of residence time and of the conversion on coke deposition. Some findings about the morphology of coke are described and linked to formation mechanisms. To illustrate this review, some particularly interesting research is referred to concerning models developed to assess coke formation during propane steam cracking.