Oxidative Cracking Research Articles

Effect of a CeyNi1−yO2−δ solid solution on the oxidative cracking of vacuum residue over NiK/CeO2

In this study, a NiK/CeO2 catalyst was employed to investigate the interaction between Ni metal and CeO2 support and its effect on the oxidative cracking of vacuum residue (VR). In the reaction results, a large amount of light oils, including naphtha, diesel and VGO, was produced through oxidative cracking over CeO2, and the quality of the liquid products was significantly improved by hydrogenation. The interaction between Ni metal and CeO2 support induced the formation of a CeyNi1−yO2−δ solid solution and increased the number of oxygen vacancies, thus enhancing the oxidative cracking of VR. Further, the addition of NiK into the supports provided Ni metallic sites for hydrogenation, producing more liquid products with a high H/C ratio. As a result, the NiK/CeO2 catalyst showed a higher diesel yield (22.87%) than that without catalyst (9.14%).

Internal oxidation and probabilistic fracture model of irradiation assisted stress corrosion cracking in stainless steels

A probabilistic fracture model is applied to irradiation assisted stress corrosion cracking effect, assuming that the oxidized part of stainless steel sample plays an essential role in the crack initiation, propagation and the sample failure. The Weibull statistical distribution of time-to-failures, estimated through the correlation with the statistical distribution of oxide strengths, including both the surface oxide layer and oxidized grain boundaries, fully describe the experimental scatter obtained in the constant load time-to-failure tests. Large failure uncertainties in these type of tests originate from an intrinsic stochastic behavior of the oxide cracking due to subcritical crack propagation process.

The degradation mechanism of Inconel 690TT induced by fretting wear in air

The research of degradation mechanisms of Inconel 690TT during fretting wear in air indicated that degradation mechanisms varied with the fretting regimes. In gross slip regime, the major degradation mechanisms were deformation, oxidation and delamination cracks in the worn subsurface. Delamination cracks in wear debris layer were inclined to nucleate at tribology transformed structure/wear debris layer interface as well as worn surface, accelerating the detachment of material. In partial slip regime, the major degradation mechanisms were deformed asperities at adhesion zone of worn center, oxidation at worn edge and fatigue cracks at the junction of adhesion/oxidation zone in the mode of transgranular and intergranular.

Oxidative Catalytic Cracking of Heavy Oil Residues

<p>The features of oxidative cracking of vacuum distillate of Aturau and Zhetybai oil have been investigated. It was found that the yield of gasoline and light gas oil depends on the ratio of natural zeolite and clay in the composite catalysts, flow rate of air and 0.2 % catalyst suspension in the raw material and temperature of the cracking. It was shown that maximum yield of gasoline, kerosene and gas oil was achieved with the composite catalyst containing 14 % of activated zeolite. Suspension of this catalyst significantly promotes gas formation during oxidative cracking as compared to that for cracking in the inert atmosphere. This proves that catalytic destruction of high molecular hydrocarbons in the presence of trace amount of air is increased. The yield of light gas oil is increased whereas yield of gasoline and gas is decreased when flow rate of 0.2 % catalyst suspension in raw material is 1.0 hour<sup>-1</sup> as compared to that for 0.1 hour<sup>-1</sup> flow rate in oxidative cracking conditions. It was found (GLC method) that formation of diene and cyclodiene hydrocarbons during oxidative cracking at 0.1 hour<sup>-1</sup> flow rate proceeds better than that for cracking in inert atmosphere. Oxidative cyclization into arenes proceeds when flow rate is 1.0 hour<sup>-1</sup> whereas oxidative olefin dehydrogenation proceeds when flow rate is 0.1 hour<sup>-1</sup>. Alpha olefins were found in gasoline, kerosene and gas oil fractions (IR-spectroscopy).</p>

Oxidative cracking of crude oil by hydrogen peroxide in the presence of iron oxide nanoparticles

The interaction of hydrogen peroxide aqueous solutions with crude oil and high-boiling refined products, such as fuel oil and vacuum gas oil, in the presence of an oxidative cracking catalyst in the form of iron oxide nanosized particles is studied. This study is aimed at modeling processes occurring in the case of using hydrogen peroxide solutions in the catalytic cracking of crude oil. It is found that, in the presence of iron particles, the reaction of hydrogen peroxide decomposition causes the cracking of petroleum hydrocarbons. This process may be accompanied by reduction in the viscosity and density of crude oil and refined products. The reaction of catalytic cracking performed under these conditions leads to a marked increase in the fraction of light hydrocarbons in the composition of crude oil and high-boiling refined products.

Comprehensive experimental investigation and numerical modeling of the combined partial oxidation-gasification zone in a pilot downdraft air-blown gasifier

A pilot downdraft gasifier is investigated experimentally using almond shell biomass as feedstock. Experimental temperature profiles along the different zones of the gasifier are measured and overlapping regions between the different gasifier stratified zones is proved: we identified two main zones (drying-pyrolysis zone and partial oxidation-reduction zone) instead of four stratified zones as proposed in the literature. In light of this experimental finding, a two dimensional mathematical model is developed for the combined partial oxidation-reduction zone based on conservation equations coupled to the heterogeneous and homogeneous chemistry. Partial oxidation and thermal cracking mechanisms of tar are proposed based on the available kinetic data. The model is developed for the quasi-steady state period of the experiment and used to simulate the heat and mass transport fields within the computational domain, analyze the interaction between the heterogeneous and homogeneous reactions and evaluate the performance of the gasifier in terms of tar conversion and syngas composition. Validation against the mean experimental temperature data and the producer gas composition at the outlet of the reactor are presented and a satisfactory agreement is observed. The influence of pyrolysis gas composition, air flow rate and bed porosity on the process and its outputs is also investigated.

On the development of a polyolefin gasification modelling approach

Gasification is a promising alternative for polymeric waste valorization when mechanical recycling is unfeasible on account of its heterogeneity or partial contamination, or simply when it yields a product of lower quality than what the market requires. Apart from its application for electricity generation, the waste-derived syngas shows a great potential for chemical waste recycling for the synthesis of hydrogen, methane, natural gas or methanol. In spite of the effort devoted so far to the experimental demonstration of these processes to enable this technology to access commercial stage, it is still necessary to develop detailed models of the process that allow a precise prediction of the resulting syngas composition, as well as tar formation and global efficiency of the process. This research work presents the development of a polyolefin gasification model for fluidized bed reactors. The model details the behaviour of primary pyrolysis and homogeneous reactions of oxidation, steam reforming, aromatization and thermal cracking. To accomplish this, it adopts new modelling strategies for the definition of primary tar species in order to reflect their twofold nature (aliphatic and aromatic), as well as to describe kinetics and stoichiometry involved in thermal cracking processes of tar species. The model is able to successfully predict the generation, volume composition and heating value of the syngas, final tar generation and global efficiency of the process.

Upgrading of Heavy Oil in Supercritical Water using an Iron based Multicomponent Catalyst

Abstract This article deals with the upgrading of bitumen in supercritical water (SCW) using a ZrO2-Al2O3-FeOx catalyst in a batch autoclave reactor. The feed bitumen is diluted using different amount of benzene as solvent in order to provide intimate contact between the solid catalyst particles. It also facilitates the diffusion of the heavy molecules into the catalyst particles. The batch reactor results show that the extra heavy oil is slightly decreased with increasing the bitumen concentration from 20 wt % to 30 wt%. No significant change in the product is observed when bitumen concentration is increased above 30 wt% level. However, the coke concentration is increased with increasing bitumen in the feed. The catalyst deactivation is also higher at higher bitumen concentrations (above 20 wt %). XRD analysis reveals that at high bitumen concentrations (above 30 wt %), transformation of magnetite into hematite is lower than that of hematite to magnetite due to consumption of lattice oxygen by the additional hydrocarbons, which also causes the catalyst deactivation. On the contrary, lower bitumen concentration (~10 wt %) maintains hematite as main phase, which is the desirable for sustained catalytic activity for the oxidative cracking reaction. It is believed that SCW acts as source of reactive hydrogen and oxygen species that favors the upgrading process. The reactive hydrogen species react with cracked hydrocarbons to produce stable lighter liquid products. On the other hand, the reactive oxygen species react with oxygen depleted catalysts, which transforms magnetite into hematite and keep the catalyst active.

Oxidative cracking of vacuum residue with steam over NiK/CeZr–Al catalysts

NiK catalysts supported on CeyZr1−yO2–Al2O3 (NiK/xCeZr–Al) (x=0, 5, 10, 20, 30, and 40) systems were prepared as novel catalysts for cracking of vacuum residue with steam and their catalytic performances were tested using a fixed bed reactor at 500°C and under atmospheric pressure. The addition of Ce and Zr was found to greatly improve the catalytic performance, increasing the total yield of liquid plus gaseous products as well as the H/C ratio of the liquid products. Active oxygen species were generated from steam over zirconia particles and then spilled over the CeyZr1−yO2 surface, where oxidative cracking of heavy oil occurred to produce CO2 and lighter molecules. Active hydrogen species obtained from steam decomposition were then added to these lighter molecules on nickel active phases, resulting in the increased H/C ratio of the liquid products. Oxidative cracking with steam over NiK/CeZr-Al catalysts has great potential as industrial processes of vacuum residue cracking.

Crack Initiation in Austenitic Stainless Steel Sanicro 25 Subjected to Thermomechanical Fatigue

Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life.

Effect of oxidation time on the tribological behavior of thermally oxidized commercially pure zirconium under dry sliding conditions

Thermal oxidation is an effective surface engineering technique to harden the surfaces of zirconium (Zr) and its alloys for improvement in friction and wear performance. In the present investigation, commercially pure zirconium (CP-Zr) is oxidized at 650°C for a wide range of times from 1h to 72h with the aim to study the effect of oxidation time on the tribological performance of CP-Zr. It is found that a dense, pore-free and adherent zirconium dioxide (ZrO2) layer can be produced at the surface for oxidation times less than 12h. Further increasing oxidation time leads to the gradual development of pores in the inner part of the oxide layer and oxidation breakaway characterized by accelerated oxidation kinetics and crack formation. Oxidation time has a significant effect on the tribological behavior of thermally oxidized CP-Zr under dry sliding conditions. The 6μm thick oxide layer produced by 6h treatment possesses the lowest friction, best wear resistance and the highest load bearing capacity. On the other hand, the thicker oxide layers produced by longer treatment times show deteriorated tribological behavior. The results are discussed in terms of the morphology of the oxide layer and crack propagation in the oxide layer and the underlying diffusion zone.

Oxidation and Thermal Cracking Behavior of Compacted Graphite Iron under High Temperature and Thermal Shock

Compacted graphite iron (CGI) was subjected to high-temperature oxidation and subsequent thermal shock to investigate its oxidation and thermal cracking behavior. The two experiments were conducted to a maximum temperature of 650 °C in air. Mass changes and micro-observations were made during the experiments. Results showed that the oxidation kinetics of CGI followed the parabolic rate law approximately over three stages. The oxidation behavior of CGI showed special mechanism owing to the existence of graphite. The interconnected graphite structure contributed to the inner iron oxidation and decarburization through the graphite–matrix interface. It was found that thermal cracks always nucleated on the surface stress concentration areas. A mutually reinforcing effect was presented between thermal cracking and oxidation. In the thickness direction, the interconnected graphite-oxide network was detected, and it would act as the potential path for thermal crack propagation during subsequent thermal shock cycles.

Extracting oxide scale cracking data from acoustic emission data using the example of oxidized cobalt

4‐point bending with in situ acoustic emission measurement has proven to be a successful route for determining the critical strain values for various oxide scales. In combination with metallographic investigations, such critical strain data can be used to establish mechanical stability diagrams for an assessment of the maximum tolerable strain before scale failure becomes imminent. The use of the acoustic emission technique enables to detect micro‐cracking in the oxide scale without metallographic inspection, thereby, providing access to the critical strain required to initiate oxide cracking. However, in some cases, acoustic emission is generated not only by oxide cracking but also from plastic deformation processes within the metal substrate. In this work, a tailored analysis procedure is developed for discriminating the signals generated by oxide scale cracking from those generated by plastic deformation in the cobalt substrate. From the resulting critical strain data, the above‐mentioned failure diagrams were deduced for cobalt oxide scales grown on high‐purity cobalt at 650 °C in dry synthetic air, and synthetic air with 10 vol% H2O, respectively.

Effect of pressure on the oxidative cracking of C2—C4 alkanes

The influence of pressure on the oxidative cracking of light alkanes C2—C4 was investigated. An elevated pressure reduces the temperature of oxycracking of light alkanes but with further increase in pressure the effect is reduced. The applied pressure decreases the temperature of the total conversion of oxygen while the maximum conversion of alkanes is not influenced. The pressure above atmospheric promotes oxidative cracking reactions but weakly affects thermal processes. At deep conversion of light alkanes, the selectivity towards main products is nearly invariable at the utilized pressures.

Multi-Dimensional Simulation of LWR Fuel Behavior in the BISON Fuel Performance Code

Nuclear fuel operates in an extreme environment that induces complex multiphysics phenomena occurring over distances ranging from inter-atomic spacing to meters, and times scales ranging from microseconds to years. To simulate this behavior requires a wide variety of material models that are often complex and nonlinear. The recently developed BISON code represents a powerful fuel performance simulation tool based on its material and physical behavior capabilities, finite-element versatility of spatial representation, and use of parallel computing. The code can operate in full three dimensional (3D) mode, as well as in reduced two dimensional (2D) modes, e.g., axisymmetric radial-axial (R-Z) or plane radial-circumferential (R-θ), to suit the application and to allow treatment of global and local effects. A BISON case study was used to illustrate analysis of Pellet Clad Mechanical Interaction failures from manufacturing defects using combined 2D and 3D analyses. The analysis involved commercial fuel rods and demonstrated successful computation of metrics of interest to fuel failures, including cladding peak hoop stress and strain energy density. In comparison with a failure threshold derived from power ramp tests, results corroborate industry analyses of the root cause of the pellet-clad interaction failures and illustrate the importance of modeling 3D local effects around fuel pellet defects, which can produce complex effects including cold spots in the cladding, stress concentrations, and hot spots in the fuel that can lead to enhanced cladding degradation such as hydriding, oxidation, CRUD formation, and stress corrosion cracking.

Correlation between microstructure and wear behavior of AZX915 Mg-alloy reinforced with 12 wt% TiC particles by stir-casting process

The present work concerns with correlation between microstructure and wear behavior of AZX915 Mg-alloy reinforced with 12 wt% of TiC particles by stir-casting process. Dry sliding tests were performed under ambient environment by using a pin-on-disc (EN8 steel) configuration with a normal load of 50 N at a constant sliding speed of 2.50 ms−1. While as-cast composite experienced delamination wear, heat treated composite suffered from delamination and oxidation wear during dry sliding contact. Moreover, the heat treated composite exhibited lower friction and higher wear rate as compared to the as-cast composite. Friction and wear behavior were correlated with microstructures based on the concept of oxidation tendency and crack nucleation/propagation. Further, a schematic model has been proposed illustrating wear mechanisms from the point of view of subsurface microstructural evolution of the AZX915-TiCp composite.

Kinetics of oxidative cracking of n‐hexane to olefins over VOx/Ce‐Al2O3 under gas phase oxygen‐free environment

The kinetics of oxidative cracking of n‐hexane to olefins using lattice oxygen of VOx/Ce‐Al2O3 is investigated. The TPR/TPO analysis shows a consistent reducibility (79%) of VOx/Ce‐Al2O3 in repeated redox cycles. The total acidity of the sample is found to be 0.54 mmol/g with 22% are strong acid sites that favors olefin selectivity. The oxidative cracking of n‐hexane in a fluidized CREC Riser simulator gives approximately 60% olefin selectivity at 30% n‐hexane conversion. A kinetic model is developed considering (1) cracking, (2) oxidative dehydrogenation (ODH), and (3) catalyst deactivations. The proposed cracking mechanism considers adsorption, C–H and C–C bond fission and desorption as elementary steps and implemented by pseudo steady state hypothesis. A Langmuir‐Hinshelwood mechanism is found to represent the ODH reactions. The developed model fits the experimental data with favorable statistical indicators. The estimated specific reaction rate constants are also found to be consistent with the product selectivity data. © 2016 American Institute of Chemical Engineers AIChE J, 63: 130–138, 2017

On the development of a wood gasification modelling approach with special emphasis on primary devolatilization and tar formation and destruction phenomena

Tar generation in biomass gasification still poses a considerable challenge regarding its access to commercial stage. Along with experimental research, it is essential to develop comprehensive models of the process that allow accurate prediction of the resulting syngas composition, as well as tar generation. Traditionally, models have focused on defining heterogeneous reactions of char conversion, devoting limited attention to gas interactions. The present research offers a new wood gasification model for fluidized beds, characterized by a detailed definition of homogeneous reactions, considering both generation and evolution of created tar through oxidation, reforming and thermal cracking. The model succeeds in predicting gas generation, composition and heating value with a relative error similar or lower than analogue existing gasification models, as well as the order of magnitude of tar generation.

Acidic and basic properties of zeolite-containing cracking catalyst in the process of butene-1 isomerization

The process of butene-1 isomerization in the presence of two groups of samples of zeolite-containing catalyst (ZCC) that earlier participated in the traditional and oxidative catalytic cracking of vacuum gasoil is investigated. It is established that the nature of the reaction mixture and conditions of the cracking process are key factors in forming the acidic and basic properties of the catalyst. It is shown that the highest activity in the butene-1 isomerization into cis-/trans-butene-2 is demonstrated by ZCC samples that participated in the oxidative catalytic cracking (oxycracking). It is suggested that the enhanced catalytic activity of this group of ZCC samples was related to the availability of acid–base centers in the form of radical-like oxygen along with protic- and aprotic-type acidic centers in the structure of the oxidative compaction products.

NiK/yCexZr1-xO2-macroporous Al2O3 catalysts for cracking of vacuum residual oil with steam

In this study, NiK/yCexZr1-xO2-macroporous Al2O3 catalysts (y=0, 10, 20, and 30; x=0.73–0.92) were synthesized by dispersing different amounts of CexZr1-xO2 phase onto macroporous γ-Al2O3 as supports, and then subsequently impregnating Ni and K into the supports, which possessed advantageous properties, such as a high surface area, ordered macropores and high oxygen storage capacity. Moreover, the introduction of Ni and K metals into these supports created more oxygen vacancies in them. We applied these catalysts to the cracking of vacuum residue with steam. It was found that the variation of the liquid yield with the amount of CexZr1-xO2 phase showed a volcano pattern. The macropores in the supports played an important role in enhancing the diffusion of large molecules to the active sites, while the high oxygen storage capacity over the CexZr1-xO2 phase improved the oxidative cracking rate, thereby increasing the lighter oil fraction from the vacuum residue.