Sequential Cracking Research Articles

Catalytic cracking of three-ring polycyclic aromatic hydrocarbons in the presence of hydrogen donors

Catalytic cracking of three-ring polycyclic aromatic hydrocarbons (PAHs) in the residue fluid catalytic cracking (RFCC) process was investigated. Catalytic cracking of mixtures of three-ring PAHs and various model hydrocarbons was carried out using RFCC equilibrium catalysts. Most of the three-ring PAHs were converted to coke in the absence of hydrogen donors, whereas coke formation was suppressed and decomposition of three-ring PAHs was promoted in the presence of aliphatic hydrocarbons, saturated rings, and aromatic ring side chains. Three-ring PAHs were first converted to noncondensed two-ring PAHs and subsequently decomposed to monocyclic aromatic hydrocarbons and condensed two-ring PAHs. This reaction can be depicted by partial hydrogenation of three-ring PAHs by hydrogen transfer reaction with coexisting hydrogen donors and sequential cracking of saturated rings. When a hydrogen donor was sufficiently present, cracking of saturated rings was the rate-determining step, and the reaction can be accelerated by increasing the reaction temperature or using a catalyst with a high acid content. These results are expected to lead to the effective utilization of heavy oil by promoting PAH cracking in the RFCC process.

Simulation of Crack Propagation in Reinforced Concrete Elements

The crack widths in reinforced concrete structures are affected by various influencing parameters, such as the tensile strength of the concrete, which is a highly variable parameter. While safe predictions of crack widths are possible with the existing models supplied in the code provisions accompanied by suitable safety factors, there are large discrepancies between the experimentally measured crack widths and the calculated values. Even within a uniaxial tensile test on reinforced concrete components, several crack parameters (spacings and widths) are usually measured. To further investigate the effects of scattering input parameters on the distribution of crack parameters, a crack propagation model (CPM) is developed, which is a powerful tool that can be used to observe sequential cracking of uniaxially loaded reinforced concrete components. In the present paper, the background of the scattering of the concrete tensile strength will be explained and the procedure for the simulation in the CPM, including the investigation of different bond–slip relationships, will be introduced. Finally, the CPM will be used to calculate experimental crack parameters and the distribution of the calculated crack parameters will be discussed.

Effect of steel fiber on the crack permeability evolution and crack surface topography of concrete subjected to freeze-thaw damage

This paper describes the steel fiber effect on the crack permeability and crack surface topography of concrete subjected to freeze-thaw damage. The sequential crack permeability of steel fiber reinforced concrete are investigated by a vacuum permeability set-up. The topographical analysis is applied on the crack surface by an invented 3-D laser scanning equipment. The results show that the crack permeability of concrete is less than the value predicted by the Poiseuille flow model and their difference decreases gradually with the crack widening. With increment of steel fiber dosage and freeze-thaw damage level, the effect of steel fiber on reducing the crack permeability becomes strong. Topographical analysis illustrates that both steel fiber and freeze-thaw damage enhance the roughness of concrete crack surface. The relationship between roughness number of crack surface and material permeability parameter α follows an exponential function, which can be employed to quickly estimate the crack permeability of concrete.

Stresses, deformations and damages of various joints in precast concrete segmental box girder bridges subjected to direct shear loading

Precast segmental concrete bridges involve multiple concrete segments joined together by posttensioning, and have the advantages of rapid construction, low-cost, and excellent quality control. Joints that represent locations of discontinuity are prominent factors affecting overall structural behavior of segmental bridges. In this study, a comprehensive analysis is performed for thirteen tested specimens with different joint types, subjected to direct shear loading, to assess the shear behavior of joints in precast box girder segmental bridges. The analysis focused on understanding the role of prestressing force, concrete compressive strength, presence of epoxy, and number of shear keys on: the stress and strain level in reinforcing bars, prestressing strands, displacements, type, time and mechanisms of sequential cracking and of damage level in the joints. The results showed that increasing of the confining pressure to 4.5 MPa and/or number of shear keys from 6 to 10, can highly improve both of the elastic stiffness and the plastic ductility of the segmental bridges, and changes the brittle behavior of the epoxied joints to a gradual strength degradation mode; the relative displacement of the dry joints is double of that in their identical epoxied joint; and the strains in the top layer reinforcement of the top slabs for keyed joints are ten times higher than that in the flat joint specimens.

Catalytic Copyrolysis of Lignocellulose and Polyethylene Blends over HBeta Zeolite

Coprocessing of Eucalyptus woodchips (EU) and low-density polyethylene (LDPE) has been investigated using two HBeta zeolites as catalyst. The reactions were carried out in a two-step (thermal/catalytic) system, varying the catalyst to feed (C/F) ratio. A maximum in the yield of the liquid organic fraction (oil*) was observed for both HBeta samples at intermediate C/F ratios due to the occurrence of sequential cracking reactions, which were accompanied by a significant reduction of the oil* oxygen content. This finding, together with the results obtained when feeding pure EU and LDPE, denotes that remarkable synergistic effects occur during the coprocesssing of lignocelllulosic biomass and polyolefinic plastics over HBeta zeolite, mainly by Diels–Alder condensation between furans and light olefins. The presence of acid sites with moderate strength in the HBeta samples is an essential factor to obtain high oil* yields since it limits the extension of severe cracking reactions.

Theoretical analysis of desiccation crack spacing of a thin, long soil layer

Soil desiccation cracking is important for a range of engineering applications, but the theoretical advancement of this process is less than satisfactory. In particular, it is not well understood how the crack spacing-to-depth ratio depends on soil material behaviour. In the past, two approaches, namely stress relief and energy balance, have been used to predict the crack spacing-to-depth ratio. The current paper utilises these two approaches to predict the approximate spacing-to-depth ratio of parallel cracks that form in long desiccating soil layers subjected to uniform tensile stress (or suction profile) while resting on a hard base. The theoretical developments have examined the formation of simultaneous and sequential crack patterns and have identified an important relationship between the stress relief and energy approaches. In agreement with experimental observations, it was shown that the spacing-to-depth ratio decreases with layer depth, and crack spacing generally increases with layer depth. The influence of the stiffness at the base interface indicated that decreasing the basal interface stiffness makes the crack spacing to increase in sequential crack formation. The experimental observations also show a decrease in cracking water content with the decrease in layer thickness, and this behaviour was explained on the basis of a critical depth concept.

Determination of Energy Release Rate Through Sequential Crack Extension

A method to determine the critical energy release rate of a peel tested sample using an energy-based approach within a finite element framework is developed. The method uses a single finite element model, in which the external work, elastic strain energy, and inelastic strain energy are calculated as nodes along the crack interface are sequentially decoupled. The energy release rate is calculated from the conservation of energy. By using a direct, energy-based approach, the method can account for large plastic strains and unloading, both of which are common in peel tests. The energy rates are found to be mesh dependent; mesh and convergence strategies are developed to determine the critical energy release rate. An example of the model is given in which the critical energy release rate of a 10-μm thick electroplated copper thin film bonded to a borosilicate glass substrate which exhibited a 3.0 N/cm average peel force was determined to be 20.9 J/m2.

Thickness-dependent Crack Propagation in Uniaxially Strained Conducting Graphene Oxide Films on Flexible Substrates

We demonstrate that crack propagation in uniaxially strained reduced graphene oxide (rGO) films is substantially dependent on the film thickness, for films in the sub-micron regime. rGO film on flexible polydimethylsiloxane (PDMS) substrate develop quasi-periodic cracks upon application of strain. The crack density and crack width follow contrasting trends as film thickness is increased and the results are described in terms of a sequential cracking model. Further, these cracks also have a tendency to relax when the strain is released. These features are also reflected in the strain-dependent electrical dc and ac conductivity studies. For an optimal thickness (3-coat), the films behave as strain-resistant, while for all other values it becomes strain-responsive, attributed to a favorable combination of crack density and width. This study of the film thickness dependent response and the crack propagation mechanism under strain is a significant step for rationalizing the application of layered graphene-like systems for flexible optoelectronic and strain sensing applications. When the thickness is tuned for enhanced extent of crack propagation, strain-sensors with gauge factor up to ∼470 are realized with the same material. When thickness is chosen to suppress the crack propagation, strain-resistive flexible TiO2- rGO UV photoconductor is realized.

Numerical modelling of mechanical behaviour of engineered cementitious composites under axial tension

In this paper, an extended finite element model is developed for accurate and effective modelling of the tensile strain-hardening and multiple-cracking behaviour of engineered cementitious composites (ECC) under uniaxial tension. The crack is modelled using the cohesive zone model with a simplified cohesive constitutive model accounting for the matrix and fibre bridging effect, and multiple cohesive zones are adaptively embedded within the model upon the occurrence of sequential cracking based on the extended finite element method (XFEM). The extended finite element model is implemented in the ABAQUS via the user element subroutine (UEL) for the numerical analysis of the tensile behaviour of ECC. Material randomness including random matrix flaws and random fibre distribution, which can significantly affect the tensile behaviour of ECC, has been accounted for in the proposed model. Three ECC mixes are modelled and good agreement between the computed and experimental results demonstrates the effectiveness of the proposed method for modelling the tensile behaviour of ECC. It is also shown that the two aspects of material randomness should be considered simultaneously in the model.

Hierarchical Formation of Intrasplat Cracks in Thermal Spray Ceramic Coatings

Intrasplat cracks, an essential feature of thermally sprayed ceramic coatings, play important roles in determining coating properties. However, final intrasplat crack patterns are always considered to be disordered and irregular, resulting from random cracking during splat cooling, since the detailed formation process of intrasplat cracks has scarcely been considered. In the present study, the primary formation mechanism for intrasplat cracking was explored based on both experimental observations and mechanical analysis. The results show that the intrasplat crack pattern in thermally sprayed ceramic splats presents a hierarchical structure with four sides and six neighbors, indicating that intrasplat crack patterns arise from successive domain divisions due to sequential cracking during splat cooling. The driving forces for intrasplat cracking are discussed, and the experimental data quantitatively agree well with theoretical results. This will provide insight for further coating structure designs and tailoring by tuning of intrasplat cracks.

Sequential Cracking and Their Openings in Steel-Fiber-Reinforced Joint-Free Concrete Slabs

AbstractNumerical and empirical models addressing the drying shrinkage cracking behavior of joint-free steel-fiber-reinforced concrete (SFRC) slabs are presented. Effect of water–cement ratio, admixtures, and free shrinkage are considered. Mechanical restrictions including base friction, fiber dosage, and interfacial bond properties restrain the growth of microcracks into main cracks and also reduce crack opening. A model based on a finite-difference equilibrium solution of a one-dimensional (1D) slab on frictional ground simulates the formation and subsequent opening of cracks in the slab. Results are compared with an empirical predictive tool for crack opening. A sensitivity study shows that correlation of predicted crack opening reduced by increasing fiber volume, base friction, and interfacial bond strength. Case studies are conducted on three slabs in service at different occasions and both models are used to predict the crack opening. While these models are developed based on different methodologies...

Cracking mechanisms in lithiated silicon thin film electrodes

The massive cracking of silicon thin film electrodes in lithium ion batteries is associated with the colossal volume changes that occur during lithiation and delithiation cycles. However, the underlying cracking mechanism or even whether fracture initiates during lithiation or delithiation is still unknown. Here, we model the stress generation in amorphous silicon thin films during lithium insertion, fully accounting for the effects of finite strains, plastic flow, and pressure-gradients on the diffusion of lithium. Our finite element analyses demonstrate that the fracture of lithiated silicon films occurs by a sequential cracking mechanism which is distinct from fracture induced by residual tension in conventional thin films. During early-stage lithiation, the expansion of the lithium-silicon subsurface layer bends the film near the edges, and generates a high tensile stress zone at a critical distance away within the lithium-free silicon. Fracture initiates at this high tension zone and creates new film edges, which in turn bend and generate high tensile stresses a further critical distance away. Under repeated lithiation and delithiation cycles, this sequential cracking mechanism creates silicon islands of uniform diameter, which scales with the film thickness. The predicted island sizes, as well as the abrupt mitigation of fracture below a critical film thickness due to the diminishing tensile stress zone, is quantitatively in good agreement with experiments.

Computational study of decomposition mechanisms and thermodynamic properties of molecular-type cracking patterns for the highly energetic molecule GZT

This study uses the Gaussian 03 program and density functional theory B3LYP with three basis set methods-[B3LYP/6-311+G(d,p), B3LYP/6-31+G(2d,p), and B3LYP/6-31G(d,p)]-to model the highly energetic ionic compound diguanidinium 5,5'-azotetrazolate (GZT) to research its decomposition mechanisms and thermodynamic properties. Molecular-type cracking patterns are proposed, which were initiated by heterocyclic ring opening, sequential cracking of the two five-membered rings of GZT, and simultaneous release of N2 molecules; whereas proton transfer, bond-breaking, and atomic rearrangements were performed subsequently. Finally, 15 reaction paths and five transition states were obtained. All possible decomposition species and transition states, including intermediates and products, were identified, and their corresponding enthalpy and Gibbs free energy values were obtained. The results revealed that (1) the maximum activation energy required is 187.8 kJ mol(-1), and the enthalpy change (ΔH) and Gibbs free-energy change (ΔG) of the net reaction are -525.1 kJ mol(-1) and -935.6 kJ mol(-1), respectively; (2) GZT can release large amounts of energy, the main contribution being from the disintegration of the 5,5'-azotetrazolate anion (ZT(2-)) skeleton (ΔH = -598.3 kJ mol(-1)); and (3) the final products contained major amounts of N2 gas, but remaining gas molecules such as HCN and NH3 were obtained, which are in agreement with experimental results. The detailed decomposition simulation results demonstrated the feasibility of this method to calculate the energies of the thermodynamic reactions for the highly energetic GZT and predict the most feasible pathways and the final products.

Salient factors controlling desiccation cracking of clay in laboratory experiments

This paper elucidates some of the controlling factors governing soil desiccation. The desiccation tests were conducted on three materials – clay, potato starch and milled quartz sand – all three featuring similar fracture energy. Two controlling factors were identified in desiccation cracking, regardless of the material. The first is the tensile stress and strain energy development within the material when the material is restrained against shrinkage. The distribution of the tensile stress will depend on the boundary conditions and material stiffness, and will dictate where cracks are likely to originate. The second factor is that the exact positions of crack initiations will be controlled by the flaws and/or pores within the material. For materials such as clay, with very fine particles, the cracking mechanism is governed by flaws, since the desaturation of fine pores would require very high suction stress, and this requirement leads to sequential cracking and orthogonal crack patterns. If the material has particles giving relatively large and uniform pore sizes with high moisture diffusivity leading to high shrinkage energy prior to cracking, then the fracture energy balance indicates that cracking can occur in near hexagonal patterns with 120° crack initiations, which occur predominantly simultaneously. However, even for materials with lower moisture diffusivity, such as for clay, high desiccation rates can give rise to an ‘effective layer' over which high suctions and strain energy develop, leading to almost simultaneous dense cracking.

Oxidation of Zircaloy-2 studied using a wedge-shaped specimen

Experiments have been carried out to vary the stress in the oxide layer during oxidation of Zircaloy-2. This was achieved by varying the thickness of the metal substrate, using a specimen with a tapered wedge-shaped cross-section. X-ray diffraction measurements confirmed that the compressive stress level in the oxide was reduced when the metal substrate was thinner. The rate of oxidation was also slower for conditions where the stress was reduced. The results can be interpreted such that transitions in the growth result from sequential cracking of the oxide when sufficient elastic strain energy accumulates and that the cracks then enhance access of oxygen to the metal interface.

Probabilistic Prediction of Crack Growth Based on Creep/Fatigue Damage Accumulation Mechanism

Abstract In this paper a deterministic creep/fatigue accumulation model from simple sequential and repetitive creep and fatigue crack growth mechanisms for creep-brittle materials is developed. In the model the maximum stress intensity factor and stress intensity factor range can be used to describe, respectively, the creep and fatigue crack growth behaviors. The probabilistic behavior of the combined creep-fatigue can be calculated by direct integration of a joint log-normal probability density function or other methods. The Monte Carlo simulation method is used in this paper to analyze the probabilistic behavior of the derived deterministic creep-fatigue model and to estimate the reliability of a creep-fatigue correlation as introduced by the uncertainties in both creep and fatigue lives. Predicted results from this new crack growth method are compared with the bi-linear creep-fatigue interaction models as adopted by ASME Code Case N-47 and API579/ASME FFS, and the results are discussed.

Hydrogen production from biomass-derived oil over monolithic Pt- and Rh-based catalysts using steam reforming and sequential cracking processes

The conversion of biomass-derived crude oil towards H 2 production was investigated using continuous catalytic steam reforming and sequential cracking/reforming processes. The performances of Pt/Ce 0.5Zr 0.5O 2 and Rh/Ce 0.5Zr 0.5O 2 catalysts deposited on cordierite monoliths were comparatively studied. The Pt-based catalyst showed better catalytic activity than Rh for steam reforming in the whole range of steam-to-carbon molar ratios (S/C) studied, the amount of added water determining the H 2 yield for both noble metals. The best H 2 yield (70%, corresponding to ∼49 mmol of H 2/g of bio-oil) was obtained with the Pt catalyst at S/C ratio of 10 at 780 °C, with CH 4 concentrations below 1%. In the case of sequential cracking, the process alternated cracking steps, during which the bio-oil is converted into H 2, CO, CO 2, CH 4 and carbon stored on the catalyst, with regeneration steps where the deposited coke was burnt under O 2. Comparison with thermal bio-oil cracking showed that the catalyst plays a major role in enhancing the H 2 productivity up to 18 mmol of H 2/g of bio-oil (∼50% of H 2 in gaseous products stream) and lowering the CH 4 formation. The steam reforming offers high yields towards H 2 but is highly endothermic, whereas the sequential cracking, despite lower H 2 yields, offers a better control of coke formation and catalyst stability, and due to lower energy input can theoretically run auto-thermally.

Continuous hydrogen production by sequential catalytic cracking of acetic acid

Abstract In this study, the mechanism of sequential cracking of acetic acid (AA) over Ni-based catalysts was investigated. During AA cracking, both thermal and catalytic reactions take place. Acetic acid is thermally decomposed into CH4, CO, CO2 and H2, which further react on Ni through WGS and steam reforming reactions, whereas some coke accumulates on the catalyst. The statistical distribution of labelled C*O2 species suggests fast isotopic exchange between CO2 and surface carbonates. During the regeneration, the coke deposited during cracking is fully burnt, and carbonates are thermally decomposed. 18O2 isotopic labelling experiments coupled with physico-chemical characterisations of catalysts, after cracking and regeneration steps, reveal that the nickel is reduced by cracking products at the beginning of each cracking step, and oxidized during the regeneration. This redox cycling progressively extracts metallic Ni, initially present as Ni2+ species incorporated in spinel-type structures.

Continuous hydrogen production by sequential catalytic cracking of acetic acid

The sequential cracking of acetic acid (AA) to produce hydrogen was investigated over noble metals and Ni-based catalysts. In this non-stationary process, AA is decomposed into a hydrogen-rich gas, with concomitant carbon deposition on the catalyst, which is periodically removed under oxygen. In this first part, Ni-based catalysts proved to be the most efficient in terms of activity and selectivity to hydrogen, compared to noble metal catalysts. The reaction conditions leading to optimal hydrogen selectivity and complete catalyst regeneration were studied. Continuous hydrogen production by the use of two reactors working alternately was demonstrated.

Hydrogen production by sequential cracking of biomass-derived pyrolysis oil over noble metal catalysts supported on ceria-zirconia

Abstract Conversion of crude pyrolysis bio-oil for H 2 production is investigated using a sequential process which alternates (i) cracking reaction steps, during which the bio-oil is converted to syngas and carbon stored on the catalyst and (ii) regeneration steps allowing to combust coke under an air flow. The performances of Pt and Rh catalysts supported on ceria-zirconia in powder form or deposited on cordierite monoliths are comparatively studied. From these data and calculated thermodynamic equilibrium, the co-existence of thermal and catalytic processes is demonstrated. A stable hydrogen productivity up to ca. 18 mmol of H 2 g −1 of bio-oil (∼50% H 2 in the gas stream) with a minimized methane formation (ca. 6%) is obtained with the monolith configuration. Both Pt and Rh-based catalysts allow a good control of carbon formation, the coke being fully combusted during the regeneration step. Slow deactivation phenomena and selectivity changes along time on stream, mostly observed for platinum powder samples, are related to changes in catalyst structure and to the peculiar role of oxygen stored in the zirconia-ceria support. The heat balance evaluation of the sequential cracking/regeneration cycle shows that the process could be auto-thermal, i.e., minimizing the energy input, being competitive with conventional steam-reforming process under the same operating temperature.