Propane oxidative dehydrogenation over vanadia catalysts supported on mesoporous silicas with varying pore structure and size

The structural characteristics and the performance of vanadia catalysts (0.7–8 wt.% V) supported on mesoporous (MCM-41, HMS, MCF, SBA-15), microporous (silicalite) and non-porous (SiO₂) silicas in oxidative dehydrogenation of propane were investigated. The structure of vanadia species, the redox and the acidic properties of the catalysts were studied using in situ Raman spectroscopy, TPD- NH₃ and H₂-TPR. The only vanadia species detected on the surface of HMS and MCM-41 for V loadings up to 8 wt.% were isolated monovanadates indicating high vanadia dispersion. Additional bands ascribed to V₂O₅ nanoparticles were evidenced in the case of SBA-15 and MCF supported catalysts while these bands were the only ones identified on the surface of the catalysts supported on silicalite and non-porous silica. The catalysts supported on mesoporous HMS and MCM-41 materials showed the best performance achieving high propane conversions (35–40%) with relatively high propene selectivities (35–47%). Lower activity due to the lower degree of vanadia dispersion, caused by the partial destruction of the pore structure was observed for the SBA-15 and MCF supported catalysts. The degree of dispersion of the V species on the catalyst surface and not the pore size and structure of the mesoporous support or the acidity/reducibility characteristics mainly determine the catalytic activity towards propene production. In addition, it was shown that the pore structure and size of the mesoporous supports did not have any significant effect in the turnover rates (TOF values) of propane conversion (and propene formation at low propane conversion, below ca. 10%). However, the highest propene yield (up to 19%) and stable catalytic behavior was attained for catalysts supported on HMS mesoporous silica, and especially for those combining framework mesoporosity and textural porosity (voids between primary nanoparticles).     

Spannungs‐ und Porenwasserdruckentwicklung infolge Schlitzwand‐Herstellung in weichen bindigen Böden

Stress and pore water pressure development during the construction process of dipahragm walls in soft clay. The influence of a diaphragm wall construction on the stress field in a soft clayey soil is investigated by the use of a three‐dimensional FE‐model of seven adjacent wall panels. The installation procedure comprises the excavation and the subsequent pouring of each panel taking into account the increasing stiffness of the placed fresh concrete. The soft clay deposit is described by a visco‐hypoplastic constitutive model considering the rheological properties and the small‐strain stiffness of the soil. The construction process considerably affects the effective earth and pore water pressures adjacent to the wall. Due to concreting, a high excess pore water pressure arises, which dissipates during the following construction steps. The earth pressure finally shows an oscillating, distinct three‐dimensional distribution along the retaining wall which depends on the installation sequence of the panels and the difference between the fresh concrete pressure and the total horizontal earth pressure at rest. In comparison to FE‐calculations adopting the earth pressure at rest as initial condition, greater wall deflections and surface ground settlements during the subsequent pit excavation can be expected, as the average stress level especially in the upper half of the wall is increased by the construction procedure of the retaining structure.     

Gilt die Minersche Regel für Sand?

Is Miner's rule applicable to sand? In calculations with an explicit (high‐cycle) accumulation model a loading with varying amplitudes is replaced by packages of cycles each with a constant amplitude. This assembling of packages according to the criteria of amplitude is based on the assumption, that the sequence of the packages is of minor importance with respect to the final value of the residual deformation, i.e. that the Miner's rule is applicable. Unfortunately, for sand only few experimental studies with a low number of cycles exist. This paper presents the results of triaxial tests with packages of cycles in which the validity of the Miner's rule was checked for sand and larger numbers of cycles (N ≥ 10⁵). It is shown that the influence of the sequence of the packages can be neglected (at least for cycles with a constant polarisation), i.e. that the procedure of explicit models is legitimate.     

Berücksichtigung des Frischbetondrucks bei der FE‐Simulation der Schlitzwandherstellung

Fresh concrete pressures as a boundary condition for FE‐simulations of a diaphragm wall construction. Underground deformations adjacent to a diaphragm wall pit excavation are generated by the pit excavation as well as by the wall construction itself. For the numerical simulation of the construction process, a 3D Finite Element model is required accounting for the construction sequence and the intermediate stress states generated by the slurry and fresh concrete pressures. Regarding the development of fresh concrete pressures, centrifuge model tests and element tests were conducted and an analytical algorithm for the calculation of the fresh concrete pressures is derived. A backcalculation of available literature data shows good agreement and enables a more detailed consideration of the fresh concrete pressures for the numerical simulation of a diaphragm wall construction.     

Dynamische Steifigkeit und Dämpfung von Sand bei kleinen Dehnungen

Dynamic stiffness and damping of non‐cohesive soils at small strains. The dynamic stiffness and material damping of sand at small strains (10^–7 ≤ γ^ampl ≤ 10^–3) were studied in resonant column tests and triaxial tests with measurements of P‐ and S‐wave velocities. The influence of the parameters stress, void ratio, strain amplitude (threshold shear strain), time (aging), fabric of grain skeleton (cyclic preloading), temperature and grain size distribution was studied. The results are compared with published test data or with the recommendations of the working committee “Soil Dynamics” of the DGGT, respectively, whereby mostly a good coincidence was observed. It is shown, that the resonant column test and the measurement of the travel time of waves deliver similar results.     

Setzungsakkumulation in nichtbindigen Böden unter hochzyklischer Belastung

Accumulation of settlement in granular materials under highcyclic loading. The numerical prognosis of the accumulation of settlements and/or stress in soils under a high‐cyclic loading (e.g. high speed railway tracks, offshore wind power plants) has gained in importance during the last years. This article presents a special constitutive model for the calculation of the accumulation in non‐cohesive soil under a high number of load cycles. The model is based on numerous cyclic laboratory tests whose results are shown in this paper.     

Investigation of the Failure Mechanism by Bending of a Rack Steering Gear

The wheel-drive system of a car has a major influence on the safety of car’s occupants. By turning the steering wheel, centrifugal forces are developed and these forces cause a bending moment on the rack steering gear. To avoid plastic deformation of the rack steering gear, sufficient strength and spring/stiffness properties are necessary for the material. In the case under investigation, the vehicle was traveling with 60 km/h on a slippery road. While negotiating a turn along the road, the driver was not able to bring the car back on the road. The car stopped without any crash, and the technical inspection which followed showed that the rack steering gear was deformed plastically at the right arm. The occupants were in great danger during this incident. Metallurgical failure analysis was conducted, revealing that plastic deformation by bending occurred because the mechanical properties of the steel were not adequate because of insufficient deoxidation practices and a defective hard coating. The lack of deoxidation and defects in the hard coating should not be allowed occur, and to avoid such failures, the quality control measures for acceptance of such parts should be modified. The new practices should include better control over the non-metallic inclusions, and measurement of the hard coating’s thickness over the whole length of the bar.     

Fracture Characteristics of Tensional–Torsional Fatigue Failure of the Archimedes Helix

“Archimedes helix” (Fig. 1) is very widely applied in worm gear systems (Fig. 2) and is a critical component of transportation systems of raw material in the ceramic industry. Failures of such components from mineral processing plants (production of ceramics) have been addressed in our lab during the last two years. The helixes are manufactured from of ferritic–perlitic steels that are usually protected during service by a hard surface coating. In the recent years, uncoated material is progressively replacing the more expensive hard coated materials primarily for economic reasons. The uncoated material frequently fails owing to the fracture of the component into two pieces (Fig. 3). A serious economic consequence of such failures is the stoppage of the production line until the broken component is replaced, while the cost of the component itself is negligible. Failure analysis was conducted, and it revealed that the primary mechanism of fracture tensional–torsional fatigue. The use of penetrant testing–non destructive testing (PT–NDT) makes it possible, in many cases, to detect the fatigue phenomenon at its early stages and to replace the part during the normal service of the machine, thereby avoiding unexpected plant shutdown.     

Shear band systems in plane strain extension: analytical solution and comparison with experimental results

Shear banding represents a local failure mechanism of a soil structure as a response to shear loading. In soil structures of different spatial scales systems of regularly spaced shear bands can be observed as a consequence of extensional loading. The phenomenon of single shear bands, defined as thin zones of localized deformation with a discontinuity of the strain field at its boundaries, is well understood. Inside the shear band the material undergoes inelastic strain softening accompanied by shearing and dilation, whereas the material outside the shear band unloads accompanied by elastic contraction in extension tests. Despite numerous experimental and numerical investigations, the physical mechanisms and parameters determining the spacing of parallel shear bands remained unknown. The paper in hand presents an analytical solution for the spacing of the shear bands and a comparison with a large base of experimental data gained from 1g and ng (geotechnical centrifuge) model experiments. The analytical solution is based on the assumption that the elastic energy rate in the unloaded zone between the shear bands tends to a minimum value. The spacing was calculated as the energetically preferred solution for a broad range of cohesive-frictional granular materials. The dependency of the calculated spacing on initial and boundary conditions as well as on material parameters was found to be in good agreement with the experimental results.     

Shear banding and strain softening in plane strain extension: physical modelling

Localization of deformation in systems of shear bands or normal faults, respectively, as a consequence of extensional loading can be observed on a wide range of spatial scales in soil and rock formations. A series of plane strain model experiments was achieved in natural (1 g) and increased (ng) gravity field (geotechnical centrifuge) with dry and moist sand as well as with dry and moist sand-clay mixtures. In these experiments, the geometry of the shear bands (inclination, width, spacing, sharpness) was detected by means of the digital image correlation (DIC) technique. Comparison with existing analytical approaches for the determination of the spacing of shear bands is presented briefly. The stress-strain behaviour of the materials was determined in a new biaxial test device, which allows for the performance of biaxial compression and extension tests. The evaluation focuses on the strain softening gradient, which is seen as a key parameter in the explanation of shear band spacing.