CATALYST FRACTURE DUE TO THERMAL SHOCK IN FLUIDIZED CATALYTIC CRACKER UNITS

The formation of fines in a fluidized catalytic cracker unit (FCCU) due to catalyst attrition and fracture is a major source of catalyst loss. In addition to standard attrition tests described in the literature, the possibility that thermal conditions could lead to catalyst fracture and fines production has been explored. Samples of fresh and used (equilibrium or e-cat) type catalysts were heated up to 600°C and mixed with cold samples to determine the impact of thermal shock on particle stability. It was found that significant fracture occurs under these conditions, leading to loss of larger catalyst particles in the bed and significant gain in the amount of fine particles. Agglomeration of particles was also evident, in some cases leading to an increase in the quantity of larger particles appearing to be present in the catalyst sample.     

反应烧结碳化硅材料的抗热震性能研究

通过对比不同温差热震后材料的残余强度 ,对反应烧结碳化硅材料的抗热震性能进行了研究。结果表明 :反应烧结碳化硅材料的抗热震性能与显微组织密切相关 ,低游离硅含量与小粒径的反应烧结碳化硅材料具有较好的抗热震断裂性能 ,而高游离硅含量或大碳化硅粒径的材料具有相对优异的抗热震损伤性。对反应烧结碳化硅材料的抗热震性与显微组织的关系进行了探讨。     

Light Degrading Properties of Size-Fractionated PM 10 Aerosol Samples Collected from an Industrial Area in Brisbane,Australia

Thirty-seven days of PM 10 aerosol samples (particles with aerodynamic diameter <10 w m) were collected in an industrial area in Brisbane during April to June 1999 to study the light extinction efficiencies of urban aerosols in different size ranges. The light scattering coefficient of the air was measured by nephelometry. The light absorption coefficient of the aerosol samples was measured by the integrating plate laser absorption method. Multiple linear regression techniques were used to investigate the relationships between the visibility degrading properties and the chemical composition of the aerosol samples. The results are comparable with those from other visual air quality studies. The absorption of light by fine (PM 2.5 ) aerosols is mainly due to elemental carbon (EC) particles smaller than 0.5 w m. The b 0 ap values of EC particles in different size ranges are 9.08 (< 2.7 w m) and 0.32 (2.7-10 w m)m 2 g -1 , respectively. The absorption of light by coarse (PM 2.5-10 ) aerosols is mainly due to soil ( b 0 ap = 0.17) and organic ( b 0 ap = 1.11) particles. The scattering of light is highly related to the concentration of fine particles in the air (mass scattering efficiency b 0 sp = 1.65) and is mainly due to the fine sulphate ( b 0 sp = 10.9), soil ( b 0 sp = 2.73), and EC ( b 0 sp = 3.89) particles. On average, fine EC (44%), sulphate (20%) and soil (7%) particles, NO 2 (9%), and Rayleigh scattering (19%) were the largest contributors of visibility degradation for the sampling days in this study.     

Effect of the particle size distribution of spinel on the mechanical properties and thermal shock performance of MgO–spinel composites

The influence of varying the amounts of spinel with a similar median particle size, but with different distribution, on the mechanical properties and thermal shock performance of MgO–spinel composites was investigated. Mechanical properties of composites decreased significantly with increasing spinel content due to the thermal expansion mismatch. However, γ_WOF values of composites increased markedly, because of a significant change in the fracture mode from transgranular to intergranular fracture. A narrow distributed spinel A (Alcoa MR66) particles resulted in shorter initial crack propagation distances from the spinel particles, but spinel B (Britmag 67) particles with a significantly broader distribution were the origins of longer interlinked cracks. The improved resistance to thermal shock in MgO–spinel composites can therefore be attributed to the microcrack networks developed around the spinel particles, associated with the high values of γ_WOF, and not to an increased K_1c. On the basis of theoretically calculated R values and experimentally found γ_WOF/γ_i ratios, resistance to thermal shock damage would be more strongly favoured with materials containing spinel B particles, rather than spinel A, for which a much larger volume% was required to achieve a similar improvement.     

Physicomathematical Modeling of Explosive Dispersion of Liquid and Powders

Studies have been conducted with respect to the generation of fine‐disperse aerosols with the use of an explosive disperser model based on hydrodynamic shock tube. The physicomathematical model for the process of explosive dispersion with the aforesaid design is described [1]. The use of elements in the model design to produce cavitation bubbles under the action of shock wave and ensurance of slower outflow process of a water‐steam mixture makes it possible to obtain liquid disperse aerosols. The experimental results for aerosol disperse parameters and the processes of initiation and propagation of an aerosol cloud produced upon explosive dispersion of a variety of liquids and condensed powders are presented. Pressure values in the combustion chamber of the disperser, velocity of aerosol particles, and aerosol disperse parameters were obtained. The design of the explosive disperser with a nozzle, which provides favorable conditions for cavitation processes inside the dispersed liquid, is shown to allow the generation of a high dispersity aerosol (about 1–100 nm), which is essential for many practical problems. The function of distribution of such particles by size cannot be measured reliably and directly by methods available; however, indirect experiments (measurement of humidity when dispersing water) permit estimating the existence of particles in the aerosol with sizes below 1 μm in quantity of approx. 90 %.     

催化裂化装置催化剂跑损原因分析

催化裂化装置催化剂非正常跑损的原因是催化剂细粉含量超高和旋风分离器分离效果差,旋风分离器效果差的常见原因是料腿堵塞,细粉含量高的原因主要是生产操作不当和原料组成变化造成催化剂的磨损严重或水热崩碎,本文在各个方面给出了具体实例和相应的解决方法。     

Effect of metals poisoning on FCC products yields: studies in an FCC short contact time pilot plant unit

In this work the effects of two metal poisons (Ni, V) on FCC products were investigated in an FCC pilot plant. The most important effects of metals were found on the gasoline, coke and H₂ yields. A comparison study of the metal distribution in the catalyst particles aged in the FCC pilot plant, a CDU and an industrial FCCU was also performed using a SEM–EDS method. SEM results showed that the metal profiles from CDU samples simulate more satisfactorily the profiles of the E-cat than that from the FCC pilot plant.     

复合添加剂对全粉料制α-A1_2O_3陶瓷抗热震性的影响

本文研究了0．3wt％TiO2和Mgo＋Y2O3＋La2O3复合添加剂对全粉料a-Al2O3抗热震性的影响人试样热震后的抗折强度保持率来看，当复合添加剂含量为0．375wt％及0．5wt％时，不仅可以促进烧结，且由于添加剂有细化晶粒的作用，可使全粉料a-A12O3抗热见性提高。     

Development of stable supports consisting of SiC---Si composite for high temperature combustion catalysts

SiC-Si composite, that is stable in oxidizing atmosphere at 1300°C and has thermal shock resistance, was prepared from a powder mixture of porous β-SiC, which was prepared from rice hulls, and Si metal. To use an SiC-Si composite as a structural support for a high temperature combustion catalyst, the foaming SiC-Si composite form with continuous bubbles was prepared from foaming SiC form and the mixture of the porous β-SiC and Si metal. The foaming SiC form was prepared from the foaming polyurethane form and a β-SiC fine particles. The β-SiC fine particles having an average diameter of 0.3 μm was coated on the foaming polyurethane form. The polyurethane part of the form was burned out and the coating β-SiC was sintered to form the foaming SiC form. The SiC form was coated on the porous SiC and Si metal powder mixtures and was heated at 1500°C in argon to prepare the foaming SiC-Si composite. The foaming composite was stable in an oxidizing atmosphere at 1300°C and was highly resistance to thermal shock. The compression stress of the foaming SiC-Si composite form (175 kg/cm²was about twice that of the a-axis of honeycomb-shaped cordierite (> 85 kg/cm²).     

Alkali deactivation of high-dust SCR catalysts used for NOx reduction exposed to flue gas from 100 MW-scale biofuel and peat fired boilers: Influence of flue gas composition

Deactivation of vanadium–titanium deNO_x SCR (selective catalytic reduction) catalysts in high-dust position have been investigated in three 100 MW-scale boilers during biofuel and peat combustion. The deactivation of the catalyst samples has been correlated to the corresponding flue gas composition in the boilers. Too investigate the effect on catalyst deactivation a sulphate-containing additive was sprayed into one of the furnaces. Increased alkali content on the SCR catalyst samples decreased the catalytic deNO_x activity. The study has shown a linear correlation between exposure time in the boilers and alkali concentration (mainly potassium) on the samples. The results imply that mainly alkali in ultra fine particles (<100 nm) in the flue gas increased the alkali accumulation on the catalyst samples. Low correlation was found between particles larger than 100 nm and the catalyst deactivation. It was not possible to decrease the deactivation of the catalyst samples by the sulphate-containing additive. Although the additive had an effect in sulphating potassium chloride to potassium sulphate, it did not decrease the amount of potassium in ultra fine particles or the deactivation of the catalyst samples.     

Bimetallic Pt/Pd diesel oxidation catalysts: Structural characterisation and catalytic behaviour

Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) studies on supported bimetallic Pt/Pd diesel oxidation catalyst (Pt:Pd weight ratio 2:1) after various treatments were compared with those of monometallic Pd and Pt catalysts prepared under similar conditions. After calcination and thermal ageing, the coexistence of alloyed bimetallic Pt/Pd particles and of tetragonal PdO has been found in the bimetallic Pt/Pd catalyst. PdO is present in form of crystals at the surface of the Pt/Pd particles or as isolated PdO crystals on the support oxide. Bimetallic Pt/Pd nanoparticles were already formed in the Pt/Pd catalyst after calcination. Hydrogen treatment causes the formation of randomly alloyed Pt/Pd nanoparticles. In the thermally aged catalyst, a strong indication for an enrichment of Pt in the interior of the particle and of Pd at its outer shell was found. In the monometallic catalyst, the Pd is found to be completely oxidised already after calcination and to consist of metallic Pd in zero-valent state exclusively after reductive treatment. Ageing under hydrothermal oxidative atmosphere leads to complete oxidation of the Pd species. After calcinations, the catalytic activity of the Pt/Pd catalyst studied is comparable to those of monometallic Pt catalysts. In contrast to monometallic Pt catalysts, the alloyed system show significant stabilisation against sintering and a much higher activity after the thermal ageing step. This stabilisation of dispersion and the presence of Pt atoms on the surface of the Pt/Pd particles are considered to cause the higher catalytic activity of metallic particles for the oxidation of carbon monoxide and propene after ageing.     

Characterization of Submicrometer Aerosol Deposition in Extrathoracic Airways during Nasal Exhalation

Submicrometer and especially fine aerosols that enter the respiratory tract are largely exhaled. However, the deposition of these aerosols under expiratory conditions is not well characterized. In this study, expiratory deposition patterns of both ultrafine (<100 nm) and fine (100–1000 nm) respiratory aerosols were numerically modeled in a realistic nasal-laryngeal airway geometry. Particle sizes ranging from 1 through 1000 nm and exhalation flow rates from 4 through 45 L/min were considered. Under these conditions, turbulence only appeared significant in the laryngeal and pharyngeal regions, whereas the nasal passages were primarily in the laminar regime. Exhaled particles were simulated with both a continuous-phase drift flux velocity correction (DF-VC) model and a discrete Lagrangian tracking approach. For the deposition of ultrafine particles, both models provided a good match to existing experimental values, and simulation results corroborated an existing in vivo–based diffusion parameter (i.e., D ^0.5 Q ^?0.28). For fine particles, inertia-based deposition was found to have a greater dependence on the Reynolds number than on the Stokes number (i.e., St^0.1 _kRe^0.9), indicating that secondary flows may significantly influence aerosol deposition in the nasal-laryngeal geometry. A new correlation was proposed for deposition in the extrathoracic airways that is applicable for both ultrafine and fine aerosols over a broad range of nasal exhalation conditions. Results of this study indicate that physical realism of the airway model is crucial in determining particle behavior and fate and that the laryngeal and pharyngeal regions should be retained in future studies of expiratory deposition in the nasal region.     

High thermal-conductivity rGO/ZrB2-SiC ceramics consolidated from ZrB2-SiC particles decorated GO hybrid foam with enhanced thermal shock resistance

Graphene derivative materials exhibit excellent mechanical and thermal properties, which have been extensively used to toughen ceramics and improve thermal shock resistance. To overcome the thermal agglomeration of graphene oxide (GO) during heating and drying process, ZrB₂-SiC particles decorated GO hybrid foam with uniformly anchored ceramic particles was synthesized by electrostatic self-assembly and liquid nitrogen-assisted freeze-drying process. Densified rGO/ZrB₂-SiC ceramics with varying microstructure, thermal physical and mechanical properties were obtained by adjusting the content of decorated ceramic particles. Although the flexural strength of rGO/ZrB₂-SiC ceramics have an attenuation compared with that of ZrB₂-SiC ceramic, the thermal conductivity, work of fracture and thermal shock resistance are greatly improved. rGO/ZrB₂-SiC ceramics exhibit delayed fracture and increasing R-curve behavior during the crack propagation. The novel preparation technology allows for the well dispersion of rGO in ZrB₂-SiC ceramics and can be easily extended to other ceramic or metal materials systems.     

The thermal shock behaviour of ductile particle toughened alumina composites

Over recent years, it has been established that the incorporation of metallic particles into a ceramic matrix can lead to enhanced fracture properties. Relatively few attempts, however, have been made to establish whether or not the improved fracture toughness typically observed in such composite systems can offer improved performance in demanding environments. The current study is concerned with the thermal shock behaviour of a ceramic matrix composite consisting of an alumina matrix containing 20 vol% of discrete iron particles. The composite material has been produced by both hot pressing and conventional sintering techniques. The hot pressed composite shows a greater resistance to thermal shock than the monolithic matrix, both in terms of the critical temperature differential and retained strength, whereas the sintered material has been found to behave as a typical low strength refractory ceramic. The calculation of thermal shock resistance parameters for the composites and the monolith has indicated possible explanations for the differences in thermal shock behaviour.     

Mechanical and thermal shock properties of size graded MgO–PSZ refractory

The effects of the mixture of coarse powder with fine PSZ powder on the thermal-mechanical properties of 10 Mg–PSZ samples were studied. The size graded specimens were injection-molded using 3.5 m% MgO–ZrO₂ powders. The physical properties of the ZrO₂ samples and five thermal shock parameters were measured and calculated. These properties included density (ρ), porosity (p), the ratio of m/(t+c+m) phase, fracture toughness (K_IC), strength (σ_f), Young's modulus (E), shear modulus (G), Poisson's ratio (ν), and the thermal expansion (α) between ambient temperature to 1100°C. The toughness and thermal shock resistance of the PSZ are controlled by the states of porous microstructure which can be represented by a parameter (nominal largest tolerable length of defects) a_t. The PSZ samples show two types of thermal shock behavior differentiated by comparing the value of a_t to the characteristic length L_f of the defects in the sintered PSZ. The states of the defects, i.e. porosity, are the microstructural evidence to explain the relationship between the thermal shock properties.     

Thermal shock behaviour of mullite–cordierite refractory materials

Abstract

The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of K _Ic measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.     

Abscheidung und Messung sehr feiner flüssiger und fester Partikeln in der chemischen Industrie

Separation and measurement of very fine liquid and solid particles in the chemical industry. In comparison to power stations, the total amounts of waste gases emitted by the chemical industry are fairly low. On the other hand, the complexity of the waste gases and their components is far higher. In the chemical industry we can therefore not be content with one overal concept for gas purification, but must use a multitude of individual solutions. These should be applied at source, i.e. to each separate production plant. To solve an aerosol separation problem successfully and economically, a knowledge of the composition, the fineness and the amount of aerosols is essential. Cascade impactors have proved to be the most suitable instruments for a fractionated measurement of solid and liquid particulates in the super and sub-micron size range. The results of these measurements make is possible to choose the right equipment and to calculate its performance. Very complex aerosols, e.g. aerosols which are composed both of liquid and solid particles, or of sticky particles, are likely to present the most difficult problems. This article deals with a small selection of cases by demonstrating the problems of separation and the different ways of solving them. It considers among others acid mists and oil mists as well as organic mists, together with very fine particulates from waste incineration plants.     

Fourier Transform Infrared Microscopy—a Tool for Speciation of Impactor-Sampled Single Particles or Particle Clusters

With the recent advent of Fourier transform infrared (FTIR) microscopy, a powerful new tool has been added to the methods available for integrated dust analysis (IDA). While the intrinsic molecular specificity of infrared spectroscopy was applied in the 1970s with the IDA strategy of impactor-sampled aerosol analysis, at that time only average information from at least two impactor spots (1–10 μg range) could be obtained after sample preparation by grinding with KBr. This preparation step for heavily loaded samples, which may induce secondary reactions within the particles and destroy part of the reactive (acidic) species originally present, is no longer required with the new technique. FTIR microscopy allows the direct observation of isolated particles (>5 μm diameter) or particle clusters (<5 μm diameter) impacted on KBr or ZnSe (by the transmission technique) after a typical sampling duration of 5 minutes in an urban environment. This technique not only shows significant superiority in the quality of the spectra obtained but also is the key to preservation of original particle acidity. By this technique inhomogeneities within one impactor spot could be shown clearly (compound mapping). Several source, city, and background aerosols have been analyzed in the micro domain for the first time for NH₄ ⁺ compounds, nitrates, sulfates, silicates, carbonates, and organics (aliphatic esters and aromatics). Acidic sulfates have been found in garage samples but not in city aerosols so far. By comparing samples at room temperature and heated up to 150°C, further information on the NH₄ ⁺ compounds and the volatility of the species could be obtained. The adaptation of millisecond-time resolution of the new rapid-scan FTIR spectrometer in our laboratory for molecular specific thermal analysis research (MOSTAR) has been started, extending the capabilities of previous work.     

Improvements on the mechanical properties and thermal shock behaviours of MgO-spinel composite refractories by ZrO2 incorporation

Microstructural features and improvements on the mechanical properties and thermal shock behaviours of MgO-spinel composite refractories with ZrO₂ addition were examined. ZrO₂ incorporation into MgO-spinel led to improvements around ∼1.5-fold ratios on mechanical properties, R_st values and thermal shock results. The basic parameters improving mechanical properties and thermal shock resistance of MgO-spinel-ZrO₂ composite refractories were determined as follows: (i) propagation of microcracks for a short distance by interlinking each other, (ii) stopping or deviation of microcracks when reaching pores or ZrO₂ particles, (iii) concurrent occurrence of mostly intergranular and some transgranular cracks on fracture surfaces, and with the addition of ZrO₂ (iv) the increase in bulk density, and (v) a significant decrease in MgO grain size. The improvements observed in thermo-mechanical properties confirmed that MgO-spinel-ZrO₂ refractories showed a low strength loss and high thermal shock damage resistance at high temperatures, leading to longer service lives for using industrial applications.     

Fracture Analyses of Alumina Subjected to Mechanical and Thermal Shock Biaxial Stresses

Disks of commercial alumina were fabricated by slip casting and sintering. Two surface finishes were performed: coarse (denoted as "C") using a 70 grit diamond wheel and fine (denoted as "F") with 120 and 320 grit SiC papers. The machined surfaces were analyzed by SEM, profilometry, and residual stresses measurements. The fracture strength was evaluated in biaxial flexure, and the thermal shock resistance was tested by cooling with a high-velocity air jet. The fracture of the specimens under both conditions was studied analyzing crack patterns and fracture surfaces in relation to the surface machining and type of loading, i.e., mechanical and thermal stresses.