Corrosion of ceramics in halogen-containing atmospheres

Most single component oxide ceramics undergo active corrosion in halogen-containing gases such as HCl to form gaseous corrosion products only. At high temperatures, such corrosion reactions are controlled by gaseous diffusion of the product gases away from the solid surface. As a result, the reaction kinetics and the corrosion rate can be predicted if the thermodynamics of the reaction are known. Similar behaviour is expected for nitride ceramics. However, several multicomponent oxides, typified by NiAl₂O₄, and carbides may react to form a porous solid product layer through which diffusion takes place and controls the corrosion rate. Nevertheless, even in these cases, if gaseous diffusion still controls, the rate of corrosion can be modeled and predicted.     

Gas–liquid interfacial areas in three-phase fixed bed reactors

A simple model, based on the theory of liquid emulsions, is proposed to develop correlations suitable for estimating gas–liquid interfacial areas for the bubble flow regime in three-phase fixed bed reactors operated with gas and liquid flowing cocurrently upflow. The model is also employed for trickle-bed reactors for estimating the increase in gas–liquid interfacial areas when these reactors are operated under high pressure conditions. It is found that developed correlations account satisfactorily for most of the data of gas–liquid interfacial areas available in the literature for cocurrent upflow three-phase fixed bed reactors and for trickle-bed reactors operated at high pressure conditions.     

Intensification of mass transfer in liquid fluidized beds with inert particles

The influence of inert particles on liquid/solid mass transfer is studied in fluidized beds by using a binary-mixture of solids of differing size and density. The addition of inert particles of higher density and smaller diameter, e.g. glass beads, exerts remarkable effects on mass transfer coefficients in comparison to that of mono-component active particles at the same liquid velocity. The extent of the effect on liquid–solid mass transfer coefficients increases with an increasing fraction of the small inert particles in the mixture. The liquid–solid mass transfer coefficients for binary-mixtures are well correlated in terms of dimensionless groups and the voidage parameter.     

On the relationship between Lagrangian micromixing models and computational fluid dynamics

The relationship between Lagrangian micromixing models, which are widely employed in chemical reaction engineering, and Eulerian computational fluid dynamic (CFD) models based on the Reynolds-averaged species conservation equation is explored. A general modeling methodology which combines the strengths of both approaches is developed in the form of a multi-environment CFD micromixing model. The formulation is shown to be equivalent to a presumed multi-scalar probability density function (PDF) approach. The four-environment generalized mixing model (GMM) model originally proposed by Villermaux and Falk (Villermaux and Falk, Chem. Eng. Sci. 49 (5127) (1994)) is used to illustrate the methodology by applying it to model a series-parallel reaction in a tubular reactor.     

Modelisticinterpretation of the impedance response of a polymer electrolyte fuel cell

In this work, several aspects related to the limiting polarization behavior of polymer electrolyte fuel cells were studied using ac impedance spectroscopy. The results were analyzed taking into account the different types of potential losses caused by the interfacial reaction kinetics, the conductance of the electrolyte in the catalyst layer, the oxygen diffusion in the gas phase, in the thin film and in the distributed agglomerate regions of the gas diffusion electrodes and the balance of water in the membrane. The main conclusion is that the water transport in the membrane plays an important role in establishing the limiting polarization behavior of polymer electrolyte fuel cells, corresponding to the only limiting factor when pure oxygen is employed as the cathodic reagent. When the oxygen source is air, the diffusion of oxygen in the gas phase becomes a limiting factor, but for pressurized systems the diffusion of water can also be observed, especially for thick membranes.     

Theoretical prediction of the minimum stirrer speed in mechanically agitated suspensions

The objective of this article is to derive theoretical equations for the minimum stirrer speed in vessels containing suspensions. There are a variety of equations for the prediction of this stirrer speed (Zwietering, Chem. Eng. Sci. 8 (1958) 244–253; Kneule and Weinspach, Verfahrenstechnik 1 (12) (1967) 531–540; Nienow, Chem. Eng. Sci. 23 (1968) 1453–1459; Einenkel, VDI-Forschungsheft Nr. 595, Düsseldorf, 1979; Einenkel and Mersmann, vt-Verfahrenstechnik 11 (2) (1977) 90–94; Niesmak, Thesis TU Braunschweig, 1982; Molerus and Latzel, Chem. Eng. Sci. 42 (6) (1987) 1423–1437; Zehner, Chem. Ing. Technol. 58 (10) (1986) 830–831). However, nearly all of them are derived from experimental results obtained in a limited range of vessel size and suspension properties [1, 2, 3]. Some relationships based on experiments carried out in small vessels are contradictory and not useful for scale-up. Reliable models are necessary not only for the design of stirred vessels but also for the reduction of power consumption. It is important to distinguish between two decisive processes: the ‘avoidance of settling’ and ‘off-bottom lifting’.     

A one-dimensional instationary heterogeneous mass transfer model for gas absorption in multiphase systems

For a physically correct analysis (and prediction) of the effect of fine, dispersed phase drops or particles on the mass transfer rate in multiphase systems, it was demonstrated that only 3-D instationary, heterogeneous mass transfer models should be used. Existing models are either homogeneous, stationary or single particle models. As a first step, a 1-D, instationary, heterogeneous multi-particle mass transfer model was developed. With this model the influence of several system parameters was studied and problems and pitfalls in the translation of modeling results for heterogeneous models into a prediction of absorption fluxes are discussed. It was found that only those particles located closely to the gas–liquid interface determine mass transfer. For these particles the distance of the first particle to the gas–liquid interface and the particle capacity turned out to be the most important parameters. Comparisons with a homogeneous model and experimental results are presented. Typical differences in results comparing a homogeneous model with the 1-D heterogeneous model developed in this work could be attributed to a change in the near interface geometry. Future work in this field should therefore be directed towards near interface phenomena. Three dimensional mass transfer models, of which a preliminary result is presented, are indispensable for this.     

Use of tendency models and their uncertainty in the design of state estimators for batch reactors

Tendency models have been successful in the modeling and optimization of batch reactor processes where a detailed understanding based on fundamental principles and detailed kinetic studies is not available. The evolutionary nature of the Tendency modeling algorithm has proven useful in updating the process model between batches, as new process data or insight become available. But optimization is not the only task that can be undertaken with a Tendency model. In this work, the use of Tendency models in the design of state estimators to estimate reactor concentrations is investigated. The primary goal is to use the knowledge of the uncertainty in the Tendency model (which, by its nature, is an approximate model) to tune an extended Kalman filter. Two examples are presented to illustrate that even though Tendency models can feature a significant amount of uncertainty, they can be used successfully in state estimators.     

Friction induced paint damage as affected by clearcoat chemistry

One form of damage to automotive painted plastic composites, in particular thermoplastic bumpers and fascias, is known to be caused from compressive shear loading placed on the surface of the composite. The loading inflicted upon the composite by a stationary force (e.g. an automobile hitting a post) or a moving object (e.g. an automobile hitting another automobile), and consequent compressive shear that is induced, can result in cohesive delamination of the substrate. The cohesive delamination in the painted composite (herein referred to as ‘gouge') is not only influenced by the cohesive strength of the substrate but also by the mechanical properties of the coating. Our studies indicate that one important attribute of the coating, namely the coating's coefficient of friction, can significantly impact the ‘gouge' resistance of the painted composite. In this work, we will describe the derivation of coating attributes as they affect ‘gouge' resistance and relate changes in the coating chemistry to useful life parameters.     

Microstructure development during constant-force drawing of poly(ethylene terephthalate) film

Structure development in PET film during high strain-rate, constant-force (CF) deformation in the temperature range 80–96°C is compared with structure development during lower strain rate, constant-extension-rate (CER) deformation in a similar temperature range. The higher (maximum) strain rates involved in CF drawing mean that much of the deformation takes place in a regime where the time available for orientational relaxation and crystallization is short. This results in high levels of ‘non-crystalline orientation' and low levels of crystallinity compared to structures obtained from CER drawing. In CER drawing, due to the lower strain rates, the degree of crystallinity always has time to reach pseudo-equilibrium values corresponding to a given level of non-crystalline orientation, and the amount of orientational relaxation occurring during drawing has the dominant influence on structure development. In CF drawing, pseudo-equilibrium crystallinity values are not reached, except when the deformation approaches the tail-end of the strain-rate spectrum. The results also provide confirmation that microstructure data obtained from rapidly quenched samples are consistent with microstructure data obtained from real-time experiments.     

Electrochemical test methods for evaluating organic coatings on metals: an update. Part III: Multiple test parameter measurements

The status of evaluating organic coated metals utilizing electrochemical means was reviewed for the period of 1988–1994. The general improvements in the overall technology are presented in three parts, each as a separate publication. Part I covers the test cell configurations, changes in testing approaches and a brief survey of measurement equipment. Part II presents the test methods involving a single test parameter such as the panel potential relative to a reference electrode, electrochemical voltage and/or current noise, as well as the dc resistance of the coating on the metal substrate. Multiple test parameter measurements such as potentiodynamic curves and electrochemical impedance spectroscopy are covered in Part III. Although the majority of data were taken from the literature, some supplementary data are included from NSWCCD studies.     

Catalytic conversions in water, part 8: carbonylation and hydrocarboxylation reactions catalyzed by palladium trisulfonated triphenylphosphine complexes

The water-soluble Pd(tppts)₃ complex (tppts=P(C₆H₄-m-SO₃Na)₃) is an active catalyst for the carbonylation of benzylic type alcohols in aqueous/organic two-phase systems in the presence of a Brønsted acid cocatalyst. For example, 1-(4-isobutylphenyl)ethanol afforded 2-(4-isobutylphenyl)propionic acid in 82% selectivity at 83% conversion and 5-hydroxymethylfurfural (HMF) gave 5-formylfuran-2-acetic acid in 72% selectivity at 90% conversion. In the latter case, use of an acid with a strongly coordinating anion led to the preferential formation of the reduction product 5-methylfurfural (MF), e.g. HI afforded MF in >99% selectivity. Pd(tppts)₃ is also an usual active catalyst (T.O.F.>2500) for the biphasic hydrocarboxylation of propene to n- and isobutyric acid, being substantially more active than the analogous Pd/PPh₃ in organic media.     

Electroorganic reactions in ionic liquids: 5 . Anodic fluorodesulfurization of phthalide, ethylene carbonate, and glucopyranosides having arylthio groups

Anodic fluorodesulfurization of 3-phenylthiophthalide, 4-phenylthio-1,3-dioxolan-2-one, and 1-arylthio-2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside in ionic liquids like Et₃N·nHF (n=4-5) and Et₄NF·nHF (n=4, 5) without any solvents provided the corresponding monofluorinated products exclusively in moderate to good yields. In sharp contrast, the anodic fluorination in Et₃N·3HF with and without co-solvents resulted in α-fluorination without desulfurization exclusively or preferentially. It was also demonstrated that the electrochemical fluorodesulfurization of 3-phenylthiophthalide under solvent-free conditions was achieved repeatedly four times by the reuse of a fluoride salt, Et₄NF·4HF. This is the first example of fluorodesulfurization by the reuse of an ionic fluoride salt.