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.     

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.     

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.     

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.     

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.     

Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography

A ‘tailor made’ computed X-ray tomographic scanner has been developed as a tool for the analysis of the distribution of gas, liquid and solid phases in packed columns. The very good spatial resolution of the scanner has first been assessed by the imaging of objects of known shape and size, called ‘physical’ phantoms. Images have then been realized on a 0.6 m diameter and 2 m height column packed with Cascade Mini-Ring 1A packing elements, which is a random polypropylene packing designed to be used in absorption columns. The solid phase distribution, leading to the void fraction distribution, has been analyzed on cross-section images of the dry packed column (without any liquid flowrate). The measured value of the bed void fraction is equal to the value provided by the manufacturer. The analysis of the axial profile of void fraction shows that the ‘end effect’ may be neglected, whereas the analysis of the radial profile evidences the existence of a non negligible ‘wall effect’. Images have then been carried out on the packed column irrigated by a liquid flowrate ranging between 0 and 6000 l h⁻¹ (0–6.10⁻³ m s⁻¹). Hold-up values have been measured in different cross-sections of the column and averaged in order to obtain the total hold-up value in the bed. The computed values are in very good agreement with those reported in the literature for similar packing. The dependence of the liquid hold-up on the liquid superficial velocity can be expressed in terms of a power law. The fitted value of the exponent, equal to 0.65, is in the range of exponent values found in correlations of the literature.     

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.     

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.     

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’.     

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.     

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.     

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.     

Behaviour of liquid/liquid/solid three-phase fixed bed reactors with application in biocatalysis

Fixed bed reactors operated by feeding with two immiscible liquids are of increasing importance in chemical as well as biochemical reaction engineering. Based on biocatalysis studies, liquid/liquid/solid three-phase fixed bed reactors have been assessed with respect to the phase distribution in such reactors and the backmixing in each liquid phase. The systems were characterized by the presence of a continuous aqueous phase. The phase distribution in the reactor differed substantially from that in the feed stream, particularly when the feed had a low content of non-polar solvent. The accumulation of the non-polar phase in fixed beds of particulate solids was accompanied by blocking of the interstitial void volume by, and channelling of, the non-polar phase as well as enhanced backmixing of both the aqueous and the non-polar phase. Studies with various non-polar solvents demonstrated that the choice of organic solvent is important for the operation of three-phase fixed bed reactors. © 1998 SCI     

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate.     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part I: Experimental

An experimental investigation of the solids flow pattern in gas-flowing solids-fixed bed contactors is presented. The apparatus and procedures for determining the dynamic and static solids holdups, solids residence time distribution and the extent and rate of the exchange between particles in the static and dynamic solids holdup are described in detail.Experiments were performed in a bench scale system, containing a column (diameter ) packed with glass beads of 16 mm in diameter packed up to the height of 0.8 m. Tracer experiments with a step input in flowing solids phase were used for determining the residence time distribution and exchange between particles. Fine solids (spheres with mean diameter of ) of two different colors (all other properties being the same) were used in the tracer experiments to determine the residence time distribution and the exchange between static and dynamic solids holdup. In both types of experiments, the response curves have been obtained via color analysis of digital photos. All experiments have been repeated at different operating conditions, with a broad variation of solids mass flux and gas velocity, and reproducibility at set conditions was checked.The obtained experimental results are discussed and the observed important characteristics of the solids flow pattern are outlined. The effects of the solids flux and gas velocity on the solids flow pattern are presented and analyzed.     

Investigation of flow boiling in circulating three-phase fluidised bed: Part II: Theoretical correlation

The following part of this paper reviews existing theoretical correlations to predict the behaviour of two-phase (liquid-solid) and three-phase (liquid-solid-vapour) fluidised beds as well as models describing heat transfer coefficients. Moreover, a theoretical correlation is developed to describe heat transfer during boiling in a three-phase circulating fluidised bed. The approach uses earlier work on two-phase (liquid-vapour) flow boiling, two-phase (liquid-solid) fluidised beds and three-phase (liquid-vapour-solid) circulating fluidised beds. The correlation developed is validated against experimental data obtained in Part I of the presented paper. The model's ability to predict the experimental data has been successfully demonstrated. The developed expression for heat transfer coefficients is written as follows:

α_T.F.B.-[(α_nb)^t_R+(α_cb)^t_R]^1/t_R     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part II: Mathematical modeling

In this paper a mathematical model for solids flow in gas-flowing solids-fixed bed contactors is developed. The presented four-parameter model assumes plug flow with axial dispersion in the dynamic, fast moving solids zone and the exchange of particles from this zone with the static zones. The complex dynamic behavior of the ‘static’ particles is described with a model of exchange between the dynamic and three static zones: (a) the ‘fast’ exchanging, (b) the ‘slow’ exchanging and (c) the ‘dead’ zone.The model parameters are optimized on the basis of two different tracer experiments: the step change response at the outlet—x_dyn, and the static holdup response—x_st. The model was tested for seven experimental series which correspond to different operating conditions (e.g., different superficial gas velocity and solids mass flux).The influence of the operating conditions on the model parameters is presented and discussed. Model reduction was also implemented in order to analyze the model simplifications and alternatives. Model sensitivity analysis was performed as well.     

Gas‐Liquid Mass Transfer in Fibrous Bed Reactor with Counter‐Current Liquid Recycle

In this work, the gas‐liquid mass transfer in a lab‐scale fibrous bed reactor with liquid recycle was studied. The volumetric gas‐liquid mass transfer coefficient, k_La, is determined over a range of the superficial liquid velocity (0.0042–0.0126 m.s^–1), gas velocity (0.006–0.021 m.s^–1), surface tension (35–72 mN/m), and viscosity (1–6 mPa.s). Increasing fluid velocities and viscosity, and decreasing interfacial tension, the volumetric oxygen transfer coefficient increased. In contrast to the case of co‐current flow, the effect of gas superficial velocity was found to be more significant than the liquid superficial velocity. This behavior is explained by variation of the coalescing gas fraction and the reduction in bubble size. A correlation for k_La is proposed. The predicted values deviate within ± 15 % from the experimental values, thus, implying that the equation can be used to predict gas‐liquid mass transfer rates in fibrous bed recycle bioreactors.     

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.     

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors. Part B: Heat, momentum and mass transport in bubbling fluidised beds

The fluid-particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the heat, momentum and mass transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user-defined function (UDF). The study completes the fast pyrolysis modelling in bubbling fluidised bed reactors.