Showing papers in "Chemical Engineering & Technology in 2003"

Microwave-Assisted Chemical Reactions

Abstract: This article reviews the state of the art of microwave-assisted reactions and the influence of microwaves on mass and heat transfer. The heating behavior of representative test reactions and single substances is compared for heating with microwaves and thermal energy. Similarities and differences between convective heating and dielectric irradiation methods are discussed with regard to the yield, the selectivity, and the enantiomeric purity of the reaction products. Furthermore, prevailing problems related to the scale-up of microwave-assisted reactions are discussed considering the energy absorption of the substances and mixtures to be heated, and their dependence on the energy consumption and the amount of substance.

Characterization of the Mixing Quality in Micromixers

Abstract: The laminar flow patterns and mixing performance of two different micromixers have been investigated and quantified using CFD. The micromixer geometries consist of a channel with either diagonal or asymmetric herringbone grooves on the channel floor. The numerical results show that a single helical flow is produced for the diagonal mixer, whereas the herringbone mixer creates a double helical flow, composed of an alternating large and small vortex. Particle tracking of a tracer shows that very little convective mixing occurs in the diagonal mixer. However, in the herringbone mixer, very good mixing occurs. Quantitative analysis methods that are traditionally used for characterizing macro-scale static mixers have been employed. Calculation of the variance of tracer dispersion and the stretching has shown to be well adapted for quantifying the mixing in the micromixers. However, methods based on the deformation rate appear to be less suitable. The results are in excellent agreement with previous experimental findings.

Microchemical Engineering: Components, Plant Concepts, User Acceptance – Part III

Abstract: Over the last five years, many activities have focused on the unexploited field of carrying out reactions on small scales. Due to the rapid development of new components, this paper deals with recent developments only in a compressed form. An important point is the analysis of possible plant concepts for microreactors and whether these are a sensible option. Due to the enormous difference in size between the microchannels and the fluid periphery of possible components this is not just a technical question. It touches on the microtechnology concept as a whole. The direction in which the field should be developed and which measures can be taken to influence its development are questions that are addressed here with respect to the big industrial interest in microreactors.

Development of a Continuous Segmented Flow Tubular Reactor and the “Scale-out” Concept – In Search of Perfect Powders

Abstract: Reference LTP-ARTICLE-2003-005doi:10.1002/ceat.200390046View record in Web of Science Record created on 2005-01-12, modified on 2017-05-10

A Stressing Model for the Description and Optimization of Grinding Processes

Abstract: The comminution behavior of a mill is characterized by the frequency of stress events, and the stress energy, acting at each stress event. Based on this, a comminution result is determined by the number of stress events, and the stress intensity. The product of stress number and stress energy is proportional to the specific energy input. The proportionality factor is the energy transfer factor, which describes the percentage of energy that is used for stressing the product particles. Using the characteriztic numbers based on a few tests, the operating parameters can be optimized and different mills can be compared. The application of the stressing model is shown for stirred media mills.

Formation of monodispersed nano- and micro-particles controlled in size, shape, and internal structure

Abstract: Size control of monodispersed particles must normally be performed during the very short nucleation period, because the final particle number is determined by the end of the nucleation and it does not change until the end of the particle growth. On the other hand, the shape control is mainly performed by adsorption of shape controllers during the growth period, although in some cases the crystal structure of the nuclei may have essential influence on the morphology of the grown particles. The internal structure of each particle is often strongly affected by the action of shape controllers. This article deals with the fundamental mechanisms of the nucleation, growth, and structural formation of monodispersed nano- and micro-particles, leading to the control of their size, shape, and internal structure.

Numerical Simulations of the Direct Shear Test

Abstract: Numerical simulations of the direct shear test have been performed under constant normal stress conditions, using the Discrete Element Method. Results are presented to show how the average shear to normal stress ratio acting on the shear band compares with the force data at the boundaries. From the calculations of the stress tensor inside the shear band the corresponding Mohr’s circles are obtained and the principal stress directions identified. The principal directions of stress and strain-rate are shown to be coaxial under steady-state conditions. Using the state of stress and strain in the shear band, the interpretation of direct shear test data is discussed in the context of traditional experimental measurements.

Inorganic Layers on Polymeric Films – Influence of Defects and Morphology on Barrier Properties

Abstract: Flexible polymeric films are not only widely used in conventional packaging as substitute for glass and aluminum foil packaging but are also proposed as encapsulation for novel products, like flexible solar cells or organic light-emitting devices. The two essential properties of the polymeric packaging are flexibility and good permeation barrier properties against gases and vapors. This article deals with vacuum web coating as a common way of increasing barrier properties of polymeric films and the problems related to this procedure. Defects caused by particles and surface imperfections are found to dominate the permeation rate for such coated polymeric films. Atomic force microscopy, electron and also optical microscopy was used for analysis of the coating layer. Three-dimensional numerical simulations were performed for modeling of the influence of defect size, spacing and film thickness. Results of numerical modeling and of many practical experiments show that the permeability is almost independent of the substrate film thickness when a critical thickness is exceeded. In most cases the defects can be treated as independent of each other. The gas permeability of vacuum web-coated polymeric films can be quantitatively predicted by a simple formula. For gases, like oxygen, it is shown that a statistic analysis of the defect sizes by optical microscopy is sufficient. For water vapor transmission, however, the structure of the coating layer itself has also to be taken into account.

Theoretical investigation of a Pd-membrane reactor for methanol synthesis

Abstract: A numerical study is performed in order to evaluate the performance and optimal operating conditions of a palladium membrane reactor for methanol synthesis. A novel reactor configuration with a Pd wall, which is perm-selective to hydrogen, has been proposed. In this configuration the reactants are added to the tube side while pure hydrogen is added to the shell side, as a result, the hydrogen diffuses across the membrane from the shell side to the tube side. In this membrane reactor, hydrogen penetrates to the reaction side in order to maintain a suitable hydrogen level in the whole length of the reactor and shift the equilibrium reaction. The effects of different parameters on the methanol output mole fraction were investigated in the co-current mode. These parameters were membrane thickness, reaction side flow rate, reaction side pressure, shell side pressure and H2/CO2 ratio in the feed.

Liquid Flow on Structured Packing: CFD Simulation and Experimental Study

Abstract: A CFD model of the two-phase countercurrent flow in the geometry of the plate-type structured packing Mellapak 250Y was built, tested and verified The model was applied to determine the effect of liquid and gas flow rates and physicochemical properties of the flowing liquids on the interfacial area formed on structured packing The CFD model allowed us to determine the minimum liquid flow rate at which an unbroken liquid film was observed on the packing surface The simulations confirmed that with an increase of the wetting rate the surface of the packing covered with a liquid film increased until the surface was totally covered up, while further slight changes of an interfacial area were the result of wave formation The effect of gas load (F factor) on the film surface was in the range of a calculation error Results of the CFD simulation allow us to predict the stages of film formation during liquid flow, to follow local velocity oscillations, film thickness and velocity profiles of phases

Nanofiltration for the Separation of Nonvolatile Products from Solutions Containing Ionic Liquids

Abstract: Nanofiltration (NF) can be used to isolate nonvolatile compounds from solutions containing ionic liquids (IL). This was shown for the mixtures bromophenol blue/IL and lactose/IL. In both cases the product was rejected while the ionic liquid permeated. Alternatively, the rejection of the ionic liquid is possible as well. Such separations are possible because nanofiltration membranes are selective towards size and charge of the components.

Applying a Thermodynamic Model to the Non‐Stoichiometric Precipitation of Barium Sulfate

Abstract: Thermodynamic models for aqueous Ba 2+ -SO 4 2- -Na + -Cl - -solutions are compared in their accuracy to predict ion activities in saturated and supersaturated solutions. The Pitzer and the Bromley model are employed, taking into account ion pair formation of barium sulfate. Such models are then used to describe particle nucleation and growth, and finally they are imbedded in a mechanistic mixing-precipitation model for a single feed semibatch process. The effect of the key operating parameters on the mean particle size is analyzed through simulations. The results are compared with previous experimental data, thus highlighting the significance of a proper choice of the thermodynamic model.

Particle Design Using Supercritical Fluids

Abstract: New supercritical fluid based techniques for particle design are reviewed in this paper. Particular attention will be given to a new and promising technique that uses supercritical fluids to assist atomization processes. Using this new and versatile process, several water soluble compounds that cannot be treated by traditional supercritical fluid based techniques can be successfully micronized. Lipophilic compounds can also be successfully processed.

Chute Performance and Design for Rapid Flow Conditions

Abstract: Many industrial chute applications are characterised by rapid flow conditions in which the bulk solid stream thickness or depth is less than the chute width. Under these conditions, it is possible to describe the stream flow by means of a lumped parameter model which takes into account the frictional drag around the chute boundaries as well as making allowance for inter-particle friction. Equations of motion to describe the chute flow are presented and their application to the determination of chute profiles to achieve optimum flow is illustrated. By means of design examples, the problems associated with the feeding of bulk solids onto belt conveyors and conveyor transfers are discussed. Criteria for the selection of the most appropriate chute geometry to minimise chute wear and belt wear at the feed point are presented. The determination of optimum chute profiles to achieve specified performance criteria is outlined.

Flow Properties of Highly Dispersed Powders at Very Small Consolidation Stresses

Abstract: With a modified standard ring shear tester yield loci of highly dispersed, dry powders were measured at preshear normal stresses down to 32 Pa and shear stresses down to 10 Pa. At small consolidation stresses stress, σ1, (< 500 Pa) the values obtained for the unconfined yield strength, σc, are proportional to the consolidation stress, σ1.

Spouted Bed Reactors

Abstract: The latest advances on conical spouted beds are discussed in this review. This contact method is especially useful for applications where a vigorous movement of the solid is required, for example, in the handling of solids that are sticky, of irregular texture, or with a large particle size distribution. Hydrodynamic correlations for calculation of the minimum spouting velocity, the operational and maximum pressure drops and bed expansion are presented. The gas flow pattern is described by means of a disperse plug flow model. Segregation is reduced and correlations are established following a similar procedure as in fluidization. Spout geometry and particle trajectories have been determined and the application of the conical spouted bed for pyrolysis of polyolefins is described.

Where is the Anion Extraction Going

Abstract: Reactive extraction processes represent efficient and smart technologies for separation and concentration of metal ions in solution, which are frequently used in industry. Despite the importance of anions in biology, medicine, environment and industry, practical examples of anion extraction are relatively limited compared to metal ion separation. Anion extraction processes are mainly based on the nonspecific ion pair formation with hydrophobic ammonium cations. In this case the phase transfer of anions is dominated by their lipophilicity. The reasons for this situation are closely connected with the specific features of anions in contrast to cations. Novel approaches for specific binding and selective transport of anionic components are based both on the better understanding of the biological role of anions and on the possibilities of supramolecular chemistry to create receptor architectures with complementary binding modes for anions. In the given review the authors discuss present research tendencies and application possibilities of new extractant types for separation and concentration of anionic species in solution.

Rigorous Modeling of Reactive Absorption Processes

Abstract: Absorption of gases in liquid solutions accompanied by chemical reactions, or reactive absorption, represents one of the most important industrial operations. The advantages of this process are enhanced solution capacity, low impurity concentrations up to the ppm range, moderate operation pressure, and selective removal of contaminants. Reactive absorption is based on a combination of various kinetically controlled phenomena, the focus being on the coupling of chemical reactions and mass transfer in multicomponent mixtures. Therefore, the process description using the equilibrium concept is often insufficient and kinetic modeling is required. This paper describes the basics of rigorous modeling of reactive absorption processes. Several case studies are used in order to validate the models. In addition, a discussion of necessary model parameters, like diffusion coefficients and reaction kinetics, is given. A reduced model with lower computation time is obtained via detailed sensitivity analysis. This model is successfully used in dynamic simulations. In this respect, a proper consideration of the film reactions appears to be crucial.

Novel Liquid‐Flow Splitting Unit Specifically Made for Numbering‐Up of Liquid/Liquid Chemical Microprocessing

Abstract: A liquid-flow splitting unit for dividing one main liquid stream into six substreams was developed without the need for active flow regulation. The tool has six fluid connectors that allow the numbering-up of a respective number of microprocess devices for liquid or liquid/liquid processes. One such liquid/liquid process for which the current tool design and tests in particular have been conceived, is the forced precipitation of precious inorganic powders by the segmented flow tubular reactor (SFTR) technique. Such investigations were performed with six micromixers of impinging-jet or interdigital separation layer type attached to the liquid-flow splitting unit. Fluid equipartition is achieved via the action of the microdevices as flow resistors, thereby increasing the pressure drop of the system. Liquid splitting was carried out in a cylindrical liquid tank with one inlet and six outlet holes that also served for pulsation dampening. By CFD simulations the impact of two important geometric parameters of this tank, namely, the height at a given diameter and the position of the inlet hole, on the fluid splitting was studied. In addition, the influence of the injection direction with respect to the normal of the tank was analyzed. It turned out that in the presence of flow resistors (typically generating a pressure drop at about 60 mbar) this impact is negligible, i.e., principally, a large flexibility on design and operation is provided. Without these resistors, the before-mentioned parameters in turn have a large impact that is discussed in detail. Experiments with a liquid-flow splitting unit and six coupled impinging-jet micromixers using water as liquid proved a reasonable fluid distribution with a standard and maximum deviation of the substream flow rates of 4 and 11 %, respectively. These deviations are mainly caused by different pressure drops (Δp) of the individual impinging-jet mixers as a result of tolerances in the microfabrication method, die sinking μEDM manufacture. Already small differences in the microstructured outlet hole diameters (d = 300 μm) of the impinging-jet mixer have a large influence due to the d -4 ∼ Δp dependence. Additionally, attaching specially adapted interdigital separation layer micromixers to the liquid-flow splitting unit, a minimum/maximum (standard) deviation of the water distribution below 5 (2) % could be achieved. To obtain this improved result, it was necessary to optimize the choice of material, the design of the microstructure inside the mixer, and the fabrication process.

The Droplet Population Balance Model – Estimation of Breakage and Coalescence

Abstract: A droplet population balance model is employed in order to describe the hydrodynamic behavior of solvent extraction columns. This model describes the axial change of local column holdup and local droplet size distributions due to the basic phenomena, like droplet rising, axial dispersion, droplet breakage and coalescence. In order to reduce experimental efforts, single and swarm droplet experiments in small lab-scale devices were performed. For this, a rotating disc contactor (RDC) with one compartment and a Venturi tube were used to investigate droplet breakage and droplet coalescence. In case of breakage the experiments were made for different droplet sizes at different rotor speeds for the EFCE system toluene/water, whereas the investigations of the coalescence phenomena depending on droplet size and holdup were done with the EFCE system n-butylacetate/water.

The Role of Particle Interactions on Suspension Rheology – Application to Submicron Grinding in Stirred Ball Mills

Abstract: A short review of different rheological models describing suspension rheology under various flow and suspension conditions is presented. Special attention is paid to models describing the influence of particle-particle interactions. The experimental part of this contribution shows how rheological properties of milling suspensions change with milling time and suspension properties in the sub-micron size range. Experimental results are related to theoretical models. The results demonstrate the key role that particle-particle interactions play in altering the flow properties of milling suspensions throughout the process.

Fed-batch Process for Pyruvate Production by Recombinant Escherichia coli YYC202 strain

Abstract: Fed-batch fermentation was applied to the production of pyruvate by using a recombinant Escherichia coli YYC202 strain. This strain is completely blocked in its ability to convert pyruvate into acetyl-CoA or acetate, resulting in acetate auxotrophy during growth in glucose minimal medium. By controlling acetate and glucose feed rate, a series of lab-scale fed-batch experiments were performed at pH 7 and 37 °C. CO2 production rate (CTR) was used for on-line regulation of the acetate feed rate. The correlation between CTR and acetate consumption rate (ACR) was determined experimentally. At optimal process conditions a final pyruvate concentration higher than 62 g/L, a space-time yield of up to 42 g/L/d and pyruvate/glucose molar yield of 1.11 mol/mol were achieved. Experimental evidence was gathered that pyruvate export is active.

Barium Sulphate Agglomeration in a Pipe – An Experimental Study and CFD Modeling

Abstract: Agglomeration effects, observed during precipitation of barium sulphate in the unpremixed feed two-dimensional tubular precipitator, are studied experimentally and interpreted theoretically. Effects of process parameters on precipitation-agglomeration phenomena are predicted using a CFD based model that describes micromixing (the multiple-time-scale turbulent mixer model is used) and precipitation (including nucleation, growth and agglomeration of crystals). Agglomeration rate is defined as a product of the collision frequency and the probability of agglomeration.

A Micro-Mechanical Model for the Rate of Aggregation during Precipitation from Solution

Abstract: The authors group has proposed that the rate of aggregation between crystals in a supersaturated solution depends on the rate of collision and on the probability of that collision surviving. It has been suggested that the probability, or efficiency, depends on the strength of the newly formed neck between the crystals and the hydrodynamic force acting to pull them apart. That strength has been quantified by assuming that the crystals first touch at a point and thus the area of the neck increases with the square of time. In this paper, over 400 data points were considered for calcium oxalate monohydrate (COM), and more than 250 for calcite, relating the rate of aggregation in a stirred tank to the stirrer speed, the supersaturation and the particle size and show that the existing model cannot account for the relationship seen. It is proposed instead that the first contact between crystals lies along a line and thus the area of the neck grows linearly with time. A dimensionless strength formulated in this way is able to account for the dependence seen.

Incorporation of Flexibility in the Design of a Methanol Synthesis Loop in the Presence of Catalyst Deactivation

Abstract: This paper presents the incorporation of process flexibility into a methanol synthesis loop operating under catalyst deactivation. A design methodology is discussed with regard to catalyst deactivation, and some limitations are identified. In the current flexibility study the size of the reactor and recycle ratio have been fixed. Attempts to maintain methanol production at the rates observed with fresh catalyst included increased pressure, increased make up gas flow rate, and the injection of carbon dioxide into the make up gas at optimized inlet temperature. In order to provide flexibility and produce a design compatible with increased production rates, the effect of interrelating equipment had to be considered. As a result of catalyst deactivation, an increased flow rate is necessary and the altered process streams entering the preheater disturb the reactor inlet temperature. These issues should be considered in the design stage and may be resolved by the flexible designs presented.

Pseudo‐polymorphic Behavior of Precipitated Calcium Oxalate

Abstract: The precipitation of calcium oxalate pseudo-polymorphs was studied in a Y-mixer in series with a reaction tube. Experiments were performed with the aim to identify the decisive variables influencing the phase composition of the precipitate. Initial supersaturation, stoichiometric ratio of the lattice ions, ionic strength and pH were varied. The resulting phase distribution was determined with the help of DSC measurements. Mixing conditions were chosen such that mixing of the reactants had no influence on the experiments. Increasing supersaturation, ionic strength and pH each favor formation of the metastable calcium oxalate dihydrate over the thermodynamic stable monohydrate, although the influence of the two latter parameters is comparatively weak. If the ratio from calcium to oxalate activities was increased in the experiments a steep change in the phase composition from approximately 50 mass percent of the monohydrate to pure monohydrate was observed. Modeling of the experimental results is successful using a combination of classical nucleation theory with an equation that relates the adsorption of potential determining ions to the interfacial tension. The interfacial tension for the uncharged particles was obtained by regression from the experimental determined dependence of the phase distribution on the supersaturation.

On the Degradation and Stabilization of Poly(Methyl Methacrylate) in a Continuous Process

Abstract: Thermally unstable polymers such as poly(Me methacrylate) are degraded considerably during industrial processing. This degrdn. and its redn. to a min. were investigated in both lab and continuous pilot-scale expts. A three-step degrdn. mechanism, starting at 180 Deg, was proved by Thermogravimetrical Anal. (TGA) and a kinetic approach to describe it was derived. The knowledge of this degrdn. behavior was then applied to a pilot-scale process with a prodn. rate of 10 kg/h and the process yield loss during the devolatilization step was investigated. Using heat stabilizers, the overall process yield could be improved by 10 %, whereas the residual org. volatiles concn. (VOC) was drastically reduced to values below 1000 ppm. In order to preserve the mol. wt. of the final product these stabilizers were added into the process, sep., at the end of the polymn. reaction but before the devolatilization step.

Turbulence Transfer Processes in Adiabatic and Condensing Film Flow in an Inclined Tube

Abstract: For the description of transfer processes in separated turbulent two-phase flow, knowledge is necessary about interactions at the interface. Particularly experimental studies were performed of the two-phase-friction coefficient, film thickness, entrainment and heat transfer for adiabatic and condensing conditions. Working systems were n-heptane/air, water/air and condensing n-heptane. The measurement of the liquid film thickness was successful with the ultrasonic method and the entrainment was measured with an isokinetic technique. Quantitative connections between hydrodynamic parameters were discussed and compared with literature models. Based on experimental data, special models are proposed. Partly the influence of the mass transfer intensity cannot be neglected for calculation of the two-phase-friction coefficient. For the description of the heat transfer for film condensation in turbulent flow the interface shear stress and the inclination angle of the tube have proved useful.

Production of Emulsions in High-Pressure Homogenizers – Part I: Disruption and Stabilization of Droplets

Abstract: In continuous mechanical emulsification, disruption and stabilization of droplets determine the resulting droplet size and, thus, the emulsion's microstructure. Without the need of adding any stabilizer, w/o emulsions provide the possibility of a high viscosity of the continuous phase and, in consequence, of decreasing the probability of coalescence. The present work presents investigations on the production of w/o emulsions in high-pressure homogenizers: the different geometries of standard valve, microfluidizer and orifice valve are compared to each other with reference to disruption and stabilization of droplets. In the orifice valve, droplets are disrupted most efficiently; however, coalescence superimposes the disruption result to a higher extent than in the microfluidizer. On the basis of simulations of the flow pattern, the geometry of the orifice valve has been improved: the new conical valve provides for higher volumetric flow rates and a lower extent of coalescence in comparison to the orifice valve.