Untersuchungen zum Hochdruck‐Permeationsverhalten reiner Gase durch mikroporöse keramische Membranen Teil 2. Analyse der Stofftransportmechanismen

The determined behaviour of a TiO₂‐membrane with an effective pore diameter below 1 nm cannot be described comprehensively by the transport mechanisms known at present. The transport mechanism is dominated by adsorption effects. The demonstrated results give an impetus for a new transport model approach termed MAT‐Model (Micro‐porous Adsorption and Transport‐Model).     

Performance and Energy Consumption of Membrane‐Distillation Hybrid Systems for Olefin‐Paraffin Separation

Light olefin and paraffin are commonly separated by energy‐intensive cryogenic distillation. Membrane/distillation hybrid systems constitute an economical alternative separation process. Different configurations of this hybrid system are studied for olefin‐paraffin separation with emphasis on C₃ separation. An approach based on the McCabe‐Thiele method is applied to analyze different process configurations. A facilitated transport membrane is considered as membrane type. Both new column design and augmentation of an existing distillation column by a membrane module are considered. Numerical examples are considered for the separation of propane from propylene through different hybridization shapes with facilitated transport membranes. The energy requirement can be halved using hybrid systems.     

Experimental and modeling analysis of diffusive release from single‐shell microcapsules

There is much experimental and mathematical work that describes chemical transport from multilayered films of planar geometries. There is less so, however, for chemical transport from multilayered spheres, a common structure for controlled‐release materials. Based on the Sturm–Liouville approach of Ramkrishna and Amundson (1974), explicit analytical solutions for the concentration profiles and release kinetics from spherical capsules are presented. Fluorescent dye‐release studies using single‐shelled microspheres called nanoparticle‐assembled capsules were performed to validate the model for uniformly and nonuniformly sized capsules. The combined experiment‐modeling approach allows optical microscopy images and release measurements to be readily analyzed for estimating diffusion coefficients in capsule core and shell walls. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Evaluation of an Entropy‐Based Combustion Model using Stochastic Reactors

The entropy transport concept (ETC) presented in this paper is a novel approach to describe reaction systems such that the dynamic behavior of a chemical system can be reproduced with a minimum in independent parameters. It is shown that, for adiabatic conditions, the mixture fraction and the reaction entropy are sufficient to describe combustion processes without significant loss of information. Entropy is used as a measure of the reaction progress in this context. In order to evaluate the applicability of the ETC for combustion modeling in turbulent systems, the entropy transport concept was implemented into a stochastic reactor model. For several test cases, the results of this ETC‐based reactor were compared with a reactor that directly integrates the species transport equations.     

Non‐Invasive Process Imaging – Principles and Applications of Industrial Process Tomography

In chemical industries, process diagnostics have traditionally been achieved using discrete sensors installed in the critical plant locations and linked into a plant control system. The usefulness of such an approach is limited, as important information about the processes is lost. This paper demonstrates a different way of looking into the spatial and temporal characteristics of the industrial processes by employing techniques of industrial process tomography. The principles behind tomographic measurements are outlined and examples illustrating the capabilities of this approach are presented. These include imaging of the processes confined within process vessels such as fluidization and nylon polymerization, as well as an example of a transport‐dominated process – flow in a pneumatic conveyor. The advantages, limitations and potential future uses of process tomography are discussed.     

CHEMICAL REACTIONS WITH MOLE CHANGES IN HETEROPOROUS CATALYSTS—PART I

The modified dusty-gas model^1.2 is used to describe the mass fluxes for zero and first order irreversible reactions with moles changes in heteroporous catalysts, and to estimate their effectiveness factors in the transition regime. It is shown that the effectiveness factors predicted by the dusty-gas model (homoporous model) are larger than those calculated by the modified dusty-gas model (heteroporous model), in some cases by about 30%. In the Knudsen and molecular regimes, the correction factors of the modified dusty-gas model approach unity and the two models predict the same values for the effectiveness factors.

Since many industrial catalysts are heteroporous and operate in the transition region of transport, the transport equations and calculational procedures presented in this work are relevant to reactor design.     

Instationary Shrinking‐Core Models for Heterogeneous Ionic Reactions

We introduce mathematical models for a shrinking‐core process in a heterogeneous reaction‐diffusion system. The first model refers to diffusive mass transport. For this case a novel numerical approach is presented, allowing for calculation of the moving boundary as an explicit system variable. To cover the additional effect of migration, which is present in our experimental set‐up, the model is modified to incorporate electrical forces. Both models are compared and experimentally validated.     

Diffusion‐controlled polymer dissolution and drug release

A model is formulated for diffusion‐controlled polymer dissolution and simultaneous drug release for amorphous, uncrosslinked polymers. Calculated fractional drug release versus time curves are similar to experimental curves reported in the literature. Solutions of the transport equations provide a way of illustrating how polymer type and molecular weight can be used in an attempt to approach a constant drug release rate. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 92–99, 2004     

Einführung in die Transportvorgänge in konzentrierten wäßrigen Elektrolytlösungen und Salzschmelzen

Introduction to transport phenomena in concentrated aqueous electrolyte solutions and molten salt mixtures . Up to this day there is no general theory of liquids. Since water-free molten salt mixtures are simpler in their structure than concentrated aqueous electrolyte solutions, it would be reasonable first to develop a theory for molten salts and then to apply this to aqueous electrolyte solutions by approximation. Such a theory does not yet exist. In this paper the system water + salt is therefore considered in the concentration range between pure water and pure salt in the liquid state. The phenomenological coefficients and friction coefficients of the different species of the system calculated from transport data are characteristic parameters for the entire concentration range which will probably offer possibilities for the analysis of transport phenomena, in close relation with the structure of the water + salt mixture. Common and essentially different properties, such as reference system and condition of electric neutrality, are discussed for concentrated aqueous electrolyte solutions and molten salt mixtures. The investigation of transport numbers from concentration cells with transference and the determination of the interdiffusion coefficients are illustrated for the system H₂O + CaCl₂ (highly concentrated electrolyte solution) and LiNO₃ + AgNO₃ (molten salt mixture). The phenomenological coefficients and the friction coefficients are calculated from the measured transport data.     

Two‐Phase Flow with Mass Transfer in Bubble Columns

Bubble columns are widely used in the chemical and biochemical industries. In these reactors a gaseous phase is dispersed into a continuous liquid phase thus the rising bubble swarm induces a circulating flow field. For the dimension of these reactors the local interfacial area and the residence time of the liquid and the gaseous phase are key parameters. In this paper an Euler‐Euler approach is used to calculate the flow field in bubble columns numerically. Therefore a transport equation for the mean bubble volume based on a population balance equation approach is coupled with the balance equations for mass and momentum. The calculations are performed for three‐dimensional, instationary flow fields in cylindrical bubble columns considering the homogeneous and the heterogeneous flow regime. For the interphase mass transfer the physical absorption of the gaseous phase into the liquid is assumed. The back mixing in the gaseous and liquid phase is calculated from the local and time dependent concentration of a tracer.     

Structured Porous Millireactors for Liquid‐Liquid Chemical Reactions

Milli‐scale reactors with an integrated microstructure offer a promising scale‐up approach for conventional microreactors. This study applies 3D‐printed structured porous millireactors to industrially relevant liquid‐liquid reactions. The underlying transport mechanisms are identified by quantifying interfacial heat and mass transfer. The structured reactors perform limited in Taylor flow compared to a packed‐bed reactor due to limited interfacial mass transfer. However, in stratified flow, their productivity increases significantly at a fraction of the pressure drop of a packed bed.     

Transport parameters for the modelling of water transport in ionomer membranes for PEM-fuel cells

The water transport number (drag coefficient) and the hydraulic permeability were measured for Nafion. The results show a significant increase of both parameters with increasing water content indicating that they are strongly influenced by the membrane microstructure. Based on these experimental studies a new model approach to describe water transport in the H₂-PEFC membrane is presented. This approach considers water transport by electro-osmosis caused by the proton flux through the membrane and by osmosis caused by a gradient in the chemical potential of water. It is parametrized by the measured data for the water transport number and the hydraulic permeability of Nafion. First simulation results applying this approach to a one-dimensional model of the H₂-PEFC show good agreement with experimental data. Therefore, the developed model can be used for a new insight into the dominating mechanisms of water transport in the membrane.     

Flavonoids Have Differential Effects on Glucose Absorption in Rats (<Emphasis Type="Italic">Rattus norvegicus</Emphasis>) and American Robins (<Emphasis Type="Italic">Turdis migratorius</Emphasis>)

Mounting evidence suggests that small birds rely largely on non-mediated intestinal absorption of glucose through the paracellular pathway, while non-flying mammals rely on mediated absorption across the enterocyte membranes by using glucose transporters SGLT-1 and GLUT-2. Relying on non-mediated transport of glucose may decrease its absorption rate at low glucose concentrations but may release small birds from the effects of glucose transport inhibitors. We evaluated transport by using flavonoids known to inhibit glucose transport in vitro. Quercetin, isoquercetrin, and phloridzin were tested in rats (Rattus norvegicus) and robins (Turdis migratirius), and naringenin, naringenin-7-glucoside, genistein, epigallocatechin gallate (EGCG), and phloretin were used only in rats. By using a pharmacokinetic approach that involves serial blood collection and area under the curve calculations, we determined the bioavailability of 3-0-methyl D-glucose, the non-metabolized analogue of D-glucose. Six of the eight flavonoids tested in rats significantly decreased the absorption of 3-0-methyl D-glucose, while none of the flavonoids tested in robins significantly decreased the bioavailability of 3-0-methyl D-glucose. We conclude that flavonoids effectively decrease glucose absorption in rats, which rely on mediated absorption of glucose, but that flavonoids do not have an effect in robins, which rely on non-mediated absorption of glucose.     

Thermodynamic approach to interfacial transport phenomena: Single‐component systems

Balancing extensive quantities at interfaces in terms of excess densities is a subtle problem because these densities change with the precise location of the dividing surface within the interfacial region. We propose to handle such ambiguities for moving interfaces by introducing a gauge degree of freedom associated with the location of the dividing surface and by studying all the normal velocities associated with different excess densities. Unambiguous interfacial balance equations can then be obtained directly from the differences between normal velocities. By assuming local equilibrium, considering the interfacial entropy balance, and identifying the entropy production rate at the interface, we are naturally led to thermodynamically consistent constitutive assumptions for the fluxes characterizing transport in the interface and for the boundary conditions characterizing transport across the interface. The usefulness and generality of the proposed approach is illustrated in the context of several examples. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1424–1433, 2014     

Theoretical investigations of electronic structure and charge transport properties in polythiophene‐based organic field‐effect transistors

Regioregular poly(3‐hexylthiophene) (P3HT) is a hole transport polymer material used in organic field‐effect transistors (OFETs) and can reach mobilities as high as 0.1 cm² V^?1 s^?1. Factors that affect the charge mobility and the transport mechanisms of P3HT‐based OFET systems are therefore of great importance. We use quantum mechanical methods to interpret the charge mobility and the transport properties of self‐assembled P3HT molecules along the intra‐chain and inter‐chain directions. Our approach is illustrated by a hopping transport model, in which we examine the variation of charge mobility with torsional angle and the intermolecular distance between two adjacent thiophene segments. We also simulate packed P3HT structures via molecular dynamics (MD) simulations. The MD results indicate that the resultant mobility along the π?π inter‐chain direction is significantly less than that along the intra‐chain direction. Accordingly, the main charge‐transfer route within the P3HT ordered domains is an intra‐chain rather than an inter‐chain one. The calculation result for the inter‐chain hole mobility is around 10^?2 cm² V^?1 s^?1, which is consistent with experimental data from P3HT single fibril. Copyright © 2009 Society of Chemical Industry     

Mesopore‐Promoted Transport in Microporous Materials

In addition to the respective pore space geometries, transport enhancement in hierarchical nanoporous materials may most decisively depend on the nature of the probe molecule under consideration. As a function of these two influences, mass transfer in hierarchical materials may follow quite different scenarios. Considering mass transfer enhancement by a network of mesopores within a microporous continuum, this work presents an approach to assess these influences quantitatively. The validity of this approach is tested by dynamic Monte Carlo simulations and by comparison with the results of diffusion measurements.     

Mixed convection flows in a square lid‐driven cavity with heat source at the bottom utilising nanofluid

This paper presents a numerical investigation of laminar mixed convection cooling of heat source embedded on the bottom wall of an enclosure filled with nanofluids. The transport equations for a Newtonian fluid are solved numerically with a finite volume approach using the SIMPLE algorithm. The influences of governing parameters, namely, Rayleigh number location and geometry of the heat source, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. The present results are validated by favourable comparisons with previously published results. The results of the problem are presented in graphical and tabular forms and discussed. © 2011 Canadian Society for Chemical Engineering     

Large‐Scale Preparation of Polymer Nanocarriers by High‐Pressure Microfluidization

Polymer nanocarriers are used as transport modules in the design of the next generation of drug delivery technology. However, the applicability of nanocarrier‐based technology depends strongly on our ability to precisely control and reproduce their synthesis on a large scale because their properties and performances are strongly dependent on their size and shape. Fundamental studies and practical applications of polymer nanocarriers are hampered by the difficulty of using the current methods to produce monodispersed nanocarriers in large quantities and with high reproducibility. Here, a versatile and scalable approach is reported for the large‐scale synthesis of polymer nanocarriers from water‐in‐oil miniemulsions. This method uses microfluidization to perform a controlled emulsification and is proven to be effective to prepare nanocarriers of different biopolymers (polysaccharides, lignin, proteins) up to 43 g min^?1 with reproducible size and distribution.     

Fluid Flow and Kinetic Modelling in Flotation Related Processes: Columns and Mechanically Agitated Cells—A Review

In this paper, fluid flow and kinetic models related to minerals flotation process are presented and the advantages and limitations of using this type of models are discussed. The modelling of such processes was firstly developed assuming perfect mixing for the whole system as a black box. Then, a more realistic approach was developed recognizing the interaction between two zones, the particle–bubble collection zone and the froth transport zone.

From a hydrodynamic point of view, experimental data showed that single large mechanical flotation cells can deviate significantly from perfect mixing, while the mixing conditions in a flotation bank of mechanical cells (three to nine cells in series) can be well described as a series of continuous perfectly mixed reactors. From plant experience, it was observed that performance of large industrial pneumatic flotation columns, originally regarded as a counter-current operation, also operate closer to a single perfectly mixed reactor.

Advances in the field of modelling and design of flotation cells and columns, have been achieved because the fluid flow regime, the mass transport conditions at the pulp/froth interface and the froth transport mechanisms are better known and understood. Key parameters such as the bubble surface area flux, related to the bubble generation and the rate of particle collection, bubble loading related to the mass transport across the pulp-froth interface and froth recovery, which is mainly related to the gas residence time in the froth, are relevant for a deeper understanding of this type of equipment.