Steffen Hardt

Chemical micro process engineering : fundamentals, modelling and reactions

Preface.List of Symbols and Abbreviations.1. A MULTI-FACETED, HIERARCHIC ANALYSIS OF CHEMICAL MICRO PROCESS TECHNOLOGY.1.1 Micro Reactor Differentiation and Process Intensification.1.2 Consequences of Chemical Micro Processing.1.3 Physical and Chemical Fundaments.1.4 Impact on Chemical Engineering.1.5 Impact on Process Engineering.1.6 Impact on Process Results.1.7 Impact on Society and Ecology.1.8 Impact on Economy.1.9 Application Fields and Markets of Micro Reactors.2. MODELLING AND SIMULATION OF MICRO REACTORS.2.1 Introduction.2.2 Flow Phenomena on the Micro Scale.2.3 Methods of Computational Fluid Dynamics.2.4 Flow Distributions.2.5 Heat Transfer.2.6 Mass Transfer and Mixing.2.7 Chemical Kinetics.2.8 Free Surface Flow.2.9 Transport in Porous Media.3. GAS-PHASE REACTIONS.3.1 Catalyst Coatings in Micro Channels: Techniques and Analytical Characterization.3.2 Micro Reactors for Gas-Phase Reactions.3.3 Oxidations.3.4 Hydrogenations.3.5 Dehydrogenations.3.6 Substitutions.3.7 Eliminations.3.8 Additions and Coupling Reactions.4. LIQUID- AND LIQUID/LIQUID-PHASE REACTIONS.<br& gt Micro Reactors for Liquid-Phase and Liquid/Liquid-Phase Recations 4.1 Micro Reactors for Liquid-phase and Liquid/Liquid-phase Reactions.4.2 Aliphatic Nucleophilic Substitution.4.3 Aromatic Electrophilic Substitution.4.4 Aliphatic Electrophilic Substitution.4.5 Aromatic Nucleophilic Substitution.4.6 Aromatic Substitution by Metal Catalysis or Other Complex Mechanisms.4.7 Free-radical Substitution.4.8 Addition to Carbon-Carbon Multiple Bonds.4.9 Addition to Carbon-Hetero Multiple Bonds.4.10 Eliminations.4.11 Rearrangements.4.12 Oxidations and Reductions.4.13 Organic Synthesis Reactions of Undisclosed Mechanism.4.14 Inorganic Reactions.5. GAS/LIQUID REACTIONS.5.1 Micro Reactors for Gas/Liquid Reactions.5.2 Aromatic Electrophilic Substitution.5.3 Free Radical Substitution.5.4 Addition to Carbon-Carbon Multiple Bonds.5.5 Addition to Carbon-Heteroatom Multiple Bonds.5.6 Oxidations and Reductions.5.7 Inorganic Reactions.Index.

Evaporation model for interfacial flows based on a continuum-field representation of the source terms

An evaporation model compatible with interface-capturing schemes for vapor-liquid flow is presented. The model formulation is largely independent of the specific realization of interface-capturing and relies on a continuum-field representation of the source terms implementable in a broad class of CFD models. In contrast to most other numerical methods for evaporating interfacial flows, the model incorporates an evaporation source-term derived from a physical relationship for the evaporation mass flux. It is shown that especially for microscale evaporation phenomena this implies significant deviations of the interface temperature from the saturation temperature. The mass source-term distribution is derived from the solution of an inhomogeneous Helmholtz equation that contains a free parameter allowing to tune the spatial localization of the source. The evaporation model is implemented into the volume-of-fluid scheme with piecewise linear interface construction. Results are obtained for three analytically or semi-analytically solvable model problems, the first two being one-dimensional Stefan problems, the third a free droplet evaporation problem. In addition, a two-dimensional film boiling problem is considered. Overall, the comparison between the CFD and the (semi)-analytical models shows good agreement. Deviations exist where convective heat transfer due to spurious currents is no longer negligible compared to heat conduction. With regard to the film boiling problem, a similar evaporation pattern as recently identified using a level-set method is found. A major advantage of the developed evaporation model is that it does not refer to intrinsic details of the interface-capturing scheme, but relies on continuum-field quantities that can be computed by virtually any CFD approach.

Microfluidic Technologies for Miniaturized Analysis Systems

Microfluidics: Fundamentals and Engineering Concepts.- Electrohydrodynamic and Magnetohydrodynamic Micropumps.- Mixing in Microscale.- Control of Liquids by Surface Energies.- Electrowetting: Thermodynamic Foundation and Application to Microdevices.- Magnetic Beads in Microfluidic Systems - Towards New Analytical Applications.- Manipulation of Microobjects by Optical Tweezers.- Dielectrophoretic Microfluidics.- Ultrasonic Particle Manipulation.- Electrophoresis in Microfluidic Systems.- Chromatography in Microstructures.- Microscale Field-Flow Fractionation: Theory and Practice.- Nucleic Acid Amplification in Microsystems.- Cytometry on Microfluidic Chips.

Integrated polymer chip for two-dimensional capillary gel electrophoresis.

Two-dimensional (2D) gel electrophoresis (GE) is one of the most powerful methods for nucleic acid and protein separation, but generally suffers from high laboratory efforts connected with high analysis costs. Therefore, we herein present the development of a miniaturized 2D capillary GE (CGE) device which allows for an efficient protein separation in analysis times of about 1.5 h. This integrated 2D-CGE chip comprises a first channel for isoelectric focussing (IEF), a second specially designed transfer channel, 300 parallel micro channels, each having a cross section of 50 microm x 50 microm, and buffer reservoirs. The present work discusses fabrication aspects, in particular the combination of different microfabrication technologies, experimental separation performances of isoelectric focussing (IEF) and CGE, and presents computer simulations and first experimental results of protein transfer from the first to the second dimension.

Electrophoretic partitioning of proteins in two-phase microflows

This work reports on protein transport phenomena discovered in partitioning experiments with a novel setup for continuous-flow two-phase electrophoresis consisting of a microchannel in which a phase boundary is formed in flow direction. Proteins can be partitioned exploiting their affinity to different aqueous phases in two-phase systems. This separation process may be enhanced or extended by applying an electric field perpendicular to the phase boundary. In this context, microsystems offer new possibilities, as interfacial forces usually dominate over volume forces, thus allowing a superior control of the formation and arrangement of liquid/liquid phase boundaries. The two immiscible phases which are injected separately into the microchannel are taken from a polyethylene glycol (PEG)-dextran system. The side walls of the channel are partially made of gel material which serves as an ion conductor and decouples the channel from the electrodes, thus preventing bubble generation inside the separation channel. The experiments show that the electrophoretic transport of proteins between the laminated liquid phases is characterized by a strong asymmetry. When bovine serum albumin (BSA) is introduced into the PEG-rich phase, it can easily be transferred into the dextran-rich phase via an applied electric field of low strength or just by diffusion. In the reverse case, up to a certain field strength the transfer to the opposing phase is strongly inhibited. Only if the field strength is further increased will the BSA molecules leave the dextran-rich phase almost completely.

Viscous mechanism for Leidenfrost propulsion on a ratchet

An evaporating drop placed on a ratchet self-propels, as discovered by Linke et al. in 2006. Sublimating platelets do the same, and we discuss here a possible viscous mechanism for these motions. We report that the flow of vapor below the levitating material is rectified by the asymmetric teeth of the ratchet, in the direction of descending slopes along each tooth. As a consequence, the resulting viscous stress can entrain the material in the same direction, and we discuss the resulting self-propelling force.

Strategies for size reduction of microreactors by heat transfer enhancement effects

One of the likely aims of reactor miniaturization in the field of chemical production and energy generation is to increase the conversion to the desired product and the selectivity of the process through better control of heat and mass transfer. In addition to the effects related to miniaturization, a further increase of the transfer coefficients is achieved by applying microstructuring techniques. In this context, three different approaches for heat transfer enhancement in miniaturized reaction systems are presented. The ideas put forward rely on entrance flow effects, inertial flows in meandering channels, and suppression of axial heat conduction. Among these ideas the entrance flow effect, realized by an arrangement of microfins with a typical dimension of a few hundred micrometers, provides the most efficient heat transfer. It is found that a heat transfer enhancement of at least one order of magnitude can be achieved compared to unstructured channels. On this basis, a miniaturized heat-exchanger reaction system is investigated, where a kinetic model of an endothermic, heterogeneously catalyzed gas-phase reaction is used. The miniaturized heat-exchanger reactor, both with and without heat transfer enhancement, is subsequently benchmarked against conventional fixed-bed technology. It is shown that, for the reaction system under study, a substantial reduction of the required amount of catalyst can be achieved in microsystems.

Automated chip-based device for simple and fast nucleic acid amplification

A chip-based PCR device is presented that is capable of rapid temperature ramping and handling sample volumes in the microliter range. The PCR chip comprises a microchannel thermally connected to three temperature zones. Inside this microchannel, the PCR sample plug is driven and precisely positioned by a ferrofluidic actuator for more than 40 cycles within 5 min. Computer simulations predict that the sample plugs are thermally equilibrated on a time scale of some 10 ms when transported to a different temperature zone. Hence, the thermal limitations on the cycle speed of the system are considerably reduced compared with conventional cyclers. The system was developed on a modular platform suitable for handling further microfluidic tasks such as DNA extraction and preparation of the PCR mix. Thus, the aspired chip-based platform represents not only a PCR system but a complete analysis system, from the injection of a patient’s blood sample to its final appraisal.

An adaptive liquid microlens driven by a ferrofluidic transducer

Ferrofluids behave superparamagnetically and can be manipulated by external magnetic fields, providing numerous applications in microfluidic systems. In this paper, an adaptive liquid microlens driven by a ferrofluidic actuator is presented. The microlens consists of a cylindrical well filled with a lens liquid connected to a microchannel containing a ferrofluid plug. When the ferrofluid plug is moved back and forth by an external magnetic field, the lens liquid is displaced, forming a liquid lens with an adaptive focus in the cylindrical well. The focal length of the lens can be changed from infinity to the scale of the radius of the cylindrical well, leading to a high optical power compared to conventional liquid lenses utilizing liquid crystals or electrowetting. The lens curvature is reversibly tunable without hysteresis when the ferrofluid plug moves with a speed below a specific threshold value. The lens can be acted on by a magnetic field of about 100 mT which can be generated by microcoils requiring much lower voltages than the electrowetting principle.

A particle-particle hybrid method for kinetic and continuum equations

We present a coupling procedure for two different types of particle methods for the Boltzmann and the Navier-Stokes equations. A variant of the DSMC method is applied to simulate the Boltzmann equation, whereas a meshfree Lagrangian particle method, similar to the SPH method, is used for simulations of the Navier-Stokes equations. An automatic domain decomposition approach is used with the help of a continuum breakdown criterion. We apply adaptive spatial and time meshes. The classical Sods 1D shock tube problem is solved for a large range of Knudsen numbers. Results from Boltzmann, Navier-Stokes and hybrid solvers are compared. The CPU time for the hybrid solver is 3-4 times faster than for the Boltzmann solver.