Klaus Schubert

Microstructure Heat Exchanger Applications in Laboratory and Industry

In this article, heat transfer in microstructure devices and its application in laboratory and industry will be described. Basic principles of microstructure heat exchangers made of metal, ceramics, and polymers will be presented. A variety of laboratory prototype applications will be shown, as well as some examples for industrial use of not only microstructure heat exchangers, but also microstructure devices as chemical reactors. A brief outlook will describe possible future application fields.

Low-Frequency Instabilities in the Operation of Metallic Multi-Microchannel Evaporators

Visualization by high-speed videography and infrared surface thermography was used to compare the spatial and temporal maldistribution of flow, manifesting itself in pulsation and hot spot formation, respectively, in water evaporators consisting of either a single metallic foil with an array of mechanically micromachined microchannels or of several such foils assembled into an electrically powered micro heat exchanger. In the single layer devices examined by high-speed videography, pulsation in the frequency range below 20 Hz was found to be dominated by the generation of large bubbles in the inlet plenum. A redesign of the inlet with microchannels instead of a large plenum eliminated the pulsation at sub-audio frequencies, at the expense of a significantly increased pressure drop across the device. Infrared thermography of an electrically powered micro heat exchanger operated as an evaporator showed the formation of metastable hot spots as the result of maldistribution among different microchannel array layers. The formation of these hot spots could be eliminated by operating the device under heater cartridge temperature control conditions instead of constant power conditions.

Reforming of Diesel Fuel in a Micro Reactor

Reforming of diesel fuel is challenging but very attractive for hydrogen production. It can facilitate the market entrance of fuel cells due to the existing infrastructure for distribution of diesel fuel. Reforming in micro reactors enables good heat transfer and therefore small and compact fuel processing systems e.g. for electrical energy generation in auxiliary power units.Due to the complexity of diesel, reforming of different diesel components and conversion intermediates in a micro reactor is investigated systematically within this work. Methane and propane were applied as conversion intermediates and hexadecane as a diesel surrogate. All experiments were conducted over a rhodium catalyst on Al2O3 or CeO2.For evaporation of the higher boiling hydrocarbons a micro structured injection nozzle was fabricated to create a fine hydrocarbon spray which evaporates in water vapour. Furthermore a complex gas chromatographic method to analyse hydrocarbons up to C16 and the permanent gases in one analysis run was developed.Experimental results show that the turnover frequency of the fuel molecules in the feed decreases linearly for straight chain hydrocarbons with an increasing number of carbon atoms. Calculations show that the observed conversions and product gas compositions are close to the thermodynamic equilibrium. The catalyst system Rh/CeO2 offers better reforming performance and higher resistance to coking apparently due to less acidic sites compared to Al2O3 and the oxygen storage capacity of CeO2.The ongoing work will examine the reforming behaviour of more model diesel fuel components e.g. mixtures of hexadecane and methylnaphthalene or synthetic diesel fuel. Experiments will be conducted in an optimised micro reformer, which disposes the heating energy by burning e.g. fuel cell off-gases. This also offers the consideration of start up and load changing behaviour.

Microstructure devices for efficient heat transfer

Microstructure devices provide unique properties with regard to heat and mass transfer. Due to the tremendously high surface-to-volume ratio they are very well suited for many thermal and chemical processes in which large amount of heat has to be transferred. Metal microstructure devices also provide very high stability against high pressure, combined with an adjustable mass flow range of up to several thousand kg of liquid per hour and per passage, depending on the size and number of the integrated microstructures. Aside of fluid driven metallic microstructure devices like the famous Karlsruhe Cube, electrically powered devices have been developed and applied for temperature ranges where thermoliquids reach their limits or the use of gases may be disadvantageous due to their high viscosity and the arising pressure drop. In this publication several microstructure devices for heating and evaporation of fluids as well as for chemical reactions are presented in overview style. Details on manufacturing and device properties are given. Some process examples and experimental data for different types of microstructure devices are shown. Fouling problems are discussed briefly by an example.

Enhanced Microstructured Reactor Performance under Forced Temperature Oscillations

The forced variation of reaction parameters is a known method to improve the performance of catalytic reactors leading to process intensification. The most often experimentally varied parameters so far were the reactant concentrations or pressure. Due to the high thermal inertia of conventional reactors it was almost impossible to achieve fast periodic reproducible temperature changes. However, it has been proven theoretically that fast periodic temperature variations may increase the reaction rate compared to the stationary temperature conditions.The possibility to thermally cycle microstructured stainless steel reactors in a periodic way with temperature differences of up to 60 K and a frequency as high as 0.06 Hz has been demonstrated. This gives the opportunity to study the influence of fast temperature changes on heterogeneously catalyzed gas phase reactions. The catalytic CO oxidation over Pt supported on Al2O3 was chosen as a test reaction. The concentrations of CO, O2 and CO2 were monitored online at the reactor outlet using FTIR and mass spectrometry. The experimental measurements under non-stationary temperature conditions have shown an increase in the measured CO2 concentration compared with the one under temperature stationary conditions. For a temperature oscillation frequency of 0.048 Hz with an amplitude of 38 K, the mean CO2 concentration is 1.72 times higher than the mean value obtained under quasi-stationary conditions. A possible explanation for this phenomenon is an increase of the reaction rate due to the presence of a transitional reactive species surface coverage resulting from the temperature oscillations.

Selective Adsorption of Solvents in a Multiscale Device

We have incorporated microspheres, 50 μm to 80 μm in diameter, of periodic mesoporous organosilica (inner surfaces up to 1000 m2 /g and pore sizes in the nanometre range) with two types of organic functionalities (benzene and ethane bridges, respectively) inside microstructured channels (each 200 μm wide and 100 μm deep) and, exemplarily, monitored by Raman microscopy that the temperature characteristics of the adsorption-desorption equilibria of benzene and ethanol vary significantly with the type of organic functionality of the microspheres and the pore morphology. The integration of this class of nanostructured material into devices by means of microchannels is a promising novel approach to, among others, substance separation in analytics, micro process engineering, and micro chemistry.Copyright