Abstract Labore ohne Glasgeräte? Werden organische Chemiker ihre Synthesen bald in Reaktorchips durchführen, die wie Computer‐Hardware organisiert sind, in Strukturen aus Silicium, Glas oder Stahl? Die Fortschritte in der Mikroreaktionstechnik der letzten zehn Jahre legen solche Fragen nahe ‐ und Umsetzungen in Mikroreaktoren gelingen häufig mit besseren Ausbeuten, erhöhter Sicherheit und weniger Zeit‐ und Materialaufwand.
A detailed mathematical model for the hot wall multiple‐disk‐in‐tube LPCVD reactor is developed by using reaction engineering concepts. This model includes the convective and diffusive mass transport in the annular flow region formed by the reactor wall and the edges of the wafers as well as the surface reactions on the reactor wall. In addition, the model describes the coupling of diffusion between and reaction on the wafers. Variations in gas velocities and diffusion fluxes due to net changes in the number of mols in the deposition are also taken into account as are nonisothermal operating conditions. The combined reactor equations are solved by orthogonal collocation. The deposition of polycrystalline Si from is considered as a specific example, and the model is employed in estimation of kinetic rate constants from published reactor measurements. The effects on the growth rates and film thickness uniformity (within each wafer and from wafer to wafer) of variations in flow rates, reactor temperature profiles, and concentration in the feed stream are analyzed. The model predictions show good quantitative agreement with published experimental data from different sources. Finally, recycle of reactor effluent is considered a typical commercial operating conditions, and it is demonstrated that this modification produces higher growth rates and better film uniformity than can be achieved in conventional LPCVD processing.
Using silicon microfabrication technology, microchemical devices have been constructed for the purpose of conducting heterogeneously catalyzed multiphase reactions. The motivation behind the design, the fabrication approach, and the experimental characterization are presented for two classes of devices. The first design involves multiple parallel channels with integrated filter structures to incorporate standard catalytic materials. These catalysts are in the form of finely divided porous particles in a packed-bed arrangement. The second device involves the incorporation of porous silicon as a catalyst support, in the form of a thin layer covering microstructured channels. These microstructured channels simulate the structure of a packed bed and enhance mass transfer relative to an open channel. The ability to incorporate features at the tens-of-microns scale can reduce the mass-transfer limitations by promoting mixing and dispersion for the multiple phases. Directly integrating the catalyst support structures into the channels of the microreactor allows the precise definition of the bed properties, including the support's size, shape and arrangement, and the void fraction. Such a design would find broad applicability in enhancing the transport and active surface area for sensing, chemical, and biochemical conversion devices. Reaction rates for the gas-liquid-solid hydrogenation of cyclohexene using the integrated catalyst with porous silicon as a support compare favorably to those rates obtained with the packed-bed approach. In both cases, the mass transfer coefficient is at least 100 times better than conventional laboratory reactors.
Hydrodynamic dispersion in microchannels can be significantly reduced by segmentation with a second immiscible phase. We address the effect of microchannel cross section on the dispersion of analytes in a segmented gas-liquid flow of alternating bubbles and liquid segments. Channels of square or nearly square cross section are considered. A significant fraction of the liquid surrounds the bubbles and wets the channel walls in the form of films or menisci. This stagnant fraction of the liquid remains when gas and liquid segments flow by, and it is connected to the liquid within the liquid segments by diffusion only and it effectively increases dispersion. We design and fabricate a microchip with integrated analyte injection and detection to investigate the effects of the influence of the stagnant liquid in segmented flow through square microchannels on the analyte bandwidth. The measured data and a corresponding model confirm the experimental trends and suggest operating conditions at which the unwanted effect of dispersion in segmented microchannel flow is minimized. Dispersion is least when the liquid flow rate is greater than the gas flow rate, and the optimum ratio of the two flow rates slightly increases with increasing bubble velocity.
Microchemical systems have evolved rapidly over the last decade with extensive chemistry applications. Such systems enable discovery and development of synthetic routes while simultaneously providing increased understanding of underlying pathways and kinetics. We review basic trends and aspects of microsystems as they relate to continuous-flow microchemical synthesis. Key literature reviews are summarized and principles governing different microchemical operations discussed. Current trends and limitations of microfabrication, micromixing, chemical synthesis in microreactors, continuous-flow separations, multi-step synthesis, and integration of analytics are delineated. We conclude by summarizing the major challenges and outlook related to these topics.
Aufbauen, anschalten, fertig: Die Kombination einer Rückkopplungssteuerung mit Durchflussoperationen in Mikroreaktoren (siehe Bild) ermöglicht eine vollautomatische Online-Reaktionsoptimierung. An einer Heck-Reaktion wird das Potenzial für eine schnelle Vielvariablen-Optimierung demonstriert, die mit minimalen Materialmengen auskommt. Die optimalen Bedingungen lassen sich schnell in einen 50-mal größeren Maßstab übertragen.
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