Channel reactors, microfluidic

Multiphase packed-bed or trickle-bed microreactor [29, 30] Standard porous catalysts are incorporated in silicon-glass microfabricated reactors consisting of a microfluidic distribution manifold, a single micro-channel reactor or a microchannel array and a 25-pm microfllter. The fluid streams come into contact via a series of interleaved high aspect ratio inlet chaimels. Perpendicular to these chaimels, a 400-pm wide channel is used to deliver catalysts as a slurry to the reaction chaimel and contains two ports to allow cross-flow of the slurry. High maldistribution, pressure drop and large heat losses may occur... 

One way to ease any difficulties that may arise in fabricating a membrane, especially in design configurations that are not planar, is to go membraneless. Recent reports take advantage of the laminar flow innate to microfluidic reactors ° to develop membraneless fuel cells. The potential of the fuel cell is established at the boundary between parallel (channel) flows of the two fluids customarily compartmentalized in the fuel cell as fuel (anolyte) and oxidant (catholyte). Adapting prior redox fuel cell chemistry using a catholyte of V /V and an anolyte of Ferrigno et al. obtained 35 mA cmr at... 

Fig. 3.8 Schematic of the Microfluidic Parallel Screening Reactor System showing a single reaction/detection channel.

Although the direct oxidation of ethane to acetic acid is of increasing interest as an alternative route to acetic acid synthesis because of low-cost feedstock, this process has not been commercialized because state-of-the-art catalyst systems do not have sufficient activity and/or selectivity to acetic acid. A two-week high-throughput scoping effort (primary screening only) was run on this chemistry. The workflow for this effort consisted of a wafer-based automated evaporative synthesis station and parallel microfluidic reactor primary screen. If this were to be continued further, secondary scale hardware, an evaporative synthesis workflow as described above and a 48-channel fixed-bed reactor for screening, would be used. 

Figure 8 A schematic of the reactor used to synthesize the nanoparticles described in this chapter. Cd and Se precursor solutions are stored in two separate syringes and injected at flow rates Ft and F2 into the two inlets of a y-shaped microfluidic device. The microfluidic device rests on a hot plate of variable temperature T. The reagent streams meet at the point of confluence and nucleation, and growth of the particles occurs as they pass along the outlet channel. The emission spectra of the particles so produced are monitored prior to collection at a detection-zone downstream of the chip using a 355-nm Nd YAG laser as an excitation source and a fiber-optic-coupled CCD spectrometer.

The combinatorial synthesis based on the combination of microfluidics and electropolymerization was realized with a microchip consisting of two areas microfluidic channels for generation of gradient of two substances and a parallel electrochemical reactor with platinum electrodes (Fig. 13.3). The system was tested for synthesis of polyaniline in the presence of polysulfonic acid. The reagent ratio providing the best efficiency of the polymer synthesis was evaluated. 

An application of microfluidic reactors is the development of a membraneless fuel cell. Two streams, one containing a fuel such as methanol, the other an oxygen-saturated acid or alkaline stream, are merged without mixing. The laminar flow pattern in the narrow channel helps to maintain separate streams without the use of membrane separators. Opposite walls function as the electrodes and are doped with catalyst. Ion exchange, protons for the add system, takes place through the liquid-liquid interface. This is an example of a solid-liquid-liquid-solid multiphase reactor. ... 

Chemical reaction and mass transfer are two unique phenomena that help define chemical engineering. Chapter 8 described problems involving chemical reaction and mass transfer in a porous catalyst, and how to model chemical reactors when the flow was well defined, as in a plug-flow reactor. Those models, however, did not account for the complicated flow situations sometimes seen in practice, where flow equations must be solved along with the transport equation. Microfluidics is the chemical analog to microelectro-mechanical systems (MEMS), which are small devices with tiny gears, valves, and pumps. The generally accepted definition of microfluidics is flow in channels of size 1 mm or less, and it is essential to include both distributed flow and mass transfer in such devices. 

The characteristic features of microsystems stem from the small size of the space in the microstructures. Therefore, microsystems are not necessarily small systems in total size. They can be large in total size as long as they contain microstructures that can be used for chemical reactions. This sharply contrasts with the concept of a lab-on-a-chip, which should be small in total size. It is also important to note that microsystems are normally set up as flow-type reactors with a constant flow of solutions through a microstructured reaction chamber or channel. Although the reactor s capacity at any one time is small, total production capacity over time is much greater than may be imagined. Therefore, microflow systems are not necessarily used solely to produce small quantities of chemical substances. In fact, a microfluidic device has been developed that fits in the palm of the hand but can produce several tons of a product per year (see Chapter 10). 

In a second screening example, Tseng and co-workers [80] demonstrated the use of a PDMS microfluidic reactorto screen bovine carbonic anhydrase II (bCAII) (138) for activity towards the dick reaction of the acetylene 139 and 10 azides (Scheme 6.34). The micro reactor consisted of several components, including a nanoliter rotary mixer, a chaotic mixer, and a microfluidic multiplexer, which enabled discrete aliquots of reactants to be introduced into the micro reaction channel, allowing multiple reactions to be performed in parallel. 

Gottschlich et al. [134] developed a microfluidic system that integrated enzymatic reactions, electrophoretic separation of the reactants from the products, and postseparation labeling of the proteins and peptides prior to fluorescence detection (see Fig. 12). Tryptic digestion of oxidized insulin p-chain was performed in 15 min under stopped flow conditions in a heated channel serving as the reactor, and the separation was completed in 60 s. Localized thermal control of the reaction channel was achieved using a resistive heating element. The separated reaction products were then labeled with naphthalene-2,3-dicarboxaldehyde (NDA) and detected by fluorescence detection. 

As seen in this brief chapter, the direct-printing process, based on laser printing of layouts on polyester films or wax paper, has the potential to become a powerful technology for the rapid prototyping of microfluidic devices at very low cost, and even a source of low-cost production of disposable devices. This is supported by the fact that the required instrumentation is commonly found at offices and chemistry laboratories. Besides the typical injection and separation channels for electrophoresis, this technology has shown that mixing, preconcentration, clean-up, reactor devices. 

Direct fluorination is a process that is generally viewed as unscaleable. The highly exothermic nature of the system ensures that large-scale reactions are problematic and the use of fluorine gas in large assay is in itself undesirable. Early work on microfluidic fluorinations showed that the direct reaction was comparable in selectivity to the industrial Schiemann process [26] and that the system could be applied to a wide range of compounds [27]. In a series of papers. Chambers et al. [28,29] showed that by the simple numbering up of reactor channels a commercially viable reactor could be constructed with no loss of reaction efficiency or selectivity. 

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