Steel Reactor

Capillary

Fig. 32.35. Cross-sectional drawing of a reactor for the parallel testing of 16 catalysts under nearly conventional conditions.

Catalyst ment. Capillaries placed in the effluent stream present the major flow resistance in the system, which means that the largest pressure drop is not over the catalyst bed but through the capillaries, meaning that differences in the catalyst packing do not significantly influence the flow through the individual wells. A nondispersive infrared sensor was used to determine CO and CO2 concentrations, and the authors report a factor of 15 increase in throughput relative to traditional methods.

Senkan and coworkers have applied an array microreactor to a number of catalytic systems including a 66-element Pd/Pt/In library for the catalytic dehydroge-nation of cyclohexane to benzene. The library consisted of 66 ternary combinations of Pt, Pd, and In prepared in 0.1 wt% increments, for a total metal loading of 1 wt% that was impregnated on 4-mm-diameter by 1-mm-long alumina supports [161, 176]. The catalyst library was prepared from solution-phase metal salt precursors using an automated liquid-handling robot. The compositions were slowly evaporated, dried, and finally calcined at 500 °C for 2 h. The catalyst pellets were placed in a reactor array consisting of 20 rectangular channels that were micro-machined on a flat nonporous silica slab. The 1-mm-wide, 1-mm-deep, and 20-mm-long channels had a 4 x 2 mm cylindrical well to hold the catalyst pellets. A similar silica slab was micromachined to fit onto the top, forming a silica block with cylindrical channels leading to and from the catalyst sample. Four microarray reactor blocks, each containing 20 samples, were stacked and placed into an aluminum heating block, making it possible to test up to 80 samples in parallel.

The entire reactor system was mounted on a stand that could be moved in three dimensions by a computer. The test samples were heated in Ar gas up to 350 °C, then reduced under hydrogen gas, cooled to the desired reaction temperature, and finally exposed to a feed stream of 10% cyclohexane in Ar. The contact time between the sample and the feed gas was approximately 4 ms. The level of reactants, products, and inert carrier gas were determined by withdrawing a small sample from each microreactor channel using a capillary sampling probe (50 mm diameter) inserted 2 mm into the channels. The gas was analyzed by a quadruple mass spectrometer. The capillary was inserted into each channel for approximately 5 s via a computer-controlled positioning system. In this manner, the entire 80-element array could be screened in approximately 10 min.

Building on an earlier microreactor design that used 15 quartz reactors [234], Claus and coworkers designed a ceramic monolith reactor with 2.2-mm-square channels, 150-mm long, that were arranged in a 16 x 16 array [178]. Each channel in the monolith represents a single fixed bed reactor, which allowed up to 256 catalysts to be tested in parallel up to 600 °C. Product gases were analyzed using a capillary mounted on an x/ y/z scanning stage, and attached to a quadruple mass spectrometer (Fig. 32.36). A series of control experiments demonstrated that the axial and radial temperature profiles between channels (as measured without catalysts) was not in excess of 5 K, and the flow rates did not differ more than 10%. The performance of the monolithic reactor was evaluated using a 36-element library of Pt/Zr/V/Al2O3 catalysts for methanol oxidation. The results showed that the reactor was valuable as a primary screen, distinguishing between poor and good catalysts at a rate of about 1 min per sample. The accuracy was lowered owing to difficulties in generating equal flows in all channels of the monolith, and, in certain cases, owing to diffusion of exhaust gases from adjacent catalysts into the end of a channel containing the catalyst currently under investigation.

Exhaust

Motion Control

Fig. 32.36. Monolithic reactor system for the parallel screening of heterogeneous catalysts.

Motion Control

Fig. 32.36. Monolithic reactor system for the parallel screening of heterogeneous catalysts.

32.10.5.2 Micromachined Array Reactors

Advancements in microfabrication have led to a new generation of microchannel reactors, allowing very large heat and mass transfer rates, and enabling safe investigations into explosive and dangerous reactions. Claus and coworkers describe a microchannel reactor that consists of a very large number of parallel microchannels having a square cross-section of approximately 500 mm on a side [178]. The reactor module consists of a stack of metallic frames with micromachined inlays. Each microstructured inlay contains a catalyst as an active coating on top of its microchannels; stacking the frames creates a system of catalyst-coated microchannels (Fig. 32.37). The reactants flow through a diffuser, through the microchannels and over the catalysts, and finally to a product outlet where products are detected by a scanning mass spectrometer. The reactor module design allows for interchangeable catalyst inlays made of different materials such as metals, silicon, ceramics, and glass, allowing many different catalysts and channel geometries to be investigated rapidly. Channel-to-channel cross-talk was tested by placing an active catalyst next to an inactive one and then measuring both product streams with a mass spec-

Fig. 32.37. Reactor module consisting of stacked metallic frames. The catalyst inlays are mounted and removed in the directions indicated by the arrow.

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