Hydrogen continued short-residence-time

One of the major advantages of flow reaetors is the short residence time of reaetions in the reaetor. This allows seleetive reaetions to pass through the system and out again before any side reaetion ean take plaee. This is very well illustrated in the synthesis of dithioketal and -aeetals, where the seleetive reaetion resulted in superior conversion using eontinuous flow when eompared to bateh synthesis. A dramatic increase of yield was noted in the hydrogenation reaetion performed by Kobayashi et al. when a residence time of less than 1 min was used. The yield inereased from 1% to 97% using the continuous-flow reactor. 

The combination of Deloxan-supported precious-metal fixed-bed catalysts and the use of liquid, near-critical, or supercritical C02 and/or propane mixtures creates new possibilities for continuous fixed-bed hydrogenations with significantly improved space-time yields and catalyst lifetimes. Short residence times and well-balanced diffusion and desorption of products and reactants results in a decrease in undesirable by-products and therefore higher selectivity. The characteristics of high-pressure hydrogenations in near-critical or supercritical fluids can be summarized as follows ... 

Noticeable deviations of (r )(t) from the equation (25) are seen at low hydrations, where effective value of a continuously decreases at t < 10 ps. These deviations should be attributed to the water molecules, which are strongly bound to lysozyme surfaces (there is about 36 water molecules, having two or more hydrogen bonds with lysozyme molecule [635, 636]). The total MSD of such water molecules quickly achieves saturation during their rather long residence times. So, the simulations indicate the presence of two main classes of water molecules with respect to the translational motion molecules with short residence times, which show anomalous diffusion due to the spatial disorder already at the short times, and molecules with long residence times, which remain bound to some centers on lysozyme surface during hundreds of picoseconds. 

Monolithic Loop Reactor A novel MLR was developed af Air Products and Chemicals (Figure 17) (144). The reactor contains a monolithic catalyst operating under cocurrent downflow condifions. Because the residence time in the monolith is short and the heat of reaction has to be removed, the liquid is continually circulated via an external heat exchanger until the desired conversion is reached. The concept was patented for the hydrogenation of dinifrofoluene fo give toluenediamine (37). 

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