Channels, monoliths

Open-channel monoliths are better defined. The Sherwood (and Nusselt) number varies mainly in the axial direction due to the formation ofa hydrodynamic boundary layer and a concentration (temperature) boundary layer. Owing to the chemical reactions and heat formation on the surface, the local Sherwood (and Nusselt) numbers depend on the local reaction rate and the reaction rate upstream. A complicating factor is that the traditional Sherwood numbers are usually defined for constant concentration or constant flux on the surface, while, in reahty, the catalytic reaction on the surface exhibits different behavior. 

GP 10] [R 18]The best HCN yield of 31% at a p-gauze platinum catalyst (70 ml h methane 70 ml h ammonia 500 ml h air 1 bar 963 °C) is much better than the performance of monoliths (Figure 3.49) having similar laminar flow conditions [2]. A coiled strip and a straight-channel monolith have yields of 4 and 16%, respectively. The micro-reactor performance is not much below the best yield gained in a monolith operated under turbulent-flow conditions (38%). 

An important factor that influences the lead distribution, especially in the channeled monolith bodies, is the character of flow. There is strong evidence that an induced change of the flow from laminar to turbulent augments the deposition of lead. An example is given in Fig. 7 by the deposition pattern of lead in a dual catalyst. Within each of the catalysts, the deposit pattern is in agreement with those shown in Figs. 4 and 5. However, the lead deposit on the inlet of the oxidation catalyst exceeds con-... 

A monolith catalyst has a much higher void fraction (between 65 and 91 percent) than does a packed bed (which is between 36 and 45 percent). In the case of small channels, monoliths have a high geometric surface area per unit volume and may be preferred for mass-transfer-limited reactions. The higher void fraction provides the monolith catalyst with a pressure drop advantage compared to fixed beds. 

Figure 3. Usual shapes of monolith catalysts. A) Square-channel monolith B) Parallel-plate monolith C) Corrugated-plate packing.

Note that, despite the typically high operating temperatures of fuel cells, radiative heat transfer was neglected. Lee and Aris (16) have discussed such effects in parallel-channel monoliths. The importance of radiative transport depends on the emissivity of the surface for the low (about 0.1) emissivity of Pt-coated catalyst-electrodes, their analysis suggests that radiative effects can be neglected. 

The overall membrane element shape comes in different types sheet, single tube, hollow fiber, and multi-channel monolith. Photographs of some commercial membrane elements are shown in Figure 5.1. The use of disks (or sheets) has been confined to medical, pharmaceutical and laboratory applications, while tubes and monoliths are employed in larger-scale applications ranging from removal of bacteria from wine and beer fermentation to oil-water separation to waste water ueatmenL... 

Multi-channel monolith (single element in a module) 300-540 ... 

Only a limited number of studies has begun to emerge in this area. The available studies encompass various levels of details. Some use CFD to focus the simulation on the How of liquid and particles through membrane pores while others model fluid flow through a single membrane tube. CFD simulation of fluid flow through a multi-channel monolithic membrane element has also been done. 

Square-channelled monolith or plate-type vanadia on titania catalysts are commonly used in SCR units of power plants. A typical monolith block (Fig. 16) has the geometrical parameters given in Tables 7 and 10. 

Catalysts were prepared on Corning EX20 cordierite, open channel monolithic substrates (nominally 62 square channels per square centimeter). High surface area supports were activated with base or noble metal components. The final composition of the fresh catalysts are shown in Table 1, where the metal content is expressed as grams of metal per liter of catalyst (including substrate). 

Once an open channel monolith was reassembled it was sealed into a demountable catalyst holder and placed in the exhaust of a diesel vehicle which was driven over a prescribed cycle on a chassis dynamometer. The vehicle was a 1977 International Harvester diesel Scout equipped with an indirect injected 3.2L, six cylinder engine. Commercial number two diesel fuel was used for all the vehicle experiments. 

For reaching the same oxygen production, single-hole tubes of 10 mm diameter would increase the amount of units to 41, comparable to multi-channel monoliths (39 units). The extreme restriction in membrane length of hollow fiber systems would result in 1,800 modules. The difference in expansion behavior of vessel wall and membrane tubes can be compensated by mounting the tubes in a... 

Berfif, G., Influence of operating conditions on the observed reaction rate in the single channel monolith reactor. Cat. Today 2001, 69 (1-4), 147-152. 

Foam monoliths behaved better than extruded or metal monoliths, which was explained on the basis of the higher mass transfer for the foam compared to straight channelled monoliths. The effect of mass transfer was confirmed by observing that HCN selectivity improved over foam monoliths with smaller pore size and extruded monoliths with higher cell density. 

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