Monohthic Reactors

In catalytic applications, monoliths can provide better control of the contact time of reactants and products with the catalyst. This leads to a potential increase in selectivity. Together with the advantages over conventional trickle-bed reactors (pressure-drop surface area, short diffusion lengths), this makes the monohth reactor very suitable for use in consecutive reaction schemes, such as selective oxidation or hydrogenation. Literature dealing with carbon monolith structures is not yet extensive, however, and a limited number of applications have been reported, as shown in Table 11.2. 

Monolith reactor This type of reactor is used extensively for the abatement of automobiles exhaust emissions. The gas flows continuously through the reactor, whereas the catalyst is a continuous phase consisting of a ceramic support and the active phase, which is dispersed onto the support. The support is structured in many channels and shapes that achieve large catalytic surface at small volume. A typical application of monohth reactors is the exliaust gas cleaning. 

Bennett CJ, Kolaczkowski ST, Thomas WJ. Determination of heterogeneous reaction kinetics and reaction rates under mass transfer controlled conditions for a monohth reactor. Process Safety and Environmental Protection Transactions of the Institution of Chemical Engineers, Part B 1991 69 209-220. 

Hoehink JHBJ, Marin GB. Modeling of monohthic reactors for automotive exhaust gas treatment, in Structured Catalysts and Reactors, (Eds.) Cybulki A, Mouhjn JA. New York Marcel Dekkei 1998. 

Stutz MJ, Poulikakos D Optimum washcoat thickness of a monohth reactor for syngas production by partial oxidation of methane, Chem Eng Sci 63 1761—1770, 2008. 

Catalytic hydrogenation is typically carried out in slurry reactors, where finely dispersed catalyst particles (<100 (tm) are immersed in a dispersion of gas and liquid. It has, however, been demonstrated that continuous operation is possible, either by using trickle bed [24] or monoHth technologies [37]. Elevated pressures and temperatures are needed to have a high enough reaction rate. On the other hand, too high a temperature impairs the selectivity of the desired product, as has been demonstrated by Kuusisto et al. [23]. An overview of some feasible processes and catalysts is shown in Table 8.1. 

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particulady active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monohthic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). For these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

Balomenou S, Tsiplakides D, Katsaounis A, et al.. Novel monohthic electrochemically promoted catalytic reactor for environmentally important reactions, Appl. Catal. B 2004 52 181-196. 

Most of the gas-liquid applications of monoliths have used a heterogeneous catalyst (be it supported noble metals or immobihzed enzymes) on the channel walls. Here, we also consider the use of monohths without a catalyst on the walls in gas-liquid applications, i.e. homogeneously catalyzed liquid-phase reactions. The fluid mechanics of the system do not change appreciably by lethng the reaction take place in the liquid bulk instead of in a washcoat layer, and it is interesting to consider such reactions in a discussion of mass transfer and power-input requirement. Of course, the mass-transfer behavior does change by changing the locahon where the reaction takes place, and we will discuss gas-hquid reactors and gas-liquid-solid reactors separately. 

The photocatalytic reduction of NO was also studied by in situ FTIR on a PtO PdOj,/Ti02 photocatalyst [23]. A special monoHthic multichannel reactor with coated catalysts was used. The catalysts were iUuminated directly in the channels... 

Structured catalysts may be used to overcome the drawbacks of conventional catalytic reactors [3]. These are reactors with monohthic converters, with catalyst-coated static mixers and arranged packings as applied in distillation and absorption columns. 

R. P. Fishwick, R. Natividad, R. Kulkarni, P. A. McGuire, J. Wood, J. M. Winterbottom, E. H. Stitt, Selective hydrogenation reactions a comparative study of monohth GDC, stirred tank and trickle bed reactors, Catal. Today2 007,128, 108-114. 

Rh/Al203/FeCr alloy microchannel monohths were compared with Rh/alumina foams for the production of hydrogen from propane. Temperature profiles obtained along the central axis were valuable in understanding the different behaviors of the reactor systems [58],... 

Michelsen, F. A., WiUielmsen, 0., Zhao, L., Asen, K. I. (2013). A distributed dymamic model of a monohth hydrogen membrane reactor. Energy Conversion and Management, 67, 160—170. 

Jung, H, Yoon, WL, Lee, H, Park, JS, Shin, JS, La, H, Lee, JD. Fast start-up reactor for partial oxidation of methane with electrically heated metaUic monohth catalyst J. Power Sources 2003 124 76-80. 

Frauhammer, J, Friedrich, G, Kohos, G, Klingel, T, Eigenberger, G, von Hippel, L, Amtz, D. Flow distribution concepts for new type monohthic co- or countercurrent reactors. Chem. Eng. Technol. 199932 1012-1016. 

In contrast to ceramic and metallic monoHths developed for automotive exhaust treatment purposes, which nowadays carry chaimels on the microscale and are actually micro-reactors by definition, the micro-reactors discussed in this chapter rather cover plate heat-exchanger technology with chaimels on the microscale. An overview of the fundamentals, practical applications, and production issues of micro-reactors for fuel-processing purposes is provided. 

All monohth-based catalytic systems summarized here were successfully used as pressure-stable catalytic reactors. Bleeding was virtually suppressed, leading even in RCM to basically ruthenium-free products with a ruthenium content of far below 0.1%. 

It is easy to use this definition for large scale fixed beds, because the porosity of the catalyst and of the catalyst bed are easy to measure and in most instances the reactor housing contributes to the overall volume only to a minor extent. However, the definition becomes doubtful when the catalyst is coated onto a monolith or foam. Here the question arises as to which volume is then being referred to the volume of the catalyst coating itself, of the monohth void fraction (channels) or of the entire monolith All these questions need to be clarified before a fair comparison of catalytic activity is feasible when gas hourly space velocity is applied for the calculations. 

Rampe et al. developed a monolithic autothermal propane reformer with feed gas pre-heating functions (see Figure 7.4). The reactor had a thermal power output of up to 1.7kW while the volume of the monohth was 283 cm [151]. The reactor was operated at a high O/C ratio of 1.33, and S/C ratio of 1.0. 

A microstructured monolith for autothermal reforming of isooctane was fabricated by Kolb et cd. from stainless steel metal foils, which were sealed to a monohthic stack of plates by laser welding [73]. A rhodium catalyst developed for this specific application was coated by a sol-gel technique onto the metal foils prior to the sealing procedure. The reactor carried a perforated plate in the inlet section to ensure flow equi-partition. At a weight hourly space velocity of 316 L (h gcat). S/C 3.3 and O/C 0.52 ratios, more than 99% conversion of the fuel was achieved. The temperature profile in the reactor was relatively flat. It decreased from 730 °C at the inlet section to 680 °C at the outlet. This was attributed to the higher wall thickness of the plate monolith compared with conventional metallic monolith technology. The reactor was later incorporated into a breadboard fuel processor (see Section 9.5). 

Figure 10.5 View into metallic monoliths produced by EMITEC (photograph courtesy of Emitec). the monolith and the reactor shell. It holds the monohth and prevents by-pass of gases through the gap [57]. A popular ceramic mat used in automotive exhaust systems is Interam produced by 3M [57j. It degrades at temperatures above 800 °C, and therefore a high temperature ceramic fibre material such as CC-Max from Unifrax must be used for monolithic reformer reactors, which are operated at higher temperature [57].

In Fig. 2.16, a close-up picture of the reactor with the capillary for temperature profiles is displayed. The FHS, the capillary inserted into one channel and the ceramic fiber paper covering FHS, catalyst, and BHS are visible. The setup can be used for in situ measurements, where axial temperature and concentration profiles can be detected from one channel of the monohthic catalyst. Figure 2.17 shows the micro volume tee mounted onto the motorized linear stage used for moving the capillary. The setup is similar to the one introduced for foam monoliths by Horn et al. (2006a,b) and then adapted to honeycomb monoliths by Donazzi et al. (2011a,b). A novel feature of our... 

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