Reactor catalytic wall

This section offers a short overview of research and process development in a field that is the target of one of the key projects in Degussa s project-house process intensification. Most of this survey has been published recently by Hiither et al. [3]. 

In order to enhance space-time yields of catalytic gas-phase reactions, two strategies are in principle possible the improvement of catalyst activity or the implementation of more intense process conditions. This usually leads to an increase of heat production that can only be partially released, if at all, using conventional reactor technology. 

A simple estimation of the temperature profile inside a tube reactor starts from an energy balance for the system [7]. Since we are particularly interested in the performances of a tube reactor and a catalytic wall reactor the following simplifying assumptions are made  

It is assumed that the reactor is an ideal tubular flow reactor. 

The final assumption is a drastic simplification that leads to an underestimation of the temperature increase. The temperature T of an arbitrary point in the catalytic bed may be related to the effective thermal conductivity A, the heat of reaction AHr and the rate of reaction r by the differential equation 

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C H20/EtOH = 6-8 Residence time = 100 100 ms. Used an autothermal flat plate catalytic wall reactor... 

Unlike SRE, the POE reaction for H2 production has been reported so far only by a few research groups.101104-108 While Wang et al. os and Mattos et r//.104-106 have studied the partial oxidation of ethanol to H2 and C02 (eqn (18)) at lower temperatures, between 300 and 400 °C using an 02/EtOH molar ratio up to 2, Wanat et al.101 have focused on the production of syngas (eqn (19)) over Rh/Ce02-monolith catalyst in a catalytic wall reactor in millisecond contact time at 800 °C. Depending on the nature of metal catalyst used and the reaction operating conditions employed, undesirable byproducts such as CH4, acetaldehyde, acetic acid, etc. have been observed. References known for the partial oxidation of ethanol in the open literature are summarized in Table 6. 

Yube, K. and Furuta, M. and Aoki, N. and Mae, K. (2007). Control of selectivity in phenol hydroxylation using microstructured catalytic wall reactors. Applied Catalysis A General 327, 278-286... 

Figure 12-13 A falling film catalytic wall reactor in which reactant in the gas must diffuse through a liquid film to react L...

It is evident, however, that this problem can be much more comphcated than either the wetted wall column or the catalytic wall reactor, because it combines the complexities of both. In fact, there are numerous additional complexities with this reactor beyond those simplified cases. 

As with the falling film reactor, the rate of mass transfer to the catalyst goes as R, while the size of the reactor goes as R, so this reactor becomes very inefficient except for very small-diameter tubes. However, we can overcome this problem, not by using many small tubes in parallel, but by allowing the gas and liquid to flow (trickle) over porous catalyst pellets in a trickle bed reactor rather than down a vertical wall, as in the catalytic wall reactor. 

Kolios et al. [106] performed an extensive study revealing that coupling of exothermic and endothermic reactions is possible under safe and stable operation conditions only in catalytic wall reactors and not in coupled packed beds. In the latter case instability and thermal runaway of the reactor may occur. Additionally, the two... 

L. D. Schmidt, Millisecond Catalytic Wall Reactors Dehydrogenation... 

Wanat, E.C., Venkataraman, K., and Schmidt, L.D. Steam reforming and water-gas shift of ethanol on Rh and Rh-Ce catalysts in a catalytic wall reactor. Applied Catalysis. A, General, 2004, 276 (1-2), 155. 

A major problem in using microstructured reactors for heterogeneously catalyzed gas-phase reactions is how to introduce the catalytic active phase. The possibilities are to (i) introduce the solid catalyst in the form of a micro-sized packed bed, (ii) use a catalytic wall reactor or (iii) to use novel designs. Kiwi-Minsker and Renken [160] have discussed in detail these alternatives. 

H. Redlingshofer, O. Krocher, W. Bock, K. Huthmacher, G. Emig, Catalytic wall reactor as a tool for isothermal investigations in the heterogeneously catalyzed oxidation of propene to acrolein, Ind. Eng. Chem. Res. 41 (2002) 1445. 

T. Giornelli, A. Lofberg, L. Guillou, S. Paul, V. Le Courtois, E. Bordes-Richard, Catalytic wall reactor. Catalytic coatings of stainless steel by VOx/TiCL and Co/Si02 catalysts, Catal. Today 128 (2007) 201. 

The use of microstructured catalytic wall reactors offers an interesting option for the revamping of existing plants. The key idea of this so-called booster concept... 

As an example of the decreasing efficiency caused by external and internal mass transfers, we consider an irreversible first-order reaction and a porous catalyst layer. The situation corresponds to a catalytic wall reactor. The relative importance between external and internal mass transfers is characterized by the ratio of the diffusion time in the porous layer tp and the characteristic time for external mass transfer called the Biot number, Bi = t /t - (L /DJk a for mass transfer. 

To compare the catalytic wall reactor with a packed bed, correct criteria must be chosen [18]. For both the reactors, the outer catalyst surface per void (V oy) volume and the space-time must be identical. Under these conditions, the following relationship between the diameter of the microchannel and the particle diameter holds... 

The catalytic wall reactor with channel diameter in the range of 50-1000 pm and a length dependent on the reaction time required circumvents the shortcomings of micro packed beds. This is discussed in more detail in Section 6.5.4. However, in most of the cases, the catalytic surface area provided by the walls alone is insufficient for the chemical transformation and, therefore, the SSA has to be increased by the chemical treatment of the channel walls, or by coating them with highly porous support layers. The thickness of the layer 5 3, depends on catalytic activity. In general, the layer thickness is sufficiently small to avoid internal heat and mass transfer influences. Catalytic layers can be obtained by using a... 

Three-phase reactions comprise gas-liquid-solid and gas-liquid-liquid reactions. Gas-liquid reactions using solid catalysts represent a very important class of reactions. Conventionally, they are carried out in slurry reactors, (bubble columns, stirred tanks), fluidized beds, fixed bed reactors (trickle beds with cocurrent downflow or cocurrent upflow, segmented bed, and countercurrent gas-liquid arrangements) and structured (catalytic wall) reactors. 

The gas-liquid-sohd reactions are carried out in various types of reactors, such as packed beds, fluidized/slurry, and catalytic wall reactors (Figure 8.1). The advantages and limitations of these reactors are described in Table 8.1. Compared to fluid-sohd systems, an additional phase makes it difficult to predict flow patterns... 

Lopez, E., Divins, N.J. and Llorca, J. (2012) Hydrogen production from ethanol over Pd-Rh/Ce02 with a metallic membrane reactor. Catalysis Today, 193, 145-150. Montane, D., Bolshak, E. and Abello, S. (2011) Thermodynamic analysis of fuel processors based on catalytic-wall reactors and membrane systems for ethanol steam reforming. Chemical Engineering Journal, 175, 519-533. 

A purchasable cross-flow heat exchanger for application in laboratory-, pilot- and production-scale plants was developed by FZK. By incorporation of a catalyst on the quadratic plates inside the heat exchanger, it can also be used as a catalytic wall reactor. Operating conditions up to 850 °C (stainless steel) and pressures of more than 100 bar are possible, and the specific inner surface area is up to 30 000 m m. The reactors can be obtained in many materials and three different sizes with a maximum flow of 6500kgh (water). Therefore, the reactors can be adjusted for various processes, and all types of catalyst deposition techniques are possible [111]. This reactor has already been applied to the catalytic oxidation of H2 by Janicke et al. [112], for example. 

Munder, Rihko-Stmckmann, and Sundmacher (2007) investigated the performance of a bilayer-solid electrolyte membrane reactor in steady-state and forced-periodic operation modes. The obtained results showed that the solid electrolyte membrane reactor has a slightly higher yield under optimal operation conditions compared with the catalytic wall reactor, and forced-periodic operation of the bilayer-solid electrolyte... 

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