Reactor monolithic-type

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 monolith reactors is the exhaust gas cleaning. 

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Catalyst Monolith. The previous discussion in this chapter focused primarily on chemical reactions taking place in packed-bed reactors. However, when a gaseous feedstream contains significant amoimts of particulate matter, dust tends to clog the catalyst bed. To process feedstreams of this type, parallel-plate reactors (monoliths) are commonly used. Figure 11-11 shows a schematic diagram of a monolith reactor. The reacting gas mixture flows between the parallel plates, and the reaction takes place on the smface of the plates. 

Shi et al.154 recently studied the steam reforming of isooctane in a monolithic type reactor simulated by a three-dimensional CFD model. They considered global reactions to represent steam reforming of isooctane, which include steam reforming of isooctane to syngas as expressed in Equation 2.48, WGS reaction as shown in Equation 2.2, and the net reaction by combining these two reactions to produce H2 and C02 as shown in Equation 2.51 ... 

Another example is monolithic-type reactors, which have found their main application in the field of combustion. A monolith bed allows better autothermic operations with a minimal pressure-drop. This concept was used to improve performances in commercial methanol into formaldehyde conversion by adding a... 

Nontraditional types of fixed-bed reactors are begmning to be considered. A gas-lift reactor with monolith type catalyst packing has been described for FT synthesis [40]. It is reported that this reactor has been tested at the laboratory scale with a cobalt... 

The majority of micro-reactors reported in the literature are stiU dedicated to catalyst evaluation. These reactors are usually monolith-type laboratory devices without heat-exchange functions, which allow for the removal of the microstructured plates after testing [26-35]. These are supplied by electrical power for heating and are StiU far away from a practical appUcation. Therefore, the design of these reactors wiU not be discussed in detail below, bearing in mind that they are useful tools for catalyst screening and characterization. 

Describe the advantages and disadvantages of the following reactor types with reference to heat and mass transfer. For each reactor discuss one reaction for which it may be appropriate to use that reactor, (a) fluidized bed reactor, (b) A continuous counter-current flow reactor, (c) A monolith reactor. 

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

The trickle-bed reactor (TBR) and slurry reactor (SR) are the most commonly used for multiphase reactions in the chemical industries. A new reactor type, the monolithic reactor (MR), offers many advantages. Therefore, these three types of reactors are discussed below in more detail. Their general characteristics are given in Table 5.4-44. With respect to slurry reactors, the focus will be on mechanically agitated slurry reactors (MASR) because these are more widely used in fine chemicals manufacture than column slurry reactors. 

Major drawbacks of the MR are the higher cost (although steadily decreasing), the lower catalyst load compared to a TBR, and the relatively little experience with this type of reactor. Due to the higher cost of monolithic catalysts only processes in which the catalyst is reasonably stable and/or easy to regenerate are feasible. 

The monolithic stirrer reactor (MSR, Figure 2), in which monoliths are used as stirrer blades, is a new reactor type for heterogeneously catalyzed liquid and gas-liquid reactions (6). This reactor is thought to be especially useful in the production of fine chemicals and in biochemistry and biotechnology. In this work, we use cordierite monoliths as stirrer blades for enzyme-catalyzed reactions. Conventional enzyme carriers, including chitosan, polyethylenimine and different are used to functionalize the monoliths. Lipase was... 

In summary, it can be concluded that the monolithic stirrer reactor is a convenient reactor type both for the laboratory and the production plant. It is user-friendly and can be used to compare different catalysts in the kinetically limited regime or hydrodynamic behavior in the mass transfer controlled regime. Stirrers or monolith samples can be easily exchanged and reloaded to suit the desired enzyme and/or reaction conditions. 

The ANL catalyst (not identified, but presumably a Pt supported on Gd-doped ceria) was also successfully used for ATR of diesel fuel. Tests of three different types of diesel fuels (n-Cie, low-sulfur diesel, and regular diesel) showed complete conversion of hydrocarbons at 800°C. The diesel surrogate n-Ci6 yielded 60% H2 on a dry, N2-free basis at 800°C, whereas the other two diesel fuels required higher temperatures (>850°C) to yield similar levels of H2 in the product gases. Similar or improved H2 yields from diesel ATR were observed with a microchannel monolith catalyst, compared with extruded pellets in a fixed-bed reactor. ... 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

Because of their low pressure drop, structured reactors in practice dominate the field for treating tail gases. Figure 2 presents the major types of reactor. The monolithic reactor represents the class of real structured catalytic reactors, whereas the parallel-passage reactor and the lateral-flow reactor are based on a structured arrangement of packings with normal catalyst particles. 

This is one order of magnitude higher than in conventional reactor types (1), which underlines the process intensification potential of monolithic reactors. 

In Table 3 the three common reactor types are compared. Obviously, the monolithic reactor in the Taylor-flow regime leads to a high degree of process intensification. When these numbers are recalculated into production rates, values of 40 mol/m3reactor-s were found. Figure 17 illustrates the high value in relation to the Weisz window of reality. This demonstrates the attractiveness of using monoliths in fast catalyzed gas-liquid-solid reactions. 

Frauhammer J., Friedrich, G., Kolios, G., Klingel, T., Eigenberger G., von Hippel, L., Arntz, D., Flow distribution concepts for new type monolithic co- or countercurrent reactors, Chem. Eng. Technol. 1999, 22, 1012-1016. 

For the experiments with increased water content or suppressed water removal, a 5 cm-long piece of coated monolith was mounted in a 500-mL autoclave. All liquid concentrations, operation conditions and catalyst hold-up were the same as in the pilot-scale plant. To maintain a gradient-less operation, a turbine-type stirrer recirculated the liquid very rapidly through the monolith channels. During the experiments, liquid samples were taken from the reactor and analyzed as described above. 

There are many different reactor designs but the two most commonly used are fixed bed and batch slurry phase. For a fixed bed reactor a given volume of solid particulate or monolith supported catalyst is fixed in a heated tube located within a furnace and liquid and/or gaseous reactants flow through the bed. This type of process is commonly used for large continuous-volume production where the reactor is dedicated to making only one product such as a bulk chemical or petroleum product. 

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