Tubular reactors monolith

Mass transfer (continued) in monolith, 27 89 in porous catalyst, 27 60-63, 68 in tubular reactor, 27 79, 82, 87 Mass transport processes, 30 312-318 convective, 30 312-313 diffusive, 30 313-315 selectivity, 30 316... 

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]. 

Figure 4 is a picture of another reactor with widely varying scales—a catalytic monolith. Like the tubular reactor, the monolith itself has two intrinsic length scales, the radius and the length, which are typically 10-20 cm and between 30 and 50 cm, respectively. The monolith cross-section has a honeycomb structure... 

Tubular reactors are probably the most common photocatalytic reactors. Their popularity stems, most likely, from their simplicity. They are characterized by a gas flow along the axis of a tube, which contains the photocatalyst in many possible forms such as a thin coated film on its wall, fluidized particles, a coated monolith, or even as a free powder resting on an appropriate support. The light sources are located, in most cases, externally to the tube, in a parallel configuration relative to its axis. Reflecting surfaces encompass the lamps array, assuring that the only absorbance of photons would be that of the photocatalyst (Figure 7). 

In a multichannel monolith with cocurrent downflow, each channel will have the same residence time and a residence-time distribution close to an ideal tubular reactor. But due to nonuniform flow distribution, the gas-liquid ratio, the volume reactant per volume catalyst, and, consequently, the conversion can be very different in different channels. 

Carbon coating can be achieved using pyrolysis of hydrocarbons at elevated temperatures [69]. Figure 2 shows a device used for carbon coating via hydrocarbon pyrolysis. In the example described here, an alumina-washcoated monolith is covered with carbon by pyrolysis of cyclohexene. A gas mixture of cyclohexene in nitrogen is passing the reactor at a certain flow rate. The monolith block to be coated is placed in the middle of the heated tubular reactor. The reaction takes place at 873-973 K, and the amount of carbon deposited can be controlled by the temperature and the time on stream. Up to 3-10 wt% carbon can be homogeneously coated onto the monolith in this way. It appears that the surface area of the carbon-coated alumina-washcoated cordierite monolith is of... 

Tubular reactors Packed bed and fluidized bed reactors are included in this cat ory. Particulate forms of enzymes/cells are bundled inside a cylindrical vessel, either in close contact with each other in a packed bed, preferably by forcing the fluid to circulate in downflow mode, or, in the form of a fluidized bed, by forcing the fluid upward, so that the bed of solid particles expands and behaves in a fluidlike manner, which favors contact between individual solid particles, hence enhancing mass transfer. The monolith reactor, where the bed of particles is replaced by a single structure, is a variation of the packed bed with minimal pressure drop. 

Traditional modeling concepts can be complemented by new elements if we consider the RTD in the channels, CFD becomes useful. The information from the CFD model can be transferred to a simplified simulation model, in which the monolith and the mixing system are described by parallel tubular reactors coupled to a mixing space. 

US Patent Appl. US2009/0176895 to Coming Inc." claims designs and methods to load and operate monolithic catalysts in multi-tubular reactors for use in the chemical processing and/or energy conversion industries. The inventors refer specifically to the use of thermally conductive, metal honeycomb catalyst supports in order to improve thermal uniformity in the reactor tubes for applications to strongly exothermic reactions, such as the partial oxidations of hydrocarbons to products... 

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

Figure 7.1. Typical membrane reactor configurations (a) reactor with plate-shaped membranes, (b) tubular-shaped membrane in double pipe configuration and (c) multichannel monolith.

Abbreviations reactors batch (B), continuous stirred tank (CST), fixed bed of catalyst (FB), fluidized bed of catalyst (FL), furnace (Fum.), monolith (M), multitubular (MT), semicontinuous stirred tank (SCST), tower (TO), tubular (TU). Phases liquid (L), gas (G), both (LG). Space velocities (hourly) gas (GHSV), liquid (LHSV), weight (WHSV). Not available, NA. To convert atm to kPa, multiply by 101.3. 

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