Catalytic reactors monolith catalysts

Schematic of the exhaust catalyst catalytic system. The NH -SCR takes place in the selective catalytic reactor (SCR) catalyst component. The other components depicted are diesel oxidative catalyst (DOC), diesel particulate filter (DPF), urea, the source of NH. The active component, most commonly a Cu-exchanged zeolite, is placed as a wash coat on a ceramic monolith.

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

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. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

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. 

Figure 25 Aspects controlling the performance of a three-phase catalytic reactor, indicating the flexibility of the use of monolithic catalysts.

Another common distinctive feature of catalysts is the shape of the catalyst pellets, which can be cylinders, rings, spheres, monoliths, and other forms. The shape has an effect in the empty space of a catalytic reactor, and, consequently, will affect mass transport as well mechanical strength. 

More complex reactors, like packed-bed reactors or catalytic monoliths, consist of many physically separated scales, with complex nonlinear interactions between the processes occurring at these scales. Figure 3 illustrates scale separation in a packed-bed reactor. The four length (and time) scales present in the system are the reactor, catalyst particle, pore scale, and molecular scale. The typical orders of magnitude of these four length scales are as follows reactor, lm catalyst particle, 10 2m (1cm) macropore scale, 1pm (10 6m) micro-pore/molecular scale, 10 A (10 9m). The corresponding time scales also vary widely. While the residence time in the reactor varies between 1 and 1000 s, the intraparticle diffusion time is of the order of 0.1s and is 10 5s inside the pores. The time scale associated with molecular phenomenon like adsorption is typically less than a microsecond and could be as small as a nanosecond. 

In principle, deposition of an active phase (metal and/or oxide) on a monolithic catalyst support can be carried out in a manner similar to that used to prepare a t) ical catalyst. However, the large dimension of a monolith can easily enhance problems of nonhomogeneous deposition. For example, if in the preparation of conventional catalyst particles the active phase would be deposited at the external surface of the support, the result would be an egg-shell-t) e catalyst, which for many processes can be advantageous. However, if this pattern of deposition were applied to a monolithic support, it could result in a monolith with only the outer charmels of the structure having a significant catalytic activity, resulting in a dramatically poor catalytic reactor. The critical steps in the s)mthesis process are the deposition and drying steps, which are discussed separately below. Calcination, reduction, etc. for monolith catalysts are not different from those used to manufacture t) ical catalysts, and these steps are therefore not discussed here. 

Modeling of catalytic combustors has been the subject of a number of studies. The models used varied in degree of complexity and could therefore answer various types of questions. General issues of modeling monolith catalytic reactors are discussed in Chapter 8 of this book and in the reviews of Irandoust and Andersson [57] and Cybulski and Moulijn [58]. Hence, only topics that are specific to the modeling of catalytic combustion in monolith catalysts are considered here. A description of some important aspects of different types of models are as follows. 

Control of the temperature throughout the reforming catalyst bed can be established by use of a monolithic catalyst. The heat transfer control can be accomplished by combining three effects that monolithic catalyst beds can impact significantly (1) direct, uniform contact of the catalyst bed with the reactor wall will enhance conductive heat transfer (2) uniformity of catalyst availability to the reactants over the length of the flow will provide continuity of reaction and (3) coordination of void-to-catalyst ratio with respect to the rate of reaction will moderate gas-phase cracking relative to catalytically enhanced hydrocarbon-steam reactions. This combination provides conditions for a more uniform reaction over the catalyst bed length. 

In quite a different application, a novel approach for producing olefins via a hydrocarbon-steam cracking process, without the use of a catalyst, was demonstrated to benefit from the use of a honeycomb monolithic catalytic reactor [28]. A typical problem associated with cracking processes of this type is maintaining the appropriate combination of heat transfer and residence time, which, if not balanced, will lead to either poor conversion... 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

In this section we develop the design equations and give the mass transfer correlations for two common types of catalytic reactors the wire screen or catalyst gauze reactor and the monolith reactor. 

Figure 5 Schematic of catalytic reactor arrangement with monolith Ni-based catalyst TAR PROTOCOLS...

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