Structured reactors pressure drop

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. 

Catalytic reactors can roughly be classified as random and structured reactors. In random reactors, catalyst particles are located in a chaotic way in the reaction zone, no matter how carefully they are packed. It is not surprising that this results in a nonuniform fiow over the cross-section of the reaction zone, leading to a nonuniform access of reactants to the outer catalyst surface and, as a consequence, undesired concentration and temperature profiles. Not surprisingly, this leads, in general, to lower yield and selectivity. In structured reactors, the catalyst is of a well-defined spatial structure, which can be designed in more detail. The hydrodynamics can be simplified to essentially laminar, well-behaved uniform fiow, enabling full access of reactants to the catalytic surface at a low pressure drop. 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

Plate-type catalysts (PTCs) consist of metal sheets, metal net, or perforated metal plates with the catalytic species deposited onto, assembled in modules that are inserted into the reactor in layers [58, 59]. Several innovative structures have been reported. Figure 9.9 gives an example of a structure permitting some vibration of the individual plates, reducing possible blockage the plates are very thin, reducing the pressure drop. 

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. 

Another disadvantage of fixed bed reactors is associated with the fact that the minimum pellet size that can be used is restricted by the permissible pressure drop through the bed. Thus if the reaction is potentially subject to diflfusional limitations within the catalyst pore structure, it may not be possible to fully utilize all the catalyst area (see Section 12.3). The smaller the pellet, the more efficiently the internal area is used, but the greater the pressure drop. 

Fig. 6. Examples of types of meshes developed to resolve laminar flow around particles (a) Chimera grid. Reprinted, with permission, from the Annual Review of Fluid Mechanics, Volume 31 1999 by Annual Reviews www.annualreviews.org (b) Unstructured grid with layers of prismatic cells on particle surfaces. Reprinted from Chemical Engineering Science, Vol. 56, Calis et al., CFD Modeling and Experimental Validation of Pressure Drop and Flow Profile in a Novel Structured Catalytic Reactor Packing, pp. 1713-1720, Copyright (2001), with permission from Elsevier.

Novel Processing Schemes Various separators have been proposed to separate the hydrogen-rich fuel in the reformate for cell use or to remove harmful species. At present, the separators are expensive, brittle, require large pressure differential, and are attacked by some hydrocarbons. There is a need to develop thinner, lower pressure drop, low cost membranes that can withstand separation from their support structure under changing thermal loads. Plasma reactors offer independence of reaction chemistry and optimum operating conditions that can be maintained over a wide range of feed rates and H2 composition. These processors have no catalyst and are compact. However, they are preliminary and have only been tested at a laboratory scale. 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

The speed of provision of the feed molecules to the adsorption/catalytic sites must be balanced with engineering issues such as pressure drop in a reactor/ adsorber, so the parhcle size and pore structure of engineered forms must be optimized for each appHcation. A hierarchy of diffusion mechanisms interplays in processes using formed zeoHtes. Micropore, molecular, Knudsen and surface diffusion mechanisms are all more or less operative, and the rate Hmifing diffusion mechanism in each case is directly affected by synthesis and post-synthesis manufacturing processes. Additional details are provided in Chapter 9. 

This study uses novel monolith structured catalysts in aiming to improve process productivity and selectivity. Apart from the advanced characteristics of low pressure drop, less backmixing, convenient change out of catalysts, the monolith reactor structure reduces the mass transfer limitation and potentially improves the selectivity. 

Fixed bed reactors still predominate for fuel processing. However, fixed beds are susceptible to vibrational and mechanical attrition. Recently, monolithic reactors, either metallic or ceramic, have attracted interest for reforming processes since they offer higher available active surface areas and better thermal conductivity than conventional fixed beds. Low-pressure drop and robustness of the structure are major advantages of monolithic reactors. 

A major problem associated with loading methods could be the inconsistency in bed structure, i.e. mean and local voidage properties, from fill to fill. Taking into consideration the fact that pressure drop is greatly influenced by the bed voidage and that pressure drop is critical for gas-phase systems, the loading of particles is of great importance, especially in gas-phase reactors (Afandizadeh and Foumeny, 2001). 

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. 

The benefits of applying micro structured reactors for autothermal reforming are manifold. Besides the well-known narrowing of the residence time distribution and the low pressure drops, hot-spot formation may well be reduced owing to the axial heat transfer of the wall material. 

The basic regulatory control structure outlined above was able to hold the process at the desired operating point for most of the disturbances. However, when manipulated variables hit constraints it was unable to prevent a unit shutdown. The disturbance IDV(6) that shuts off the fresh feed flowrate F,A is probably the most drastic. The resulting imbalance in the stoichiometric amounts of components A and C drives the concentration yA down quite rapidly. The reaction rate slows up, reactor temperature drops, and the process shuts down on high pressure. Since one degree of freedom has been removed by this disturbance, the control structure must be modified with overrides to handle the component balances. 

The first three types (pellets, extrudates and granules) are primarily used in packed bed operations. Usually two factors (the diffusion resistance within the porous structure and the pressure drop over the bed) determine the size and shape of the particles. In packed bed reactors, cooled or heated through the tube wall, radial heat transfer and heat transfer from the wall to the bed becomes important too. For rapid, highly exothermic and endothermic reactions (oxidation and hydrogenation reactions, such as the ox-... 

Structured catalysts and reactors promise high precision at different length scales because each scale can be optimized independently short diffusion lengths correspond to thin catalyst layers, and the dimensions of the interstitial space may be independently chosen to provide well-defined residence times and minimal pressure drops. 

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