Monolithic reactors channels

Taylor flow have many similarities with monolith reactors. A lot of work on Taylor microchannel flow has been aimed at understanding and improving the conditions within monolithic reactor channels [23]. Because of its low axial mixing properties, Taylor flow can be used in high-throughput screening [4]. Even microfiltration efficiencies have been found to improve in the Taylor flow regime [24]. 

Figure 13.2 Internally illuminated monolith reactor (IIMR) scheme with a detail of the cross section of the monolith channels and the fiber-optic bundle.

The difference in reactor performance is due to the difference in hydraulic diameters of the reaction channels, i.e. related to varying mass-transfer limitations. The micro channels of the p-gauze platinum catalyst amount to 70 pm, whereas the monoliths have channel/pore diameters of 500-1200 pm. 

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. 

It has been demonstrated that kg can be estimated by analogy with the Graetz-Nusselt problem governing heat transfer to a fiuid in a duct with constant wall temperature (SH= Nut) [30] and that the axial concentration profiles of NO and of N H 3 provided by the 1D model are equivalent and almost superimposed with those of a rigorous multidimensional model of the SCR monolith reactor in the case of square channels and of ER kinetics, which must be introduced to comply with industrial conditions for steady-state applications characterized by substoichiometric NH3 NO feed ratio, that is, a[Pg.401]

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. 

A brief review of the development history of monolith reactor models for TWC applications can be found in Koltsakis and Stamatelos (1997). Various workers have looked at 1-, 2- and 3-dimensional models considering both the whole monolith and just a single channel. A multidimensional model for the whole monolith is required for investigating the effects of a flow maldistribution across the front face of the monolith, but is probably unnecessary when the flow is uniform. Other workers have studied multidimensional single channel models, where the gas flow within the channel is modelled in detail. In general, for a model to be useful in practice, some compromise has to be made between having a reasonable runtime versus detail/complexity, both in terms of the chemical kinetics and the description of the flow field within the channels of and across the monolith. 

With a monolith reactor, diffusion from the channels to the catalyst coated on the channel walls is the sole means by which reactants are able to reach the catalyst (Section III). It seems reasonable that a similar diffusion process occurs in a coated filter. 

In this section the models employed for simulation of catalytic monolith reactor are discussed, focusing on effective description of heat and mass transfer phenomena in monolith channel. The number of different mathematical models developed for converters of automobile exhaust gases over the last decades is huge—cf., e.g. Heck et al. (1976), Young and Finlayson (1976), Oh and Cavendish (1982), Zygourakis and Aris (1983), Chen et al. (1988),... 

In many situations, the monolith reactor can be represented by a single channel. This assumption is correct for the isothermal or adiabatic reactor with uniform inlet flow distribution. If the actual conditions in the reactor are significantly different, more parallel channels with heat exchange have to be simulated (cf., e.g. Chen et al., 1988 Jahn et al., 1997, 2001 Tischer and Deutschmann, 2005 Wanker et al., 2000 Young and Finlayson, 1976). In this section we will further discuss effective single channel models. 

Scale-up is in principle straightforward. Larger channel geometries (e.g., in the internally finned monolith channels) allow countercurrent operation of gas and liquid. Monolith reactors are intrinsically safer. The monolith channels have no radial communication in terms of mass transport, and the development of runaway by local hot spots in a trickle-bed reactor cannot occur. Moreover, when the feed of liquid or gas is stopped, the channels are quickly emptied. 

An early application of a combined steam reformer/catalytic combustor on the meso scale was realized by Polman et al. [101]. They fabricated a reactor similar to an automotive metallic monolith with channel dimensions in the millimeter range (Figure 2.65). The plates were connected by diffusion bonding and the catalyst was introduced by wash coating. The reactor was operated at temperatures between 550 and 700 °C 99.98% conversion was achieved for the combustion reaction and 97% for the steam reforming side. A volume of < 1.5 dm3 per kW electrical power output of the reformer alone was regarded as feasible at that time, but not yet realized. 

Figure 2.89 CO conversion vs. reaction temperature in (a) micro channel and (b) monolith reactors. CO feed concentration 5000 ppm 02/C0 ratio 1.0. GHSV (micro channels) ...

The same groups also reported on a 64-fold ceramic block reactor and a ceramic monolithic reactor for the screening of up to 250 catalysts in parallel (Figure 3.41). The catalyst array was prepared via an incipient wetness method by combination of different amounts of Pt, Zr and V on the alumina walls of the monolith by means of an automatic liquid handler. Gas samples from each channel of the monolith were analyzed sequentially by a quadrupole mass spectrometer by moving a capillary sampling line into the channels with the help of a three-dimensional positioning system [69],... 

Several length-scales have to be considered in a number of applications. For example, in a typical monolith reactor used as automobile exhaust catalytic converter the reactor length and diameter are on the order of decimeters, the monolith channel dimension is on the order of 1 mm, the thickness of the catalytic washcoat layer is on the order of tens of micrometers, the dimension of the pores in the washcoat is on the order of 1 pm, the diameter of active noble metal catalyst particles can be on the order of nanometers, and the reacting molecules are on the order of angstroms cf. Fig. 1. The modeling of such reactors is a typical multiscale problem (Hoebink and Marin, 1998). Electron microscopy accompanied by other techniques can provide information on particle size, shape, and chemical composition. Local composition and particle size of dispersed nanoparticles in the porous structure of the catalyst affect catalytic activity and selectivity (Bell, 2003). 

Fig. 1. The multiple scales in the catalytic monolith reactor (a) catalytic monolith (10 cm), (b) channel with catalyst washcoat on the walls (1mm), (c) SEM image of the washcoat layer (10 pm), (d) TEM image of meso-porous y-Al203 with dispersed Pt (200 nm).

Ideally, in contrast to scale-up of packed beds, scale-up of monolithic reactors is simple. As a consequence of fhe accurate fabrication process, the variation in charmel size is small, and to a good first-order approximation, the channels can be regarded as identical. Thus, when we know the behavior of one channel, we should be able to predicf that of the whole reactor. 

A recent example is the optical fiber monolith reactor, reported by Lin and Valsaraj (208). They used a monolith for photocatalytic wastewater treatment with the channels of the monolith completely filled with flowing liquid. The monolith structure was used merely as the distributor of the optical fibers, but the benefits of monolith, such as low-pressure drop and excellent mass transfer characteristics for multiphase systems, were not fully exploited. 

When comparing film flow monolithic reactors with conventional catalytic packed reactors, one can conclude that the critical hydrod)mamic characteristics (hydraulic capacity, pressure drop, and volumetric mass transfer rates) are similar, but monoliths have distinct advantages greater flexibility, easier scale-up, the susceptibility of fhe surface to coating procedures, and advances in control of flooding—all allowing the use of very small channels and therefore efficienf cafalysf ufilizafion. 

The main features of monolith reactors (MR) combine the advantages of conventional slurry reactors (SR) and of trickle-bed reactors (TBR), avoiding their disadvantages, such as high pressure drop, mass transfer limitations, filtration of the catalyst, and mechanical stirring. Again, care must be taken to produce a uniform distribution of the flow at the reactor inlet. Scale-up can be expected to be straightforward in most other respects since the conditions within the individual channels are scale invariant. 

Analyses of monolith reactors specific for SCR applications are limited in the scientific literature Buzanowski and Yang [43] have presented a simple one-dimensional analytical solution that yields NO conversion as an explicit function of the space velocity unfortunately, this applies only to first-order kinetics in NO and zero-order in NH3, which is not appropriate for industrial SCR operation. Beeckman and Hegedus [36] have published a comprehensive reactor model that includes Eley-Rideal kinetics and fully accounts for both intra- and interphase mass transfer phenomena. Model predictions reported compare successfully with experimental data A single-channel, semianalytical, one-dimensional treatment has also been proposed by Tronconi et al [40] The related equations are summarized here as an example of steady-state modeling of SCR monolith reactors. 

D.J. Worth, S.T. Kolaczkowski, and A. Spence, Modelling channel interaction in a catalytic monolith reactor, Trans. ICheniE. 77 331 (1993). 

The use of monoliths as catalytic reactors focuses mainly on applications where low pressure drop is an important item. When compared to fixed beds, which seem a natural first choice for catalytic reactors, monoliths consist of straight channels in parallel with a rather small diameter, because of the requirement of a comparably large surface area. The resulting laminar flow, which is encountered under normal practical circumstances, does not show the kinetic energy losses that occur in fixed beds due to inertia forces at comparable fluid velocities. Despite the laminar flow, monolith reactors still may be approached as plug-flow reactors because of the considerable radial diffusion in the narrow channels [1]. 

The route from reactant to product molecule in a monolith reactor comprises reactant transport from the bulk gas flow in a channel toward the channel wall, simultaneous diffusion and reaction inside the porous washcoat on the channel wall, and product transport from the wall back to the bulk flow of the gas phase. 

Laminar flow is the usual flow regime met in monolith reactors, given that the typical Reynolds number has values below SOO. The radial velocity profile in a single channel develops from the entrance of the monolith onward and up to the position where a complete Poiseuille profile has been established. The length of the entrance zone may be evaluated from the following relation [3] ... 

Equation 2 expresses whether radial diffusion, which in the case of laminar flow is due to molecular diffusion, is fast enough to outlevel radial concentration profiles. This approximation usually holds for monolithic reactors because of the rather small channel diameter. The corresponding axial dispersion coefficient can be calculated [1] from the following ... 

Considerations along the above lines lead to analogous correlations for the Sherwood number for the description of mass transfer in a single channel. The application of the rather simple Nusselt and Sherwood number concept for monolith reactor modeling implies that the laminar flow through the channel can be approached as plug flow, but it is always limited to cases in which homogeneous gas-phase reactions are absent and catalytic reactions in the washcoat prevail. If not, a model description via distributed flow is necessary. 

Almost similar results were obtained experimentally by Votruba et al. [19], who studied evaporation of water and hydrocarbons from porous monoliths. These results predict Nu and Sh values clearly lower than does Eq. (13), and moreover suggest that Nu or Sh values would fall under their theoretically predicted lower limit at a low Reynolds number [16,20]. It is not unlikely that the discrepancy is due to a maldistribution of flow over the different monolith channels, as a result of the low pressure drop, similar to the effect signalized for fixed beds at low Reynolds numbers [7]. Experimental work [4], which was carried out with an inert fixed bed in front of the monolith reactor to assure an even distribution, gave data that come quite near to the results of Hawthorne, Eq. (13) [2]. 

In this chapter, after a general description of possible flow patterns in monolith channels, the main features and properties of monoliths will be discussed. Following this, the monolith reactor will be compared to some other conventional reactors that are widely used. Next, applications of monolith reactors in catalytic gas-liquid processes will be summarized. Finally, some ideas concerning the future needs in this field will be presented. 

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