Monolithic reactors applications

Schildhauer T., Kapteijn F. and Moulijn J. (2005). Reactive stripping in pilot plant scale monolith reactors - application to esterification. Chemical Engineering and Processing 44 (2005), 695-699. 2.6... 

Therefore, there is a strong motivation to develop a dynamic model of the SCR monolithic reactor suitable for extended temperature operation and to study the fast SCR reaction in view of future possible applications. In the following, we will focus on these two issues. 

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. 

This chapter discusses both the development of models and their application. One way of organising this chapter would be to discuss model development first and then go on to consider the applications. However, as the entire reason for developing these models is to have a practical tool for system design, it was decided to start with the application of the models. The next section discusses the physical model for a monolith reactor, which is common to all technologies (except diesel particulate filters) discussed later. Our approach to model development will then be covered in detail, using TWCs as an example. The final section will outline work done on the various technologies used for diesel exhaust aftertreatment. 

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. 

The catalyst and particulate filter models were developed individually with different university partners. They are described in the following sections. A key issue for all models is robustness and scalability as the applications range from passenger cars to heavy-duty commercial vehicles. The models are physical and chemically based, consisting of a transport model for heat and mass transfer phenomena in the monolith in gas and solid phases, cf. Fig. 6. The monolith reactor modeling is discussed in more detail in Section III. 

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

Avila P, Montes M, Miro EE. Monolithic reactors for environmental applications A review on preparation technologies. Chemical Engineering Journal. 2005 109(1—3) 11—36. 

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). 

In single-phase applications, a monolithic reactor will have a design similar to that of a classical packed-bed reacfor, excepf for characferistics associated with specific properties of fhe monolifh. The laffer properties include the low pressure drop and the mechanical robustness. The low-pressure drop calls for a careful inlef design in order to prevent preferential flow fhrough parf of fhe reacfor (usually fhe center part). The mechanical robustness allows the placement of fhe reactor in any desired position (horizontal, vertical, etc.), a useful property for the automobile manufacturer. 

For many systems involving gas-phase reactants, when the pressure drop is a key parameter, monolithic reactors are state of the art, whereas for the other sectors, industrial application of monoliths, in particular in postreactors, is increasing or is the subject of intense research and... 

TABLE 5 Applications of Monolithic Reactors (Commercial or Demonstration Stage). 

Monolithic reactors, similar to many other structured reactors, offer high precision combined with a high efficiency. They are a valuable tool for process intensification. Most practical applications of these reactors are found in environmental catalysis, motivated by the ambition to realize low-pressure drops at high flow rates for gas-phase reactants. Monoliths are the state-of-the-art reactors in these applications. 

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. 

J.E. Aniia and R. Govind, Applications of binderless zeolite-coated monolithic reactors, Appl. Cat. A General 131 07 (1995). 

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]. 

Although the present considerations are valid for monolith reactors in general, independent of the actual chemical reactions, details will refer mostly to the application of monoliths in automobile exhaust gas treatment, which has received most attention in the past and still is dominant in the practical use of monoliths. Several reviews treat the extensive literature on monoliths, among which is a very recent one [2]. 

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. 

The model description just proposed has never been validated or used in simulation studies. It does incorporate, however, all phenomena that have been reported as potentially important for monolith reactor modeling. It is obvious that many simplifications can be made for specific applications, and this indeed has been done in the literature. 

In the last 15 years, the use of monoliths has been extended to include applications for performing multiphase reactions. Particular interest has been focused on the application of monolith reactors in three-phase catalytic reactions, such as hydrogenations, oxidations, and bioreactions. There is also growing interest in the chemical industries to find new applications for monoliths as catalyst support in three-phase catalytic reactions. 

Oxidation of organic and inorganic species in aqueous solutions can find applications in fine chemical processes and wastewater treatment. Here, the oxidant, often either air or pure oxygen, must undergo all the mass transfer steps mentioned above in order for the reaction to proceed. During the last decade, increased environmental constraints have resulted in the application of novel processes to the treatment of waste streams. An example of such a process is wet air oxidation. Here, the simplest reactor design is the cocurrent bubble column. However, the presence of suspended organic and inert solids makes the use of monolith reactors favorable. 

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. 

The balance of advantages and drawbacks of the monolith reactors is positive, making this reactor type very attractive for applications in multiphase processes. The modeling of monolith reactors and some concepts for reactor design are presented in Chapter 10. 

Of primary interest for the industrial application of monolith reactors is to compare them with other conventional three-phase reactors. Two main categories of three-phase reactors are slurry reactors, in which the solid catalyst is suspended, and packed-bed reactors, where the solid catalyst is fixed. Generally, the overall rate of reactions is often limited by mass transfer steps. Hence, these steps are usually considered in the choice of reactor type. Furthermore, the heat transfer characteristics of chemical reactors are of essential importance, not only due to energy costs but also due to the control mode of the reactor. In addition, the ease of handling and maintenance of the reactor have a major role in the choice of the reactor type. More extensive treatment of conventional reactors can be found in the works by Gianetto and Silveston [11], Ramachandran and Chaudhari [12], Shah [13,14], Shah and Sharma [15], and Trambouze et al. [16], among others. 

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