Monolithic reactor

New reactor technologies are currently under development, and these include meso- and micro-structured reactors or the use of membranes. Among meso-structured reactors, monolithic catalysts play a pre-eminent role in environmental applications, initially in the cleaning of automotive exhaust gases. Beside this gas-solid application, other meso-structures such as membranes [57, 58], corrugated plate or other arranged catalysts and, of course, monoliths can be used as multiphase reactors [59, 60]. These reactors also offer a real potential for process intensification, which has already been demonstrated in commercial applications such as the production of hydrogen peroxide. 

The whole process consists of a sequence of consecutive steps which allows separation into production modules. Some of them already exist and will just need slight modifications. Others have to be developed. At the very beginning, one makes use of the newly developed reel-to-reel etching technology for micro structured stainless-steel foils similar to the process described in [166], The structured foils are then coated with catalyst, structured again with a laser tool and finally folded by sheet metal forming to a reactor monolith. The readily mass-produced reactors will be sealed by laser welding. 

A very limited radial mixing inside the channel and no mass exchange between individual channels with resulting zero mixing over the reactor (monolithic catalysts)... 

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]. 

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Catalyst Monolith. The previous discussion in this chapter focused primarily on chemical reactions taking place in packed-bed reactors. However, when a gaseous feedstream contains significant amoimts of particulate matter, dust tends to clog the catalyst bed. To process feedstreams of this type, parallel-plate reactors (monoliths) are commonly used. Figure 11-11 shows a schematic diagram of a monolith reactor. The reacting gas mixture flows between the parallel plates, and the reaction takes place on the smface of the plates. 

Spinning disk reactor Static mixer reactor Monolithic reactors Microreactors HEX reactors... 

Figure 7.9 Variation of bacteria coverage density (number of cells per iOO square pm) as a function of distance from the micro-reactor inlet. The intercept of the dashed line indicates the optimum thickness of the micro-reactor monolithic support...

Jet injectors may also be combined with monolith reactors. Monoliths are usually tube reactors with channeled flow. The reaction occurs at the gas-liquid interface as well as on the channel wall, which are usually catalytic or coated with catalytic material. Monoliths can be made into vertical (similar to bubble column) or horizontal tubes, airlift devices (whereby the riser would a monolith), or even into a mechanically stirred device. Usually, however, monoliths are designed like bubble columns or airhft reactors (Broekhuis et al., 2001). 

Fixed-Bed Reactors - Monoliths 1189 Figure 9.10 Stack of SCR honeycombs. 

Technologies Reactors spinning disk reactor, static mixer reactor, microreactors, heat exchange reactors, monolithic reactors, oscillatory flow reactors, trickle bed reactors... 

Conventional reactor technologies such as fixed beds and slurry reactors suffer from serious drawbacks. Mass transfer resistance is the crucial factor in the scaleup of processes. Laboratory experiments are often carried out with catalyst particles with diameters clearly less than 1 mm, whereas industrial reactors typically operate with larger catalyst particles ranging from 1 mm to 1 cm. The scale dimensions are illustrated in Figure 9.1. Intrinsic kinetics is thus inevitably coupled to the modeling of mass transfer, as has been illustrated in previous chapters. Internal mass transfer limitations can be suppressed by decreasing the particle size, but the particle sizes in industrial processes cannot be diminished limitlessly, because this would lead to a tremendous increase in the pressiue drop. To overcome this problem, new innovations and structured reactors have been developed, such as catalytic packing element reactors, monoliths, and fiber structures. The aim of these innovations has... 

Figure 7.1. Particulate and monolith structures, (a) Exlrudates, tablets and spheres, (b) Structured reactors (monoliths or honeycombs).

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

Scale-up in fixed-bed reactors is limited by the maximum size of the matrix that can be manufactured as a monolith. Hence, this system is appHcable for small- to medium-scale production of antibodies and other proteins, usually for the diagnostic market. This system has been described in greater detail ia the Hterature (22). 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

The analysis of the transient behavior of the packed bed reactor is fairly recent in the literature 142-145)- There is no published reactor dynamic model for the monolith or the screen bed, which compares well with experimental data. 

Electrochemical Promotion Using a Bipolar Monolithic Reactor... 

Figure 12.12. Near full scale monolithic Dinex reactor for electrochemically promoted soot combustion.18 20 Reprinted with permission from the Society of Automotive Engineers.

There is a general trend toward structured packings and monoliths, particularly in demanding applications such as automotive catalytic converters. In principle, the steady-state performance of such reactors can be modeled using Equations (9.1) and (9.3). However, the parameter estimates in Figures 9.1 and 9.2 and Equations (9.6)-(9.7) were developed for random packings, and even the boundary condition of Equation (9.4) may be inappropriate for monoliths or structured packings. Also, at least for automotive catalytic converters. 

I., Mikkola, J.-P., and Salmi, T. (2004) The development of monolith reactors general strategy with a case study. Chem. Eng. Sci., 59, 5629-5635. 

Autocatalysts, based on monoliths, are probably the most extensively used catalytic reactors around a hundred million have been installed and are performing well in car exhaust systems [10-12]. Reduction of volatile organic carbon (VOC) emissions [13] and removal of NOj, from stationary sources [14, 15] are also... 

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... 

Metal monoliths show good thermal characteristics. A typical support with herringbone channels made from Fecralloy performed satisfactory in automotive applications [27]. Modeling showed that overall heat transfer was about 2 times higher than for conventional pellets [28,29]. Hence, there is potential for structured catalysts for gas-phase catalytic processes in multitubular reactors. 

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... 

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

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