Microstructured reactors characteristics

J.-M. Commenge, L. Falk, J.-P. Corriou, M. Matlosz, Analysis of Microstructured Reactor Characteristics for Process Miniaturization and Intensification, Chem. Eng. Technol. 28 (2005) 446. 

Rebrov, E. V., Duinkerke, S.A., de Croon, M. H. J. M., Schouten, J. C., Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/ heat-exchanger for optimal peformance in platinum catalyzed ammonia oxidation, Chem. Eng. 93 (2003) 201-216. 

To avoid mass and heat transfer resistances in practice, the characteristic transfer time should be roughly 1 order of magnitude smaller compared to the characteristic reaction time. As the mass and heat transfer performance in microstructured reactors (MSR) is up to 2 orders of magnitude higher compared to conventional tubular reactors, the reactor performance can be considerably increased leading to the desired intensification of the process. In addition, consecutive reactions can be efficiently suppressed because of a strict control of residence time and narrow residence time distribution (discussed in Chapter 3). Elimination of transport resistances allows the reaction to achieve its chemical potential in the optimal temperature and concentration window. Therefore, fast reactions carried out in MSR show higher product selectivity and yield. 

Mass Transfer in Catalytic Microstructured Reactors 247 Table 6.3 Mass transfer characteristics for different channel geometries [53],... 

Especially fast reactions benefit from the excellent mass transfer characteristics of microstructured devices. In addition, heat management for highly exothermic reactions is greatly facilitated because of efficient removal of heat produced during the reaction. Selective examples of different gas-liquid reactions that have been studied in the microstructured reactors are listed in Table 7.14. 

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

Therefore, one of the major drivers for running Friedel-Crafts alkylations in microstructured reactors is to improve the selectivity of monoalkylation products under reasonable stoichiometric conditions, in particular by achieving significantly accelerated and intensified mixing and mass transport than achievable in macroscopic processes. Moreover, it is also expected that the exothermic alkylation reactions additionally benefit from the improved heat transfer characteristics of microreactors. 

Appropriate reactor structuring therefore appears as a relevant intensification strategy by offering new dimensions for operation, microstructured reactors can be used to modify selectively the hierarchy and choose the phenomenon that should impose its efficiency on the system. For example, reducing the characteristic dimension accelerates transfer phenomena with respect to homogeneous reactions, enabling one to eliminate detrimental temperature effects. 

As the specific surface area (Equation 2.24) and the heat transfer coefficient (Equation 2.21) increase with decreasing diameters, it follows that microstructured channel reactors are characterized by very short cooling times, thus improving temperature control and reducing the risk of reactor runaway. As a consequence, microstructured reactors can be operated under harsh reaction conditions such as high temperature and pressure. The chemical kinetics is speeded up drastically reducing the characteristic reaction time. This concept is often called novel process windows [20]. 

The amount of iodide formed given by [I3-] + [I2] is a measure for the quality of mixing. Based on the reaction kinetics and the characteristic reaction time 1 2. Commenge and Falk [14] calculated the formation of iodine as a function of the mixing time in microstructured reactors. For the theoretical predictions, the authors used the relatively simple interaction by exchange with the mean (I EM) model described by ViHermaux [10]. The results obtained for different experimental... 

Compared with laboratory fixed-bed reactors or conventional extruded monoliths, such a microstructured monolith is smaller in characteristic dimensions, lower in pressure loss by optimized fluid guiding and constructed from the catalytic material solely [3]. The latter aspect also leads to enhanced heat distribution within the micro channels, giving more uniform temperature profiles. 

Table 97 Characteristics of the microstructured body of the reactor (O. Goerke...

A characteristic of this type of reactor is the steel substrate, which is preferably used as the reaction chamber (it can also be titanium or aluminum). This allows the use of microstructures under robust experimental conditions such as high temperatures. 

M 39] [P 37] Using an azo-type competitive reaction, the selectivities were compared for the P- and V-type micro mixers having straight and oblique fluid injection, respectively [41]. In this way, laminar- and turbulent-flow mixing achieved by vertical interdigital microstructured mixers can be compared. The selectivities of the turbulent V-type mixer are better to some extent as compared with the P-type device however, neither approaches the characteristics of the ideal tubular reactor. The micro devices, however, are better than a conventional jet mixer. 

The characteristic features of microsystems stem from the small size of the space in the microstructures. Therefore, microsystems are not necessarily small systems in total size. They can be large in total size as long as they contain microstructures that can be used for chemical reactions. This sharply contrasts with the concept of a lab-on-a-chip, which should be small in total size. It is also important to note that microsystems are normally set up as flow-type reactors with a constant flow of solutions through a microstructured reaction chamber or channel. Although the reactor s capacity at any one time is small, total production capacity over time is much greater than may be imagined. Therefore, microflow systems are not necessarily used solely to produce small quantities of chemical substances. In fact, a microfluidic device has been developed that fits in the palm of the hand but can produce several tons of a product per year (see Chapter 10). 

We first present general criteria for the rational use of MSRs on the basis of fundamentals of chemical reaction engineering [21-24], The main characteristics of MSRs are discussed, and the potential gain in reactor performance relative to that of conventional chemical reactors is quantified (Section 2). Subsequently, the most important designs of fluid-solid and multiphase reaction systems are described and evaluated (Sections 3 and 4). Because microstructured multichannel reactors with catalytically active walls are by far the most extensively investigated MSRs for heterogeneous catalytic reactions, we present their principal design and recent synthetic methods separately in Section 5. 

Yeong et al. [100,101] used a microstructured film reactor for the hydrogenation of nitrobenzene to give aniline in ethanol at a temperature of 60 °C, a H2 partial pressure of 0.1-0.4 MPa, and residence times of 9-17 s. Palladium catalysts were deposited as films or particles on a microstructured plate. Confocal microscopy was used to measure the liquid film thickness, which increased from 67 to 92 pm as flow rates were increased from 0.5 to 1.0 cm3 min-1. The value of kha characteristic of this system was estimated to be 3-8 s 1 at an interfacial surface area (per reactor volume) of 9000-15000 m2 m 3. Conversion was found to be affected by both liquid flow rate and H2 partial pressure, and the reactor operated between the kinetic and mass transfer-controlled regimes. 

All metals used in a reactor have crystalline structures. Crystalline microstructures are arranged in three-dimensional arrays called lattices. This chapter will discuss the three most common lattice structures and their characteristics. 

Comparison of a single-tube packed-bed reactor with a traditional batch reactor was also published in the case of o-nitroanisole hydrogenation, not for productivity purposes but rather as laboratory tools for kinetic studies (Scheme 9.11) [46]. It was shown that the better efficiency of mass transfer enables the microreactor to obtain intrinsic kinetic data for fast reactions with characteristic times in the range 1-100 s, under isothermal conditions, which is difficult to achieve with a stirred tank reactor. However, the batch reactor used in this study was not very well designed since a maximum mass transfer coefficient (kia) of only 0.06 s was measured at 800 rpm, whereas kia values of up to 2 s are easily achieved in small stirred tank reactors equipped with baffles and mechanically driven impellers [25]. This questions the reference used when comparing microstructured components with traditional equipment, with the conclusion that comparison holds only when the hest traditional technology is used. 

Material and structural issues to be addressed are primarily related to the potential for corrosion and stress corrosion cracking under irradiation at the high temperatures and pressures associated with the SCWR. Materials for cladding and structural components must be identified and tested to demonstrate their performance in thermal and fast-spectrum reactors. Radiolysis and water chemistry at supercritical conditions must be investigated to understand the effect on reactor materials. Specific material properties to be investigated include dimensional and microstructure stability, and strength, embrittlement, and creep resistance characteristics of the materials. 

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