Reactor miniaturization

The miniplant concept was proposed first by Ponton at the University of Edinburgh (UK) in 1993 [1,58-60], The potential benefits of distributed processing, as outlined above and reported elsewhere [2, 4], were identified, in particular focusing on an increase in process safety. However, the critical evaluation of the miniplant concept also revealed a number of potential drawbacks. For instance, the specific production costs of a small production units not only depend on reactor miniaturization, but are also determined by control instruments and other peripheral equipment. 

The interaction between reactor miniaturization and catalyst nanotailoring to achieve an effective PI has been emphasized by Charpentier [28] and Dautzen-berg [29]. The role of membranes in PI has been discussed by Drioli ef al. [30]. This short overview of the recent state-of-the-art, although limited to reviews published in the last few years, evidences the intense research effort and broad-range type of applications for PI, from refinery and petrochemistry to biotechnology, fine and specialty chemical production. 

The volume ratio quantifies reactor miniaturization which is one objective of process intensification. A value lower than unity indicates effective miniaturization. 

Oxygenation reactions lend themselves to scaling up studies. As an example, in the oxygenation of a-pinene via singlet oxygen, it has been found that the key factors for optimization are reactor miniaturization, the intensity and the spectral... 

Segregrated flow model The fluid in a flow reactor is assumed to behave as a macrofluid. Each clump functions as a miniature batch reactor. Mixing of molecules of different ages occurs as late as possible. 

Dream reactions can be performed using chemical micro process engineering, e.g., via direct routes from hazardous elements [18]. The direct fluorination starting from elemental fluorine was performed both on aromatics and aliphatics, avoiding the circuitous Anthraquinone process. While the direct fluorination needs hours in a laboratory bubble column, it is completed within seconds or even milliseconds when using a miniature bubble column. Conversions with the volatile and explosive diazomethane, commonly used for methylation, have been conducted safely as well with micro-reactors in a continuous mode. 

Micro Total Analysis Systems (pTAS) are chip-based micro-channel systems that serve for complete analytics. The word Total refers to the monolithic system character of the devices, integrating a multitude of miniature functional elements with minimal dead volumes. The main fields of application are related to biology, pharmacology, and analytical chemistry. Detailed applications of pTAS systems are given in Section 1.9.8. Recently, pTAS developments have strongly influenced the performance of organic syntheses by micro flow (see, e.g., [29]). By this, an overlap with the micro-reactor world was made, which probably will increase more and more. 

Many other, less obvious physical consequences of miniaturization are a result of the scaling behavior of the governing physical laws, which are usually assumed to be the common macroscopic descriptions of flow, heat and mass transfer [3,107]. There are, however, a few cases where the usual continuum descriptions cease to be valid, which are discussed in Chapter 2. When the size of reaction channels or other generic micro-reactor components decreases, the surface-to-volume ratio increases and the mean distance of the specific fluid volume to the reactor walls or to the domain of a second fluid is reduced. As a consequence, the exchange of heat and matter either with the channel walls or with a second fluid is enhanced. 

More favorable for miniaturization are processes with an operation time-scale proportional to or (d f. For a linear dependence on the channel diameter, the product N L df is conserved under the conditions described above. This means that with shrinking df and for fixed efficiency, the reactor volume decreases proportionally with the channel diameter. For a quadratic dependence of the operation time-scale with channel diameter, the product N L is conserved and the reactor volume decreases as the channel diameter squared. 

Table 1.7 Numerical example illustrating the increased heat transfer on miniaturization of industrial reactors [110].

Worz et al. give a numerical example to illustrate the much better heat transfer in micro reactors [110-112]. Their treatment referred to the increase in surface area per unit volume, i.e. the specific surface area, which was accompanied by miniaturization. The specific surface area drops by a factor of 30 on changing from a 11 laboratory reactor to a 30 m stirred vessel (Table 1.7). In contrast, this quantity increases by a factor of 3000 if a 30 pm micro channel is used instead. The change in specific surface area is 100 times higher compared with the first example, which refers to a typical change of scale from laboratory to production. 

Jensen stresses the great flexibility in reactor design that can be achieved by means of microfabricahon, in particular when parallel, MEMS-based mask processes are followed, having a mulhtude of miniature designs on one mask [75], This should invigorate the innovahve nature of reactor design and allow one to overcome... 

Among the hazardous chemical weapons scheduled class 1-3, methyl isocyanate becoming more and more important as a precursor [83]. This is just one among a number of substances which could be made via micro-reactor synthesis. Especially in the case of so-called binary weapons, where two relatively harmless substances are mixed to give a weapon, on-site mixing is demanded this can be accomplished with high performance by micro reactors. Pocket-sized miniature plants can neither be monitored nor detected. 

Rinard dedicated his research to a detailed analysis of methodological aspects of a micro-reactor plant concept which he also termed mini-plant production [85] (see also [4, 9, 10] for a commented, short description). Important criteria in this concept are JIT (Just-in-time) production, zero holdup, inherent safety, modularity and the KISS (keep it simple, stupid) principle. Based on this conceptual definition, Rinard describes different phases in plant development. Essential for his entire work is the pragmatic way of finding process solutions, truly of hybrid character ]149] (miniaturization only where really needed). Recent investigations are concerned with the scalability of hybrid micro-reactor plants and the limits thereof ]149], Expliddy he recommends jointly using micro- and meso-scale components. 

An impressive example of the impact of miniaturization on the explosion limit has been given for the oxyhydrogen reaction [18]. For a conventional reactor of 1 m diameter, explosive behavior sets in at 420 °C at ambient pressure (10 Pa). An explosion occurs at about 750 °C, when the reactor diameter is decreased to about 1 mm. A further reduction to 100 pm shifts the explosive regime further to higher pressures and temperatures. 

Die Fabrik auf dem Chip, Spektrum der Wissenschafi, October 2002 Miniaturization and modularization of parts of future chemical apparatus general advantages of micro flow expert opinions specialty and fine chemical applications leading position of German technology flexible manufacture large-capacity micro reactors reformers for small-capacity applications compatible and automated micro-reaction systems process-control systems temperature and pressure sensors [209]. 

Original citation Miniature chemical reactors will pave the way for the future. These reactors will cut today s monster chemical plants down to the size of a car, with huge financial and environmental gains [219],... 

Miniaturization and parallelization key approaches for drug development apparatus for combinatorial chemistry UHTS 1536 titer-plate format modular construction of apparatus applications of UHTS fine-chemical synthesis by micro reactors numbering-up nature as model general advantages of micro flow vision of plants-on-a-desk [233]. 

Tonkovich, A. L. Y, Zilka, J. L., Powell, M. R., Gall, C. J., The catalytic partial oxidation of methane in a micro-channel chemical reactor, in Ehrfeld, W, Rinard, I. H., Wegeng, R. S. (Eds.), Process Miniaturization 2nd International Conference on Microreaction Technology, IMRET 2, Topical Conf. Preprints, pp. 45-53, AIChE, New Orleans (1998). 

WoRZ, O., Jacket, K. P., Richter, T, Wole, A., Microreactors, new efficient tools for optimum reactor design. Microtechnologies and Miniaturization, Tools, Techniques and Novel Applications for the BioPharmaceutical Industry, IBC Global Conferences, London (1998). 

For the reasons mentioned above, the development of miniature test processes becomes increasingly important for such biotransformation reactions [84]. Micro reactors can handle small volumes and process them in a well-defined manner and have been shown to have high test throughput frequencies. Hence waste reduc-... 

Micro reaction systems may help to overcome or at least reduce some of the above-mentioned limitations [69]. Electrochemical micro reactors with miniature flow cells where electrodes approach to micrometer distances should have much improved field homogeneity. As a second result of confined space processing, the addition of a conducting salt may be substantially reduced. In addition, benefits from a uniform flow distribution and efficient heat transfer may be utilized. 

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