Micro structured -Reactors

Felcht reports on Degussa s activities in cooperation with partners from academia and industry to develop innovative industrial-scale micro-structured reactors for making large-tonnage products by liquid- and gas-phase reactions [137]. The aim is to make the potential of micro reactors more widely available for a larger variety of processes, naturally with focus on Degussa s fine and specialty chemical productions. Also, the aim is to circumvent traditional problems of scale-up. 

The study is based on interviews with about 100 selected companies and insti-tutes/universities. Representatives from companies made up 70% of all interviewees. The study initially predicted a small increase in worldwide turnover according to general economic development. It predicts at the same time, however, that an interaction of an increasing acceptance and a significant improvement of the technical suitability of micro-structured reactors (e.g. for chemical production) could lead to an amplified steep increase of the market turnover also in the short term. 

There is not much to be said about the use of micro reactors for bulk chemicals and commodities. Worz et al. are so far the only ones who have disclosed their work on the potential of micro-structured reactors for the optimization of chemical processes performed on a large scale ofindustrial relevance [110,112,154,288-290]. This included a fast exothermic liquid/liquid two-phase reaction, which was used for the industrial production of a vitamin intermediate product, and a selective oxidation reaction for an intermediate, a substituted formaldehyde derivative. 

Rebrov, E. V., de Croon, M. H. J. M., ScHOUTEN, J. C., Design of a micro-structured reactor with integrated heat-exchanger for optimum performance of highly exothermic reaction, Catal. Today 69 (2001) 183-192. 

WuNSCH, R., Eichtner, M., Gorke, O., Haas-Santo, K., Schubert, K., Process of applying AljOj coatings in microchannels of completely manufactured micro-structured reactors, Chem. Eng. Technol. 25, 7 (2002) 700-703. 

Maurer, R., Claivaz, C., Eichtner, M., Schubert, K., Renken, A., A micro-structured reactor system for the methanol dehydrogenation to water-free formaldehyde, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, pp. 100-105 (5-9 March 2000), AIChE Topical Conf Proc., Atlanta, USA. 

Jahnisch, K., Hessel, V., Lowe, H., Baerns, M., Chemistry in micro-structured reactors, Angew. Chem. Int. 

The equilibrium values are not reached at a rhodium catalyst on a micro structured reactor within the limits of the experimental conditions and the constructional constraints [3]. As possible explanations post-catalytic reactions at lower temperatures or, more likely, insufficient catalyst activity concerning the short residence times are seen. 

Yoshida, J.-I., Nagaki, A., Suga, S., Highly selective reactions using micro-structured reactors, in Proceedings of the Micro Chemical Plant - International Workshop, 4 February 2003, pp. L2 (3-9), Kyoto (2003). 

Kolb G, Hessel V (2004) Micro-Structured Reactors for Gas Phase Reactions. Chem Eng J 98 1-38... 

Figure 4 shows the impact of process intensification for this hypothetical case. With a temperature increase of only 41°C, the number of reactors for such comparatively big units is reduced from 20, hardly feasible in view of costs and process control, to four, feasible for the same reasons. Thus, the costs decrease by almost a factor of five (not exactly, since fixed costs have a small share). Another 21°C temperature increase halves the number of reactors again, and at 249°C, which is 149°C higher than the base temperature, an equivalent of 0.2 micro-structured reactors is needed. This means that practically one micro-structured reactor is taken and either reduced in plate number or in the overall dimensions. The costs of all microstructured reactors scales largely with their number only at very low numbers do fixed costs for microfabrication lead to a leveling off of the cost reduction. 

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. 

In recent years, micro-structured reactors have attracted considerable attention for a variety of applications [61, 62]. Such micro devices are characterized by a laminar flow, and the very high surface-to-volume ratio they provide leads to increased mass and heat transfer, offering the potential for process intensification. 

It has also been recognized that micro-structured components, because of their low mass and thermal inertia, are able to offer short response times for unsteady state periodic operations. Micro-reactors have been used successfully for fluorination, oxidations and both heterogeneous [63-65] and homogeneous hydrogenations [66]. A review on gas-liquid micro-structured reactors has been published [67]. The very small material inventory when using micro devices offers another advantage, notably as a laboratory tool for screening applications, kinetics determination and process data acquisition, where the main concern is... 

While batch reactors remain the workhorse in fine chemical production, the need to switch to continuous processes will increase the use of meso- and micro-structured reactors both at the laboratory scale (for discovery, process data determination, demonstration, small-scale production) and at the production level. 

Abdallah, R. and Fumey, B. and Meille, V. and de Bellefon, C. (2007). Micro-structured reactors as a tool for chiral modifier screening in gas-liquid-solid asymmetric hydrogenation. Catalysis Today, 125, 34-39. 

Hessel V, Serra C, Loewe GH, Hadziioannou G (2005) Polymerizations in micro-structured reactors overview. Chem Ing Tech 77 1693... 

Design Concepts of Micro Structured Reactors for Fuel Processing Applications... 

Depending on the targets heading for, various design concepts of micro structured reactor have been realized ... 

Numerical calculations revealed that isothermal operation was feasible throughout a catalyst layer as thick as 60 pm in a micro structured reactor under the conditions... 

The benefits of applying micro structured reactors for autothermal reforming are manifold. Besides the well-known narrowing of the residence time distribution and the low pressure drops, hot-spot formation may well be reduced owing to the axial heat transfer of the wall material. 

Early work by Tonkovich et al. [46] dealt with a heat exchanger/reactor containing catalyst powder for the partial oxidation of methane for distributed hydrogen production. The intention was to run the reaction safely in a micro structured reactor owing to the short residence times applied and the improved heat removal avoiding hot-spot formation. 

TeGrotenhuis et al. [58] performed a 1 000 h stability test in a micro structured reactor for the steam reforming reaction with a catalyst not specified. A mixture of 74% isooctane, 20% xylene and 5% methylcyclohexane as simulated gasoline was fed to the reactor at a S/C ratio of three and a 650 °C reaction temperature. A regeneration step was performed after 500 h and finally the catalyst converted 97% of the feed. 

At a 300 °C reaction temperature, 1 200 ppm methane was found under standard conditions. Testing in the micro structured reactor at a very high GHSV of 500 000 h-1, a 230 °C reaction temperature and an 02/CO ratio of 1.0 revealed a very low CO concentration of < 20 ppm at the reactor outlet regardless of the feed concentration. Less than 30 ppm methane was detected under these conditions. Increasing the 02/CO ratio above 1.0 did not further improve this performance (Figure 2.62). 

Integrated Micro Structured Reactor Fuel Processing Concepts... 

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