Microprocess Technology
Microprocess technology as a relatively new
technology has gained significant importance in recent years. In simple
terms microreaction technology can be characterized as the
miniaturization of reactor systems operating in a continuous mode. By
now quite a number of companies are on the market which provide special
systems for this technology like micromixers, micro heat exchangers and
microrectors. Companies like the “Institut für Mikrotechnik”,
Mainz (IMM) or the “Forschungszentrum Karlsruhe” deal mainly with this
kind of technology. Universities offer lectures on microreaction
technology and a series of books are available. Since the 90s the
potential of microprocess technology for chemical and biological
reactions has been investigated. The international conference for
Microtechnology (IMRET) takes place each year in Europe and the USA and
the Industrial Platform of Modular Micro Chemical Technology
(MicroChemTec) was founded in partnership with DECHEMA. Microstructured
reactors are reactors with three-dimensional structures. The inner
dimensions are under a millimeter in size. The common range is between
ten and one hundred micrometers.
Figure 1: Scale of micromixer (Copyright IUV)
Listed below are some important notes concerning microreactors in comparison with conventional chemical reactors:
- High surface-area-to-volume ratio as a result of the dimension
- Higher heat-transfer coefficient and therefore heat exchange
The high heat exchanging efficiency allows fast heating and cooling in reaction mixtures within the microstructures. The development of hot spots or the accumulation of reaction heat is suppressed. Due to these properties high selectivity, yield and product quality can be received. Simultaneously undesirable side reaction and fragmentation are hindered. The microstructured reactors can be used successfully for strongly exothermic and fast reactions. In addition, mass transport is also considerably improved in microstructured reactors. The mixing times are smaller than in conventional systems (down to several milliseconds). Due to the miniaturization the diffusion times are very short, so that the influence of mass transport on the speed of a reaction can be considerably reduced.
Figure 2: Laminar flow in micro mixer (Copyright IUV)
Regarding the hydrodynamic flow in the microchannel it was possible to show that flows are mostly laminar, directed, and highly symmetric. This is of great value with regards to the prediction of the residence time distribution. In comparison to conventional reactors parameters like residence time, flow rate, pressure and temperature are more easily controlled. Due to a better control of these parameters the hazard potential of strongly exothermic or explosive reactions can be reduced. It is possible to carry out reactions with toxic substances or at high pressures more safely. In conjunction with microreactors a relatively fault-tolerant safety level is achieved due to these intrinsic conditions. Microstructured reactors give opportunities to new production concepts. In comparison to conventional processes a scaling-up is not essential. The obtained results from the laboratory can be directly used for the industrial process. This can be achieved by a so called “numbering-up”. Nevertheless the application of microstructured reactors is limited to specific production areas. Due to the small dimension the right handling is important, so there are special pump systems required which do not show any pulsation, and the purity of the reactants is very important. The microreaction technique is applicable in the area of fine chemicals or is used for reactions with higher risk potential like fluorination reactions or two-phase exothermic reactions (Evonik).
Literature:
Chemistry in Microstructured Reactors, K.Jänisch, V. Hessel, H.Löwe and M.Baerns,
Angewandte Chemie Int. Ed., 2004, 43, 406-446 and the literature quoted in this paper
Transport Phenomena in Micro Process Engineering (Heat and Mass Transfer), N.Kockmann, Springer 2007 Elements22 Evonik Service Newsletter 2008, 20-24
Authors: Marko Hoffmann, Claudia M. Müller, Daniela Hameister