Fast heating and cooling for high temperature chemical microreactors

Many chem. reactions, like the dehydrogenation of gaseous methanol to formaldehyde, require fast heating and cooling of the gases to control precisely the residence time at the reaction temp. (on the order of hundreds of ms) to minimize consecutive reactions. For these applications, microsystems are of great interest thanks to their characteristic high surface- to-vol. ratio and low thermal inertia. Heater and cooler units were developed with compatible micromachining technologies in view of a future integration in a complete microreactor system. The heater is composed of a silicon duct, capped with a silicon nitride membrane heated by a platinum filament. Within the mean residence time of approx. 4 ms nitrogen was heated to a max. temp. of 600 Deg. Numerical simulations are in agreement with the performed temp. measurements. However, the large temp. gradient of this prototype due to single-sided heating is not well suited for chem. reactions. A soln. with two heated membranes face-to-face is being investigated. The cooler is a co- or counter-current flow heat exchanger machined into silicon by deep plasma etching. The measured overall heat transfer coeffs. are lower than expected. This can be explained by the longitudinal heat conduction in the fins, which is neglected in the basic theory. New exchangers with thinner fins are therefore being developed but the cooling needed for our particular application (from 700 Deg to 200 Deg in a few ms) is already possible with the current structures. [on SciFinder (R)]