The aqueous Kolbe-Schmitt synthesis using resorcinol to yield2,4-dihydroxy benzoic acid was performed in a microreactorrig. This small-scale plant was equipped initially with onecapillary reactor
and one microstructured cooler only. Later,two upgraded versions were constructed, having in addition amicrostructured cooler
and a microstructured mixer, respectively. The chemical protocol was significantly varied ascompared to st
andard laboratory operation as described in theliterature. Higher temperatures (up to 220
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C)
and pressures(up to 74 bar) were employed in a facile manner, termed high-p,T processing. In this way, the reaction time could be shortenedby orders of magnitude, from about 2 hours to less than oneminute, in some cases to some seconds. This resulted in aremarkable increase of the space-time yield by a factor of 440at best. Productivity was in the L/h range
and yielded at best111 g/h product, corresponding to 4 t/a. Scale-out solutions areindicated. Drawbacks of the microreactor operation were alsoidentified such as high sensitivity to fouling
and delicateregulation of the system pressure, leading to partly unstableplant operation. Possibly even a considerable part of the productwas rearranged to 2,6-dihydroxybenzoic acid
and then thermally decomposed under the harsh reaction conditions. Solutions to overcome or at least diminish these restrictions areenvisaged,
and in the hope that this may be achieved, a processinnovation
and business perspective for the high-p,T microreactor processing is depicted.