Measurements on the turbulent flow around and through monoliths for application in a monolith stirred reactor

Catalyst impregnated monolith blocks are used almost exclusively in the catalyst converters found in automotive exhaust pipes. Recent research has shown that these monolith blocks can also be applied in the process and fine chemical industry as a possible replacement for existing powdered catalysts in e.g. stirred tank reactors. In this research the feasibility of a monolith stirred reactor (MSR) was investigated with LDA measurements on the flow field around and through monoliths (i) in a uniform flow field and (ii) as impeller blades in a stirred tank. Both;the flow fields around monoliths in a uniform flow fleld and in a stirred tank are found to depend on the cylinder used, blocked or monolith, as weU as on the monolith channel diameter and the speed of the flow fleld. Downstream of the obstacle, two distinct flow regimes are possible: (i) flow towards the cylinder for blocked cylinders or monoliths at low flow rates or low speeds of rotation and (ii) flow away from the cylinder for monoliths at higher flow rates or higher speeds of rotation. The wake behind the impeller blade travels out- and downward for increasing distances behind the impeller blade, until it dissipates before the next blade passage. For sufficient flow through the channels the turbulent kinetic energy levels directly behind the monohth impeller are reduced drastically. Through measurements on the monolith surface, with a tilted probe set-up, the flow through the monolith channels has been obtained. For a given monolith channel diameter, the channel velocity is approximately proportional to the speed of rotation. For increasing channel diameters the velocity increases, but the increase is less than proportional to the channel diameter. The results of a model for predicting these channel velocities, are in agreement with the measurements. A model chemical reaction inside the MSR has been used to estimate the efliciency of an MSR. Decreasing the channel diameter from 3 to 1 mm always yields an enhanced efficiency, while increasing the speed of rotation has a diminishing effect on the efliciency, depending on the reaction rate. The model can be used to flnd the appropriate speed of rotation, where a substantial improvement in efficiency is stiff reached.Kramers Laboratorium voor Fysische TechnologieApplied Science