I have invented a new type of spinning disc reactor!
Performing heterogeneous catalysis in chemical reactors either requires the immobilisation of the solid phase, or a downstream recovery of expensive catalyst. The first option evidently reduces the external mass transfer rate from the bulk of the fluids in the reactor to the catalyst surface, while the second one generally introduces a costly unit operation in the process.
One way of immobilising a heterogeneous catalyst in a rotor-stator spinning disc reactor is by coating the disc according to the procedure described by Meeuwse et al. (2010). In this work, liquid-solid mass transfer coefficients were obtained as high as kLSaLS = 0.25 m3L m–3R s–1. An improvement to this configuration could be made, however, if very small catalyst particles could be freely suspended in the reactor, while simultaneously preventing them from leaving without introducing high pressure drop solutions like filters.
The general idea is shown in the figure above and it is as follows. When a particle-laden flow is pumped through a hollow axis into the reactor, the centrifugal force acts on the heavier particles to move them radially outwards. Inside the hollow rotor assembly the drag force is directed radially inwards, but when the centrifugal force is high enough, we can effectively entrap particles. Now, what normally happens, because of conservation of angular momentum, the radially inwards flowing liquid will have an increasing azimuthal velocity towards the centre of the reactor so that at some point the liquid’s rotation will be larger than that of the disc. This then leads to the formation of centripetal Bödewadt layers that act as a focus to drag in the catalyst particle, completely undermining the idea behind the design. This problem is easily remedied by the introduction of a highly porous reticulated carbon foam inside the rotor assembly. This will prevent the formation of these boundary layers, effectively entrapping the particles, solely by hydrodynamical means!
The contents of this blog are based on the following publication: