Catalysts for Moving-Bed Reactors

Moving beds are prone to the same pressure-drop constraints as fixed beds and require, therefore, particles with diameters greater than about 3 mm. 

Mechanically strong spheres of this size are produced by extrusion, granulation, or drip coagulation. 

As mentioned in Section 9.2.2, spherical bodies can be produced directly by means of granulation, but the mechanical strength of granules strongly depends on the type and amount of binder applied with the spray. 

Simpler dripping methods have been reported where the solidification starts immediately after contact of the drops with the liquid. An example represents freezing of slurry droplets in liquid nitrogen and subsequent freeze-drying and calcination of the bodies [50]. However, such methods produce particles of anisotropic shape, and the size distributions are broad because many droplets break apart upon bouncing against the liquid surface. 

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