Scale Up of Fluidized Bed Reactors: What Can Be Calculated and What Needs to Be Measured

Freireich, B., Particulate Solid Research, Inc.

Fluidized beds are commonly used for the manufacturing of a wide range of products including syngas, fuels including gasoline, acrylonitrile, polyolefins, and ultra-high purity silicon. These units provide unsurpassed  heat transfer along with  the ability to flow solids during operation.  However, scale-up of these units can be challenging with respect to productivity and reliability.  For fast reactions, the fluidized bed is usually mass-transfer limited and mass-transfer is strongly dependent on bed hydrodynamics.  Thus, scale-up parameters need to include gas compression, bubble growth, solids mixing, and particle cohesive forces all of which need to be obtained on a large-scale.  Many of the available correlations were either obtained on too small of a test unit to be relevant with commercial application, or simply don’t consider parameters that affect to mass transfer.

Thus, understanding key hydrodynamic parameters such as bed density profiles, bubble hydrodynamics, entrainment, pressure loop profiles and jet-penetration lengths, all of which are depend on particle properties, can yield a commercial-scale design that mimics pilot-plant performances with reliability goals similar to other unit operations.  While modeling efforts can provide good approximations of what is and is not important with scale up. Yet, most models are based on “ideal” or “standard” conditions and properties and rarely consider micro-scale effects such as interparticle forces (coulombic, capillary, van der Waals) or other surface interactions (collisional stresses).   Thus, experiments from cold flow to pilot studies are needed to understand controlling hydrodynamic parameters as relevant to the operation conditions of the process and the physical properties of the powder or catalyst.