The Dos and Don'ts of Fluidized Bed Scale-Up

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.

Unit reliability also needs to be addressed in the design stage of fluidized bed scale-up. Particle-particle collisions and particle wall collisions need to be managed to ensure low particle attrition and limited vessel erosion. Thus, areas of high particle shear such as that from gas jets and cyclone separators need to be understood. The addition of shrouds and vortex stabilizers can extend the lifetime of a unit by years.

Thus, understanding key hydrodynamic parameters such as bed density profiles, bubble hydrodynamics, pressure loop profiles and jet-penetration lengths can yield a commercial-scale design that mimics pilot-plant performances with reliability goals similar to other unit operations. Details of these parameters and how they can be obtained are discussed.