(573e) Transition from Pack Bed Flow to Slurry Flow with Biomass Materials

Authors: 
Johanson, K., Material Flow Solutions, Inc.

Biomass handling is a very diverse field.  Often the question of how to handle the biomass solids depends on the final use or process.  Generally, there are two choices.  The first is to handle the biomass as a bulk solid and transport it via mechanical conveyors and store it in hoppers.  The second is to dilute the biomass with liquid and then transport the material using hydraulic conveying.  The choice to hydraulically convey the material often depends on the distance traveled and the solids content of the resulting slurry.  The problem is that biomass transport mode depends on the particle size and the strength of the mass of material in the submerged liquid.  If the particle size is small and the solids concentration is low, the resulting slurry develops rheological consistency and one can treat the flow using non-Newtonian fluid models.  The solid and liquid travel together and do not separate.  However, if the particle size is increased or the solids concentrations are high, then the material behaves as a two-phase system where the flow of solids is driven by the local permeability of the bed.  Here the driving force is the pressure drop due to liquid flow across the bed.  In this case, there will always be a slip velocity between the particles and the fluid.  Transition to this fully packed bed mode of flow is through a series of meso-scale transitions.  However, at some point the flow in a confined geometry will induce pluggages as material transitions from a consistent rheological material to a two-phase compressible system with a permeable solid submerged in a fluid.   If the flow behavior in this transition zone could be explained, then guidance could be given to avoid poor operational regimes for a particular biomass solid.   This paper will compare measured viscosity versus strain rates curves for various particle sizes and solids consistencies, and compute the expected pressure drop across traditional pipe flow assuming a Hershel Buckley fluid approximation as a means of determining the critical pressure drop for very concentrated systems.  This paper will also present measured hydraulic permeability values as a function of the material compressibility.  We will compute the expected pressure drop using typical two-phase flow calculations assuming a permeable solid, and then compare the pressure drop across the pipe with the rheological method.  It is assumed that transition from fully packed bed flow to rheological flow will be a function of the ratio of these two pressure gradients.  The actual transition will go through a meso-scale transition, but the beginning or the end of the transition relates to the balance of these pressure drops.