Multiphase flow

Lignocellulosic biomass is a complex, multicomponent material that can be converted into valuable chemicals and fuels. The material should be processed at high solids concentration to minimize energy and capital costs. At high solids concentration, the material is difficult to mix and transport. An experimental study of pressure driven pipe flow of concentrated corn stover revealed a positive pressure curvature at steady state, and the material was observed to continue to flow for several minutes after the pump was turned off. Occasionally, the velocity at the outlet displayed apparent stick-slip behavior along with fluctuations in the pressure, which was associated with the presence of compositional heterogeneities within the pipe. One- and two-fluid models of lignocellusosic biomass as a compressible viscoplastic material with a variable compressibility and a composition-dependent yield stress were developed and solved numerically. The one-fluid model predicts the positive pressure curvature observed experimentally, using reasonable physical parameters, and also predicts expansion after cessation of flow.

Compressibility and a composition-dependent yield stress are shown to be model features that are required to predict a positive pressure curvature. A variable compressibility is necessary to match the steady-state pressure data when the material is flowing and when the material is not flowing, with a single set of parameters. The model predicts a pressure profile with a positive curvature that becomes increasingly linear at elevated pressures and the compressibility decreases with increasing pressure.

A two-fluid model is used to study the impact of concentration heterogeneities. Here, the biomass solids are treated as a Bingham plastic phase and the water and air are treated as a single Newtonian fluid phase. The two-fluid model retains the features of the one-fluid model and predicts a positive curvature at steady state, and expansion after cessation. The two fluid model also predicts a decrease in pressure when a high solids concentration region is expelled from the pipe. The model is also capable of producing fluctuations in the pressure and velocity when compositional heterogeneities are present, which suggests that apparent stick-slip behavior can arise even when a no-slip boundary condition at the wall is employed.