(297j) Rheology and Mass Transport of Biomass Slurries by Fiber-Level Simulation

Biomass has been considered as a promising alternative sustainable fuel source. To lower capital and operating costs and increase the economic viability of biofuels, the conversion processes need to be conducted at high solids concentration. However, high solids concentrations result in large yield stresses which create challenges for mixing, heating, and transporting the biomass.

Rheological and mass transport properties of biomass slurries at high fiber concentration were studied by fiber-level simulations. The specific viscosity of the suspensions increases with increasing fiber curvature, stiffness, friction between mechanical contacts, and solids concentration. The specific viscosity increases linearly with concentration in the dilute regime, increases with the cube of the concentration in the semi-dilute regime, and approaches a plateau at high concentrations. Concentrated fiber suspensions are highly viscous, and exhibit significant yield stresses and normal stress differences, which can be explained by the formation of fiber networks in the suspensions. Yield stresses scale with volume concentration and fiber aspect ratio in the same way as that observed in experiments.

Shear-induced diffusion in simple shear flow for suspensions of both spheres and fibers was also studied. Two different models for the fibers were employed: one in which each fiber consisted of a flexible series of linked rigid cylinders, and another in which each fiber consisted of a rigid chain of hard spheres. The scaled diffusivity (D/(α²g¢), where D is the diffusivity matrix, α is radius, and g¢ is shear rate) increases for both kinds of suspensions as particle concentration or friction between mechanical contacts are increased, and the scaled diffusivity (D/(ng¢l⁵), where n is number density and l is fiber half length) decreases as the fiber aspect ratio is increased. The rheological properties and shear-induced diffusion in fiber suspensions are controlled by mechanical contacts between fibers, and thus depend on the suspension microstructure. The microstructure, as measured by the fiber orientation distribution function, evolves slowly with time. Steady state is reached only after a sufficient number of fiber collisions, which depends on the fiber and suspension parameters described above.