Date on Master's Thesis/Doctoral Dissertation

5-2008

Document Type

Master's Thesis

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Berson, Robert E. (Eric)

Author's Keywords

High-solids saccharification; Computational fluid dynamics; Rheology of biomass slurries; Scale-up; Power consumption; Semibatch processing; Viscous slurries; Biomass

Subject

Corn syrup--Research; Corn--processing

Abstract

High solids processing of biomass slurries provides the following benefits: maximized product concentration in the fermentable sugar stream, reduced water usage, and reduced reactor size. However, high solids processing poses mixing and heat transfer problems above about 15% for pretreated corn stover solids due to their high viscosities. Also, highly viscous slurries require high power consumption in conventional stirred tanks since they must be run at high rotational speeds to maintain proper mixing. An 8 liter scraped surface bio-reactor (SSBR) is employed here that is designed to efficiently handle high solids loadings for enzymatic saccharification of pretreated corn stover (PCS) while maintaining power requirements on the order of low viscous liquids in conventional stirred tanks. The determination of the rheological behavior of biomass slurries is vital for process design at industrial scale. The viscosities of biomass slurries are seen here to be a function of initial solids concentration and initial biomass particle size. An extensive study has been conducted to investigate the effect of solids loading and viscosity on the rates and extent of enzymatic hydrolysis reactions. For batch testing with 25% (highest loading studied) initial PCS solids concentration, about 10% more glucose is released in the SSBR than in the shake flask after 168 hours of the saccharification reaction. The role of the viscosity of biomass slurries in power consumption of the reactor is presented. A semi-batch approach is employed to maintain lower slurry viscosity and, therefore, improved glucose release rates and reduced power consumption when operating with higher solids content. A processing efficiency is defined as sugar released per unit energy input. The 20% semi-batch saccharification test efficiency is about 27% higher than the 20% batch saccharification test efficiency. The settling of biomass particles presents a serious problem for measuring the viscosity of the slurries. Maintaining homogeneity by uniformly suspending all the particles is necessary for accurate viscosity measurements. Therefore, a new viscosity measuring technique has been developed here that incorporates the uniform suspension speed (USS) for particles in the viscometer cup that can be applied to any type of particulate suspension. The USS has been determined experimentally and computationally by a Computational Fluid Dynamics (CFD) model developed here that is well validated by experimental results. The wet density of PCS solids, which is not reported in the literature, is determined from the CFD model to be 1100 ± 50 kg/m3 based on the volume fraction distribution of solids at 305 rpm, the USS of a 5% solids slurry.

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