(602a) Axial Dispersion in a Horizontally Rotating Scraped Surface Bioreactor

Horizontally rotating bioreactors offer many advantages for high-solids enzymatic saccharification.  By operating horizontally: (1) particle settling and dead zone formation are avoided by horizontal rotation of the blades, which scrape against the surface of the reactor. (2) Impellers in conventional stirred tank reactors need to be run above the just suspended speed to maintain an off-bottom/uniform suspension, which demands significant power. The horizontal scraping action of the impellers in the SSBR requires very low rotation rates and, hence, much less power while more effectively preventing particle settling. (3) The scraping action of the blades maintains a clear reactor surface and, thereby, improves heat transfer. 

Despite these advantages, all the transport motion occurs only in the angular direction.  Multi-directional transport is important for dispersion of enzyme and carrying away product from the reaction sites.  The purpose of this study was to investigate the degree of axial dispersion, if any, in a horizontally rotating scraped surface bioreactor (SSBR) during high solids enzymatic saccharification.

Computational fluid dynamic modeling qualitatively showed clear evidence of axial motion within the slurries. Axial dispersion coefficients were then quantified as a function of the solids concentrations and fluid viscosity.  The dispersion coefficients were obtained experimentally by measuring the transient concentration of an injected electrolytic tracer and then fitting the concentration/time data to an axial dispersion model.  Axial dispersion was strongly dependent on viscosity and there were order of magnitude differences between coefficient values for water and biomass slurries approaching 20% solids content.  The degree of axial transport was still significant enough to consider horizontally rotating reactors as a viable option for processing high solids biomass slurries.