Sunday, April 29, 2007

Rheological parameter determination for Zymomonas mobilis fermentation broths with high substrate loading

Byung-Hwan Um, Chemical Engineering, University of Maine, 5737 Jenness Hall, University of Maine, Orono, ME 04469-5737 and Thomas R. Hanley, Chemical Engineering, Auburn University, 149 Ross Hall, Auburn, AL 36849.

To obtain the five percent (v/v) alcohol production needed for an economically viable industrial-scale ethanol distillation, high carbohydrate concentrations are required.  This carbohydrate level can be achieved only when a high initial cellulose concentration is combined with a favorable conversion yield of cellulose into soluble sugars.  High substrate concentration in the form of fibrous, solid materials produces a high viscosity broth that is difficult to mix efficiently. 

Accordingly, the determination of rheological properties of concentrated slurries is critical in designing equipment and providing statistically significant full-scale strategies.  Initial efforts in this work focused on characterizing the rheology of highly concentrated fermentation broth during a simultaneous saccharification fermentation process.

The experimental determination of the shear stress-shear rate relationships of the various formulation broths with different concentrations was performed with a variable speed rotational viscometer (2 to 200 RPM).  The effect of temperature (30oC and 50oC) and concentration (10, 15, and 20 per cent total solids) was determined for fermentation times up to four hours.

The fluid behavior of the broth slurries in Zymomonas mobilis ethanol fermentation was modeled using the power-law model, the Herschel-Bulkley model and the Bingham model. The results showed that broth slurries were pseudoplastic with a yield stress.  The model slope increased and the model intercept decreased with increasing fermentation time at shear rates normal for the fermentor. The broth slurries exhibited Newtonian behavior at high and low shear rates during initial SSF process.