Challenges of cellulosic ethanol mixing design & optimization

Cellulosic ethanol poses unique challenges from a mixing perspective. High solids cellulosic slurries (20-40 wt. % solids) are not liquid, but more like damp solids. They cannot be mixed with equipment designed to agitate liquids, so solids mixers must be used. Unfortunately, such mixers are very expensive on a volumetric basis, and are therefore not practical to use for the complete saccharification or fermentation reactions. Usually, they are used only for adding enzyme to the unhydrolyzed cake.

Fortunately, one can take advantage of the fact that the saccharification process ultimately converts the stiff, fibrous structure into soluble sugars, leading the final solution to be a low viscosity liquid.

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The rheology starts out as Herschel Bulkley (shear thinning with a yield stress), then power law, and sometimes ends up nearly Newtonian.

Practical reaction schemes include fed-batch, in which the mixed fluid properties are sufficiently liquid to allow the use of turbine agitators, and continuous flow, in which the first reactor is a backmixed design with sufficient retention time to allow enough hydrolysis to permit use of a turbine agitator. (Benz Technology International does not represent any agitator manufacturer.)

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Each feedstock, solids concentration, pre-treatment method and enzyme dosing level results in different fluid properties as a function of time and conversion. Normal fluid viscometers are unable to accurately characterize the rheology, so “wet” testing with turbine agitators is needed both to determine rheology for mixer design and to scale-up the agitation needed for avoiding dead spots.

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