(783a) Numeric Optimization of Power Consumption for Simultaneous Saccharification and Fermentation of Cellulose to Ethanol

Using second-generation bioethanol, cellulose, the most abundant raw material in nature, can be converted to ethanol. Despite intensive research efforts at laboratory scale, the secondary effects of scale-up to large systems are still largely uncertain. A threedimensional numerical model of a laboratory scale fermenter is used to predict the effects of particulate mixing and reaction kinetics. This allows for case studies or optimizations of the reactor design to be carried out. The potential design variables are manifold ranging from agitation speed to impeller geometry while objectives are conversion rates and power consumption.

A two step reaction kinetics for simultaneous saccharification and fermentation (SSF) of Avicel (microcrystalline cellulose) particles to ethanol was presented by [van Zyl, 2012]. This separation of the two primary cellulase enzyme-kinetics gives the capability to predict the heterogeneous behavior of the enzyme-substrate interactions. This model improves the understanding of these systems while maintaining sufficient simplicity for implementation alongside a commercial computational fluid dynamics environment.

Viscosity is a mayor factor for the designing of a mixing vessel, as viscosity influences both: the shear rates and fluid dynamics of the system. To get the viscosity of the particle-fluid mixture an apparent dynamic viscosity model is used.

Cross-coupling the reaction models with computational fluid dynamic simulations provide a novel approach to capture the secondary effects of substrate conversion and particle distribution on the performance of the fermentation vessel. The first effect is the reduction in the average particle diameter due to the conversion of the cellulose. The lower drag forces of these smaller particles allow â?? in a second effect - for a better distribution in the vessel. It is important to maintain a fully suspended particle distribution to maximize the surface contact with the rest of the mixing fluid. These effects enable a reduced impeller rotation rate at later stages of the conversion decreasing the power consumption.

Difficulties in the modeling arise due to the different time scales in the simulation. The mixing with rotation rates of 150 rpm compared to the fermentation lasting for several hours. An approach to couple these phenomena in one application is presented.

In this contribution we show how to model this analysis efficiently using the commercial CFD software STAR-CCM+by CD-adapco.

References:
van Zyl, Josebus Maree. Three-dimensional modelling of simultaneous saccharification and fermentation of cellulose to ethanol. Diss. Stellenbosch University, 2012.