A Semi-Empirical Approach to Estimate Kinetic Parameters for the Batch and Continuous Settling of Flocculating Yeast Slurries

Increased productivity is one of the most important objectives in continuous production of ethanol. It is generally accepted that a high concentration of yeast favors an increased ethanol yield of the fermentation process . Several techniques that promote the accumulation ofbiomass in fermenters have been used in continuous fermentation processes, including cell immobilization in polymeric matrices and cell recycle by centrifugation or filtration through membranes. Good performances have been reported for processes using immobilized cells; however, procedures and materials involved in this technique are inaccessible to a large number of small industrial units . Centrifugation and filtration equipments are extremely expensive and prone to mechanical problems; the use of these equipments is limited due to high investment and maintenance costs. Since yeast separation by conventional methods is an expensive operation in alcoholic fermentation plants, alternative separation techniques using cheaper equipment such as hydrocyclones and settlers have been investigated aiming to reduce the costs involved in ethanol production.

Although solid particles can settle under the action of a centrifugal force, gravitational sedimentation is understood as the settling of particles within a fluid under the force of gravity. The factors that controlthe settling velocity are the solids concentration in the slurry, the densities of solid and liquid, the size and shape of the particles and the viscosity of the medium. However, the most important factors are the particle size and particle density.