Otimização dinâmica da fermentação alcoólica no processo embatelada alimentada

Abstract

This work presents an experimental and theoretical study of the dynamic optimization of the alcoholic fermentation in fed-batch mode utilizing the Saccharomyces cerevisiae yeast in a bioreactor of 5 L. Twelve experiments were carried out, three them in batch and nine in fed-batch mode, with initial sucrose concentrations between 154 and 195.6 g/L for the experiments in batch and between 217 and 285 g/L for the experiments in fed-batch. The filling times for the bioreactor varied from 3 to 5 hours. The inoculum volume corresponded to 30% of the bioreactor volume with alcohol concentration like 4%, typical of the industrial processes. The results about the influence of the filling time and of the fed sucrose concentration about the substrate, product and cells concentration profiles and about the yields and productivities in the fed-batch fermentation are presented. The estimation of the parameters of the non structured kinetic model proposed by Tosetto (2002, Dissertation in Chemical Engineering Campinas Estadual University, Campinas-SP. 82p), using the experimental results of the batch and fed-batch fermentation, was obtained through the algorithm of differential evolution by Storn and Price (1995, available in http://www.ICSI. Berkeley.edu/~storn/code.html, access: november 13 2007). The analysis of parametric sensitivity and the estimate of the rank correlation between the parameters through the sensitivity matrix of the models were realized through the code DDASPK (Petzold et al., 2000, Code DDASPK. Copyright. University of Califórnia, 1-206) and by using the methodology proposed by Sanz e Voss (2006, Advances in Water Resources, 29, 439-457) respectively. The kinetic model proposed represented satisfactorily the fermentation in fed-batch despite of the high correlation between the parameters, and the Dynamic Optimization Problem was formulated for a specific experimental case that resulted in the biggest ethanol production (fed substract concentration = 285 g/L and filling time = 5 h). The simulated optimal profile of the fed substract rate that maximizes the ethanol production, subject to constraints in the volume, in the feed rate and in the yield, was obtained by the methodology proposed by Lobato et al. (2006, XXII IACChE (CIIQ) 2006 / V CAIQ AAIQ Asociación Argentina de Ingenieros Químicos IACChe - Interamerican Confederation of Chemical Engineering), which utilizes the Switching Function to define the sequence of singular and non singular arcs and the switching times. The optimal profile is formed by the following sequence: minimum feed rate-singular feed rate-maximum feed rate, with time of fermentation of 5.11 h, defined on the basis of the analysis of the operational conditions that favored the cellular growth in the initial phase and the formation of product in the final phase, as indicated by Modak et al. (1986, Biotechnol. Bioeng., 28, 1396-1407 ).