Modelagem e simulação da fermentação alcoólica contínua extrativa. Verificação da influência dos subprodutos inibidos.

Abstract

In view of the ethanol being one of the most promising fuel alternatives obtained by renewable sources, the alcoholic fermentation has been intensively investigated by researchers all over the world. The increasing search for ethanol in the 70s and 80s, led the researchers and the companies linked to such sector to find high productivity processes. With the consolidation of fermentation process at the end of 80's decade, new processes were developed which can remove ethanol from fermented stock to eliminate ethanol inhibition effect and hence increasing the yield and productivity of the process. The vacuum flash separation connected to a fermenter, show many advantages such as: less productivity costs, practical facility concerning the whole process operation being that of the system which best connects to the Brazilian industries. It also eliminates the heat exchanging need that reduces the fixed costs and the possibility of using high sugar concentration in the reactor thus reducing the distillation costs. Such processes can increase the by-products concentration so that they can become toxic to the yeast. The vacuum fermentation process concentrates non-volatile components in the fermented stock. The aim of the present work was to study the influence of higher alcohols (1-propanol nbutanol, iso-butanol and iso-amyl alcohol) on the inhibition of process, finding the ways to minimize this inhibition, controlling the concentration of ethanol and by-products in reactor below their inhibiting levels, so that to increase the yield and productivity of the extractive alcoholic fermentation system, using vacuum flash tank as a separation unit of produced ethanol. The plant was modeled on the basic of mass and energy balance equations and kinetic model applying the fourth order resolution method of Runge-Kutta and computer simulation. The optimization was carried out using factorial design and response surface analysis method. The variables of optimization process studied were: initial sugar concentration (So), initial cell concentration (Xo), residence time (tr), reactor temperature (To), water temperature (Tw), cell recycle rate (R) and flash tank liquid recycle rate (r). With the help of experimental planning, it was verified that the obtained quadratic mode! for yield and productivity had a statistical significance of 99% confidence. The values that optimize the yield and the productivity were: So = 180 g/1, R = 0.3, tr = 1.4 h and r = 0.4, having yield of 82.55% and productivity of 21.49 g/!.h. These response variable values when compared with the SILVA (1998), were less by 1% for yield and 0.1 g/l.h for productivity. Operating in temperatures conditions of northeast regions and comparing with those of southeast regions, it was verified that lass 5% in yield and 2 g/l.h in productivity, where all the comparison made were within the range of variables studied in the present work.