Gas-Solid Fluidized Bed Fermentation

For ethanol production a substantial improvement over the packed bed reactor is offered by the gas-solid fluidized bed fermenter which uses a liquid instead of a solid substrate and replaces the packed bed of substrate or micro-organisms with a fluidized bed of yeast pellets (Smith et al, 1997). This system can be viewed as a standard fermentation against which the performance of different bioreactors may be assessed. 

A theoretical model (Beck and Bauer, 1989), based on ethanol inhibition alone as the limiting factor in gas-solid fluidized bed fermenters run with recirculating inert gas, suggested that the potential of this technique has not been fully explored. Hayes (1998) suggested significant improvements to the model and provided experimental confirmation of its validity. 

A summary of the factors which are known to influence ethanol production from glucose in a gas-solid fluidized bed fermenter, or which may have an influence based on observations with submerged fermentations, is shown in Figure 6.1. In anaerobic beds, the key factors are the fermentation temperature and ethanol inhibition, both of which have a dramatic effect on the specific rafe of ethanol production. Bed dehydration and its influence on yeast pellet moisture content is also important, since a failure of fermentation may occur if the pellets become too dry (Bauer, 1986). 

Figure 6.1 Factors affecting ethanol production from glucose using baker s yeast (S. cerevisiae) in a gas-solid fluidized bed fermenter. Reproduced from Hayes (1998) with permission.

The two inert gases used in anaerobic gas-solid fluidized bed fermenters are carbon dioxide (Moebus and Teuber, 1982a) and nitrogen (Rottenbacher, 1985). The former found that the addition of air at the beginning of the run substantially increased carbon dioxide production compared to a bed run under pure carbon dioxide. An identical bed operated under strict anaerobiosis appeared to ferment glucose... 

Figure 6.2 is a schematic diagram of the anaerobic gas-solid fluidized bed fermenter used by Hayes and co-workers (Hayes, 1998 Smith... 

The methods most generally used for the calculation of activity coefficients at intermediate pressures are the Wilson (1964) and UNIQUAC (Abrams and Prausnitz, 1975) equations. Wilson s equation was used by Sato et al. (1985) to predict the composition of fhe condensate gas stripped from a packed bed fermenter at 30°C, whilst Beck and Bauer (1989) used the UNIQUAC equation, with temperature-dependent parameters given by Kolbe and Gmehling (1985), for their model of an anaerobic gas-solid fluidized bed fermenter at 36°C. In this case it was necessary to go beyond the temperature range of fhe source data down to 16°C in order to predict the composition of the fluidizing gas leaving the condenser. 

The model of Beck and Bauer (1989) predicts the ethanol productivity, and the ethanol concentrations in the bed and the condensate, assuming equilibrium conditions in an anaerobic gas-solid fluidized bed fermenter using a partial condenser (see Figure 6.6). This model does not predict the build-up of ethanol in the bed nor the increase in the rate of ethanol production at the partial condenser. Rather, it is assumed that this start-up phase is already complete, and that the ethanol concentration in the bed and the rate of ethanol production at the partial... 

Bauer, W. and Rottenbacher, L., Regelung der Substratzufuhr eines Gas/ Festhoff-Wirbelschichtfermenters iiber die Bestimmung des Fluidisations-verhaltens [Gontrol of material feed into a gas/solids fluidized bed fermenter on the basis of fluidization behaviour], Int. Z. Lebensmittel Tech. Verfahrenstechnik, 35 (1984) 18-23. 

Beck, M. and Bauer, W., Energy balance of ethanol production with a gas-solid fluidized bed fermenter, Bioproc. Eng., 4 (1989) 123-128. 

Moebus, O. and Teuber, M., Direkte Umwandlung von Starke in Ethanol in einem Gas-Festhoff-Wirbelschichtfermenter mit technischen Amylasen und Backerhefe [Direct conversion of starch into ethanol in a gas-solid fluidized bed fermenter with technical amylases and baker s yeast], Wissenschaft und Umwelt, 1 (1985) 80-84. 

Development and modelling of a gas/solid fluidized bed fermenter for the production of ethanol with S. cerevisiae. Dissertation, Technical University of Hamburg-Harburg, 1985. 

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