Anaerobic gas-solid fluidized bed fermenter

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... 

Figure 6.6 Schematic diagram of an anaerobic gas-solid fluidized bed fermenter for ethanol production. Reproduced from Hayes (1998) with permission.

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... 

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). 

The term three-phase fluidization, in this chapter, is taken as a system consisting of a gas, liquid, and solid phase, wherein the solid phase is in a non-stationary state, and includes three-phase slurry bubble columns, three-phase fluidized beds, and three-phase flotation columns, but excludes three-phase fixed bed systems. The individual phases in three-phase fluidization systems can be reactants, products, catalysts, or inert. For example, in the hydrotreating of light gas oils, the solid phase is catalyst, and the liquid and gas phases are either reactants or products in the bleaching of paper pulp, the solid phase is both reactant and product, and the gas phase is a reactant while the liquid phase is inert in anaerobic fermentation, the gas phase results from the biological activity, the liquid phase is product, and the solid is either a biological carrier or the microorganism itself. 

If the fermentation contains sohds, then special precautions may be required to prevent sedimentation. One is a fluidized bed, where a dense slurry phase in the bottom of the tank is kept in gentle motion via upflow of the hquid or some induced gas/liquid momentum. The upflow anaerobic sludge bed (UASB) reactor is an example. In biogas installations, sohds may sediment and bacterial consortia may take several months to convert ah the material into methane and CO2. Because of the slow reaction rates, these instahations can be extremely large, up to 20 000 m. Other devices are packed-bed bioreactors, where ceUs are immobihzed on a solid support and the liquid is percolated or trickled down to supply nutrients and remove products. These can be found in specific antibiotic or food fermentations. Because of the lack of free fluid flow, the performance characteristics of such vessels are usually poor compared to suspensions. 

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