Anaerobic ethanol production

The production of ethanol from cooked rice starch (Moebus and Teuber, 1985) differs from the normal process of spraying the carbon source into the bed since all of the carbon source is made available at the start of the run, subject only to the breakdown of starch to glucose (and maltose) by amylases. The starch (0.3 mm particles), amylases and yeast pellets were mixed in the bed and water sprayed in to maintain the fermentative activity of the yeast. The fermentation was carried out at 31.5°C. 

Bauer and co-workers (Rottenbacher, 1985 Rottenbacher et ah, 1987) carried out anaerobic fermentation under nitrogen gas at 18-20°C in a gas-tight closed circuit with a partial condenser followed by a gas adsorber for ethanol and water vapour recovery. The highest rate of ethanol production was about 50gh with a bed of 2.4 kg of extruded cylindrical baker s yeast pellets of diamefer 800 rm. This rate is less 

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Table 6.3 The composition of glucose-based feed solutions used for anaerobic ethanol production (Hayes, 1998).

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 6.3 shows the time course of ethanol production as a function of feed composition (Table 6.3). The best feed solution was feed D which permitted the bed to run for eight hours without quenching and resulted in a higher ethanol yield (0.46) than feed A (0.35) which contained only glucose. Potassium and magnesium ions are known to stimulate fermentation (Jones and Greenfield, 1984 Maynard, 1993). Feed C caused the bed to quench after only two hours because, under anaerobic conditions, the yeast extract supplied was unable to be assimilated sufficiently quickly and the remainder was therefore... 

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

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... 

Hayes, W.A., Ethanol production from glucose by Saccharomyces cerevisiae in an anaerobic gas-solid fluidised bed fermenter, PhD thesis. University of Lincolnshire and Humberside, 1998. 

Fermentation is the anaerobic decomposition of organic compounds, basically carbohydrate, into alcohols by the action of bacterial enzymes. Ethanol is the desired product of carbohydrate fermentation, but other alcohols, acids, ketones, and aldehydes can form as by-products. One common example of ethanol production from biomass is the fermentation of com. For illustration, this process will be summarized. 

Measuring the C02 evolution, CER, is an excellent way to monitor the ethanol production since the dominant source of C02 production in anaerobic fermentations is directly linked to ethanol formation (13). One of the main goals of the present project was to maximize ethanol production. The feed rate, F(t), which maximizes the overall ethanol production, 1(F) (Eq. 1), for a given time, t, should therefore be found (16) ... 

The NREL Pichia strain NPw9 was equivalent to the D5A yeast in ethanol production however, it did not utilize xylose under the anaerobic conditions used in the screening experiments (data not shown). However, the Pichia strain NPw9 has been shown to ferment xylose in toxic pretreated softwood hydrolysates under microaerophilic conditions (23). 

Ethanol Production in Yeast When grown anaerobically on glucose, yeast (S. cerevisiae) converts pyruvate to acetaldehyde, then reduces acetaldehyde to ethanol using electrons from NADH. Write the equation for the second reaction, and calculate its equilibrium constant at 25 °C, given the standard reduction potentials in Table 13-7. 

Many strains over 200 were isolated from seawater samples and many of them showed adhesive growth to culture flasks and/or flocculated growth. More than 10 strains were tested to examine algal productivity, starch content and ethanol production. Table 1 shows some strains having a productivity of ca. 30 g/m2 d, accumulated a starch more than 30 % (vs dry weight), but had a variety of starch conversion rate to ethanol. One of the excellent strains, Chlamydomonas sp. YA-SH-1, which was isolated from the Red Sea showed (1) a growth rate of 30 g-dry biomass/m d, (2) a starch content of 30 %(dry base), and (3) a conversion rate from starch to ethanol of 50 % in the dark and anaerobic condition. 

Figure 1 shows the time course of the ethanol production in the dark and anaerobic algal slurry. It was evident that the ethanol concentration increased during intial 44 hours and were remained constant thereafter. 

Figure 1. The time course of the ethanol production by Chlamydomonas sp. YA-SH-1 strain under dark and anaerobic conditions.

Although CO2 is inhibitory to microbes, compressed hydrocarbon solvents may be appropriate for extractive bioconversions and extractions in biphasic (aqueous-compressed solvent) systems. Our laboratory investigated the metabolic activity of the anaerobic, thermophilic bacteria Clostridium ther-mocellum as a model system (45). Thermophilic bacteria have a distinct advantage over conventional yeasts for ethanol production in their ability to use a variety of inexpensive biomass feedstocks. Extractive fermentation using compressed solvents is an approach to address the end-product toxicity of these bacteria to ethanol and improve the economic viability of biofuel production by thermophilic organisms. 

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