3.2. ABE production in continuous fermentation–PV coupled process
As demonstrated in Fig. 1, the shortage of substrate in the batch fermentation with PV was the main reason leading to the halt of butanol production. Concurrently, ABE concentration in the permeate solution decreased over time as ABE concentration decreased in the bioreactor (see Fig. 2a). A continuous operation with liquefied cassava feeding was conducted to explore the ability of continuous ABE fermentation with in situ product removal by PV. Fig. 3a shows
the profiles of ABE production using liquefied cassava in continuous fermentation–PV coupled process for 320 h. In the continuous operation, the glucose concentration in the reactor was regulated at around 20–30 g/L by continuously introducing concentrate liquefied cassava with additional nutrients. As can be seen from Fig. 3a, the coupled process gave relatively stable ABE production. It should be noted that the butanol concentration in the broth was below 5 g/L throughout the fermentation to ensure the toxicity of butanol to microorganism was negligible. The average concentrations
of acetic and butyric acids in the fermentation broth were found to be 1.16 g/L and 1.18 g/L, respectively, which were lower than those in control fermentation, while the number of viable cells in late fermentation phase was up to 200 ± 13 105 CFU/ mL. These results indicated that the pervaporation separation was effective in removing butanol from the fermentation broth. In the continuous fermentation, total ABE production was estimated based on the amounts of ABE present in the fermentation broth and permeate solution. 244.0 g ABE was produced from 736.0 g cassava (equivalent to 647.7 g glucose). The average ABE yields from cassava were 0.38 g/g, which was 15% higher than that in batch experiment. Similar improvements in butanol and ABE yields were also reported by Hecke et al. (2012). This could be
attributed to the alleviation of product inhibition by pervaporation. The ABE productivity and the average glucose consumption rate were 0.76 and 2.02 g/L h, respectively, which were 81% and 58% higher than those in batch fermentation without PV (see Table 1).
3.2. ABE production in continuous fermentation–PV coupled process
As demonstrated in Fig. 1, the shortage of substrate in the batch fermentation with PV was the main reason leading to the halt of butanol production. Concurrently, ABE concentration in the permeate solution decreased over time as ABE concentration decreased in the bioreactor (see Fig. 2a). A continuous operation with liquefied cassava feeding was conducted to explore the ability of continuous ABE fermentation with in situ product removal by PV. Fig. 3a shows
the profiles of ABE production using liquefied cassava in continuous fermentation–PV coupled process for 320 h. In the continuous operation, the glucose concentration in the reactor was regulated at around 20–30 g/L by continuously introducing concentrate liquefied cassava with additional nutrients. As can be seen from Fig. 3a, the coupled process gave relatively stable ABE production. It should be noted that the butanol concentration in the broth was below 5 g/L throughout the fermentation to ensure the toxicity of butanol to microorganism was negligible. The average concentrations
of acetic and butyric acids in the fermentation broth were found to be 1.16 g/L and 1.18 g/L, respectively, which were lower than those in control fermentation, while the number of viable cells in late fermentation phase was up to 200 ± 13 105 CFU/ mL. These results indicated that the pervaporation separation was effective in removing butanol from the fermentation broth. In the continuous fermentation, total ABE production was estimated based on the amounts of ABE present in the fermentation broth and permeate solution. 244.0 g ABE was produced from 736.0 g cassava (equivalent to 647.7 g glucose). The average ABE yields from cassava were 0.38 g/g, which was 15% higher than that in batch experiment. Similar improvements in butanol and ABE yields were also reported by Hecke et al. (2012). This could be
attributed to the alleviation of product inhibition by pervaporation. The ABE productivity and the average glucose consumption rate were 0.76 and 2.02 g/L h, respectively, which were 81% and 58% higher than those in batch fermentation without PV (see Table 1).
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