PROCESS TECHNOLOGIES FOR PRODUCING BIOFUELS AND COPRODUCTS FROM LIGNOCELLULOSIC FEEDSTOCKS
Location: Bioenergy Research Unit
Title: Prolonged conversion of n-butyrate to n-butanol with Clostridium saccharoperbutylacetonicum in a two-stage continuous culture with in-situ product removal
Submitted to: Biotechnology and Bioengineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 2011
Publication Date: November 17, 2011
Citation: Richter, H., Qureshi, N., Heger, S., Dien, B.S., Cotta, M.A., Angenent, L.T. 2012. Prolonged conversion of n-butyrate to n-butanol with Clostridium saccharoperbutylacetonicum in a two-stage continuous culture with in-situ product removal. Biotechnology and Bioengineering. 109:913-921.
Interpretive Summary: Butanol is an important chemical and biofuel that can be produced from agricultural crops and residues. Production of butanol from agricultural residues requires four stages: i) Pretreatment of the biomass to prepare the biomass for enzymatic digestion, ii) conversion of sugars to butyric acid, iii) transformation of butyric acid into butanol, and iv) butanol recovery. One method for producing butyric acid is to ferment the pretreated biomass using a mixed bacterial culture. However, this requires an efficient process for converting butyric acid into butanol. The aim of the present study was to develop a continuous process for conversion of butyric acid to butanol with online butanol recovery. Several important discoveries were made that allowed for butanol production to be stabilized over longer periods of time. Over a 42-day operating period, butyric acid was transformed to butanol with a 93% conversion efficiency. Successful conversion of lignocellulosic biomass to butanol would benefit the U.S. agricultural community, the biofuel industry, and the transportation industry.
A 2-stage process was described for continuous bioconversion of n-butyrate into n-butanol with planktonic cells of Clostridium saccharoperbutylacetonicum N1-4. Online product removal via gas stripping was integrated within the system. Our work focused on a continuous fermentation system specifically designed to optimize the conversion of n-butyrate into n-butanol. Glucose was used to supply carbon, energy, and reducing equivalents to the fermentation. Metabolic oscillations (i.e., periodical changes of solventogenic activity) were dampened to a great extent by using a 2-stage fermentation approach when compared to a 1-stage fermentation approach. Culture degeneration (i.e., an irreversible loss of solventogenic activity) was avoided by periodical heat shocking and re-inoculation of stage 1 (1/10 volume of stage 2) and by maintaining the concentration of undissociated n-butyric acid in stage 2 at 3.4 mM with a pH-auxostat. On average for a 42-day operating period, the rate of n-butanol production was 0.39 g/(L*h), the rate of n-butyrate consumption was 0.251 g/(L*h), the conversion efficiency for n-butyrate to n-butanol was 93%, the molar yield of n-butanol produced to n-butyrate consumed was 2.0, the molar yield of n-butanol produced to glucose consumed was 0.718, the molar ratio of n-butyrate and glucose consumed was 0.358, and the molar yield of carbon in n-butanol produced to carbon in n-butyrate and glucose consumed was 0.386. These data illustrate that efficient conversion of n-butyrate into n-butanol by solventogenic Clostridia is feasible and that this can be performed in a continuous system for more than a month.