Acetic acid removal from corn stover hydrolysate using ethyl acetate and the impact on Saccharomyces cerevisiae bioethanol fermentation

Abstract

Acetic acid is introduced into cellulose conversion processes as a consequence of composition of lignocellulose feedstocks, causing significant inhibition of adapted, genetically modified and wild-type S. cerevisiae in bioethanol fermentation. While adaptation or modification of yeast may reduce inhibition, the most effective approach is to remove the acetic acid prior to fermentation. This work addresses liquid-liquid extraction of acetic acid from biomass hydrolysate through a pathway that mitigates acetic acid inhibition while avoiding the negative effects of the extractant, which itself may exhibit inhibition. Candidate solvents were selected using simulation results from Aspen Plus™, based on their ability to extract acetic acid which was confirmed by experimentation. All solvents showed varying degrees of toxicity toward yeast, but the relative volatility of ethyl acetate enabled its use as simple vacuum evaporation could reduce small concentrations of aqueous ethyl acetate to minimally inhibitory levels. The toxicity threshold of ethyl acetate, in the presence of acetic acid, was found to be 10 g L(-1) . The fermentation was enhanced by extracting 90% of the acetic acid using ethyl acetate, followed by vacuum evaporation to remove 88% removal of residual ethyl acetate along with 10% of the broth. NRRL Y-1546 yeast was used to demonstrate a 13% increase in concentration, 14% in ethanol specific production rate, and 11% ethanol yield. This study demonstrated that extraction of acetic acid with ethyl acetate followed by evaporative removal of ethyl acetate from the raffinate phase has potential to significantly enhance ethanol fermentation in a corn stover bioethanol facility. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:929-937, 2016.

Techno‐economic analysis for incorporating a liquid–liquid extraction system to remove acetic acid into a proposed commercial scale biorefinery

“…A simple vacuum evaporation was able to recover ethyl acetate . Two flash drums were implemented following the extraction column connected to the two exiting streams from the extraction column.…”

“…In flash drum 2, 99.6 wt.% of ethyl acetate, which was dissolved in the aqueous phase, is evaporated. This decreases the ethyl acetate concentration below its inhibition in the stream that is entering the fermentation reactor; while the vapor is being recycled to the solvent storage tank. These two flash drums collect over 99.9 wt.% of ethyl acetate in each run (3,499,000 kg/h from a total of 350,000 kg/h).…”

“…Therefore 14% of higher ethanol production rate will increase the fermentation broth flow rate by about 14% as well and this will translate to 514,203 kg/h of beer exiting the bioreactor instead of 451,055 kg/h and $20 M/year of excess ethanol (about 12% increase) at 5.4% wt. of ethanol content.…”

“…When ethyl acetate is used as the organic solvent, it can be removed from the fermentation broth and therefore has no lasting toxicity on downstream fermentation. LLE can enhance bioethanol fermentation performance by increasing the reaction rate and final specific ethanol yield by 14% and 11%, respectively …”

“…The recyclability of the extraction solvent promotes sustainability and is encompassed within the overall long‐term goal of embracing second‐generation bioethanol as viable energy source. As a previous study suggested, adopting an LLE system is an efficient technique to remove known fermentation inhibitors regardless of the biomass type and the species of fermenting microorganism. Though exact kinetics of the inhibition of acetic acid and other known inhibitors on the ethanol producing microorganisms requires additional study, the straightforward, linear equations that have been used in these calculations are expandable for use at various flow rates, with different materials of construction, a range of years of operation, and different compositions of feedstocks.…”