Techno-economic analysis of CO2 fermentation to acetic acid

Acetic acid is one of the most widely used carboxylic acid, with applications in textile, fibre, pharma and food industries. Nowadays, the majority of this chemical is produced via energy-intensive and fossil fuel-based processes, contributing to greenhouse gas emissions and environmental pollution. As a consequence, interest in the production of bio-acetic acid is growing. A promising route could be gas fermentation using CO2 and H2 as gaseous substrates. This path could potentially overcome the limits associated to sugar fermentation, i.e. the competition with food production and deforestation associated with first-generation biomass and the expensive pretreatment linked to second-generation one. Although CO2 fermentation is already practiced at an industrial scale for ethanol production, to the best of our knowledge scientific literature lacks any techno-economic analyses focused on the manufacture of acetic acid through the same approach. Therefore, the aim of this work is to present a techno-economic analysis for the production process of acetic acid via fermentation, using captured CO2 from a waste stream and green hydrogen as substrates. The process was simulated on Aspen Plus® and is aimed at the production of 37 kton.y-1 of glacial acetic acid (99.9 wt%). The fermentation section is preceded by an upstream phase to obtain the reactants at the required purity and is followed by a downstream step to concentrate the very diluted culture broth until glacial acetic acid is obtained. H2 is produced with alkaline water electrolysis (AEL), whereas CO2 is captured with chemical absorption with mono ethanolamine (MEA). In particular, CO2 derives from the upgrade process of biogas into bio-methane of an anaerobic biodigester starting from OFMSW (Organic Fraction of Municipal Solid Waste). The fermentation step has been simulated based on the results of a study in which the fermenter was simulated as a bubble column. The purification of acetic acid occurs with a hybrid process, which combines liquid-liquid extraction and azeotropic distillation using methyl tert-butyl ether (MTBE) as solvent. To maximize energy recovery minimizing utilities consumption, an energy integration was conducted using Aspen Energy Analyzer (HX-NET). Subsequently, an economic analysis was carried out using the Net Present Value (NPV) method to determine a minimum selling price for acetic acid that would cover all investment costs during the plant-life. The estimated price is more than twice the actual market price of glacial acetic acid. Although less economical than traditional methods, gas fermentation offers a sustainable alternative reducing fossil fuels consumption and utilizing CO2 otherwise emitted into the atmosphere.