Design, Simulation, and Techno-Economic Evaluation of a Novel Process for Ethanol Recovery from Fermentation Broths

Abstract

Addressing rising energy needs, particularly in industry, transportation, and heating, while simultaneously decreasing the rate of greenhouse gas emissions, is a critical challenge facing society nowadays. Liquid biofuels produced from renewable sources, such as ethanol, have received increased research attention because they show long-term viability potential and fit the current transportation infrastructure. Ethanol is produced by the anaerobic fermentation of sugars obtained from renewable biomass. Conventionally, ethanol is recovered from the aqueous fermentation broth using at least two distillation steps combined with a dehydration step. However, this configuration requires large amounts of energy and is particularly inefficient, mainly due to the low ethanol concentrations achieved in the fermentation broth, and the formation of an ethanol-water azeotrope. In this thesis, a new extraction-pervaporation system was proposed and investigated for use in ethanol recovery. In this process, ethanol is extracted from the fermentation broth using organic solvents and recovered from this mixture by pervaporation. It was envisioned that this new process configuration would lead to improved energy efficiency and economics for the ethanol production process. The proposed process was investigated using numerical and experimental techniques. New polymeric membranes were developed and were shown to be effective in separating ethanol/2-ethylhexanol and ethanol/pentanol mixtures. The permeates were obtained at similar or higher concentration than that achievable in enriching distillation columns. The pervaporation mass transfer process was modelled accounting for the effect of concentration polarization and for concentration-dependent diffusion in the membrane layer, and results showed that, under the conditions studied, the mass transfer resistance in the membrane controls the pervaporation process. It was also shown that higher temperatures and feed concentrations result in higher fluxes through the membrane. A pervaporation numerical model incorporating a membrane permeability model based on the results achieved in previous sections was developed. Ethanol extraction from the broth was also modelled, as well as the fermentation process. A techno-economic analysis was carried out and results indicated that, although the current economic performance is lacking, the extraction-pervaporation process is more energetically efficient than the distillation process, achieving similar ethanol concentrations while requiring less than half of the amount of energy input.