461248 Continuous Butanol Extraction Using Supercritical Carbon Dioxide
Butanol has promise as a drop-in biofuel or as a fuel additive that can be blended with gasoline at much higher proportions than ethanol. To date, economical production of butanol has been hampered largely by its cytotoxicity, which becomes limiting at levels as low as several weight percent. In this work, we demonstrate n-butanol extraction from dilute aqueous solutions used to simulate fermentation broths using supercritical carbon dioxide (scCO2). In this talk, we will focus on process engineering of the n-butanol extraction. Our process engineering work is complemented by microbiological and metabolic engineering work that has identified a scCO2-tolerant bacterial strain and genetically modified it for biofuel production. Compared to other butanol recovery approaches, scCO2-extraction has sterilization and potential energy balance advantages; because scCO2 selectively extracts butanol instead of water, a highly concentrated butanol stream can be recovered which requires minimal post-processing purification.
Specifically in this work, we studied n-butanol extraction performance and material/energy balance analysis. For butanol extraction performance, we used batch-wise extraction to study the effects of initial n-butanol concentration, extraction vessel pressure, and scCO2 volumetric flow rate on n-butanol extraction rate. Additionally, we modeled the data using a standard liquid-liquid mass transfer model to determine values for the mass transfer coefficient, kla. Figure 1 shows representative extraction data compared to the model predictions using fit mass transfer coefficients. In all cases, the mass transfer model adequately described the experimental data. Best-fit values of kla did not vary within our estimated limits of uncertainty for variation in extraction pressure (from 10.3 to 13.7 MPa the range over which the scCO2-tolerant bacterial strain has exhibited growth); therefore, we conclude that operation at lower pressures should be favored to achieve better process economy. Similar analysis was performed to interpret the mass transfer coefficient from correlations developed for gas-liquid and liquid-liquid extraction and compared to interfacial area results obtained from scCO2-droplet size analysis. In addition to n-butanol, we investigated extraction of other alcohol products (e.g., iso-butanol, pentanol, and hexanol) as these are potential metabolic targets for a modified scCO2-tolerant organism. Finally, we studied the extraction of the aldehyde intermediate, butanal, to set limits on its accumulation in the fermentation products.
Figure 1. Representative n-butanol extraction data obtained at 10.3 MPa and 40 °C.
In addition to extraction experiments and modeling, we performed preliminary material and energy balance calculations of a conceptual continuous process. Thermodynamic and extraction data were used to model n-butanol removal from the fermenter and fermentation kinetics were modeled based on activity of similar organisms. The Peng-Robinson equation of state was used for modeling thermodynamic properties. To improve energy efficiency, the scCO2 extraction stream was only partially de-pressurized from 10 to 5 MPa, resulting recovery of n-butanol at 79 wt% purity. Energy was recovered during the de-pressurization, which partly offset the energy required for CO2 compression. Analysis was performed assuming the availability of either 10 or 0.1 MPa CO2. Overall, we found that n-butanol with 95% purity could be produced at an energy cost of 3.9 MJ kg-1 (assuming that a 1 bar CO2 source is available) or 3 MJ kg-1 (assuming a 10 MPa CO2 source). These figures compare well with existing methods of butanol recovery, which range from 9 to more than 20 MJ kg-1, and the lower heating value of the butanol fuel itself (33.1 MJ kg-1). Further analysis is needed to account for heat loss and heat integration and to optimize process efficiency.