Separation of butanol fromin acetone-butanol-ethanol fermentation by a hybrid distillation-extraction process combining extraction and distillation

Abstract:
Butanol has been identified as a possible fuel from renewable resources. Its combustion and physical properties would allow the introduction to the market with almost no changes in the infrastructure or combustion engine. The major advantages of butanol over ethanol as bio-fuel are the higher energy content by mass and volume and that it is not hygroscopic, such that pipeline transport is possible. Like ethanol, butanol can be produced by fermentation from biomass. The process typically uses the bacterium clostridium acetobutylicum to facilitate the conversion of glucose to butanol. The side products of the fermentation are ethanol and acetone. Product concentrations are fairly low due to the inhibition of clostridium acetobutylicum by acetone and butanol. Typical product concentrations are about 12g of butanol, 6g of acetone and 2g of ethanol per liter of fermentation broth1. Although biotechnological production of butanol as a solvent has been industrial standard until the 1950s, the Hhigh substrate costs and the costs for the separation of butanol from theis dilute fermentation broth in the downstream processing have so far prohibited the industrial-scale production of bio-butanol as a fuel1. Due to the large content of water, conventional distillation processes are particularly expensive. As a consequence, alternative separation technologies and hybrid processes have been investigated. These include gas stripping, pervaporation, adsorption and extraction2,3. In this work, we propose a process flowsheet for the separation of butanol from fermentation broth by a combination of liquid-liquid extraction and distillation. The butanol and the by-products are continuously removed from the fermentation in order to prevent intoxication of the microorganisms. The products are then separated from the water in an extraction column. Here, we study the use of novel and highly selective solvents, which are generated and tested via computer- aided molecular design (CAMD). The separation of the solutes from the significantly higher boiling solvent can then be efficiently performed by distillation. The whole downstream process including the separation of the butanol from the by-products is modeled with the help of shortcut methods for extraction and distillation. The algorithmic shortcut methods allow a robust and efficient mathematical optimization of the process recycles and the operating points of the individual units. The performance of the solvents identified by CAMD will be tested and compared to known solvents from the literature4. In addition, various flowsheet alternatives and recycle policies are studied. The optimal flowsheet is benchmarked against alternative distillation and hybrid downstream processes. It will be shown that the proposed hybrid process combining extraction and distillation compares favorably to these alternative processes. 1. Dürre, P., Fermentative butanol production - bulk chemical and biofuel, Ann. N.Y. Acad. Sci., 1125, 353-362, 2008. 2. Qureshi, N., Ezeji, T. C., Butanol, ‘a superior biofuel’ production from agricultural residues (renewable biomass): recent progress in technology, Biofuels, Bioprod. Bioref., 2, 319-330, 2008. 3. Qureshi, N., Hughes, S., Maddox, I. S., Cotta, M. A., Energy-efficient recovery of butanol from model solutions and fermentation broth by adsorption, Bioprocess. Biosyst. Eng., 27, 215-222, 2005. 4. Roffler, S. R., Blanch, H. W., Wilke, C. R., In-situ recovery of butanol during fermentation. Part 1: Batch extractive fermentation, Bioprocess Eng., 2, 1-12, 1987.

Keywords:
Biofuels, butanol, solvent, separation

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