(594c) Ethanol Recovery From Carbon Dioxide Stripped Ethanol-Water Vapour Mixture Using Adsorption

The heavy reliance on fossil fuels is leading to the rapid depletion of this source of energy since coal, oil and natural gas are essentially non-renewable energy resources. One study predicts the depletion times for oil, coal and natural gas at approximately 35, 107, and 37 years, respectively (Shafiee and Topal, 2009). This has led to extensive research in alternative energy streams and in particular bio-ethanol production as this fuel source is both cleaner and renewable. The most viable avenue for the production of bioethanol is to use lignocellulosic agricultural or forestry residuals as the feedstock. Ethanol produced using lignocellulosic materials, with xylose and glucose being the most abundant fermentable sugars, is limited by the relatively low (typically 4-6 %wt) ethanol tolerance of the microorganism used to convert these sugars to ethanol. One technique to partly alleviate inhibition effects is to remove ethanol as it is produced. Carbon dioxide stripping represents a potential method to remove ethanol with carbon dioxide absorbing ethanol from the fermentation broth. Since carbon dioxide is also produced within the fermentation broth, additional carbon dioxide does not introduce an impurity for ethanol removal. The productivity of today's fuel ethanol fermentor will vary, depending on factors such as the amount of sugars added and the final ethanol concentration, but in most plants it is no more than approximately 2 g l-1 h-1, even in continuous-cascade fermentors. Using gas stripping, the productivity of ethanol has been experimentally shown to increase to as much as 15.8 g l-1 h-1 (Taylor, 1997). With a significant amount of ethanol in the vapour phase through carbon dioxide stripping, finding an efficient means for ethanol recovery is paramount to the successful stripping operation. Adsorption appears to be a viable option and is the focus of this study.

A study on the adsorption potential for carbon dioxide stripped ethanol-water vapour mixture is analyzed. Literature review showed that activated carbon and hydrophobic zeolites would be the most appropriate adsorbents for this application. A series of adsorption screening experiments were performed to compare four activated carbon adsorbents (Filtrasorb 200, Nuchar RGC 40, Sorbonorit B4, WV-B 1500) and two hydrophobic ZSM-5 type zeolites (HiSiv 3000, CBV 8014). The vapour composition used for the comparison simulated the vapour outlet fermentor concentration after stripping with carbon dioxide. Activated WV-B 1500 provided the highest ethanol capacity with all four activated carbons having higher capacities than the two zeolites.

Multi-temperature isotherms were conducted using WV-B 1500 for ethanol and water in the presence of carbon dioxide with both the Langmuir and Freundlich isotherms providing satisfactory fits. Ethanol adsorption experiments with and without the presence of water were conducted and showed similar ethanol capacities. Therefore these results show the presence of water has negligible effects for ethanol adsorption.

Modelling of the system was followed by preliminary economic analysis to study the economic viability of the system.

References:

Shafiee, S., Topal, E., ?When will fossil fuel reserves be diminished?? Energy Policy, 37, 181 (2009)

Taylor F., Kurantz, M. J., Goldberg, N., Craig Jr., J. C., ?Effects of ethanol concentration and stripping temperature on continuous fermentation rate,? Applied Microbiology and Biotechnology, 48, 311 (1997)