With the depletion of fossil fuels, research in alternative energy sources that can provide the world's energy demand has increased significantly during the past decade. One alternative energy source in particular that has gained worldwide recognition as a potential replacement to oil is bioethanol. It is renewable and environmentally friendlier than fossil fuels. In this study, adsorption is used to increase the efficiency of ethanol production by decreasing the effect of product inhibition using carbon dioxide stripping technology. The reduction in product inhibition is particularly important when ethanol is produced from lignocellulosic biomass because microorganisms that are able to use all fermentable sugars are less tolerant to ethanol. Carbon dioxide removes ethanol from the fermentation broth and reduces the level of ethanol toxicity, while adsorption is used to recover the entrained ethanol from the vapor phase. The literature review showed that activated carbon and hydrophobic zeolites would be the most appropriate adsorbents for ethanol recovery in the vapor phase. A series of adsorption screening experiments were performed to compare four activated carbon adsorbents (Filtrasorb 200, Nuchar RGC 40, Sorbonorit B4, and WV-B 1500) and two hydrophobic ZSM-5-type zeolites (HiSiv 3000 and CBV 8014). The vapor composition used was controlled to resemble the fermenter outlet vapor concentration after stripping with carbon dioxide. Activated WV-B 1500 exhibited the highest ethanol capacity among activated carbons, having higher ethanol adsorption capacities than the two zeolites. Adsorption isotherms for ethanol and water in the presence of carbon dioxide at different temperatures were determined using WV-B 1500 as the adsorbent with the temperature-dependent Toth isotherm model providing satisfactory fits for these isotherms. Ethanol adsorption experiments with and without the presence of water were conducted and showed similar ethanol adsorption capacities, indicating that the presence of water has a negligible effect on ethanol adsorption.

Carbon dioxide stripping of ethanol and adsorption can be used in a bioethanol fermentation system to reduce product inhibition. To gain a better understanding of the bioethanol adsorption process, a mathematical model was used to predict the adsorption performance of activated carbon WV-B 1500 for ethanol vapor adsorption in the presence of carbon dioxide and water. The model takes into account changes in velocity due to adsorption, heat effects during adsorption, and heat losses to the surroundings. The model, developed without resorting to fitting parameters, was validated successfully by comparing the model predictions with experimental adsorption data. Finally, the model was used to predict the adsorption working capacities to assess the performance of the adsorption process in an industrial process.

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