272003 Improvements in Bio-Butanol Separation by Adsorption: Adsorbent Screening, Kinetics and Equilibrium
The rising cost of crude oil combined with depletion of the resources, political instability in oil producing countries and environmental concerns are some of the reasons that have motivated different waves of interest for renewable and sustainable fuels. To achieve a greater use of biofuels, it is paramount to develop innovative techniques to convert biomass to biofuel as efficiently as possible in order to make these production processes more economically viable. Recent developments in biotechnology offer an avenue for bio-production of fuels from renewable and sustainable resources instead of petroleum-based fuels.
Bio-butanol offers several strong attributes and has advantages over other biofuels such as lower volatility, flammability and corrosiveness. The net heat of combustion (NHOC) for butanol is 29.2 MJ/L, which is 83% of gasoline NHOC (which is 32.5 MJ/L) whereas values for ethanol and methanol are 65 and 45% of gasoline NHOC, respectively. Butanol is a very hydrophobic compound with a solubility of 7.7 g/100 mL (at 20°C) in water,whereas lower alcohols (methanol, ethanol and n-propanol) are miscible in water. Also, butanol can be used in car engines without any modification. Butanol was initially produced from fermentation, but in the 1950s, butanol produced from petroleum via the oxo alcohol process became much cheaper and the chemical route remains the only economic source of butanol today. However, the resurgence of interest in the production of butanol from fermentation is being witnessed from research institutions and major players, including large multinational petroleum companies. However, the production of biobutanol via fermentation is facing significant engineering challenges due the very low final concentration, low yield and severe butanol toxicity to microorganisms. It is therefore important to find an efficient separation technique to recover butanol at the end of fermentation or during the fermentation to reduce the level of toxicity and prolong the fermentation in addition to making it an economically feasible process.
Adsorption is an energy-efficient process that can be used to achieve this objective. In selecting a suitable adsorbent for the adsorption process, many factors need to be considered such as : adsorption rate, adsorption capacity, ease of desorption, selectivity for the desired product and cost of the adsorbent. Activated carbon and zeolites are the most common adsorbents used in this process since they are hydrophobic materials having high adsorption capacity for butanol. In this study, two activated carbons (AC F600 and AC F400) with different particles sizes (0.55-0.75 and 1 mm) and two zeolites (ZSM-5 and NaY) with different SiO2/Al2O3 ratios (80 and 1.8) were tested. Adsorbent screening was done based on the kinetic and equilibrium experiment results. In kinetic experiments, AC F400 and AC F600 showed much faster adsorption kinetics in comparison to zeolites., The results obtained from kinetic experiments for both activated carbons showed that the adsorption reaches equilibrium in approximately 100 minutes while for zeolites, the system did not yet reach equilibrium even after 240 min.
Results for equilibrium experiments showed that the highest adsorption capacity for butanol was obtained with AC F400. With an equilibrium butanol concentration of 15 g/L, the butanol adsorption capacity for AC 400 was 283 mg/g while butanol adsorption capacities for AC F600, ZSM-5 and NaY were 150, 155 and 116 mg/g, respectively. Based on these results, AC F400 was selected as the adsorbent to carry out further experiments. Due to the presence of other components such as acetone and ethanol in the fermentation broth in the butanol production process, it is essential to assess the impact of other components o[J1] n butanol adsorption capacity. To investigate the competitive effect of these components on butanol adsorption, ternary solutions of butanol-acetone-water and butanol-ethanol-water with different initial concentrations of acetone and ethanol (1, 5 and 10 g/L) were tested. In this series of experiments, results showed that the presence of ethanol has a negligible effect on butanol adsorption whereas the addition of acetone decreased the butanol adsorption capacity. The negative effect of acetone was more pronounced at higher acetone concentrations. For the 10 g/L butanol equilibrium concentration, in the presence of 1, 5 and 10 g/L initial acetone concentration, butanol adsorption capacity decreased from 258 mg/g to 239, 212 and 206 mg/g, respectively.
In this study, the selectivity of AC F400 for the three main fermentation species (butanol, acetone and ethanol) was investigated using binary solutions of butanol-water, acetone-water and ethanol-water separately. The results showed that AC F400 is more selective to butanol in comparison to the other components. At an equilibrium concentration of 10 g/L, adsorption capacity of AC F-400 for butanol was 258 mg/g, while for acetone and ethanol, the adsorption capacities at the same equilibrium concentration were 115 and 55 mg/g, respectively.
From all the results obtained in this investigation, activated carbon F400 has been chosen to be the best adsorbent for butanol separation due to its high affinity, fast kinetics and higher adsorption capacity.