Recovery of biochemicals from ABE fermentation broths using MFI adsorbents: A comparison between traditional beads and a structured adsorbent

Sammanfattning: Energy demands of the world are ever increasing in this industrial era. Over reliance on petroleum-based fuels has a negative impact on the environment. In addition, with depleting reservoirs of fossil fuels, the need for new, sustainable fuels and chemicals is more urgent than ever. One such chemical is 1-butanol (or simply butanol), which has great potential as a gasoline substitute because of its favorable fuel properties. Butanol can be produced from acetone, butanol and ethanol (ABE) fermentation using e.g. Clostridium acetobutylicum. However, the concentration of butanol in fermentation in the resulting broth is limited to ca. 20 g/L due to its toxicity for microorganisms. Butyric acid is a precursor to butanol, which is produced prior to butanol in ABE fermentation. Butyric acid is an important industrial chemical, which can be further derived into a number of commercial compounds e.g. acetate butyrate, butyl acetate and butanol. In this study, hydrophobic MFI zeolite was evaluated for the recovery of butanol and butyric acid from both model and real fermentation broths. Adsorption isotherms of the main components viz. butanol, butyric acid, acetone, ethanol and acetic acid were determined at room temperature. The experimentally determined isotherms were than fitted to the Langmuir adsorption model with good fit. Butyric acid and butanol showed high affinity for the hydrophobic MFI zeolite. The butanol saturation loading was determined to be 0.11 g-butanol/g-zeolite for both binary (water-butanol) and multicomponent (ABE) model solutions in concert to previous findings. However, adsorption of butyric acid was found to be strongly pH dependent, with high adsorption below and little adsorption above the pKa value of the acid. Thermal desorption experiments showed that adsorbed water and butanol starts to desorb at 100 °C and 118 °C respectively. The hydrophobic MFI zeolite was also evaluated for the recovery of bio-butanol from real fermentation broth produced by Clostridium acetobutylicum using xylose recovered from birch Kraft black liquor. The results showed that even in the presence of phenolic compounds, which may interfere with the adsorption of butanol, the zeolite was very selective towards the targeted molecules i.e. butyric acid and butanol. In addition, butyric acid adsorption could be suppressed by increasing the pH of the solution to facilitate better selectivity towards butanol. The selectivity of butanol over acetone and ethanol was found to be 25 and 250 respectively at pH 8 and room temperature for batch adsorption experiments. A structured adsorbent in the form of steel monolith coated with a silicalite-1 film was prepared. X-ray diffractometry and scanning electron microscopy was used to characterize the adsorbent. The performance of the structured adsorbent was evaluated by performing breakthrough experiments at room temperature using model ABE fermentation broths and the performance was compared with that of traditional adsorbents in the form of beads. The structured silicalite-1 adsorbent required less amount of solution to achieve saturation as compared to the commercial ZSM-5 beads. Desorption studies showed that a high quality butanol product with purity up to 97% for butanol-water system and 89% for the ABE system can be recovered with the structured silicalite-1 adsorbent. The commercial ZSM-5 beads also showed good selectivity but the concentration of butanol in the desorbed product was limited to 71% for the butanol-water system and 61% for ABE system, probably as a result of entrained liquid between the beads.