Integrating microfiltration with cocrystallization for separating glucose from ethanol aqueous solution

Membrane filtration has strong potential for bioethanol purification because of its high product selectivity, low energy consumption, and high system flexibility. A method combining microfiltration with cocrystallization was investigated for purifying ethanol from fermentation broth. After yeast removal, unreacted glucose and ethanol must be separated to diminish the product inhibition effect and to enable continuous operation. The simulated fermentation broth was used in experiments to focus salt-sugar cocrystals removal in this study. Sodium chloride (NaCl) was used as a precursor to form crystal nucleus, leading to cocrystalization with glucose. The effect of NaCl concentration and crystallization time on crystal size and growth rate was investigated. The Feret diameters of crystals increased with time, but the crystal growth rate reduced exponentially during crystallization. The optimal mole ratio of NaCl to glucose was determined as 25. Constant pressure microfiltration was subsequently conducted to separate the crystals from ethanol solution. The filtration flux attenuated with time because of membrane fouling, which was majorly attributed to cake formation. The number of crystals increased with the NaCl concentration, resulting in heavier cake mass and therefore higher glucose rejection. Specific filtration resistance was inversely proportional to the square of the crystal size. Moreover, filtration flux increased with the filtration pressure because of the higher driving force. The cake formed by salt-sugar cocrystals was slightly compressible (cake compressibility = 0.23). An increase in salt concentration led to higher crystal formation, higher cake growth, and therefore higher glucose rejection. The optimal NaCl concentration, at which the highest crystal formation and the glucose rejection were obtained, was 165 kg/m3. The proposed method can be used to predict filtration performance from operating conditions and crystal characteristics and applied to other biomass sources in the processes of bioethanol purification.