Bio-butanol recovery by adsorption/desorption processes

he Acetone-Butanol-Ethanol (ABE) fermentation route to produce chemicals and fuels from renewable resources still suffers from several issues that severely limit the industrial development. In particular, the low butanol concentration in the fermentation broth - due to butanol toxicity to microorganisms – affects the energy sustainability of technologies for butanol recovering. Novel techniques have been investigated in the last years. Among these, adsorption-based technique has been proposed as the most promising one. Present study reports on the assessment of the efficiency of butanol recovery according to adsorption and desorption processes. Tests were carried out using Amberlite XAD-7 as adsorbent material with respect to butanol, ethanol, acetone, and acetic/butyric acids in aqueous solutions. Amberlite XAD-7 was successfully used in fixed-bed column lab apparatus. Two desorption techniques were investigated – thermal drying/desorption and displacement desorption by methanol – to select the best candidate in terms of mass and energy efficiency. This study proved that Amberlite XAD-7 is a potential good adsorbent material to be successfully used in the process of butanol recovery. Amberlite XAD-7 was characterized by high adsorption capacity and selectivity towards butanol. Indeed, the adsorption capacity for acetone, butanol, ethanol, acetic and butyric acid was 17.1, 102.1, 4.2, 14.1 and 21.3 mg/g, respectively when using a solution of 13 g/L of butanol, 5.8 g/L of acetone, 1.6 g/L of ethanol, 6 g/L of acetic acid and 9 g/L of butyric acid. Moreover, adsorbed butanol was recovered as a high butanol concentration solution according to thermal (butanol concentration higher than 800 g/L) and chemical desorption processes (butanol concentration in methanol solution higher than 20 g/L, that is about 1.7 times the concentration in the stream used to saturate the resin). The stability of the performance of Amberlite XAD-7 bed with respect to butanol capture and concentration was provided by carrying out the adsorption–desorption cycle ten times. Butanol recovery was always higher than 97% for both the recovery techniques investigated. The overview of the energy demand for the proposed adsorption/thermal drying/desorption process was reported. The energy demand to concentrate butanol from diluted aqueous mixtures (about 13 g/L) is about 13 MJ/kgB, which is a fraction of the energy content of butanol (36 MJ/kgB)