Continuous multi-stage extraction of n-butanol from aqueous solutions with 1-hexyl-3-methylimidazolium tetracyanoborate

The separation of n-butanol from fermentation broth is very challenging. Due to low n-butanol mass fractions in the broth, purification by distillation is energy- and thus cost-intensive. Therefore, alternative processes are required. One potential alternative process is liquid-liquid extraction, which is promising due to mild operating conditions. In this paper, a continuous multi-stage extraction of n-butanol with the ionic liquid 1-hexyl-3-methylimidazolium tetracyanoborate is investigated experimentally and theoretically. Liquid-liquid equilibrium compositions of the system ionic liquid/synthetic medium are analysed in laboratory scale and compared to pilot scale experiments in a mixer-settler unit at 35 °C. The results confirm that a continuous separation of n-butanol is feasible. The solvent-to-feed ratio and n-butanol mass fraction in the aqueous feed influence the n-butanol recovery which lies between 85% and 99% in the mixer-settler unit depending on the process parameters. In the theoretical part of our study, the NRTL model is applied successfully to correlate experimental liquid-liquid equilibrium compositions of the ternary system ionic liquid/n-butanol/water. Using this correlation, the multi-stage extraction is modelled and validated with pilot scale experiments. The results confirm the use of an equilibrium approach for the multi-stage extraction model without the need of stage efficiency. Furthermore, they allow for a detailed process analysis, which reduces the experimental and financial effort.