The role of ionic liquid pretreatment and recycling design in the sustainability of a biorefinery: a sugarcane to ethanol example

The use of lignocellulosic (LC) materials, especially residues, as feedstock in biorefinery applications is a promising alternative to the oil refinery production of fuels, power and chemicals, reducing the global warming potential (GWP) related to these activities. The conversion of LC's carbohydrates into useful sugars is entirely dependent on the efficiency of the pretreatment (PT) step. Ionic liquids (ILs) have been explored as tailored solvents for the solubilization of LC's complex structure during PT, which can overcome the existing hurdles related to PT. This work assesses the impact of PT variables and the IL recycling through freezing concentration (FC) in an IL-based biorefinery. The influence of temperature, solid loading and IL dilution was systematically studied and the mass and energy balances, economic and environmental outcomes calculated. Life cycle analysis (LCA) was employed in a cradle-to-gate approach. Results showed that solid loading and IL dilution, rather than PT temperature, have the major influence on the energy requirements to produce 1 kg of ethanol. IL make up is a critical parameter to minimize the environmental impact and operational costs associated with the process. Product selling price and IL recycling were the most impacting variables concerning profitability. Extra investment for improved IL recycling is advantageous up to 99% of recovery. Product diversification can improve the economic feasibility even if it is associated with increased capital expenditure. The results show the importance of the pretreatment design and solvent recycling from an integrated perspective, thus challenging the criteria defined while assessing these steps alone.