Low energy recycling of ionic liquids via freeze crystallization during cellulose spinning

A new method for recycling ionic liquids (ILs) from a cellulose spinning process is suggested. The method involves the combination of freeze crystallization and evaporation of H2O from IL + H2O mixtures to recycle the ILs. Processes with EmimAc and EmimDep were used as references to develop this IL recycling method.

EmimAc + 12.5 wt% H2O and EmimDep + 4 wt% H2O were selected for a quantitative mass and energy analysis of the cellulose spinning and IL recycling process (the maximal initial H2O levels in the ILs + H2O mixtures for cellulose dissolution were determined experimentally). The energy requirement for the freeze crystallization + evaporation method was compared to evaporation only for recycling of EmimAc and EmimDep.

To produce 1 kg dry cellulose fiber, 45.4 MJ and 62.6 MJ are required for recycling EmimAc and EmimDep respectively by the freeze crystallization + evaporation recycling method. Using evaporation only, 66.9 MJ is required for EmimAc recycling and 99.9 MJ for EmimDep recycling per kg cellulose fiber produced.

Thus, to fabricate 1 kg dry cellulose fiber using freeze crystallization + evaporation rather than evaporation, 21.5 MJ can be saved for EmimAc and 37.3 MJ for EmimDep recycling. We also show that compared to a classical Lyocell fiber production method using N-methylmorpholine-N-oxide (NMMO) as solvent, use of ILs is energy saving in itself.

Hence, significantly less H2O is required in the cellulose spinning process with ILs than with NMMO, and in turn less H2O has to be evaporated for the solvent recycling.