110
A. R. Abouelela et al.
5.6.2 Ionic Liquid Re-Concentration
It is essential to develop robust and cost-effective IL recycle and recovery technologies in order for ionic liquid processes to be economically and ecologically viable
[104]. The non-volatility of ionic liquids and, therefore, their near-quantitative recovery and recycling is one of the main reasons for their early popularity. However,
studies on ionic liquids’ recyclability in operational environments are still scarce
[118]. In ionic liquid-based biomass pretreatment processes, the IL is diluted with
a large amount of water during the product recovery step (ionic liquid dissolution
process) or during the lignin precipitation step (ionic liquid fractionation process)
[121]. The diluted IL-water mixture stream must be re-concentrated prior to ionic
liquid recycling. Ionic liquids used for biomass pretreatment processes (both fractionation and dissolution) are hydrophilic and miscible in water, which makes their
re-concentration and recovery a very challenging task compared to hydrophobic IL
recovery and recycles [118]. Thermal evaporation of water is one of the first options to
consider for IL recovery and recycle. However, thermal evaporation technologies are
highly energy-intensive, especially when the solvent to be removed is non-volatile,
such as water [122]. In addition, the IL–water mixture represents a real challenge due
to the presence of strong hydrogen-bond interactions between the water molecules
and the IL ions, especially at low water concentrations [123, 124]. In addition, boiling point elevation in the IL–water binary mixture due to the strong water–ions
interaction makes thermal distillation more challenging and energy-intensive [121,
125].
Membrane-based technologies are considered as the most cost-effective and
energy-efficient choice, and they are also industrially mature. Membrane-based technologies, such as nanofiltration (NF), reverse osmosis (RO), pervaporation and electrodialysis, were investigated for IL re-concentration and dehydration application.
Haerens et al. have studied the use of pressure-driven membrane technologies, such
as NF and RO, to re-concentrate 5 vol.% Ethaline200 aqueous solution [126]. Only
a five-fold concentration of the IL was possible, reaching a maximum concentration
of 20–25 vol.%. The intrinsically high osmotic pressure of ILs and low ILs rejection,
especially at high IL concentrations, limit the application of membrane-pressure
processes for IL dehydration. Therefore, the authors suggested using RO and NF
to re-concentrate dilute IL solutions by a factor of four to five followed by another
re-concentration process to achieve higher water removal. Lynam et al. studied a
vacuum membrane distillation process to concentrate [C 2 C 1 im][Cl] solution at high
feed concentration (>20 wt%) using a hydrophobic polyacrylonitrile (PAN)-based
membrane [127]. At optimal operating conditions, the processes showed good performance achieving a 65% final concentration and 99.5% IL recovery. However, the
loss of performance with time due to membrane pore wetting was highlighted as a
potential operational challenge.
Pervaporation technology was also recently investigated for [C 2 C 1 im][Ace] reconcentration in a biomass pretreatment process [121]. The process was very effective
in re-concentrating the IL from 20 wt% initial concentration up to 80 and 99 wt% at
A. R. Abouelela et al.
5.6.2 Ionic Liquid Re-Concentration
It is essential to develop robust and cost-effective IL recycle and recovery technologies in order for ionic liquid processes to be economically and ecologically viable
[104]. The non-volatility of ionic liquids and, therefore, their near-quantitative recovery and recycling is one of the main reasons for their early popularity. However,
studies on ionic liquids’ recyclability in operational environments are still scarce
[118]. In ionic liquid-based biomass pretreatment processes, the IL is diluted with
a large amount of water during the product recovery step (ionic liquid dissolution
process) or during the lignin precipitation step (ionic liquid fractionation process)
[121]. The diluted IL-water mixture stream must be re-concentrated prior to ionic
liquid recycling. Ionic liquids used for biomass pretreatment processes (both fractionation and dissolution) are hydrophilic and miscible in water, which makes their
re-concentration and recovery a very challenging task compared to hydrophobic IL
recovery and recycles [118]. Thermal evaporation of water is one of the first options to
consider for IL recovery and recycle. However, thermal evaporation technologies are
highly energy-intensive, especially when the solvent to be removed is non-volatile,
such as water [122]. In addition, the IL–water mixture represents a real challenge due
to the presence of strong hydrogen-bond interactions between the water molecules
and the IL ions, especially at low water concentrations [123, 124]. In addition, boiling point elevation in the IL–water binary mixture due to the strong water–ions
interaction makes thermal distillation more challenging and energy-intensive [121,
125].
Membrane-based technologies are considered as the most cost-effective and
energy-efficient choice, and they are also industrially mature. Membrane-based technologies, such as nanofiltration (NF), reverse osmosis (RO), pervaporation and electrodialysis, were investigated for IL re-concentration and dehydration application.
Haerens et al. have studied the use of pressure-driven membrane technologies, such
as NF and RO, to re-concentrate 5 vol.% Ethaline200 aqueous solution [126]. Only
a five-fold concentration of the IL was possible, reaching a maximum concentration
of 20–25 vol.%. The intrinsically high osmotic pressure of ILs and low ILs rejection,
especially at high IL concentrations, limit the application of membrane-pressure
processes for IL dehydration. Therefore, the authors suggested using RO and NF
to re-concentrate dilute IL solutions by a factor of four to five followed by another
re-concentration process to achieve higher water removal. Lynam et al. studied a
vacuum membrane distillation process to concentrate [C 2 C 1 im][Cl] solution at high
feed concentration (>20 wt%) using a hydrophobic polyacrylonitrile (PAN)-based
membrane [127]. At optimal operating conditions, the processes showed good performance achieving a 65% final concentration and 99.5% IL recovery. However, the
loss of performance with time due to membrane pore wetting was highlighted as a
potential operational challenge.
Pervaporation technology was also recently investigated for [C 2 C 1 im][Ace] reconcentration in a biomass pretreatment process [121]. The process was very effective
in re-concentrating the IL from 20 wt% initial concentration up to 80 and 99 wt% at