112
A. R. Abouelela et al.
requirement by ~80% (0.9 MJ kg
−1 ). However, IL re-concentration is more difficult
to heat-integrate with only a 14% potential reduction in energy requirement after
heat integration due to the substantial amount of energy needed during the phase
change. The immense amount of energy required for IL re-concentration makes it
imperative to dedicate more research efforts toward developing cost-effective and
robust dehydration technologies to recover ILs from aqueous solutions with minimal
IL losses.
5.6.3 Cellulose Pulp Washing
Washing the cellulose-rich pulp after the pretreatment is essential for three main reasons: (1) to reduce ionic liquid losses, (2) to remove residual inhibitory compounds,
and (3) to maintain high pulp quality for downstream processing [128, 129]. For
bioethanol production, the presence of small traces of an IL can inhibit commercial
enzymes in the enzymatic hydrolysis unit or the microbes in the fermentation unit
[129, 130]. Klein-Marcuschamer et al. tracked the residual amount of [C 2 C 1 im][Ace]
in the recovered solid and liquid fractions to investigate the washing efficiency and
its impact on the performance of commercial enzymes [100]. At high [C 2 C 1 im][Ace]
concentrations of 16–46% (1st to 3rd water wash), the glucose yield of pretreated
switchgrass did not exceed 60%. After the 5th water wash, the IL concentration
dropped to 5.7% and the glucose yield increased to 80%. This highlights the importance of extensive washing to remove the residual IL. However, the extensive water
washing step adds to the process of operating costs and the process capital cost for the
wastewater treatment unit. The issue led to the exploration of options to reduce the
water footprint, such as the development of the “single-pot” or “one-pot” wash-free
process [101, 131]. The process combines the pretreatment and enzymatic saccharification in one step by using a specialty thermophilic enzyme cocktail, called JTherm,
that can tolerate the presence of the ionic liquid [C 2 C 1 im][Ace] [132, 133]. Shi et al.
showed that at 10 and 20% [C 2 C 1 im][Ace] concentrations and at 70 °C, JTherm
retains 81 and 68% of its activity, respectively, while the activity of CTec2 commercial at 50 °C dropped significantly to 37 and 19%, respectively [134]. The authors
found that the ability to perform enzymatic saccharification with the presence of
10% [C 2 C 1 im][Ace] can potentially reduce water use by 2–15 times compared to
the conventional water wash configuration.
Although the one-pot configuration seems promising in terms of reducing the
process of water consumption, the process is still in early stages of development, and
the enzymes’ tolerance to other ILs is still unknown. Recently, more ionic liquidbased pretreatment studies started to consider the use of IL–water mixtures instead
of 100% neat ionic liquid [108, 135]. The use of IL–water mixtures should potentially reduce the excessive amount of water needed for washing prior to enzymatic
saccharification. Therefore, it is important to target low initial IL concentration not
only to reduce the IL use and cost in the process but also to achieve a less intensive and more cost-effective washing configuration. For example, in the ionoSolv
A. R. Abouelela et al.
requirement by ~80% (0.9 MJ kg
−1 ). However, IL re-concentration is more difficult
to heat-integrate with only a 14% potential reduction in energy requirement after
heat integration due to the substantial amount of energy needed during the phase
change. The immense amount of energy required for IL re-concentration makes it
imperative to dedicate more research efforts toward developing cost-effective and
robust dehydration technologies to recover ILs from aqueous solutions with minimal
IL losses.
5.6.3 Cellulose Pulp Washing
Washing the cellulose-rich pulp after the pretreatment is essential for three main reasons: (1) to reduce ionic liquid losses, (2) to remove residual inhibitory compounds,
and (3) to maintain high pulp quality for downstream processing [128, 129]. For
bioethanol production, the presence of small traces of an IL can inhibit commercial
enzymes in the enzymatic hydrolysis unit or the microbes in the fermentation unit
[129, 130]. Klein-Marcuschamer et al. tracked the residual amount of [C 2 C 1 im][Ace]
in the recovered solid and liquid fractions to investigate the washing efficiency and
its impact on the performance of commercial enzymes [100]. At high [C 2 C 1 im][Ace]
concentrations of 16–46% (1st to 3rd water wash), the glucose yield of pretreated
switchgrass did not exceed 60%. After the 5th water wash, the IL concentration
dropped to 5.7% and the glucose yield increased to 80%. This highlights the importance of extensive washing to remove the residual IL. However, the extensive water
washing step adds to the process of operating costs and the process capital cost for the
wastewater treatment unit. The issue led to the exploration of options to reduce the
water footprint, such as the development of the “single-pot” or “one-pot” wash-free
process [101, 131]. The process combines the pretreatment and enzymatic saccharification in one step by using a specialty thermophilic enzyme cocktail, called JTherm,
that can tolerate the presence of the ionic liquid [C 2 C 1 im][Ace] [132, 133]. Shi et al.
showed that at 10 and 20% [C 2 C 1 im][Ace] concentrations and at 70 °C, JTherm
retains 81 and 68% of its activity, respectively, while the activity of CTec2 commercial at 50 °C dropped significantly to 37 and 19%, respectively [134]. The authors
found that the ability to perform enzymatic saccharification with the presence of
10% [C 2 C 1 im][Ace] can potentially reduce water use by 2–15 times compared to
the conventional water wash configuration.
Although the one-pot configuration seems promising in terms of reducing the
process of water consumption, the process is still in early stages of development, and
the enzymes’ tolerance to other ILs is still unknown. Recently, more ionic liquidbased pretreatment studies started to consider the use of IL–water mixtures instead
of 100% neat ionic liquid [108, 135]. The use of IL–water mixtures should potentially reduce the excessive amount of water needed for washing prior to enzymatic
saccharification. Therefore, it is important to target low initial IL concentration not
only to reduce the IL use and cost in the process but also to achieve a less intensive and more cost-effective washing configuration. For example, in the ionoSolv