114
A. R. Abouelela et al.
(CR) of metals in their operational environments are classified as follows: excellent
(CR < 0.05 mm/y), very good (0.05 < CR < 0.5 mm/y), satisfactory (0.5 < CR
< 1.27 mm/y), unsatisfactory (1.27 mm/y < CR) [143]. In the scale-up study of
IL-based biomass pretreatment, Li et al. investigated the corrosion behavior and
compatibility of Hastelloy C276 as the construction material of a Parr reactor in the
scale-up of an acid-assisted IL pretreatment process [144]. The Hastelloy coupons
were immersed in 1-butyl-3-methylimidazolium chloride [C 4 C 1 im][Cl] and 1-ethyl3-methylimidazolium chloride [C 2 C 1 im][Cl] and biomass slurry for 144 h at 140 °C.
At an HCl concentration of 0.6%, corrosion rates of all coupons were <0.025 mm/y.
Increasing the HCl concentration to 1.8% increased corrosion rates by two to three
times; however, corrosion rates were still <0.025 mm/y. Interestingly, at both HCl
concentrations, [C 4 C 1 im][Cl] was more corrosive than [C 2 C 1 im][Cl]. However, the
use of expensive construction material, such as Hastelloy, is not favorable for the
process economics, and the use of a less expensive material (e.g., stainless steel) is
more economical for a large-scale operation [145].
Preliminary data on the corrosivity of the [HSO 4 ]-based ILs and water mixtures
used in the ionoSolv process suggest that at room temperature, the corrosion resistance of stainless steel (SS-316 and SS-304) is excellent at water contents between
0–80 wt%. However, corrosion rates of both steel grades significantly increase by
an order of magnitude at 70 °C. Since pretreatment takes place at higher temperatures (≥150 °C), it is unlikely that conventional steels 304/316 could be used
as construction materials for the pretreatment reactor. On the contrary, stainless
steel showed very high corrosion resistance in ILs with fluorinated anions, such as
bis(trifluoromethylsulfonyl)imide [NTf 2 ]
− and triflate [OTf]
− [137, 142, 146]. This
highlights how the chemical structure differences of ILs (e.g., protic-aprotic, acidicbasic, and hydrophilic-hydrophobic) can have a significant impact on its interaction
with the construction material.
The use of very highly corrosion-resistant metals and alloys, such as tantalum,
titanium, and zirconium as construction materials, is often uneconomical as these
metals are several times more expensive than steel (Fig. 5.9). One way to use these
exotic corrosion-resistive metals is to apply them as a coating material for pipes and
equipment. For example, Tantaline
® is a product line of steel coated with tantalum,
which provides superior corrosion resistance performance in very harsh environments, such as hot concentrated acid.
A more cost-effective way to reduce the material-of-construction cost is to use
glass or fluoropolymer liners, such as polytetrafluoroethylene (PTFE), for pipes and
equipment. However, the feasibility of implementing such liners is highly dependent
on the process conditions. In addition, liners might pose some problems, such as
permeation of the fluid, collapse or damage of the liners, joint creep or cold flow,
and/or charge build-up. Because the interaction and the compatibility of ILs with
these liners or exotic metal-coated steel are unknown, a dedicated study along with
discussion with vendors would be crucial prior to testing. It is clear that there is a
lack of information regarding the selection of construction materials for IL scale-up
studies. However, the interaction of ILs with materials has gained much less attention
compared to other process aspects. Since ILs present a new solvent paradigm that is
A. R. Abouelela et al.
(CR) of metals in their operational environments are classified as follows: excellent
(CR < 0.05 mm/y), very good (0.05 < CR < 0.5 mm/y), satisfactory (0.5 < CR
< 1.27 mm/y), unsatisfactory (1.27 mm/y < CR) [143]. In the scale-up study of
IL-based biomass pretreatment, Li et al. investigated the corrosion behavior and
compatibility of Hastelloy C276 as the construction material of a Parr reactor in the
scale-up of an acid-assisted IL pretreatment process [144]. The Hastelloy coupons
were immersed in 1-butyl-3-methylimidazolium chloride [C 4 C 1 im][Cl] and 1-ethyl3-methylimidazolium chloride [C 2 C 1 im][Cl] and biomass slurry for 144 h at 140 °C.
At an HCl concentration of 0.6%, corrosion rates of all coupons were <0.025 mm/y.
Increasing the HCl concentration to 1.8% increased corrosion rates by two to three
times; however, corrosion rates were still <0.025 mm/y. Interestingly, at both HCl
concentrations, [C 4 C 1 im][Cl] was more corrosive than [C 2 C 1 im][Cl]. However, the
use of expensive construction material, such as Hastelloy, is not favorable for the
process economics, and the use of a less expensive material (e.g., stainless steel) is
more economical for a large-scale operation [145].
Preliminary data on the corrosivity of the [HSO 4 ]-based ILs and water mixtures
used in the ionoSolv process suggest that at room temperature, the corrosion resistance of stainless steel (SS-316 and SS-304) is excellent at water contents between
0–80 wt%. However, corrosion rates of both steel grades significantly increase by
an order of magnitude at 70 °C. Since pretreatment takes place at higher temperatures (≥150 °C), it is unlikely that conventional steels 304/316 could be used
as construction materials for the pretreatment reactor. On the contrary, stainless
steel showed very high corrosion resistance in ILs with fluorinated anions, such as
bis(trifluoromethylsulfonyl)imide [NTf 2 ]
− and triflate [OTf]
− [137, 142, 146]. This
highlights how the chemical structure differences of ILs (e.g., protic-aprotic, acidicbasic, and hydrophilic-hydrophobic) can have a significant impact on its interaction
with the construction material.
The use of very highly corrosion-resistant metals and alloys, such as tantalum,
titanium, and zirconium as construction materials, is often uneconomical as these
metals are several times more expensive than steel (Fig. 5.9). One way to use these
exotic corrosion-resistive metals is to apply them as a coating material for pipes and
equipment. For example, Tantaline
® is a product line of steel coated with tantalum,
which provides superior corrosion resistance performance in very harsh environments, such as hot concentrated acid.
A more cost-effective way to reduce the material-of-construction cost is to use
glass or fluoropolymer liners, such as polytetrafluoroethylene (PTFE), for pipes and
equipment. However, the feasibility of implementing such liners is highly dependent
on the process conditions. In addition, liners might pose some problems, such as
permeation of the fluid, collapse or damage of the liners, joint creep or cold flow,
and/or charge build-up. Because the interaction and the compatibility of ILs with
these liners or exotic metal-coated steel are unknown, a dedicated study along with
discussion with vendors would be crucial prior to testing. It is clear that there is a
lack of information regarding the selection of construction materials for IL scale-up
studies. However, the interaction of ILs with materials has gained much less attention
compared to other process aspects. Since ILs present a new solvent paradigm that is