Environmental Engineering Reference
In-Depth Information
2014
). In the separation step (3), products are separated from the ionic liquid by
adding an additional substance that acts as an anti-solvent. Adding water or acetone
to the biomass and ionic liquid solution causes the cellulose fraction to precipitate
as amorphous cellulose. In the recycle step, the ionic liquid is purified by washing
and extraction. Recovery of the ionic liquid from aqueous streams is a research
topic but it can be accomplished by ion exchange resin (Anthony et al.
2001
), by
adsorption onto activated carbon (Lemus et.al.
2013a
,
b
) or the adsorption of the
ionic liquid onto functionalized carbon materials (Qi et al.
2013
).
Supercritical fluids play several important roles in the processing of biomass
with ionic liquids. Firstly, the viscosity of many ionic liquids that dissolve biomass
is high, with viscosity values being 50-300 times higher than water. Dissolution of
biomass into the ionic liquid causes the viscosity of the solution to greatly increase.
Carbon dioxide, which has low viscosity, has good solubility in many ionic liquids
while ionic liquids, do not dissolve into the gas or supercritical phase of CO
2
. Thus,
CO
2
can act as a viscosity reducing agent for the dissolution and reaction steps
in Fig.
5.11
. Secondly, if reactions are desirable, CO
2
can act as a mass transfer
agent to promote reactant solubility in the ionic liquid phase. A good example is
the promotion of hydrogenation reactions by using molecular hydrogen in the ionic
liquid phase. Thirdly, many small non-polar molecules are soluble in the supercriti-
cal CO
2
. Thus, supercritical CO
2
can be used in the separation step of Fig.
5.11
to
extract products (Brennecke et al.
2002
).
5.4
Conclusions
An overview has been given of energy applications with supercritical fluids. Us-
ing fluids in their supercritical state can allow the development of energy-efficient
cycles, and robust biofuel syntheses. Supercritical fluids used with ionic liquids
show great promise in developing biomass as a sustainable energy resource for
chemicals and biofuels.
References
Angelino, G., and C. Invernizzi. 2009. Carbon dioxide power cycles using liquid natural gas as
heat sink.
Applied Thermal Engineering
29:2935-2941.
Anthony, J., E. Maginn, and J. Brennecke. 2001. Solution thermodynamics of imidazolium-based
ionic liquids and water.
Journal of Physical Chemistry B
105:10942-10949.
Arai, K., R. L. Smith Jr., and T. Aida. 2009. Decentralized chemical processes with supercritical
fluid technology for sustainable society.
Journal of Supercritical Fluids
47:628-636.
Brennecke, J., L. Blanchard, J. Anthony, Z. Gu, I. Zarraga, and D. Leighton. 2002. Separation of
species from ionic liquids.
Clean Solvents
819:82-96.
Fang, Z., R. L. Smith Jr., and X. Qi, eds. 2014.
Production of biofuels and chemicals with ionic
liquids
. Volume 1 in Biofuels and Biorefineries Series, Springer, Dordrecht.
