of water. Equation 1 shows the substantial change in rate
constants on increasing the reaction pressure in supercritical
reactions, which is attributed to the change in volume of
activation (Shaw et al. 1991).
ð1Þ
where k x , P, ΔV
╪ , R and T are rate constant, pressure, molar
volume of activation, gas constant and temperature,
respectively. These results show that the reaction kinetics
and physical characteristics of water can be controlled significantly by adjusting pressure and temperature. The
resulting properties make supercritical water a tunable
reaction solvent and medium for transformation of organic
wastes to useful chemical compounds, which needs high
reaction temperature and pressure to activate the reactants.
2.2 Sub- and Supercritical Water as an Efficient
Solvent for Biomass Processing
Water is a green solvent that can be used as a reaction
medium at both sub- and supercritical environment in different domains for the treatment of biomass and production
of a variety of chemicals.
Subcritical environment in the biomass conversion reaction is very favorable, especially for the extraction of
nutrients, photo-chemicals from various natural substances.
Besides, for food and pharmaceutical industries, the subcritical water is an excellent source due to less toxic nature,
and free organic solvent products are formed which lead to
no or any expensive removal of organic solvent (Peterson
et al. 2008).
In hydrothermal processing studies, especially for the
liquefaction section, mostly subcritical water is recommended as a solvent for the enhancement of higher oil or
bio-crude yields, and higher conversion rates as compared to
other organic solvents. For example, Zhu et al. and Jindal
et al. reported the HTL of barely straw and wood, respectively (lignocellulosic biomasses), and both reported high
bio-crude yields with the lower amount of solids at subcritical conditions due to efficient transformation of cellulose
and hemicellulose through alkali catalyst K 2 CO 3 via
hydrolysis (Zhu et al. 2014; Jindal and Jha 2016). Malins
et al. employed subcritical water for the liquefaction of
sewage sludge and observed an increase in energy recovery
corresponding to higher ratio of weight fractions of sewage
sludge and water (Malins et al. 2015). However, bio-crude
showed a slight decrease in HHV values ranging from 35.95
to 31.35 MJ/kg, which is associated with the hydrolysis
reactions of polysaccharides, lipids, and peptides that formed
oxygenated compounds like acids and alcohols, etc.
Moreover, it was also reported that subcritical water can also
be utilized as an extractive medium for carbohydrate and
protein. In one of the studies, microalgae was treated at
(180–374 °C), which resulted in maximum carbohydrate and
14.2/100 and 31.2/100 (g/g) protein yields, respectively at
277 °C with 5% biomass loading (Awaluddin et al. 2016).
Besides the extraction of carbohydrate and protein, some
publications focused on the isolation of phenolic compounds
from the biomass. These phenolic compounds are associated
with carbohydrates and proteins. By using subcritical water,
the segregation and hydrolysis are carried out altogether and
more active aglycones are formed in the product. These free
aglycones are tended to be extreme antioxidant properties
than bonded glycosides. Singh et al. studied the influence of
subcritical water on potato peel, and noticed that the optimal
temperature of 180 °C and 60 min for the extraction
of phenolic acids like (hydroxyl benzoic, ferulic and coumaric acids) as listed in Table 1 (Singh and Saldaña 2011).
2.2.1 Supercritical Water as Reaction Medium
Water in supercritical environment is a sustainable solvent
that possesses unique ability to dissolve a variety of organic
substances and feedstocks for chemical synthesis and production of valuable products like oil, gas and char.
The feedstock includes cellulose, lignin, hemicellulose,
plastics and other wastes, e.g., rubber tire, inorganics and
wastewater. It has been found from the literature that the
temperature above the critical point does not facilitate the
liquefaction (Peterson et al. 2008). However, other technologies, like supercritical water gasification and supercritical water oxidation are strongly suggested at supercritical
conditions. The supercritical water plays a crucial role in
hydrolysis and pyrolysis reactions in supercritical water
gasification. However, supercritical water oxidation holds
reactions like depolymerization, defragmentation where
water acts like: (1) a reactant or product for hydrolysis and
hydration, hydrogen provider, (2) an acid/base catalyst or
precursor, (3) responsible for intermolecular interactions, to
enhance higher hydrophobicity, and (4) medium for energy
transfer, diffusion and phase behavior (Knez et al. 2018).
Hydrolysis is a primary reaction in supercritical conditions, particularly for the destruction of non-polar organic
waste. At supercritical state, higher temperature and pressure
speed up the rate of hydrolysis reaction even when no catalyst is added. It is the breaking up of glycosidic, peptide,
triglycerides, ester and amide bonds. Saski et al. observed
the effect of supercritical water with cellulose decomposition
in a detailed manner (Sasaki et al. 1998). Gasification is one
of the most dominant hydrothermal processing methods in
the supercritical region, and it has been extensively used
worldwide to reform the organic matter and recover useful
gaseous products like H 2 , CO, CO 2 and CH 4 . Cherad et al.
had already optimized the aqueous phase produced from
26
K. Sharma et al.
constants on increasing the reaction pressure in supercritical
reactions, which is attributed to the change in volume of
activation (Shaw et al. 1991).
ð1Þ
where k x , P, ΔV
╪ , R and T are rate constant, pressure, molar
volume of activation, gas constant and temperature,
respectively. These results show that the reaction kinetics
and physical characteristics of water can be controlled significantly by adjusting pressure and temperature. The
resulting properties make supercritical water a tunable
reaction solvent and medium for transformation of organic
wastes to useful chemical compounds, which needs high
reaction temperature and pressure to activate the reactants.
2.2 Sub- and Supercritical Water as an Efficient
Solvent for Biomass Processing
Water is a green solvent that can be used as a reaction
medium at both sub- and supercritical environment in different domains for the treatment of biomass and production
of a variety of chemicals.
Subcritical environment in the biomass conversion reaction is very favorable, especially for the extraction of
nutrients, photo-chemicals from various natural substances.
Besides, for food and pharmaceutical industries, the subcritical water is an excellent source due to less toxic nature,
and free organic solvent products are formed which lead to
no or any expensive removal of organic solvent (Peterson
et al. 2008).
In hydrothermal processing studies, especially for the
liquefaction section, mostly subcritical water is recommended as a solvent for the enhancement of higher oil or
bio-crude yields, and higher conversion rates as compared to
other organic solvents. For example, Zhu et al. and Jindal
et al. reported the HTL of barely straw and wood, respectively (lignocellulosic biomasses), and both reported high
bio-crude yields with the lower amount of solids at subcritical conditions due to efficient transformation of cellulose
and hemicellulose through alkali catalyst K 2 CO 3 via
hydrolysis (Zhu et al. 2014; Jindal and Jha 2016). Malins
et al. employed subcritical water for the liquefaction of
sewage sludge and observed an increase in energy recovery
corresponding to higher ratio of weight fractions of sewage
sludge and water (Malins et al. 2015). However, bio-crude
showed a slight decrease in HHV values ranging from 35.95
to 31.35 MJ/kg, which is associated with the hydrolysis
reactions of polysaccharides, lipids, and peptides that formed
oxygenated compounds like acids and alcohols, etc.
Moreover, it was also reported that subcritical water can also
be utilized as an extractive medium for carbohydrate and
protein. In one of the studies, microalgae was treated at
(180–374 °C), which resulted in maximum carbohydrate and
14.2/100 and 31.2/100 (g/g) protein yields, respectively at
277 °C with 5% biomass loading (Awaluddin et al. 2016).
Besides the extraction of carbohydrate and protein, some
publications focused on the isolation of phenolic compounds
from the biomass. These phenolic compounds are associated
with carbohydrates and proteins. By using subcritical water,
the segregation and hydrolysis are carried out altogether and
more active aglycones are formed in the product. These free
aglycones are tended to be extreme antioxidant properties
than bonded glycosides. Singh et al. studied the influence of
subcritical water on potato peel, and noticed that the optimal
temperature of 180 °C and 60 min for the extraction
of phenolic acids like (hydroxyl benzoic, ferulic and coumaric acids) as listed in Table 1 (Singh and Saldaña 2011).
2.2.1 Supercritical Water as Reaction Medium
Water in supercritical environment is a sustainable solvent
that possesses unique ability to dissolve a variety of organic
substances and feedstocks for chemical synthesis and production of valuable products like oil, gas and char.
The feedstock includes cellulose, lignin, hemicellulose,
plastics and other wastes, e.g., rubber tire, inorganics and
wastewater. It has been found from the literature that the
temperature above the critical point does not facilitate the
liquefaction (Peterson et al. 2008). However, other technologies, like supercritical water gasification and supercritical water oxidation are strongly suggested at supercritical
conditions. The supercritical water plays a crucial role in
hydrolysis and pyrolysis reactions in supercritical water
gasification. However, supercritical water oxidation holds
reactions like depolymerization, defragmentation where
water acts like: (1) a reactant or product for hydrolysis and
hydration, hydrogen provider, (2) an acid/base catalyst or
precursor, (3) responsible for intermolecular interactions, to
enhance higher hydrophobicity, and (4) medium for energy
transfer, diffusion and phase behavior (Knez et al. 2018).
Hydrolysis is a primary reaction in supercritical conditions, particularly for the destruction of non-polar organic
waste. At supercritical state, higher temperature and pressure
speed up the rate of hydrolysis reaction even when no catalyst is added. It is the breaking up of glycosidic, peptide,
triglycerides, ester and amide bonds. Saski et al. observed
the effect of supercritical water with cellulose decomposition
in a detailed manner (Sasaki et al. 1998). Gasification is one
of the most dominant hydrothermal processing methods in
the supercritical region, and it has been extensively used
worldwide to reform the organic matter and recover useful
gaseous products like H 2 , CO, CO 2 and CH 4 . Cherad et al.
had already optimized the aqueous phase produced from
26
K. Sharma et al.