**2.1. Subcritical and supercritical water extraction**

Subcritical and supercritical water extractions have been employed extensively in biomass utilization due to the tunable physical and chemical properties of water, potentially valuable products, and environmental friendliness. Furthermore, these two fractionation methods are known as the promising methods to make the biorefinery concept more practical with sufficient and sustainable profit.

Typically, subcritical water is defined as the use of water at a temperature between the boiling point and critical temperature (373–647 K) under pressure, which is high enough to maintain its liquid state. Supercritical water occurs at a temperature and pressure higher than its critical point (22.1 MPa and 647 K). In the supercritical region, the properties of liquid and vapor fuse [6, 7]. The behavior of subcritical and supercritical water near critical point mainly depends on pressure and temperature; therefore, some important properties of water could be tuned

**Figure 4.** Static dielectric constant of water at various temperatures and pressures [14].

certainly takes great effects. The essential function of water in common fractionation includes the following: (a) it acts as a mass transfer medium, (b) it plays as a reactant constructing a mild acidic state due to the mitigation of pKw at an increased temperature, (c) it performs as a heat transfer medium, and (d) it represents as an explosion medium for explosion pretreatment to tear biomass into small pieces. Due to the advantages of water-lignocellulose interaction and efficacy, many attempts have practically focused on applying water into the fractionation process to separate the mixture of lignocellulosic biomass into an individual composition called aqueous fractionation, hydro-based fractionation, or "hydro-fractionation." The overall process of bio-based product production from lignocellulosic biomass is shown in **Figure 3**. Hydro-fractionations or the processes utilizing water as a medium, reactant, or catalyst for separating mixture compositions including subcritical extraction, supercritical extraction, and steam explosion are mainly discussed in terms of their fractionation principle, current

Subcritical and supercritical water extractions have been employed extensively in biomass utilization due to the tunable physical and chemical properties of water, potentially valuable products, and environmental friendliness. Furthermore, these two fractionation methods are known as the promising methods to make the biorefinery concept more practical with suf-

Typically, subcritical water is defined as the use of water at a temperature between the boiling point and critical temperature (373–647 K) under pressure, which is high enough to maintain its liquid state. Supercritical water occurs at a temperature and pressure higher than its critical point (22.1 MPa and 647 K). In the supercritical region, the properties of liquid and vapor fuse [6, 7]. The behavior of subcritical and supercritical water near critical point mainly depends on pressure and temperature; therefore, some important properties of water could be tuned

status and potential uses, life cycle and bioeconomy.

**Figure 3.** Process for sugar-based chemical platform production from biomass.

**2.1. Subcritical and supercritical water extraction**

**2. Principle of hydro-fractionation**

ficient and sustainable profit.

72 Renewable Resources and Biorefineries

by varying the temperature and pressure for particular conditions of biomass fractionation. In this section, the important properties of water at the subcritical and supercritical state related to biomass fractionation including dielectric constant, ionization constant, density, and viscosity are demonstrated and discussed. A better understanding of water properties under various temperatures and pressures can allow an appropriate experimental design and suitable operating conditions for some specific proposes.

Dielectric constant is a dimensionless value showing the relative permittivity of a material compared with the permittivity of free space. Typically, the high dielectric constant of a solvent means that it has high polarity and vice versa. **Figure 4** shows the influence of temperature and pressure on the dielectric constant. The value of the dielectric constant tends to decrease with the increasing temperatures while it is slightly affected by pressure around the critical point. This phenomenon hints that the polarity of water can be reduced by increasing the temperature which indicated that the solubility of hydrophobic organic compounds and low molecular biopolymers in biomass could be enhanced by using low polarity of water generated at elevated temperatures [8–10]. It is worth mentioning that low polarity of water also reduces the solubility of salt in the process, especially type 2 salts (classified by solubility behavior) such as Na2 SO<sup>4</sup> , Na2 CO<sup>3</sup> , and K<sup>2</sup> SO<sup>4</sup> [11], and the participation of salt might cause fouling that diminishes the efficiency of the process or even terminates the process. Therefore, the water supply should be treated to eliminate type 2 salts before its use in the process; also, a special design of a reactor might be required in case of raw material containing high contents of type 2 salts [12, 13].

The ionization constant of water is the ratio between the concentration of ionic ([H3 O]+ and [OH]− ) products and the reactant at the equilibrium condition. The influence of temperature and pressure on the ionization constant is shown in **Figure 5**. In the subcritical region, the ionization constant increases with the raising of temperature and is slightly affected by the increase of pressure. On the other hand, beyond critical temperature, the ionic constant

**Figure 5.** Negative log (base 10) of ionization constant of water at various temperatures and pressures [19].

declines with the increase of reaction temperature and dramatically drops with the reduction of pressure. Therefore, the reaction of biomass degradation takes place in ionic media for subcritical water extraction. On the other hand, the supercritical water extraction provided a radical-oriented environment for biomass fractionation [15–18].

(433–533 K) for a period ranging from seconds to several minutes and then suddenly depressurizing it to atmospheric pressure, making the biomass undergo an explosive decompression. This pretreatment is the combination of mechanical forces and chemical effects due to autohydrolysis of the acetyl group in hemicellulose. Autohydrolysis takes place from the formation of acetic acid from the acetyl group in the hemicellulose structure at high temperature where water acts as an acid at high temperature. The hemicellulose and lignin bonds are cleaved during the explosion, allowing the hemicellulose become water soluble; water-soluble lignin from plant cell wall is also released from the cleavage action into water phase. The mechanical effect is caused by explosive decompression that occurred from suddenly dropped pressure at the termination of the pretreatment, which induced the cell walls in biomass to undergo structural disruption and expansion. Because of these effects, a part of hemicellulose hydrolyzed and solubilized; lignin was redistributed, lignocellulosic matrix polymer was broken down, particle size was decreased, the degree of polymerization was reduced, and porosity was increased; moreover, cellulose was slightly depolymerized, which led to the improvement of lignocellulose digestibility [22–24].

**Figure 6.** Viscosity of water at various temperatures and pressures [21].

Supercritical extraction in terms of operating conditions, reaction mechanism, and preferred

>647 >22.1 Radical reaction Extract desired product

expansion of water

**Fractionation route Application**

Liquid ionic reaction Extract desired product

Reduce crystallinity of

biopolymer

Hydro-Fractionation for Biomass Upgrading http://dx.doi.org/10.5772/intechopen.79396 75

**Pressure range (MPa)**

liquid phase)

373–647 >0.001 (maintain

Steam explosion 433–533 0.69–4.83 Rapid volume

biomass is shown in **Table 1**.

**Temperature range (K)**

**Table 1.** Comparison of different hydro-fractionation methods.

**Hydro-fractionation** 

Subcritical water extraction

Supercritical water extraction

**method**

Density of water is defined as the ratio between the mass and volume of water at a specific temperature and pressure. The density of water is decreased with the increase of temperature due to the expansion of the volume. With the increase of pressure, the density of water increases. The higher density of water at specific conditions provides a better chance to penetrate the biomass structure [20].

The viscosity of water is the resistance of water from the external stress such as tensile strength and shear strength. It refers to the resistivity of the water over movement or deformity. The viscosity of water decreased with the increase of temperature but only a slight change was observed when the pressure increased in the subcritical region. However, a more effect of higher pressure was found in the supercritical region on the higher value of viscosity. The viscosity has a direct effect on biomass fractionation. Since the small value of water viscosity provides better wettability of the biomass, the penetration of water to destroy the biomass structure increases (**Figure 6**).

#### **2.2. Steam explosion**

Steam explosion, one of the most widely employed hydrothermal technologies for pretreating lignocellulose in industrial applications to convert biomass into useful chemicals, has been recognized as an environmental friendly pretreatment method that can effectively enhance subsequent enzymatic hydrolysis without the necessity of using chemicals, except water, which can lower environmental impact, lower capital investment, bring more potential for energy efficiency, and give rise to less hazardous process chemicals and conditions; this offers several attractive features when compared to hydrolytic acid and oxidative processes. Steam explosion involves exposing wet lignocellulosic biomass to high-pressure saturated steam (0.69–4.83 MPa) and temperature

**Figure 6.** Viscosity of water at various temperatures and pressures [21].

declines with the increase of reaction temperature and dramatically drops with the reduction of pressure. Therefore, the reaction of biomass degradation takes place in ionic media for subcritical water extraction. On the other hand, the supercritical water extraction provided a

**Figure 5.** Negative log (base 10) of ionization constant of water at various temperatures and pressures [19].

Density of water is defined as the ratio between the mass and volume of water at a specific temperature and pressure. The density of water is decreased with the increase of temperature due to the expansion of the volume. With the increase of pressure, the density of water increases. The higher density of water at specific conditions provides a better chance to pen-

The viscosity of water is the resistance of water from the external stress such as tensile strength and shear strength. It refers to the resistivity of the water over movement or deformity. The viscosity of water decreased with the increase of temperature but only a slight change was observed when the pressure increased in the subcritical region. However, a more effect of higher pressure was found in the supercritical region on the higher value of viscosity. The viscosity has a direct effect on biomass fractionation. Since the small value of water viscosity provides better wettability of the biomass, the penetration of water to destroy the biomass structure increases (**Figure 6**).

Steam explosion, one of the most widely employed hydrothermal technologies for pretreating lignocellulose in industrial applications to convert biomass into useful chemicals, has been recognized as an environmental friendly pretreatment method that can effectively enhance subsequent enzymatic hydrolysis without the necessity of using chemicals, except water, which can lower environmental impact, lower capital investment, bring more potential for energy efficiency, and give rise to less hazardous process chemicals and conditions; this offers several attractive features when compared to hydrolytic acid and oxidative processes. Steam explosion involves exposing wet lignocellulosic biomass to high-pressure saturated steam (0.69–4.83 MPa) and temperature

radical-oriented environment for biomass fractionation [15–18].

etrate the biomass structure [20].

74 Renewable Resources and Biorefineries

**2.2. Steam explosion**

(433–533 K) for a period ranging from seconds to several minutes and then suddenly depressurizing it to atmospheric pressure, making the biomass undergo an explosive decompression. This pretreatment is the combination of mechanical forces and chemical effects due to autohydrolysis of the acetyl group in hemicellulose. Autohydrolysis takes place from the formation of acetic acid from the acetyl group in the hemicellulose structure at high temperature where water acts as an acid at high temperature. The hemicellulose and lignin bonds are cleaved during the explosion, allowing the hemicellulose become water soluble; water-soluble lignin from plant cell wall is also released from the cleavage action into water phase. The mechanical effect is caused by explosive decompression that occurred from suddenly dropped pressure at the termination of the pretreatment, which induced the cell walls in biomass to undergo structural disruption and expansion. Because of these effects, a part of hemicellulose hydrolyzed and solubilized; lignin was redistributed, lignocellulosic matrix polymer was broken down, particle size was decreased, the degree of polymerization was reduced, and porosity was increased; moreover, cellulose was slightly depolymerized, which led to the improvement of lignocellulose digestibility [22–24].

Supercritical extraction in terms of operating conditions, reaction mechanism, and preferred biomass is shown in **Table 1**.


**Table 1.** Comparison of different hydro-fractionation methods.
