**1. Introduction**

The fossil fuel demand from industrialization and domestic utilization has been continually rising, which is in contrast to the depleting supply of petroleum resources that leads to public concerns for the adequacy of long-term energy supply and also environmental issues due to greenhouse gases being drastically released. In addition, the expanding consumption of natural resources also drives the global community to force with economic problems. The replacement of supplies from fossil fuels, which is one of the challenging tasks, has been of intense concern. The use of alternative energy from renewable resources is a promising solution not only for long-term environment sustainability but also in economic aspects. Plant biomass including agricultural, forestry, herbaceous,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and residue, which is a sufficiently abundant natural renewable resource, has been considered as a suitable alternative carbon source that can be converted into useful sustainable products and varieties of chemicals. Among these, the exploitation and utilization of biomass energy have motivated and attracted a great deal of interest from around the world due to a power opportunity to improve energy security, reduce the trade deficit, dramatically lower greenhouse gas emissions, and improve price stability [1]. Besides the advantages mentioned above, agricultural biomass such as crop residues are generated with large quantity annually, making them promising sources for further utilization due to their abundance, diversity, and low-cost. Therefore these potential biomass residues can play important roles as sustainable carbon sources.

*Hemicellulose*: the second most abundant polymer is a complex, random, and amorphous branched carbohydrate comprising of different polysaccharides, including hexoses (d-glucose, d-mannose, d-galactose), pentoses (l-arabinose, d-xylose), and uronic acid with 50–200 units. The backbone of hemicellulose is either a homopolymer or a heteropolymer with short branches linked by β-(1, 4) glycosidic linkage or β-(1, 3) glycosidic linkage and groups of acetates were randomly attached with ester linkages to the hydroxyl groups of the sugar rings [3]. Hemicellulose has a lower molecular weight when compared to cellulose. Moreover, hemicellulose has short lateral chains, which provide linkage between cellulose and lignin, making hemicellulose easier to hydrolyze and degrade into monosaccharides than cellulose

Hydro-Fractionation for Biomass Upgrading http://dx.doi.org/10.5772/intechopen.79396 71

*Lignin*: it is a complex hydrophobic, large molecular structure containing cross-linked heteropolymers of three different main phenolic components which are trans-p-coumaryl alcohol, trans-coniferyl alcohol, and trans-sinapyl alcohol, which shield the polysaccharide fibers from external environment stress, microbial attacks, and oxidative stress. Lignin is recognized as the cellular glue and encrusting material due to the existence of strong carbon▬carbon bond connection (C▬C) and ether linkages (C▬O▬C), which together provide compressive strength to different compositions and individual fibers of lignocellulosic biomass (**Figure 2**). The high crystallization region, high degree of polymerization, different connection forces between each composition, the protection effect from hemicellulose, and lignin of the lignocellulosic agricultural residue cell wall are stable and make it hard to be degraded for utilization in a further step; therefore, to convert lignocellulosic agricultural residue to biofuels, energy, or chemical platforms, a large number of pretreatment approaches have been investigated on a wide variety of feedstocks to deconstruct and fractionate the complex network structure to its simpler molecules in order to increase the efficiency of biomass composition utilization. Several fractionation technologies have been developed during the last decades. Those methods are usually classified into physical, biological, chemical, and physicochemical pretreatments. The several key properties to take into consideration for low-cost and advanced pretreatment processes are (a) the large amount of yield and harvesting time of feedstock, (b) the large volume of accessible pretreated substrate, (c) less sugar degradation, (d) a minimum number of inhibitors generated after the reaction, (e) a reasonable size and cost of reactor, (f) less solid waste production, (g) effectiveness at low moisture content, and (h) the minimum heat and power requirement [5].

Considering the concerns above, the most cost-effective processes in the biomass upgrading in the industry utilize the dispensable pretreatment and fractionation process where water most

**Figure 2.** Examples of cellulose (left) hemicellulose (middle) and lignin (right) structures.

[4]. This allows hemicellulose to be removed under mild reaction conditions.

The term "lignocellulosic agricultural residues" is used for describing all organic materials which are produced as by-products from harvesting and processing agricultural crops. Chemically, lignocellulosic agricultural residue can be generally regarded as being composed of three polymers including 40–50% of cellulose, which is a major component, 25–30% of hemicellulose, and 15–20% of lignin along with smaller amounts of pectin, protein, nitrogen compounds, and inorganic ingredients [1]. Crystalline and amorphous bundles of cellulose form a skeleton surrounded by the covalently linked matrix of hemicellulose and lignin [2]. These polymers are associated with each other in a hetero-matrix and varying relative compositions depending on the system, type, species, age, stage of growth, and even source of biomass, and they can be in the form of liquids, slurries, or solids. **Figure 1** displays three main components of lignocellulosic biomass.

According to **Figure 1**, each component of lignocellulosic biomass is described below.

*Cellulose*: the most enormously bountiful biopolymer in the world and the main source of the C6 sugar unit is a linear homo-polysaccharide of d-glucose linked together by β-(1, 4) glycosidic linkages, with cellobiose as the smallest repetitive unit. The long cellulose chains linked together with β-(1, 4) orientation results in the formation of intermolecular and intramolecular hydrogen and van der Waals bonds, which cause cellulose to be packed into microfibrils; they are fine structures bundled up together to form cellulose fibers with highly crystalline structure causing its stable properties, insoluble in water unless at high temperatures or with the presence of a catalyst, and are resistant to enzyme attacks [1, 3].

**Figure 1.** Lignocellulosic biomass composed of cellulose, hemicellulose, and lignin.

*Hemicellulose*: the second most abundant polymer is a complex, random, and amorphous branched carbohydrate comprising of different polysaccharides, including hexoses (d-glucose, d-mannose, d-galactose), pentoses (l-arabinose, d-xylose), and uronic acid with 50–200 units. The backbone of hemicellulose is either a homopolymer or a heteropolymer with short branches linked by β-(1, 4) glycosidic linkage or β-(1, 3) glycosidic linkage and groups of acetates were randomly attached with ester linkages to the hydroxyl groups of the sugar rings [3]. Hemicellulose has a lower molecular weight when compared to cellulose. Moreover, hemicellulose has short lateral chains, which provide linkage between cellulose and lignin, making hemicellulose easier to hydrolyze and degrade into monosaccharides than cellulose [4]. This allows hemicellulose to be removed under mild reaction conditions.

and residue, which is a sufficiently abundant natural renewable resource, has been considered as a suitable alternative carbon source that can be converted into useful sustainable products and varieties of chemicals. Among these, the exploitation and utilization of biomass energy have motivated and attracted a great deal of interest from around the world due to a power opportunity to improve energy security, reduce the trade deficit, dramatically lower greenhouse gas emissions, and improve price stability [1]. Besides the advantages mentioned above, agricultural biomass such as crop residues are generated with large quantity annually, making them promising sources for further utilization due to their abundance, diversity, and low-cost. Therefore these potential

The term "lignocellulosic agricultural residues" is used for describing all organic materials which are produced as by-products from harvesting and processing agricultural crops. Chemically, lignocellulosic agricultural residue can be generally regarded as being composed of three polymers including 40–50% of cellulose, which is a major component, 25–30% of hemicellulose, and 15–20% of lignin along with smaller amounts of pectin, protein, nitrogen compounds, and inorganic ingredients [1]. Crystalline and amorphous bundles of cellulose form a skeleton surrounded by the covalently linked matrix of hemicellulose and lignin [2]. These polymers are associated with each other in a hetero-matrix and varying relative compositions depending on the system, type, species, age, stage of growth, and even source of biomass, and they can be in the form of liquids, slurries, or solids. **Figure 1** displays three

According to **Figure 1**, each component of lignocellulosic biomass is described below.

*Cellulose*: the most enormously bountiful biopolymer in the world and the main source of the C6 sugar unit is a linear homo-polysaccharide of d-glucose linked together by β-(1, 4) glycosidic linkages, with cellobiose as the smallest repetitive unit. The long cellulose chains linked together with β-(1, 4) orientation results in the formation of intermolecular and intramolecular hydrogen and van der Waals bonds, which cause cellulose to be packed into microfibrils; they are fine structures bundled up together to form cellulose fibers with highly crystalline structure causing its stable properties, insoluble in water unless at high temperatures or with

biomass residues can play important roles as sustainable carbon sources.

the presence of a catalyst, and are resistant to enzyme attacks [1, 3].

**Figure 1.** Lignocellulosic biomass composed of cellulose, hemicellulose, and lignin.

main components of lignocellulosic biomass.

70 Renewable Resources and Biorefineries

*Lignin*: it is a complex hydrophobic, large molecular structure containing cross-linked heteropolymers of three different main phenolic components which are trans-p-coumaryl alcohol, trans-coniferyl alcohol, and trans-sinapyl alcohol, which shield the polysaccharide fibers from external environment stress, microbial attacks, and oxidative stress. Lignin is recognized as the cellular glue and encrusting material due to the existence of strong carbon▬carbon bond connection (C▬C) and ether linkages (C▬O▬C), which together provide compressive strength to different compositions and individual fibers of lignocellulosic biomass (**Figure 2**).

The high crystallization region, high degree of polymerization, different connection forces between each composition, the protection effect from hemicellulose, and lignin of the lignocellulosic agricultural residue cell wall are stable and make it hard to be degraded for utilization in a further step; therefore, to convert lignocellulosic agricultural residue to biofuels, energy, or chemical platforms, a large number of pretreatment approaches have been investigated on a wide variety of feedstocks to deconstruct and fractionate the complex network structure to its simpler molecules in order to increase the efficiency of biomass composition utilization. Several fractionation technologies have been developed during the last decades. Those methods are usually classified into physical, biological, chemical, and physicochemical pretreatments. The several key properties to take into consideration for low-cost and advanced pretreatment processes are (a) the large amount of yield and harvesting time of feedstock, (b) the large volume of accessible pretreated substrate, (c) less sugar degradation, (d) a minimum number of inhibitors generated after the reaction, (e) a reasonable size and cost of reactor, (f) less solid waste production, (g) effectiveness at low moisture content, and (h) the minimum heat and power requirement [5].

Considering the concerns above, the most cost-effective processes in the biomass upgrading in the industry utilize the dispensable pretreatment and fractionation process where water most

**Figure 2.** Examples of cellulose (left) hemicellulose (middle) and lignin (right) structures.

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

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**.

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

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

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].

) 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

The ionization constant of water is the ratio between the concentration of ionic ([H3

[11], and the participation of salt might cause fouling that diminishes the

SO<sup>4</sup> ,

O]+ and

Hydro-Fractionation for Biomass Upgrading http://dx.doi.org/10.5772/intechopen.79396 73

suitable operating conditions for some specific proposes.

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

Na2 CO<sup>3</sup>

[OH]−

, and K<sup>2</sup>

SO<sup>4</sup>

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 status and potential uses, life cycle and bioeconomy.
