Water is an ecologically safe substance that is widespread throughout nature. Below the critical point, the vapor pressure curve separates the liquid and vapor phases (Franck and Weingartner, 1999) and ends at the critical point (Tc = 373°C, pc = 22.1 MPa, and pc = 320 kg m-3). Beyond the critical point, the density of the supercritical water (SCW) can be varied continuously from liquid-like to gas-like values without any phase transition over a wide range of conditions.

Water plays an essential role in HTU. It is therefore critical to understand the fundamentals of water chemistry when subjected to high-temperature conditions. Water is rather benign and will not likely react with organic molecules under standard environmental conditions (20°C and 1 bar). However, when the temperature increases, two properties of water mole­cules change substantially. First, the relative permittivity (dielectric constant), er, of water
decreases quickly when the temperature increases. When the thermal energy increases, the shared electron between oxygen and hydrogen atoms tends to circulate more evenly and the electronegativity of the oxygen molecule is reduced (becomes less polar). For example, when temperature increases from 25°C to 300°C, the relative permittivity decreases from 78.85 to 19.66, resulting in water molecules from very polar to fairly nonpolar, in relative terms. This polarity change makes water more affinitive to the organic hydrocarbons, most of which are nonpolar molecules.

Second, the dissociation of water dramatically increases with the increase in temperature. Water, like any other aqueous solution, splits into H+ and OH~ ions in hydrolysis or disso­ciation. This process is reversible and the rate is sufficiently rapid that it can be considered to be in equilibrium at any instant (Zhang, 2010).

The complete miscibility of supercritical water and gases as well as many organic com­pounds makes SCW an excellent solvent for homogeneous reactions of organic compounds with gases, like the oxidation of organic compounds with oxygen and air. The absence of phase boundaries leads to a rapid and complete reaction. From the macroscopic point of view, SCW is a nonpolar solvent; from a microscopic view, water is a molecule with a strong dipole moment of 1.85 D. Water in the supercritical state is able to react with different compounds. Therefore water is both solvent and reactant in a variety of reactions.

The ionization constant of water increases with temperature and reaches a maximum near 250° C; the amount of dissociation is three times what it would be at ambient temperatures and pressures. Therefore, subcritical water in the 220-300°C region offers opportunities as both a benign solvent and a self-neutralizing catalyst. Here, water acts as both reactant and reaction medium. Water as reactant leads to hydrolysis reactions and rapidly degrades the polymeric structure of biomass to water-soluble products (Kumar, 2010).

Hot compressed water in the sub — and supercritical states exhibits exciting physical and chemical properties, which can be varied continuously from gas-like to liquid-like behavior. This opens up several promising opportunities for separation processes and chemical reactions.