Under peculiar conditions, water’s properties change a lot compared to other material. The sub critical water or pressurized hot water is obtained at temperatures between 100°C (the boiling point) and 374°C (critical temperature) and at high pressure as the water in the M&F Technology process consists of saturated pressurized vapor (see saturated vapor pressure).
The change in the water’s properties are due to very strong hydrogen bonding : bond alignment is measured by relative permittivity which is already high at normal temperature for water and this permittivity allows water to dissolve salts.
At the sub critical state, water behaves as an organic solvent and an ionic product.
Viscosity and surface tension of water drop and diffusivity increases with increasing temperature. Self-ionization of water increases with temperature, and the pKw of water at 250 °C is closer to 11 than the more familiar 14 at 25 °C. This means the concentration of hydronium ion (H3O+) is higher, and hence the pH is lower (although the level of hydroxide (OH−) is increased by the same amount so the water is still neutral). Specific heat capacity at constant pressure also increases with temperature, from 4.187 kJ/kg at 25 °C to 8.138 kJ/kg at 350 °C. A significant effect on the behaviour of water at high temperatures is decreased dielectric constant (relative permittivity). These properties may be then used for separating lignin, cellulose and hemicellulose of the biomass.
The pressurized hot water can be more corrosive, that is why M&F Technology puts a lot of emphasis on the materials used for the reaction chamber.
If the needed energy for superheated water extraction is less than one sixth that needed for steam distillation, with the M&F Technology process the energy needed is even lower as the heat is mainly provided by the collisions between particles. Only the rotated torque requires energy, but for a very short time as the reaction time is between 30 seconds and 1 minute (depending to the input). Due to the sharp rise in the temperature, the M&F Technology process and therefore machine are closely monitored by an electronic system.
The results of water effluent analysis show that no fatty acids are recovered in the expelled water at the end of the process : the.y can have been destroyed by the heat or the action of the hot pressurized water is limited.