A hydrothermal process is one that involves water at elevated temperatures and pressures. High-temperature water (HTW) refers to water in its liquid state below its critical temperature and pressure (374 °C, 221 bar), whereas it becomes a highly compressible fluid called supercritical water (SCW) above this point.
An advantage of hydrothermal processing for biomass is that hot water can serve as a solvent, a reactant, and even a catalyst or catalyst precursor. While many biomass compounds (e.g., lignin, cellulose) are not water-soluble at ambient conditions, most are readily solubilized in HTW or SCW. These soluble components can then be subject to hydrolytic attack, engendering fragmentation of bio-macromolecules. Water, both in its dissociated and native form, can help catalyze hydrolysis and other reactions.
Hydrothermal processing relies on the unique properties of water at high temperature and pressure. Most importantly, elevated temperatures diminish hydrogen bonding and reduce water’s dielectric constant (see figure below). Thus, many organic compounds become completely miscible in HTW. From a solubility perspective, HTW behaves much like a non-polar organic solvent. Furthermore, the ion product (KW = [OH–][H+]) for liquid water increases with temperature, being nearly three orders of magnitude higher at 250 °C than it is at ambient temperatures. Accordingly, HTW boasts higher H+ and OH– concentrations than ambient liquid water, increasing the effectiveness of hydrolysis and dehydration reactions, which are known to be acid or base catalyzed. The properties and applications of SCW, which is more gas-like, are discussed elsewhere in detail.
Figure taken from: P. Kritzer, E. Dinjus, Chem. Eng. J., 2001, 83, 207.
Text adapted from: Savage, P.E., R.B. Levine, C.M. Huelsman. “Hydrothermal Processing of Biomass.” Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals. Ed. M. Crocker. Royal Society of Chemistry, 2009.