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Właściwości i zastosowania wody pod- i nadkrytycznej

Światła-Wójcik D.

Wiadomości Chemiczne

2010

[Z] 64, 3-4

225-242

In the last decades, sub- and supercritical water has received continuously increasing attention as a reaction medium. As safe, non-toxic, readily accessible it is used in chemical synthesis, waste destruction and biomass processing [1–4]. A broad area of technological and industrial applications of sub- and supercritical water arises from its physical and transport properties falling between those of a gas and a liquid. The solvent properties of water can rapidly change with increasing pressure and temperature [2, 5, 10]. Above the critical point (Tc = 647.1 K, Pc = 22.06 MPa) water becomes highly compressible and diffusive. The static dielectric constant approaches values characteristic for low-polar solvent (Fig. 5). Contrary to liquid water at ambient conditions, supercritical water is a poor solvent for ionic species but is well miscible with hydrocarbons and gases (Fig. 6). The ionic product of supercritical water can be a few orders of magnitude higher than in ambient water (Fig. 4) with consequent effect on the kinetics and mechanisms of chemical reactions. By adjustment of thermodynamic conditions one can tune density, viscosity, polarity or pH of water to the desired solvation properties without any change in the chemical composition. An alternation in the character of water solvent near and above the critical point is the consequence of the structural transformations in the hydrogen-bonded network. As evidenced by many experimental and simulation studies the average number of hydrogen bonds per molecule and the lifetime of H-bonds decrease with increasing temperature and decreasing density [2, 10, 19]. With respect to experiment computer simulation plays an equal, and sometimes pivotal role, in quantitative characterization and understanding of water under extreme conditions. Precise definition of an H-bond employed in computer simulation allows one to examine size and topology of clusters of hydrogen-bonded molecules for various thermodynamic states [17, 19]. Such knowledge is invaluable to link features of the hydrogen bonding with the macroscopic properties of water [10, 19]. This article provides an overview of three aspects concerning water from ambient to supercritical conditions. In Chapter 1 the physical and transport properties are reviewed. Features of hydrogen bonding and a relationship between the molecular engagement in hydrogen-bonded clusters and macroscopic properties of water are discussed in Chapter 2. Chapter 3 focuses on technological and industrial applications of sub- and supercritical water. The summary concludes on main research needs.

Determination of heat effects during the oxidation of biomass in subcritical water

Puczyńska I., Skrzypski J., Imbierowicz M., Troszkiewicz M.

Proceedings of ECOpole

2009

Vol. 3, No. 2

365--371

The search of alternative energy sources is one of the most important elements of sustainable development policy. The biomass is a valuable source of renewable energy but its transformation into biofuel needs a big amount of energy because many kinds of biomass have high hydrations level. The wet oxidation (in subcritical water conditions) of substances containing biopolymers gives a beneficial energetic effect of the process. We do not lose energy for the dehydration of biomass, which is directed towards the burning process. The waste containing cellulose (or another similar biopolymers) is a potential, valuable source of renewable energy. The wet oxidation process in subcritical water conditions can become an attractive alternative for the conventional burning processes.

Poszukiwanie alternatywnych źródeł energii jest jednym z ważniejszych elementów polityki zrównoważonego rozwoju. Biomasa jest w tym kontekście cennym źródłem energii odnawialnej, jednak ze względu na fakt, iż wiele rodzajów biomasy ma wysoki stopień uwodnienia, przekształcenie jej w biopaliwo wymaga dużych nakładów energetycznych. Stosując utlenianie substancji zawierających biopolimery w warunkach wody podkrytycznej (tzw. mokre utlenianie), uzyskujemy korzystny efekt energetyczny procesu, gdyż nie tracimy energii na odwadnianie strumieni kierowanych do spalania. W tym ujęciu odpady zawierające celulozę (lub inne podobne biopolimery) są potencjalnym, cennym źródłem energii odnawialnej, a proces utleniania w warunkach wody podkrytycznej może stać się atrakcyjną alternatywą dla konwencjonalnych procesów spalania.