Water-molten salt systems

Fig. 4. In the Solar Two Project a molten salt system shown in the scheme replaces Solar One s water/steam system. In operation, "cold" molten salt is pumped from a storage tank to a receiver on a tower. Sunlight reflected from a field of sun-tracking mirrors heats the salt in the receiver to 1050°C. The heated salt then flows down into a hot storage tank where it is pumped to a heat exchanger to produce the steam that drives a turbine. Some of the hot molten salt can also be stored to produce steam on demand at a later time. Salt cooled to 550°C in the steam generator recirculates through the system and...

Based on the results of the Solar One plant. Southern California Edison formed a consortium that included DOE and EPRI to constmct a Solar Two Project. Solar Two will convert the idle Solar One central receiver plant from a water/steam system to a molten salt system, thereby improving efficiency and operating performance. With the molten salt technology, solar energy can be collected during the day and stored in the salt to produce electricity when needed. The three-year demonstration is scheduled to begin in late 1996. 

Because of the inherent technical difficulties, investigations of transport properties in molten salts are much less common than those of aqueous solutions. However, interpretation of the phenomena seems to be even simpler in molten salts where water is not involved. Molten salt systems are considered to be the simplest liquid electrolytes. Data have been compiled largely due to the great efforts of the Janz group." "... 

Heat is removed from the regenerator by means of the circulating catalyst, supplemented by water-cooled tubes. This technique eliminates the need for the complex and expensive temperature-control system used in the Houdry process (the closely spaced, perforated inlet and outlet pipes, and the circulating molten-salt system). 

Electrochemically, the system metal/molten salt is somewhat similar to the system metal/aqueous solution, although there are important differences, arising largely from differences in temperature and in electrical conductivity. Most fused salts are predominantly ionic, but contain a proportion of molecular constituents, while pure water is predominantly molecular, containing very low activities of hydrogen and hydroxyl ions. Since the aqueous system has been extensively studied, it may be instructive to point out some analogues in fused-salt systems. 

Mixed solvent systems containing water are included below even in case water is only a minor constiment, molten salts are listed after liquid solvent-based systems. 

Heat transfer operations. Heat transfer fluids other than steam and cooling water utilities are sometimes introduced into the design of the heat exchange system. These heat transfer media are sometimes liquid hydrocarbons used at high pressure. When possible, higher boiling liquids should be used. Better still, the flammable material should be substituted with a nonflammable medium such as water or molten salt. 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

Ionic liquids are, quite simply, liquids that are composed entirely of ions. Thus, molten sodium chloride is an ionic liquid a solution of sodium chloride in water (a molecular solvent) is an ionic solution. The term ionic liquids was selected with care, as it is our belief that the more commonly used phrase molten salts (or simply melts) is referential, and invokes a flawed image of these solvents as being high-temperature, corrosive, viscous media (cf. molten cryolite). The reality is that room-temperature ionic liquids can be liquid at temperatures as low as — 96°C, and are typically colorless, fluid, and easily handled. To use the term molten salts to describe these novel systems is as archaic as describing a car as a horseless carriage. Moreover, in the patent and recent academic literature, ionic... 

Catalyst-philic liquid phases can be used to promote the catalytic activity of heterogeneous catalysts, and to facilitate product-catalyst separation. A variety of different constituents of such catalyst-philic phases can be used, the most attractive being quaternary ammonium and phosphonium salts, PEGs, as well as water and other kinds of low-temperature molten salts. In each system, the catalyst-philic phase is characterized as being separate from the remainder of the reaction mixture, and the catalyst should reside within this phase. 

However, there are no known SB systems with Mg in aqueous solutions. The Mg anode s irreversibility in aqueous solutions is thought to be due, in part to the existence of monovalent Mg ions during the electrochemical discharge, in part to the selfcorrosion and film formation, and in part caused by other factors (136,140). All attempts to deposit this metal on the negative electrode from aqueous electrolytes have failed. It is claimed that the Mg cell with molten salt electrolyte, LiCl-KCl eut., is reversible (141) it operates at temperatures above the eutectic melting point, i.e. about 400°C. Small amounts of water might decrease the operating temperature. 

Prospects for TR Electrolyte SBs. In view of the harmful effects often cited in the literature of even small traces of water on the operation of non-aqueous batteries with alkali metal anodes, it might be supposed that electrolytes of the TR composition cannot be applied in such batteries. This same idea may dominate when molten salt SBs are considered. Such a general conclusion cannot be justified. A dilute solution of water in a salt has the structure either of this salt proper or its adjacent hydrate, and the energy, properties and reactions of this water are quite different from those of pure water or of dilute solutions of various compounds in it. On the other hand, a small amount of water in the electrolyte system will decrease its melting point and increase its conductivity. Mixtures of water with such liquids as some alcohols or dioxane and other aprotic and even proton-forming substances, may open new prospects for... 

Pure ILs have a dual nature since they are actually molten salts or a mixture of cations and anions. They were found to have a relatively high solvent polarity, comparable to that of short-chain alcohols [4-5]. Since CCC needs to work with a Diphasic liquid system, water-insoluble ILs should be selected if an aqueous phase is desired, l-butyl-3-methylimidazolium hexa-fluorophosphate ([C4CiIm][PFg]) has limited water solubility (18 g/L or 1.3% or 63 mM [5]) and is easy to synthesize. It was the first IL used in CCC [6]. 

Dense ionic fluids are not all that new if one examines the many applications of molten salt use in chemistry to date. A good deal of the work is in electrochemistry where the relatively high temperatures are less of a limitation but the relation between low-temperature molten salts and ionic fluids certainly exists. It would be wise neither to completely depend on nor to completely ignore all that has been learned with molten salts and molten salt chemistry. Some highly reactive, easily oxidized metals are readily purified in molten salt solvent systems without the problems with oxygen or the decomposition of water with release of hydrogen. 

The chemistry of molten salts as nonaqueous solvent systems is one that has developed extensively from the 1960s till the present, and only a brief survey can be given here. The most obvious difference when compared with the chemistry of aqueous solutions are the strongly bonded and stable nature of the solvent, a concomitant resistance to destruction of the solvent by vigorous reactions, and higher concentrations of vanous species, particularly coordinating anions, than can be obtained in saturated solutions in water. 

Mixtures of equisized charged spheres were also treated by the MSA. Such a system is then uniquely characterized by the ratio of the critical temperatures of the pure components. Harvey [235] found that a continuous critical curve from the dipolar solvent to the molten salt is maintained until the critical temperature of the ionic component exceeds that of the dipolar component by a factor of about 3.6. This ratio is much higher than theoretically predicted for nonionic model fluids. We recall that for NaCl the critical line is still continuous at a critical temperature ratio of about 5. Thus, the MSA of the charged-hard-sphere-dipolar-hard-sphere system captures, at least in part, some unusual features of real salt-water systems with regard to their critical curves. 

---