Liquid metals isothermal system

Mass-transfer deposits can lead to blockages in non-isothermal circulating systems, cis in the case of liquid-metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum e.g. by eliminating oxidising impurities or by maintaining reducing conditions over the melt . 

The corrosion resistance of various metals and alloys in high-temperature liquid lithium is shown in Figure 11. Unfortunately, lithium is much more corrosive than sodium. Consequently, it will be impossible to take full advantage of its many attractive heat-transfer properties until a satisfactory container material is found. The most corrosion-resistant pure metals in a static isothermal system are molybdenum, niobium, tantalum, tungsten, and iron. Of the commercially available structural materials, no alloys tested to date have had satisfacto corrosion resistance at a temperature above 1400 F. for extended time periods in systems where temperature differentials exist. Even though iron has good resistance in static isothermal lithium, iron and iron-base alloys suffer from mass trans-... 

Although the term mass transfer as used in liquid metal technology normally refers to the phenomenon described above, a second type of mass transfer has been observed in isothermal liquid metal systems due to the presence of more than one container metal or alloy. For example, nickel will transfer to and deposit on molybdenum in sodium at 1800°F and will dissolve from type 304 stainless steel to precipitate on iron in lithium at 1800°F. The possibility of such dissimilar metal mass transfer must be considered every time an additional material is proposed for use in an engineering system as a valve seat, impeller bearing, and so forth. Not much data are available on usable material combinations, and the tendency is to design for a single container alloy whenever possible. 

The ternary systems of these kinds of metals with boron reveal a more complex structure because of the presence of many other ternary phases denoted to as tp-and co-phases. The stoichiometries of these solid solutions thereof. Other ternary phases have the composition and M M B2,v, e.g., TaNiB2, Mo2peB4, and MosCoB. As an example, an isothermal section of the B-Co-Mo system is shown in Fig. 27 in which both the x- and the (p-phases are linked with Co as the binder [128], However, in systems with Fe replacing Co, a <,c-phase does not exist. Hence co is in equilibrium with liquid metal and is thus likely to form a cermet material with Fe like the x-phase Mo2Fei3Bs (Fig. 28). Phase compositions located in the pseudo-binary equilibria with a metal can easily be pressureless liquid phase sintered at temperatures between 1500°C and 1700°C. Wear-resistant parts have been developed from Mo2FeB2-Fe cermets with Ni or Cr additives [129-131, 307]. Figure 29 presents an isothermal section of the Ni-Ta-B system at 950°C [126] with three ternary phases where only X is in equilibrium with metallic Ni. 

Besides masking, elemental transfer can occtir between two dissimilar materials in contact with the same liquid metal (even isothermally) due to an activity gradient. Again, experiments with only one solid material in contact with the liquid metal would avoid this complication. However, if this type of system cannot be used, careful surface analysis of as-exposed specimens shordd be used to try and understand mass transfer contributions from dissimilar material driving forces. 

As noted above, the mass transfer kinetics of temperature gradient loops are usually described with reference to dissolution in the hot leg. It is possible to quantitatively study the dissolution step using the rotating cylinder technique. Unlike loop studies, this technique allows one to study dissolution in a system where the hydrodynamic conditions are fully defined. Experimentally, solid cylinders of the test material are rotated at various speeds in an isothermal liquid-metal bath. Changes in the concentration of solid in the liquid and changes in the cylinder radius are determined as a function of time. With these data it is possible to determine the mass transfer coefficient and the rate-controlling step for dissolution. 

In an isothermal system, with no forced convection of the liquid metal, the dissolution process will stop when the solubility limit of the dissolved species is reached. However, in general, the system is anisothermal, the solubility increasing with temperature, dissolution will occur in hot zones and deposition will occur in cold zones of the system. 

Transition Region Considerations. The conductance of a binary system can be approached from the values of conductivity of the pure electrolyte one follows the variation of conductance as one adds water or other second component to the pure electrolyte. The same approach is useful for other electrochemical properties as well the e.m. f. and the anodic behaviour of light, active metals, for instance. The structure of water in this "transition region" (TR), and therefore its reactions, can be expected to be quite different from its structure and reactions, in dilute aqueous solutions. (The same is true in relation to other non-conducting solvents.) The molecular structure of any liquid can be assumed to be close to that of the crystals from which it is derived. The narrower is the temperature gap between the liquid and the solidus curve, the closer are the structures of liquid and solid. In the composition regions between the pure water and a eutectic point the structure of the liquid is basically like that of water between eutectic and the pure salt or its hydrates the structure is basically that of these compounds. At the eutectic point, the conductance-isotherm runs through a maximum and the viscosity-isotherm breaks. Examples are shown in (125). 

Both systems are suitable to check whether or not there is a directly proportional relationship between the width of the homogeneity range of a compound and the growth rate of its layer, predicted by the diffusional theory.5 It is clear that in view of the presence of the liquid zinc phase during preparation of Ni-Zn and Co-Zn reaction couples, all the inter-metallic phases had equal and favourable conditions to form their nuclei at the interface between nickel or cobalt and zinc, which could then readily grow during subsequent isothermal annealing. 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

Furter [91] has analyzed the state of the art from the point of view of employing the salt effect in industrial processes, especially in extractive distillation. In addition, he ha.s made up a list of references covering the years 1966 to 1977 [91 a]. Schubert et al. [92] investigated the effect of some metal chlorides and other salts on the isothermal = 60°C) phase equilibrium behaviour of the systems n-propanol-water, n-butanol-water and methanol-water. Using CH30H/H20/NaBr as an example, the method of predicting salt effects for vapour-liquid equilibria as developed by Schuberth has been extended to uusaturated solutions [92a]. 

Density and conductivity isotherms for fluid mercury are shown in Fig. 2.4 (Gotzlaff, 1988). These data are qualitatively similar to those of cesium shown in Fig. 2.3. An important quantitative difference, however, is the value of the conductivity in the immediate vicinity of the critical point. The conductivity of mercury near the critical point is about two orders of magnitude lower than that of cesium near its critical point. This simple comparison shows that there is no universal behavior of the electronic properties of fluid metals. Moreover, such data raise the possibility that a continuous MNM transition may be distinct from the liquid-vapor transition in some fluid metal systems. 

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