Eutectic points solidification

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

As a final note regarding the Fe-C phase diagram, the eutectic temperature corresponding to the minimum melting point of the Fe-C system is 1,130°C. As the liquid is cooled at the eutectic temperature, solidification of ledeburite will occur. The microstructure of ledeburite is tiny austenite crystals embedded in a matrix of cemen-tite. At carbon concentrations less than the eutectic i.e., 4.3 wt% C), ledeburite and austenite will form a solid solution. By contrast, increasing carbon concentrations will result in ledeburite/cementite solutions. 

If a 20% fructose solution is selected. Young [17] shows in diagram (Fig. 2.63), that pure ice freezes out from this mixture at appr. -2.5°C this is a result of the known phenomenon of freezing point reduction in solutions. The freezing-out of pure ice, however, causes an increase in the sugar concentration and thus a further freezing point depression. This continues until a mixture of ice and fmctose dihydrate is present at -10°C. This lowest common solidification point is called eutectic point . 

On continued cooling of the system both the components, A and C, will crystallize simultaneously and the composition of the melt will move on the boundary line ei —et until the ternary eutectic point et is attained, where also the component B begins to crystallize. At the eutectic temperature the system has no degree of freedom (k = 3, / = 4, V = 0), which means that its cooling will stop. The system maintains the eutectic temperature until its whole solidification. Completely analogical is the crystallization process of mixtures lying in the crystallization fields of the components B and C. 

Curve No. 3 shows freezing of the eutectic mixture. Only one delay can be seen on the curve, caused by evolution of the crystallization heat of both the components. At the eutectic point the binary system has no degree of freedom, since two solid phases coexist two solid phases and the melt saturated by both the components (k = 2, f = 3, v = 0). The temperature of the system thus stays constant again until its total solidification. In practice, however, we can see again at the end of the delay a decrease in temperature caused by the evolution of more heat to the surroundings, than it can be evolved by the crystallization of components. 

Frozen Aqueous Solutions Concentration Effects. As solute is rejected by the growing ice and as its concentration increases in the shrinking liquid phase, the temperature drops toward the eutectic point, where the entire system approaches complete solidification. Of major importance is the fact that a highly concentrated liquid phase can persist indefinitely at any point above the eutectic temperature. 

In this case, as in the case of those systems where the pure components are deposited, a minimum freezing-point is obtained. In the latter case, however, there are two freezing-point curves which intersect at a eutectic point in the case where mixed crystals are formed there is only one continuous curve. On one side of the minimum point the liquid phase contains relatively more, on the other side relatively less, of the one component than does the solid phase while at the minimum point the composition of the two phases is the same. At this point, therefore, complete solidification and complete liquefaction will occur without change of temperature, and the solid solution will accordingly exhibit a definite melting-point. ... 

Somewhat different, however, is the result when the solution has an intermediate composition, as represented by or x. In the former case the dodecahydrate will first of all separate out, but on further withdrawal of heat the temperature will fall, the solution will become relatively richer in ferric chloride, owing to separation of the hydrate, and ultimately the eutectic point D will be reached, at which complete solidification will occur. Similarly with the second solution. Ferric chloride dodecahydrate will first be formed, and the temperature will gradually fall, the composition of the solution following the curve CB until the cryohydric point B is reached, when the whole will solidify to a conglomerate of ice and dodecahydrate. 

Concentration-Temperature Diagram.—In this diagram the temperatures are taken as the abscissae, and the composition of the solution, expressed in atoms of chlorine to one atom of iodine, is represented by the ordinates. In the diagram, A represents the melting-point of pure iodine, 114°. If chlorine is added to the system, a solution of chlorine in liquid iodine is obtained, and the temperature at which solid iodine is in equilibrium with the liquid solution will be all the lower the greater the concentration of the chlorine. We therefore obtain the curve ABF, which represents the composition of the solution with which solid iodine is in equilibrium at different temperatures. This curve can be followed down to 0°, but at temperatures below 7 9 (B) it represents metastable equilibria. At B iodine monochloride can be formed, and if present the system becomes invariant B is therefore a quadruple point at which the four phases, iodine, iodine monochloride, solution, and vapour, can co-exist. Continued withdrawal of heat at this point will therefore lead to the complete solidification of the solution to a mixture or conglomerate of iodine and iodine monochloride, while the temperature remains constant during the process. B is the eutectic point for iodine and iodine monochloride. 

When the formulated solution contains essentially saline or organic solutes that crystallize easily, the interstitial phase will crystallize out abruptly as an eutectic or a mixture of eutectics. The crystallization results in an immediate hardening of the frozen system, which becomes fully rigid. At this point, the system has reached its maximum temperature for complete solidification (eutectic point, Te), which is a basic parameter of the freeze-drying process. 

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