Isothermal evaporation

Based on differences in melting points and Hquid-phase solubilities four modes of operation possible drown-out, isothermal evaporation, adiabatic evaporation, and cooling (choice depends on stream characteristics). 

Order of preference adiabatic evaporation > isothermal evaporation > cooling >drown-out. 

Isothermal annealing, 23 288—290 transformation diagram for, 23 289 Isothermal dehydrogenation, 23 337 Isothermal evaporation, general separation heuristics for, 22 319-320 Isothermal forging, of titanium, 24 859 Isothermal furnace liners, 13 239-240... 

Water evaporation occurs when the vapor pressure of the water at the surface, which is temperature dependent, is greater than the water pressure in the subsurface, which is dependent on relative humidity and temperature. The isothermal evaporation process is described by Stumm and Morgan (1996) via a reaction progress model, in which the effects of the initial reaction path are based on the concept of partial equilibrium. Stumm and Morgan (1996) describe partial equilibrium as a state in which a system is in equilibrium with respect to one reaction but out of equilibrium with respect to others. As an example, Stumm and Morgan (1996) indicate (Fig. 7.1) that water with a negative residual alkalinity (i.e.. 

Fig. 7.1 Isothermal evaporation (25°C) of natural water leading to CaCO precipitation 2[Ca +] > [Aik] (with pCOj = 10 atm). The concentration factor is the degree by which the water has been reduced by volume compared to the initial solution. (Stumm and Morgan 1996)...

O- and S-donor ligands. [Tl2Cu(C03)2] has been reported. Isothermal evaporation of a solution of CuSO and CsSO at 80°C gives [Cs3Cu2(OH)2-(S04)3],2H20. The reaction of HCIO with Cu(OAc)2 in acetic acid shows the equilibria... 

The isothermal evaporation of a sat. soln. of two salts with a common ion leads to the separation of both salts, but with sat. soln. of three salts, this may or may not occur. The isothermal evaporation of solution II furnishes as solid phases the same three salts as are present as solutes. These soln. were called by W. Meyer-hofer congruent solutions, because, on isothermal evaporation, they furnish as solid phases the same salts as are present as solutes in soln. On the other hand, with solution I the salts which separate as solid phases are different from those present in soln., for sodium chloride appears in place of ammonium hydrocarbonate. In contrast with congruent soln., W. Meyerhofer styled them incongrucnt solutions here the sat. soln. contains a different salt from those which are present as solid phases. The gradual addition of sodium chloride to solution I changes the composition of the soln. until the incongruent solution I passes into II ammonium hydrocarbonate does not change when added in a similar manner to solution II. 

Equations 1 and 2 in the forms stated are attributable to Probert 98), and derivations for the case of heat transfer controlling are given by Kumm 70), Marshall and Seltzer 87), Godsave 37-39), and Goldsmith and Penner 42). The case of mass transfer controlling is derived by Fuchs 31) and compared by Gilbert, Howard, and Hicks 36) to isothermal evaporation into a maintained vacuum. 

As an example, consider the isothermal evaporation of a liquid interface into an unbounded space, previously considered by Danckwerts (Dl). The governing equation is now... 

The spring waters of the Sierra Nevada result from the attack of high C02 soil waters on typical igneous rocks and hence can be regarded as nearly ideal samples of a major water type. Their compositions are consistent with a model in which the primary rock-forming silicates are altered in a closed system to soil minerals plus a solution in steady-state equilibrium with these minerals. Isolation of Sierra waters from the solid alteration products followed by isothermal evaporation in equilibrium with the eartKs atmosphere should produce a highly alkaline Na-HCO.rCOA water a soda lake with calcium carbonate, magnesium hydroxy-silicate, and amorphous silica as precipitates. 

E. C. C. Baly and H. M. Duncan studied the decomposition of ammonia by means of a hot platinum wire. Two types of ammonia may be prepared—an inactive and an active modification—which are decomposed to different extents by the same quantity of energy. The active form is obtained by the slow withdrawal of ammonia from a cylinder containing the compressed gas by warming the cone. aq. soln. and drying the gas by quicklime and by isothermal evaporation of the liquefied gas at its b.p. The inactive form is obtained by the rapid evaporation of the liquefied gas. The inactive gas slowly recovers its activity on remaining in contact with the liquefied gas. The same effect can be produced by gently warming the gas by means of a platinum wire heated at 200°. In order to observe these phenomena, the platinum wire must be activated in the same way as is customary in W. Ostwald s process for the catalytic oxidation of ammonia in air to nitric acid. Alternatively,... 

Now consider the isothermal evaporation of water from a surface and the subsequent diffusion through a stagnant air layer, as shown in Fig. 11-4. The free surface of the water is exposed to air in the tank, as shown. We assume that the system is isothermal and that the total pressure remains constant. We further assume that the system is in steady state. This requires that there be a slight air movement over the top of the tank to remove the water vapor which... 

If the mass dm is supposed to be transferred by isothermal evaporation the work done is ... 

Compute the pH variation resulting from isothermal evaporation (25 Cr) of an incipiently 10 M NaHCOa solution that remains in equilibrium with the partial pressure of CO2 of 3 X 10 atm. 

Water composition is also affected by concentration resulting from evaporation (and evapotranspiration). Example 15.1 will illustrate the principles, procedures, and calculations of the effect of concentrating natural waters by isothermal evaporation for a few simplified systems. These calculations illustrate how the reaction path of natural waters during evaporation depends on the Ca /UCO ratio. In a reaction progress model the effects of initial reactions on a reaction path, for example, on the appearance of a solid stable phase and on the redistribution of aqueous species, are described. Reaction progress models are usually based on the concept of partial equilibrium. Partial equilibrium describes a state in which a system is in equilibrium with respect to one reaction, but out of equilibrium with respect to others. 

Example 15.1. Isothermal Evaporation of a Natural Water Leading to CaCOj Precipitation Compute pH and solution composition during isothermal evaporation (25 C) for three types of calcium bicarbonate-containing waters... 

Only solid solutions were present under the conditions of the investigation of the Fe-Co [242], Ni-Cu [250], and SnTe-PbSe [262] systems given in Table 7. The thermodynamic activities, excess Gibbs energies, mixing enthalpies and excess entropies were determined by the use of the ion intensity ratio method for the Fe-Co and Ni-Cu systems as described for the melts. The partial pressure of the molecules SnTe, SnSe, PbTe, and PbSe were obtained for different compositions of the quasi-binary system SnTe-PbSe using the isothermal evaporation technique. 

It may be pointed out, parenthetically, that in the isothermal evaporation of a liquid we have a case in which there may be a large volume change, the pressure remaining constant, namely, the pressure of the saturated vapour Suppose a cylinder fitted with a piston contains some liquid and saturated vapour Let us consider what is the value of the maximum work which will be done when one grain-mole of the liquid is vaporised We suppose that there is more than one gram-mole... 

Isothermal Evaporation.— The isothermal solubility curves are of great importance for obtaining an insight into the behaviour of a solu-... 

If the formation of double salt is possible, but if the temperature lies within the transition interval, the relations will be represented by a diagram like Fig. 126. Isothermal evaporation of the solution X will lead to the deposition of the component A, and the composition of the solution will alter in the direction DE at the point E, the double salt will be formed, and the composition of the solution will remain... 

Since the salts were originally present in equimolccular proportions, the final result of evaporation will be the pure double salt. If when the solution has reached the point E the salt A which had separated out is removed, double salt only will be left as solid phase. At a given temperature, however, a single solid phase can exist in equilibrium with solutions of different composition. If, therefore, isothermal evaporation is continued after the removal of the salt A, double salt will be deposited, and the composition of the solution will change in the direction EF. At the point F the salt B will separate out, and on evaporation both double salt and the salt B will be deposited. In the former case (when the salt A disappears on evaporation) we are dealing with an incoftgruently saturated solution but in the latter case, where both solid phases continue to be deposited, the solution is said to be congruently saturated -... 

A congruently saturated solution is one from which the solid phases are continuously deposited during isothermal evaporation to dryness, whereas in the case of incongruently saturated solutions, at least one of the solid phases disappears during the process of evaporation. 

Lastly, if the temperature lies outside the transition interval, isothermal evaporation of an unsaturated solution of the composition X (Fig. 127) will lead to the deposition of pure double salt from beginning to end. If a solution of the composition Y is evaporated, the component A will first be deposited and the composition of the solution will alter in the direction of E, at which point double salt will separate out. Since the solution at this point contains relatively more of A than is present in the double salt, both the double salt and the single salt A will be deposited on continued evaporation, in order that the composition of the solution shall remain unchanged. In the case of solution Z, first component B and afterwards the double salt will be deposited. The result will, therefore, be a mixture of double salt and the salt B (congruently saturated solutions). 

How much water must be evaporated off before sodium chloride begins to be deposited, and how much pure sodium chloride can be obtained by isothermal evaporation of the solution at 25° ... 

With the help of the solubility data given on page 299, it is also possible to follow the crystallisation process in a quantitative manner, and to calculate the amounts of the salts successively deposited on isothermal evaporation. Thus, when a solution containing i gram-molecule of potassium sulphate (174 3 grams) and i gram-molecule of... 

Ksenzenko, V.I., Khojamamedov, A.M., Kuliev, Ch.A., and Ovezova, O., The behaviour of rare and isolated elements during the isothermic evaporation of underground mineralized waters at the Gaurdak deposit , Izvestia AS TSSR, Series of Physical-technical chemical and geological sciences, 1978, (3), 104-107. 

The scale of operation often has an overriding importance on the selection of the equipment because of the means used for heat transfer. For very small-scale crystallization work it is common to use radiation. The capacity of such equipment varies from a few liters up to several hundreds of liters per day (of solution cooled). For operation on scales up to several thousand liters per day, it is possible to use tanks with water-cooled coils and an agitator. For large-scale applications where the quantity of solution is thousands of liters per day, it is almost universal practice to use vacuum evaporation to remove the solvent this is true whether the solution is cooled by adiabatic evaporation or in equipment where crystallization occurs because of isothermal evaporation. 

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