Ammonia, enthalpy-concentration

Figure 3.3. Enthalpy-concentration diagram for aqueous ammonia. Reference states enthalpies of liquid water at 0°C and liquid ammonia at —77°C are zero. (Bosniakovic, Technische Thermodynamik, T. Steinkopff,... 

For a long time efforts to improve the efficiency of industrial ammonia production concentrated on synthesis gas production, and major progress was achieved over the years. In ammonia synthesis itself considerable progress was made in converter design and recovery of the reaction enthalpie at high temperature, but there has been no substantial improvement in the catalyst since the 1920s. The standard commercial iron... 

Enthalpy-concentration charts are particularly useful for two-component systems in which vapor and liquid phases are in equilibrium. The Gibbs phase rule (Equation 6.2-1) specifies that such a system has (2 -I- 2 - 2) = 2 degrees of freedom. If as before we fix the system pressure, then specifying only one more intensive variable—the system temperature, or the mass or mole fraction of either component in either phase—fixes the values of all other intensive variables in both phases. An H-x diagram for the ammonia-water system at 1 atm is shown in Figure 8.5-2. 

Figure 8 -2 Enthalpy-concentration diagram for the ammonia-water system at 1 atm. (From G. G. Brown el al.. Unit Operations, 1950. Figure 551. Reprinted by permission of John Wiley Sons.)...

Suppose the mass fraction of ammonia in a liquid solution of NH3 and H2O al 1 atm is specified to be 0.25. According to the phase rule, the system temperature and the mass fraction of NH3 in the vapor phase are uniquely determined by these specifications. (Verify.) A tie line may therefore be drawn on the enthalpy-concentration chart from x - 0.25 on the liquid-phase curve to the corresponding point on the vapor-phase curve, which is at y = 0.95 and the... 

Use the NH3-H2O enthalpy-concentration chart in Appendix I to determine for 1 atm pressure what the mass fraction ammonia is at equilibrium in the gas and liquid phases for a liquid temperature of 130°F. 

FIG. 2-7 Enthalpy-concentration diagram for aqueous ammonia. From Thermodynamic and Physical Properties NH3-H20, Int Inst. Refrigeration, Paris, France, 1994 (88 pp.). Reproduced by permission. In order to determine equilibrium compositions, draw a vertical from any liquid composition on any boiling line (the lowest plots) to intersect the appropriate auxiliary curve (the intermediate curves). A horizontal then drawn from this point to the appropriate dew line (the upper curves) will establish the vapor composition. The Int. Inst. Refrigeration publication also gives extensive P-v-xtah es from —50 to 316°C. Other sources include Park, Y. M. and Sonntag, R. E., ASHRAE Trans., 96,1 (1990) 150-159 x, h, s, tables, 360 to 640 K) Ibrahim, O. M. and S. A. Klein, ASH E Trans., 99, 1 (1993) 1495-1502 (Eqs., 0.2 to 110 bar, 293 to 413 K) Smolen, T. M., D. B. Manley, et al.,/. Chem. Eng. Data, 36 (1991) 202-208 p-x correlation, 0.9 to 450 psia, 293-413 K)i Ruiter, J. P, 7nf. J. R rig., 13 (1990) 223-236 gives ten subroutines for computer calculations. 

Enthalpy-Concentration Mass fraction) Ammonia-Water... 

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. 

The enthalpy of solution of the ammoniate LaI3-6NH3 in liquid ammonia at 25°C is -178.5 kJ mol-1. This value is close to that at infinite dilution, for it was determined at a concentration of Lal3 in the product solution of only 1 in 18,500 (224). Comparisons of this value with those for other iodides, or for aqueous media, are complicated by the fact that the ammoniate rather than anhydrous Lal3 was used for this calori-... 

Similar data for the two-component system water-ammonia are also available and complete, because of the use of this system in absorption refrigeration. The data of Scatchard and coworkers are the most recent 24), and are unique among such compilations in that the availability function is tabulated as well as the usual enthalpy, entropy, and Gibbs free energy. These data have been converted to graphical form (charts) by Kohloss and Scott 16) and Bulkley and Swartz 4). Older data, but more complete in the low concentration range, are those of Jennings and Shannon 11). 

The concentrations of acid sites on all catalysts, measured by aqueous pH titration, appear in the Table, along with molar enthalpies of neutralisation in water, average molar enthalpies of ammonia adsorption in the absence of solvent, and catalytic data. 

This early study is eoncemed with the preparation and the investigation of the properties of thorium metal and of a large number of eompounds oxide, hydroxide, various halides, oxyhalides, halogenocomplexes, their hydrates and adducts with ammonia. Elemental analyses were given for many of the compounds studied. Enthalpies of solution at 288 K of the metal in cQ. 3.6 hd OCl and of a number of compounds in cq. 16000 H2O were reported. Values were given without any detail. Although the concentration of thorium in the calorimetric solutions was not specified in every case, it was assumed by this review to be the same throughout. 

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