Ammonia, enthalpy-concentration chart

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. 

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. 

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). 

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