Ammonia-water equilibrium curve

Figure 9-77. Equilibrium curves for ammonia-water an operating system for production of 8% aqua (by weight) at total pressure of 150 psig.

Suppose, for example, that a mixture of ammonia and water that is 40% NH3 by mass is contained in a closed vessel at 140°F and 1 atm. Point A on Figure 8.5-2 corresponds to this condition. Since this point lies between the vapor and liquid equilibrium curves, the mixture separates into two phases whose compositions are found at the extremities of the 140°F tie line (points B and C),... 

Although the transformation of ammonium carbamate to urea is not a direct dehydration, the presence of water is, however, a limiting factor with respect to the extent that the reaction occurs. It has been found that at any temperature a definite equilibrium is established that can be approached from either direction. Krase and Gaddy have shown that a real shift in the carbamate-urea-water equilibrium is obtained by employing an excess of anhydrous ammonia, which functions as a strong dehydration agcnt. The employment of ammonia, up to 280 per cent of that combined as carbamate, gives conversions to urea between 81 and 85 per cent of the carbamate ammonia. These results are shown in the curve of Fig. 8-8. 

We have selected as base cases those of dry air and of humid air with ammonia water interactions, both with the liquid fraction of = 0.85. These were evaluated in more detail using the model AERCLOUD. Figures 21.1a to d show the computed numbers of moles of ammonia (vapor and liquid phases). The headlines of figures show also the selected initial droplet sizes (1,000 /u,m and 100 /um). The curves marked with HE-limit show the model predictions in the homogeneous equilibrium limit. 

FIGURE 27.7 Number of moles of ammonia vapor and liquid versus time predicted by AERCLOUD, for 85% liquid releases in dry air and in 99.99% humid air allowing for ammonia-water interactions. The initial droplet radius ranges from 1000 /xm to 100 /xm. The curves have been computed using three model options including and excluding droplet ventilation and in the homogeneous equilibrium limit. 

Generally speaking, whenever a substance is distributed between two insoluble phases, a dynamic equilibrium of this type can be established. The various equilibria are peculiar to the particular system considered. For example, replacement of the water in the example considered above with another liquid such as benzene or with a solid adsorbent such as activated carbon or replacement of the ammonia with another solute such as sulfur dioxide will each result in new curves not at all related to the first. The equilibrium resulting for a two-liquid-phase system bears no relation to that for a liquid-solid system. A discussion of the characteristic shapes of the equilibrium curves for the various situations and the influence of conditions such as temperature and pressure must be left for the studies of the individual unit operations. Nevertheless the following principles are common to all systems involving the distribution of substances between two insoluble phases ... 

At a given temperature and atmospheric pressure, the molar ratio of ammonia saturated in the outlet air and in the inlet water can be assumed to remain constant according to Hemy s law, which can thus be used to determine the respective moles of ammonia in a mole of anas a function of the moles of ammonia in a mole of water. Tchobanoglous (14) has prepared a set of curves showing the equilibrium distribution of ammonia in air and water at various temperatures under the condition of atmospheric pressure (Fig. 6). Using Eq. (15) and Fig. 6, the theoretical requirement of air for the ammonia-stripping operation at 100% efficiency can be calculated. For example, at a water temperature of 20°C and an influent ammonia concentration of 20 mg/L, the theoretical air requirement is calculated as follows ... 

If we now inject additional ammonia into the container, a new set of equilibrium concentrations will eventually be established, with higher concentrations in each phase than were initially obtained. In this manner, we can eventually obtain the complete relationship between the equilibrium concentrations in both phases. If the ammonia is designated as substance A, the equilibrium mole fractions in the gas (yA) and liquid ( A) give rise to an equilibrium-distribution curve as shown in Figure 3.1. This curve results irrespective of the amounts of air and water that we start with, and is influenced only by the temperature and pressure of the system. It is important to note that at equilibrium the concentrations in the two phases are not equal instead, the chemical potential of the ammonia is the same in both phases. It is this equality of chemical potentials which causes the net transfer of ammonia to stop. The curve of Figure 3.1 does not, of course, show all the equilibrium concentrations existing within the system. For example, water will partially vaporize into the gas phase, the components of the air will also dissolve to a small extent into the liquid, and equilibrium concentrations for these substances will also be established. For the moment, we need not consider these equilibria, since they are of minor importance. 

If a quantity of a single gas and a relatively nonvolatile liquid are brought to equilibrium in the manner described in Chap. 5, the resulting concentration of dissolved gas in the liquid is said to be the gas solubility at the prevailing temperature and pressure. At fixed temperature, the solubility concentration will increase with pressure in the manner, for example, of curve A, Fig. 8.1, which shows the solubility of ammonia in water at 30 C. 

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