Ammonia-Water Equilibrium

Ammonia is the principal basic gas in the atmosphere. Absorption of NH3 in H20 leads to 

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Carbon Dioxide/Water Equilibrium 345 Sulfur Dioxide 348 Ammonia/Water Equilibrium 353 Nitric Acid/Water Equilibrium 355 Equilibrium of Other Important Atmospheric Gases Aqueous-Phase Reaction Rates 361 S(IV) to S(VI) Transformation and Sulfur Chemistry 363... 

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

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10% ammonia-air mixture is scrubbed with water and the concentration of ammonia is reduced to 0.1%. If the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interface, derive from first principles an expression for the rate of absorption at any position in the column. At some intermediate point where the ammonia concentration in the gas phase has been reduced to 5%. the partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/nr and the transfer rate is ]0 3 kmol/m2s. What is the thickness of the hypothetical gas film if the diffusivity of ammonia in air is 0.24 cm2/s ... 

The water equilibrium always exists in aqueous solution. In general, we can focus our initial attention on the equilibria involving other major species (NH3 in this example). Nevertheless, the water equilibrium does exert its effect on the concentrations of OH and H3 O. In this example, the concentration of hydroxide anion is established by the ammonia equilibrium, but the concentration of hydronium cations must be found by applying the water equilibrium. We use this feature in several of our examples in this chapter. 

Ammonia is to be recovered from a 5 per cent by volume ammonia-air mixture by scrubbing with water in a packed tower. The gas rate is 1.25 m3/m2s measured at 273 K and 101.3 kN/m2 and the liquid rate is 1.95 kg/m2s. The temperature of the inlet gas is 298 K and the temperature of the inlet water 293 K. The mass transfer coefficient is Kaa = 0.113 kmol/m3s (mole ratio difference) and the total pressure is 101.3 kN/m2. What is the required height of the tower to remove 95 per cent of the ammonia. The equilibrium data and the heats of solutions are ... 

TABLE 2. Ammonia-Water Vapor-Liquid Equilibrium... 

Pour 2 ml of a dilute ammonia solution into a test tube and heat it. How does equilibrium shift in the ammonia-water system when the temperature is changed ... 

Distillation of Ammonium Bicarbonate Solutions. Vapor-liquid equilibrium data for ammonium bicarbonate solutions at the boil are apparently not available in the literature. The data in the literature, however, do indicate that when the temperature of such a solution is increased, or the pressure on it decreased, the gas that is evolved is predominantly carbon dioxide. Thus, it appears that such a distillation would be two consecutive processes first, a steam stripping of the carbon dioxide in the solution, followed by a distillation of ammonia from an ammonia-water mixture containing perhaps some carbon dioxide. Possibly the ammonia, carbon dioxide, and water in the distillate product would recombine completely in the condenser to form an ammonium bicarbonate solution. Perhaps an absorption tower would be necessary to effect the recombination. 

Probably when ammonia dissolves m water, equilibrium is set up between the several molecules as indicated below ... 

Construct the equilibrium line. The equilibrium line relates the mole fraction of ammonia in the gas phase to that in the liquid phase when the two phases are at equilibrium. Equilibrium is assumed to exist between the two phases only at the gas-hquid interface. For dilute systems, Henry s law will apply. It applies for liquid mole fractions less than 0.01 in systems in general, and, as can be seen in Example 11.5, for the ammonia-water system it applies to liquid mole fractions as high as... 

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. 

The gas stream leaving the cyclone contains hot air, the excess ammonia, water evaporated from the nitric acid solution in the reactor and from the collected liquid in the cyclone, and 3% of the ammonium nitrate in the reactor effluent. The stream leaves the separator at 233°C, passes through the air preheater, and enters a partial condenser where some of the water and ammonia and essentially all of the nitrate are condensed. The equilibrium relationship between the compositions of the vapor and liquid streams leaving this unit may be expressed in the form... 

For ionic melts, we should mention that solvents of the second kind have been studied more intensively than those belonging to the first kind. Also, the treatment of the results former media is simpler. According to the data obtained on equilibrium constants, the general oxoacidity scale may be presented to a sufficient accuracy in the following manner (Fig. 1.1.3). The extremely wide range of liquid state of ionic liquids presented there forces us to picture solvents which cannot coexist at the same temperature in the same scale, but a similar situation is observed for protic solvents (liquid ammonia, water). So, the destruction of nitrate melts runs at temperatures near... 

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. 

An example of the use of the standard-addition method is the determination of ammonia in aquaria and sea water [19] using an ammonia-gas electrode. To a 100 ml sample is added a sufficient number of NaOH pellets to raise the pH above 11 (to convert all ammonium ion to ammonia). The equilibrium potential is read, then a sufficient volume of standard NH4CI solution to approximately double the concentration is added, and the new equilibrium potential is read. The effective detection limit for this method is approximately 1 ppb NH3 but below about 10 ppb considerable time is required for the electrode to stabilize, making the method somewhat impractical at the 1-10 ppb level. Results compare favorably with the phenol-hypochlorite spectrophotometric method for ammonia. 

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