Another concept for handling discharges of ammonia from a refrigeration system is to absorb the ammonia in water. ANSI/ASHRAE Standard 15–94has traditionally listed the discharge of ammonia into a tank of water as an option. The recommended proportions of water to ammonia are 1 m3 of water for each 120 kg of ammonia (1 gallon of water per lb of ammonia). Field tests of the absorption of ammonia in vapor form in a vessel of water showed the effectiveness of this process, as illustrated in Fig. 13.12. During a time interval of approximately 11 minutes, 90 percent of the released ammonia was absorbed to bring the ammonia concentration in the water to the end condition of 120 kg of ammonia in 1 m3 of water (1 lb of ammonia per gallon of water). Figure 13.12 shows the temperatures of the solutions during absorption of both vapor and liquid. In absorbing ammonia vapor, the temperature of the solution rises from the starting temperature of about 20°C (68°F) to 60°C (140°F) in the 11-minute absorption period. Water in the vessel will absorb liquid ammonia equally well, as Fig.13.12 also shows, and the temperature rise is only about 15°C (27°F). The greater rise in temperature when vapor is absorbed is explained by the need to condense the vapor in the absorption process.
Sometimes regulating agencies prefer a facility absorb the entire charge of the plant in a vessel of water. This requirement is normally not practical for a large plant with a charge of, for example, 5,000 kg (11,000 lb) or greater. Even for water vessels capable of absorbing modest-sized discharges, the water in these vessels must be kept from freezing in cold climates.
If a water tank for absorption of the ammonia is used, a question that arises is what to do with the aqua-ammonia solution after ammonia has been absorbed in it. In some localities, firms are available to haul away the solution for reclamation. If the solution is relatively free of oil, it may be used as agricultural fertilizer. If direct discharge to atmosphere is permitted, the ammonia can be vented off to the atmosphere naturally, sometimes even heating the solution slightly to speed the release. In connection with the absorption studies reported in Fig. 13.12, some desorption tests showed that when a vessel was vented through a tube, it required about 50 days to bring the ammonia concentration down to one-half of its starting concentration.
One of the methods of disposing of the aqua-ammonia solution is to dilute and discharge the mixture into either the sanitary or storm sewer. This action should be taken only after consultation with the appropriate authorities who
are likely to specify a maximum permitted pH value of the diluted mixture. It is appropriate, then, to explain pH values and also to emphasize what some magnitudes of pH mean with respect to the mass concentration of ammonia in the water. The pH value, which means potential of hydrogen, is defined as:
H+=number of gram-ions of hydrogen per liter
Pure water contains 0.00000001 gram-ions of H+ per liter, so pure water possesses a pH value of log (10,000,000) or 7. Absorption of ammonia in water decreases the number of hydrogen ions, so the pH value of an ammonia/water solution exceeds 7, and high pH values indicate a high concentration of ammonia. An important point is that only a small amount of ammonia can rapidly raise the pH value from 7. Table 13.3 shows the percentage concentrations by mass resulting in various pH values. The table shows that even one part of ammonia in 100,000 parts of water elevates the pH to 10. The consequence of this distribution of pH values is that if an ammonia/water mixture can be discharged only if the pH value is below 11, for example, then enormous quantities of dilution water will be required.