Dissociated Ammonia Ref 5 6

Heating metallurgical-grade anhydrous ammonia in the presence of a catalyst causes the ammonia to dissociate into a mixture of hydrogen and nitrogen molecular gases in a 3-to-1 ratio (75% hydrogen and 25% nitrogen):

Dissociated ammonia is pure, consistent, and dry; the dew point is typically less than -51 °C (-60 °F). Residual undissociated ammonia is generally well below 250 ppm.

Although ammonia molecules begin dissociating at 315 °C (600 °F), 980 °C (1800 °F) provides a practical operating compromise between dissociated rate and equipment life. Higher temperatures must be used to ensure that virtually no undissociated ammonia molecules remain. Generally 1 kg of liquid ammonia yields —1.4 m3 of undissociated ammonia vapor or —2.8 lrf of dissociated ammonia (DA) (hydrogen and nitrogen in a 3-to-l ratio). In English units, 1 lb of liquid ammonia yields 22.5 standard ft3 of undissociated ammonia vapor or 45 standard ft3 of DA.

A typical ammonia dissociator produces 2 to 140 m3/h (70 to 4950 ft3/h) of dissociated ammonia at an outlet pressure of 35 to 100 kPa (5 to 15 psi). Liquid ammonia is fed from a storage tank into a vaporizer, which transforms it into ammonia vapor. A heater in the storage tank maintains gas operating pressure if the temperature of the liquid ammonia falls below the recommended level.

The vapor then passes through a pressure regulator, is preheated, and enters an externally heated retort (typically made from Inconel) that contains a nickel- and iron-bearing catalyst. The ammonia molecules dissociate into molecular hydrogen and nitrogen at 925 to 1040 °C (1700 to 1900 °F). The resulting mixture leaves the retort and passes through a heat exchanger; heat is subsequently transferred to the incoming ammonia vapor. If necessary, the dissociated ammonia can be pressure regulated and piped to the furnace.

Output can be easily controlled between 0 and 100% of capacity by regulating flow at any point along the processing line. This control feature is unavailable on exo and endo gas generators, which typically operate between 70 and 100% of their rated capacity. Some recent designs of endo generators allow greater output reduction.

The most important component of the dissociator is the catalyst chamber or retort, which is generally fabricated from Inconel. This chamber is heated either electrically or by natural gas. If electrically heated, a dissociator requires 0.5 to 1.0 kW (depending on the size of dissociator) of electricity per cubic meter (14 to 28 W/ft3) of dissociated ammonia produced. Figure 13 shows a schematic of a typical ammonia dissociator. Ammonia dissociators are relatively maintenance free compared to endo and exo gas generators.

Outlet dissociated Inlet ammonia a mmon i a{3H2 - N2) vapor (N H3)

Fig. 13 Schematic of ammonia dissociator

Because dissociated ammonia is very dry (low dew point), it is highly reducing to surface oxides, as the high hydrogen-to-water ratio given in Table 3 indicates. Consequently, it is frequently used where the high reducing capability of stable oxides is desirable, such as in sintering of stainless steel. Thermodynamically, the high hydrogen content in dissociated ammonia is decarburizing to carbon steel; kinetically, however, the decarburizing reaction is very slow when the dew point in the furnace remains low (below -40 °C, or -40 °F).

The decarburizing reaction can be further slowed by diluting dissociated ammonia with a lower dew point inert gas such as nitrogen when sintering carbon steel that cannot have any decarburization at the surface. Because dissociated ammonia is very dry, it does not provide optimum burning of the lubricant during delubing in the preheating zone. However, because dissociated ammonia is 75% hydrogen, it has a thermal conductivity higher than nitrogen-rich atmospheres.

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