The effect of ammonia on low-temperature shift catalysts
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In current low-variation water circulation processes, ammonia water, synthetic vent gas, or ammonia tank vent gas is added to the system in order to prevent equipment corrosion. However, in some enterprises, internal leakage occurred in the synthesis reactor; after steam was added at a reduced pressure, it was found that the catalyst activity decreased, even though the pH value of the circulating water was not high. Actually, both of these situations are normal. Ammonia is usually added to the system in the hot water tower; even if there is a sudden excess of ammonia, it will be carried away by the shifted gas (the outlet temperature of the hot water tower is higher than that of the saturated tower, and the volume of the shifted gas is 1.25 times that of the semi-water gas). In fact, under pressurized shift conditions, by maintaining the pH value of the water system between 8.4 and 13.5, the ammonia partial pressure in the gas exiting the saturated tower remains at the PPm level, thus preventing any damage to the catalyst. However, if the added steam contains ammonia, it will cause ammonia in constant concentrations to enter the catalyst layer directly, forming ammonium salts that coat the surface of the catalyst and render it inactive. Even ammonia can form cobammine complexes with cobalt, permanently deactivating the catalyst.
Ammonia water is weakly alkaline and acts as a buffer solution; its pH cannot reach 13.5, with the maximum value being 10
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I think it’s better to add pure water in order to reduce the total solids in the system and minimize the formation of ammonium salts； It is beneficial for the long-term use of the catalyst
I would like to ask: what is the upper limit for the ammonia concentration that can cause poisoning when copper-based low-temperature catalysts react with ammonia? It seems that there are signs of poisoning in my company’s ammonia, with a concentration of 100PPM! :Q
Copper-based catalysts must not come into contact with ammonia, as even trace amounts can have an effect; whereas cobalt-molybdenum-based catalysts are less affected by this