In a recent Stainless Steel World article Mark Brouwer discussed the materials challenges presented during the production of urea.
Urea is produced by the reaction of ammonia and carbon dioxide in a reactor, under a pressure of 150-220 bar and temperatures in the range 170-200°C. The corrosive conditions are severe in all high pressure parts where the intermediate component ammonium carbamate is present. Several factors govern the corrosion of stainless steel in carbamate solutions.
Corrosion rates are influenced by the temperature, the amount of carbamate and possibly by the ammonia/carbon dioxide ratio. In 1953 the invention of oxygen addition allowed the application of (austenitic) stainless steels. If there is not sufficient oxygen present, austenitic stainless steels will corrode actively at high rates, whereas only relatively small amounts of oxygen are sufficient to keep them in the passive state.
It all started with 316L type stainless steels. The amount of oxygen necessary is determined by the type of steel being used. To avoid active corrosion, oxygen is added continuously to the feed of a urea plant. The effect of dissolved oxygen on the corrosion rate of different stainless alloys is illustrated in Table 1. The table confirms the better performance of ASTM 310MoLN (x2CrNiMoN 25.22.2), which needs less oxygen to passivate and shows less loss of material in both the active and passive states in comparison with ASTM 316L.
The urea synthesis conversion improves at higher temperatures, but at the same time high temperatures will also increase the corrosion rate of metallic materials. A continuous challenge to develop better materials is therefore a must in urea plant technologies.
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