Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 88.4.10 Minimisation of NH 3 emissions from granulationDescriptionThe liquid urea feedstock to the solidification section, either as a melt or as a concentratedsolution, contains some dissolved NH 3 . This arises from residual traces of ammoniumcarbonates, urea decomposition products and the dimerisation to biuret. This residual NH 3 isstripped/flashed during the solidification process and is released along with cooling air to theair.Gaseous formaldehyde, vaporised from a fine liquid mist injected into the hot air stream at thegranulator vent, reacts preferentially with the stripped NH 3 to form HMTA (hexamethylenetetramine). In this hot dry atmosphere, this is the preferential reaction rather than the standardurea-formaldehyde reaction. The urea-formaldehyde reaction would be prevalent in the diluteurea solution stage of the scrubbing operation downstream. The unstable HMTA compound isdissolved in the dilute scrubbing liquor (in this example, process water condensate) and isrecycled to the vacuum concentration section, where HMTA decomposes to NH 3 andformaldehyde. The formaldehyde is kept in solution, where it reacts with the large excess ofurea and finally becomes a part of the granulation additive. By absorption in the process watercondensate, NH 3 is recycled to the urea synthesis section. The formaldehyde finally ends up inthe product urea, where it is anyway used as a standard additive.Achieved environmental benefits• 50 % reduction in NH 3 emission.Cross-media effectsNone believed likely, if it is assumed that formaldehyde would have been used as a standardadditive anyway.Operational dataSee description.ApplicabilityGenerally applicable.EconomicsNo information provided.Driving force for implementationReduced emission levels.References to literature and example plants[133, Hydro Fertilizer Technology, 2000], YARA, Sluiskil and Incitec, Brisbane354 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers