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

Chapter 9Scrubbing devices• packed columns• venturi scrubbers• irrigated sieve plates.AN emissions from neutralisers are very difficult to remove because the particles are very fine.A combination of droplet separators and scrubbers can be used. For all the above, scrubberswould require the addition of acid, normally nitric acid, to neutralise any free ammonia and tooptimise its removal. Process to process heat exchange is preferred where practicable forcondensation of the steam. Alternatively, water or air cooled exchanger(s) are required.9.2.5 Prilling and granulationThe prilling technique (the formation of spheres by the solidification of liquid droplets offertiliser material) is used in many plants for the production of AN and in some plants for CAN.The granulation of AN may be carried out in a dedicated plant, or in one which can also produceCAN. CAN and NPK fertilisers can be produced in the same installation.9.2.5.1 PrillingThe AN from a prilling tower needs to be virtually anhydrous. To achieve this, the slurry issprayed through one-component nozzles, perforated plates, or perforated centrifuges at the topof the tower. Cold air is drawn through the tower countercurrently to extract the heat ofcrystallisation. The droplets solidify into round granules as they fall through the tower, and areremoved at the foot of the prilling tower, where they are then cooled and screened. When CANis being produced, ground filler material (limestone and/or dolomite) is added before the slurryis prilled. Sometimes prills from the prilling tower may be increased in size by processing in afattening drum.NH 3 and AN (and filler material in CAN production) become entrained in the air discharged atthe top of the prilling tower. A lower melt temperature can reduce emissions. Ammonia isusually removed by absorption in a wet scrubber. Small particles of ammonium nitrate(miniprills) are carried out with the air and these can be removed using comparatively simpleequipment. AN fume is also lost from the surface of the prills, which are submicron in size andmore difficult to remove.9.2.5.2 GranulationGranulation, in contrast to the prilling technique, needs a more complicated plant and a varietyof equipment, including rotating pans and drums, fluidised beds and other more specialisedequipment. The main advantage of granulation, environmentally, is that, although the nature ofthe effluent is similar, the quantity of air to be treated is much smaller so typically theabatement equipment is more cost effective. The granular product can also be made in a widerange of particle sizes, which are larger than prills. Some granulation processes can use ANcontaining up to 8 % water but this water still has to be removed in the process. However, as thetemperature needed is lower, a better overall energy economy is possible.368 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers