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

Chapter 88.2.1.1 Particle formationPrillingConcentrated urea melt is sprayed through a spinning bucket or prill head located at the top of atower. The liquid droplets (“prills”) that are formed fall through the tower and are cooledagainst a countercurrent flow of air which allows the prills to solidify. Further cooling is usuallyrequired. With some plant designs, the cooler is incorporated into the base of the prill tower; inothers, a cooling takes place after the prills have been removed from the tower.GranulationVarious types of granulation equipment are available, including drum granulators, pangranulators and fluidised bed granulators. The basic principle of the process involves thespraying of concentrated melt onto recycled seed particles circulating in the granulator. Anincrease in granule size and drying of the product take place simultaneously. Air passingthrough the granulator solidifies the melt deposited on the seed material. All commercialprocesses are characterised by product recycling. After the formation of the granules, it may benecessary to dry and/or to cool the product.8.2.2 UANFor production of UAN using the partial recycle CO 2 stripping process, see Section 8.4.14.Continuous and batch type processes are used for the production of UAN. In both processesconcentrated urea and ammonium nitrate solutions are measured, mixed and then cooled. Figure8.2 gives an overview.In the continuous process, the ingredients of the UAN solution are continuously fed to andmixed in a series of appropriately sized static mixers. Raw material flow as well as finishedproduct flow, pH and density are continuously measured and adjusted. The finished product iscooled and transferred to a storage tank for distribution.In the batch process, the raw materials are sequentially fed to a mixing vessel fitted with anagitator and mounted on load-cells. The dissolving of the solid raw material(s) can be enhancedby recirculation and heat exchange as required. The pH of the UAN product is adjusted prior tothe addition of the corrosion inhibitor.Process waterUrea (solid or in solution)AN (solid or in solution)SolutionmixingpH adjustment (HNO 3or NH 3)corrosion inhibitorUAN solutionfor storage ordistributionFigure 8.2: Overview of the production of UAN solution[128, EFMA, 2000]326 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers