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

Chapter 9automatically adjusted by a small flow of gaseous NH 3 . The pipe reactor process can beoperated with 63 % HNO 3 (preheated to 60 ºC) and can yield a 97 % w/w ANS without furtherconcentration/evaporation.ANS is the product from the neutraliser. The concentrations are variable depending on the feedmaterials and/or operating conditions. The product solution may be sent to storage withoutfurther processing. If it is to be used in the manufacture of solid AN, CAN or NPK fertiliser, itis normally concentrated by evaporation.The starting materials AN and calcium carbonate may be derived from a nitrophosphate process,see Section 7.4.8.9.2.3 EvaporationThe ANS is normally concentrated in an evaporator to the water content required for theparticular product finishing. The water concentration is normally below 1 % for a prilledproduct and up to 8 % for some granulation processes.Steam from the neutraliser or from a steam raising facility on the site is used as the heat sourcefor evaporation. Saturated steam at a controlled temperature must be used to avoid thedecomposition of AN. Evaporation may be carried out at atmospheric pressure or undervacuum. The latter allows for the re-use of neutraliser steam, but it is more capital intensive.There is a large variety of evaporator systems in industrial use; these include circulatory, shelland tube heat exchangers and falling film evaporators. Falling film evaporators have theadvantage of utilising a small working volume and a short residence time. Steam formed duringthe evaporation process might be contaminated with ammonia, along with droplets ofammonium nitrate. Techniques for decontamination include:• droplet separators, as used in the neutralisers• scrubbers, as used for the removal of fine dusts and fumes in the production of solid product• using steam, which can be condensed and used in the system to purify neutralisercondensate.The AN solution produced must be kept at a concentration and temperature that preventscrystallisation of the AN. In order to minimise effluents from downstream equipment, thesolution from the evaporator may need to be cooled.9.2.4 Process steam purificationThe process steam leaving the neutraliser can be used directly or is purified, or it can becondensed and then purified. The steam may be used in the evaporator or it may be used topreheat and evaporate ammonia, and also to preheat the nitric acid. The following techniqueshave been used for the purification of the steam:Droplet separation techniques:• knitted wire mesh demister pads• wave plate separators• fibre pad separators using, for example PTFE fibres.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 367