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

Chapter 99.2.2 NeutralisationThe exothermic neutralisation of HNO 3 with NH 3 gas produces ANS (ammonium nitratesolution) and steam. The HNO 3 is normally preheated in corrosion-resistant equipment,particularly if the concentration of nitric acid is at the lower end of the 50 – 70 % range.Preheating using steam or hot condensate from the AN process is the most effective use of thisexcess heat.The amount of preheat required can be estimated from an enthalpy energy balance using theconcentration of the HNO 3 and the concentration of AN from measured physical data.Neutralisation is carried out in a single-stage or in two-stages. A two-stage neutraliser operateswith a low pH in the first stage (acidic conditions) and a neutral pH in the second stage. Theequipment can operate at a variety of operating pressures and temperatures. Pressure,temperature and concentrations in most neutralisers are linked by the boiling pointcharacteristics of the AN solutions, with only two of these variables being independent. As anexample, when operating a neutraliser at 4 bar and controlling the ANS at 76 % w/w, thetemperature is 180 ºC.For safety reasons, the temperature in the neutraliser has to be limited. To achieve this, a part ofthe condensed steam may be recycled back to the neutraliser. Alternatively this may be achievedby limiting the acid strength led to the neutraliser. The same temperature would be achieved byfeeding the neutraliser with a 55 % HNO 3 . Hence, in a conventional process, it is not possible totake full advantage of using a high concentrated HNO 3 for reduction of the water content.NH 3 gas may contain small quantities of unreacted H 2 and inert N 2 . These are vented from theneutraliser system at a suitable point, depending on the characteristics of the particular process.At the operating temperature of the neutraliser, impurity control is of great importance becausea safety incident would constitute a significant environmental incident. Some manufacturers donot recycle AN screenings to the neutraliser for this reason. Recycling is especially to beavoided if the screenings are contaminated by an organic anticaking additive. It should be notedthat an acidic ANS is more unstable than an alkaline solution.The selection of neutraliser pressure involves, amongst other things, a trade off betweeninherent safety and energy efficiency.Neutralisers may be free boiling vessels, circulating systems, or pipe reactors. The factors whichinfluence the choice of neutraliser are:• a two-stage neutraliser produces most of the boil-off steam in the first stage and most of theNH 3 emission in the second stage. This reduces the total NH 3 emission• a single-stage neutraliser is simpler and cheaper• neutralisation at an elevated pressure will produce steam at a higher temperature. This steamis more usable for downstream processes such as evaporation and drying• the control of the neutraliser is a critical parameter. The pH and the temperature must bothbe strictly controlled to limit losses from the neutraliser. All installations must, therefore,include pH and temperature controls, using reliable equipment, which must be tested on aroutine basis.An example of a neutraliser is the pipe reactor. The neutralisation reaction is carried out in apipe. The design allows an efficient mixing of HNO 3 and NH 3 , which in turn allowsminimisation of the residence time. Both, NH 3 and HNO 3 are preheated using a part of theprocess steam generated in the reaction. Raw material dosing is optimised by flowratemeasurement. Because of the mixing and the reaction, the pressure decreases within the pipefrom about 4 – 7 bar at the reactor head to about 1 bar in the following separator tank, where theoutlet of the pipe reactor is located. In the separator, the ANS flows down to the bottom and thegenerated process steam flows up. The ANS overflows by gravity to a buffer, where the pH is366 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers