Tank-type neutralisers
The use of an atmospheric or pressurized tank-type neutraliser offers maximum flexibility in managing the acid/ammonia reactions and obtaining the critical heat/liquid phase criteria needed for good granulation when producing a wide variety of granular NPK grades. Because the acid/ ammonia reactions are most often only partially completed in these tank-type neutralisers, they are often referred to as “preneutralisers.” Such preneutralisers are commonly used in most of today’s ammonium phosphate plants and in many NPK plants.
When a preneutraliser is used, large amounts of acid can be partially reacted with ammonia. The degree of reaction performed in the preneutralizer is determined by a number of factors; however, the most important criterion is the production of a fluid slurry that is easy to transport (pump) to the granulator and uniformly distribute onto the rolling bed of material in the granulator.
The fluidity of the preneutralized slurry is maintained through careful control of the NH3:H3PO4 mole ratio (Figure 1), temperature, and free water content of the reacted slurry. The neutralization reactions that are only partially completed in the preneutralizer are completed in the granulator where additional ammonia is added beneath the rolling bed of material. In some cases, phosphoric acid may be added to the granulator to adjust the NH3:H3PO4 mole ratio to achieve optimum granulation.
When sulphuric acid is reacted in combination with phosphoric acid in the preneutralizer, special precautions must be taken to select construction materials that will resist the more corrosive environment caused by the presence of sulphuric acid. Type 316L stainless steel (SS) is not a suitable construction material if sulphuric acid is neutralized in combination with phosphoric acid, a more noble alloy is required. The selection of construction materials for the preneutralizer system should be made only after careful testing of possible construction materials under actual operating conditions. Also, at the higher mole ratio and pH when sulphuric acid is present (for example, about 1.5 and 6.8, respectively), the presence of ammonium sulphate crystals tends to thicken the slurry and make pumping difficult; thus, operation at a lower NH3:H3PO4 mole ratio and pH (about 0.4 and 2.0, respectively) is often preferred, when sulphuric acid is fed to the preneutralizer.
Pipe-type reactors
In the early 1970s, the Tennessee Valley Authority (TVA) demonstrated the feasibility of replacing a conventional tank-type preneutralizer with a novel device referred to as a pipe or “tee” reactor (Salladay and Paulson, 1984). This type of reactor was a radical departure from the conventional tank-type preneutralizer normally used to react large amounts of ammonia with phosphoric acid. The pipe reactor consists basically of a length of corrosion-resistant pipe (about 5·- 15 m long) to which phosphoric acid, ammonia, and often water are simultaneously added to one end through a piping configuration resembling a tee, thus the name “tee reactor.” The acid and ammonia react quite violently, pressurizing the unit and causing the superheated mixture of ammonium phosphate slurry (“melt”) and water vapour to forcefully discharge from the opposite end of the pipe that is positioned inside the granulator. Uniform distribution of the “melt” on top of the rolling bed of material in the granulator is achieved by varying the configuration and orientation of the discharge opening(s) of the pipe. A primary advantage of the pipe reactor over conventional preneutralizers is that the preneutralized slurry pump and piping system is eliminated and a much more concentrated slurry can be delivered directly to the granulator. Thus the chemical heat of reaction is more effectively used to evaporate unwanted water from the process compared with the operation of a conventional preneutralizer. The tee reactor was modified by TVA to also accept an additional flow of sulphuric acid through another pipe inlet located opposite the phosphoric acid inlet, giving the unit a “cross” configuration and thus the name “pipe-cross reactor” (PCR) (Green et.al., 1978; Salladay et al.,1985).
Use of the PCR makes it possible to react a wide variety of phosphoric/sulphuric acid mixtures with ammonia. This capability is particularly useful in NPK granulation plants and allows a greater choice in the selection of raw materials to improve granulation and optimize the overall cost of production. In addition, the problem of ammonium chloride fumes is eliminated because the sulfuric acid does not come in direct-contact with the Muriate of Potash (MOP) that may be in the formulation.
In general, the mixture discharged from the PCR does not require further reaction with ammonia in the granulator. ln some cases, however, the level of reaction in the PCR may be altered (decreased) to minimize the escape of ammonia or to obtain improved granulation characteristics of the “melt” when it is combined with the solids in the granulator.
Several variations of pipe-type reactors (and materials of construction) are currently used in NPK, DAP, and MAP plants; sometimes the pipe-type reactor is used in combination with a conventional tank-type preneutralizer. Perhaps one of the greatest advantages offered by the use of pipe reactor technology in the NPK industry is that it provides an opportunity to effectively use a greater variety of raw materials including, for example, larger quantities of dilute acids and scrubber liquor. This added flexibility in raw material choices can often result in more favourable production costs and at the same time provide a method for disposing of certain problem materials such as excess scrubber liquor. It should be noted, however, that the technology does not fit all situations equally well. Therefore, its potential should be carefully examined with regard to the particular circumstances.
References
Green, H,D.J., Brunner, R., Britt, O. and Super, A.P. (1978). Panel discussion: Recent pipe cross experiences. In Proceedings of the 28th Annual Meeting Fertilizer Industry Round Table, pp. 46-54, Atlanta, GA, U.S.A.
Salladay, D.G., and Paulson, W.H. (1984). Development of IVA pressure reactor for production of ammonium phosphates and its retrofit in conventional plants. 34th Annual Meeting of the Fertilizer Industry Round Table, October 30 -November 1, 1984.
Salladay, D.G., Traweek, M. S. and Louizos, N.H. (1985). Production of monoammonium phosphate sulfate with TVA Pressure Pipe-Cross Reactor. In Proceedings of the 35th Annual Meeting Fertilizer Industry Round Table, pp. 102-113, Atlanta, GA, U.SA
Links to related IFS Proceedings
244, (1986), Adapting a Pipe Reactor to a Blunger for NPK Production,
R J Milborne, D W Philip.
Links to external resources
Ivell, D.M. and Ward, N.D. (1984). ‘The Use of a Pressure Neutraliser for Slurry Granulation,’ IFA Technical Conference, Paris, France, 5-8 November.
Leyshon, D.W., Day, D. (1989). ‘MAP Manufacture Using the Pipe Reactor,’ 1989 Joint Meeting Central Florida and Peninsular Florida Sections of the AIChE, Clearwater Beach, Florida.
Mangat, I.S. and Toral, J.L. (1978). ‘Pipe Reactor – An Innovation for Improvement of Granulation Plant,’ ISMA Technical Conference, Orlando, Florida, USA, 23-27 October.
Moraillon, P. and Cotonea, Y. (1982). ‘Savings on Energy in Granulation of Fertilisers by a New Method of Using Pipe Reactors,’ IFA Technical Conference, Kallithea, Greece, 5-7 October.
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