Other Early AlternativesAn improvement in the TVA-type process for making DAP is to substitute a pipe reactor for the preneutralizer. Ammonia and phosphoric acid react in a pipe which discharges directly into the granulator. Steam generated by the reaction is released in the granulator and swept out with a current of air. This arrangement is simpler and less expensive than the use of a tank preneutralizer. An additional advantage is that it uses more concentrated phosphoric acid; thus, slurry discharged into the granulator contains less water and the recycle ratio is decreased. Fittel and Hollingsworth report that a plant in Australia using the pipe reactor has a recycle ratio of 3:1 (as compared with a 5:1 ratio for tank preneutralization) [9], A somewhat similar system is used in Spain by the firm of S.A. Cros to produce DAP and other grades of compound fertilizers [10]. The use of pipe reactors in granulation processes will be described further in a later section of this chapter and in the chapter “Compound Fertilizers” (16).
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