The Pipe Reactor Process can be used with superphosphoric acid and anhydrous ammonia to produce liquid fertilizers that have very high polyphosphate contents. Polyphosphates sequester or solubilize the metallic impurities present in wet process acid. The increased solubility of high poly N-P Base solutions prevent the formation of crystals of phosphate salts that precipitate as bothersome sludges in storage tanks and applicator equipment. Because of the abundance of long chain species of polyphosphate, 10-34-00 or 11-37-00, storage life is extended allowing off season production. Formulation costs of NPK grades are reduced because larger quantities of phosphate Fertilizer (P205) from 54 percent orthophosphoric acid can be utilized.
In the pipe reactor, orthophosphoric acid is polymerized to form chains of polyphosphates. The heat of reaction of the two anhydrides, superphosphoric acid and ammonia, is used to drive off molecular water thereby, linking the orthophosphate units into polyphosphates.
The pipe reactor process was developed at a division of the TVA-National Fertilizer Development Center in Alabama by Bob Meline and his associates. The process is continuous-the super acid, anhydrous ammonia and water are pumped into the plant and the product continuously pumps out to storage as liquid polyphosphate fertilizer. This necessitates paying strict attention to process controls, such as flow rates, temperatures, pressures, specific gravity, and pH.
Super phosphoric acid (SPA) is shipped in special insulated railroad cars. It is loaded at the manufacturing facility at a temperature of about 200°F. This allows the acid to arrive at the pipe reactor plant at a temperature at which it is free flowing and easily pumped. Super acid tends to get very viscous and resistant to flow below 70°F. A positive displacement pump with a variable speed drive is used to “meter” the flow of SPA into the plant.
Anhydrous ammonia vapor is used in the pipe. Liquid ammonia is pumped through a vaporizer (heat exchanger) through which the product going to storage is also pumped. The temperature of the hot 10-34-00 is reduced and the liquid ammonia is vaporized. Ammonia vapor is needed in the pipe to insure a more complete reaction and a hotter pipe temperature.
The pipe is typically constructed out of a six inch type 316 stainless steel pipe. The pipe is mounted in an upright configuration about 10 feet high that turns down and drops into the hotwell. The acid enters the side and the ammonia enters the bottom about six inches above the acid entrance.
The super acid and ammonia converge in the bottom of the pipe and react instantly with a release of energy as heat. The temperature quickly reaches 600-650°F and a molten material forms, commonly called “melt”. (At room temperatures, the melt is a glass-like solid and has an analysis of about 10-63-00). The heat of the reaction causes several chemical changes. Water is removed as steam. Phosphorous atoms in the super acid combine to form polyphosphate molecules. The anhydrous ammonia reacts with these molecules to produce the ammonium polyphosphate melt.
The hot melt discharges from the pipe and is immediately cooled and dissolved in recycled 10-34-00. Water is metered into the system to control the product concentration of 34 percent P205. About two-thirds of the ammonia required to make 10-34-00 is fed to the pipe reactor and the balance is added to the recycled solution. The amount of ammonia going to the pipe reactor determines the reaction temperature and thus the percent of polyphosphate. The ammonia fed to the recycled mixture is a control for the final 10-34-00 grade.
It is important to cool the product going to storage to less than 90°F. This is usually accomplished by a two step process. The finished solution is circulated in an evaporative cooling tower, which lowers the recycled product from 170°F to 130°F. The product going to storage is then pumped through a heat exchanger, resulting in two things: 1) it cools the product from 130°F to 90°F; and 2) it vaporizes the liquid ammonia.
The final product grade is controlled by using a simple set of manually operated instruments that measure specific gravity and pH. The specific gravity is determined with a hydrometer. A reading is obtained that allows the determination of the percent of P205. This reading is typically around 1.390 at 70°F. It will vary depending on the source and impurity level of the super acid. The specific gravity is changed by varying the water flow rate into the process. The pH measurement determines the nitrogen to P205 ratio. The ratio for 10-34-00 is .294. Typically, the pH of 10-34-00 is around 6.00. A pH meter that measures out to the hundredth of a unit works best. The pH is controlled by varying the supplemental ammonia that is added to the recycled stream.
Benefits of Pipe Reactor Process and High Polyphosphates
Sequester or dissolve impurities present in the wet process phosphoric acid. This prevents sludge formation.
Provide maximum nutrient concentration.
Have low freezing point – below 10°F.
Polyphosphates can dissolve up to 40 times more micronutrients by weight than orthophosphates.
Produce high quality clear liquids and low viscosity suspensions.
Polyphosphates permit year-round production and storage.
Sludge formation in tanks has been eliminated.
Higher nutrient concentration, 10-34-00/11-37-00 vs. 08-24-00 represent savings in storage and transportation.
Off season production of NPK grades.
Lower cost ortho acid can be added to NPK formulations to produce a more competitive fluid.
Hopwood, L.E., Harwell, A.O., “Super Acid and the Pipe Reactor Process”, American Chemical Society Paper, 1973.
Texas Gulf Research Data, 1973.
Meline, R. S., Lee, R. G., and Scott, W. C., Jr., “Use of a Pipe Reactor in Production of Liquid Fertilizers with Very High Polyphosphate Content”, Fertilizer Solutions, V. 16 #2, 32-45 (1972).
Jernigan, J. D., and Boyd, J. A., “Use of Super Acid in Fluid Fertilizer”, Fertilizer Roundtable Paper, 1972.