Tower prilling

Concentration. Evaporation procedures depend on the concentration of the solution produced during neutralization and the water content required for the subsequent production of soHd product. Neutralizer solutions can contain as Httle as 2% and as much as 25% water feeds to drum granulators can contain 5% water, prill towers 0.3 to 0.5% water. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Dilute This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon each other. Specifically, the solids phase is so fuUy dispersed in the gas that the den sity of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal setthng velocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Gravity settling chambers such as prilling towers and countercurrent-flow spray diy-ers are two exceptions in which gas velocity is insufficient to entrain the sohds completely. 

Breakup of a low-velocity liquidjet (Ih/elocity) . This governs in special applications like prilling towers and is often an intermediate step in liquid breakup processes. 

Special requirements such as size uniformity in prilling towers can dictate stiU other approaches to dispersion. Here plates are drilled with many holes to develop nearly uniform columns. 

Prilling tower NH4, NO3 Proper operation control, scrubbers... 

Urea Plants - In urea plants, wet scrubbers or fabrie filters are used to control fugitive emissions from prilling towers fabric filters are used to eontrol dust emissions from bagging operations. These equipment are an integral part of the operations, to retain product. New urea plants should achieve levels of particulate matter in air emissions of less than 0.5 kg/t of product for both urea and ammonia. 

Ammonium Nitrate Plants - In ammonium nitrate plants, wet scrubbers can be considered for prill towers and the granulation plant. Particulate emissions of 0.5 kg/t of product for the prill tower and 0.25 kg/t of product for granulation should be the target. Similar loads for ammonia are appropriate. Other effluents that originate in a nitrogenous fertilizer complex include boiler blowdown, water treatment plant backwash, and cooling tower blowdown from the ammonia and nitric acid plants. 

Urea Plant 6. Use total recycle processes in the synthesis process reduce microprill formation and carryover of fines in prilling towers. 

Figure 10-8. The CT-BISA (Chiyoda Corp.) process for producing bis-phenol A from acetone and phenol. (1) reactor, (2-4) distillation columns, (5) phenol distillation column, (6) crystallizer, (7) solid/liquid separator, (8) prilling tower.

A two-stage evaporative concentration under vacuum, with a limited residence-time in the evaporator to limit biuret formation, produces a urea stream containing about 0.5 per cent water which can be sprayed into a prilling tower. 

In the Prilling Process, such as practiced by the Lion Oil Co (Division of Monsanto Chemical Co, El Dorado, Arkansas), the production centers around a prilling tower, where a coned soln of AN forms into small droplets which flow downward against a stream of air. The resulting slightly moist prills are screened, dried and cooled by air Slid sgmn screened... 

The ammonium nitrate solution (from the neutralizer) is evaporated to ca. 96% concentration in the steam heated vacuum evaporator. The concentrated solution is sprayed into the top of the prilling tower, where it falls through an upward flow of air supplied by blowers at the base of the tower. During its fall it is cooled and solidifies to round pellets or prills of the desired size. 

Figure 12.16. A prilling tower for ammonium nitrate, product size range 0.4-2.0 mm. The dryer is not needed if the moisture content of the melt is less than about 0.5%.

Prilling towers convert molten materials into droplets and allow them to solidify in contact with an air stream. Towers as high as 60 m are used. Economically the process becomes competitive with other granulation processes when a capacity of 200-400tons/day is reached. Ammonium nitrate prills, for example, are 1.6-3.5 mm dia in the 5-95% range. 

Emissions from the prilling and granulation sections of AN and CAN plants can be treated by a range of abatement equipment. Particulate material from some granulation plants is relatively coarse in particle size, whereas the prill tower emissions contain very fine particles. Candle filters are normally required for a prill tower emission. They can abate particulate emissions down to 15 mg/m3 of air. For coarser material dry devices such as bag filters or dry cyclones are used. Particulate emissions can be of higher concentration, perhaps up to 30 to 50 mg/m3, but the recovered material is a solid that can more readily be recycled to the process. 

A schematic diagram of a prill tower is given in Fig. 7.7. Some details of an installation for fertilizer materials are listed in Table 1.2. Melt is provided to the prilling devices at the top of the tower from reactor/evaporator systems in the case of a fertilizer material such as ammonium nitrate or from a melt tank in the case of fusible materials such as petroleum wax and coal tar pitch. Melt droplets travel counter-current to cooling air and the solid prills are conveyed away from the bottom of the tower to appropriate downstream treatments such as cooling, clay treatment and storage. 

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