Prilling refers to formation of solid particles or “prills” in an open tower via solidification as droplets fall from a prill head. Exemplary methods of prilling are disclosed in U.S. Pat. Nos. 7,175,684 and 7,575,365, the disclosures of which are incorporated by reference herein.
The quality of prills formed is dependent on several factors, including the consistency of the pressure at the prill plate. Typical methods of controlling the pressure include controlling the height of liquid or slurry in fluid communication with the prill plate. This liquid or slurry may be positioned directly above the prill plate, or may be located in a separate prill head tank in fluid communication with the prill head. In typical systems, in order to aid in maintaining a consistent supply of liquid or slurry to the prill head, a supply of the liquid or slurry to be prilled in excess of the amount required by the prill plate is provided to the prill head, and the excess liquid or slurry is returned to a supply tank located at a lower elevation.
In one typical system, feed liquid or slurry is pumped into a head tank having a weir at a predetermined height. The weir maintains the height of the liquid or slurry in the head tank, while excess liquid or slurry is discharged down the weir pipe. The excess overflows to a supply tank located at a lower elevation than the head tank.
In another typical system, a feed stream is made available to a head tank and a return stream in fluid communication with the feed stream returns excess liquid or slurry not flowing into the head tank to a supply tank located at a lower elevation. The flow of liquid or slurry from the feed stream to the head tank is regulated by a valve. The valve opens and closes in response to commands from a level indicator controller monitoring the level of liquid or slurry in the prill head tank. As liquid or slurry from the prill head tank exits through the prill head, the level indicator controller detects a drop in the tank level and adjusts the valve to allow more liquid or slurry from the feed stream to enter the tank. As more liquid or slurry enters the prill head tank, the level indicator controller detects a rise in the tank level and adjusts the valve to allow less liquid or slurry from the feed stream to enter the tank.
In still another typical system, a feed stream is provided to a prill head tank that feeds a prill head. A return stream returns excess liquid or slurry from the prill head tank to a supply tank located at a lower elevation. A valve in the return stream line opens and closes in response to commands from a level indicator controller monitoring the level of liquid or slurry in the prill head tank. As liquid or slurry from the prill head tank exits through the prill head, the level indicator controller detects a drop in the tank level and adjusts the valve to allow less liquid or slurry to exit through the return stream. If the level indicator controller detects a rise in the tank level, the controller adjusts the valve to allow more liquid or slurry to exit through the return stream.
In other typical systems, a head tank is not used and the liquid or slurry is fed directly to a vessel containing the prill plate.
Maintaining a consistent pressure at the prill plate provides a consistent quality and size distribution in the prills formed from the prilling process. However, small changes in liquid levels during operation can result in a decrease in the quality of prills produced and a wider distribution of prill sizes produced. For example, even in a large head tank, changes of only a few inches in the level of the liquid or slurry may have a profound effect on prill quality and size distribution. Maintaining fine control of the pressure is complicated by several factors. These factors include the highly viscous nature of some liquids or slurries used in the prilling process, the resolution and reaction speed of level sensors, controls, and valves associated with supplying the liquid or slurry, and variations in the pressure and flow rate associated with pumping the liquid or slurry to the top of a prilling tower.
Improvements in the foregoing processes are desired.