An American fertilizer manufacturer experienced rapid scaling in an ammonia circulation cooler, likely in the form of calcium phosphate and calcium carbonate. The scaling occurred due to an issue with various manual flow control valves that were inadvertently set incorrectly.
The cooling water used as the cooling medium for these exchangers comes from an open evaporative cooling tower system. This cooling water is supersaturated with calcium-based salts (CaCO3 and CaPO4) due to the concentrating effect of evaporation at the cooling tower. While the cooling water chemical program is designed to inhibit the formation of calcium-based scale in the supersaturated environment, the incorrectly set valves resulted in reduced flows, increased water temperatures and reduced water velocity that resulted in scale formation. The fertilizer manufacturer turned to Veolia for a chemical cleaning solution that would prevent a costly shutdown for offline cleaning, while minimizing the risk of future shutdowns
Veolia recommended its specialty chemical cleaner, Kleen AC9502, to be introduced into the circulating water of the cooling exchanger to remove the calcium deposition. To prevent a decrease in pH levels in the bulk cooling water, the solution was pumped into the inlet of the ammonia cooler in 5-minute intervals. After the initial injection of Kleen AC9502, the injection interval was extended from 5 to 25 minutes. Veolia monitored the cooling tower bulk water pH to ensure that it did not drop to a point where significant mild steel corrosion could occur in the overall system. Veolia also collected water samples from the ammonia cooler outlet which were sent off for analysis to determine the amount of scale removed.
Result
Calcium deposit removal occurred immediately after the first dose of Kleen AC9502 was introduced. Through monitoring of the concentration of calcium in the outlet water, it was estimated that 33 pounds of scale was removed through this cleaning process (Figure 1). The cooling tower bulk water pH remained within control range during the entire procedure, never dropping below 6.7 (see Figure 2), and quickly recovered to within target range in a matter of hours. This indicates that targeting specific exchangers with intermittent injection of the Kleen AC9502 cleaner while a heat exchanger is in operation can effectively remove deposits without risking elevated corrosion rates in the cooling water network due to decreased pH.
With the help of the new chemistry, the IMI Fabi Benwood plant was able to keep the cooling system and its heat exchangers free of deposit during operation, which enabled the system to run in dry-mode during the winter without performance issue and preventing significant operating costs.
Aside from confirming the removal of scale from heat exchanger outlet water analyses, the performance of the cleaning was evident from the immediate 5-degree drop in process outlet temperature. This boosted efficiency and allowed for a ~2.2% increase in UAN production.
Figure 1: Ammonia cooling water outlet calcium concentration during Kleen AC9502 injection. These concentrations in relationship to outlet flow translates to ~33 lbs of calcium deposit removed.
Figure 2: Cooling tower basin pH levels