Challenge

 A processing unit in this petrochemical plant utilized a cross flow cooling tower with conventional wood slat fill, and a  recirculation rate of 45,000 gallons per minute for process cooling. Chlorine gas was used at an average rate of 600 pounds per day, supplemented with sodium bromide. A chronic leak of process contaminants into the cooling water caused rapid microbiological growth. The high chlorine/ bromide feedrate coupled with a healthy biofilm throughout the system resulted in excessive corrosion rates of around 10 mpy on carbon steel with a pitting index routinely > 20, frequently reaching 50-60, as measured by an in-line corrator. The algae-biofilm mat blocked many of the distribution nozzles allowing the water to short-circuit through the tower. The drift eliminator was also heavily fouled with algae/ biofilm, restricting airflow throughout the tower, causing poor heat transfer and inefficient cooling. Foaming and odors resulting from hydrogen sulfide produced by sulfate reducing bacteria (SRB’s) present in the biofilm were also problems. The customer required an effective alternative treatment program at no additional cost to the plant.

Solution

 A two-month chlorine dioxide trial was proposed. The treatment program included three phases: cleanup, optimization, and maintenance. During the cleanup phase, chlorine dioxide was fed continuously for 72 hours at 1-2 mg/L based on recirculation rate. The system was then treated intermittently for several hours on a daily basis. A maintenance treatment of 5-hours/ day, 3 times/ week was initiated once the tower was clear of biofilm and algae.

Results

Corrosion rates of < 5 mpy were observed after the chlorine dioxide program was fully implemented. The pitting index was reduced to < 1 mpy, consistent with a well-treated cooling system. Within 48 hours of commencement of the cleanup phase, the basin temperature dropped 10°F and a small decrease in the condenser temperature was observed. Plant personnel also observed a slight increase in amps of the fan motors, providing confirmation of improved cooling. The foaming and odor problems were also eliminated. The annual program cost is less than the original chlorine program. The trial was converted to a permanent installation and the customer continues to be extremely satisfied with the program.