Chemistry behind Ammonia conversion NO2/NO3 in cooling water system.

 Chemistry behind Ammonia conversion NO2/NO3 in cooling water system.

Cooling Towers installed at Urea Plants face severe challenges of high nitrates due to ammonia ingress in cooling water system. Ammonia gas is highly soluble in water(32 g per 100 mL at 25 °C). It usually enters in cooling tower due to induced draft fans of tower from below sources;

Evaporation Ponds

Ammonia flare system

Ammonia compressors due to gasket related issues

Ammonia heat exchangers due to leakages

Wind direction towards cooling tower increases ammonia concentration in cooling water and results nitrites and nitrates in the water. In many cases ammonia impact is so much higher that after a few days of ammonia ingress, cooling water pH drops below 7.0 and soda ash is dosed to bring it normal.Similarly because of high ammonia ,higher bacterial counts observe resulting black deposits on coupon(high corrosion rate on carbon steel).That’s why proper bacterial control is utmost important during high ammonia in cooling water system.

Nitrification Process Overview

The biological conversion of ammonia to nitrate is called nitrification. There are two types of bacterias in cooling water system i.e aerobic and anaerobic .Bacterias which converts Ammonia into NO2/NO3 are aerobic bacteria and can be tested through TVC dipslides method as well.

There are 2 types of microbes that can nitrify ammonia

 1) Both soil and the ocean contain archaeal microbes, that convert ammonia to nitrites. They may enter in cooling tower through air or through uncleaned raw water reservoir.

2) Nitrifying bacteria such as Nitrosomonas and Nitrobacter that are present in freshwater are also capable of nitrifying ammonia. 

The C02 produced by organisms in water, is converted into Carbonic acid. But Carbonic acid is unstable. It breaks down to form bicarbonate or carbonate and hydrogen ions

H20 + C02 => H2C03 

H2C03 => HC03- + H+ => C03 + 2H 

Ammonia oxidizing bacteria (AOB) like Nitrosomonas, Nitrospira, Nitrosococcus, Nitrosolobus and Nitrosovibrio oxidize ammonia to gain energy. The energy is used in metabolizing their food i.e. Carbonate.

In the first step, ammonia is converted into hydroxylamine with the help of an enzyme called mono-oxidase. NH3 + O2 + 2 H+ + 2 e- => NH2OH + H2O 

In the second step, hydroxylamine is converted into nitrite NH2OH + H2O => NO2- + 5H+ + 4e-

In the second step, hydroxylamine is converted into nitrite NH2OH + H2O => NO2- + 5H+ + 4e-

The total reaction can be written as NH3+ 1.5 02 => H+ + H20 + N02- 

Since ammonia is just a source of energy, this reaction produces very little sludge/slime. Conversion of Ammonia to Nitrite produces acid (H+) and it requires minimum 3.9 hours to convert ammonia to NO2.

 In the second step, Nitrite oxidizing bacteria (NOB) like Nitrobacter, Nitrospina, Nitrococcus and Nitrospira bacteria convert nitrite to nitrate with the help of nitrite oxido-reductase. N02- + 0.5 02 => N03- 

The total reaction of Nitritation and Nitratation can be written as 

NH3 + 202 => N03- + H+ + H20 

Research has shown that both nitrite formation and nitrate formation run at different velocities. Therefore, if nitrite formation is faster than nitrate formation, there will be buildup of nitrite as well as nitrous acid (HN02-) If nitrate formation is faster, nitrite may be undetectable. Nitrite formation produces 274.91 kJ/mol energy whereas nitrate formation produces 74.16kJ/mol. Due to the high energy available, ammonia oxidizing bacteria (AOB) can double in 7-8 hours where as nitrite oxidizing bacteria (NOB) takes 10-13 hours

What to Do incase of Ammonia Ingress in Cooling Water?

Cooling water system can be successfully operated in presence of ammonia. In many cases <0.5mpy corrosion rates were maintained on carbon steel coupon while operating system under ammonia leakages(20ppm NH3 in CWS).

As ammonia enters in cooling water system it initially increases pH and if proper bacterial control is not in place it will decrease pH after few days. It will also increase Total Bacterial Counts.

During ammonia ingress,acid dosing should be increased to maintain cooling water pH as per normal operating limit.

Maintaining 1ppm free chlorine in the system twice /day(evening and night) is the ultimate solution for cooling water system.

Non-oxidizing biocide addition will further decrease nitrification process.

TBC must be maintained <10^4 cfu/ml during ammonia ingress period.

Conclusion

Ammonia ingress in cooling water systems is major problem at urea plants as it is bacteria food. It increases bacterial growth exponentially resulting pitting corrosion .Prompt response as per ammonia concentration in cooling water will mitigate nitrification process. It takes around 14 hours to convert ammonia into nitrates(nitric acid).Ammonia in cooling water will increase pH initially and will shift system towards scaling side. During ammonia ingress period rise in biocide demand has been observed. Ammonia reduction from source should be first choice to overcome nitrification process. Chlorine/Bromine are very effective oxidizing biocide for nitrifying bacterias. Isothiazolin based biocide will decrease nitrates within 24-48hours.

With best regards,

Dr. Amar Nath Giri