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Dissolved Oxygen Scavenger Dosing Calculations For Water Treatment




Dissolved Oxygen Scavenger Dosing Calculations For Water Treatment Oxygen scavengers are dosed in water where even the smallest amount of oxygen can cause corrosion to the system which could include piping and equipment made of commonly used materials of construction such as carbon steel .

Beside the well-known application of treating Boiler Feed Water (BFW) with oxygen scavengers there are other applications which may require that the dissolved oxygen content in the water be reduced. One such application could be injection water for oil recovery from oil producer wells. Another application could be handling of produced water in upstream oil and gas where carbon steel piping and equipment is employed. The applications mentioned aforementioned are generally at or near ambient temperatures.

It is a well known fact that water has higher capacity to retain oxygen at lower temperatures than at higher temperatures or in simple terms lower the temperature higher the dissolved oxygen in water and vice-versa.

I had prepared an excel workbook wherein the various oxygen scavengers and their dosing rates are mentioned based on a reputed company standard. In the same workbook I had also provided a table and a graph plotted from the table for dissolved oxygen values in ppm against water temperature. The table is provided as an attachment for the benefit of the readers. The excel workbook is programmed to calculate the dosing rate for a particular oxygen scavenging chemical when the temperature of the water and the water flowrate has been specified as an input.

This particular excel workbook is applicable for dosing rates of oxygen scavenging chemicals in water at or around ambient temperatures. This workbook is not applicable for BFW since BFW is normally at or above its saturation temperature (100°C or above) and oxygen levels at these temperatures tend to be sub-ppm levels or parts per billion levels which is not accounted for in the dissolved oxygen versus water temperature table.

Some of the most commonly used oxygen scavenging chemicals are: 1) Carbohydrazide 2) Diethylhydroxylamine (DEHA) 3) Hydroquinone 4) Methyl Ethyl Ketoxime 5) Sodium Sulfite 6) Catalyzed Sodium Sulfite 7) Ammonium Bisulfite. The attached table provides the names and dosing rates based on per unit ppm of dissolved oxygen in the water to be treated.

Additional dosing is generally required over and above the stoichiometric requirements given in the attached table in order to ensure complete reaction and removal of dissolved oxygen from the water. As a rule-of-thumb dose 20 mg/L of scavenger in excess to the stoichiometric requirement.

Let us consider an example of 45% by weight ammonium bisulfite solution used as an oxygen scavenger. If 10,000 Liter of water containing 8 mg/L oxygen is to be treated with this solution then what would be the quantity of 45% ammonium bisulfite required.

The feed ratio for 100% ammonium bisulfite to oxygen ppm is 8 as defined in the attached table. Hence 45% ammonium bisulfite required:

[8*10,000 L*8 mg/L + 10,000 L*20 mg/L] / 0.45 = 1,866,666 mg or approximately 2 kg of 45% ammonium bisulfite solution.

The calculations are simple and anyone with basic knowledge of excel can program the oxygen scavenger dosing rates based on the attachments I am providing. At higher water temperatures (above 90°C) the calculations are not valid due to the lack of information on dissolved oxygen levels.

Hope to get some comments from the readers of my blog.

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Regards,
Ankur.




I recommend periodically checking any automated DO scavenge system using the CHEMets colorimetric DO test kit (or similar). You should be able to shut off your chemical injection system and see the O2 level increase. Turn it back on and see the sample show in the low ppb range (or where ever you expect to operate). As a side note make sure the person performing the test is not color blind. 

Dear Sir,
Another method that is used as an internal chemical treatment is the addition of chemicals for filming, the goal is to avoid contact with the metal oxygen. within the scope of the industry, which method is used as the reference standard?

Dear Ankur,

 

After dosing what is the O2 concentration in the water needs to be indicated.

Dear Ankur,

 

After dosing what is the O2 concentration in the water needs to be indicated.

Jatin,

 

If you use in excess of the stoichiometric quantity, then theoretically you do not have any residual O2 left in the water.

 

Regards,

Ankur.

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VIKRAM SINGH
Apr 08 2013 06:00 AM

please send calculation for hydrazine hydrate also.

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Mirza Chughtai
Apr 16 2013 05:47 AM

Dear Ankur sahib

We are facing BFW treatment problem at a waste heat recovery power plant using cement mills exhaust gases with a Rankin Cycle. The HRSG's operating at 16 Bar with a little supper heat. 95% good quality condensate returns back to the system through a surface condensor operating at - 0.089Mpa.

Proper water treatment is employed but the feed water system from Vacuum Deaerator to the inlet of economizer is showing corrosion of Carbon Steel.

We are dosing immediately after the vacuum deaerator N2H4 which we religiouly maintaining at 0.1 ppm minimum.

Please let me know what else we must add or do to eliminate this corrosion. By the way the BFW temperature is hardly 40 Celcius.

Dear Ankur sahib

We are facing BFW treatment problem at a waste heat recovery power plant using cement mills exhaust gases with a Rankin Cycle. The HRSG's operating at 16 Bar with a little supper heat. 95% good quality condensate returns back to the system through a surface condensor operating at - 0.089Mpa.

Proper water treatment is employed but the feed water system from Vacuum Deaerator to the inlet of economizer is showing corrosion of Carbon Steel.

We are dosing immediately after the vacuum deaerator N2H4 which we religiouly maintaining at 0.1 ppm minimum.

Please let me know what else we must add or do to eliminate this corrosion. By the way the BFW temperature is hardly 40 Celcius.

Dear Mirza,

Corrosion caused by BFW cannot solely be attributed to DO in the water. There are other agents that can cause corrosion one of them being dissolved Carbon Dioxide in water which can cause Carbonyl Acid formation and corrode Carbon Steel.

 

Anti-corrosion treatment is still an art and still involves a lot of trial and error to get the right combination of dosing chemicals to prevent corrosion.

 

One of the leading water treatment chemical manufacturer and specialist in the world is NALCO chemicals. They have years of experience in providing specific dosing programs using a combination of anti-corrosion chemicals. My suggestion to you would be to get in touch with them and provide them all the details of your BFW system including sample analysis of your BFW to design a specific chemical dosing program for you in order to mitigate corrosion. You may request their specialist to make a visit to your plant and study your system and provide you the correct dosing program to prevent corrosion.

 

Regards,

Ankur.

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Mirza Chughtai
Apr 20 2013 12:36 AM

Dear Ankur sahib

 

We are maintaining a condensate return pH > 8.5 by dosing a combination of neutralizing amines as Nalco would suggest.

Could you please explain to me why there is a better and faster DO removal from BFW during positive pressure( above STP) De-aeration in a properly designed de-aerator compared to a system under vacuum.

There must me some thermodynamics involved?

Since they can't increase the de-aerator temperature above 40 Celcius, shall we add DEHA or MEKO or Sodium Sulphite inaddition to Hydrazine to control this rapid corrosion.

Please also note that Carbon Dioxide must get flashed out during the Vacuum Deaeration say at 40 Celcius!

 

Awaiting suggestions!

 

Regds

Mirza

Dear Mirza,

 

I cannot provide you an exact recipe for the combination of chemicals to be dosed. That will be provided to you by a chemical supplier such as NALCO.

 

I can send you some additional information on Deaerators if you provide me your mail ID. This information should help you in finding out whether your Deaerator is designed and operated properly.

 

Regards,

Ankur.

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Technical Bard
Mar 30 2014 08:48 PM

A warning to users - Ankur's work here is great if the BFW is CLEAN.  In oil-production environments where produced water is re-used to generate steam, the water often contains significant salt quantities that alter the solubility of oxygen.   Some produced waters also contain organic species that create a chemical oxygen demand (COD), which may actually reduce the dissolved O2 to very low levels, making active scavenger dosing easier or potentially unnecessary.

The recommended dissolved O2 content for water Injection wells is < 20 ppb. But our water is saturated with H2S and CO2 and no amount of Ammonium bi-sulphite can bring down the dissolved O2 even less than 100 ppb. I do see some literature by Snavely and Blount, recommends that Sodium Sufite will not work in the presence of H2S as a O2 Scavenger. Can any one recommended me a better way to handle this situation?

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Shryli Shreekar
Jul 09 2018 06:54 AM

Dear Sir,

  I am Shryli, a Molecular Biology student from Mysore University. Currently I'm working on a project which includes the study of the behaviour of some nematodes, C. elegans to be specific when they are exposed to low oxygen concentration. I am using Sodium Sulfite for my experiments. Can you please tell me at what concentration of Sodium Sulfite i can get 2% oxygen in the solution? And are there any experiments done on time against DO for different concentrations of Sodium Sulfite solution? I have tried standardising the Sodium Sulfite concentration by finding DO using DO meter but I am getting a lot of fluctuations which are making it worse. Please guide me.

Regards,

Shryli K S

Email: shrylishreekar.k@gmail.com   

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