Sodium Sulfite as Oxygen Scavenger

The dosage of sodium sulfite or other oxygen scavengers may be calculated by looking at their reaction rate to remove the oxygen and then their residual dosage to provide a feedwater or boiler water residual.

Sodium Sulfite Example

The reaction rate is:
2Na2SO3 (7.88 ppm) + O2 ( 1 ppm) → 2Na2SO4

For the residual, 1.6 ppm Na2SO3 will yield 1 ppm SO3.

The total calculation equation to remove residual oxygen and provide a boiler water residual is:

(Feedwater O2 x 7.88) + (1.6 x Boiler Water SO3 Residual) / Feedwater Cycles = Feedwater Dosage of 100% sodium sulfite in ppm.

Spreadsheets can be made as shown at the end of the article that illustrate the impact of changes in variables such as dissolved oxygen concentration, feedwater cycles, steam production, desired sulfite residual, and applied dosages. Examine the results shown on the examples.

If the dissolved oxygen concentration varies from 0.01 ppm to 1.0 ppm in the feedwater, to maintain the 30 ppm boiler water residual, the daily dosage demand increases by almost 500% and the reaction demand increases from 3.9% of the dosage to 80%. Clearly a variable oxygen content is very difficult and costly to control.

Looking at cycle control reveals that even though the dosage to obtain a residual may increase dramatically at low cycles, only a small portion of the sulfite total dosage is required to remove oxygen, so residual control is not as important if the deaerator is operating properly. The reaction dosage at 5 to 50 cycles only varies from 0.8% to 7.6% of the total required dosage.

Changing the residual in the boiler if the deaerator is operating properly can change consumption significantly, but should have little impact on results, unless the deaerator dissolved oxygen level spikes above the 0.01 ppm level.

Variations in deaerator oxygen removal obviously would have a great impact on sulfite consumption. If the dosage is fixed at 1.92 ppm at 25 cycles, and the oxygen content from the deaerator increases from 0.01 ppm to 0.1 ppm, the sulfite residual will drop from 29 ppm to 18 ppm. The reaction demand increased from 4.1% of the total dosage to 41%.

Clearly, the control of sulfite feed and maintaining control will depend upon the specifics of each system. A deaerator that provides consistent, low oxygen residuals will be much easier to control than one that is giving unsteady results. Feedwater heaters with variable oxygen levels will be nearly impossible to control economically. Where dissolved oxygen levels are high or variable and when using feedwater heaters, applying a passivator such as erythorbate or DEHA would be a valuable addition to the sulfite.