Features of the use of sodium bisulfite in the water treatment of steam boilers
Ivan Tikhonov
The article discusses the serious consequences of incorrect application of sodium bisulfite solution in a steam boiler room.
Sodium bisulfite (NaHSO3) in water treatment is usually used for chemical binding of oxygen in the treated water. Sodium bisulfite attaches oxygen and becomes sodium bisulfate. In the presence of alkali, bisulfate transforms into sodium sulfate. The main disadvantage of this technology is that after dosing the reagent into water, the salinity of the treated water increases significantly. After treating water with an oxygen concentration of 8 mg/l, its salinity will increase by about 60 mg/l (NaHSO4).
There is another drawback. Sodium bisulfite is an acidic salt of sodium and Sulfurous acid. Sulfurous acid is formed during the hydrolysis of sulfur dioxide (sulfurous anhydride – SO2). When sulfurous anhydride is interacting with water, it forms sulfurous acid H2SO3. If caustic soda is added to the water, sodium bisulfite will be formed,
SO2+NaOH = NaHSO3
Or sodium sulfite
SO2+2NaOH = Na2SO3 + Н2О
In accordance with the requirements of the regulatory document, the content of sulfur dioxide must meet the requirements of the test. The test requires an analysis for the methyl orange alkalinity of an aqueous solution of sodium bisulfite. The solution will be considered to have passed the test if after adding the indicator the sample does not acquire a red color.
This suggests that the presence of sulfur dioxide in the solution does not cause its increased acidity (due to the formation of sulfurous acid). I.e. bicarbonates are present in the solution (there are no strong acids) and the pH value of such a solution is at least 4.4.
The sodium bisulfite contains the potential for acid formation and it can be dangerous when it is added in large quantities.
The fact is that if you increase the content of caustic soda in a solution of sodium bisulfite, then bisulfite will begin to turn into sodium sulfite, which will significantly reduce the efficiency of oxygen binding in the treated water. Therefore, the sodium bisulfite solution always has an acidic reaction.
As a rule, sodium bisulfite is used for binding residual oxygen after the deaerator. In this case, small quantity of the sodium bisulfite is added. At the same time, hot feed water significantly contributes to an increase in the reaction rate of chemical binding of oxygen.
But if sodium bisulfite is added in large quantities, this can lead to extremely unpleasant consequences.
Let’s consider an example of how the dosing of sodium bisulfite in a large volume led to a complete violation of the water-chemical mode of a steam boiler.
The salinity of the source water for the steam boiler was 800-900 mg/l. Therefore, as one of the stages of the water treatment system for steam boilers, it was decided to use a reverse osmotic desalination plant. After the reverse osmosis the water was sent to a thermal atmospheric deaerator. To remove oxygen from the reverse osmosis filtrate (make-up water), a solution of sodium bisulfite was dosed before the deaerator.
This scheme has the advantage that the thermal deaerator is used as a feed tank of steam boilers, which receives make-up water and condensate. Preliminary removal of oxygen from the make-up water will significantly reduce the steam consumption in the deaerator for thermal removal of oxygen. But the setup of the sodium bisulfite dosing station was done carelessly. As a result, a significant excess of sodium bisulfite was formed in the make-up water befor the deaerator. First of all, this happened because the make-up water (filtrate) has practically no pressure. The reagent dosing unit is always adjusted to a certain water pressure in the pipeline into which the reagent is added. If there is no pressure in the pipe, then the installation will dose a much larger amount of reagent (in this case, sodium bisulfite).
As a result, a significant amount of sulfur dioxide got into the “deaerator – steam boiler – steam condensate tract – deaerator” circuit.
It is clear that if there is a bicarbonate ion (alkalinity) in the source water, then in the process of removing carbon dioxide with steam, caustic soda is naturally formed in the boiler water. This condition determines the alkalinity of boiler water by phenolphthalein and high pH. But in that case (after osmosis), no more than 0.15-0.2 mg-eq/l bicarbonate ion remained in the make-up water. At the same time, there was clearly more sulfur dioxide. As a result, all the caustic soda that was formed in the boiler was neutralized with sulfurous acid obtained from sulfur dioxide. The boilers had an acidic environment. The pH value was from 4.4 to 6.5. The returned condensate had a pH value of no more than 4.4. Severe corrosion of boilers, steam condensate tract and feed pipelines was observed.
Naturally, after the dosing of sodium bisulfite was discontinued, the quality of boiler water began to meet the standards a few days later. Phenolphthalein alkalinity appeared, the pH value of the condensate increased. The corrosion processes in the boiler and the active corrosion of the steam condensate tract stopped.
This example shows how important timely and accurate monitoring of the operation of reagent dosing systems is. Especially if the dosing is used as the main stage of water treatment. If it is impossible to organize timely and competent control at the facility, then it is necessary to apply well-studied mass transfer processes that allow ensuring high quality of water treatment and the entire boiler room even in the absence of operational control. Otherwise, the boiler house may stop function after several months.
It is important to understand how much acid the sodium bisulfite solution contains.
Let’s plot the forms of sulfur dioxide in water depending on its pH value.
The hydrolysis of sulfur dioxide goes in two stages:
- SO2+H2O = H2SO3 = Н + НSO3
- HSO3 = H + SO3
As a result, bisulfite anions are formed in the first stage and sulfate anions in the second. The dissociation constants: for the first stage – 1.55 * 10-2, for the second stage – 6.31*10-8.
The equations of chemical reactions of hydrolysis in stages can be written as follows:
where,
SO2, HSO3, SO3 – concentration in mol/l.
By setting different ratios of sulfur dioxide, bisulfite and sulfite according to these equations, it is possible to determine at what pH which form will prevail. According to these formulas graphs of the dependence of each form of sulfur dioxide on the pH value were constructed. The graphs are shown in Figure 1.
Figure 1
According to the graphs, it can be traced that the largest amount of bisulfite in water is observed at a pH value in the range from 4 to 5. In this case, bisulfite in solution is almost 100%. At a pH value below 4, the content of bisulfite decreases in water and sulfur dioxide appears. The more sulfur dioxide is in the water, the lower the pH value will be. When the pH value is greater than 5, sulfite begins to appear in the water from bisulfite. This process is a second-stage hydrolysis and can only be carried out if alkali is added to the water or sulfur dioxide is distilled.
With the help of these graphs it is clearly shown what happens when the concentration of sulfur dioxide in an aqueous solution of sodium bisulfite is determining in accordance with the requirements of the regulatory document. The methyl orange indicator is added to the sample and its color is checked. If the color of the sample immediately turns red, it means that the pH of the water is below 4.14, which automatically indicates the presence of sulfur dioxide. If the indicator color is orange, then the pH value of the solution is greater than 4.14. This suggests that sulfur dioxide is found in a small permissible amount.
Some manufacturers of aqueous solutions of sodium bisulfite indicate a pH value of 5.1, which is quite justified. At this pH value, sulfites are just beginning to appear in the solution, but there is no sulfur dioxide, i.e. there is no potential for the formation of an acidic medium under certain conditions.
If someone independently prepares an aqueous solution of sodium bisulfite from a dry reagent for water treatment purposes, I recommend checking the resulting pH value. Most likely, the pH value will be below 4. Therefore, the addition of alkali to the resulting solution will be required to bind excess acidity. But the final pH of the solution should be no more than 5.1.
