Problem 13.5: Feedwater contains 3 ppm dissolved oxygen a

From Chapter 13 Problem Set. Level 2: Applied Thinking.

Question

Feedwater contains 3 ppm dissolved oxygen. (a) Calculate the stoichiometric sulfite requirement. (b) If the boiler is operating at 10 cycles of concentration and the target boiler residual is 30 ppm, what total sulfite concentration would you target in the boiler feedwater?

Answer

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(a) Stoichiometric sulfite requirement: 3 ppm O₂ × 7.88 = 23.6 ppm sodium sulfite in the feedwater to neutralize the dissolved oxygen. (b) The sulfite that reacts with oxygen is consumed – it does not concentrate in the boiler. Only the excess sulfite that survives the reaction concentrates with the boiler water. To achieve a 30 ppm residual in the boiler at 10 cycles, the excess sulfite in the feedwater must be: 30 ppm ÷ 10 = 3.0 ppm excess in feedwater. Total feedwater sulfite target = stoichiometric demand + excess for residual = 23.6 + 3.0 = 26.6 ppm sodium sulfite.