Module 4: Liquid Characteristics - pH - Answers

#1: A packed bed wet scrubber is used to remove hydrogen chloride from a gas stream. The pH of the aqueous liquid entering the scrubber is measured at 6 and the aqueous liquid leaving the scrubber is measured at 5. Has the solution become more or less acidic? By what factor has the H⁺ ion concentration changed?

: Answer: The aqueous liquid has become more acidic. The [H⁺] concentration has increased from 10^-6 to 10^-5 or by a factor of 10.; Note that since pH is measured on a logarithmic scale, a change in pH of 1 signifies a ten-fold increase in either acidity or alkalinity.

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: Answer:
Point A (pH = ~7). City water, which has a pH of approximately 7, reduces the temperature of the gas stream as the gas passes through the evaporative cooler.; Point B (pH = ~7). City water, which has a pH of approximately 7, is used to remove build-up on the mist eliminator in the cyclonic separator.; Point C (pH = 5 - 6). The scrubbing liquid exits the cyclonic separator (at Point C) after the liquid has come into contact with the acidic gas stream within the venturi scrubber, thereby reducing the pH of the liquid to approximately 5 - 6.; Point D (pH = 6 - 9). The aqueous liquid entering the venturi scrubber is generally maintained at a pH of 6 - 9 to optimize the acid gas removal efficiency.; Point E (pH = ~7) and Point F (pH = 9 - 11). City water, which has a pH of approximately 7, is added to the alkaline powder in the mixing tank (at Point E) to form an alkaline slurry (Point F) having a pH of 9 - 11.; Point G (pH = 6 - 9) The aqueous liquid sent to the clarifier consists of the same liquid that enters the venturi scrubber at Point D. Therefore, it has the same pH range, which is 6 - 9.

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