Question: What is the dissolved CO 2 concentration in the product stripped water

Question

Question: What is the dissolved CO2 concentration in the product stripped water?

Wastewater containing 380 mg/L as CaCO3 bicarbonate with a pH of 7.9 is being pumped to a small treatment. It is pumped at a rate of 450 m3/h via a pipe . Finally, after biological treatment, the water passes through a degasser to strip the CO2 from the water using air containing 0.2% CO2 flowing at 35 kg/min. The offgas produced contains 5% CO2.

Instruction:-

This is a mass balance question. In the stripper the CO2 from the water goes into the gas stream. It has to be assumed (for convenience) that all of the bicarbonate is converted to CO2. The change in CO2 mass flow (in kg/h) in the gas stream must be the same as the corresponding change in the water stream. The change in the gas stream concentration is given, as is the gas flow, so the mass flow of CO2 is determined by this. Dividing the CO2 mass flow by the flow of water gives the CO2 concentration in the water (in mass per unit volume).

Explanation / Answer

Answer:

For de gassar, the air containts 0.2% CO2 flowing @ 35 kg/ minutes, so 35*0.2% = 0.07 Kg/min of CO2.

Waste water is pumped at a rate of 450 m3 / hr i.e., 7.5 m3/min.

So, the total dissolved CO2 = mass/unit volumne

                                        = 0.07/7.5

                                        =0.010 Kg / m3

So, Answer is 0.010 Kg/m3.

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