# 2. A clarifier in a wastewater treatment plant contains wastewater with a bulk concentration of TCE of 0.010 kmol m-3.

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2. A clarifier in a wastewater treatment plant contains wastewater with a bulk concentration of TCE of 0.010 kmol m-3.

The clarifier operates at 1.0 atm and a constant temperature of 20 oC. The overall diameter is 2.0 m and it is enclosed to contain the (sometimes smelly) gases which are emitted from the wastewater. Fresh air is blown into this enclosure to remove the gases emitted from the clarifier and is then sent to an incinerator. The TCE content in the gas leaving the clarifier is 4.0 mol %. The liquid film mass transfer coefficient for the clarifier is estimated as kx=0.200 kmol m-2s-1 and the gas film mass transfer coefficient is ky=0.0001 kmol m-2s-1. Assuming that this wastewater has the same molar density and TCE equilibrium relationship as in Question 1, determine the following:

(a) Determine the mole fraction xA and molar ratio XA of TCE in the wastewater

(b) What would be the mole fraction yA* of TCE in the air, if in equilibrium with 0.010 kmol m-3 TCE in the wastewater? (i.e. saturation mole fraction in gas phase)

(c) What would be the mole fraction xA* of TCE in the wastewater, if in equilibrium with 4 mol% TCE in the air? (i.e. saturation mole fraction in liquid phase)

(d) Plot the Henry’s law relation y*=mx determined in Q1, and indicate xA, xA*, yA, yA* for the clarifier on that plot. What will be the direction of mass transfer of TCE? (i.e. from wastewater to air, or from air to wastewater?) 8 marks (e) Determine the overall mass transfer coefficients Kx and KL

(f) Determine the overall mass transfer coefficients Ky and KG (kmol m-2 s-1 atm-1). 6 marks (g) Determine the rate of mass transfer of TCE to/from the surface of the clarifier

(h) Determine xAi and yAi, the mole fraction of TCE in the liquid and vapour phase, respectively, at the water/air interface. Plot these values on graph from Q2d.

(i) If the concentration of TCE in the wastewater entering the clarifier is 0.050 kmol m-3, what flowrate of wastewater (m3/h) is required to ensure that the concentration of TCE in wastewater leaving the clarifier is 0.01 kmol m-3? If the depth of liquid in the clarifier is 4 m, what average residence time is required for the wastewater?

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