1. Henry’s Law can be expressed variously as: pA*=H CA , where
the units of pA are atm and
units of CA are kmol m-3; pA*=H’ xA; and yA*=m xA
(a) Write an expression relating m, H and H’
3 marks
(b) Table 1 contains vapour-liquid equilibrium data for
trichloroethylene (TCE) in wastewater at
20 oC. Plot the data (remember to label axes, including
units)
3 marks
(c) Determine the Henry’s law constants (H, H’, m) with correct
units for TCE in wastewater at
20 oC, assuming total pressure is 1.0 atm and molar density of
wastewater is 66 kmol m-3.
Plot pA*=H CA on the graph.
3 marks
Table 1: Experimental data for vapour-liquid equilibrium of TCE in
wastewater

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
9 marks
(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)
2 marks
(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)
2 marks
(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
6 marks
(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
8 marks
(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.
7 marks
(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?
9 marks