Q1. Water is used to absorb the vapor of methanol at 300 K and 101.325 kPa. The concentration of methanol in both gas and liquid phases are very low, with the equilibrium relation following Henry's Law. The known parameters are:
solubility of methanol in water = 1.995 kmol/(m3 .kPa)
local mass transfer coefficient for gas film kG = 1.55E-5 kmol/(m2.s.kPa)
local mass transfer coefficient for liquid film kL = 2.08E-5kmol/(m2.s.kmol/m3)
(1) Please determine the overall mass transfer coefficient for gas phase, KG,
(2) And determine the percentage of mass transfer resistance in gas film out of the overall resistance.
(3) At steady operation in certain section of the absorption column, the partial pressure of methanol is 5 kPa and the liquid composition is 2.11 kmol/m3. Please determine the methanol absorption rate NA, [kmol/(m2.h)] . (2 points)
Q2. Water is used to absorb NH3 (mole fraction 0.1) from air at 1 atm in a counter current operation. The gas mixture at 313 K is fed to the bottom of the column at a flow rate of 0.556 m3/s and a velocity of 1.2 m/s. Water is introduced to the column from the top. Due to the enthalpy of dissolution, the temperature of the ammonium water is higher at the bottom of the column. The temperature and composition distribution in the liquid phase from top to bottom of the column, with corresponding gas phase equilibrium compositions are described by Table 1.
Table 1.
|
temperature of NH3 solution
|
composition of NH3 solution
|
gas phase composition
|
|
deg C
|
X, kmol (NH3) / kmol (H2O)
|
Y*, kmol (NH3) / kmol (air)
|
|
20
|
0
|
0
|
|
23.5
|
0.005
|
0.0056
|
|
26
|
0.01
|
0.01
|
|
29
|
0.015
|
0.018
|
|
31.5
|
0.02
|
0.027
|
|
34
|
0.025
|
0.04
|
|
36.5
|
0.03
|
0.054
|
|
39.5
|
0.035
|
0.074
|
|
42
|
0.04
|
0.097
|
|
44.5
|
0.045
|
0.125
|
|
47
|
0.05
|
0.156
|
(1) If the recovery of NH3 is 95%, assuming air is totally inert, please determine the theoretical minimum liquid flow rate Lmin, (kg/s)
(2) If the real liquid flow rate is 1.1 times of the theoretical minimum, and if the KGa is known to be 0.0556 kmol/(m3.s), please determine the diameter of the column, (m).
(3) Please determine the height of the packings (m), based on the above information.
Q3. Clean water is used to remove SO2 from air through a counter current scrubbing process to reach a 95% recovery, in which the flow rate of gas is 5000 std m3/h containing 10%(v/v) SO2. The equilibrium for the solute between gas and liquid is Y* = 26.7 X.
(1) Please determine the required liquid flow rate, assuming 1.5 fold of the theoretical minimum, (kmol/h).
(2) Please determine the number of theoretical plate.
(3) Using the number of plate determined from (2), if the recovery is to be increased from 95% to 98%, what will be the new water consumption (kmol/h)?
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