CL 351                                   Homework # 2

 

Due date: 18 August 2005

 

1.  20 cc of Ether and 100 cc of water, which are mutually immiscible, are contacted in a container such that the interfacial area is 12.2 cm2.  Water forms the bottom layer being heavier than ether.  An iodine-like solute is originally present in both phases at 3 x 10-3 M.  However, it is 700 times more soluble in ether.  Diffusion coefficients in both phases are around 10-5 cm2/sec.  Resistance to mass transfer in the ether is across a 0.01 cm film; resistance to mass transfer in the water involves a surface renewal time of 10 seconds.  What is the solute concentration in the ether after 20 minutes?

 

2.  A packed, absorption tower was used to absorb compound A from a gas mixture into solvent B.  At one point in the tower, the partial pressure of A in the gas stream was 1.52 x 104 Pa and the concentration of A in the contacting liquid stream was 1.0 x 10-3 kg mole/m3.  The mass transfer between the gas stream and the liquid stream at that point in the tower was 4 x 10-5 kg mole/(m2 sec).  The individual mass transfer coefficient on the gas side was 3.95 x 10-9 kg mole/(m2 sec Pa).  A laboratory experiment verified that the system satisfied Henry’s law and the liquid composition 1 x 10-3 kg mole/m3 was in equilibrium with a partial pressure of 3.04 x 103 Pa.  Find individual liquid side mass transfer coefficient, overall mass transfer coefficient based on liquid side and gas side and the corresponding driving forces.

 

3.  We are studying gas absorption into water at 2.2 atmospheres total pressure in a packed tower.  From earlier experiments with ammonia and methane, we believe that for both gases the mass transfer coefficient times the packing area per tower volume is 18 lb mol/(hr ft3) for the gas side and 530 lb mol/(hr ft3) for the liquid side.  The values for these two gases may be similar because methane and ammonia have similar molecular weights.  However, their Henry’s law constants are different: 9.6 atmosphere for ammonia and 41,000 atmospheres for methane.  What is the overall gas-side mass transfer coefficient for each gas?  What fraction of resistance does gas offer for each gas?

 

4.  A solution with oxygen dissolved in water containing 0.5 mg oxygen per 100 grams of water is brought in contact with a large volume of atmospheric air at 283 K and a total pressure of a1 atmosphere.  The Henry’s law constant for the oxygen-water system at 283 K equals 3.27 x 104 atm/mole fraction.  Will the solution gain or lose oxygen?  What will be the concentration of oxygen in the final equilibrium solution?

 

5.  Various methods have been investigated to reduce evaporation from large water reservoirs in semiarid regions.  One method that has been tested is to spray a non volatile chemical on the reservoir surface.  In an attempt to determine the effectiveness of this method, the surface of a 1 m x 5 m rectangular container filled with water was covered with a 0.002 m thick layer of the chemical.  Since the water and the chemical are only slightly miscible, the rate of evaporation can be determined by calculating the diffusion of water through the stagnant chemical film.  Using the data given below, calculate the mass transfer coefficient and the rate at which water evaporates.  The diffusion coefficient for water through the film is 2.3 x 10-9 m2/sec. The concentration of water in the chemical at the liquid-liquid interface is 0.3 kg mole/m3.  Concentration of water in the chemical at the gas-liquid interface is 0.05 kgmole/m3.  Assume that the molar density of the chemical is 0.35 kgmol/m3.