CL 351                                   Homework # 4

 

Due date: 1 September 2005

 

           

1.       A wastewater stream of 600 gallons per minute containing 10 ppm (by weight) of benzene is to be stripped with air in a packed column operating at 25oC and 2 atm to produce water containing 0.005 ppm of benzene.  The packing used is 2-in Intalox saddles which offer an interfacial area of 131.4 m2/m3.The vapour pressure of benzene at 25oC is 95.2 torr.  The solubility of benzene in water at 25oC is 0.18 g/100 g.  An expert in volatile organic chemical (VOC) stripping with air has suggested use of 1,000 scfm of air (60oF, 1 atm), at which condition the mass transfer coefficients for benzene are: kLa = 0.067 sec-1 and kga = 0.8 sec-1. 

 

a)       Determine the minimum air stripping rate in scfm.  Is it less than the rate suggested by the expert? 

b)       The overall mass transfer coefficient, Kga, in units of mol/(m3 sec) and sec-1.

c)       The volume of packing in cubic meters.

 

2.       A 2-feet diameter packed column with a packing of 20 feet of 1-in Berl saddles contains air at 1 atmosphere and 20oC.  The tower is apparently close to flooding , and Δp = 24 inches of water.  The mass velocity of liquid is 8.5 times that of the gas.

 

a)       If the tower is repacked with 1.5-in polypropylene Pall rings, what would the pressure drop be?

b)       How much higher a liquid flow rate could be used if the pressure drop were the same as it was with Berl saddles?

 

  1.  To remove 95% of the ammonia from an air stream containing 40% ammonia by volume, 488 lbmol/hr of  a pure absorbent per 100 lbmol/hr of entering gas are to be used, which is greater than the minimum requirement.  Pressure is 1 atm and temperature is 298 K.  Calculate the height of packing needed for this separation, if the tower diameter is 1 foot.  The overall gas side mass transfer resistance is 0.02 meters per second.  The tower uses Pall rings which provide an interfacial area of 102 m2/m3.  Equilibrium data is tabulated below

 

x, mole faction of ammonia in liquid

0

0.02

0.04

0.06

0.08

y, mole faction of ammonia in gas

0

0.026

0.091

0.2

0.365

 

 

  1. Determine the diameter of an absorber that is packed with 1-inch Berl saddles and the gas pressure drop is limited to 0.25 inches of water per foot of the tower.  The gas enters the tower at a concentration of mercaptan (solute that is absorbed) of 5 mole% and leaves at 0.3 mole%.  A non-volatile mercaptan-free solvent enters the tower at 0.236 m3/sec and leaves with a concentration of 3.05 mole% mercaptan.  Specific gravity of solvent is 0.81, its viscosity is 3 x 10-3 Pa.s.  The operating temperature and pressure are 293 K and 1 atmosphere.  The gas enters the tower with an average molecular weight of 30.1 g/gmol and the average liquid molecular weight is 180 g/gmol.