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					Tutorial 4. “Absorption in packed columns”

i) Consider the expression for the overall gas mass transfer coefficient:

  1    m 1
       
  K y kx k y

in the case where m is much greater then one (low solubility). How would changes in
the gas film (ky) and liquid film (kx) mass transfer coefficients affect the overall gas
phase coefficient? What impact would these manipulations have on the height of a
transfer unit (HTU)?

ii) In the class we discussed the relation between the overall mass transfer coefficients
and gas and liquid films mass transfer coefficients. In particular, starting from the
expression for the overall gas phase mass transfer coefficient:

1   m 1
    
K y kx k y

and using the following mass balance relation for the gas phase:
 Vdy  K y a ( y  y*)SdZ

we obtained an expression for the height of a packed absorption tower:

          yb
   V /S        dy
Z
   K ya    y  y*  H
          ya
                         Oy   N Oy


Follow similar steps and derive an expression for the height of a packed absorption
tower starting with the overall liquid phase mass transfer coefficient:
1   1    1
    
K x k x mk y

iii) Using the obtained expression for the height of a packed tower, repeat the problem
considered in the class:

A gas stream of 3.0% A is passed through a packed column to
remove 99% of A by absorption in water. The absorber operates at
25°C and 1 atm, V/S=20mol/h/ft2, L/S=100mol/h/ft2

y*=3.1x
kxa=60mol/h/ft3
kya=15mol/h/ft3

Find NOx, HOx, ZT neglecting changes in the gas and liquid flow




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