J[Xfi 1 [Xf[x [xf I Vmx T

ZT is the tower height and the expressions with overbars are average values between elevations 1 and 2, respectively.

If the ZT's of the previous equations are solved, they will be expressed in terms of the product of the reciprocal of the overbarred factors by the respective integrals. Thus, the tower height may be expressed as the product of two factors. Consider the first as the height of a mass transfer unit, H, and the second as the number of mass transfer units, N. Therefore,

H based on the gas side will be designated as Hy and H based on the liquid side will be designated as Hx. The corresponding designations for Nt are N^ and Ntx, respectively. Thus,

1VmJ

Example 9.12 A packed absorption tower is designed to removed SO2 from a coke oven stack. The stack gas flow rate measured at one atmosphere and 30°C is 10 m /s, and the SO2 content is 3.0%. Using an initially pure water, 90% removal is desired. The equilibrium curve of SO2 in water may be approximated by yt = 30xi. Determine the water requirement if 150% of the minimum flow rate is deemed adequate. Calculate the height of the tower. Assume Kyfa = 0.024 kgmols/m • s- mol fraction. Assume the total cross-sectional area of tower equals 11.0 m2.

Solution:

To get the minimum flow rate, assume that at the bottom, the operating line is at equilibrium point; thus, y = 30 x;; 0.03 = 30xf j; xf j = 0.001

Therefore,

G = 10 (3027273:)( 103 )( ¿ï)( 10-3 )( 1-0.03 ) = 0.39 kg • mol/s Therefore,

294 84 294 84 L = ( 1.5 )(28 )(0.39 )( 18) = 294.84 kg/s = P84 =

p water 981

9 J[yf1]( 1- [ Vf ])([ yf ] - [ y* ]) Average yf = (0.03 + 0.0031 )/2 = 0.017

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