An engineer at a pulp plant is considering the feasibility of sparging a waste g

Question

An engineer at a pulp plant is considering the feasibility of sparging a waste gas stream containing a small concentration of chlorine gas into a chlorine-bleaching unit in order to augment the chlorine requirements for the unit. In this processing unit, the waste gas stream at 1.013 x 10° Pa and containing only 0.2 mole% chlorine is bubbled countercurrent to the absorbing water stream. At the flow rate of the streams, the individual gas film coefficient, ky, is 1 kg mole/m2 hr (Ay mole fraction) and the liquid-film coefficient, k, is 10 kg mole/m2 hr (Ax mole fraction). At the temperature of the system, the Henry's law constant, H, is 6.13 x 104 Pa(kg mole/m) The gas stream, containing 0.2 mole% chlorine gas, is in contact with the aqueous stream containing 2.6 x 10 kg mole Cl2/m 3. Determine (a) the overall coefficient, Kx, (b) the chlorine molar flux, (c) the interfacial composition, and (d) percent resistance to mass transfer in the liquid

Explanation / Answer

Ky=1

kx=10

m=6.13*10^4

1/Kx=1/(m*ky)+1/kx

=>Kx=1/(1/(6.13*10^4*1)+1/10)=9.99 kg mol/m2/hr/deltax

b) Vapor Phase mole fraction =.2/100

=>Partial Pressure of Cl2 in vapor phase = .2/100*101325=202.65 Pa

Partial Pressure = Henry Law constant*Interfacial liquid concentration

=> Interfacial Liquid concentration = 202.65/61300=3.3 mol/m3

Density of water =1000 kg/m3

=>Interfacial mole fraction of Cl2 = 3.3/(1000*1000/18)=5.94*10^-5

Molecular weight of Water = 18 g/mol

Bulk liquid concentration = 2.6 mol/m3

=>Bulk Cl2 mole fraction = 2.6/(1000*1000/18)=4.68*10^-5

=> Molar flux=Kx*(Delta x)=9.99*10^3*(5.94-4.68)*10^-5=0.12 mol/m2/hr

1/Ky=1/ky+m/kx

=>Ky=1/(1/1+61300/10)=.00016 kgmol/hr/m2/deltaY

% Resistance in liquid phase = 1/Kx/(1/Kx+1/Ky)*100=1/9.99/(1/9.99+1/.00016)*100=.0016%

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