Absorption Resistance Chemical Engineering Notes | EduRev

Mass Transfer

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4.9.5. Absorption resistance 
During absorption, when transport from the bulk of the gas phase to that of the liquid phase, the possibility of the transport resistance arising on the gas side. This is relatively infrequent occurrence in absorption process. But it can be found under certain circumstances where fast and instantaneous reactions are involved, especially when combined with low gas phase of the reactant.
The absorption rate for fast pseudo-fast-order reaction is
Absorption Resistance Chemical Engineering Notes | EduRev                                                                 (4.76)
The relation for instantaneous reaction is
Absorption Resistance Chemical Engineering Notes | EduRev                                      (4.77)
Using Henry’s law (pAi=HcAi), the interface concentration can be removed from Equation (4.77) to give
Absorption Resistance Chemical Engineering Notes | EduRev                                                            (4.78)
when the reaction is at the interface:
Absorption Resistance Chemical Engineering Notes | EduRev                                                                (4.79)
If
Absorption Resistance Chemical Engineering Notes | EduRev                                                               (4.80)
then transport resistance is mainly on the gas side and the absorption rate is
Absorption Resistance Chemical Engineering Notes | EduRev                                                                                     (4.81)

Example Problem 4.4: 
In a reactor Carbon dioxide is absorbed in NaOH. The CO2 partial pressure ranges from 2.5 bar at the reaction inlet and (0.025) at the outlet. The liquid phase has a roughly constant composition of 0.96 M NaOH. Find out the absorption rate at reactor inlet.
Data: kL = 0.025 cm/s, kG = 2.5×10-5 mol.cm-2s-1bar-1 , H = 56.2 bar l mol-1 , DCO2 = 1.6×10-5 cm2s-1 , DOH/DCO2 = 1.7, k2C0OH = 7000S-1 , C0OH = 0.40

Solution 4.4: 
Parameter M is calculated as an initial step
Absorption Resistance Chemical Engineering Notes | EduRev  
The enhancement factor Ei is
Absorption Resistance Chemical Engineering Notes | EduRev      
As Absorption Resistance Chemical Engineering Notes | EduRev = p/H, it follows that
Absorption Resistance Chemical Engineering Notes | EduRev
Hence, Ei = 16.28 at the reactor inlet and Ei = 1528.64 at the outlet. E is determined iteratively from the Wellek’s equation In the present problem its value is 3.492. Hence, absorption rate at the reactor inlet is 
Absorption Resistance Chemical Engineering Notes | EduRev

Nomenclature 
ā  Specific interfacial contact area between gas and liquid, m/m3
Ls  Liquid flow rate per unit area basis, (solute free basis) mol/h.m2
Ā, An  Absorption factor M Parameter defined in Equation (4.77)
G/  Gas flow rate per unit area basis, mol/h.m
NtG   Number of gas phase transfer units
Gs   Gas flow rate per unit area basis,(solute free basis) mol/h.m2
Absorption Resistance Chemical Engineering Notes | EduRev  Stripping factor
hT Packing height, m
U Overall gas phase conversion
HtG Height if transfer units, m
Absorption Resistance Chemical Engineering Notes | EduRev  Moles of Nth component in the liquid stream per mole of solvent entering the absorber
KY Overall gas phase mass transfer coefficient, kmol/m2h (ΔX)
x, y Mole fraction of solute in liquid and gas
kx, ky Individual gas phase mass transfer coefficients, kmol/m2 h (ΔX)
X, Y Mole ratio of solute in liquid and gas
KG Overall gas phase mass transfer coefficient, kmol/m2 h (Δp)
Absorption Resistance Chemical Engineering Notes | EduRev  Moles of Nth component in the gas stream leaving any plate
L/ Liquid flow rate per unit area basis, mol/h.m2

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