Measurement Of Liquid Phase Diffusion Coefficient | Mass Transfer - Chemical Engineering PDF Download

2.5 Measurement of liquid-phase diffusion coefficient 
There are few methods of determination of liquid-phase diffusivities. The most commonly used method is Diaphragm cell method.

2.5.1 Diaphragm cell method 
Diaphragm cell contains two compartments as shown in Figure 2.8. These are separated by diaphragm. Let the area of the diaphragm is ‘a’ and porosity is ‘ε’. Effective area for diffusion = a ε
                 (2.49)

where τ is tortuosity. As the solute concentration is very low, bulk flow term can be taken as zero. Hence,

Figure 2.8: Diaphragm cell

                      (2.50)
                                                 (2.51)

and                                             (2.52)
Adding Equations (2.51) and (2.52) and substituting NA from Equation (2.50) we have,
        (2.53)
Boundary conditions 

Therefore,                 (2.54)
 

2.5.2 Semi-empirical Equation: Wilke-Chang Equation

                        (2.55)
where, D⁰_AB  is diffusivity of solute A in solvent B, m²/s, φ is association factor [for H₂O=2.26; MeOH=1.9; EtOH=1.5; non-associated solvent=1.0]; M_B is molecular weight of B; T is absolute temperature in K; μ_B is solution viscosity, kg/m.s; ν_A is solute molar volume at normal boiling point, m3 /kmol [ν_A=0.0756 m³/kmol for H₂O as solvent].

2.5.3 Stokes-Einstein Equation 
Liquid diffusivity varies linearly with absolute temperature and inversely proportional to viscosity of the medium. Hence,
                                                 (2.56)