MASS TRANSFER- 1
Mass transfer may occur in a gas mixture
Further mass transfer may occur in a liquid solution
Also, mass transfer may occur in a solid.
The basic mechanisms of mass transfer are the same whether the phase is a gas, or a liquid, or a solid. Driving force for mass transfer is concentration gradient of a species. Mass transfer due to concentration gradient is called diffusion which is very analogous to heat transfer by conduction.
It is the movement of particles in a medium from an area of high concentration to an area of low concentration, resulting in the uniform distribution of the substance.
Diffusion in gases >> diffusion in liquids>>diffusion in solids.
Four terms frequently used in mass transfer are
- Molar concentration or Mole concentration or 3.5 < ReD< 7.6×104,
0.71 < Sc < 380 and 1.0 < (μ/μs) < 3.2 or volume concentration, ’ CA’
ii. Mass concentration, ‘ ρA’
iii. Mole fraction ‘ xA’
iv. Mass fraction ‘mA*’
Definition of Mole concentration’ CA’ of Species A in a mixture
Number of molecules of species A present per unit volume of the mixture (atoms/m3 or molecules/m3 or moles/m3 or kmol/m3)
Definition of mass concentration, ‘ρA’, of Species A in a mixture
Mass concentration of species A = Mass of species A per unit volume of the mixture ( kg of species A / m3 mixture)
Definition of Mole fraction’ xA’ of Species A in a mixture
It is a ratio of molar concentration of A to the total molar concentration
xA = CA/C = CA/(CA + CB)
It has no units.
Definition of mass fraction ‘mA*’ of Species A in a mixture
It is a ratio of mass concentration of A to the total mass concentration
mA* = ρA/( ρA+ ρB)
It has no units.