MASS TRANSFER-1

MASS TRANSFER- 1

Introduction

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.
Diffusion

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

  1. 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.