Introduction to Mass Transfer

Introduction

Three fundamental transfer processes:

i) Momentum transfer

ii) Heat transfer

iii) Mass transfer

Mass transfer may occur in a gas mixture, a liquid solution or solid.

Mass transfer occurs whenever there is a gradient in the concentration of a species.

The basic mechanisms are the same whether the phase is a gas, liquid, or solid.

Definition of Concentration

i) Number of molecules of each species present per unit volume (molecules/m3)

ii) Molar concentration of species i = Number of moles of i per unit volume (kmol/m3)

iii) Mass concentration = Mass of i per unit volume (kg/m3)

Diffusion phenomena

Fick’s law: linear relation between the rate of diffusion of chemical species and the concentration gradient of that species.

Thermal diffusion: Diffusion due to a temperature gradient. Usually negligible unless the temperature gradient is very large.

Pressure diffusion: Diffusion due to a pressure gradient. Usually negligible unless the pressure gradient is very large.

Forced diffusion: Diffusion due to external force field acting on a molecule. Forced diffusion occurs when an electrical field is imposed on an electrolyte ( for example, in charging an automobile battery)

Knudsen diffusion: Diffusion phenomena occur in porous solids.

Whenever there is concentration difference in a medium,

  nature tends to equalize

  things by forcing a flow

  from the high to the low

  concentration region.

The molecular transport process of mass is characterized by the general equation:

    Rate of transfer process = driving force  resistance

Example of Mass Transfer Processes

Consider a tank that is divided into two equal parts by a partition.

Initially, the left half of the tank contains nitrogen N2 gas while the right half contains O2 at the same temperature and pressure.

When the partition is removed the N2  molecules will start diffusing into the air while the O2 molecules diffuse into the N2.

If we wait long enough, we will have a homogeneous mixture of N2 and O2 in the tank.

Liquid in open pail of water evaporates into air because of the difference in concentration of water vapor at the water surface and the surrounding air.

A drop of blue liquid dye is added to a cup of water. The dye molecules will diffuse slowly by molecular diffusion to all parts of the water.

Molecular Diffusion Equation

Fick’s Law

is the molar flux of component A in the z     direction in kg mol A/s.m2.

is the molecular diffusivity of the molecule A in B in m2/s

is the concentration of A in kg mol/m3.

z  is the distance of diffusion in m

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