Transfer phenomenon

1. Subject : Transport Phenomena
Topic :Molecular Mass & Molar Flux, Convective Mass &
Molar Flux
Sem : 8 th
Prepared by;

2. Transfer processes are either:
• Molecular (rate of transfer is only a function of molecular activity), or
• Convective (rate of transfer is mainly due to fluid motion or convective
currents)
Unlike momentum and mass transfer processes, heat transfer has an added
mode of transfer called as radiation heat transfer.
Transfer processes
In chemical engineering, we study three transfer processes (rate
processes), namely;
•Momentum transfer or Fluid flow
•Heat transfer
•Mass transfer
The study of these three processes is called as transport phenomena.

3. Rate Process Driving force
Conductance
(Transport property)
Law for molecular
transfer
Momentum
transfer
Velocity gradient
Viscosity
(Kinematic viscosity)
Newton’s law of
viscosity
Heat transfer
Temperature
gradient
Thermal conductivity
(Thermal diffusivity)
Fourier’s law
Mass transfer
Concentration
gradient (chemical
potential gradient)
Mass diffusivity Fick’s law
Summary of transfer processes

4. Convective mass transfer

5. Total Flux
• Amount of a quantity passing through a unit surface area per unit time.
Convective Flux:
• Amount of a quantity passing through a unit surface area per unit time that is carried
by some reference velocity.
Diffusive Flux:
• Amount of a quantity passing through a unit surface area per unit time due to
diffusion.
• The difference between the total flux and the convectiveflux.
• Cannot be defined independently of the total & convective fluxes!
Types of Flux

6. Diffusive Fluxes
Total mass flux
of component i
Mass diffusive flux of
component i relative to a
mass-averaged velocity.
Convective flux of
component i due to a
mass-averaged velocity
• The total flux is partitioned into a convective and diffusive flux.
• The convective flux is defined in terms of an average velocity.
• The diffusive flux is defined as the difference between the total and
convective fluxes. It is only defined once we have chosen a convective flux.
ji = ni – piv
= pi(ui - v )
= ni - int

7. Diffusion
Flux
Total
Flux
Convective
Flux
Comments
Diffusive Fluxes
Diffusion fluxes are defined as motion relative to some reference velocity.
(Diffusion flux) = (Total Flux) - (convective flux)
j i
ju
i
piv
piu
ni
ni
Mass diffusive flux relative to a
mass-averaged velocity
Mass diffusive flux relative to a
molar-averaged velocity
J i N i
ciu Molar diffusive flux relative to a
molar-averaged velocity
J v
i N i ic v Molar diffusive flux relative to a
mass-averaged velocity
Ji = ni — piv
= pi(ui — v )
= ni —⇥ int
J = N i - c u
= ci (ui —u)
= N i —x i N t

8. Molecular mass and molar flux

10. Convective Mass and Molar Flux