Thermal diffusion principle, factors, types of equipment, separation factor

1. Thermal Diffusion
VIKAS KARANDE,
202120008.

2. Concepts
• Principle
• Theory
• Equipment
• Separation Efficiency
• Design Factors

3. Two Effects
 Soret effect
Mass flux due to a temperature gradient and appears in the species continuity
equation when you have a multicomponent mixture.
It is also known as the thermal diffusion effect.
Doufor effect
Enthalpy/Energy flux due to a concentration gradient and appears in the energy
equation for multicomponent mixture.
It is also called as the diffusion-thermo effect.
Soret effect is the reciprocal of doufor effect

4. Thermal Diffusion
 When a temperature gradient is applied to a
homogeneous mixture of gases/liquid , each component
diffuses at a different velocity, dependent upon
molecular mass and interaction between molecules.
 In general two gases are exposed to a temperature
gradient between two surfaces, the gas with the lower
molecular weight will tend towards the hot surface and
heavier molecular weight will move towards cold
surface.

5.  Thermal diffusion disturbs the homogeneity
of mixture composition: the concentration
of components in the regions of increased
and decreased temperatures, respectively,
becomes different. Since the establishment
of a concentration gradient causes, in turn,
ordinary diffusion, in a stationary non
uniform temperature field a steady state
inhomogeneous state is possible in which
the separation effect of thermal diffusion is
balanced by the counteraction of
concentration diffusion.

6. Separation of isotopes
mixture:
uranium hexafluoride(UF6) =
U-235: lighter molecular wt.
U-238: large molecular wt.

7.  In a binary gaseous mixture with non uniform composition at
constant pressure and with no external forces the total diffusion
mass flux of each component i is
 J1=c1(v1—v) = -D12 grad. C1+(DT/T) grad. T
DT - Thermal diffusion coefficient
D12 – Binary diffusion coefficient
KT - Thermal diffusion ratio
(v1—v)–Relative velocity betn component 1 and
avg velocity of Gas mixture
C1 – Concentration of the molecule in the 1 component

8. .
In an binary fluid mixture with non-uniform concentration and
temperature, the mass flow J of component 1
 J1 = −ρD.∇c − ρc(1 − c).DT. ∇T
D -diffusion coefficient
DT- the thermal diffusion coefficient, ρ the mass density
c- the concentration of component 1 in weight fractions
 the diffusion flow J1 vanishes, the Soret coefficient ST is given by
 ST = DT/D= − (1/c(1 − c))*|∇c|/|∇T |
 A large Soret coefficient indicates a strong separation of the
components. The sign of the Soret coefficient for one of the
components denotes if its mass fraction increases towards
the cold or warm region of the system

9.  How KT and α are related
KT=DT/D12 KT α C1*C2
KT = αC1*C2
α – Thermal diffusion factor
KT and ST is depends on only molecules size and shape (ratio),
concentration .
Typical values of α
 α less dependent on the composition than KT and for mixtures whose
molecule properties are similar it does not change significantly.
 In gaseous mixtures α does not practically exceeds 0.4
 For mixture of Isotope a typical value of α is 0.01
 water and organic solutions α varies within the range 10−3 to 10−2

10. Thermal Diffusion Equipments
Thermal diffusion carried out in two broad types of equipment
(a) Static cells in which natural convection is avoided
(b) (Clusius - Dickel) thermogravitational columns in which use is made up of the
natural convection to enhance the separation.
Types of columns:
Inclined column - deflects particles, and reduce remixing
Rotary wired - Wire helix within annulus

11.  Convection-free static cells are primarily
utilized to determine the thermal diffusion
factor α. In these cells, the temperature
gradient is applied in such a manner that
convection is inhibited.
 Static equipment is constructed of horizontal
flat plates with the hot plate on the top this cell
is very simple. Because of the temperature
difference between top hot plate and bottom
cold plate, one component (usually the lighter)
migrates towards the hot plate due to thermal
diffusion and concentrates there while the other
one concentrates in the region of the bottom
plate. The migration due to thermal diffusion is
opposed by ordinary diffusion.

12.  Agitated cell:A fritted glass disk to separate the hot and cold regions. Mixing in
hot and cold regions is provided by magnetic stirrers. The localization of both
temperature and concentration gradients in a membrane or fritted glass disk
permits operation of the cell with a horizontal temperature gradient
 Two bulb apparatus: For gas systems two bulb apparatus is used

13.  Thermogravitational column :
 The thermogravitational column consists of two
metal surfaces, one hot and another cold arranged
vertically, the fluid mixture to be separated being
held in between them.
Natural convection in the fluid mixture occurs
primarily because of changes in density due to
temperature but in some cases the effect of
concentration gradient may also be of substantial
magnitude

14. .
 In an ideal column a temperature gradient exists only in the direction
normal to the plates. The flux of component 1 due to thermal diffusion,
J1x=(DT/T)*dT/dx ;
KT=DT/D12= αC1*C2
J1x=(D12. α.C1.C2 )/T*dT/dx
 By combining the thermal-diffusion flux with simultaneously acting
fluxes due to ordinary diffusion and convective current
J1=D12*(dc/dx)+(dc/dy) - (D12. α.C1.C2 )/T*dT/dx - V*dc/dy

15.  a hot wire column with reservoirs This column was very simple in operation and
very effective. the effectiveness of their column by obtaining chlorine isotopes6
in very high purity.
 A central calrod heater is sometimes used in place of the hot wire.

16.  The construction of concentric cylindrical column is simpler than that of hot wire
columns.
 A drawback of hot wire columns is vibration of the hot wire which causes back
mixing.

17.  Multicolumn cascade system: Thermal diffusion column:

18. Separation efficiency:
 Separation factor (q):
 Impermeable barrier can reduce the remixing effect.
 It may effectively increases the degree of separation and
Increase the concentration of top product and decreases the
Concentration of bottom product.
 Optimal position of the barrier to obtain the maximum degree
of separation.

19.  Factors on which separation depends:
 Temperature difference between plate
 Size and Shape of particle
 heat and thermal conductivity
 Flow rate feed and product
 Annular space of concentric tube
 Diameter of the wire was essentially equal to the annular spacing
 Space between plates
 Dimensions of the column

20.  Applications:
 Separation of isotopes of He, Ne, Ar, Kr, Xe, U.
 Separation of liquid organic mixtures
 Drug discovery
 Particle separation by field flow fractionation
 Advantages:
 Simple and low capital cost.
 Separation of isotopes are done and effective by this process.
 Disadvantages:
 Thermal requirements are high.
 Forglation effect (isotopic equilibrium)

21. Thank you