Diffusion it is a phenomenon which deals the microscopic movements of the molecules in all the phases, learning and studying the concept of diffusion will help to understand mass transfer operations, chemical reaction engineering and transportation concepts. Kinetic Theory of gases supports the diffusion occurrence in a system,
Diffusion is understood as a process of movement of particles from higher concentration section to lower concentration section of the system. Diffusion of the component is caused by this concentration difference which is called as the concentration gradient. For example when a bottle containing perfume is opened the smell is sensed as the molecules diffuse into the air due the concentration difference, so till the equilibrium is attained these molecules will move all over the room if an exit fan is present at top of the room and push out the air with some constant flow rate and the with the same flow rate the molecule will transfer into the system from the bottle.
This concept of molecule behaviour is used in separation operation and purification operations as distillation, extraction, absorption, reverse osmosis etc. Diffusion can also be occurred by pressure gradient even with temperature gradient or even with external force field which they act as the activity gradient. Mostly diffusion concept is developed by the physical properties of the molecules. For a system, at equilibrium, no net diffusion occurs. which is explained as when a molecule diffuse from one phase to another phase in a system containing two phases through an interface, then at the same time another molecule will diffuse from opposite direction which the net diffusion is set to zero due to the cancellation of the opposite direction, as said this occurs at equilibrium only.
Diffusion is understood as a process of movement of particles from higher concentration section to lower concentration section of the system. Diffusion of the component is caused by this concentration difference which is called as the concentration gradient. For example when a bottle containing perfume is opened the smell is sensed as the molecules diffuse into the air due the concentration difference, so till the equilibrium is attained these molecules will move all over the room if an exit fan is present at top of the room and push out the air with some constant flow rate and the with the same flow rate the molecule will transfer into the system from the bottle.
This concept of molecule behaviour is used in separation operation and purification operations as distillation, extraction, absorption, reverse osmosis etc. Diffusion can also be occurred by pressure gradient even with temperature gradient or even with external force field which they act as the activity gradient. Mostly diffusion concept is developed by the physical properties of the molecules. For a system, at equilibrium, no net diffusion occurs. which is explained as when a molecule diffuse from one phase to another phase in a system containing two phases through an interface, then at the same time another molecule will diffuse from opposite direction which the net diffusion is set to zero due to the cancellation of the opposite direction, as said this occurs at equilibrium only.
Classification:
1. Molecular diffusion
2.Eddy or Turbulent diffusion
Importance of Molecular diffusion:
Due to the thermal energy in a molecule, it gains the ability to move through another set of molecules in a system is known to be thermal diffusion. So in the same way molecules will tend to travel due to the driving force of concentration difference. To understand this let us discuss with an example which we involve regularly during the rush hour.
Let us consider a road which you want to cross from one side, say ‘A’ at the same time there are a group of people waiting to cross the same from the opposite side, say ‘B’. Now when the traffic is halted by the traffic signal, all the sudden you start to move toward the zebra crossing and diffuse into the mass of people who are in hurry as you, things to be considered are:
You cannot move without colliding (facing each other) with anyone
You cannot move in straight line, you have to take some zig-zig turn into the gaps between the people,
Finally, you will reach the point ‘B’ let us compare to some physical phenomena
You will be replaced by the one molecule of the solute in solution ‘A’(which contain high concentration of solute A) and other mass of people will take the position of the solvent molecules present in the solution ‘B’, as you and opposite people cross the road in the same way solute molecule will cross the interface and move into the solution B, at the same time solvent molecules (of solution B which are on opposite side) will cross the interface and move towards the solution A.
Molecule will have the rate as you have so it also covers a distance in zig-zig mode colliding with other molecules, the distance travelled per second will be the rate of diffusion, when certain fixed area is considered along the path which the molecule move than the rate become as the flux which is moles/ (area X time) and this rate depend on the molecular weight also and the number of molecule present which we say as concentration, the rate will exist till the molecule reaches the destination, that is the space in solution B to be occupied sufficiently and saturated with molecules of B, this state is called equilibrium.
Gas mixture system diffusivity is used for mass transfer calculations and to design the
Let us consider a road which you want to cross from one side, say ‘A’ at the same time there are a group of people waiting to cross the same from the opposite side, say ‘B’. Now when the traffic is halted by the traffic signal, all the sudden you start to move toward the zebra crossing and diffuse into the mass of people who are in hurry as you, things to be considered are:
You cannot move without colliding (facing each other) with anyone
You cannot move in straight line, you have to take some zig-zig turn into the gaps between the people,
Finally, you will reach the point ‘B’ let us compare to some physical phenomena
- You will move at some rate i.e. speed
- The distance you covered is less than the displacement
- Opposite people also move with some speed or rate
- Your body weight also will affect the speed you travel, the heavy you are slow you will move
You will be replaced by the one molecule of the solute in solution ‘A’(which contain high concentration of solute A) and other mass of people will take the position of the solvent molecules present in the solution ‘B’, as you and opposite people cross the road in the same way solute molecule will cross the interface and move into the solution B, at the same time solvent molecules (of solution B which are on opposite side) will cross the interface and move towards the solution A.
Molecule will have the rate as you have so it also covers a distance in zig-zig mode colliding with other molecules, the distance travelled per second will be the rate of diffusion, when certain fixed area is considered along the path which the molecule move than the rate become as the flux which is moles/ (area X time) and this rate depend on the molecular weight also and the number of molecule present which we say as concentration, the rate will exist till the molecule reaches the destination, that is the space in solution B to be occupied sufficiently and saturated with molecules of B, this state is called equilibrium.
Diffusivities (cm2/s) of gases at standard atmospheric pressure, (101.325 KPa), T in Kelvin
s.no
|
System
|
200
|
273.15
|
293.15
|
373.15
|
473.15
|
573.15
|
673.15
|
1
|
Ar-CH4
|
-
|
-
|
-
|
0.306
|
0.467
|
0.657
|
0.876
|
2
|
Ar-CO
|
0.168
|
0.187
|
0.290
|
0.439
|
0.615
|
0.815
| |
3
|
Ar-CO2
|
0.129
|
0.078
|
0.235
|
0.365
|
0.517
|
0.689
| |
4
|
Ar-H2
|
0.698
|
0.794
|
1.228
|
1.876
|
2.634
|
3.496
| |
5
|
Ar-He
|
0.381
|
0.645
|
0.726
|
1.088
|
1.617
|
2.226
|
2.911
|
6
|
Ar-Kr
|
0.064
|
0.117
|
0.134
|
0.210
|
0.323
|
0.456
|
0.605
|
7
|
Ar-N
|
0.168
|
0.190
|
0.290
|
0.439
|
0.615
|
0.815
| |
8
|
Ar-Ne
|
0.160
|
0.277
|
0.313
|
0.475
|
0.710
|
0.979
|
1.283
|
9
|
Ar-O2
|
0.166
|
0.189
|
0.285
|
0.430
|
0.600
|
0.793
| |
10
|
Ar-SF6
|
-
|
-
|
-
|
0.128
|
0.202
|
0.290
|
0.389
|
11
|
Ar-Xe
|
0.052
|
0.095
|
0.108
|
0.171
|
0.264
|
0.374
|
0.498
|
12
|
CH4-H2
|
-
|
-
|
0.782
|
1.084
|
1.648
|
2.311
|
3.070
|
13
|
CH4-He
|
-
|
-
|
0.723
|
0.992
|
1.502
|
2.101
|
2.784
|
14
|
CH4-N2
|
-
|
-
|
0.220
|
0.317
|
0.480
|
0.671
|
0.890
|
15
|
CH4-O2
|
-
|
-
|
0.210
|
0.341
|
0.523
|
0.736
|
0.978
|
16
|
CH4-SF6
|
-
|
-
|
0.167
|
0.257
|
0.363
|
0.482
| |
17
|
CO-CO2
|
-
|
-
|
0.162
|
0.250
|
0.38
| ||
18
|
CO-H2
|
0.408
|
0.686
|
0.772
|
1.162
|
1.743
|
2.423
|
3.196
|
19
|
CO-He
|
0.365
|
0.619
|
0.698
|
1.052
|
1.577
|
2.188
|
2.882
|
20
|
CO-Kr
|
0.131
|
0.581
|
0.227
|
0.346
|
0.485
|
0.645
| |
21
|
CO-N2
|
0.133
|
0.208
|
0.231
|
0.336
|
0.491
|
0.673
|
0.878
|
22
|
CO-O2
|
-
|
-
|
0.202
|
0.307
|
0.462
|
0.643
|
0.849
|
23
|
CO-SF6
|
-
|
-
|
-
|
0.144
|
0.226
|
0.323
|
0.432
|
24
|
CO2-C3H8
|
-
|
-
|
0.084
|
0.133
|
0.209
|
--
|
-
|
25
|
CO2-H2
|
0.315
|
0.552
|
0.412
|
0.964
|
1.470
|
2.066
|
2.745
|
26
|
CO2-H2O
|
-
|
-
|
0.162
|
0.292
|
0.496
|
0.741
|
1.021
|
27
|
CO2-He
|
0.300
|
0.513
|
0.400
|
0.878
|
1.321
|
-
|
-
|
28
|
CO2-N2
|
-
|
-
|
0.160
|
0.253
|
0.392
|
0.553
|
0.733
|
29
|
CO2-N2O
|
0.055
|
0.099
|
0.113
|
0.177
|
0.276
|
-
| |
30
|
CO2-Ne
|
0.131
|
0.227
|
0.199
|
0.395
|
0.603
|
0.847
|
-
|
s.no
|
System
|
200
|
273.15
|
293.15
|
373.15
|
473.15
|
573.15
|
673.15
|
31
|
CO2-O2
|
0.159
|
0.248
|
0.38
|
0.535
|
0.710
| ||
32
|
CO2-SF6
|
0.099
|
0.155
| |||||
33
|
D2-H2
|
0.631
|
1.079
|
1.219
|
1.846
|
2.778
|
3.866
|
5.103
|
34
|
H2-He
|
0.775
|
1.320
|
1.490
|
2.255
|
3.394
|
4.726
|
6.242
|
35
|
H2-Kr
|
0.340
|
0.601
|
0.682
|
1.053
|
1.607
|
2.258
|
2.999
|
36
|
H2-N2
|
0.408
|
0.686
|
0.772
|
1.162
|
1.743
|
2.423
|
3.196
|
37
|
H2-Ne
|
0.572
|
0.982
|
0.317
|
1.684
|
2.541
|
3.541
|
4.677
|
38
|
H2-O2
|
0.692
|
0.756
|
1.188
|
1.792
|
2.497
|
.299
| |
39
|
H2-SF6
|
0.208
|
0.649
|
0.998
|
1.400
|
1.851
| ||
40
|
H2-Xe
|
0.513
|
0.122
|
0.890
|
1.349
|
1.885
|
2.493
| |
41
|
H2O-N2
|
0.242
|
0.399
| |||||
42
|
H2O-O2
|
0.244
|
0.403
|
0.645
|
0.882
|
1.147
| ||
43
|
He-Kr
|
0.330
|
0.559
|
0.629
|
0.942
|
1.404
|
1.942
|
2.550
|
44
|
He-N2
|
0.365
|
0.619
|
0.698
|
1.052
|
1.577
|
2.188
|
2.882
|
45
|
He-Ne
|
0.563
|
0.948
|
1.066
|
1.592
|
2.362
|
3.254
|
4.262
|
46
|
He-O2
|
0.641
|
0.697
|
1.092
|
1.640
|
2.276
| ||
47
|
He-SF6
|
1.109
|
0.592
|
0.871
|
1.190
| |||
48
|
He-Xe
|
0.282
|
0.478
|
0.538
|
0.807
|
1.201
|
1.655
|
2.168
|
49
|
Kr-N2
|
0.131
|
0.149
|
0.227
|
0.346
|
0.485
|
0.645
| |
50
|
Kr-Ne
|
0.131
|
0.228
|
0.258
|
0.392
|
0.587
|
0.812
|
1.063
|
51
|
Kr-Xe
|
0.035
|
0.064
|
0.073
|
0.116
|
0.181
|
0.257
|
0.344
|
52
|
N2-Ne
|
0.258
|
0.483
|
0.731
|
1.021
|
1.351
| ||
53
|
N2-O2
|
0.202
|
0.307
|
0.462
|
0.643
|
0.849
| ||
54
|
N2-SF6
|
0.148
|
0.231
|
0.328
|
0.436
| |||
55
|
N2-Xe
|
0.107
|
0.123
|
0.188
|
0.287
|
0.404
|
0.539
| |
56
|
Ne-Xe
|
0.111
|
0.193
|
0.219
|
0.332
|
0.498
|
0.688
|
0.901
|
57
|
O2-SF6
|
0.097
|
0.154
|
0.238
|
0.334
|
0.441
|
