Molecular Diffusion of A through Non-diffusing B

In summary: Transport Phenomena by Bird, Stewart, and Lightfoot? It explains the concept of bulk flow in much more detail.
  • #1
AAMAIK
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Diffusion of A through Non-diffusing B
Dry air is required for burning of sulphur in a sulphuric acid plant. Moisture (A) diffuses through a film or layer of air (B), reaches the acid and gets absorbed in it. But air being virtually insoluble in sulphuric acid will not diffuse. So air is non-diffusing.
If we consider steady-state molecular diffusion through a constant area. A diffuses because it has a non zero gradient of partial pressure at each point of the diffusion path. But Why is it so that B (Blue circles) is not diffusing if at each point along the diffusion length its partial pressure varies due to the constant outflux of molecules A (red circles) in the direction of decreasing concentration or partial pressure?
 

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  • #2
A (moisture) and B (air) are actually diffusing to the different locations in micro-environment. In your specific case, moisture preferentially diffuse to sites where high electric fields (sulfate anions to protons connection) is present and incorporation of polarizable water actually decrease free energy, adding "drift" component to entropy-driven "diffusion". Air molecules are nearly non-polarizable, and for them net attractive force (and therefore pressure gradient) to high electrical field areas is much smaller. For air molecules, diffusion is nearly purely entropy-driven, therefore less efficient (no "drift" component).
As result, you can have different pressure gradients for different gases across the same macroscopic surface.
 
  • #3
The overall flux of B is comprised of the bulk flow times the concentration of B in the bulk flow plus the diffusive flux of B relative to the bulk flow. The bulk flow is away from the boundary and the diffusive flux of B is toward the boundary, so the net flux of B is zero.
 
  • #4
Chestermiller said:
The overall flux of B is comprised of the bulk flow times the concentration of B in the bulk flow plus the diffusive flux of B relative to the bulk flow. The bulk flow is away from the boundary and the diffusive flux of B is toward the boundary, so the net flux of B is zero.
So B carries A towards the phase boundary A diffuses through the phase boundary and at the same time B moves away from the boundary?
 
  • #5
AAMAIK said:
So B carries A towards the phase boundary A diffuses through the phase boundary and at the same time B moves away from the boundary?
Almost. A and B are transporting by mean transport toward the boundary together. Superimposed on that, A is diffusing toward the boundary while B is diffusing away from the boundary. The net result of all that is that the flux of B is zero, and the flux of A is toward the boundary.
 
  • #6
For a much more detailed discussion on all of this, see the chapters on mass transfer in Transport Phenomena by Bird, Stewart, and Lightfoot.
 
  • #7
Chestermiller said:
Almost. A and B are transporting by mean transport toward the boundary together. Superimposed on that, A is diffusing toward the boundary while B is diffusing away from the boundary. The net result of all that is that the flux of B is zero, and the flux of A is toward the boundary.
I can't visualize how these two phenomena could take place at the same time. If we were to place a stationary observer then he would note a flux of A and B towards the boundary, A diffuses through the phase boundary and reaches the acid surface. The stationary observer would also note a flux of dry air away from the boundary.
I have another question, what is physically meant by bulk flow?
The equation for the molar flux of the diffusive flux relative to a stationary observer contains the term responsible for bulk flow which to me is a matter of whether the frame of reference is moving or stationary, diffusion is still occurring by the molecular diffusion mechanism.
I am sorry for bothering you @Chestermiller
 
  • #8
AAMAIK said:
I can't visualize how these two phenomena could take place at the same time. If we were to place a stationary observer then he would note a flux of A and B towards the boundary, A diffuses through the phase boundary and reaches the acid surface. The stationary observer would also note a flux of dry air away from the boundary.
I have another question, what is physically meant by bulk flow?
The equation for the molar flux of the diffusive flux relative to a stationary observer contains the term responsible for bulk flow which to me is a matter of whether the frame of reference is moving or stationary, diffusion is still occurring by the molecular diffusion mechanism.
I am sorry for bothering you @Chestermiller
Have you consulted the reference that I recommended?
 

1. What is molecular diffusion?

Molecular diffusion is the process by which molecules move from an area of high concentration to an area of low concentration. This is driven by the natural random motion of molecules and occurs until there is an equal distribution of the molecules throughout the space.

2. What is the role of A and B in molecular diffusion?

A and B refer to different types of molecules. A is the diffusing molecule, meaning it is the one moving from an area of high concentration to an area of low concentration. B is the non-diffusing molecule, meaning it does not move and serves as a barrier for the diffusing molecule.

3. What factors affect the rate of molecular diffusion?

The rate of molecular diffusion is affected by several factors, including the concentration gradient (the difference in concentration between two areas), temperature (higher temperatures increase the rate of diffusion), and the size and shape of the molecules (smaller molecules diffuse faster).

4. How does molecular weight affect the diffusion of A through B?

Molecular weight can play a role in the rate of diffusion. Generally, larger molecules will diffuse slower than smaller molecules due to their larger size and mass. However, other factors such as temperature and concentration gradient can also affect the diffusion rate.

5. What are some real-life applications of molecular diffusion?

Molecular diffusion plays a crucial role in many natural and industrial processes. It is important in biological systems for processes such as gas exchange in the lungs and the transport of nutrients in cells. In industry, molecular diffusion is used in processes such as distillation, filtration, and chemical reactions.

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