Molecular Diffusion of A through Non-diffusing B

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Discussion Overview

The discussion centers on the molecular diffusion of moisture (A) through a non-diffusing layer of air (B) in the context of a sulphuric acid plant. Participants explore the mechanisms of diffusion, the interaction between A and B, and the implications of bulk flow and pressure gradients in this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes the diffusion of moisture through air, questioning why air does not diffuse despite a varying partial pressure along the diffusion path.
  • Another participant proposes that moisture preferentially diffuses to areas with high electric fields, suggesting that this creates a "drift" component that enhances diffusion compared to air molecules.
  • A claim is made that the overall flux of B consists of bulk flow and diffusive flux, leading to a net flux of zero for B.
  • There is a reiteration of the idea that B carries A towards the phase boundary while A diffuses through it, with a clarification that both A and B are moving toward the boundary together.
  • One participant expresses difficulty visualizing the simultaneous transport of A and B, suggesting that a stationary observer would perceive both moving towards the boundary while air moves away.
  • A question is raised about the physical meaning of bulk flow and its relation to diffusion, indicating a concern about the frame of reference in the context of molecular diffusion.

Areas of Agreement / Disagreement

Participants express differing views on the interaction between A and B, particularly regarding the nature of their movement and the implications of bulk flow. The discussion remains unresolved with multiple competing perspectives on the mechanisms at play.

Contextual Notes

Some participants reference the complexity of mass transfer and the need for further exploration of the concepts discussed, indicating that there may be limitations in their current understanding or definitions used.

Who May Find This Useful

This discussion may be of interest to those studying mass transfer, diffusion processes, or related fields in chemistry and engineering, particularly in industrial applications involving chemical reactions and transport phenomena.

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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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.
 
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.
 
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?
 
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.
 
For a much more detailed discussion on all of this, see the chapters on mass transfer in Transport Phenomena by Bird, Stewart, and Lightfoot.
 
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
 
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?
 

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