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Molecular Diffusion of A through Non-diffusing B

  1. Dec 2, 2018 #1
    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. jcsd
  3. Dec 2, 2018 #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.
  4. Dec 3, 2018 #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.
  5. Dec 3, 2018 #4
    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?
  6. Dec 3, 2018 #5
    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.
  7. Dec 3, 2018 #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.
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