Chapter 7 Differential Equations of Mass Transfer

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Chapter 7 Differential Equations of Mass Transfer
? Control volume for the conservation of mass
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Mass transfer with chemical reaction in C.V.
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Equation of continuity of component A
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The equation of continuity for the binary
mixture Since
where
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In terms of molar units
(7.1)
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Where binary mixture of A and B
If
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? Special forms for mass transfer
By eq. (7.1)
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P1. If ?constant, it implies P2. If RA0 and
constant DAB
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P3. If RA0 and no fluid motion (v0), Ficks
second law P4. Steady-state process for P1P3
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Boundary conditions P1. Gas-liquid surface
without reaction, ideal gas phase and
species A is weakly soluble in liquid
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P2. A reacting surface boundary Ex. The flux of
A and B move in the opposite direction to
the flux C with reaction
and on a surface
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P3. Zero flux at the boundary
P4. Convective mass transfer at the boundary
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? Steps for modeling process S1. Draw the
physical system S2. List of assumptions S3. To
reduce the physical system, e.g.
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S4. Boundary conditions
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Example 1. Develop a differential model for CVD
process
Note (1) In diffusion zone, RA0 (2)
Feed gas is exceeded
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(3) Molecular diffusion (4)
The flux is one-dimensional along z (6)
Steady-state process Modeling S1. By eq. (7.1)
for rectangular coordinates
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S2. In diffusion zone
where
S3. Reactant flux is opposite in the direction to
the product flux
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S4. Boundary conditions
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Example 2. Determine the flux of WF6 gas (species
A) onto the surface in CVD process
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S2. In one-dimensional diffusion zone
For gaseous reactants
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Exercises Problems 25.5, 25.10, 25.12, 25.15