Homework Set # 12: Shell mass/molar balance for components in a multi
component system
Problem #1:
Recall the example of evaporation of component A contained in a test tube in the lecture
note (or see section 18.2, 2
nd
edition, 17.2, 1
st
edition).
(a)
By neglecting the convective flux of component A, show that this leads to
(
)
(
)
1
1
2
2
1
A
A
z
z
cD
N
x
z
z
=
=
−
−
A
x
(b)
What error is made in the determination of D in the example 18.22 (or 17.22, 1
st
edition) if the result in (a) is used?
Answer:
a.
Without the convection term, the total molar flux of A is equal to the diffusion flux of
A. For steady state
.
A
A
dx
N
cD
const
dz
= −
=
Since c and D are constant, thus
(
)
(
)
1
2
2
1
.
A
A
A
x
x
dx
const
dz
z
z
−
=
= −
−
Therefore,
(
)
(
)
1
1
2
2
1
A
A
z
z
cD
N
x
z
z
=
=
−
−
A
x
b.
Error in the determination of D in the example 18.22
To get the solution of Example 18.22 by using the result in (a), we make the following
calculation with the help of the equation of state for an ideal gas.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '08
 Peters
 #, 0.79%

Click to edit the document details