5.12: Untitled Page 110
- Page ID
- 18243
Chapter 5
In this case we are given the inlet mole fractions, ( x ) 0 and A o
( y )
0 . 010 , and the system parameters, K
2 5
. 3 , M
90 kmol/h
A N
1
eq,A
and M
30 kmol/h . For these particular values, we want to know how
Figure 5.9. Multi‐stage gas‐liquid contacting device
many stages, N, are required to reduce the exit mole fraction of acetone to ( y ) 0 001
.
. In this case we express Eq. 5‐91 as
A 1
( y )
N
A N
0 010
.
2
3
1
1 A A A .... A
10 . 0
(1)
( y )
0 . 001
A 1
Two‐Phase Systems & Equilibrium Stages 201
in which the value of the absorption coefficient is given by A M
M
K
1 186
.
. This leads to the values listed in Table 5.9
eq,A
where we see that 5 stages are insufficient to achieve the desired result, ( y ) 0 001
.
. In addition, the use of 6 stages reduces the exit mole A 1
fraction of acetone in the air stream to ( y ) 0 00081
.
which is less than
A 1
the desired result. For this type of situation, it is the responsibility of the chemical engineer to make a judgment based on safety considerations, environmental constraints, requirements for other processing units, and economic optimization. Such matters are covered in a future course on process design, and we have alluded to some of these concerns in Sec. 1.1.
Table 5.9. Number of stages, N, versus ( y )
( y )
A N
1
A 1
Number of stages, N
1 + A + A 2 + A 3 +….+ AN
1
2.186
2
3.592
3
5.260
4
7.238
5
9.584
6
12.366
Type II: Given the inlet mole fractions, ( x ) and ( y )
, the
A o
A N1
system parameters, and the number of stages, N, we would like to determine the value of ( y ) .
A 1
In this case we consider an existing unit in which there are 7 stages. The inlet mole fractions are given by ( x ) 0 and ( y ) 0 . 010 , and the A o
A 8
parameters associated with the system are specified as K
2 5
. 3 ,
eq,A
M
90 kmol/h and M
30 kmol/h . In order to determine the mole
fraction in the ‐phase (air) leaving the cascade, we make use of Eq. 5‐91
to express ( y ) as
A 1
( y )
A 8
( y )
(2)
A 1
2
3
4
5
6
1
7
A A A A A A A This leads to the following value of the mole fraction of acetone leaving the top of the cascade illustrated in Figure 5.9: