This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ) (2) NR = 6 and NA = 0
Therefore, Eq. (1) becomes, ND ND
all elements Let N1 = number of rectifying stages and N2 = number of stripping stages.
Using the values of ND for the elements in Table 5.3:
Rectifying section
2N 1 + 2C + 5
Feed stage with heat transfer
3C + 8
Stripping section
2N 2 + 2C + 5
Partial reboiler
C+4
N D e = 2(N1 + N2) + 8C + 22 (3) all elements Substituting Eq. (3) into (2),
N D unit = 2 N 1 + N 2 + 8C + 22 − 6 C + 2 = 2 N1 + N 2 + 2C + 10 (4) However, in Operation (e) of Table 5.4, the feed stage and the partial reboiler are counted
in N.
Therefore, N1 + N2 = N  1  1 = N  2. Substituting this into Eq. (4), gives ( N D )unit = 2( N − 2) + 2C + 10 = 2 N + 2C + 6
which verifies the result given in Table 5.4. (5 Exercise 5.29 (continued)
Analysis: (continued)
If a vapor sidestream is added, we add 2 degrees of freedom, one for the location and one
for the sidestream flow rate.
Then, ( N D ) unit with sidestream = 2 N + 2C + 6 + (2) = 2 N + 2C + 8
Operation (f), reboiled stripper:
Using the values of ND for the elements in Table 5.3:
Section of N  1 stages
Partial reboiler 2(N  1) + 2C + 5
C+4
N D e = 2N +3C + 7 (6) all elements Substituting Eq. (3) into (2), with NR = 2 and NA = 0, ( N D )unit = 2 N + 3C + 7 − 2 ( C + 2 ) = 2 N + C + 3
If a vapor sidestream is added, we add 2 degrees of freedom, then, ( N D )unit with sidestream = 2 N + C + 3 + (2) = 2 N + C + 5 (7) Exercise 5.30
Subject:
Verification of numbers of degrees of freedom for one operation, and
effect of adding a liquid sidestream..
Given: Degrees of freedom for operation (h) in Table 5.4. Find: Verify value of ND and determine effect of adding a liquid sidestream
Analysis: Operation (h), extractive distillation:
From Eq. (570),
N D unit =
ND e − NR C + 2 + N A (1) NR = 13 and NA = 0
Therefore, Eq. (1) becomes,
N D unit = (2) all elements ND
all elements e − 13(C + 2) Let N1 = number of rectifying stages above the MSA feed,
N2 = number of intermediate stages between the MSA feed and the feed,
N3 = number of stripping stages.
Using the values of ND for the elements in Table 5.3:
Total condenser
C+4
Reflux divider
C+5
Rectifying section
2N 1 + 2C + 5
MSA Feed stage with heat transfer
3C + 8
Intermediate section
2N 2 + 2C + 5
Feed stage with heat transfer
3C + 8
Stripping section
2N 2 + 2C + 5
C+4
Partial reboiler
N D e = 2(N1 + N2 + N3 ) + 15C + 44
(3)
all elements Substituting Eq. (3) into (2),
N D unit = 2 N 1 + N 2 + N 3 + 15C + 44 − 13 C + 2 = 2 N 1 + N 2 + N 3 + 2C + 18 (4)
However, in Operation (h) of Table 5.4, the two feed stages and the partial reboiler are
counted in N. Therefore, N1 + N2 + N3 = N  3. Substituting this into Eq. (4), gives
(5)
( N D )unit = 2( N − 3) + 2C + 18 = 2 N + 2C + 12
which verifies the result in Table 5.4.
If a liquid sidestream is added, we add 2 degrees of freedom, then,
( N D )unit with sidestream = 2 N + 2C + 12 + (2) = 2 N + 2C + 14 Exercise 5.31
Subject: Alternative design variables for distillation Given: Distillation design variables in Table 5.4
Find: Conditions under which the alternative varia...
View Full
Document
 Spring '11
 Levicky
 The Land

Click to edit the document details