Unformatted text preview: (4) valve, with NR = 4 and NA = 0.
(a) and (b)
From Eq. (568),
NV system =
NV u − N R C + 3 + N A
(1)
all units From Eq. (569), NE system = NE
all units u − NR (2) For the distillation column, NV and NE can be obtained by adding a second feed to the
results given in Example 5.5. This adds C + 3 to NV and 1 to NE. Therefore,
NV = (7N+2NC+5C+20) + (C+3) = 7N+2NC+6C+23
and NE = (5N+2NC+4C+9) + 1 = 5N+2NC+4C+10
For the reboiled stripper, the determination of NV and NE is given in Exercise 5.36.
For the cooler, use the total condenser in Table 5.1.
For the valve, use the total condenser in Table 5.1, less one variable for no heat transfer.
Unit (or element)
NV
NE
Distillation column
7N+2NC+6C+23
5N+2NC+4C+10
Reboiled stripper
7M+2MC+C+4
5M+2MC+1
Cooler
2C+7
C+3
Valve (no heat transfer)
2C+6
C+3
Total
7(N+M)+2(N+M)C+11C+40 5(N+M)+2(N+M)C+6C+17
Using Eqs. (1) and (2),
( NV )system = 7(N +M )+2(N +M )C +11C +40 − 4 ( C + 3) ( N E )system = 7(N +M )+2(N +M )C + 7C + 28
= 5(N +M )+2(N +M )C +6C +17 − 4 = 5(N +M )+2(N +M )C + 6C + 13
(c) From Eq. (571),
( N D )system = ( NV )system − ( N E )system = [ 7(N +M )+2(N +M )C + 7C + 28] − [5(N +M )+2(N +M )C + 6C + 13]
= 2( N + M ) + C + 15 Exercise 5.39 (continued)
Analysis:. (d)
A reasonable set of specifications is:
Pressures of stages, 2 reboilers, condenser
Pressures of cooler, reflux divider, valve
Heat transfer for stages and divider
Feed completely specified
Feed stage and sidestream stage locations
Distillate rate
Reflux ratio
Number of stages for N and M
Sidestream rate
Boilup ratio in stripper
Saturated liquid reflux
Exit temperature from cooler
Total N+M+1
3
N +M1
C+2
2
1
1
2
1
1
1
1 2(N + M) + C + 15 Exercise 5.40
Subject: Allowable specifications for system of extractive distillation and distillation
Given: System in Fig. 5.32
Find: If given specifications are sufficient. If not, give additional specifications.
Analysis: System consists of a distillation column with 2 feeds (Case h in Table 5.4), a
distillation column (Case b in Table 5.4), a cooler (use total condenser in Table 5.3), and
a mixer (Table 5.4). From these sources, the degrees of freedom areas follows:
Unit
ND
Extractive distillation
2N1+2C+12
Distillation
2N2+C+9
Cooler
C+4
Mixer
2C+6
Total
2(N1+N2)+6C+31
From Eq. (570),
ND unit = ND
all elements e − NR C + 2 + N A (1) NR = 4 and NA = 0
Therefore, Eq. (1) becomes,
N D unit = 2 N 1 + N 2 + 6C + 31 − 4 C + 2 = 2 N 1 + N 2 + 2C + 23
A reasonable set of specifications is:
Pressures of stages, 2 reboilers, 2 condensers N1 + N2 + 2
Pressures of cooler, 2 reflux dividers, mixers
4
Heat transfer for stages, 2 dividers, mixer
N 1 + M2 + 1
Feed completely specified
C+2
Makeup specified except flow rate
C+1
Two feed stage locations
2
Distillate rate
1
Two reflux rates
2
Number of stages for N1 and N2
2
Phenol recycle rate
1
Total 2(N1 + N2) + 2C + 18 Therefore, 5 more specifications are needed. They might be the following:
Saturated liquid reflux from 2 condensers
2
Feed stage...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
 Spring '11
 Levicky
 The Land

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