capital cost considerations
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CAPITAL ENERGY TRADE-OFFS
By Siti Shawalliah Idris, AMIChemE
CPE 633 PROCESS ENGINEERING II
Capital Energy Trade-Offs
The correct setting for DTmin is ECONOMIC
COST
Capital
Total
1 2 DTmin
OPT
Capital Energy Trade-Offs
Energy cost targets as a function of DTmin
Energy Cost
DTmin
Capital Energy Trade-Offs
But what about capital cost ?
Number of units
Number of heat exchanger units: NMER
NMER =(Sabove -1 ) + (Sbelow -1)
S = number of streams including utilities
Heat Transfer Area Target
Heat transfer area – for an enthalpy
interval
Concept of calculation:
Construct the composite curves
Put heat exchangers on all streams in each
vertical section of the composite curves
Calculate the area in each section, taking into
account the specific heat transfer coefficients
and correction factors of each stream (U*f)
T
H
Network Area
We can set overall area target based on the following equation
Network Area, Amin
T
H
1
2
3
4
5 6
Trading off Energy and Capital Targets
We can track the variation of area target with DTmin
Trading off Energy and Capital Targets
T
H
DTmin1
E1
T
H
DTmin2
E2
1 2 Area
DTmin E1
E2
A2
A1
1
2
Area and no. of units can be obtained to give variation of capital cost Area
DTmin
N
DTmin
Capital cost
DTmin
Total Annual Cost
Capital cost
DTmin
Energy Cost
DTmin
Capital cost
DTmin
Total
Energy
Capital
DTopt
Then we design for DTminOPT (or a resonable value of DTmin
But, we should still optimise the design
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
What are the degrees of freedom when optimising the design?
Loop & Path
A loop is a path that begins and
ends at the same point.
A path is a sequence of distinct
lines that are connected to each
other.
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-Y (W)
-Y (W) +Y (W)
+Y (W) Path E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
Loop
Loop & Path
Heat duties can be changed within a loop without changing the utility consumption
This changes both loads and temperature differences
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-U (W)
-U (W)
+U (W)
+U (W)
Loop
Another Loop
Changes both loads and temperature differences
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
+V (W) -V (W)
-V (W) +V (W)
+V (W) -V (W)
-V (W) +V (W)
Loop
Heat Duties can be changed alonh a utility path to change the utility consumption
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-Y (W)
-Y (W) +Y (W)
+Y (W) Utility
There are other utility paths in this problem
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-Y (W)
-Y (W) +Y (W)
+Y (W) Utilit
y
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-X (W)
-X (W) +X (W)
+X (W) Utility +X (W)
+X (W) -X (W)
-X (W)
a b
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-Z (W)
-Z(W) +Z (W)
-Z (W) Utility
Path
+Z (W)
+Z (W)
-Z (W)
+Z (W)
E
E
A
A
1
2
3
4
40 oC
80 oC
20 oC
140 oC
250 oC B
B
C
C
D
D
C
200 oC
180 oC
230 oC H
-U (W)
-U(W) +U (W)
-U (W) Utility
Path
+U (W)
+U (W)
-U (W)
+U (W)
c d
What is optimum ?
U, V, X,Y, Z must be varied simultaneously to minimise cost
MULTIVARIATE OPTIMISATION
ALSO, for designs with stream splits
Branch flowrates are additional degrees of freedom
CP T
0.94 180o
1.5 300o
2.0 350o
CP T
2.06 354o
1.5 300o
1.0 200o
1
3
T>180oC
2
500 oC
480 oC
460 oC
T>160oC
500 oC
300 oC
180 oC
160 oC
3
CP
1
1
Summary
Energy and capital cost targets can be set prior to design
Energy and capital cost can be traded off ahead of design
Network designs can be optimised by exploiting the degrees of
freedom in loops, utility paths and stream splits
Summary
Some problems exhibit a threshold – only hot or cold utility required.
True threshold problems have large temperature driving force and no
pinch.
Most threshold problems turn out to be pinched problems after
multiple utilities used
Thank you for your attention
Capital Energy Trade-offs
Working Example
Maximum Energy Recovery Design
Identify the degrees of freedom for network optimisation
A
A
1
2
4
6
42 oC
90 oC 160 oC
B
B
C
C
C
100 oC
140 oC
150 oC H
D
D
HP
H
3
100 oC
80 oC
80 oC
5 25 oC 79 oC
25 oC
75 oC 40 oC
C
4500
2100
16000
7050
1925
16670
4330
2425
DTmin = 20oC
A
A
1
2
4
6
42 oC
90 oC 160 oC
B
B
C
C
C
100 oC
140 oC
150 oC H
D
D
H
3
100 oC
80 oC
80 oC
5 25 oC 79 oC
25 oC
75 oC 40 oC
C
4500
2100
16000
7050
1925
16670
4330
2425
Solution
A
A
1
2
4
6
42 oC
90 oC 160 oC
B
B
C
C
C
100 oC
140 oC
150 oC H
D
D
HP
H
3
100 oC
80 oC
80 oC
5 25 oC 79 oC
25 oC
75 oC 40 oC
C
4500
2100
16000
7050
1925
16670
4330
2425
Loop
1 A
A
1
2
4
6
42 oC
90 oC 160 oC
B
B
C
C
C
100 oC
140 oC
150 oC H
D
D
H
3
100 oC
80 oC
80 oC
5 25 oC 79 oC
25 oC
75 oC 40 oC
C
4500
2100
16000
7050
1925
16670
4330
2425
Loop
A
A
1
2
4
6
42 oC
90 oC 160 oC
B
B
C
C
C
100 oC
140 oC
150 oC H
D
D
H
3
100 oC
80 oC
80 oC
5 25 oC 79 oC
25 oC
75 oC 40 oC
C
4500
2100
16000
7050
1925
16670
4330
2425
Utility path 1
Utility path 2
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