37
CHAPTER 9
Process Furnace Design Case
Study
9.1 PROBLEM STATEMENT AND SOLUTION
An economic process model was developed [1, 2] for an industrial metallurgical application. The
annual cost for a furnace operation in which the slag-metal reaction is a critical factor of the process
was considered and a modified version of the problem is presented. The objective was to minimize
the annual cost and the primal equation representing the model was:
Y = C
1
/(L
2
D T
2
) + C
2
L D + C
3
L D T
4
. (9.1)
The model was subject to the constraint that:
D L.
The constraint must be set in geometric programming for which would be:
(D/L) 1 (9.2)
where
D = Depth of the furnace (ft)
L = Characteristic Length of the furnace (ft)
T = Furnace Temperature (K) .
For the specific example problem, the values of the constants were:
C
1
= 10
13
($ ft
2
K
2
)
C
2
= 100($/ft
2
)
C
3
= 5 10
11
(ft
2
K
4
).
From the coefficients and signs, the signum values for the dual are:
σ
01
= 1
σ
02
= 1
σ
03
= 1
σ
11
= 1
σ
1
= 1 .
38 9. PROCESS FURNACE DESIGN CASE STUDY
D
L
T=?
Figure 9.1: Process furnace.
The dual problem formulation is:
Objective Function ω
01
+ ω
02
+ ω
03
= 1 (9.3)
L terms 2ω
01
+ ω
02
+ ω
03
ω
11
= 0 (9.4)
D terms ω
01
+ ω
02
+ ω
03
+ ω
11
= 0 (9.5)
T terms 2ω
01
+ 4ω
03
= 0 . (9.6)
Using Equations (9.3)to(9.6), the values of the dual variables were found to be:
ω
01
= 0.4
ω
02
= 0.4
ω
03
= 0.2
ω
11
=−0.2 .
The dual variables cannot be negative and the negative value implies that the constraint is
not binding, that is it is a loose constraint. Thus, the problem must be reformulated without the
constraint and the dual variable is forced to zero, that is, ω
11
= 0 and the equations resolved. This
means that the constraint D L will be loose, that is D will be less than L in the solution. The new
dual becomes:
Objective Function ω
01
+ ω
02
+ ω
03
= 1 (9.7)
L terms 2ω
01
+ ω
02
+ ω
03
= 0 (9.8)
D terms ω
01
+ ω
02
+ ω
03
= 0 (9.9)
T terms 2ω
01
+ 4ω
03
= 0 . (9.10)
Now the problem is that it has 4 equations to solve for three variables. If one examines
Equations (9.8) and (9.9), one observes that Equation (9.8) is dominant over Equation (9.9) and
thus Equation (9.9) will be removed from the dual formulation. The new dual formulation is:
9.1. PROBLEM STATEMENT AND SOLUTION 39
Objective Function ω
01
+ ω
02
+ ω
03
= 1 (9.11)
L terms 2ω
01
+ ω
02
+ ω
03
= 0 (9.12)
T terms 2ω
01
+ 4ω
03
= 0 . (9.13)
The new solution for the dual becomes:
ω
01
= 1/3
ω
02
= 1/2
ω
03
= 1/6
and by definition
ω
00
= 1.0 .
The dual variables indicate that the second term is the most important, followed by the first
term and then the third term. The degrees of difficulty are now equal to:
D = T (N + 1) = 3 (2 + 1) = 0 .
The objective function can be found using the dual expression:
Y = d(ω) = σ
M
m=0
T
m
t=1
(C
mt
ω
mo
mt
)
σ
mt
ω
mt
σ
(9.14)
= 1[[{(C
1
1/(1/3)1)}
(1/31)
][{(C
2
1/(1/2)}
(1/21)
][{(C
3
/(1/6))}
(1/61)
]]
1
= 1[[{(1 10
13
1/(1/3)1)}
(1/31)
][{(100 1/(1/2)}
(1/21)
][{(5 10
11
/(1/6))}
(1/61)
]]
1
= $11, 370 .
This can be expressed in a general form in terms of the constants as:
Y = (3C
1
)
1/3
(2C
2
)
1/2
(6C
3
)
1/6
. (9.15)
The values for the primal variables can be determined from the relationships between the
primal and dual which are:
C
1
L
2
D
1
T
2
= ω
01
Y (9.16)
C
2
L D = ω
02
Y (9.17)
and C
3
L D T
4
= ω
03
Y. (9.18)

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